HST Observations of Leo I

doi:10.1086/306967

Hubble Space Telescope Observations of the Local Group Dwarf Galaxy Leo I

Carme Gallart et al 1999 ApJ 514 665-674 doi: 10.1086/306967

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Carme Gallart¹, Wendy L. Freedman¹, Mario Mateo², Cesare Chiosi³, Ian B. Thompson¹, Antonio Aparicio⁴, Gianpaolo Bertelli^3,5, Paul W. Hodge⁶, Myung G. Lee⁷, Edward W. Olszewski⁸, Abhijit Saha⁹, Peter B. Stetson¹⁰ and Nicholas B. Suntzeff¹¹
¹ Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101
² Department of Astronomy, 821 Dennison Building, University of Michigan, Ann Arbor, MI 48109
³ Dipartimento di Astronomia, dell'Università di Padova, Vicolo dell'Osservatorio 5, I-35122-Padova, Italy
⁴ Instituto de Astrofísica de Canarias, E-38200 La Laguna, Canary Islands, Spain
⁵ National Council of Research, CNR-GNA, Rome, Italy
⁶ Department of Astronomy, University of Washington, Seattle, WA 98195
⁷ Department of Astronomy, Seoul National University, Korea
⁸ Steward Observatory, The University of Arizona, Tucson, AZ 85721
⁹ National Optical Astronomy Observatory, P.O. Box 26732, Tucson, AZ 85726
¹⁰ National Research Council, Hertzberg Institute of Astrophysics, Dominion Astrophysical Observatory, Canada
¹¹ Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, Casilla 603, La Serena, Chile

ABSTRACT. We present deep HST F555W (V) and F814W (I) observations of a central field in the Local Group dwarf spheroidal (dSph) galaxy Leo I. The resulting color-magnitude diagram (CMD) reaches I simeq 26 and reveals the oldest simeq 10-15 Gyr old turnoffs. Nevertheless, a horizontal branch is not obvious in the CMD. Given the low metallicity of the galaxy, this likely indicates that the first substantial star formation in the galaxy may have been somehow delayed in Leo I in comparison with the other dSph satellites of the Milky Way. The subgiant region is well and uniformly populated from the oldest turnoffs up to the 1 Gyr old turnoff, indicating that star formation has proceeded in a continuous way, with possible variations in intensity but no big gaps between successive bursts, over the galaxy's lifetime. The structure of the red clump of core He-burning stars is consistent with the large amount of intermediate-age population inferred from the main sequence and the subgiant region. In spite of the lack of gas in Leo I, the CMD clearly shows star formation continuing until 1 Gyr ago and possibly until a few hundred Myr ago in the central part of the galaxy.

Subject headings: galaxies: evolution; galaxies: individual (Leo I); galaxies: photometry; galaxies: stellar content

Print publication: Issue 2 (1999 April 1)
Received 1998 March 19, accepted for publication 1998 November 6

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Hubble Space Telescope Observations of the Local Group Dwarf Galaxy Leo I

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