DIISC-IV. DIISCovery of Anomalously Low Metallicity H II Regions in NGC99: Indirect Evidence of Gas Inflows Alejandro J. Olvera1 , Sanchayeeta Borthakur1 , Mansi Padave1 , Timothy Heckman1,2 , Hansung B. Gim1,3 , Brad Koplitz1 , Christopher Dupuis1 , Emmanuel Momjian4 , and Rolf A. Jansen1 1 School of Earth and Space Exploration, Arizona State University, 781 Terrace Mall, Tempe, AZ 85287, USA; ajolver2@asu.edu 2 Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA 3 Department of Physics, Montana State University, P.O. Box 173840, Bozeman, MT 59717, USA 4 National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801, USA Received 2024 March 28; revised 2024 July 24; accepted 2024 August 11; published 2024 November 25 Abstract As a part of the Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium (DIISC) survey, we investigate indirect evidence of gas inflow into the disk of the galaxy NGC 99. We combine optical spectra from the Binospec spectrograph on the MMT telescope with optical imaging data from the Vatican Advanced Technology Telescope, radio H I 21 cm emission images from the NSF Karl G. Jansky’s Very Large Array, and UV spectroscopy from the Cosmic Origins Spectrograph on the Hubble Space Telescope. We measure emission lines (Hα, Hβ, [O III]λ5007, [N II]λ6583, and [S II]λ6717, 31) in 26 H II regions scattered about the galaxy and estimate a radial metallicity gradient of −0.017 dex kpc−1 using the N2 metallicity indicator. Two regions in the sample exhibit an anomalously low metallicity (ALM) of 12+ log(O/H)= 8.36 dex, which is ∼0.16 dex lower than other regions at that galactocentric radius. They also show a high difference between their H I and Hα line of sight velocities on the order of 35 km s−1. Chemical evolution modeling indicates gas accretion as the cause of the ALM regions. We find evidence for corotation between the interstellar medium of NGC 99 and Lyα clouds in its circumgalactic medium, which suggests a possible pathway for low metallicity gas accretion. We also calculate the resolved Fundamental Metallicity Relation (rFMR) on subkiloparsec scales using localized gas- phase metallicity, stellar mass surface density, and star formation rate surface density. The rFMR shows a similar trend as that found by previous localized and global FMR relations. Unified Astronomy Thesaurus concepts: Circumgalactic medium (1879); Galaxy abundances (574); Galaxy accretion (575); Galaxy chemical evolution (580); H II regions (694) Materials only available in the online version of record: animation 1. Introduction Cosmological simulations confirm that gas cycling in and out of galaxies, also known as the baryon cycle, is crucial to their growth. Gas inflows into galactic disks are necessary to maintain star formation over cosmic time, whereas galactic outflows are important to regulating star formation rates (D. Kereš et al. 2005; P. F. Hopkins et al. 2014; C.-A. Faucher-Giguère & S. P. Oh 2023). The interstellar medium (ISM) serves as the center stage for the baryon cycle as it is where accreted gas may collapse to form stars and where outflows from stellar remnants are produced. Thus, in order to further understand the physical mechanisms affecting the ISM, it is vital to know its atomic gas content (MHI), molecular gas content (MH2), star formation rate (SFR), and metallicity (Z). Measurement of metal abundances in galaxies provides an observational approach to inferring gas in/outflows and star formation since most metals are the result of the nucleosynth- esis within stars and in their supernovae explosions (K. Finlator & R. Dave 2008; S. J. Lilly et al. 2013; F. Belfiore et al. 2016). Therefore, the radial metallicity gradient of galaxies is of great interest to study how galaxies grow (e.g., inside-out or outside- in; G. Pezzulli & F. Fraternali 2016; R. B. Larson 1976; F. Matteucci & P. Francois 1989; A. Acharyya et al. 2020; E. Wang & S. J. Lilly 2022). In the local Universe, most massive galaxies show a negative metallicity gradient with metallicity dropping with galactocentric radius (M. B. Vila-Costas & M. G. Edmunds 1992; M. S. Oey & R. C. J. Kennicutt 1993; S. F. Sánchez 2020). Studies with large samples of galaxies, such as the Calar Alto Legacy Integral Field Area (CALIFA) survey or the Sloan Digital Sky Survey (SDSS), have measured a characteristic gradient of approximately−0.1 dex/Re from their distribution of slopes, where Re is the disk effective radius (S. F. Sánchez et al. 2014; L. Sánchez-Menguiano et al. 2016b; T. Parikh et al. 2021). However, individual galaxies can have multiple metallicity slopes within the disk of the galaxy (L. Sánchez-Menguiano et al. 2018). In our current understanding of galaxy evolution, gas from the regions surrounding galaxies, namely, the circumgalactic medium (CGM) or intergalactic medium, is accreted onto the outer parts of the galactic disk (R. Roskar et al. 2010; A. S. Font et al. 2011; C. N. Lackner et al. 2012; S. M. Moran et al. 2012; J. Sánchez Almeida et al. 2014). The accretion process is thought to be crucial to supplying galaxies with gas to continue forming stars. However, directly observing active gas accretion is challenging due to the low densities in the CGM. Indirect evidence of gas accretion can be found via observations of the bright star-forming regions within galactic disks since inflows of ex situ gas can have a chemical The Astrophysical Journal, 976:205 (18pp), 2024 December 1 https://doi.org/10.3847/1538-4357/ad8238 © 2024. The Author(s). Published by the American Astronomical Society. 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Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. 1 https://orcid.org/0000-0002-2819-0753 https://orcid.org/0000-0002-2819-0753 https://orcid.org/0000-0002-2819-0753 https://orcid.org/0000-0002-2724-8298 https://orcid.org/0000-0002-2724-8298 https://orcid.org/0000-0002-2724-8298 https://orcid.org/0000-0002-3472-0490 https://orcid.org/0000-0002-3472-0490 https://orcid.org/0000-0002-3472-0490 https://orcid.org/0000-0001-6670-6370 https://orcid.org/0000-0001-6670-6370 https://orcid.org/0000-0001-6670-6370 https://orcid.org/0000-0003-1436-7658 https://orcid.org/0000-0003-1436-7658 https://orcid.org/0000-0003-1436-7658 https://orcid.org/0000-0001-5530-2872 https://orcid.org/0000-0001-5530-2872 https://orcid.org/0000-0001-5530-2872 https://orcid.org/0000-0003-1739-3640 https://orcid.org/0000-0003-1739-3640 https://orcid.org/0000-0003-1739-3640 https://orcid.org/0000-0003-3168-5922 https://orcid.org/0000-0003-3168-5922 https://orcid.org/0000-0003-3168-5922 https://orcid.org/0000-0003-1268-5230 https://orcid.org/0000-0003-1268-5230 https://orcid.org/0000-0003-1268-5230 mailto:ajolver2@asu.edu http://astrothesaurus.org/uat/1879 http://astrothesaurus.org/uat/574 http://astrothesaurus.org/uat/575 http://astrothesaurus.org/uat/575 http://astrothesaurus.org/uat/580 http://astrothesaurus.org/uat/694 https://doi.org/10.3847/1538-4357/ad8238 https://doi.org/10.3847/1538-4357/ad8238 https://crossmark.crossref.org/dialog/?doi=10.3847/1538-4357/ad8238&domain=pdf&date_stamp=2024-11-25 https://crossmark.crossref.org/dialog/?doi=10.3847/1538-4357/ad8238&domain=pdf&date_stamp=2024-11-25 http://creativecommons.org/licenses/by/4.0/ composition different from that of disk gas. If the newly introduced gas is metal poor, it can dilute the metal content of the star-forming regions (B. M. Tinsley 1973; F. Bournaud & B. G. Elmegreen 2009; A. Dekel et al. 2009; F. Vincenzo et al. 2016a; Z. J. Pace et al. 2021). The diluted regions can then become anomalies in the metallicity pattern across the galaxy. Several studies of individual galaxies have found these anomalously low-metallicity (ALM) regions (J. Sánchez Almeida et al. 2014; J. C. Howk et al. 2018a, 2018b; Y. Luo et al. 2021; M. Ju et al. 2022). Recently, integral-field surveys with more statistical robustness, like the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey, have found a large sample of ALM regions with a preference for low-mass galaxy hosts (<1010Me; H.