http://www.noao.edu/noao/staff/mighell/umi/
WFPC2 OBSERVATIONS OF THE URSA MINOR DWARF SPHEROIDAL GALAXY
Mighell, K. J., & Burke, C. J.
1999, AJ, 118, 366-380
We present our analysis of archival Hubble Space Telescope Wide Field Planetary
Camera 2 (WFPC2) observations in F555W (~V) and F814W (~I) of the central
region of the Ursa Minor dwarf spheroidal galaxy. The V versus V-I
color-magnitude diagram features a sparsely populated blue horizontal branch, a
steep thin red giant branch, and a narrow subgiant branch. The main sequence
reaches ~2 magnitudes below the main-sequence turnoff (V_TO ~ 23.27 +- 0.11
mag) of the median stellar population. We compare the fiducial sequence of Ursa
Minor with the fiducial sequence of the Galactic globular cluster M92
(NGC 6341). The excellent match between Ursa Minor and M92 confirms that the
median stellar population of the UMi dSph galaxy is metal poor ([Fe/H]_UMi ~
[Fe/H]_M92 ~ -2.2 dex) and ancient (age_UMi ~ age_M92 ~ 14 Gyr). The B-V
reddening and the absorption in V are estimated to be E(B-V) = 0.03 +- 0.01 mag
and A_V = 0.09 +- 0.03 mag. A new estimate of the distance modulus of Ursa
Minor, (m-M)_0 = 19.18 +- 0.12 mag, has been derived based on
fiducial-sequence fitting with M92 and the adoption of the apparent V distance
modulus for M92 of (m-M)_V = 14.67 +- 0.08 mag (Pont et al. 1998, A&A, 329,
87). These HST observations indicate that Ursa Minor has had a very simple star
formation history consisting mainly of a single major burst of star formation
about 14 Gyr ago which lasted <=2 Gyr. If the ancient Galactic globular
clusters, like M92, formed concurrently with the early formation of
the Milky Way galaxy itself, then the Ursa Minor dwarf spheroidal is probably as
old as the Milky Way.
Kenneth Mighell
Associate Scientist
Kitt Peak National Observatory
National Optical Astronomy Observatories
EMAIL: mighell@noao.edu
MAIL: P.O. Box 26732, Tucson, AZ 85726-6732
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FAX: (520) 318-8360
URL: http://www.noao.edu/noao/staff/mighell/