Spectroscopic confirmation of the dwarf spheroidal galaxy d0944+71 as a member of the M81 group of galaxies

TL;DR

This study confirms that the dwarf spheroidal galaxy d0944+71 is a member of the M81 group by measuring its velocity and metallicity using Keck/DEIMOS spectroscopy, marking the first such measurement beyond the Local Group.

Contribution

First spectroscopic measurement of velocity and metallicity of a dwarf spheroidal galaxy beyond the Local Group, confirming its association with the M81 group.

Findings

d0944+71 has a heliocentric velocity consistent with M81

Metallicity of d0944+71 is [Fe/H] = -1.3 ± 0.3

Galaxy shows no signs of strong tidal stripping

Abstract

We use Keck/DEIMOS spectroscopy to measure the first velocity and metallicity of a dwarf spheroidal (dSph) galaxy beyond the Local Group using resolved stars. Our target, d0944+71, is a faint dSph found in the halo of the massive spiral galaxy M81 by Chiboucas et al. We coadd the spectra of 27 individual stars and measure a heliocentric radial velocity of $-38\pm10$~km/s. This velocity is consistent with d0944+71 being gravitationally bound to M81. We coadd the spectra of the 23 stars that are consistent with being red giant branch stars and measure an overall metallicity of ${\rm [Fe/H]}=-1.3 \pm 0.3$ based on the calcium triplet lines. This metallicity is consistent with d0944+71 following the metallicity$-$luminosity relation for Local Group dSphs. We investigate several potential sources of observational bias but find that our sample of targeted stars is representative of the metallicity distribution function of d0944+71 and any stellar contamination due to seeing effects is negligible. The low ellipticity of the galaxy and its position in the metallicity$-$luminosity relation suggest that d0944+71 has not been affected by strong tidal stripping.