Browsing by Author "Stern, Daniel"

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A Chandra and HST View of WISE-selected AGN Candidates in Dwarf Galaxies
(American Astronomical Society, 2021-06) Latimer, Lilikoi J.; Reines, Amy E.; Hainline, Kevin N.; Greene, Jenny E.; Stern, Daniel
Reliably identifying active galactic nuclei (AGNs) in dwarf galaxies is key to understanding black hole (BH) demographics at low masses and constraining models for BH seed formation. Here we present Chandra X-ray Observatory observations of 11 dwarf galaxies that were chosen as AGN candidates using Wide-field Infrared Survey Explorer (WISE) mid-infrared (mid-IR) color–color selection. Hubble Space Telescope images are also presented for 10 of the galaxies. Based on Sloan Digital Sky Survey spectroscopy, six galaxies in our sample have optical evidence for hosting AGNs and five are classified as star-forming. We detect X-ray point sources with luminosities above that expected from X-ray binaries in the nuclei of five of the six galaxies with optical evidence of AGNs. However, the X-ray emission from these AGNs is generally much lower than expected based on AGN scaling relations with infrared and optical tracers. We do not find compelling evidence for AGNs in the five optically-selected star-forming galaxies despite having red mid-IR colors. Only two are detected in X-rays and their properties are consistent with stellar-mass X-ray binaries. Based on this multiwavelength study, we conclude that two-color mid-IR AGN diagnostics at the resolution of WISE cannot be used to reliably select AGNs in optically-star-forming dwarf galaxies. Future observations in the infrared with the James Webb Space Telescope offer a promising path forward.
Clumpy Star Formation and AGN Activity in the Dwarf–Dwarf Galaxy Merger Mrk 709
(American Astronomical Society, 2021-05) Kimbro, Erin; Reines, Amy E.; Molina, Mallory; Deller, Adam T.; Stern, Daniel
Nearby, low-metallicity dwarf starburst galaxies hosting active galactic nuclei (AGNs) offer the best local analogs to study the early evolution of galaxies and their supermassive black holes (BHs). Here we present a detailed multi-wavelength investigation of star formation and BH activity in the low- metallicity dwarf-dwarf galaxy merger Mrk 709. Using Hubble Space Telescope Hα and continuum imaging combined with Keck spectroscopy, we determine that the two dwarf galaxies are likely in the early stages of a merger (i.e., their first pass) and discover a spectacular ∼ 10 kpc-long string of young massive star clusters (t . 10 Myr; M? & 105 M ) between the galaxies triggered by the interaction. We find that the southern galaxy, Mrk 709 S, is undergoing a clumpy mode of star formation resembling that seen in high-redshift galaxies, with multiple young clusters/clumps having stellar masses between 107 and 108 M . Furthermore, we present additional evidence for a low-luminosity AGN in Mrk 709 S (first identified by Reines et al. 2014 using radio and X-ray observations), including the detection of the coronal [Fe X] optical emission line. The work presented here provides a unique glimpse into processes key to hierarchical galaxy formation and BH growth in the early Universe.