The Stellar Initial Mass Function in the Giant HII Region NGC 595

Abstract

As part of a larger effort to study the resolved and composite properties of the giant H II regions in Messier 33, we have analyzed multiband HST/WFPC-2 images of NGC 595 in terms of the ionizing cluster's resolved stellar population. Photometric reductions of the PC images yield 100 stars on the UV image, 272 stars on the U image, 345 stars on the B image, and 561 stars on the V image. A total of 267 stars are common to the U, B, and V images while 86 stars are detected on all 4 images. Although some clustering is evident, the degree of central concentration is less than that seen in 30 Doradus. The resulting U-B vs B-V diagram of the resolved stars is used to determine the reddening of each star. The average reddening derived from this diagram is E_B-V_ = 0.36+/-0.28 mag. The dereddened color-M_V_ diagram is best fit by a model cluster having an age of 4.5 +/- 1.0 Myr, and hence initial masses no greater than 51 M_sun_. A total of 13 supergiant stars and 10 candidate WR stars [Drissen et al. (AJ, 105,1400, 1993), and references therein] are identified with M_V_ = -5 to -8 mag. The remainder are main-sequence O-type (98) and early B-type (>145) stars with M_V_ = 1 to -6 mag. The ratio of WR to O stars is WR/O = 0.11+/- 0.01, roughly the same as found in the core of 30 Doradus. The resulting luminosity function has a slope of α = - 0.71. The derived IMF has a slope of {GAMMA} = -1.32+/-0.02 before subtracting a background component, and {GAMMA}= - 1.00+/-0.05 after subtracting a background based on photometry of the surrounding WF images. Integration of the derived IMF down to a lower mass limit of 4 M_sun_ yields a total mass of 7350 M_sun_, while integration down to 0.1 M_sun_ yields a total mass of 18000 M_sun_. The total estimated ionizing luminosity is 5.0 x 10^50^ photons s^-1^, roughly half that which is inferred from the Hα luminosity in this region. This shortfall of ionizing photons can be reconciled by allowing for a spread in the stellar ages, and/or by increasing the modeled EUV luminosity of the stars at the inferred cluster age.