We have obtained V and I-band photometry for about 17500 stars in the field 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 with \sim 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 arcmin ( or 1.8 kpc ) above and below the plane of this galaxy .
For these old stars we derive an age of > 10 ^ { 10 } yr and a metallicity of [ Fe / H ] = -1.8 \pm 0.2 .
We construct a deep luminosity function , obtaining an accurate distance modulus ( m - M ) _ { 0 } = 25.62 \pm 0.1 for this galaxy based on the I-magnitude of the red giant branch ( RGB ) tip and adopting E ( V - I ) = 0.05 .