CCD photometry of resolved dwarf irregular galaxies. I - Sextans A

Aparicio, A.; Garcia-Pelayo, J. M.; Moles, M.; Melnick, J.
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Astronomy and Astrophysics Supplement Series (ISSN 0365-0138), vol. 71, no. 2, Nov. 1987, p. 297-338. Research supported by the Consejo Superior de Investigaciones Cientificas and Universidad de Chile.

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CCD UBV photometry of 2279 stars in Sextans A is presented. Differences with the photometric scales of Sandage and Carlson (1985) and Hoessel et al. (1983) are discussed; recalibration of the Sandage and Carlson data for Cepheids in Sextans A results in a smaller value for the distance modulus, μ = 25.6. A similar value for the distance modulus is obtained from the colour-colour diagram. Colour-colour and colour-magnitude diagrams for the whole galaxy and, for different parts, are presented indicating a general star formation activity throughout Sextans A. The contribution of stars brighter than Mpg = -3 to the total blue light of the galaxy amounts to 25%, a much higher value than is found for other dwarf irregular galaxies in the Local Group. The integrated birthrate per unit mass of massive stars for the galaxy is found to be much higher than in the Magellanic Clouds, suggesting that the rate in Sextans A was probably smaller in the past. Different slopes for high mass and low mass IMF parts are needed to account for its low surface brightness. The formation of massive stars seems to be favoured for systems of decreasing mass or, given the relation between total mass and metal content, for galaxies of decreasing metallicity.