AHSTStudy of the Stellar Populations in the Cometary Dwarf Irregular Galaxy NGC 2366

We present V and I photometry of the resolved stars in the cometary dwarfirregular galaxy NGC 2366, using Wide Field Planetary Camera 2 images obtainedwith the Hubble Space Telescope. The resulting color-magnitude diagram reachesdown to I~26.0 mag. It reveals not only a young population of bluemain-sequence stars (age <30 Myr) but also an intermediate-age population ofblue and red supergiants (20 Myr100 Myr) and red giant branch(RGB) stars (age >1 Gyr). The measured magnitude I=23.65+/-0.10 mag of the RGBtip results in a distance modulus m-M=27.67+/-0.10, which corresponds to adistance of 3.42+/-0.15 Mpc, in agreement with previous distancedeterminations. The youngest stars are associated with the bright complex ofHII regions NGC 2363=Mrk 71 in the southwest extremity of the galaxy. As aconsequence of the diffusion and relaxation processes of stellar ensembles, theolder the stellar population is, the smoother and more extended is its spatialdistribution. An underlying population of older stars is found throughout thebody of NGC 2366. The most notable feature of this older population is thepresence of numerous relatively bright AGB stars. The number ratio of AGB toRGB stars and the average absolute brightness of AGB stars in NGC 2366 areappreciably higher than in the BCD VII Zw 403, indicating a younger age of theAGB stars in NGC 2366. In addition to the present burst of age <100 Myr, therehas been strong star formation activity in the past of NGC 2366, from ~100 Myrto <3 Gyr ago.Comment: 32 pages, 15 figures, accepted for publication in the Astrophysical Journa