-C. Hwang et al. 2019). Theory and a growing amount of observations suggest that these ALM regions are a result of low Z gas accretion into the disk that forms young stars (J. C. Howk et al. 2018a; H.-C. Hwang et al. 2019; L. Scholz-Diaz et al. 2021). 1.1. NGC 99 As a pilot study, we selected NGC 99 to explore radial metallicity gradients, search for ALM H II regions, and investigate a resolved fundamental metallicity relation (FMR). NGC 99 is a star-forming spiral galaxy at a distance of 79.4 Mpc. As part of the Deciphering the Interplay between the Interstellar medium, Stars, and the Circumgalactic medium (DIISC) survey (S. Borthakur et al. 2024), NGC 99 has extensive multiwavelength data coverage allowing us to probe the chemical compositions of its star-forming regions, ISM, and CGM and connect that information to other physical properties such as stellar mass (Må) and SFR. To do this, we combine data from the Very Large Array, the Vatican Advanced Technology Telescope, the Hubble Space Tele- scope, the LBT, and our newly obtained multiobject spectra from the MMT. M. Padave et al. (2024a) recently measured the global stellar mass (Må,global) and star formation rate (SFRglobal) of NGC 99 to be 4.17× 1010Me and 2.45Me yr−1, respec- tively. H. B. Gim et al. (2024, in preparation) found the total H I mass to be 1.68× 1010Me. Other properties of NGC 99 can be found in Table 1. We combine optical spectra of 26 H II regions with optical band and radio H I 21 cm imaging to study the disk of NGC 99. We also probe the CGM of the galaxy at 159 kpc from its center with a UV-bright QSO sightline. The paper is organized as follows. In Section 2, we describe the suite of data available and its reduction. In Section 3, we analyze radial gradients of the dust content and metallicity. In Section 4, we discuss our results in the context of other studies, explore chemical evolution models, and derive a resolved FMR. We summarize our findings in Section 5. 2. Observations and Data Analysis 2.1. Optical Spectroscopy of H II Regions We obtained multislit spectra for NGC 99 using the new spectroscopy instrument, Binospec, on the MMT 6.5 m telescope located on Mount Hopkins in Arizona. The observations were taken over a period of four nights between 2020 September and 2021 November with an average seeing of 1 15. A catalog of far-ultraviolet (FUV) bright targets was created using SExtractor (E. Bertin & S. Arnouts 1996) on an archival FUV image from the Galaxy Evolution Explorer (GALEX; D. C. Martin et al. 2005; P. Morrissey et al. 2007). The image is presented in M. Padave et al. (2024a). Regions with a FUV magnitude brighter than 25 were added to the catalog. UV regions are generally spatially correlated with Hα regions as both are used as star formation indicators. Hence, we are targeting and detecting H II regions. The BinoMask software (J. Kansky et al. 2019) then automatically selected targets from the catalog by optimizing for the highest number of targets per mask design and placing slits to avoid the effects of differential atmospheric refraction. BinoMask selected a total of 31 UV-bright H II regions to observe. Each mask was observed for a total of 2400 s using four exposures of 600 s each during the same night. We used the 270 lines mm−1 grating of Binospec with a central wavelength of 5800Å for each slit at a dispersion of 1.3Å pix−1. This allowed us to cover the entire wavelength range of emission lines observed from 3900 to 9240Å in one setting. The data were reduced using the Binospec Data Reduction Pipeline, which carried out the bias correction, flat-fielding, wavelength calibration, relative flux correction, coaddition, and extraction to 1D spectra (J. Kansky et al. 2019). Of the 31 regions observed, the spectra of 26 regions had measurable emission lines. The slit locations can be seen in Figure 1. Figure 2, a spectrum of region 16, shows a representative H II region Table 1 Properties of NGC 99 Parameter Value R.A.a (α2000) 00h23m59 422 Decl.a (δ2000) 15 46 13. 04+  ¢  Morphological Typeb SABc Inclinationc 20° Redshiftd 0.01771 Distancee 79.4 Mpc Stellar mass (Må,global) c 4.17 × 1010 Me R25 c 11.46 kpc Re c 8.51 kpc E(B − V )f 0.0481 SFRglobal g 2.45 Me yr−1 RH I h 45.8 kpc H I massh 1.68 × 1010 Me Circular velocityh 292 km s−1 Gas dispersioni 14 km s−1 Halo massj 5.0 × 1011 Me Virial radiusj 207 kpc Notes. a M. F. Skrutskie et al. (2006). b R. J. Buta (2019). c M. Padave et al. (2024a). d C. M. Springob et al. (2005). e Calculated using the distance modulus value from S. F. Sánchez et al. (2012). f E. F. Schlafly & D. P. Finkbeiner (2011). g Estimated from far-ultraviolet and 22 μm data adopted from M. Padave et al. (2024a). h Based on the peak of the distribution of velocity dispersions in the Very Large Array D-configuration H I image. We adopt the peak value because some of the values at the central regions are impacted by beam smearing of the steeply varying rotation curve. i Based on Very Large Array D-conf. observations (H. B. Gim et al. 2024, in preparation). j Halo mass and virial radius were estimated from the stellar mass using prescriptions by A. V. Kravtsov et al. (2018) and applying modifications based on the findings of R. Mandelbaum et al. (2016). The full sample will be described in S. Borthakur et al. (2024). 2 The Astrophysical Journal, 976:205 (18pp), 2024 December 1 Olvera et al. spectrum produced by the Binospec instrument with the lines we aim to measure. After correcting the spectra for redshift, emission lines were found using the find_lines_derivative method from the specutils python package (N. Earl et al. 2022) and identified by matching the line center with an emission line database. The local continuum level within a ∼100Å window was measured using specutils’ fit_generic_continuum function and subtracted from each emission line. The total integrated flux of each line was then calculated using the amplitude and standard deviation of a modeled Gaussian. Using pyFIT3D (E. A. D. Lacerda et al. 2022), we modeled the stellar spectrum of our H II region with the strongest continuum level to investigate the effect of stellar absorption, which we found to be minimal. Therefore, for other targets where the continuum is weaker, the stellar absorption is not modeled as it would not be significant compared to the strength of the emission lines. Our absolute flux calibration has some uncertainty. How- ever, this should have no impact on our metallicity measure- ments and very little on the Balmer decrement. To ensure an accurate SFR estimate, we cross calibrated our Hα fluxes with VATT narrowband Hα imaging (M. Padave et al. 2024b). The Hα flux of each region is reported in Table A1. We investigated the shape of the Hα emission line, which is the strongest line in these spectra, to look for evidence of multiple component emission. We found that all but one of the emission lines fit well with a single Gaussian profile. The spectrum from Region 2 shows a slight wing toward shorter wavelengths. 2.2. Optical g- and r-band Data Optical continuum g- and r-band imaging of NGC 99 was obtained on UT 2020 October 7 using the VATT4k CCD imager at the 1.8 m Vatican Advanced Technology Telescope (VATT) operated by the Mount Graham Observatory. The total exposure times in g and r are 600 and 1200 s, respectively. The observation setup and data reduction are described in M. Padave et al. (2024a). We utilize the r-band image and statmorph (V. Rodrigu- ez-Gomez et al. 2019) to estimate position angle, ellipticity, and inclination. We then use these parameters and the R.A. and decl. of each region to calculate the H II region’s semimajor axis value. We assume the galaxy to be circular in projection, thus taking the semimajor value to be the H II region’s galactocentric radius. 