Leo A: A Late-blooming Survivor of the Epoch of Reionization in the Local Group

Abstract

As part of a major program to use isolated Local Group dwarf galaxies as near-field probes of cosmology, we have obtained deep images of the dwarf irregular galaxy Leo A with the Advanced Camera for Surveys aboard the Hubble Space Telescope. From these images we have constructed a color-magnitude diagram (CMD) reaching apparent [absolute] magnitudes of (M 475 , M 814 ) (29.0 [+4.4], 27.9 [+3.4]), the deepest ever achieved for any irregular galaxy beyond the Magellanic Clouds. We derive the star-formation rate (SFR) as a function of time over the entire history of the galaxy. We find that over 90% of all the star formation that ever occurred in Leo A happened more recently than 8 Gyr ago. The CMD shows only a very small amount of star formation in the first few billion years after the Big Bang; a possible burst at the oldest ages cannot be claimed with high confidence.The peak SFR occurred ≈1.5-4 Gyr ago, at a level 5-10 times the current value. Our modelling indicates that Leo A has experienced very little metallicity evolution; the mean inferred metallicity is consistent with measurements of the present-day gas-phase oxygen abundance. We cannot exclude a scenario in which all of the ancient star formation occurred prior to the end of the era of reionization, but it seems unlikely that the lack of star formation prior to ≈8 Gyr ago was due to early loss or exhaustion of the in situ gas reservoir.

The star formation history of the Sculptor dwarf irregular galaxy

“…Similar patterns of globally decreasing SFR during the past 1−2 Gyr have been observed in a number of LG galaxies with similar luminosity to SDIG, e.g. Leo A (Cole et al 2007).…”

“…In practice, the density of stars within a defined color and magnitude grid along the observed CMD is compared with the density of the synthetic stars drawn from the model CMD (for a review see Tolstoy et al 2009;Aparicio 2002), and the best model is adopted as the SFH. The approach taken here was developed by Cole to study the stellar populations of dIrr galaxies in the LG (see, e.g., Skillman et al 2003b;Cole et al 2007;Cignoni et al 2012). As with any such approach, the power of the method is greatly reduced by lack of photometric depth (e.g., Weisz et al 2011b).…”

A close look at the Centaurus A group of galaxies

“…The choice of when to change the size of the bins in (logarithmic) age is based on the appearance of different age-sensitive features in the CMD (e.g., MS turnoffs and BL stars). As shown in Cole et al (2007) and discussed in Aparicio & Hidalgo (2009), SFH solutions obtained in this way are generally robust to minor variations in the exact binning scheme adopted. Our CMDs are not deep enough to include features like the old MS turnoff, the red clump or the horizontal branch, due to the distance of our targets, and it is thus difficult to accurately constrain the star formation episodes for ages > ∼ 1 Gyr from the evolved stellar populations A131, page 4 of 16 Upper panels: star formation histories derived for the five studied galaxies (HIPASS J1348-37, KK196, KK182, ESO384-016, and ESO269-58, ordered by increasing absolute magnitude).…”

“…We can now look for the synthetic SFH producing a CMD that has the maximum likelihood to match the observed one. This is done by exploring the parameter space via a simulated annealing algorithm (described in Skillman et al 2003;Cole et al 2007). This procedure gives for each galaxy the SFR at each chosen time interval, and a corresponding best-fit metallicity value.…”

“…The first systematic HST study of the resolved stellar populations of its dwarf members was carried out by Karachentsev et al (2002b), followed by more recent, even deeper HST observations (Karachentsev et al 2007). In Crnojević et al (2011a) we derive recent ( < ∼ 1 Gyr) SFHs for five dIrr members of the M 83 subgroup with the synthetic CMD modeling technique (e.g., Dolphin 2002;Cole et al 2007;Cignoni & Tosi 2010).…”