Imaging of the Stellar Population of IC 10 with Laser Guide Star Adaptive Optics and the Hubble Space Telescope

Abstract

We present adaptive optics (AO) images of the central starburst region of the dwarf irregular galaxy IC 10. The Keck II telescope laser guide star was used to achieve near diffraction-limited performance at H and K^' (Strehl ratios of 18% and 32%, respectively). The images are centered on the putative Wolf-Rayet (W-R) object [MAC92]24. We combine our AO images with F814W data from HST. By comparing the K^' versus [F814W]-K^' color-magnitude diagram (CMD) with theoretical isochrones, we find that the stellar population is best represented by at least two bursts of star formation, one ~10 Myr ago and one much older (150-500 Myr). Young, blue stars are concentrated in the vicinity of [MAC92]24. This population represents an OB association with a half-light radius of about 3 pc. We resolve the W-R object [MAC92] 24 into at least six blue stars. Four of these components have near-IR colors and luminosities that make them robust WN star candidates. By matching the location of C stars in the CMD with those in the SMC we derive a distance modulus for IC 10 of about 24.5 mag and a foreground reddening of E(B-V)=0.95 mag. We find a more precise distance by locating the tip of the giant branch in the F814W, H, and K^' luminosity functions. We find a weighted mean distance modulus of 24.48+/-0.08. The systematic error in this measurement, due to a possible difference in the properties of the RGB populations in IC 10 and the SMC, is +/-0.16 mag.