Reading between the (Spectral) Lines: Magellan/IMACS Spectroscopy of the Ultrafaint Dwarf Galaxies Eridanus IV and Centaurus I

Abstract

We present a spectroscopic analysis of Eridanus IV (Eri IV) and Centaurus I (Cen I), two ultrafaint dwarf galaxies of the Milky Way. Using IMACS/Magellan spectroscopy, we identify 28 member stars of Eri IV and 34 member stars of Cen I. For Eri IV, we measure a systemic velocity of ${v}_{\mathrm{sys}}=-{31.5}_{-1.2}^{+1.3}\ \mathrm{km}\,{{\rm{s}}}^{-1}$ , and velocity dispersion ${\sigma }_{v}={6.1}_{-0.9}^{+1.2}\ \mathrm{km}\,{{\rm{s}}}^{-1}$ . Additionally, we measure the metallicities of 16 member stars of Eri IV. We find a metallicity of $[\mathrm{Fe}/{\rm{H}}]=-{2.87}_{-0.07}^{+0.08}$ , and resolve a dispersion of σ _[Fe/H]=0.20 ± 0.09. The mean metallicity is marginally lower than all other known ultrafaint dwarf galaxies, making it one of the most metal-poor galaxies discovered thus far. Eri IV also has a somewhat unusual right-skewed metallicity distribution. For Cen I, we find a velocity v _sys = 44.9 ± 0.8 km s^‑1, and velocity dispersion ${\sigma }_{v}={4.2}_{-0.5}^{+0.6}\ \mathrm{km}\,{{\rm{s}}}^{-1}$ . We measure the metallicities of 27 member stars of Cen I, and find a mean metallicity [Fe/H] = ‑2.57 ± 0.08, and metallicity dispersion ${\sigma }_{[\mathrm{Fe}/{\rm{H}}]}={0.38}_{-0.05}^{+0.07}$ . We calculate the systemic proper motion, orbit, and the astrophysical J-factor for each system, the latter of which indicates that Eri IV is a good target for indirect dark matter detection. We also find no strong evidence for tidal stripping of Cen I or Eri IV. Overall, our measurements confirm that Eri IV and Cen I are dark-matter-dominated galaxies with properties largely consistent with other known ultrafaint dwarf galaxies. The low metallicity, right-skewed metallicity distribution, and high J-factor make Eri IV an especially interesting candidate for further follow-up.