The H α Concentration of Local Star-forming Galaxies: Implications for Galaxy Structure Formation

Abstract

In this work, we present a study on the H α emission line flux concentration of 3098 low-redshift star-forming galaxies (SFGs) using the MaNGA data available in the Data Release 17 from the Sloan Digital Sky Survey (SDSS). We define the H α flux concentration index ( C H α ) as C Hα = F H α , 0.8 R e / F H α , 1.5 R e , where F H α , 0.8 R e and F H α , 1.5 R e are the cumulative H α flux inside 0.8 and 1.5 r -band effective radii, respectively. We find that C H α is strongly correlated with the luminosity-weighted stellar age gradient. C H α is also sensitive to environmental effects, in the sense that low-mass satellite galaxies below the star formation main sequence (SFMS) tend to have higher C H α . For central galaxies, we find that massive disk galaxies with enhanced star formation rates tend to have higher C H α , while such a phenomenon is not seen in the low-mass regime. We interpret this as evidence that compaction events more frequently occur in the high-mass regime, which eventually results in the buildup of prominent bulges in massive SFGs. Implications of these findings for galaxy structure formation are discussed.