EAPSI: Determining Disk Galaxy Evolutionary State through Spatially-Resolved Gas Mass Fractions

Pace Zachary J, Madison WI

Investigators

Abstract

Current research in galaxy formation and evolution seeks to answer one main question: how did the heterogeneous universe we observe--with its vast tracts of virtually-empty space between immensely compact islands of stars and hot gas--form from an overwhelmingly homogeneous beginning, the Big Bang? This is indicated robustly in individual galaxies by the gas mass fraction, the percentage of mass contained in both hot plasma and cool atomic & molecular hydrogen--as opposed to mass contained in stars. A small gas fraction indicates--in general--an evolved system with reduced capacity for forming new stars, while a large gas fraction suggests the potential for large star formation and metal-enrichment of the interstellar medium. In collaboration with Prof. Yanmei Chen (Nanjing University: Nanjing, Jiansu, China), the PI will construct statistically-robust estimates of stellar mass, and compare to his existing measurements of spatially-resolved gas mass. The radial variation of gas mass fraction will be compared with estimates of total galaxy mass, star formation rate, and galaxy environment density. In the new era of integral-field spectroscopic surveys (such as MaNGA, SAMI, and CALIFA), responsive, robust, and cheap methods of stellar-continuum fitting are necessary counterparts to large, nightly datastreams. Most methods of stellar-mass estimation are computationally costly, and do not take advantage of modern advances in statistics and machine learning. Prof. Yanmei Chen has developed a method of fitting the stellar continuum, which relies on the one-off principal-component analysis (PCA) of a library of single stellar population (SSP) models, which yields order-of-magnitude improvements in compute time and will enable the use of larger, more flexible SSP libraries. The PI has obtained a set of fully-theoretical SSP models, with a spectral resolution exceeding that of MaNGA. Such SSPs include near-IR features like the CaII triplet (necessary for breaking the age-metallicity degeneracy), and improve on existing models of the thermally-pulsating asymptotic giant branch (TP-AGB). Combining the high-resolution stellar library with Chen's PCA code (translated by the PI from IDL to Python) will yield measurable improvements in resolved stellar-mass estimates. When paired with resolved total gas mass (HI + H2), estimated using new optical-spectroscopic methods (rather than from expensive, radio-band follow-up), a census of stars and gas in nearby galaxies will result. These single-source estimates of galaxy evolutionary state will improve current knowledge of galactic chemical evolution and mass assembly, and place the Milky Way in a cosmic context. This award under the East Asia and Pacific Summer Institutes program supports summer research by a U.S. graduate student and is jointly funded by NSF and the Ministry of Science and Technology of China.