The H<i>α</i> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>C</mml:mi> <mml:mi>Hα</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:msub> <mml:mi>F</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mi>α</mml:mi> <mml:mo>,</mml:mo> <mml:mn>0.8</mml:mn> <mml:mspace width="0.25em"/> <mml:msub> <mml:mi>R</mml:mi> <mml:mi mathvariant="normal">e</mml:mi> </mml:msub> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mi>F</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mi>α</mml:mi> <mml:mo>,</mml:mo> <mml:mn>1.5</mml:mn> <mml:mspace width="0.25em"/> <mml:msub> <mml:mi>R</mml:mi> <mml:mi mathvariant="normal">e</mml:mi> </mml:msub> </mml:mrow> </mml:msub> </mml:math> , where <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>F</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mi>α</mml:mi> <mml:mo>,</mml:mo> <mml:mn>0.8</mml:mn> <mml:mspace width="0.25em"/> <mml:msub> <mml:mi>R</mml:mi> <mml:mi mathvariant="normal">e</mml:mi> </mml:msub> </mml:mrow> </mml:msub> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>F</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mi>α</mml:mi> <mml:mo>,</mml:mo> <mml:mn>1.5</mml:mn> <mml:mspace width="0.25em"/> <mml:msub> <mml:mi>R</mml:mi> <mml:mi mathvariant="normal">e</mml:mi> </mml:msub> </mml:mrow> </mml:msub> </mml:math> 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.