2.3. VLA H I 21 cm Data The H I 21 cm emission from NGC 99 was observed with the Karl G. Jansky Very Large Array (VLA) in the D-configuration as a part of the VLA-DIISC project (H. B. Gim et al. 2024, in preparation). The observations were performed from 2019 November 9 through November 16 (program ID: 19B-183) for a total of 6.5 hr with a channel spacing of 5.208 kHz within the bandwidth of 16MHz at the central frequency of 1395MHz. The data reduction was carried out with the Common Astronomy Software Application (CASA; J. P. McMullin et al. 2007) version 5.6.1 according to the general reduction schemes for H I spectroscopy. The absolute flux density scale and bandpass were calibrated using 3C 48, while the complex gain was calibrated using J2340+1333. Hanning smoothing was applied to the data to mitigate Gibbs ringing phenomena, resulting in an effective velocity resolution of 2.2 km s−1, doubling the original velocity width. The image cube was made by the CASA task tclean with a pixel size of 6″, the Briggs weighting function, and a robust value of 0.5 for the optimal sensitivity and synthesized beam size. The cleaning was performed until the maximum residual reached 1.5σ, where σ is the rms noise in the image. The image cube was spatially smoothed to the synthesized beam size of 50 5× 47 5 over all the channels. The final image cube has a sensitivity of 0.69 mJy beam−1 (2.2 km s−1)−1 corresponding to a column density of 6.81× 1018 cm−2. The H I 21 cm emission from NGC 99 was recovered by the Source Finding Application-2 (SoFiA-2, P. Serra et al. 2015; T. Westmeier et al. 2021) above 3σ, where it was found within the velocity range of 5125.4–5312.3 km s−1. The velocity width of the H I spectrum isW50= 130.54 andW20= 154.78 km s−1 at 50% and 20% of the maximum intensity, respectively. The H I mass was estimated to be MHI= (1.68± 0.03)× 1010Me assuming a luminosity distance of 79.4Mpc. 2.4. COS Spectra The ultraviolet spectrum of the quasi-stellar object (QSO) SDSS J002330.58+154744.9, which is at an impact parameter of 159 kpc from the center of NGC 99, was obtained using the G130M medium resolution grating of the Cosmic Origin Spectrograph (COS; S. Osterman et al. 2011; J. C. Green et al. 2012) on board the Hubble Space Telescope under observing program GO-14071 (PI: Borthakur). After processing with the standard COS pipeline, the multiple G130M spectra were coadded and binned by 3 pixels, resulting in a spectral bin size Figure 1. A r-band image of NGC 99 obtained at the Vatican Advanced Technology Telescope with all the included Binospec slits placed on the H II regions labeled by their assigned number. The yellow arrow points in the direction of QSO SDSS J002330.58+154744.9, which is a projected distance of 159 kpc away from the galaxy center. 3 The Astrophysical Journal, 976:205 (18pp), 2024 December 1 Olvera et al. of ∼7 km s−1 in the observed wavelength coverage of λ= 1152–1453Å. All absorption features associated with the QSO, the Milky Way’s ISM, and NGC 99ʼs CGM were identified through visual inspection. We detected H I λ1215 (Lyα) in absorption in the CGM of NGC 99. To fit the Lyα profile, we determined the continuum bracketing the absorption by using feature-free regions within ±1000 km s−1. The continuum was estimated using a Legendre polynomial of order 2 using the procedure of K. R. Sembach et al. (2004), which was then used to produce the normalized spectrum. We fit Voigt profiles to each feature using the software of E. L. Fitzpatrick & L. J. Spitzer (1997), and techniques similar to J. Tumlinson et al. (2013). These fits derived the velocity centroids, Doppler b-values, and column densities for the profile, which are listed in Table 2. The measurement uncertainties were derived using the error analysis methods of K. R. Sembach & B. D. Savage (1992). We find four components (labeled A–D) of Lyα and fit each separately. Due to saturation, the best-fit Voigt profile for the dominant component (A) gives a lower limit on the column density of N(H I)> 2.69× 1014 cm−2 (Figure 3). The three weaker components (B–D) have column density estimates of N(H I)= 1.67× 1013, 1.13× 1013, and 1.20× 1013 cm−2. The centroid for the dominant component is −44 km s−1 offset Figure 2. A typical MMT/Binospec spectrum of an H II region in NGC 99. The flux is shown in log scale to show the level of the underlying stellar continuum. The three insets highlight the emission lines of interest in this study. Table 2 Measurements from QSO Absorption Spectroscopy Tracing the CGM of NGC 99 Species λrest Wrest a Component Centroidb Doppler b log N (Å) (mÅ) Label (km s−1) (km s−1) (log cm−2) H I 1215 513 ± 52 (A) −44 ± 8 42 ± 0.1 �14.4 ± 0.2 L L L (B) 34 ± 14 17 ± 42 13.2 ± 0.6 L L L (C) 99 ± 11 15 ± 41 13.1 ± 0.5 L L L (D) 366 ± 21 30 ± 1 13.1 ± 0.4 Si II 1302 �119c L L L � 12.86 Si III 1206 �144c L L L � 12.83 Si IV 1393 �144c L L L � 13.21 C II 1334 �143c L L L � 13.85 N V 1238 �124c L L L � 13.77 Notes. a Equivalent widths as estimated directly from the data. The error is calculated from both continuum fitting and statistical errors. b Velocity centroid values are reported with respect to the rest frame of NGC 99 (z = 0.0177). c The values represent 3σ upper limits. Figure 3. COS spectra of the Lyα transition plotted at the rest frame of NGC 99 (z = 0.0177). The best combined profile is plotted in red, and the resulting column density estimate is shown in the upper right. The dominant absorption component (A) is −44 km s−1 offset from the systemic velocity of the galaxy, with three weaker components (B–D) located at 34 km s−1, 99 km s−1, and 366 km s−1. Lyα is the only detected species associated with the galaxy’s CGM. Measurements for Lyα and upper limits for the metal lines are presented in Table 2. 4 The Astrophysical Journal, 976:205 (18pp), 2024 December 1 Olvera et al. from the systemic velocity of NGC 99, while the weaker components are located at 34 km s−1, 99 km s−1, and 366 km s−1. 3. Results 3.1. Dust Extinction We investigated the dust content of the H II regions using the Balmer decrement, F(Hα)/F(Hβ) flux, across the galaxy. The Balmer decrement of each region is listed in Table A1. The left panel of Figure 4 shows the Balmer decrement for the 26 H II regions as a function of galactocentric radius. All the points show F(Hα)/F(Hβ) values greater than the expected intrinsic value of 2.86 from Case B recombination in the absence of dust. Within 10 kpc of the center, the regions show a flat Balmer decrement, while at radii greater than 10 kpc, the points show a wide scatter. One point in particular, region 7 at 11 kpc, shows a high F(Hα)/F(Hβ) value of ≈5.14, while the lowest value of 3.19 was observed in region 3 at 11.7 kpc. The right panel of Figure 4 reveals a connection between the Balmer decrement and the SFR of the H II regions where a higher SFR is associated with higher Balmer ratios. From the Schmidt–Kennicutt (SK) law, we expect higher SFRs with larger cold gas surface densities, Σgas (M. Schmidt 1959; R. C. J. Kennicutt 1998). For a fixed dust-to-mass ratio, the dust column density and the gas column densities are coupled; the observed correlation is not surprising. Dust has also been used as a proxy for gas metallicity; however, the color-coded points of the right panel of Figure 4 reveal that the Balmer decrement of the H II regions has little dependence on the gas-phase metallicity of the regions. Therefore, we conclude that metallicity is not the primary factor affecting the trend between the dust content and the SFR. 3.2. Gas-phase Metallicity We investigate how the metallicity of the H II regions in the galaxy varies as