Variations in the Milky Way's Stellar Mass Function at [Fe/H] < -1

TL;DR

This study tests IMF universality by measuring the Milky Way's stellar mass function for metal-poor, low-mass stars ($M_*\,\sim\,0.2$–$0.5\,M_\odot$) with [Fe/H]$<-1$ within ~1 kpc. It combines a probabilistic, halo-leaning kinematic selection with Gaia XP-based metallicities calibrated against SDSS-V to build a high-purity metal-poor sample ($\sim$53,000 stars). A forward-modeling approach accounts for Gaia's selection via a mass- and metallicity-dependent effective volume, yielding a non-parametric MF that varies with metallicity: transitioning from near-flat ($\alpha_1 \approx 0$) at [Fe/H]$\sim -2$ to strongly bottom-heavy ($\alpha_1 \approx -1.8$ to $-2.7$) near [Fe/H]$\sim -1.2$–$-1.5$, with the mass ratio $\xi_{\mathrm MR}$ rising from ~$0.42$ to ~$0.86$. These results indicate that the Milky Way's low-mass MF is not universal and support metallicity-dependent IMF formulations such as IGIMF, with implications for early star formation and galaxy assembly. Future work will refine metallicity calibration at $M_*<0.5\,M_\odot$ and explicitly model binaries to sharpen these constraints.

Abstract

We present the first determination of the Galactic stellar mass function (MF) for low-mass stars (0.2-0.5 M_sun) at metallicities [Fe/H] < -1. A sample of ~53,000 stars was selected as metal-poor on the basis of both their halo-like orbits and their spectroscopic [Fe/H] from Gaia DR3 BP/RP (XP) spectra. These metallicity estimates for low-mass stars were enabled by calibrating Gaia XP spectra with stellar parameters from SDSS-V. For -1.5 < [Fe/H] < -1, we find that the MF below 0.5 M_sun exhibits a "bottom-heavy" power-law slope of alpha ~ -1.6. We tentatively find that at even lower metallicities, the MF becomes very bottom-light, with a near-flat power-law slope of alpha ~ 0 that implies a severe deficit of low-mass stars. This metallicity-dependent variation is insensitive to the adopted stellar evolution model. These results show that the Galactic low-mass MF is not universal, with variations in the metal-poor regime. A further calibration of XP metallicities in the regime of M < 0.5 M_sun and [Fe/H] < -1.5 will be essential to verify these tentative low-metallicity trends.

Figures (4)

• Figure 1: Color-Magnitude Diagrams (CMDs) of the Gaia XP sample. Left: The CMD of Gaia sources within 1 kpc is color-coded by $\eta$, defined in Eq. \ref{['eq:eta']}, which is the likelihood that a star's orbit is halo-like rather than disk-like. Blue colors denote stars that are more likely to be on halo-like orbits. Right: CMD of the sample with halo-like orbits ($\eta>10$), color-coded by XP-derived metallicity. Typical absolute-magnitude error is dominated by distance uncertainties, with $\sigma_{M_G}\approx 0.08~\mathrm{mag}$ at $d=800~\mathrm{pc}$. The gray background represents sources within 100 pc. The dot--dashed colored lines show loci from the PARSEC stellar evolution models evaluated at a fixed age of 5 Gyr for different metallicities. The black dotted lines indicate iso-mass lines from the PARSEC models.
• Figure 2: Left: Metallicity distribution of the kinematically pre-selected sample (halo-like orbits, $\eta>10$) and the final metal-poor sample. The orange histogram shows the [Fe/H] distribution for the kinematically selected parent sample within 1 kpc, while the blue histogram shows the distribution for our final metal-poor sample, selected using both kinematic ($\eta$) and chemical ($\text{[Fe/H]} < -1.1$) criteria. The vertical dashed line indicates our metallicity cut. Right: The CMD for our final metal-poor sample with $\eta>10$ and [Fe/H]<-1.1.
• Figure 3: The estimated effective volume $\tilde{V}_{\mathrm{eff}}$ as a function of absolute magnitude $M_{G0}$. The plot shows how the volume accessible to the survey is much larger for brighter (lower $M_{G0}$), more massive stars than for fainter, less massive ones.
• Figure 4: Stellar mass functions for the metal-poor sample. The logarithmic mass density, $\log_{10}\Phi$ [number kpc$^{-3}$ M$_\odot^{-1}$], is shown as a function of stellar mass $M$ for different metallicity bins (color-coded by [Fe/H], see color bar). Stellar masses are derived from PARSEC models (solid lines) and BaSTI models (dotted lines). Symbols show the binned densities with shaded bands indicating $1\sigma$ uncertainties. The dashed gray line shows the canonical Kroupa IMF, scaled for visual comparison.