Galactic Stellar Halo Luminosity Function

TL;DR

This study measures the luminosity function of the Milky Way's stellar halo using Gaia DR3 data, revealing detailed features from faint subdwarfs to bright giants, and compares it with previous studies to inform galaxy mass estimates.

Contribution

First continuous measurement of the Milky Way halo luminosity function across a wide magnitude range using Gaia data, accounting for sample selection effects.

Findings

Halo LF shows a peak at M_G=10 and a flattening near M_G=7.

Local stellar halo density is 1.7×10^{-4} stars/pc^3.

Disk-to-halo ratio by number density is 480:1.

Abstract

We measure the luminosity function (LF) of the Milky Way's stellar halo, using a magnitude complete, distance limited sample of stars from $Gaia$ DR3. Stars with high transverse velocities are selected, to isolate a high purity sample of the local halo. We adopt a cutoff transverse velocity of 250$\,$km$\,$s$^{-1}$, yielding 24,471 stars, and compute the halo LF, taking into account the effects of sample selection criteria. The LF displays similar features as are found in the well-probed LF of nearby, metal-rich disk stars, showing a strong peak at an absolute magnitude of around $M_G=10$, and a flattening near $M_G\sim7$ (Wielen dip). The $Gaia$ sample yields the first measurement of the LF continuously from the dimmest main sequence halo stars (subdwarfs) at an absolute $M_G$ magnitude near 13 mag to bright giants at $M_G\sim-3$. We obtain a local stellar halo number density of $1.7\times10^{-4}$ stars$\,$pc$^{-3}$ and disk-to-halo ratio by stellar number density of 480:1. We convert the $Gaia$ $G$ band measurements for our sample stars to Johnson-Kron-Cousins $V$ band, compute the $V$-band halo LF, and compare it to previous studies published over many decades that cover a wide range of techniques used. We discuss applications of the LF to the measurement of the luminosity and stellar mass of the Milky Way halo.