a function of galactocentric radius. We measured the flux of the strong emission lines Hα, Hβ, [O III] λ5007, [N II]λ6583, and [S II]λ6717, 31. The ratios of these fluxes provide metallicity indicators N2 and O3N2, which are defined as ⎛ ⎝ ⎞ ⎠ [ ] ( )N2 log N II 6583 H 1 l a = ⎜ ⎟ ⎛ ⎝ ⎞ ⎠ [ ] [ ] ( )O3N2 log O III 5007 H H N II 6583 , 2 l b a l = where the lines refer to line fluxes (L. Searle 1971; D. Alloin et al. 1979; G. Denicolo et al. 2002; L. J. Kewley & M. A. Dopita 2002; M. Pettini & B. E. J. Pagel 2004; T. Nagao et al. 2006; R. A. Marino et al. 2013; M. A. Dopita et al. 2016; M. Curti et al. 2017; F. Bian et al. 2018; R. Maiolino & F. Mannucci 2019; M. Curti et al. 2020). The N2 and O3N2 values for each region are given in Table A1. Both the indicators have the benefit of being insensitive to dust attenuation since the two lines in each line ratio (Hα/[N II] λ6583 and [O III]λ5007/Hβ) are close in wavelength space (∼20Å). The indicators serve well as proxies for the metallicity of the systems and can be converted into oxygen gas-phase metallicities ( )12 log O H+ . We follow the pre- scription from M. Curti et al. (2020) to obtain gas-phase metallicities. Figure 5 shows N2 and O3N2 as a function of galactocentric radius with the corresponding gas-phase metallicity on the right y-axis. The black solid line shows the best linear fit between the galactocentric radius and gas-phase metallicity, while uncer- tainty is given by the shaded regions. The slope, intercept, and associated errors were estimated using polyfit from NumPy. When fitting for 12+ log(O/H) instead of N2 and O3N2 as a function of galactocentric radius, the radial gradients are −0.017 and −0.020 dex kpc−1, respectively. Further information can be found in Table A2. The filled circles are color coded by |VHα− VHI|, the absolute difference between the centroid of the Hα emission line and the mass-weighted H I 21 cm velocity of each region. Both indicators show the expected trends of decreasing metallicity with increasing radius in general, with N2 and O3N2 having a Spearman-ρ value of −0.60 and −0.63, respectively. Interestingly, two regions at radii 6.1 and 7.2 kpc (labeled 15 and 19 in Figure 1, respectively) show an abnormal drop in Figure 4. The Balmer decrement of each H II region as a function of its galactocentric radius (left) and star formation rate (right). The associated error bars are overplotted in red. The observed H II regions within 10 kpc of the center of NGC 99 show a flat radial distribution. H II regions outside of 10 kpc show wide scatter. 5 The Astrophysical Journal, 976:205 (18pp), 2024 December 1 Olvera et al. metallicity compared to others at similar radii, regardless of which metallicity indicator is used. These regions are more than 3σ below the linear fits shown in Figure 5. In addition, these regions also show a high velocity offset between VHα and VHI. Both trace the spiral arms and, on visual inspection, do not show any morphological differences. We performed a Cook’s distance statistical outlier test and confirmed that these regions are outliers in the data. Masking the two ALM H II regions, we refit the data and found that the radial gradient for both N2 and O3N2 became steeper by 0.03 dex. The new gradients are shown by the blue dashed line in Figure 5. Figure 6 shows the spatial locations of the H II regions with filled circles color coded based on their metallicity and velocity offset of the Hα emission line from the ISM kinematics. Region 15 lies in one of the spiral arms of NGC 99 and is surrounded by points with higher metallicity while also having a higher velocity offset. Region 19 shows a negative velocity offset despite its colocated ISM showing a positive velocity consistent with rotation. Combining the metallicity and velocity information presented in Figure 5, we suggest that the star formation in these regions is fueled by low-metallicity gas that has been accreted into the disk of the galaxy. Figure 5. The metallicity radial gradient from both the N2 (left) and the O3N2 (right) indices. The right-side y-axis in both panels shows the corresponding gas-phase metallicity calculated from the calibration equations derived in M. Curti et al. (2020). The black solid line shows the best linear fit to all of the data, and the red and dark red shaded regions signify the 3σ and 5σ, respectively, confidence intervals of the fit. For y = 12 + log(O/H), the slopes of the fit are −0.017 and −0.020 dex kpc−1, respectively. The color bar shows the absolute difference between the Hα emission line velocity offset and the mass-weighted H I 21 cm velocity of the ISM in the region. Regions 15 and 19, which are labeled and are at 6.1 and 7.2 kpc, show both anomalously low metallicity and a high velocity difference. The blue line shows the fit without the two ALM regions. The fits (slopes and intercepts) are presented in Table A2. Figure 6. r-band images of the NGC 99 with the locations H II regions indicated by the circles, which are color coded based on their gas-phase metallicity (left) and velocity offset of the Hα emission line (right). Regions 15 and 19 show abnormally low metallicities and high velocities relative to their surrounding regions. 6 The Astrophysical Journal, 976:205 (18pp), 2024 December 1 Olvera et al. Alternative explanations for the ALM H II regions may be azimuthal and arm–interarm variations in the oxygen abun- dance and deviations from the general radial profiles, which have been observed and described in many galaxies (R. C. J. Kennicutt & D. R. Garnett 1996; P. Martin & J. Belley 1996; B. Cedrés & J. Cepa 2002; F. F. Rosales-Ortega et al. 2011; B. Cedrés et al. 2012; Y. Li et al. 2013; S. F. Sán- chez et al. 2015; L. Sánchez-Menguiano et al. 2016a, 2017, 2019). It is also possible that these H II regions are extraplanar and look like part of the disk due to projection effects. However, that is not likely in this case as NGC 99 is almost face-on (i= 20°), so projection effects are minimal. Therefore, we conclude that the ALM H II regions are due to the accretion of low-metallicity gas. Similar conclusions were also made by other recent studies, such as J. C. Howk et al. (2018a), H.-C. Hwang et al. (2019), and L. Scholz-Diaz et al. (2021). 3.3. [S II] Deficiency We make measurements of the [S II]λ6717, 31 emissions lines and measure [S II]-based metallicity indicators such as S2, N2S2Hα, and RS32 (G. Denicolo et al. 2002; S. Y. Yin et al. 2007; M. A. Dopita et al. 2016; M. Curti et al. 2017; R. Mai- olino & F. Mannucci 2019; M. Curti et al. 2020). For example, the S2 index, defined as ⎛ ⎝ ⎞ ⎠ [ ] ( )S2 log S II 6717, 31 H , 3 l a = shows no radial dependence with galactocentric radius. Over- all, the [S II]-based indicators show large variation compared to N2 or O3N2. To investigate it further, we constructed a line ratio diagram (Figure 7) using the line ratios of [O III]/Hβ and [S II]/Hα from our 26 H II regions. The left panel of Figure 7 shows SDSS galaxies from H. Aihara et al. (2011) as the binned background, while the right panel background shows H II regions from the CALIFA survey (C. Espinosa-Ponce et al. 2020). Our regions show a deficiency between 0.5 and 1.0 dex from the expected value of S2 for a set value of the [O III]/Hβ ratios, regardless of which sample we compare to. Our observations are consistent with those observed by J. Wang et al. (1997), albeit for galaxies. They inferred that this [S II] deficiency is caused by the leaky H II regions where Lyman-continuum (LyC) photons are escaping into the surrounded ISM. Since [S II] is mostly produced at the boundary between the ionized and neutral zones of H II regions, the width of our slits, 1 or 0.378 kpc, may not encompass the entirety of the [S II] produced due to the interaction of LyC photons with neutral gas (E. W. Pellegrini et al. 2012). For this reason, we refrain from using any indicator involving [S II]. 