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The ACS LCID Project: Quantifying the Delayed Star Formation in Leo A

Published online by Cambridge University Press:  01 December 2006

Andrew A. Cole
Andrew A. Cole
Affiliation:
Department of Astronomy, University of Minnesota, 116 Church St. SE, Minneapolis, MN, 55455, USA email: cole@astro.umn.edu
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Abstract

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We have obtained deep images of the Local Group dwarf irregular galaxy Leo A using the Hubble Space Telescope Advanced Camera for Surveys. From these images we have constructed the deepest ever color-magnitude diagram (CMD) for an 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 Leo A's star formation has occurred more recently than 8 Gyr ago. Our measurements are not inconsistent with a scenario in which all of the ancient star formation occurred prior to the end of the era of reionization, with subsequent suppression until lookback times corresponding to redshift z ≈ 1. However, given Leo A's strongly isolated state and its dynamical mass of ≳ 108 M⊙, its large gas supply was more likely to have been merely heated than driven out of the galaxy altogether.

Keywords

galaxies: dwarfcolor-magnitude diagramsgalaxies: evolution

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