College of Letters & Science
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The College of Letters and Science, the largest center for learning, teaching and research at Montana State University, offers students an excellent liberal arts and sciences education in nearly 50 majors, 25 minors and over 25 graduate degrees within the four areas of the humanities, natural sciences, mathematics and social sciences.
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Item Origins of the Evil Eye: M64's Stellar Halo Reveals the Recent Accretion of an SMC-mass Satellite(American Astronomical Society, 2023-06) Smercina, Adam; Bell, Eric F.; Price, Paul A.; Bailin, Jeremy; Dalcanton, Julianne J.; de Jong, Roelof S.; D’Souza, Richard; Gozman, Katya; Jang, In Sung; Monachesi, Antonela; Nidever, David; Slater, Colin T.M64, often called the "Evil Eye" galaxy, is unique among local galaxies. Beyond its dramatic, dusty nucleus, it also hosts an outer gas disk that counter-rotates relative to its stars. The mass of this outer disk is comparable to the gas content of the Small Magellanic Cloud (SMC), prompting the idea that it was likely accreted in a recent minor merger. Yet, detailed follow-up studies of M64's outer disk have shown no evidence of such an event, leading to other interpretations, such as a "flyby" interaction with the distant diffuse satellite Coma P. We present Subaru Hyper Suprime-Cam observations of M64's stellar halo, which resolve its stellar populations and reveal a spectacular radial shell feature, oriented ∼30° relative to the major axis and along the rotation axis of the outer gas disk. The shell is ∼45 kpc southeast of M64, while a similar but more diffuse plume to the northwest extends to >100 kpc. We estimate a stellar mass and metallicity for the southern shell of M⋆ = 1.80 ± 0.54 × 108M⊙ and [M/H] = −1.0, respectively, and a similar mass of 1.42 ± 0.71 × 108M⊙ for the northern plume. Taking into account the accreted material in M64's inner disk, we estimate a total stellar mass for the progenitor satellite of M⋆,prog ≃ 5 × 108M⊙. These results suggest that M64 is in the final stages of a minor merger with a gas-rich satellite strikingly similar to the SMC, in which M64's accreted counter-rotating gas originated, and which is responsible for the formation of its dusty inner star-forming disk.Item Co-formation of the thin and thick discs revealed by APOGEE-DR16 and Gaia-DR2(Oxford University Press, 2020-12) Beraldo e Silva, Leandro; Debattista, Victor P.; Nidever, David; Amarante, João A S; Garver, BethanySince thin disc stars are younger than thick disc stars on average, the thin disc is predicted by some models to start forming after the thick disc had formed, around 10 Gyr ago. Accordingly, no significant old thin disc population should exist. Using 6D coordinates from Gaia-DR2 and age estimates from Sanders & Das, we select ∼24 000 old stars (${\tau \gt 10\mbox{$\, \mathrm{Gyr}$}}$, with uncertainties $\lesssim 15$) within $2\mbox{$\, \mathrm{kpc}$}$ from the Sun (full sample). A cross-match with APOGEE-DR16 (∼1000 stars) reveals comparable fractions of old chemically defined thin/thick disc stars. We show that the full sample pericentre radius (rper) distribution has three peaks, one associated with the stellar halo and the other two having contributions from the thin/thick discs. Using a high-resolution N-body + SPH simulation, we demonstrate that one peak, at $\mbox{$r_\mathrm{per}$}\approx 7.1\mbox{$\, \mathrm{kpc}$}$, is produced by stars from both discs that were born in the inner Galaxy and migrated to the Solar Neighbourhood. In the Solar Neighbourhood, ∼1/2 (∼1/3) of the old thin (thick) disc stars are classified as migrators. Our results suggest that thin/thick discs are affected differently by radial migration inasmuch as they have different eccentricity distributions, regardless of vertical scale heights. We interpret the existence of a significant old thin disc population as evidence for an early co-formation of thin/thick discs, arguing that clump instabilities in the early disc offer a compelling explanation for the observed trends.Item APOGEE Net: An Expanded Spectral Model of Both Low-mass and High-mass Stars(American Astronomical Society, 2022-04) Sprague, Dani; Culhane, Connor; Kounkel, Marina; Olney, Richard; Covey, K. R.; Hutchinson, Brian; Lingg, Ryan; Stassun, Keivan G.; Román-Zúñiga, Carlos G.; Roman-Lopes, Alexandre; Nidever, David; Beaton, Rachael L.; Borissova, Jura; Stutz, Amelia; Stringfellow, Guy S.; Ramírez, Karla Peña; Ramírez-Preciado, Valeria; Hernández, Jesús; Kim, Jinyoung Serena; Lane, Richard R.We train a convolutional neural network, APOGEE Net, to predict Teff, {log}g, and, for some stars, [Fe/H], based on the APOGEE spectra. This is the first pipeline adapted for these data that is capable of estimating these parameters in a self-consistent manner not only for low-mass stars, (such as main-sequence dwarfs, pre-main-sequence stars, and red giants), but also high-mass stars with Teff in excess of 50,000 K, including hot dwarfs and blue supergiants. The catalog of ∼650,000 stars presented in this paper allows for a detailed investigation of the star-forming history of not just the Milky Way, but also of the Magellanic clouds, as different type of objects tracing different parts of these galaxies can be more cleanly selected through their distinct placement in Teff, {log}g parameter space than in previous APOGEE catalogs produced through different pipelines.