3.4. Spectroscopic Observations of Neighboring Dwarf Galaxy SDSS J002353.17+154356.8 is a dwarf galaxy located 57 kpc away from NGC 99 in projection. The left panel of Figure 8 shows an optical image of NGC 99 with SDSS J002353.17+154356.8 near the bottom right corner of the image. We detail its properties in Table A3. While observing NGC 99 with the MMT as described in Section 2.1, we also placed a slit on the center of SDSS J002353.17 +154356.8 oriented along its major axis and obtained a Figure 7. Distribution of 26 H II regions (filled gold circles) on a background of hexagon bins from SDSS-DR8 galaxies (left) and from CALIFA H II regions (right). The color bar shows the number of galaxies or H II regions in each bin. The data points show a 0.5–1.0 dex offset in [S II] from the majority of SDSS galaxies and CALIFA H II regions. Figure 8. Optical image (left) of NGC 99 (top left of image) and its companion J002353.17+154356.8 (bottom right of image) from the DESI Legacy Survey (A. Dey et al. 2019). The projected distance between the objects is 57 kpc. (right) The optical spectrum showing the Hα and [N II]λ6583 emission lines of J002353.17+154356.8 obtained with the MMT using the Binospec instrument. The redshift was measured to be 0.01747. 7 The Astrophysical Journal, 976:205 (18pp), 2024 December 1 Olvera et al. spectrum with measurable emissions lines present. The right panel of Figure 8 shows a cutout of the optical spectrum of the dwarf galaxy. The spectrum was reduced using the same procedure as the spectra in NGC 99. By measuring the center of the Hα emission line, we found a redshift of 0.01747, which is slightly lower than the redshift of NGC 99. The close proximity of the dwarf galaxy leads us to believe it is a satellite of NGC 99. Shifting the spectrum into NGC 99ʼs frame of reference, we measured a velocity offset of −74.51 km s−1 with respect to NGC 99. We calculated that the dwarf galaxy has an N2 value of −0.46 corresponding to 12+ log(O/H)= 8.71 (M. Curti et al. 2020). We utilize theMå map from M. Padave et al. (2024a) and the H I 21 cm image from H. B. Gim et al. (2024, in preparation) to measure the Må and H I mass of the dwarf galaxy to be 8.62× 108Me and 5.8× 109Me, respectively. The stellar mass of J002353.17+154356.8 is comparable to the Small Magellanic Cloud (SMC; Må,SMC= 3.1× 108Me) but has an H I mass an order of magnitude higher than the SMC’s gas mass (Mgas,SMC= 4.2× 108Me; G. Besla 2015). J002353.17 +154356.8 does not show any signs of tidal disruption due to its host galaxy, so we can conclude that the presence of this dwarf galaxy does not affect any of the properties of NGC 99. 4. Discussion 4.1. Metallicity Radial Gradient Comparison We compared our radial metallicity gradient to four galaxies from the CHemical Abundances Of Spirals (CHAOS) project that are similar in mass to NGC 99. D. A. Berg et al. (2020) measured the radial metallicity gradient of NGC 0628, NGC 3184, NGC 5194, and NGC 5457 using the direct method to measure the metallicity of multiple H II regions observed with the Multi- Object Double Spectrographs (MODS; R. W. Pogge et al. 2010) on the Large Binocular Telescope (LBT). The left panel of Figure 9 shows the metallicity distribution of NGC 99 and the four CHAOS galaxies as a function of galactocentric radius normalized by each galaxy’s isophotal radius (R25). In compar- ison to NGC 99, the four CHAOS galaxies are less massive ( ( )M M10.0 log 10.5< < ) and much closer (7.2 4, where SFRMS is the global SFR from the star-forming galaxy main sequence (MS). We utilize the time-dependent MS equation of J. S. Speagle et al. (2014) to determine SFRMS= 1.64Me yr−1. Therefore, SFRglobal/SFRMS= 1.74, so NGC 99 is not currently in a starbursting phase. For cases where η is much greater than 1, the mass loading factor becomes nonphysical for a galaxy of this mass (J. Chisholm et al. 2017; X. Xu et al. 2022). Figure 11. The mass loading factor η, modeled using Equation (7), of 26 H II regions as a function of galactocentric radius assuming yO = 0.009 (left), yO = 0.014 (middle), and yO = 0.037 (right). The points are color coded by their gas-phase metallicity. The mass loading factor, as represented in the chemical analysis, gets impacted by metal dilution from low-metallicity gas accretion if M Minflow =  and thus deviates from the traditional definition. Figure 10. (Left) The gas-phase metallicity of the 26 H II regions is shown as a function of gas fraction with the points color coded by effective yield, yeff. A closed- box model is assumed to calculate effective yield. The lines show the true yield, yO, found by C. Leitherer et al. (2014; dash) and F. Vincenzo et al. (2016b; solid and dashed–dotted, respectively). (Right) For the same H II regions, the effective yield is now shown as a function of galactocentric radius color coded by gas fraction. All of the H II regions have an effective yield lower than the true yield found by F. Vincenzo et al. (2016b). The effective yield is constant across the disk of the galaxy. The two ALM H II regions, numbers 15 and 19, are marked. 10 The Astrophysical Journal, 976:205 (18pp), 2024 December 1 Olvera et al. Therefore, the most likely value of true yield is around yO= 0.009. Regardless of yO, a correlation between η and R is evident. η increases with radius and shows an inverse correlation with stellar density. The mass loading factor is highest in the outermost H II regions, which is contradictory to the traditional expectation for η. Instead, the high mass loading factors are an artifact of the model we assumed. Equation (7) shows that η and Zgas are roughly inversely related; therefore, a lower metallicity can result in higher mass loading factor values. The color coding in Figure 11 confirms that η has an inverse relation with gas-phase metallicity. The outer H II regions with lower metallicities have the highest mass loading factors. This suggests that η, in this situation, is tracing gas dilution due to inflow. Both processes, the outflow of metal-rich gas and inflow of low-metallicity gas accretion, can lower the metallicity of the H II regions, causing their mass loading factor to be high. Even if NGC 99 had an outflow (which we cannot be certain about), it would have η proportional to the star formation rate, which can be approximated by the stellar mass surface density today if a significant fraction of the stellar mass was created in that burst. Since that is not the case, the most likely process leading to the increasing η trend with radius would be the accretion of lower metallicity gas. This is supported by the fact that gas accretion is believed to occur mostly at the outskirts of the galactic disks along their major axis (C. Peroux et al. 2020). We acknowledge that the bathtub chemical evolution models are not sophisticated enough to precisely model the processes of mixing and diffusion. Another limitation of the model is that it does not account for differential accretion, i.e., when M Minflow ¹ and Minflow is independent of M . Nevertheless, this model helps us understand the balance of gas outflow and inflow in a qualitative manner, which is what we advise the reader to take away from this discussion. 