Farthest Neighbor: The Distant Milky Way Satellite Eridanus II (1611.05052v2)

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 $\sigma_{\rm [Fe/H]} = 0.47 ^{+0.12}_{-0.09}$. 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 $\gtrsim10~\mathrm{M_\odot}$.

Overview and Implications of "Farthest Neighbor: The Distant Milky Way Satellite Eridanus II"

The paper presented in the paper "Farthest Neighbor: The Distant Milky Way Satellite Eridanus II" by Li et al. provides a comprehensive spectroscopic analysis of Eridanus II (Eri II), a dwarf galaxy at the edge of the Milky Way's influence. Utilizing data gathered from the 6.5-meter Magellan Telescopes at Las Campanas Observatory, this research outlines significant findings regarding the kinematics, metallicity, and structure of Eri II, contributing to the broader understanding of ultra-faint dwarf galaxies in cosmological frameworks.

Key Findings and Methodology

1. Spectroscopy and Membership: The authors identified 28 member stars of Eri II through Magellan/IMACS spectroscopy, determining a systemic radial velocity of $v_{\rm hel} = 75.6 \pm 1.3~\mbox{km s}^{-1}$ and a velocity dispersion of $6.9^{+1.2}_{-0.9}~\mbox{km s}^{-1}$. These values affirm Eri II's nature as a dispersion-supported system in dynamical equilibrium.
2. Mass and Dark Matter Dominance: With a derived mass within the half-light radius of $1.2^{+0.4}_{-0.3} \times 10^{7} M_\odot$, Eri II displays a mass-to-light ratio of $420^{+210}_{-140} M_\odot/L_\odot$, confirming it as dark matter dominated.
3. Metallicity and Dispersion: The mean metallicity of Eri II, obtained from CaT lines of red giant stars, is $\feh = -2.38 \pm 0.13$ with a considerable metallicity dispersion of $\sigma_{\feh} = 0.47 ^{+0.12}_{-0.09}$.
4. Galactic Dynamics and Orbit: Eri II's velocity in the Galactic Standard of Rest frame suggests it may be on its initial approach to the Milky Way or may have passed its orbital apocenter. Further, Eri II is highlighted as one of the furthest Milky Way satellites known, at approximately 370 kpc.
5. Stellar Population and Star Formation: The lack of associated bright blue stars—previously thought to imply recent star formation—suggests that Eri II currently lacks young stars and significant gaseous content, a surprising result given its mass and position relative to the Milky Way.

Implications for Cosmology and Dark Matter Research

• Quenching Mechanisms: The findings challenge existing models of star formation quenching in dwarf galaxies, especially those located near the virial radius of the Milky Way. Eri II's characteristics suggest that, despite its isolation, it has been quenched of star formation, providing a case paper for understanding feedback and gas-stripping processes.
• Dark Matter Constraints: The large velocity dispersion and spatial structure of Eri II, together with its central star cluster, offer a rare opportunity to place constraints on the presence of massive compact halo objects (MACHOs) as candidates for dark matter. This enhances our ability to test theoretical models of dark matter composition and distribution.
• Future Directions: Prospective studies could further elucidate Eri II's interaction history and its role within the dark matter rich environment of the Milky Way’s halo. Considerations for additional observations, such as deep imaging and proper motion measurements, will be vital in pinpointing Eri II’s orbital evolution and verifying theoretical predictions concerning interaction-induced morphological transformations in dwarf galaxies.

In conclusion, Li et al.'s work provides a detailed account of Eri II's properties, setting a precedent for future studies in understanding the dynamics of remote satellite galaxies and their implications for galactic formation and structure, as well as offering potential insights into the nature and behavior of dark matter within these systems.