Farthest Neighbor: The Distant Milky Way Satellite Eridanus II

TL;DR

This work provides the first spectroscopy of Eridanus II, a very distant Milky Way satellite, to measure its kinematics and metallicity. Using Magellan/IMACS CaT spectroscopy, the authors identify 28 member stars, deriving a systemic velocity $v_{\rm hel} = 75.6 \pm 1.3~\mathrm{(stat.)} \pm 2.0~\mathrm{(sys.)}~\mathrm{km\,s^{-1}}$ and a velocity dispersion $\sigma_v = 6.9^{+1.2}_{-0.9}~\mathrm{km\,s^{-1}}$, which yield a half-light dynamical mass $M_{1/2} = 1.2^{+0.4}_{-0.3} \times 10^{7}~M_\odot$ and $M/L_V \approx 420^{+210}_{-140}$. The mean metallicity is ${\rm [Fe/H]} = -2.38 \pm 0.13$ with a dispersion of $0.47^{+0.12}_{-0.09}$ dex, supporting the conclusion that Eri II is a dark matter-dominated dwarf galaxy. The orbit appears bound to the Milky Way and most consistent with a second infall on an eccentric orbit, with no evidence for recent star formation or a young stellar population, despite its mass and distance. The results place constraints on quenching mechanisms, halo gas content, and MACHO dark matter scenarios through the presence of a central star cluster and the inferred DM density, while also highlighting Eri II as a key testbed for DM halo profiles at the smallest galactic scales.

Abstract

We present Magellan/IMACS spectroscopy of the recently-discovered Milky Way satellite Eridanus II (Eri II). We identify 28 member stars in Eri II, from which we measure a systemic radial velocity of $v_{\rm hel} = 75.6 \pm 1.3~\mbox{(stat.)} \pm 2.0~\mbox{(sys.)}~\mathrm{km\,s^{-1}}$ and a velocity dispersion of $6.9^{+1.2}_{-0.9}~\mathrm{km\,s^{-1}}$. Assuming that Eri~II is a dispersion-supported system in dynamical equilibrium, we derive a mass within the half-light radius of Eri II is $1.2^{+0.4}_{-0.3} \times 10^{7}~\mathrm{M_\odot}$, indicating a mass-to-light ratio of $420^{+210}_{-140}~\mathrm{M_\odot}/\mathrm{L_\odot}$ and confirming that it is a dark matter-dominated dwarf galaxy. From the equivalent width measurements of the CaT lines of 16 red giant member stars, we derive a mean metallicity of ${\rm [Fe/H]} = -2.38 \pm 0.13$ and a metallicity dispersion of $σ_{\rm [Fe/H]} = 0.47 ^{+0.12}_{-0.09}$. The velocity of Eri II in the Galactic Standard of Rest frame is $v_{\rm GSR} = -66.6~\mathrm{km\,s^{-1}}$, indicating that either Eri II is falling into the Milky Way potential for the first time or it has passed the apocenter of its orbit on a subsequent passage. At a Galactocentric distance of $\sim$370 kpc, Eri II is one of the Milky Way's most distant satellites known. Additionally, we show that the bright blue stars previously suggested to be a young stellar population are not associated with Eri II. The lack of gas and recent star formation in Eri II is surprising given its mass and distance from the Milky Way, and may place constraints on models of quenching in dwarf galaxies and on the distribution of hot gas in the Milky Way halo. Furthermore, the large velocity dispersion of Eri II can be combined with the existence of a central star cluster to constrain MACHO dark matter with mass $\gtrsim10~\mathrm{M_\odot}$.

Figures (10)

• Figure 1: IMACS spectra of three Eri II member stars at various magnitudes and S/N levels. (Left) The Fraunhofer A-band region of the spectrum, used for the corrections of velocity errors caused by the mis-centering of the stars within the slits. (Right) The CaT region, used for measuring radial velocities and metallicities of candidate member stars. The three Ca lines are marked with dashed red lines. The gaps in the spectra are caused by the gaps between IMACS CCDs.
• Figure 2: The telluric correction $v_{\rm tel}$ as a function of slit position in the direction parallel to the slits ($x$-dimension, left panel) and perpendicular to the slits ($y$-dimension, right panel). The RMS of $v_{\rm tel}$ residuals in the $x$-dimension with respect to a fitted second-order polynomial is $\sim$1 $\mathrm{\,km}\xspace\,\mathrm{\,s}\xspace^{-1}$.
• Figure 3: The results of radial velocity uncertainty estimation tests using 38 pairs of repeated observations from the October run ($v_1$, $\sigma_1$) and November run ($v_2$, $\sigma_2$). The probability distribution function (PDF, left panel) and cumulative distribution function (CDF, right panel) show the distributions of the velocity difference normalized by the quadrature sum of their uncertainties. The red dashed curves show a standard normal distribution with zero mean and unit variance. The p-value from a K-S test between the sample and the model is 0.98. This indicates that our estimation of the velocity uncertainties is reasonable.
• Figure 4: (Left) Color-magnitude diagram of Eri II using DES Y1A1 photometry. Stars within 8 of the center of Eri II are plotted as small black dots, and stars selected for spectroscopy (as described in §\ref{['sec:targets']}) are plotted as filled gray circles. Points surrounded by black outlines represent the stars for which we obtained successful velocity measurements. Those we identify as Eri II members are filled in with red. Non-members that have velocities close to the velocity of Eri II are filled in with cyan. A PARSEC isochrone bressan12 with age = 12.0 Gyr and $\hbox{[Fe/H]} = -2.2$ is displayed as the solid magenta line. The other two dashed magenta lines show the boundaries of the selected high priority RGB candidates as discussed in §\ref{['sec:targets']}. (Middle) Spatial distribution of the observed stars. Symbols are as in the left panel. The elliptical half-light radius of Eri II is outlined as a black ellipse. The yellow star indicates the location of the central star cluster of Eri II. (Right) Radial velocity distribution of observed stars. The clear narrow peak of stars at $v \sim 75$$\mathrm{\,km}\xspace\,\mathrm{\,s}\xspace^{-1}$ highlighted in red is the signature of Eri II. The hatched histogram indicates stars that are non-members of Eri II, among which the hatched cyan histogram corresponds to the cyan filled circles in the left and middle panels.
• Figure 5: Example of rest frame spectra of a foreground main sequence star (left) and an Eri II member star (right) around the Na1 $\lambda$8190 Å doublet, shown in black. The two Na1 lines are marked with dashed blue vertical lines. The foreground dwarf has strong Na1 lines, while the Eri II member is a giant star with low surface gravity and therefore the Na1 lines are hardly detectable. Overplotted dashed red lines are the spectrum of the telluric standard star, indicating the absorption from the Earth's atmosphere.