The H$\alpha$ concentration of local star-forming galaxies: implications for galaxy structure formation

Abstract

In this work, we present a study on the H$\alpha$ 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. We define the H$\alpha$ flux concentration index ($C_{\mathrm{H}\alpha }$) as $C_{\mathrm{H}\alpha }=F_{\mathrm{H}\alpha ,0.8{\mathrm{R}}_{\mathrm{e}}}/F_{\mathrm{H}\alpha ,1.5{\mathrm{R}}_{\mathrm{e}}}$, where $F_{\mathrm{H}\alpha ,0.8{\mathrm{R}}_{\mathrm{e}}}$ and $F_{\mathrm{H}\alpha ,1.5{\mathrm{R}}_{\mathrm{e}}}$ are the cumulative H$\alpha$ flux inside $0.8$ and $1.5$ $r-$band effective radius, respectively. We find that $C_{\mathrm{H}\alpha }$ is strongly correlated with the luminosity weighted stellar age gradient. $C_{\mathrm{H}\alpha }$ is also sensitive to environmental effects, in the sense that low-mass satellite galaxies below the star formation main sequence tend to have higher $C_{\mathrm{H}\alpha }$. For central galaxies, we find that massive disk galaxies with enhanced star formation rate tend to have higher $C_{\mathrm{H}\alpha }$, 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 resulting in the buildup of prominent bulges in massive SFGs. Implications of these findings on galaxy structure formation are discussed.

13 pages, 10 figure, ApJ accepted