UNRAVELLING STELLAR POPULATION PUZZLES USING GAIA, HST, AND MANGA DATA

Tathagata Pal

doi:10.7273/000007211

This dissertation contains detailed description of all the work done by me in Washington State University (WSU), Pullman with Prof. Dr. G. Worthey and other collaborators. The overarching theme of this dissertation revolves around the idea of using Gaia, Hubble Space Telescope (HST), and Mapping Nearby Galaxies at the Apache point observatory (MaNGA) data to look into the formation and evolution of galaxies. This dissertation includes two projects focused on improving stellar population synthesis model ingredients and one application of the model to external galaxies. Carbon stars (C-stars) are intermediate stars that form during the asymptotic giant branch (AGB) phase when the carbon-to-oxygen ratio exceeds one at the surface. These luminous stars contribute significantly to the light of a stellar population. However, current models on C-star temporal evolution heavily rely on C-star counts from the Magellanic Clouds (MC), which have low heavy element content compared to the Milky Way (MW). We explore the rate of C-star production as a function of mass in the MW using C-star catalog and open cluster catalogs inferred from Gaia DR2. Other than the presence of a ∼7M⊙ C star in MW, we find good agreement in terms of C-star production with results from MCs. The presence of such a high-mass C-star is intriguing, as it challenges current models of stellar evolution, which must account for the mechanisms that allow carbon dredging to the surface, even in stars of relatively high mass. One of the major ingredients of any stellar population synthesis model is stellar spectral libraries with comprehensive wavelength coverage and good spectral resolution. To obtain stellar spectra extending into the ultraviolet (UV), the Hubble Space Telescope’s (HST) Space Telescope Imaging Spectrograph (STIS) observed 556 stars through proposals GO9088, GO9786, GO10222, and GO13776. The observations cover a wavelength regime of 2000˚A< λ <10000˚Awith a resolution of λ/∆λ ∼1000. We applied scattered-light corrections and flux corrections due to pointing errors relative to the center of the 0.”2 slit. In total, we present 513 fully reduced stellar spectra, corrected for flux, dereddened, and cross-correlated to zero velocity. Using this stellar library we also looked into Hα and MgII 2800 stellar chromospheric activity indicators. Apart from three cool dwarfs and one giant with mild Hα flares, only Be stars exhibit strong Hα emission. In contrast, we show that Mg2800 emission is strongly inversely correlated with temperature: warm stars display absorption, while stars cooler than 5000 K consistently show chromospheric emission, regardless of whether they are dwarfs or giants, or their [Fe/H]. The MaNGA survey provides spatially resolved spectroscopic data for ∼10000 galaxies with wavelength coverage from 3000 ˚A to 10000˚A with resolution of R∼2000. We analyzed 2417 early-type galaxies (ETGs) using metallicity-composite stellar population synthesis model to infer their age, [Fe/H], and C, N, Na, and Mg abundances and their corresponding radial gradients within 0.65 of the effective radius. We found that age shows a positive correlation with velocity dispersion σ, exhibiting a slope of 0.7 dex/log(σ) i.e. large ETGs are older. In contrast, [Fe/H] displays a slight negative correlation with σ, with an overall slope of -0.23 dex/log(σ). At the same time, the light metals C, N, Na, and Mg show a positive slope, so that larger ETGs are more metal rich overall but deficient in Fe. We also observed a flat radial gradient in age across all σ bins, while the radial gradient for [Fe/H] is negative, with an average slope of -0.04 dex/log(R/Re). Most elements exhibit a mean radial gradient of approximately -0.03 dex/log(R/Re), except for Na, which shows a steeper mean radial gradient of around -0.10 dex/log(R/Re). Well quantified errors in the MaNGA data allow us to measure the intrinsic scatter (δi) in the measured astrophysical parameters for the first time. We found that δi in age and [Fe/H] steadily decreases with increasing σ whereas their corresponding radial gradients remain almost constant. This points to the fact that smaller ETGs are more varied in age and abundance structure than larger ones. Astrophysical scatter δi in elemental abundances show low values (around 0.08 dex) for all σ bins except for Na, for which absorption in the interstellar medium is a concern.

UNRAVELLING STELLAR POPULATION PUZZLES USING GAIA, HST, AND MANGA DATA

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