The stellar populations of NGC 3109: Another dwarf irregular galaxy with aPopulation II stellar halo

We have obtained V- and I-band photometry for about 17,500 stars in the hel d of the dwarf irregular galaxy NGC 3109, located in the outskirts of the L ocal Group. The photometry allows us to study the stellar populations prese nt inside and outside the disk of this galaxy. From the VI color-magnitude diagram, we infer metallicities and ages for the stellar populations in the main body and in the halo of NGC 3109. The stars in the disk of this galax y have a wide variety of ages, including very young stars of age similar to 10(7) yr. Our main result is to establish the presence of a halo consistin g of Population II stars, extending out to about 4".5 (or 1.8 kpc) above an d below the plane of this galaxy. For these old stars we derive an age of o ver 10(10) yr and a metallicity of [Fe/H] = -1.8 +/- 0.2. We construct a de ep luminosity function, obtaining an accurate distance modulus (m - M)(0) = 25.62 +/- 0.1 for this galaxy based on the I-magnitude of the red giant br anch tip and adopting E(V - I) = 0.05.