photoD with <i>Rubin</i> ’s Data Preview 1: First stellar photometric distances and faint blue star deficits

Aims . We investigate the utility of Rubin ’s Data Preview 1 (DP1) for estimating stellar number density profiles across the Milky Way halo. Methods . We used stellar broad-band near-UV to near-IR ugrizy photometry released in Rubin ’s DP1 to estimate distance and metallicity for blue main sequence stars brighter than r = 24 in three ~1.1 sq. deg. fields at southern Galactic latitudes. Results . Compared to TRILEGAL simulations of the Galaxy’s stellar content, we found a likely deficit of blue main sequence turn-off stars with 22 &lt; r &lt; 24. We interpreted this discrepancy as a signature of a steeper halo number density profile at galactocentric distances 10–50 kpc than the canonical ~1/ r 3 profile assumed in TRILEGAL simulations. Conclusions . This interpretation is consistent with earlier suggestions based on observations of more luminous, but much less numerous, evolved stellar populations, along with a few pencil beam surveys of blue main sequence stars in the northern sky. These results bode well for the future Galactic halo exploration with Rubin ’s Legacy Survey of Space and Time (LSST).