DELVE Milky Way Satellite Galaxy Census I: Satellite Population and Survey Selection Function in DES, DELVE, and Pan-STARRS

Abstract

The properties of Milky Way satellite galaxies have important implications for galaxy formation, reionization, and the fundamental physics of dark matter. However, the population of Milky Way satellites includes the faintest known galaxies, and current observations are incomplete. To understand the impact of observational selection effects on the known satellite population, we perform rigorous, quantitative estimates of the Milky Way satellite galaxy detection efficiency in three wide-field survey datasets: the Dark Energy Survey Year 6, the DECam Local Volume Exploration Data Release 3, and the Pan-STARRS1 Data Release 1. Together, these surveys cover $\sim$13,600 deg$^2$ to $g \sim 24.0$ and $\sim$27,700 deg$^2$ to $g \sim 22.5$, spanning $\sim$91% of the high-Galactic-latitude sky ($|b| \geq 15^\circ$). We apply multiple detection algorithms over the combined footprint and recover 49 known satellites above a strict census detection threshold. To characterize the sensitivity of our census, we run our detection algorithms on a large set of simulated galaxies injected into the survey data, which allows us to develop models that predict the detectability of satellites as a function of their properties. We then fit an empirical model to our data and infer the luminosity function, radial distribution, and size-luminosity relation of Milky Way satellite galaxies. Our empirical model predicts a total of $265^{+79}_{-47}$ satellite galaxies with $-20 \leq M_V \leq 0$, half-light radii of $15 \leq r_{1/2} (\rm pc) \leq 3000$, and galactocentric distances of $10 \leq D_{\rm GC} (\rm kpc) \leq 300$. We also identify a mild anisotropy in the angular distribution of the observed galaxies, at a significance of $\sim$$2σ$, which can be attributed to the clustering of satellites associated with the LMC.

Figures (14)

• Figure 1: Galactocentric locations of Milky Way satellites with $r_{1/2} > 15$ pc. In total, our census recovered 49 out of 62 known Milky Way satellites within our survey footprint. Filled circles indicate systems recovered by our analysis, while open circles represent unrecovered systems. Each satellite is colored according to the survey region that it is located in. The red, blue, and green regions show the coverage areas of DES Y6, DELVE DR3, and PS1 DR1 surveys, respectively. The gray region marks areas excluded from our census, either because they are at low Galactic latitudes, are near other stellar systems (e.g., globular clusters), or lack sufficient survey coverage. We note that four relatively bright satellite (LMC, SMC, Sagittarius, and Antlia II) are outside of our survey footprint.
• Figure 2: Equatorial McBryde--Thomas projection of the coverage area of DES, DELVE, and PS1 surveys used in our census. Darker shades indicate regions with deeper magnitude limits, which is particularly apparent in the relatively inhomogeneous DELVE survey. We also highlight in black the masked regions not considered in our census. This includes regions around the Galactic plane with high interstellar reddening ($E(B-V) > 0.2$) or high stellar density ($\rho_{G<21}>$ 8 stars/arcmin$^2$), and regions around other types of stellar systems (such as globular clusters and the Magellanic Clouds). Additionally, we represent in yellow the regions where we do not have coverage from any of our three surveys.
• Figure 3: Stellar completeness of the DES Y6, DELVE DR3, and PS1 DR1 stellar catalogs relative to the HSC-SSP PDR3 stellar catalog as a function of $r$-band magnitude limit. For the inhomogeneous DELVE survey, stellar completeness is illustrated at the median $r$-band depth ($m_{\mathrm{lim},r} = 23.5$) with a solid line, and at additional depths $m_{\mathrm{lim}}\sim\{23.0, 24.0, 24.5, 25.0\}$ using dashed lines, with darker lines indicate deeper limiting magnitudes.
• Figure 4: Diagnostic plots for one of the census galaxies, Hydra II, constructed using DELVE DR3 data. Left: Smoothed, isochrone-filtered stellar density map centered on Hydra II. Center Left: Color–magnitude diagram (CMD) of stars near the system, with points colored by their angular distance from the Hydra II centroid. A best-fit PARSEC isochrone is overlaid as a dashed maroon line. Center Right: Spatial distribution of stars, colored by their ugali membership probability. Right: CMD of stars, colored by their ugali membership probability. For this illustration, we adopt an elliptical Plummer profile using the best-fit structural parameters as the ugali galaxy model. However, in our actual search, we use a radially symmetric Plummer profile, with galaxy parameters drawn from a predefined grid.
• Figure 5: Distribution of "hotspots" identified by the search algorithms in the DES and DELVE data. The $x$-axis represents the detection threshold value, $\chi$. The red line shows hotspots returned by simple that pass the SIG$_{gr}>\chi$ threshold, while the blue line shows those returned by ugali that meet the $\sqrt{\mathrm {TS}_{gr}}>\chi$ threshold. The green and purple lines represent subsets of ugali hotspots that meet additional detection significance thresholds on $\sqrt{\mathrm {TS}_{gi}}$ (based on $g-i$ bands) and SIG$_{gr}$ (from simple), both of which are required for inclusion in the census within the DES and DELVE regions, respectively. The dashed black lines indicate the number of previously known dwarf galaxy systems in our DES and DELVE footprint that meet the ugali$\sqrt{\mathrm{TS}_{gr}}>\chi$ threshold.