Statistical studies of HII regions in nearby galaxies

Abstract

Aims. We systematically study the relations between the Hαluminosity and the near-IR luminosity and temperature of a set of selected HII regions in the disc of M100. Methods. By using the technique of cross-reference between the optical Hα image (the catalogue of HII regions in M100) and the Spitzer IRAC images, we locate the selected HII regions in the IRAC image frames. By using the chi-squared non-linear fitting technique, we estimate the IRAC-3-band colour temperature Tcol(IRAC) of selected HII regions, and calculate their near-IR luminosities. Results. The spectral fits allow us to separate the PAH emission in the 3.6 μm band from the thermal dust emission that dominates the other three bands, thereby assigning values for the colour temperature of the dust. These values lie in the range 250 K to 350 K. There is a well defined linear relation between the Hα luminosity and the derived IRAC luminosity for the HII regions, but no apparent relation between the latter and the colour temperature. While the range in Hα luminosity is almost two orders of magnitude, the range in temperature is less than a factor two. We also find that the colour temperature is independent of galactocentric radius, in contrast to previous findings for electron temperatures in HII regions. These results could be understood in scenarios where the OB association pushes the surrounding dust and gas out to radii that are larger for higher luminosity regions or, alternatively, where the dust we are measuring consists of tiny stochastically heated grains.