Willman 1 Revisited: The Kinematics, Chemistry, and Orbital Properties of a Potentially-Disrupting Dwarf Galaxy

Abstract

The ultra-faint Milky Way satellite Willman 1 (W1; $M_V = -2.6$; $r_{\rm half} \sim27$ pc) was the first stellar over-density found via resolved stars in the Sloan Digital Sky Survey, yet its classification as either a dwarf galaxy or star cluster remains ambiguous. Using new Keck/DEIMOS spectroscopy, HST/ACS photometry, and orbital modeling, we re-examine the nature of W1. From our updated sample of 57 member stars, we find that past analyses included four binaries and seven non-members, identified here using Gaia proper motions and updated velocities. We continue to find a velocity dispersion consistent with previous analyses, measuring $σ_v = 4.7^{+1.5}_{-1.3}$ km s$^{-1}$ from 49 stars out to $3~r_{\rm half}$. If W1 is in equilibrium, this suggests a dynamical mass of $5.9^{+3.7}_{-3.4} \times 10^5~M_{\odot}$ and a mass-to-light ratio of $(M/L)_V = 660 \pm 590$. Based on Ca II triplet measurements, we estimate an iron abundance of [Fe/H] $= -2.45^{+0.12}_{-0.13}$ and a metallicity dispersion of $σ_{\rm [Fe/H]} = 0.30^{+0.15}_{-0.11}$ dex. We confirm that W1 does not exhibit mass segregation inside $\sim1~r_{\rm half}$. Our best-fit orbital model predicts that W1 is at apocenter, implying that W1 has been closer to the Milky Way in the recent past, reaching a pericentric distance $\lesssim 25$ kpc from the Galactic center $\sim0.3$ Gyr ago. Given its internal kinematics, metallicity spread, and lack of mass segregation, we conclude that W1 is a galaxy. However, given its orbit and structural properties, which suggest that W1 might be tidally disrupted, and the difficulty of identifying a pure member sample, we caution that the measured internal velocity dispersion may not accurately reflect the dynamical mass of this system.

Figures (10)

• Figure 1: From our five criteria for membership (panels a, b, c, d), the membership probability of each star was determined (panels e, f, g, h). In each plot (left to right: CMD, velocity, proper motion, and metallicity profile), all stars with photometric and radial velocity measurements are color-coded according to the membership probability of the criterion in question, with clear non-members ($P_{\rm criterion} < 0.1$) colored gray. Stars with variable velocities are circled in black. The priors for each marker are delineated in the top panels willman2011Pace2022, and the properties derived from our member sample are shown in the bottom panels (see §\ref{['sec:results']}, Table \ref{['table_properties']}).
• Figure 2: Upper: Spatial distribution of the 57 W1 member stars, color-coded according to $P_{\rm mem}$. Members with variable velocities are circled in black and non-members are in colored gray. The three concentric dotted ellipses represent the space within 1, 2, and 3 $\, r_{\rm half,ell}$. All stars with photometry from munoz2018a within $3\, r_{\rm half,ell}$, $P_{\rm CMD} > 0.5$, and $r_0 < 23$ are shown in black. The field of view of the HST/ACS imaging is denoted red (see §\ref{['ssec:hst_redux']}). Lower: CMD of W1 member stars overlaid with an old, metal-poor PARSEC isochrone in red. All stars with photometry from munoz2018a within $3 \, r_{\rm half,ell}$ are shown in black. The liberally defined red giant branch for which we measure spectroscopic metallicities is boxed.
• Figure 3: Heliocentric radial velocities and uncertainties of the 49 members as a function of elliptical half-light radius (upper) and projected distance with respect to the major axis (lower). For both plots, stars to the left and the right of the center axis of W1 are plotted as squares and circles, respectively. All 49 stars are color-coded purple according to their membership probability and non-members are in gray. In both panels, the dotted black line represents the membership probability weighted 7-star rolling velocity average, and the gray shaded region represents the uncertainty in the rolling average calculated via bootstrapping.
• Figure 4: Upper: Posterior samples for the two-component Gaussian mixture model to measure the systemic velocity and velocity dispersion of W1 based on a velocity sample of 65 potential member stars within $3 \, r_{\rm half,ell}$. The dashed lines represent the median value and 16th and 84th percentiles. Lower: Best-fit mixture model of two-component Gaussian representing the Milky Way and W1. The 65 potential members used for our fitting are over-plotted, and the member sample (§\ref{['ssec:final_members']}) is outlined in purple. We note the asymmetric velocity distribution and comment on this in §\ref{['ssec:tidal']}.
• Figure 5: CaT equivalent width as a function of V-band magnitude ( upper panel) and the CaT-based spatial metallicity distribution ( lower panel) for the eight RGB stars in our member sample. The solid line denotes the calibration range of the empirical relationship of Carrera2013 plotted at the mean metallicity of W1 while the dotted lines represents the metallicity dispersion determined from MCMC sampling.