4.3. Estimation of Accreted Mass We can estimate the accreted amount of low-metallicity gas needed to dilute the original gas present in the two ALM regions to the metallicity value we observe today. The accreted mass MH,acc can be given by the following equation ( )M M M , 8H,acc H,now H,orignal= - where MH,now is the hydrogen mass of the region now, and MH,original is the original hydrogen mass of the system. We assume that the metals in the H II regions are primarily made up of oxygen, so the mass in metals we currently see MO,now can then be given by ( )M M M , 9O,now O,original O,acc= + where MO,original is the original oxygen mass of the region before dilution and MO,acc is the oxygen mass that was accreted in. If we assume that the gas accreted was pristine, that is MO,acc= 0, then MO,now=MO,original. The oxygen mass can then be estimated using metallicity and H I 21 cm mass values using MO,now=MHnow,× Znow,ALM where Znow,ALM is the measured metallicity of the two ALM regions. We convert 12+ log(O/H) to Znow,ALM using Equation (5). We estimate the mass of oxygen originally present in regions 15 and 19 to be 8.8× 103Me and 8.2× 103Me, respectively. The original amount of hydrogen is MH,original,=MO,original/Zoriginal where Zoriginal is the metallicity of the H II regions before dilution. Zoriginal is estimated using the linear fitted metallicity gradient from the N2 indicator , where we excluded the ALM regions from the fit, which can be found in Table A2. Zoriginal for the two regions is 8.57 and 8.55. Thus, MH,original of the H II regions is 2.01× 106Me and 1.98× 106Me. Using Equation (8), we estimate the accreted hydrogen mass needed to dilute the systems to be 1.26× 106Me and 1.08× 106Me for regions 15 and 19, respectively. These accreted masses should be considered lower limits since we assume that the accreted gas is pristine. The masses of the accreted gas clouds are similar to those found in high- velocity clouds (HVC) around the Milky Way (M. E. Putman et al. 2012). 4.4. CGM of NGC 99 The absorption spectra at the position of the QSO J0023 +1547 trace four clouds in Lyα, the strongest of which matches the velocity of the H I 21 cm emitting ISM within the disk of the NGC 99. Figure 12 shows the ISM kinematics as traced by H I 21 cm with the galaxy’s optical map overlaid. The position of the QSO is indicated with a filled circle that is color coded to the velocity of the strongest component (A). The strongest component (see Table 2) is corotating with the ISM with an uncertainty of less than 5 km s−1. The large synthesized beam (50 5× 47 5) of the VLA smears the small-scale velocity structure of the H I data. Therefore, the precise measurement of velocity at the edge of the H I disk is not possible with our data. The corotating component has a column density, N(H I)� 1014.4 cm−2, which is more than an order of magnitude higher than all the other three components put together, indicating that most of the neutral material is corotating. The three weaker components are noncorotating, i.e., their velocities are reversed from what is seen on the blueshifted side of the galaxy. If the covering fraction of the strong component is large, then we are looking at the extended disk material associated with the disk. Galaxy simulations have seen the presence of large extended corotating disks (K. R. Stewart et al. 2011). Recent simulations by Z. Hafen et al. (2022) also found that gas accreting into thin- disk galaxies in the FIRE simulations is dominated by rotating cooling flows. The hotter accreting gas cools down to 104 K, and its geometry transitions from a quasi-spherical distribution to a cool extended disk. The CGM gas distributions and kinematics suggest two possible entryways for gas into the disk of the galaxies. First is through the extended disk traced by the dominant component. While the gas in the extended disk is much further from where the ALMs are seen (at galactocentric radii of <10 kpc), the gas may flow inward without much metal mixing for galaxies with low gas dispersion (σg∼ 14 km s−1) through radial flows in the ISM (P. Sharda et al. 2021). The steep increase in the mass loading fraction as a function of radius (see Figure 11) suggests that gas dilution via accretion is increasingly important at large radii. This scenario is consistent with the radial flow of low- metallicity gas along the disk. M. Padave et al. (2024a) find no correlation between the neutral gas content of the CGM and inside-out disk growth, suggesting that gas likely transverses large distances to get to the inner stellar disk before forming stars. The second pathway is through the halo vertically to the disk. HVCs are one such example that shows an overall infall 11 The Astrophysical Journal, 976:205 (18pp), 2024 December 1 Olvera et al. that is believed to support star formation in the Milky Way (N. Lehner et al. 2022). Our estimate of the inflowing cloud masses needed to produce the ALMs falls within the HVC mass range of the Milky Way (M. E. Putman et al. 2012) and anomalous velocity clouds analogous to intermediate-velocity clouds seen in NGC 4321 (Gim et al. 2021). No metal absorbers associated with the hydrogen clouds were detected (Table 2). This is not surprising as even at solar metallicity, the metal lines are expected to be very weak for gas clouds of N(H I)� 1014−15 cm−2. We do not expect the Lyα absorbers to be tracing highly ionized gas as we did not detect high-ionization species such as Si IV and N V. 4.5. Fundamental Metallicity Relation on Resolved Scales J. Lequeux et al. (1979), D. Zaritsky et al. (1994), and C. A. Tremonti et al. (2004) established a positive correlation between global metallicity and the Må of galaxies, known as the mass–metallicity relation (MZR). Modern studies, such as MaNGA, have shown that the MZR evolves over cosmic time (K. Bundy et al. 2015; A. Camps-Fariña et al. 2022). In addition, observations have shown that metallicity and Må depend on SFR (M. A. Lara-López et al. 2010; F. Mannucci et al. 2010; M. Curti et al. 2020). The relation between the three properties is known as the FMR. M. A. Lara-López et al. (2010) and F. Mannucci et al. (2010) found that at a fixed Må, a higher SFR yields a lower metallicity. Additionally, as Må increases, so do metallicity and SFR. Results from F. F. Rosal- es-Ortega et al. (2012), which were later confirmed by J. K. Barrera-Ballesteros et al. (2016), established that the MZR and FMR also occur on localized (subkiloparsec) scales. We will further explore the FMR here. We derived the SFR from the Hα emission line using ( ) ( )M L Clog SFR yr log log 10x x 1  = -- where Clog 41.27x = and Lx, in units of erg s−1, is the Hα luminosity derived from the integrated and dust-corrected Hα flux at a distance of 79.4 Mpc (C.-N. Hao et al. 2011; E. J. Murphy et al. 2011; R. C. Kennicutt & N. J. Evans 2012). We convert Må and SFR to stellar mass surface density (Σå) and star formation rate surface density (ΣSFR), respectively, by dividing Må and SFR by the area of each slit, 0.35 kpc2. We investigate the relationship between Σå, ΣSFR, and metallicity by plotting the data in 3D space to gain a better visual representation. Figure 13 shows the Σå, ΣSFR, and 12+ log(O/H) parameter space with the filled circles color coded based on their galactocentric radius in kpc. We fit a plane to the data and find that the relationship between the three properties is described by the equation: ( ) ( ) ( ) ( ) ( ) ( ) ( ) 12 log O H 0.144 0.044 log 0.225 0.050 log 6.443 0.455 . 