http://arxiv.org/abs/1604.06239
We present the star formation history of the extremely metal-poor dwarf galaxy DDO~68, based on our $V-$ and $I-$ band photometry with the Advanced Camera for Surveys on board of the Hubble Space Telescope. With a metallicity of only $12+\log(O/H)=7.15$ and an isolated location in the periphery of the nearby Lynx-Cancer void, DDO~68 is one of the most metal poor galaxies known. It has been argued in the past that DDO~68 is a young system that started forming stars only $\sim 0.15$~Gyr ago. Our data provide a deep and uncontaminated optical color-magnitude diagram that now allows us to disprove this hypothesis, since we find a population of at least $\sim 1$~Gyr old stars. The star formation activity has been fairly continuous over all the look-back time. The current rate is quite low, and the highest activity occurred between 10 and 100 Myr ago. The average star formation rate over the whole Hubble time is \mbox{$\simeq 0.01$ M$_{\odot}$ yr$^{-1}$}, providing a total mass of formed stars of \mbox{$\simeq 1.3 \times 10^8$ M$_{\odot}$}. Our photometry allows us to infer the distance to this galaxy: based on the tip of the red giant branch we estimate $D = 12.08 \pm 0.67$~Mpc, or $(m-M)_0 = 30.41 \pm 0.12$~mag, while to let our synthetic color-magnitude diagram reproduce the observed ones we need a slightly higher distance, $D=12.65$~Mpc, or $(m-M)_0 = 30.51$, still inside the errors of the previous determination. DDO~68 shows a very interesting and complex history, with its quite disturbed shape and a long tail probably due to tidal interactions.
E. Sacchi, F. Annibali, M. Cignoni, et. al.
Fri, 22 Apr 16
26/54
Comments: 17 pages, 28 figures, submitted to ApJ
You must be logged in to post a comment.