A First Measurement of Circumgalactic Dust Reddening from Only 4.6 deg squared of the Rubin Observatory's Data Preview 1

We present the first measurement of circumgalactic dust reddening from the Vera C. Rubin Observatory, using only 4.6 deg squared of ComCam Data Preview 1 - roughly 0.03% 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 10 kpc to 1 Mpc. Interpreting average E(g-z) with a Milky Way extinction curve, we find A V = (1.2 plus or minus 0.4) times 10-1 (r / 20, kpc)-1.8 plus or minus 0.4 within 120 kpc. The amplitude and radial dependence agree with earlier SDSS, KiDS, and DES results despite the about1000 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 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, shows substantially more reddening within 50 kpc than previously measured around more massive LRGs and implies a dust-to-stellar-mass ratio of about 1%, 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.