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Leo I - The youngest Milky Way dwarf spheroidal galaxy?

1993; Institute of Physics; Volume: 106; Linguagem: Inglês

10.1086/116736

1538-3881

Myung Gyoon Lee, Wendy F. Freedman, Mario Mateo, I. B. Thompson, Miguel Roth, M. T. Ruíz,

Astronomy and Astrophysical Research

We have obtained deep CCD photometry of ~16000 stars in the Leo I dwarf spheroidal galaxy (dSph), the most distant known satellite of the Milky Way Galaxy. The resulting color-magnitude diagrams (CMDs) of Leo I reveal several striking features. (1) There is a well-defined red giant branch (RGB) which is very blue and has a flatter slope than all other dwarf dSphs and Galactic globular clusters. The tip of the RGB is at I_TRGB_ = 18.25+/-0.10 mag and (V - I)_TRGB_ = 1.43+/-0.02 mag. (2) Unlike all of the other known dSphs; Leo I has no obvious horizontal branch; however, it does show a strong clump of red giants at V ~ 22.4 mag and (V - I) ~ 0.8 mag. (3) The CMD of Leo I shows ~50 anomalous Cepheid candidates. (4) There are ~50 asymptotic giant branch stars above the tip of the RGB, including 15 known carbon stars. (5) A large number of stars is seen at V ~> 23 mag (M_V_ ~> +1 mag) and 0 < (V - I) < 1 mag. The present data do not go sufficiently deep to determine conclusively the nature of these faint stars. However, despite increasing crowding effects, the number density of these stars increases from V = 23 mag to V = 24 mag, consistent with the presence of a young population of ~3 Gyr. From the I magnitude of the tip of the RGB we estimate the distance moduli of Leo I to be (m-M)_0_ = 22.18+/-0.11 mag, corresponding to a distance of 270+/-10 kpc for an adopted I-band extinction of 0.04 mag. We estimate the mean color of the RGB at M_I_ = -35 mag to be (V - I)_-35_ = 1.29+/-0.02 mag with an intrinsic dispersion of +/-0.07 mag. These results suggest that the mean metallicity of the RGB stars is [Fe/H]= -2.0+/-0.1 dex with a dispersion of ~0.3 dex. Applying timing arguments to the pair of our Galaxy and Leo I, we obtain an estimate for the mass of our Galaxy of M = 1.7 x 10^12^M_sun_ for an assumed age of the Universe of t = 14 Gyr.