Ships Passing in the Night: Spectroscopic Analysis of Two Ultra-Faint Satellites in the Constellation Carina

Abstract

We present Magellan/IMACS, Anglo-Australian Telescope/AAOmega+2dF, and Very Large Telescope/GIRAFFE+FLAMES spectroscopy of the CarinaII (Car II) & Carina III (Car III) dwarf galaxy candidates, recently discovered in the Magellanic Satellites Survey (MagLiteS). We identify 18 member stars in Car II, including 2 binaries with variable radial velocities and 2 RR Lyrae stars. The other 14 members have a mean heliocentric velocity $v_{\mathrm{hel}}=477.2\pm 1.2$ km/s and a velocity dispersion of $\ddot{I}{ƒ}_v=3.4_{-0.8}^{+1.2}$ km/s. Assuming Car II is in dynamical equilibrium, we derive a total mass within the half-light radius of $1.0_{-0.4}^{+0.8}\times 10^6{M}_{\odot }$, indicating a mass-to-light ratio of $369_{-161}^{+309}{M}_{\odot }/L_{\odot }$. From equivalent width measurements of the calcium triplet lines of 9 RGB stars, we derive a mean metallicity of [Fe/H] = $-2.44\pm 0.09$ with dispersion $\ddot{I}{ƒ}_{[Fe/H]}=0.22_{-0.07}^{+0.10}$. Considering both the kinematic and chemical properties, we conclude that Car II is a dark-matter-dominated dwarf galaxy. For Car III, we identify 4 member stars, from which we calculate a systemic velocity of $v_{\mathrm{hel}}=284.6_{-3.1}^{+3.4}$ km/s. The brightest RGB member of Car III has a metallicity of [Fe/H] $=-1.97\pm 0.12$. Due to the small size of the Car III spectroscopic sample, we cannot conclusively determine its nature. Although these two systems have the smallest known physical separation ($\hat{I}”d\sim 10kpc$) among Local Group satellites, the large difference in their systemic velocities, $\sim 200$ km/s, indicates that they are unlikely to be a bound pair. One or both systems are likely associated with the Large Magellanic Cloud (LMC), and may remain LMC satellites today. No statistically significant excess of ${\hat{I}}^3$-rays emission is found at the locations of Car II and Car III in eight years of Fermi-LAT data.

20 pages, 9 figures. Submitted to ApJ