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Astron. Astrophys. 352, 363-370 (1999)

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1. Introduction

The Sagittarius dwarf irregular galaxy (SagDIG) is a blue, low surface brightness galaxy which was found on the ESO and SERC survey plates by Cesarsky et al. (1977) and Longmore et al. (1978). Both teams detected the galaxy in the 21-cm line with a negative radial velocity which indicates its probable membership in the Local Group. Its brightest blue stars are asymmetrically distributed, being concentrated on the eastern side of the galaxy. Based on their apparent magnitude, [FORMULA], Cesarsky et al. (1977) estimated the galaxy distance modulus to be [FORMULA]. Skillman et al. (1989) estimated the metallicity of its ionized gas to be 3% of the solar value and Strobel et al. (1991) give an [FORMULA] map of the galaxy. Later on, Lo et al. (1993) and Young & Lo (1997) undertook detailed investigations of SagDIG in the H I line with the VLA and showed that its velocity field is dominated by chaotic motions rather than by rotation ([FORMULA] km/s). For the H I mass and the total (virial) mass of SagDIG Lo et al. (1993) derived [FORMULA] and [FORMULA] respectively.

Surprisingly, SagDIG remains the least optically studied dIrr galaxy among the Local Group members (see, for example, bibliography in Gallart et al. 1996a and Mateo 1998). Even the existing estimates of its integrated magnitude are spread over a range of 15.5-13.8. The scarcity of studies devoted to SagDIG encouraged us to undertake the detailed CCD imaging of the galaxy that we present here. The paper is organized as follows. Sect. 2 gives a short description of the observations. In Sect. 3 the colour-magnitude diagram (CMD) is described and the distance and metallicity of the galaxy are derived from the tip of the red giant branch (TRGB) and the colour of the red giant branch (RGB), respectively. In Sect. 4 the radial distribution of stars is studied and the integrated magnitude obtained. The star formation history is derived in Sect. 5. Sect. 6 presents a summary of the global properties of SagDIG. Finally, the conclusions of the paper are summarized in Sect. 7.

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© European Southern Observatory (ESO) 1999

Online publication: December 2, 1999