EDP Sciences
Free access
Volume 384, Number 2, March III 2002
Page(s) 393 - 402
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20020047

A&A 384, 393-402 (2002)
DOI: 10.1051/0004-6361:20020047

The Sagittarius dwarf irregular galaxy: Metallicity and stellar populations

Y. Momany1, 2, E. V. Held3, I. Saviane4 and L. Rizzi3, 2

1  European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
2  Dipartimento di Astronomia, Università di Padova, vicolo dell'Osservatorio 2, 35122 Padova, Italy
3  Osservatorio Astronomico di Padova, vicolo dell'Osservatorio 5, 35122 Padova, Italy
    e-mail: held, rizzi@pd.astro.it
4  European Southern Observatory, Casilla 19001, Santiago 19, Chile
    e-mail: isaviane@eso.org

(Received 1 August 2001 / Accepted 2 January 2002 )

We present deep BVI observations of the dwarf irregular galaxy UKS 1927-177 in Sagittarius (SagDIG). Statistically cleaned V, (B-I) color-magnitude diagrams clearly display the key evolutionary features in this galaxy. Previously detected C stars are located in the color-magnitude diagrams and shown to be variable, thus confirming the presence of a significant upper-AGB intermediate age population. A group of likely red supergiants is also identified, whose magnitude and color is consistent with a 30 Myr old burst of star formation. The observed colors of both blue and red stars in SagDIG are best explained by introducing a differential reddening scenario in which internal dust extinction affects the star forming regions. Adopting a low reddening for the red giants, $E(B-V) = 0.07 \pm 0.02$, gives [ Fe/H $]=-2.1 \pm 0.2$ for the mean stellar metallicity, a value consistent with the [O/H] abundance measured in the regions. This revised metallicity, which is in accord with the trend of metallicity against luminosity for dwarf irregular galaxies, is indicative of a "normal", although metal-poor, dIrr galaxy. A quantitative description is given of the spatial distribution of stars in different age intervals, in comparison with the distribution of the neutral hydrogen. We find that the youngest stars are located near the major peaks of emission on the shell, whereas the red giants and intermediate-age C stars define an extended halo or disk with scale length comparable to the size of the hydrogen cloud. The relationship between the distribution of ISM and star formation is briefly discussed.

Key words: galaxies: fundamental parameters -- galaxies: individual: UKS 1927-177 -- galaxies: dwarf -- galaxies: local group -- Galaxy: stellar content

Offprint request: Y. Momany, momany@pd.astro.it

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