A spectroscopic confirmation of the bootes II dwarf spheroidal

Andreas Koch*, Mark I. Wilkinson, Jan T. Kleyna, Mike Irwin, Daniel B. Zucker, Vasily Belokurov, Gerard F. Gilmore, Michael Fellhauer, N. Wyn Evans

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

96 Citations (Scopus)


We present a new suite of photometric and spectroscopic data for the faint Bootes II dwarf spheroidal galaxy (dSph) candidate. Our deep photometry, obtained with the Isaac Newton Telescope/Wide Field Camera, suggests a distance of 46 kpc and a small half-light radius of 4.0′ (56 pc), consistent with previous estimates. Follow-up spectroscopy obtained with the Gemini/GMOS instrument yielded radial velocities and metallicities. While the majority of our targets covers a broad range in velocities and metallicities, we find five stars that share very similar velocities and metallicities and that are all compatible with the colors and magnitudes of the galaxy's likely red giant branch. We interpret these as a spectroscopic detection of the Bootes II system. These stars have a mean velocity of -117 km s-1, a velocity dispersion of (10.5 ± 7.4) km s-1, and a mean [Fe/H] of -1.79 dex, with a dispersion of 0.14 dex. At this metallicity, Boo II is not consistent with the stellar-mass-metallicity relation for the more luminous dwarf galaxies. Coupled with our distance estimate, its high negative systemic velocity rules out any physical connection with its projected neighbor, the Bootes I dwarf spheroidal, which has a velocity of +100 km s-1. The velocity and distance of Bootes II coincide with those of the leading arm of Sagittarius, which passes through this region of the sky, so that it is possible that Bootes II may be a stellar system associated with the Sagittarius stream. Finally, we note that the properties of Bootes II are consistent with it being the surviving remnant of a previously larger and more luminous dSph galaxy.

Original languageEnglish
Pages (from-to)453-462
Number of pages10
JournalAstrophysical Journal
Issue number1
Publication statusPublished - 2009
Externally publishedYes


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