Stellar Surface Density Modulates MgII Cool-gas Outflow Absorption in DESI Star-forming Galaxies
Abstract
Galaxy outflows are usually ordered by stellar mass and star-formation rate (SFR), but the same feedback budget may couple differently to gas in diffuse and compact galaxies. We use Dark Energy Spectroscopic Instrument (DESI) Data Release 1 stacked spectra of massive star-forming galaxies at 0.35<z<1.0 to test whether stellar surface density, \(Σ=M/(2πRe2)\), is an independent empirical coordinate of down-the-barrel singly ionized magnesium () cool-gas absorption. In AGN-clean samples matched in stellar mass, and in a stricter sample matched in both stellar mass and a Balmer-line SFR proxy, the \ outflow equivalent width (EW) rises monotonically with \(Σ\) in every redshift bin. From the lowest to highest \(Σ\) tertile, EW out increases by \(0.37\)--\(0.61\)~Å, while the absolute outflow velocity changes only weakly. DESI therefore shows that cool-gas outflow strength in massive star-forming galaxies is not set only by how much stellar mass or star formation a galaxy has, but also by how tightly the galaxy is built. The structural dependence points to changes in the absorbing velocity distribution and/or the effective covering fraction of cool outflowing gas.
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