SIGNALS of Giant HII Regions: A Spatially Resolved Analysis of NGC 604

TL;DR

This study presents a spatially resolved analysis of NGC 604, the most luminous giant H II region in M33, using SITELLE imaging Fourier transform spectroscopy as part of the SIGNALS survey. By constructing maps of key emission-line ratios, extinction, density, excitation, and kinematics across the region, the authors show that ionization-driven variations dominate over chemical inhomogeneities, and they identify discrete sources such as a supernova remnant and a planetary nebula that would be invisible in integrated spectra. They derive an integrated oxygen abundance of $12+\log(O/H) = 8.48 \pm 0.07$ and an ionized gas mass of $M_{{\rm HII}} \approx 8.9\times10^{5}\,M_\odot$, along with a mild ionization parameter, while also revealing a central high-excitation core, a low-excitation eastern ridge, and complex kinematics linked to stellar feedback. The work highlights the importance of spatially resolved data for interpreting H II regions and cautions against applying integrated-calibration strong-line diagnostics to individual spaxels, informing future analyses of GHRs in the SIGNALS program and similar surveys.

Abstract

Observing giant HII regions at fine spatial scales uncovers detailed structures and reveals variations in ionization, abundance, and dynamical properties of ionized gas and the effect of stellar feedback. Using emission line data of M33 observed with SITELLE as part of the Star-formation, Ionized Gas, and Nebular Abundances Legacy Survey (SIGNALS), we present maps of the principal optical emission line ratios for NGC 604, the most luminous HII region in M33. The excitation maps align well with the H$α$ morphology and are clearly related to the location of the central stellar cluster and secondary stellar groups. The maps of 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 are primarily caused by changes in ionization, which in turn affect the observed line ratios. We present the H$α$ kinematics of the region and connect it to the excitation structure, showing how the dynamic motions influence the spatial distribution of ionized gas. We note two distinct sources identified in these excitation maps: a known supernova remnant and a previously unknown planetary nebula. Such parsec-scale features contribute only a small percentage to the overall light and would remain undetected without the use of high-resolution spatial data. Throughout the paper, we make comparisons to and raise concerns about single-aperture and long-slit spectroscopic measurements of giant HII regions, highlighting the limitations and potential inaccuracies of such methods.

Figures (11)

• Figure 1: Left: sitelle deep image of NGC 604. For each pixel, the information from all three filters were summed: SN1 (blue), SN2 (green), and SN3 (red). Middle: an RGB image showing only the gas emission lines [O2]$\lambda$3727 (blue), [O3]$\lambda$5007 (green), and H$\alpha$ (red). Right: the same image as in the middle panel with particular features annotated. The outer red contour defines NGC 604 as is ued for the integrated spectrum and corresponds to a flux level of 2e-17s□cm. The green contours define a known supernova remnant and a point source (see the text for details). Bubbles noted by maizapellaniz2004 are marked with letters. The red crosses are the sites of wr stars and OB stars from drissen2008 and bruhweiler2003, respectively. The green cross near the point source is a star detected in near-infrared imaging by farina2012. The coordinate system is the offset from the center coordinate [angle-symbol-degree=h, angle-symbol-minute=m, angle-symbol-second=s]1;34;33.11, +30;47;6.8. North is up and east is to the left.
• Figure 2: The integrated spectrum of NGC 604 with the apparent strong lines indicated. The black line represents the spectrum, while the red line is the fit performed by orcs using a sincgauss function. The black line below the fit is the fit residuals. Please note the changing wavelength along the broken $x$-axis.
• Figure 3: The visual dust extinction, $A_V$, map for NGC 604 as estimated from the Balmer decrement. The characteristic median uncertainty is 0.17. The contours show the distribution of the JWST NIRCam F770W flux at levels of [list-units=single]20;50 in red and purple, respectively sarbadhicary2024.
• Figure 4: Left: the [S2]$\lambda$6717/[S2]$\lambda$6731 emission line ratio. Characteristic uncertainties are $\sim$0.4. Right: the electron density, $\log n_e$, calculated with the models of proxauf2014. Characteristic uncertainties are 1. The black contours show the distribution of the reddening-corrected H$\alpha$ flux at levels of $\log(F_{\text{H$\alpha$}}) = -16.5, -15.5, -14.5 \, \erg\per s\per\square cm$.
• Figure 5: From left to right: the $\log(\text{[\ion{N}{2}]}\lambda6584/\text{H$\alpha$})$, $\log(\text{[\ion{S}{2}]}\lambda\lambda6717,6731/\text{H$\alpha$})$, and $\log(\text{[\ion{O}{3}]}\lambda5007/\text{H$\beta$})$ emission line ratio maps. Characteristic uncertainties are 0.08;0.08;0.11, respectively. North is up, east to the left.