Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published June 10, 2013 | Submitted + Published
Journal Article Open

Segue 2: The Least Massive Galaxy


Segue 2, discovered by Belokurov et al., is a galaxy with a luminosity of only 900 L_☉. We present Keck/DEIMOS spectroscopy of 25 members of Segue 2—a threefold increase in spectroscopic sample size. The velocity dispersion is too small to be measured with our data. The upper limit with 90% (95%) confidence is σ_v < 2.2 (2.6) km s^(–1), the most stringent limit for any galaxy. The corresponding limit on the mass within the three-dimensional half-light radius (46 pc) is M_(1/2) < 1.5 (2.1) × 10^5 M_☉. Segue 2 is the least massive galaxy known. We identify Segue 2 as a galaxy rather than a star cluster based on the wide dispersion in [Fe/H] (from –2.85 to –1.33) among the member stars. The stars' [α/Fe] ratios decline with increasing [Fe/H], indicating that Segue 2 retained Type Ia supernova ejecta despite its presently small mass and that star formation lasted for at least 100 Myr. The mean metallicity, 〈[Fe/H]〉 = -2.22 ± 0.13 (about the same as the Ursa Minor galaxy, 330 times more luminous than Segue 2), is higher than expected from the luminosity-metallicity relation defined by more luminous dwarf galaxy satellites of the Milky Way. Segue 2 may be the barest remnant of a tidally stripped, Ursa Minor-sized galaxy. If so, it is the best example of an ultra-faint dwarf galaxy that came to be ultra-faint through tidal stripping. Alternatively, Segue 2 could have been born in a very low mass dark matter subhalo (v_(max) < 10 km s^(–1)), below the atomic hydrogen cooling limit.

Additional Information

© 2013 American Astronomical Society. Received 2013 March 11; accepted 2013 April 21; published 2013 May 20. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. We are grateful to the many people who have worked to make the Keck telescope and its instruments a reality and to operate and maintain the Keck Observatory. The authors wish to extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. Without their generous hospitality, none of the observations presented herein would have been possible. We thank the anonymous referee for a courteous report on our article and Josh Simon for a helpful discussion. We are also grateful to Shunsaku Horiuchi for confirming that Segue 2 shows no signal in sky maps from the Fermi Gamma-ray Space Telescope. E.N.K. and M.B.K. acknowledge support from the Southern California Center for Galaxy Evolution, a multicampus research program funded by the University of California Office of Research, and partial support from NSF grant AST-1009973. J.G.C. thanks NSF grant AST-0908139 for partial support. Facility: Keck:II (DEIMOS)

Attached Files

Published - 0004-637X_770_1_16.pdf

Submitted - 1304.6080v3.pdf


Files (1.8 MB)
Name Size Download all
755.1 kB Preview Download
1.1 MB Preview Download

Additional details

August 22, 2023
October 24, 2023