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Astron. Astrophys. 361, 491-499 (2000)

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1. Introduction

NGC 3109 (DDO 236), a late-type dwarf spiral galaxy, has been classified as Sm IV (Sandage & Tammann 1981) and is seen almost edge-on with an inclination close to 80o (Carignan 1985). Since this galaxy has no nucleus it has been assigned the morphological type Ir (van den Bergh 1999). It is a southern Magellanic dwarf galaxy well resolved into stars and is one of the largest Magellanic dwarfs close to the Local Group at a distance of 1.36[FORMULA]0.10 Mpc (Musella et al. 1997) with an apparent major-axis diameter of 30´ (12 kpc) and minor-axis diameter of 6´ (Demers et al. 1985).

NGC 3109 has a similar dimension as the LMC. With an absolute magnitude comparable to the SMC of [FORMULA] mag it is rather underluminous. Estimates of the total mass run from 0.6 to [FORMULA]. The galaxy is surrounded by a huge HI envelope, quite larger than its optical size (Materne 1980; Huchtmeier et al. 1980; Jobin & Carignan 1990). Jobin & Carignan (1990) derive an HI mass of [FORMULA]. The rotation curve requires a large dark matter halo.

On deep exposures NGC 3109 exhibits spiral structure. Associations of knots of stars are visible along spiral arms. A globular cluster search revealed ten candidates. Bright blue stars are found evenly distributed over the face of the galaxy but particularly along the spiral arms, indicating that star formation is taking place on a galaxy-wide scale (Demers et al. 1985). The metallicity of NGC 3109 has been found to be low, similar to the SMC (Richer & McCall 1995). Minniti et al. (1999) establish the existence of an extended halo of old and metal-poor stars.

NGC 3109 belongs as most luminous member to a subgroup of the Local Group dwarfs which is relatively isolated (Mateo 1998). Another member of this group is the dwarf spheroidal galaxy Antila at a distance of only 1.o2 from NGC 3109. From this apparent separation on the sky the lower limit to the distance between both galaxies is [FORMULA]26 kpc (Whiting et al. 1997).

In this article we derive the X-ray population in the field of NGC 3109 from ROSAT PSPC observations. We classify a few of the X-ray sources which coincide with the HI extent of NGC 3109 from their X-ray spectral properties. We find two candidate AGN which we use to probe the gas content of NGC 3109.

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© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000