WFPC2 Observations of the Carina Dwarf Spheroidal Galaxy

Abstract

We present our analysis of Hubble Space Telescope Wide Field Planetary Camera 2 observations in F555W (~V) and F814W (~I) of the Carina dwarf spheroidal galaxy. The resulting V vs (V-I) color-magnitude diagrams reach V ~ 27.1 mag. The reddening of Carina is estimated to be E(V-I) = 0.08 +- 0.02 mag. A new estimate of the distance modulus of Carina, (m-M)_0 = 19.87 +- 0.11 mag, has been derived primarily from existing photometry in the literature. The apparent distance moduli in V and I were determined to be (m-M)_V = 20.05 +- 0.11 mag and (m-M)_I = 19.98 +- 0.12 mag, respectively. These determinations assumed that Carina has a metallicity of [Fe/H] = -1.9 +- 0.2 dex. This space-based observation, when combined with previous ground-based observations, is consistent with (but does not necessarily prove) the following star formation scenario. The Carina dwarf spheroidal galaxy formed its old stellar population in a short burst (=< 3 Gyr) at about the same time the Milky Way formed its globular clusters. The dominant burst of intermediate-age star formation then began in the central region of the galaxy where stars formed for several billion years before the process of star formation became efficient enough in the outer regions of the galaxy to allow for the formation of large numbers of stars. There has been negligible star formation during the last few billion years. This observation provides evidence that at least some dwarf galaxies can have complex global star formation histories with local variations of the rate of star formation as a function of time and position within the galaxy.