A pilot method to determine the high mass end of the stellar initial mass function in galaxies using UVIT, Hα-MUSE observations and applied to NGC 628

Abstract

We present a pilot method to estimate the high-mass initial mass function (IMF) across the arm, interarm, and spur regions in galaxies and apply it to NGC 628. We extracted star-forming complexes (SFCs) from the Hα Very Large Telescope/Multi Unit Spectroscopic Explorer and Ultraviolet Imaging Telescope (far-ultraviolet (FUV) and near-ultraviolet (NUV)) observations of NGC 628 and used Atacama Large Millimeter/submillimeter Array observations to define the molecular gas distribution. We find that the extinction-corrected Hα and FUV luminosities correlate well. Using the fact that O stars have a shorter lifetime (107 yr) compared to B stars (108 yr), we estimated the approximate number of O stars from Hα emission, and the number of B0 (M* > 10M⊙), and B1 (10M⊙ ≥ M* ≥ 3M⊙) stars using FUV and NUV observations. We derived the IMF index (α) for different regions using O to B0 (α1) and B0 to B1 (α2) stellar ratios. Our findings indicate that if we assume Hα arises only from O8-type stars, the resulting α1 value is consistent with the canonical IMF index. It steepens when we assume O stars with masses up to 100 M⊙ with mean α1 = 3.16 ± 0.62. However, the α2 does not change for large variations in the O-star population, and the mean α = 2.64 ± 0.14. When we include only blue SFCs (FUV − NUV ≤ 0.3), mean α2 is 2.43 ± 0.06. The IMF variation for SFCs in arms and spurs is insignificant. We also find that α2 correlates with different properties of the SFCs, the most prominent being the extinction-corrected UV color (FUV − NUV).