Stellar Surface Density Modulates MgII Cool-gas Outflow Absorption in DESI Star-forming Galaxies

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, $\Sigma_\star=M_\star/(2\pi R_e^2)$, is an independent empirical coordinate of down-the-barrel singly ionized magnesium (\mgii) 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 \mgii\ outflow equivalent width (EW) rises monotonically with $\Sigma_\star$ in every redshift bin. From the lowest to highest $\Sigma_\star$ tertile, EW$_{\rm out}$ increases by $0.37$--$0.61$~\AA, 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.