Exploring Metallicity and Gas Flows in NGC 99 through Multi-Wavelength Spectroscopy

We present a resolved multi-wavelength study of the local Milky Way-type galaxy, NGC 99. We utilized optical multi-object spectroscopy of 26 HII regions to measure a radial metallicity gradient of -0.017 dex/kpc (-0.20 dex/R25), consistent with the mean metallicity gradient of large surveys. Among our sample, we find anomalously low metallicity (ALM) HII regions with H-alpha gas kinematically offset from the HI gas in the surrounding interstellar medium, theorizing that these result from metal-poor gas accretion. Our chemical evolution modeling indicates that outflows have expelled metals from the HII regions, while inflows of metal-poor gas have diluted the surrounding gas. Using the bathtub model, we find that the mass-loading factor increases due to gas dilution at greater galactocentric radii, where metal-poor gas is more likely to be accreted. We obtain the star-formation rate surface density from H-alpha emission and the stellar mass surface density from optical band imaging to derive the resolved Fundamental Metallicity Relationship (rFMR), which is an extension of the Mass-Metallicity Relation (MZR) on sub-galactic scales. Our rFMR reveals that metallicity correlates inversely with star-formation rate for a given stellar mass. The rFMR is crucial to understanding the connection between local and global physics and helps measure metallicities in higher redshift galaxies. This study provides insight into the flow of metals in and out of galaxies like ours.