Hα to FUV ratios in resolved star-forming region populations of nearby spiral galaxies

Abstract

We present a new study of Hα/FUV flux ratios of star-forming regions within a sample of nearby spiral galaxies. We search for evidence of the existence of a cluster-mass-dependent truncation in the underlying stellar initial mass function (IMF). We use an automated approach to identification of extended objects based on the SEXTRACTOR algorithm to catalogue resolved H II regions within a set of nearby spiral galaxies. Corrections due to dust attenuation effects are applied to avoid artificially boosted Hα/FUV values. We use the BPASS stellar population synthesis code to create a benchmark population of star-forming regions to act as a reference for our observations. Based on those models, we identify a zone of parameter space populated by regions that cannot be obtained with a cluster-mass-dependent truncation in the stellar IMF imposed. We find that the investigated galaxies display small subpopulations of star-forming regions falling within our zone of interest, which appears to be inconsistent with imposing an IMF truncation at maximum stellar mass dependent on the total cluster mass. The presence of those regions is expected in models using both stochastic and sorted sampling of the full extent of the stellar IMF. This result persists after taking dust attenuation effects into account. We highlight the significance of stochastic effects in environments with low star formation activity and in studies describing systems associated with small physical scales.