Stellar Populations at the Center of IC1613*

Andrew A. Cole; Eline Tolstoy; John S. Gallagher III; John G. Hoessel; Jeremy R. Mould; Jon A. Holtzman; Abhijit Saha; Gilda E. Ballester; Christopher J. Burrows; John T. Clarke; David Crisp; Richard E. Griffiths; Carl J. Grillmair; Jeff J. Hester; John E. Krist; Vikki Meadows; Paul A. Scowen; Karl R. Stapelfeldt; John T. Trauger; Alan M. Watson; James R. Westphal

doi:10.1086/301042

Stellar Populations at the Center of IC 1613^*

Andrew A. Cole¹, Eline Tolstoy², John S. Gallagher III¹, John G. Hoessel¹, Jeremy R. Mould³, Jon A. Holtzman⁴, Abhijit Saha⁵, Gilda E. Ballester⁶, Christopher J. Burrows^7,8, John T. Clarke⁶, David Crisp⁹, Richard E. Griffiths¹⁰, Carl J. Grillmair¹¹, Jeff J. Hester¹², John E. Krist⁷, Vikki Meadows⁹, Paul A. Scowen¹², Karl R. Stapelfeldt⁹, John T. Trauger⁹, Alan M. Watson¹³, and James R. Westphal¹⁴

© 1999. The American Astronomical Society. All rights reserved. Printed in U.S.A.
The Astronomical Journal, Volume 118, Number 4 Citation Andrew A. Cole et al 1999 AJ 118 1657 DOI 10.1086/301042

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Author affiliations

¹ Department of Physics and Astronomy, University of Massachusetts, Amherst, Massachusetts, 01003

² European Southern Observatory, Karl-Schwarzschild-Strasse, Garching bei München, Germany

³ Mt. Stromlo and Siding Spring Observatories, Australian National University, Private Bag, Weston Creek Post Office, ACT 2611, Australia

⁴ Dept. of Astronomy, New Mexico State University, Box 30001 Department 4500, Las Cruces, NM 88003-8001

⁵ National Optical Astronomy Observatories, 950 N. Cherry Ave., Tucson, AZ 85726

⁶ Dept. of Atmospheric and Oceanic Sciences, University of Michigan, 2455 Hayward, Ann Arbor, MI 48109

⁷ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218

⁸ Astrophysics Division, Space Science Department, European Space Agency

⁹ Jet Propulsion Laboratory, 4800 Oak Grove Dr., Mail Stop 183-900, Pasadena, CA 91109

¹⁰ Carnegie-Mellon University, Department of Physics, 5000 Forbes Ave., Pittsburgh, PA 15213

¹¹ SIRTF Science Center, California Institute of Technology, 770 S. Wilson, Pasadena, CA 91125

¹² Dept. of Physics and Astronomy, Arizona State University, Tyler Mall, Tempe, AZ 85287

¹³ Instituto de Astronomía UNAM, 58090 Morelia, Michoacan, Mexico

¹⁴ Division of Geological and Planetary Sciences, MS 170-25, California Institute of Technology, Pasadena, CA 91125

Dates

Received 1999 March 29
Accepted 1999 May 26

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Abstract

We have observed the center of the Local Group dwarf irregular galaxy IC 1613 with the WFPC2 aboard the Hubble Space Telescope in the F439W, F555W, and F814W filters. We analyze the resulting color-magnitude diagrams (CMDs) using the main-sequence and giant-branch luminosity functions and comparisons with theoretical stellar models to derive a preliminary star formation history for this galaxy. We find a dominant old stellar population (age ≈7 Gyr), identifiable by the strong red giant branch (RGB) and red clump populations. From the (V-I) color of the RGB, we estimate a mean metallicity of the intermediate-age stellar population [Fe/H] = -1.38 ± 0.31. We confirm a distance of 715 ± 40 kpc using the I magnitude of the RGB tip. The main-sequence luminosity function down to I ≈ 25 provides evidence for a roughly constant star formation rate (SFR) of approximately 3.5 × 10^-4 M_⊙ yr^-1 across the WFPC2 field of view (0.22 kpc²) during the past 250–350 Myr. Structure in the blue loop luminosity function implies that the SFR was ≈50% higher 400–900 Myr ago than today. The mean heavy-element abundance of these young stars is around one-tenth solar. The best explanation for a red spur on the main sequence at I ≈ 24.7 is the blue horizontal branch component of a very old stellar population at the center of IC 1613. We have also imaged a broader area of IC 1613 using the 3.5 m WIYN telescope under excellent seeing conditions. The WIYN CMD reveals a prominent sequence of asymptotic giant branch stars and red supergiants that is less prominent in the WFPC2 CMD because of the smaller field of view. The asymptotic giant branch star luminosity function is consistent with a period of continuous star formation over at least the age range 2–10 Gyr. We present an approximate age-metallicity relation for IC 1613, which appears to be similar to that of the Small Magellanic Cloud. We compare the Hess diagram of IC 1613 with similar data for three other Local Group dwarf galaxies and find that IC 1613 most closely resembles the nearby, transition-type dwarf galaxy Pegasus (DDO 216).

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Footnotes

*
Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555, and with the WIYN telescope at the Kitt Peak National Observatory.

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Stellar Populations at the Center of IC 1613*