The episodic star formation history of the Carina dwarf spheroidal galaxy

Abstract

We present deep photometry of the Carina dwarf spheroidal galaxy in the B and V filters from CTIO/MOSAIC out to and beyond the tidal radius of r ell ≈ 0.48 degrees. The accurately calibrated photometry is combined with spectroscopic metallicity distributions of red giant branch (RGB) stars to determine the detailed star formation and chemical evolution history of Carina. The star formation history (SFH) confirms the episodic formation history of Carina and quantifies the duration and strength of each episode in great detail as a function of radius from the centre. Two main episodes of star formation occurred at old (>8 Gyr) and intermediate (2−8 Gyr) ages, both enriching stars starting from low metallicities ([Fe/H] < −2 dex). By dividing the SFH into two components, we determine that 60 ± 9 percent of the total number of stars formed within the intermediate-age episode. Furthermore, within the tidal radius (0.48 degrees or 888 pc) a total mass in stars of 1.07 ± 0.08 × 10 6 M was formed, giving Carina a stellar mass-to-light ratio of 1.8 ± 0.8. By combining the detailed SFH with spectroscopic observations of RGB stars, we determined the detailed age-metallicity relation of each episode and the timescale of α-element evolution of Carina from individual stars. The oldest episode displays a tight age-metallicity relation during ≈6 Gyr with steadily declining α-element abundances and a possible α-element "knee" visible at [Fe/H] ≈ −2.5 dex. The intermediate-age sequence displays a more complex age-metallicity relation starting from low metallicity and a sequence in α-element abundances with a slope much steeper than observed in the old episode, starting from [Fe/H] = −1.8 dex and [Mg/Fe] ≈ 0.4 dex and declining to Mg-poor values ([Mg/Fe] ≤ −0.5 dex). This clearly indicates that the two episodes of star formation formed from gas with different abundance patterns, which is inconsistent with simple evolution in an isolated system.

The Carina Project

“…Indeed, the stars range from being α-poor ([Mg/Fe] < −0.3) to α-enhanced ([Mg/Fe] ∼ 0.3). These findings have been independently supported by the detailed star formation history performed by de Boer et al (2014). They found evidence of different age-metallicity relations and different trends in the α-element distributions between old-and intermediate-age subpopulations.…”

“…Finally, we note that the age-metallicity pairs found for individual Carina stars by de Boer et al (2014) and by A&A 580, A18 (2015) Fig. 2.…”

“…They found evidence of different age-metallicity relations and different trends in the α-element distributions between old-and intermediate-age subpopulations. More recently, VandenBerg et al (2015) used the star formation history provided by de Boer et al (2014) and found that specific sets of cluster isochrones, covering a broad range in iron and in α-element abundances, take account of old horizontal branch (HB) stars and red clump (RC) stars in Carina. It is worth mentioning that these analyses are typically based on stellar ages estimated by comparing the position of the stars in color-magnitude diagrams (CMDs) with specific stellar isochrones.…”

A Hubble Space Telescope Study of the Enigmatic Milky Way Halo Globular Cluster Crater*

“…Both have had continuous star formation throughout their lifetimes (e.g., Weisz et al 2014a)and have present-day stellar metallicities similar to Crater. However, the mean metallicity of Carina, as presented in de Boer et al (2014), does not reach Crater's value until ∼5 Gyr ago, indicating that it is not an exact match. Of course, such quantities highly depend on the SFH of a particular system, and this type of mismatch may suggest that Crater's progenitor happened to enrich slightly more quickly than Carina.…”

Estimating the evolution of gas in the Fornax dwarf spheroidal galaxy from its star formation history: an illustrative example

“…Our estimate of the reionization time can be improved significantly by using detailed CDM simulations to estimate halo evolution instead of the model of Zhao et al (2009), which only gives the median halo growth history. We note that data on SFH are available for other dwarf galaxies of the Local Group (e.g., de Boer et al 2012ade Boer et al , 2014Weisz et al 2014). Combining CDM simulations with our approach to study these systems may provide an interesting probe of reionization.…”