The mass-to-light ratios of the draco and ursa minor dwarf spheroidal galaxies. I. Radial velocities from multifiber spectroscopy

Taft E. Armandroff, Edward W. Olszewski, Carlton Pryor

Research output: Contribution to journalArticle

92 Scopus citations

Abstract

We have measured 206 radial velocities for 94 probable members in the Ursa Minor dwarf spheroidal galaxy and 167 velocities for 91 probable members in the Draco dwarf spheroidal galaxy. These velocities were obtained using the KPNO 4 m telescope with the Hydra multifiber positioner and bench spectrograph in 1992, 1993, and 1994. Both simulations and repeated measurements show that the median uncertainty in these velocities is 4.0 km/s. Our velocities are compared with the other major datasets for Draco and Ursa Minor. Aside from a small zero-point difference, there is excellent agreement between the Hydra velocities and the Olszewski et al. MMT echelle velocities. Comparison with the Hargreaves et al. Ursa Minor velocities reveals inconsistencies which we ascribe to underestimated errors in the Hargreaves et al. data. After correcting for zero-point differences, we combine the three major sets of velocities. Our spectra recovered all of the known Carbon (C) stars in Draco and Ursa Minor and revealed one new C star in Draco and two in Ursa Minor. In addition, one star whose velocity is consistent with membership in Ursa Minor has a deep and wide Hβ absorption line in its spectrum. An initial discussion of the kinematics of the two galaxies is presented. The systemic velocities of Draco and Ursa Minor are -293.3±1.0 and -247.4±1.0 km s-1, respectively. Ursa Minor shows apparent rotation about a position angle of 75° with an amplitude of about 3 km s-1; the morphological major axis is 530°±5°. No such rotation is found in Draco. We find a velocity dispersion of 10.4±0.9 km s-1 for Ursa Minor including all stars and 8.8±0.8 km s-1 excluding the star with the most extreme velocity. In Draco we find 10.7±0.9 km s-1 including all stars and 8.5±0.7 km s-1 excluding the three stars with the most extreme velocities. These dispersions are consistent with the dispersions measured in the other two studies and confirm that Draco and Ursa Minor have the largest mass-to-light ratios of any of the dwarf spheroidals.

Original languageEnglish (US)
Pages (from-to)2131-2165
Number of pages35
JournalAstronomical Journal
Volume110
Issue number5
DOIs
StatePublished - Nov 1995

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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