MATs: Akshara Viswanathan & Eltha Yu-Hsuan Teng

3:00pm -  3:30pm, Akshara Viswanathan
Tracing the Milky Way's ancient footsteps: Insights from chemodynamical investigations of bright and distant metal-poor stars

Galaxy formation and evolution is a complex process. The chemodynamical study of metal-poor stars in the Milky Way provides a unique and powerful way of unlocking the secrets of star formation, and the growth of galactic structures. Leveraging large datasets including Gaia's precision astrometry, the Pristine survey's metallicity estimates for low-metallicity stars and our own spectroscopic follow-up observations, we conduct comprehensive chemodynamical analyses of some of the most metal-poor stars in the Galaxy. Here I present the first spectroscopic follow-up of ~250 stars with predicted [Fe/H] < -2.5 (extremely metal-poor, EMP) in the Milky Way from this new dataset. We find that 75% of the stars have indeed [Fe/H] < -2.5, while all of them are very metal-poor ([Fe/H] < -2). This means a large improvement over the existing methods that search for EMP stars. Additionally, we probe further out into the outer galactic halo, going beyond 30 kpc. This resulted in the confirmation of three brightest members of the most metal-poor structure known in the universe, the C-19 stellar stream, one of which is 50 degrees away from the detected main body of the stream. Some of the EMP stars we discovered are on prograde disk-like orbits, which needs further investigation. 

We also unveil an invaluable catalog of over 1,000 very metal-poor (VMP) stars from archival Gaia RVS spectra, with a significant portion lacking any literature parameters. This catalog serves as a treasure trove for high-resolution spectroscopic research, shedding light on VMP star characteristics across diverse Galactic regions. (https://ui.adsabs.harvard.edu/abs/2023arXiv230906137V/abstract) 

Together, these works offer a fresh and comprehensive perspective on the Milky Way's assembly and growth from a chemodynamical standpoint, utilizing bright and distant metal-poor stars as invaluable guides to the early universe.

3:30pm -  4:00pm, Eltha Yu-Hsuan Teng
Revealing the drivers of CO-to-H2 conversion factor variation and its impact on star formation efficiency

Star formation in galaxies is governed by the amount of molecular gas and the efficiency that gas is converted into stars. However, assessing the amount of molecular gas relies on the CO-to-H2 conversion factor (α_CO), which is known to vary with molecular gas conditions like density, temperature, and dynamical state – the same conditions that also alter star formation efficiency. The variation of α_CO, particularly in galaxy centers where α_CO can drop by nearly an order of magnitude, thus causes major uncertainties in current molecular gas and star formation efficiency measurements. Using ALMA observations of multiple 12CO, 13CO, and C18O lines in several barred galaxy centers, we found that α_CO is primarily driven by CO opacity changes and therefore shows strong correlations with observables like velocity dispersion and 12CO/13CO line ratio.

Motivated by these results, we have constructed a new α_CO prescription which accounts for emissivity effects in galaxy centers and verified it on a set of barred and non-barred galaxies with measured α_CO values from dust. Applying our new prescription to 65 galaxies from the PHANGS survey, we found an overall 3x higher star formation efficiency in barred galaxy centers than in non-barred centers, and such a trend is obscured when using a MW α_CO or other existing prescriptions. Our results suggest that the high star formation rates observed in barred centers are not simply due to an increased amount of molecular gas but also an enhanced star formation efficiency compared to non-barred centers or disk regions.