Crater 2: An Extremely Cold Dark Matter Halo

TL;DR

Crater 2 is an extremely cold, low-density dwarf galaxy with a significant dark matter halo, challenging existing scaling relations among Milky Way dwarf spheroidals.

Contribution

This study provides the first detailed spectroscopic analysis of Crater 2, revealing its unique cold velocity dispersion and extreme properties among dwarf galaxies.

Findings

Crater 2 has the coldest resolved velocity dispersion among dwarf galaxies.

It possesses a high mass-to-light ratio indicating a dominant dark matter halo.

Crater 2 is an outlier in structural and dynamical scaling relations.

Abstract

We present results from MMT/Hectochelle spectroscopy of red giant candidate stars along the line of sight to the recently-discovered Galactic satellite Crater 2. Modelling the joint distribution of stellar positions, velocities and metallicities as a mixture of Crater 2 and Galactic foreground populations, we identify 62 members of Crater 2, for which we resolve line-of-sight velocity dispersion 2.7 +/- 0.3 km/s about mean velocity of 87.5 +/- 0.4 km/s. We also resolve a metallicity dispersion 0.22 about a mean of [Fe/H]=-1.98 +/- 0.1 that is 0.28 +/- 0.14 poorer than is estimated from photometry. Despite Crater 2's relatively large size (projected halflight radius R(h)=1 kpc) and intermediate luminosity (M_V =-8), its velocity dispersion is the coldest that has been resolved for any dwarf galaxy. These properties make Crater 2 the most extreme low-density outlier in dynamical as well as structural scaling relations among the Milky Way's dwarf spheroidals. Even so, under assumptions of dynamical equilibrium and negligible contamination by unresolved binary stars, the observed velocity distribution implies a gravitationally dominant dark matter halo, with dynamical mass 4.4 x 10^6 Msun and mass-to-light ratio M/L=53 enclosed within a radius of 1 kpc, where the equivalent circular velocity is 4.3 km/s.