SIGNALS of Giant HII Regions: Resolved Ionization & Abundance Properties of NGC 604

Abstract

Although we often simplify them as such, real HII regions are not spherical ionized gas clouds with homogeneous physical properties. To study variations in chemical and ionization properties, high spatial resolution is required, which nearby giant HII regions satisfy well. Using emission line data of M33 observed with SITELLE as part of the Star-formation, Ionized Gas, and Nebular Abundance Legacy Survey (SIGNALS), we present maps of the principle optical emission line ratios for NGC 604, the most luminous HII region in M33. While much of the region falls within the low-density limit, the electron density map reveals clumps of higher-density material, suggesting that geometry may be a contributing factor. The excitation maps align well with the Hα morphology and are clearly related to the location of the central stellar cluster and secondary stellar groups. We note two distinct sources identified in these excitation maps: a known supernova remnant and an unknown point source. Maps of abundance- and ionization-sensitive line ratios show substantial variations across the face of NGC 604. We demonstrate that these variations are unlikely to be due to chemical inhomogeneities but instead are primarily caused by changes in ionization. Finally, we present the Hα kinematics of the region and connect it to the excitation structure. Throughout the paper, we make comparisons to and raise concerns about single-aperture and long-slit spectroscopic measurements of giant HII regions.

Ray Garner

TAMU Astronomy Postdoctoral Researcher

I’m a scientist, Star Wars fan, and amateur photographer raised in Georgia. My research interests include galaxy evolution, star formation, satellite galaxies, and nebular diagnostics.