Nearest Neighbor: The Low-mass Milky Way Satellite Tucana III*

Abstract

We present Magellan/IMACS spectroscopy of the recently discovered Milky Way satellite Tucana III (Tuc III). We identify 26 member stars in Tuc III, from which we measure a mean radial velocity of v hel = −102.3 ± 0.4 (stat.) ± 2.0 (sys.) km s −1 , a velocity dispersion of 0.1 +0.7 −0.1 km s −1 , and a mean metallicity of [Fe/H] = −2.42 +0.07 −0.08 . The upper limit on the velocity dispersion is σ < 1.5 km s −1 at 95.5% confidence, and the corresponding upper limit on the mass within the half-light radius of Tuc III is 9.0 × 10 4 M . We cannot rule out mass-to-light ratios as large as 240 M / L for Tuc III, but much lower mass-to-light ratios that would leave the system baryon-dominated are also allowed. We measure an upper limit on the metallicity spread of the stars in Tuc III of 0.19 dex at 95.5% confidence. Tuc III has a smaller metallicity dispersion and likely a smaller velocity dispersion than any known dwarf galaxy, but a larger size and lower surface brightness than any known globular cluster. Its metallicity is also much lower than those of the clusters with similar luminosity. We therefore tentatively suggest that Tuc III is the tidally-stripped remnant of a dark matter-dominated dwarf galaxy, but additional precise velocity and metallicity measurements will be necessary for a

Can tides disrupt cold dark matter subhaloes?

“…A basic implementation of this method is made available online together with code to generate equilibrium N-body models, see section 3.1. Luminosity and projected half-light radius of the stellar component are chosen so that the evolved model approximately matches the Tucana III dwarf galaxy at redshift z = 0: L = 780 +350 −240 L , R h = (44 ± 6) pc (Drlica-Wagner et al 2015), with an upper limit on the line-of-sight velocity dispersion of σ < 1.5 km s −1 (Simon et al 2017). Using the upper limit on σ, these authors find an upper limit on the mass enclosed within the half-light radius of M(< R h ) < 9 × 10 4 M and an upper limit on the dynamical mass-to-light ratio averaged within R h of M/L = 2M(< R h )/L < 2.4 × 10 2 M L −1 .…”

Stellar Streams Discovered in the Dark Energy Survey

“…Spectroscopic observations have been unable to conclusively classify Tucana III as an ultrafaint galaxy or star cluster (Simon et al 2017 ) are both unusual for a globular cluster. In addition, Simon et al (2017) argue that the mass-to-light ratio of the core of Tucana III is larger than that of a globular cluster,…”

“…The core of Tucana III lies slightly offset from the luminosity-metallicity relationship for ultrafaint galaxies (Kirby et al 2013). Simon et al (2017) note that if Tucana III has been stripped of~70% of its stellar mass, then it would lie directly on the metallicity-luminosity relation of ultrafaint dwarfs. We find that the total stellar mass of the Tucana III stream, including the core and the tidal tails, iś  M 3.8 10 3 , which is 4.75 times the stellar mass of the Tucana III core .…”

“…In addition, Simon et al (2017) argue that the mass-to-light ratio of the core of Tucana III is larger than that of a globular cluster,…”

Chemical Abundance Analysis of Three α-poor, Metal-poor Stars in the Ultrafaint Dwarf Galaxy Horologium I*

“…Spectroscopic velocity measurements also yield a measure of the mass-to-light (M/L) ratio and a determination of whether a satellite is a dark-matter-dominated dwarf galaxy or a baryondominated stellar cluster (see Willman & Strader 2012, for a comprehensive definition). The DES-discovered candidate satellites considered most likely to be nearby ultrafaint dwarf galaxies have been selected for follow-up spectroscopy; six have subsequently been confirmed to be highly dark-matterdominated, low-luminosity satellites: Reticulum II (Ret II; Simon et al 2015a), Tucana II (Tuc II) and Grus I (Walker et al 2016), Tucana III (Tuc III; Simon et al 2017), Eridanus II , and Horologium I (Hor I; , the last being the subject of this paper.…”