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Astron. Astrophys. 352, L63-L68 (1999)


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Letter to the Editor

A new investigation on the Antlia Dwarf Galaxy *

A.M. Piersimoni 1, G. Bono 2, M. Castellani 2, G. Marconi 2, S. Cassisi 1, R. Buonanno 2 and M. Nonino 3,4

1 Osservatorio Astronomico di Collurania, via M. Maggini, 64100 Teramo, Italy
2 Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy
3 European Southern Observatory, 85748 Garching bei München, Germany
4 Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 40131 Trieste, Italy

Received 1 September 1999 / Accepted 19 September 1999

Abstract

We present deep [FORMULA] and [FORMULA] color-magnitude diagrams (CMDs) of the Antlia dwarf galaxy, based on Science Verification (SV) data collected with the FORS I camera on the ESO Very Large Telescope (VLT). The CMDs present two key features: a well-defined Red Giant Branch (RGB), and a sample of bright blue stars, belonging to a young stellar component. The comparison between theory and observations confirms that the bright stars are consistent with the occurrence of a star formation episode [FORMULA] Gyr ago. In agreement with Sarajedini et al. (1997) and Aparicio et al. (1997), we find that the young stars are more centrally concentrated than the old ones.

By adopting the new calibration of the Tip of the RGB (TRGB) provided by Salaris & Cassisi (1998) we estimated that the Antlia distance modulus is [FORMULA] mag , and therefore a distance [FORMULA] Mpc . This distance determination is [FORMULA] larger than the values suggested in previous investigations. We estimated that the mean metallicity of Antlia is of the order of [FORMULA]. This estimate is at least 0.3 dex more metal-rich than similar evaluations available in the literature. The disagreement both in the distance and in the metallicity determinations are mainly due to difference in the calibration of the TRGB method and in the [FORMULA] color index vs. metallicity relation. The differential RGB luminosity function shows an excess in the observed counts -at the [FORMULA] level- when compared with theoretical predictions. This discrepancy might be due to a stellar component with an age approximately equal to 0.7 Gyr .

Key words: stars: luminosity function, mass function – galaxies: distances and redshifts – galaxies: individual: Antlia – galaxies: Local Group – galaxies: stellar content

* Based on observations collected with VLT-UT1 telescope of ESO in Paranal, during the Science Verification Program

© European Southern Observatory (ESO) 1999

Online publication: November 23, 1999

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