2025ApJ...987...11A

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).