A First Measurement of Circumgalactic Dust Reddening from Only 4.6 deg${}^2$ of the Rubin Observatory's DP1

We present the first measurement of circumgalactic dust reddening from the Vera C. Rubin Observatory, using only 4.6 deg${}^2$ of ComCam Data Preview 1 - roughly $0.03\mathrm{\%}$ of the final LSST footprint. Using photometric redshifts, we stack background-galaxy colors around foreground-galaxy positions and detect a chromatic reddening profile from $r_{\perp }\simeq 10$ kpc to $1$ Mpc. Interpreting average $E(g-z)$ with a Milky Way extinction curve, we find $A_V=(1.2\pm 0.4)\times {10}^{-1}(r_{\perp }\mathrm{/}20\mathrm{k}\mathrm{p}\mathrm{c}{)}^{-1.8\pm 0.4}$ within $120$ kpc. The amplitude and radial dependence agree with earlier SDSS, KiDS, and DES results despite the $\sim 1000\times$ smaller survey area and a foreground sample extending 3-6 mag fainter and 1-2 dex lower in stellar mass. The innermost 10-15 kpc bin reaches $A_V\simeq 0.3$ mag, comparable to high-latitude extinction through the Milky Way disk near the Solar circle; the steep power-law slope implies a dust distribution that does not simply trace the halo-gas profile. Splitting by rest-frame $g-r$ shows stronger extinction around red foreground galaxies (rest-frame $g-r>0.5$), although the blue subsample is too noisy to establish a significant color dependence. This red sample, with median halo mass $5\times {10}^{11}{M}_{\odot }$, shows substantially more reddening within 50 kpc than previously measured around more massive LRGs and implies a dust-to-stellar-mass ratio of $\sim 1\mathrm{\%}$, nearly saturating the dust budget allowed by stellar metal yields. These pathfinder data demonstrate LSST’s promise for high-precision galaxy-dust measurements across galaxy mass, environment, and redshift.