Infrared photometry of the inner regions of the dwarf irregular galaxy NGC 3109

We present VIJHK photometry for stars in a 0.8×2.0 kpc₂ inner field of the dwarf irregular galaxy NGC 3109. The infrared photometry reaches 3 magnitudes fainter than previous work. Optical and infrared color-color and color-magnitude diagrams, and a bolometric luminosity function are constructed, confirming the presence of an intermediate-age stellar population. We also present a logL/L_⊙ vs logT_{ef f} diagram of the galaxy and compare the results with the theoretical isochrones of Bertelli et al. (1994) for Z = 0.004 and Y = 0.24. The comparison suggests that most of the stars detected in the IR are 10⁹ yr or younger. We compare the infrared color-magnitude diagrams of NGC 3109 with the composite color-magnitude diagrams of LMC clusters and of the outer disk of the Scd galaxy M33. They show similar stellar content in these late-type systems.