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

We have obtained V- and I-band photometry for about 17,500 stars in the held of the dwarf irregular galaxy NGC 3109, located in the outskirts of the Local Group. The photometry allows us to study the stellar populations present 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 galaxy 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 consisting of Population II stars, extending out to about 4".5 (or 1.8 kpc) above and below the plane of this galaxy. For these old stars we derive an age of over 10(10) yr and a metallicity of [Fe/H] = -1.8 +/- 0.2. We construct a deep 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 branch tip and adopting E(V - I) = 0.05.