11 SFR+ = -  ´ S +  ´ S +   Figure 13 is available as an online video where we rotate about the z-axis one time so that the entirety of the plane is seen. The coefficients in front of the two surface density parameters describe how and to what strength gas-phase metallicity depends on local Σå and ΣSFR. The resolved FMR (rFMR), given by Equation (11), shows that for a given Σå, metallicity relates inversely with ΣSFR. Moreover, for a given ΣSFR, metallicity depends proportionally on Σå. The value of the coefficients, −0.144 and 0.225, for ΣSFR and Σå, respectively, reveals that the metallicity of the H II regions has a stronger dependence on Σå than ΣSFR. The scatter in metallicity around the best-fit plane is σFMR= 0.07 dex, which is comparable to the scatter of the surface fit by M. Curti et al. (2020). W. M. Baker et al. (2023) recently found similar results using 56,000 H II regions from the MaNGA survey and measure σFMR= 0.06 dex. Our rFMR equation presents the resolved FMR at spatial scales of 4″ (1.5 kpc), which provides a view of kpc scale physics that may be responsible for the observed relationship. The spatial resolution we achieve is comparable to some of the IFU surveys; however, the multiobject spectrograph allows us to sample a much larger region of the galaxy going well beyond Figure 12. An optical image of NGC 99 overlaid with VLA H I 21 cm map showing extended H I emission beyond the optical disk of the galaxy. The spatial location of QSO (J0023+1547) is shown relative to NGC 99, with the color of the point corresponding to the velocity centroid of the dominant component of the Lyα line. The primary component of the CGM is corotating at roughly the same velocity as the ISM closest to the sightline. This suggests a large disk extending out to the CGM. 12 The Astrophysical Journal, 976:205 (18pp), 2024 December 1 Olvera et al. R25. Moreover, the size of our Binospec slits encompasses the gas and stars within the observed regions allowing us to probe their chemical characteristics. Studies on the rFMR have been done using spatially resolved data from the MaNGA sample, which samples spatial scales of about 1.5 kpc (J. K. Barrera-B- allesteros et al. 2016; L. Sánchez-Menguiano et al. 2019; B. B. Teklu et al. 2020). We will increase our sample size and spatial resolution in a future study. 5. Summary In this pilot study, we study the properties of 26 H II regions in the galaxy NGC 99. We measured the radial metallicity gradient from strong-line metallicity indicators of N2 and O3N2 to investigate indirect evidence of gas accretion. Additionally, we analyze the chemical evolution of the systems by assuming a closed-box, accreting box, and accreting/outflow box models. We measure an effective yield and mass loading factor for each region. Finally, we derive a resolved Fundamental Metallicity Relation by connecting the metallicity information to Σå and ΣSFR. The results are summarized below: 1. Using oxygen gas-phase metallicities calibrated using the N2 metallicity index, we measure the radial metallicity gradient of NGC 99 to be −0.017 dex kpc−1 or, in terms of R25, −0.19 dex/R25. Our radial gradient matches with the peak of the distribution of radial gradients found by other studies (D. A. Berg et al. 2020; L. S. Pilyugin et al. 2023). 2. We discover two anomalously low-metallicity regions with a high difference between the H I and Hα gas line- of-sight velocities. We believe that these two regions are an indirect sign of the accretion of low-metallicity gas from the CGM that diluted the gas used to form the stars in these regions. 3. Assuming the closed-box model, we find that the effective yield of each observed H II region is below the true nucleosynthetic yield. Additionally, the effective yields are roughly constant with a galactocentric radius. These results lead us to believe that the galaxy has a varying degree of inflows and outflows. 4. When inflow and outflows are incorporated, we find that the H II regions have a mass loading factor between 0.5 and 15, depending on the choice of yO. Additionally, we suggest higher accretion rates of low-metallicity gas in the outskirts of the galaxy as the reason for higher η values at higher radii. This occurs as a consequence of using the bathtub model, which does not account for differential accretion. 5. We find Lyα absorbers associated with the CGM of the galaxy at an impact parameter 159 kpc. The strongest component of the absorption feature kinematically matches the disk kinematics closest to the sightline. This may hint at the presence of an extended disk, assuming that the covering fraction of the absorbing media is high. We suggest that a large H I disk may be a possible pathway for low-metallicity gas to flow into the ISM of this galaxy. 6. We derive Equation (11) describing the resolved Funda- mental Metallicity Relation relating Z, Σå, and ΣSFR. We find that for a given Σå, ΣSFR increases with decreasing metallicity. From this result, we believe that the local physics within galaxies affects the global scale physics. Our equation is useful for estimating metallicities in galaxies where only imaging data might be available, which is especially the case for higher redshift galaxies. Future work will expand this analysis to other galaxies in the DIISC sample to look for other ALM H II regions and grow our sample size for the rFMR to improve the precision of the plane. Multiwavelength studies of local galaxies are critical to obtaining a detailed look into how galaxies obtain their gas and how it is processed for star formation. Figure 13. 3D visualization of the resolved Fundamental Metallicity Relation for the 26 H II regions in the galaxy NGC 99 color coded by galactocentric radius. The x-axis and y-axis are the stellar mass and star formation rate surface densities, respectively, for each region. Gas-phase metallicities from the N2 indicator are shown on the z-axis. The points are well fit my a linear plane described by Equation (11). An animated version of this figure is available online, where we rotate around the z-axis for one full rotation in an 18 s animation. (An animation of this figure is available in the online article.) 13 The Astrophysical Journal, 976:205 (18pp), 2024 December 1 Olvera et al. https://doi.org/10.3847/1538-4357/ad8238 Acknowledgments We thank Jackie Monkiewicz, Ben Weiner, Chris Howk, Mirko Curti, and Patrick Kamieneski for the valuable and useful discussions during the course of this work. We thank the referee for constructive comments. We thank the support staff at the MMT Observatory, the Steward Observatory, the Vatican Advanced Technology Telescope, the Very Large Array, the National Radio Astronomy Observatory, and the Space Telescope Science Institute for help with this project. All HST data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science Institute. The specific observations analyzed can be accessed via doi:10.17909/rf6m-3y73. The Arizona State University authors acknowledge the 23 Native Nations that have inhabited this land for centuries. Arizona State University's four campuses are located in the Salt River Valley on ancestral territories of Indigenous peoples, including the Akimel O’odham (Pima) and Pee Posh (Maricopa) Indian Communities, whose care and keeping of these lands allows us to be here today. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under grant No. 2233001. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. A.O., S.B., M.P., B.K., H.G., and C.D. are supported by the NSF grants 2108159 and 2009409. M.P., S.B., and R.J. are supported by NASA ADAP grant 80NSSC21K0643. S.B., H.G., and T.H. are also supported by Hubble Space Telescope (HST) grant HST-GO-14071 administrated by STScI, operated by AURA under contract NAS 5-26555 from NASA. Observations reported here were obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona. This work is also partly based on observations with the VATT: the Alice P. Lennon Telescope and the Thomas J. Bannan Astrophysics Facility. