photoD with Rubin's Data Preview 1: first stellar photometric distances and deficit of faint blue stars. Stellar distances with Rubin's DP1

TL;DR

This study tests Rubin's Data Preview 1 photometry as a tool to map the Milky Way halo by focusing on blue main-sequence turn-off stars and deriving distances and metallicities with the photoD framework. It finds a significant deficit of faint blue stars relative to TRILEGAL predictions, implying a much steeper halo density profile at galactocentric radii of $10$–$50$ kpc, with the halo slope $n$ estimated between $5$ and $8$ under plausible oblateness ranges. Metallicity distributions are broadly consistent with a halo peak near $[Fe/H]\, hicksim\, -1.5$ at ∼20 kpc, though uncertainties in the $u$ band color term introduce systematic shifts in $[Fe/H]$ and distances. The results support a more complex outer-halo structure than used in prior priors and bode well for large-scale Rubin/LSST halo mapping, while underscoring the need for robust photometric transformations and priors to fully exploit the data and break degeneracies between halo shape parameters.

Abstract

Aims: We investigate the utility of Rubin's Data Preview 1 for estimating stellar number density profile in the Milky Way halo. Methods: Stellar broad-band near-UV to near-IR $ugrizy$ photometry released in Rubin's Data Preview 1 is used to estimate distance and metallicity for blue main sequence stars brighter than $r=24$ in three $\sim$1.1. sq.~deg. fields at southern Galactic latitudes. Results: Compared to TRILEGAL simulations of the Galaxy's stellar content by (Dal Tio, 2022), we find a significant deficit of blue main sequence turn-off stars with $22 < r < 24$. We interpret this discrepancy as a signature of a much steeper halo number density profile at galactocentric distances $10-50$ kpc than the cannonical $\sim1/r^3$ 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, and 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.

Figures (16)

• Figure 1: The sky coverage of Rubin Data Preview 1 dataset. The seven LSSTComCam fields are shown as yellow dots (with names) in the context of the LSST's planned regions: North Ecliptic Spur (purple), South Celestial Pole (yellow), the dusty plane, the Galactic Plane and Magellanic Clouds (brown and green), and low-dust-extinction regions of the Wide-Fast-Deep (blue) program. Here we analyze data from three fields: ECDFS, EDFS and Rubin SV 95 -25. Adapted from 10.71929/rubin/2570308 (https://dp1.lsst.io/).
• Figure 2: Difference between the PSF (point spread function) magnitude and the CModel magnitude in the $gri$ bands, and the mean $gri$ value (bottom right) vs. magnitude for the ECDFS field (analogous to Figure 1 from 2020AJ....159...65S). The colormap encodes the number of stars per bin. The bimodal distribution of stars and galaxies (more precisely, unresolved and resolved sources) is evident.
• Figure 3: Analogous to Figure \ref{['fig:SG']}, except that the abscissa is zoomed in on, and the left panels show the $u$ and $y$ band diagrams, respectively. The two vertical lines in the bottom right panel show the separation boundary between unresolved and resolved sources (dashed at 0.016 mag: Rubin default value for single-band classification; solid at 0.04 mag: adopted here for the mean $gri$ values and designed to ensure stellar completeness to faint magnitude limits).
• Figure 4: A comparison of color-magnitude diagrams of unresolved (top) and resolved (bottom) sources selected from Rubin DP1 fields ECDFS (left) and SV 95 $-$25 (right). Note the low fractions of unresolved sources (approximately 8% in the ECDFS field and 24% in the SV 95 $-$25 field, for $r<25$).
• Figure 5: A comparison of color-magnitude diagrams of unresolved sources selected from Rubin DP1 (top row), TRILEGAL simulation (middle row) and DES and DELVE data (bottom row). Each column corresponds to one of the Rubin DP1 fields (as designated in the top left corner of each panel). Color scale is according to the probability density. While bin sizes are smaller in the rightmost column, the same color scale is shared between all panels. TRILEGAL, DELVE and DES data have been selected so that the area of each field is equal to the approximate area covered by the corresponding DP1 fields. Recall that $r<24$, star-galaxy separation and $0 < g-r < 1.4$ selection criteria were applied. DELVE data contain only a few stars fainter than r$\approx$22. Please note a deficit of faint ($r>22$) blue stars ($g-r< 0.6$), compared to TRILEGAL simulation, in all three fields.