The stellar populations of the Carina dwarf spheroidal Galaxy. 1: A new color-magnitude diagram for the giant and horizontal branches

Abstract

We report on the first in a series of studies of the Carina dwarf spheroidal galaxy, a nearby satellite of our Galaxy. Our two major results are: 1) precise BI photometry (σ B−I < ∼ 0.05 for V < ∼ 22) for 11,489 stars in the Carina field, and 2) the detection of two, morphologically distinct, horizontal branches in Carina, which confirms that star formation occurred in two well-separated episodes. The old horizontal branch and RR Lyrae instability strip belong to a > ∼ 10 Gyr stellar population, while the populous red-clump horizontal branch belongs, presumably, to a ∼ 6 Gyr stellar population. We derive a distance modulus (m − M ) 0 = 20.09 ± 0.06 for Carina from the apparent magnitudes of the old horizontal branch and the tip of the red giant branch (RGB), and discuss modifications to the previously estimated distance, total magnitude, and stellar ages of Carina. Using the color of the RGB, we estimate the metallicities of the younger and older populations to be [Fe/H] ≃ −2.0 and -2.2, respectively.

VLT/FLAMES spectroscopy of red giant branch stars in the Carina dwarf spheroidal galaxy

“…The most simple scenario assumes that the small and unevolved dwarf spheroidal (dSph) galaxies that we see around the Milky Way should be related to the type of objects that originally merged to form the Milky Way. This suggests that the stellar populations at the ages when mergers occurred should be sim-seen in deep CMDs (first shown by Smecker-Hane et al, 1996). This is the unequivocal signature of periods of active star formation separated by similarly long periods of no star formation at all.…”

“…We noticed that the ages of the RGB stars in the Carina dSph (Table 10) fall broadly into two groups, as might be expected from the SFH (Smecker-Hane et al, 1996;Monelli et al, 2003;Rizzi et al, 2003;Bono et al, 2010). Because our ages have large error bars, our aim is simply to split our sample into an old population and an intermediate age population.…”

“…The Carina dSph has a particularly complex SFH, with at least two, and possibly three or four distinct bursts of star formation over its history as shown by multiple MSTOs in its CMD (e.g., Smecker-Hane et al, 1996;Hurley-Keller et al, 1998). The extremely narrow RGB shows the strong presence of an age-metallicity degeneracy.…”

Baryonic Properties of the Darkest Galaxies

“…Another "exceptional" dSph is the Galactic companion Carina, the only dSph known to have experienced clearly episodic star formation interrupted by periods of quiescence (e.g., Smecker-Hane et al 1994;Monelli et al 2003;Koch et al 2006). It is not understood what lead to the pauses and the repeated onset of star formation.…”

Element Abundances In Nearby Galaxies

“…The dEs display a variety of complex multi-episode star formation histories. For example, Carina shows evidence for several distinct star formation events spread over several Gyr (Smecker-Hane et al 1994); Sculptor and Fornax appear morphologically similar, but Sculptor appears to formed the bulk of its stars at an earlier time than Fornax (Tolstoy et al 2001). Only Ursa Minor appears to have something like a simple, monometallic stellar populations based on its color-magnitude diagram (Mighell & Burke 1999, but see below).…”