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under a cooperative agreement by Associated Universities, Inc. Facility: HST, GALEX, MMT (Binospec), VATT and VLA. Software: astropy (Astropy Collaboration et al. 2013, 2018, 2022), CASA (J. P. McMullin et al. 2007), ipython/ jupyter (F. Perez & B. E. Granger 2007; T. Kluyver et al. 2016), matplotlib (J. D. Hunter 2007), NumPy (C. R. Harris et al. 2020), pyFIT3D (E. A. D. Lacerda et al. 2022), SoFiA-2 (P. Serra et al. 2015; T. Westmeier et al. 2021), statmorph (V. Rodriguez-Gomez et al. 2019), specutils (N. Earl et al. 2022), and Python from https://www.python.org. Appendix The Appendix section contains tables that were referenced to in the main text. Table A1 contains the position of each H II region as well as the measured properties of the regions such as Hα flux, Balmer decrement, metallicity indicator values, and gas velocities. In addition, Table A2 contains the fitted line Table A1 NGC 99 H II Region Position, Flux, Balmer, and Metallicity Indicator Values H II R.A. Decl. Radius F(Hα) F(Hα)/F(Hβ) N2 O3N2 S2 VHα VH I Region (deg) (deg) (kpc) (erg s−1 cm−2) (km s−1) (km s−1) 1 5.9966 15.7598 14.4 1.27 4.44 ± 0.41 −0.966 ± 0.127 1.2726 ± 0.1349 −0.699 ± 0.051 −66.33 −26.41 2 5.9907 15.7627 14.4 2.56 3.37 ± 0.31 −0.974 ± 0.098 1.4556 ± 0.1050 −0.651 ± 0.067 −23.78 −35.86 3 5.9952 15.7621 11.7 1.71 3.19 ± 0.14 −0.802 ± 0.048 0.8396 ± 0.0580 −0.611 ± 0.043 −39.04 −29.07 4 6.0006 15.7620 12.2 4.26 3.98 ± 0.38 −0.975 ± 0.069 1.5007 ± 0.0815 −0.658 ± 0.043 −17.45 −13.18 5 6.0026 15.7620 13.6 4.73 3.65 ± 0.16 −0.919 ± 0.024 1.3667 ± 0.0352 −0.668 ± 0.020 −3.44 −3.20 6 6.0009 15.7628 11.4 13.13 3.81 ± 0.17 −0.917 ± 0.025 1.2389 ± 0.0373 −0.740 ± 0.025 −10.99 −3.98 7 6.0014 15.7634 11.0 79.82 5.14 ± 0.15 −1.006 ± 0.017 1.4924 ± 0.0265 −0.913 ± 0.016 −33.62 −3.98 8 5.9934 15.7662 8.3 2.79 4.29 ± 0.21 −0.823 ± 0.035 0.9739 ± 0.0482 −0.537 ± 0.029 −47.36 −25.09 9 5.9976 15.7658 6.1 2.67 4.45 ± 0.28 −0.776 ± 0.039 0.8317 ± 0.0530 −0.658 ± 0.456 −28.33 −7.96 10 6.0032 15.7662 10.0 6.72 4.17 ± 0.10 −0.826 ± 0.021 1.0075 ± 0.0250 −0.737 ± 0.019 23.09 10.26 11 6.0033 15.7678 9.1 14.58 3.80 ± 0.23 −0.778 ± 0.025 0.9274 ± 0.0429 −0.705 ± 0.024 48.38 14.89 12 5.9945 15.7682 5.3 2.96 4.13 ± 0.28 −0.658 ± 0.034 0.2986 ± 0.0581 −0.625 ± 0.037 −30.18 −16.45 13 5.9952 15.7684 4.3 5.90 4.16 ± 0.22 −0.610 ± 0.020 0.1732 ± 0.0420 −0.725 ± 0.025 −30.31 −16.45 14 5.9977 15.7686 2.3 3.52 3.90 ± 0.19 −0.470 ± 0.026 0.1633 ± 0.0397 −0.570 ± 0.030 −6.32 −3.24 15 5.9935 15.7696 6.1 12.79 4.39 ± 0.42 −1.104 ± 0.059 1.3145 ± 0.0749 −0.940 ± 0.068 −66.76 −17.18 16 5.9922 15.7714 8.1 19.80 4.46 ± 0.10 −0.954 ± 0.019 1.3544 ± 0.0261 −0.725 ± 0.016 −50.78 −16.78 17 5.9994 15.7716 3.2 10.49 4.34 ± 0.14 −0.629 ± 0.024 0.2426 ± 0.0356 −0.661 ± 0.024 38.13 13.55 18 5.9920 15.7753 10.8 1.60 3.60 ± 0.24 −1.018 ± 0.042 1.4242 ± 0.0565 −0.562 ± 0.088 15.93 −6.40 19 5.9946 15.7744 7.2 24.62 4.23 ± 0.19 −1.107 ± 0.027 1.5927 ± 0.0338 −0.956 ± 0.024 −35.09 3.64 20 6.0028 15.7742 9.3 2.12 4.44 ± 0.41 −0.560 ± 0.026 0.6094 ± 0.0555 −0.414 ± 0.019 41.59 32.41 21 5.9926 15.7763 11.1 1.64 3.88 ± 0.30 −0.898 ± 0.054 0.9573 ± 0.0772 −0.533 ± 0.046 −1.43 2.89 22 5.9980 15.7763 8.3 1.56 3.90 ± 0.30 −0.727 ± 0.038 0.9263 ± 0.0517 −0.546 ± 0.037 73.33 21.22 23 6.0008 15.7776 11.0 9.27 4.25 ± 0.14 −0.998 ± 0.016 1.4131 ± 0.0227 −0.801 ± 0.018 32.00 30.57 24 6.0007 15.7816 16.0 2.37 4.12 ± 0.19 −1.212 ± 0.052 1.7082 ± 0.0563 −0.842 ± 0.035 34.68 37.89 25 6.0020 15.7819 17.1 2.40 3.85 ± 0.16 −1.070 ± 0.033 1.2779 ± 0.0429 −0.638 ± 0.023 55.76 42.14 26 6.0034 15.7825 18.6 1.09 3.97 ± 0.20 −1.12 ± 0.085 1.6681 ± 0.0901 −0.638 ± 0.038 48.44 47.21 Note. Flux is in units of ×10−16 erg s−1 cm−2. 14 The Astrophysical Journal, 976:205 (18pp), 2024 December 1 Olvera et al. https://doi.org/10.17909/rf6m-3y73 http://www.python.org Table A2 Metallicity Radial Gradients y x Slope m Intercept b Comment Spearman ρ N2 R −0.032 ± 0.007 −0.564 ± 0.073 12 + log(O/H)N2 R −0.017 ± 0.004 8.646 ± 0.038 −0.60 12 + log(O/H)N2 R −0.020 ± 0.003 8.693 ± 0.029 ALMs removed −0.80 12 + log(O/H)N2 R/R25 −0.191 ± 0.041 8.644 ± 0.038 O3N2 R 0.083 ± 0.015 0.249 ± 0.166 12 + log(O/H)O3N2 R −0.020 ± 0.004 8.703 ± 0.041 −0.63 12 + log(O/H)O3N2 R −0.023 ± 0.003 8.746 ± 0.035 ALMs removed −0.77 Note. Linear fit equations to the metallicity vs. galactocentric radius data. Slopes are all in dex kpc−1 except for when x = R/R25 which gives slope units of dex R25 1- . Intercepts are all in dex. Table A3 Properties of SDSS J002353.17+154356.8 Parameter Value R.A.a (α2000) 00h23m53 17 Decl.a (δ2000) 15 43 56. 87+  ¢  Redshift 0.01747 Projected distance to NGC 99 57 kpc Velocity offset to NGC 99 (ΔVHα) −74.51 km s−1 Stellar massb 8.62 × 108 Me H I massc 5.8 × 109 Me N2 −0.46 12 + log(O/H) 8.71 Notes. a M. F. Skrutskie et al. (2006). b M. Padave et al. (2024a). c H. B. Gim et al. 2024, in preparation. 15 The Astrophysical Journal, 976:205 (18pp), 2024 December 1 Olvera et al. Table A4 Calculated Physical Properties of H II Regions H II Radius ( )12 log O H N2+ ( )12 log O H O3N2+ log(ΣSFR) ( )log S ( )Mlog HI fgas ( )ylog eff η η η Region (kpc) (Me yr−1 kpc−2) (Me kpc−2) (Me) for yO = 0.009 for yO = 0.014 for yO = 0.037 1 14.4 8.43 ± 0.07 8.456 ± 0.036 −2.79 7.02 6.38 0.47 −2.38 1.72 3.40 10.70 2 14.4 8.43 ± 0.05 8.405 ± 0.030 −2.49 7.09 6.43 0.46 −2.39 1.73 3.40 10.70 3 11.7 8.52 ± 0.03 8.565 ± 0.014 −2.64 7.08 6.43 0.46 −2.30 1.83 2.54 8.51 4 12.2 8.43 ± 0.04 8.392 ± 0.024 −2.26 7.29 6.42 0.34 −2.53 1.98 3.43 10.70 5 13.6 8.46 ± 0.01 8.430 ± 0.010 −2.21 7.21 6.37 0.36 −2.48 1.26 3.13 9.92 6 11.4 8.46 ± 0.01 8.465 ± 0.010 −1.77 7.68 6.45 0.19 −2.69 2.34 3.13 9.92 7 11.0 8.41 ± 0.01 8.394 ± 0.008 −0.99 8.12 6.45 0.08 −2.93 1.78 3.64 11.25 8 8.3 8.51 ± 0.02 8.533 ± 0.012 −2.43 7.56 6.51 0.25 −2.56 0.88 2.68 8.73 9 6.1 8.53 ± 0.02 8.567 ± 0.013 −2.44 7.75 6.53 0.19 −2.63 2.34 2.52 8.29 10 10.0 8.51 ± 0.02 8.525 ± 0.007 −2.05 7.56 6.48 0.24 −2.57 1.39 2.68 8.73 11 9.1 8.53 ± 0.01 8.544 ± 0.011 −1.71 7.87 6.49 0.14 −2.70 1.91 2.52 8.29 12 5.3 8.59 ± 0.02 8.686 ± 0.012 −2.38 7.92 6.53 0.14 −2.64 2.84 2.06 7.09 13 4.3 8.62 ± 0.01 8.712 ± 0.009 −2.07 8.12 6.53 0.09 −2.69 2.42 1.86 6.55 14 2.3 8.70 ± 0.02 8.714 ± 0.009 −2.26 8.31 6.54 0.20 −2.44 2.18 1.38 5.28 15 6.1 8.36 ± 0.03 8.444 ± 0.021 −1.80 7.96 6.51 0.12 −2.90 1.76 4.20 12.75 16 8.1 8.44 ± 0.01 8.433 ± 0.008 −1.59 7.89 6.48 0.13 −2.80 0.97 3.33 10.43 17 3.2 8.61 ± 0.02 8.698 ± 0.007 −1.82 8.26 6.52 0.22 −2.51 1.36 1.92 6.73 18 10.8 8.41 ± 0.02 8.414 ± 0.017 −2.69 6.99 6.43 0.51 −2.34 1.26 3.58 11.25 19 7.2 8.36 ± 0.02 8.364 ± 0.010 −1.51 7.94 6.49 0.12 −2.90 1.12 4.20 12.75 20 9.3 8.65 ± 0.02 8.619 ± 0.013 −2.51 7.66 6.46 0.19 −2.50 1.66 1.67 6.05 21 11.1 8.47 ± 0.03 8.537 ± 0.019 −2.67 7.04 6.43 0.48 −2.32 0.84 2.98 9.67 22 8.3 8.56 ± 0.02 8.544 ± 0.013 −2.67 7.65 6.47 0.20 −2.57 1.37 2.28 7.67 23 11.0 8.42 ± 0.01 8.417 ± 0.007 −1.93 7.59 6.44 0.21 −2.70 1.11 3.53 10.97 24 16.0 8.30 ± 0.03 8.328 ± 0.018 −2.54 7.24 6.35 0.33 −2.68 0.71 4.97 14.78 25 17.1 8.38 ± 0.02 8.454 ± 0.012 −2.52 7.18 6.34 0.36 −2.56 0.52 3.97 12.13 26 18.6 8.35 ± 0.05 8.341 ± 0.029 −2.87 7.44 6.28 0.21 −2.77 1.63 4.32 13.07 16 T h e A stro ph y sica l Jo u rn a l, 976:205 (18pp), 2024 D ecem ber 1 O lvera et al. parameters from Figure 5 while Table A3 shows the basic properties of NGC 99's companion SDSS J002353.17 +154356.8. Table A4 consists of the calculated properties of the H II regions such as gas-phase metallicity, stellar mass surface density, and SFR surface density. ORCID iDs Alejandro J. Olvera https://orcid.org/0000-0002-2819-0753 Sanchayeeta Borthakur https://orcid.org/0000-0002- 2724-8298 Mansi Padave https://orcid.org/0000-0002-3472-0490 Timothy Heckman https://orcid.org/0000-0001-6670-6370 Hansung B. Gim https://orcid.org/0000-0003-1436-7658 Brad Koplitz https://orcid.org/0000-0001-5530-2872 Christopher Dupuis https://orcid.org/0000-0003-1739-3640 Emmanuel Momjian https://orcid.org/0000-0003- 3168-5922 Rolf A. Jansen https://orcid.org/0000-0003-1268-5230 References Acharyya, A., Krumholz, M. R., Federrath, C., et al. 2020, MNRAS, 495, 3819 Aihara, H., Allende Prieto, C., An, D., et al. 2011, ApJS, 193, 29 Alloin, D., Collin-Souffrin, S., Joly, M., & Vigroux, L. 1979, A&A, 78, 200 Asplund, M., Amarsi, A. M., & Grevesse, N. 2021, A&A, 653, A141 Astropy Collaboration, Price-Whelan, A. 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