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Wide Field Planetary Camera 2 Observations of Leo A: A Predominantly Young Galaxy Within the Local Group


Tolstoy, E and Gallagher, JS and Cole, AA and Hoessel, JG and Saha, A and Dohm-Palmer, RC and Skillman, ED and Mateo, M and Hurley-Keller, D, Wide Field Planetary Camera 2 Observations of Leo A: A Predominantly Young Galaxy Within the Local Group, The Astronomical Journal, 116, (3) pp. 1244-1262. ISSN 0004-6256 (1998) [Refereed Article]

DOI: doi:10.1086/300515


The unprecedented detail of the Wide Field Planetary Camera 2 (WFPC2) color-magnitude diagrams (CMDs) of the resolved stellar population of Leo A presented here allows us to determine a new distance and an accurate star formation history for this extremely metal-poor Local Group dwarf irregular galaxy. From the position of the red clump, the helium-burning blue loops, and the tip of the red giant branch, we obtain a distance modulus, m - M = 24.2 ± 0.2, or 690 ± 60 kpc, which places Leo A firmly within the Local Group. Our interpretation of these features in the WFPC2 CMDs at this new distance based upon extremely low-metallicity (Z = 0.0004) theoretical stellar evolution models suggests that this galaxy is predominantly young, i.e., less than 2 Gyr old. A major episode of star formation 900-1500 Gyr ago can explain the red clump luminosity and also fits in with our interpretation of the number of anomalous Cepheid variable stars seen in this galaxy. We cannot rule out the presence of an older, underlying globular cluster age stellar population with these data. However, using the currently available stellar evolution models, it would appear that such an older population is limited to no more than 10% of the total star formation to have occurred in this galaxy. Leo A provides a nearby laboratory for studying young metal-poor stars and investigations of metal-poor galaxy evolution, such as is supposed to occur for larger systems at intermediate and high redshifts.

Item Details

Item Type:Refereed Article
Research Division:Physical Sciences
Research Group:Astronomical sciences
Research Field:Galactic astronomy
Objective Division:Expanding Knowledge
Objective Group:Expanding knowledge
Objective Field:Expanding knowledge in the physical sciences
UTAS Author:Cole, AA (Associate Professor Andrew Cole)
ID Code:45300
Year Published:1998
Web of Science® Times Cited:123
Deposited By:Physics
Deposited On:2007-07-10
Last Modified:2007-07-10

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