Investigating HII Regions in the Disk of NGC 7331 with the Circumgalactic H$α$ Spectrograph

TL;DR

This study uses the Circumgalactic H$\alpha$ Spectrograph (CH$\alpha$S) to map ionized gas kinematics in 136 HII regions across the disk of NGC 7331, deriving H$\alpha$ luminosities and velocity dispersions with high spatial resolution. The authors find that the observed L$_{\mathrm{H}\alpha}$–$\sigma$ relation is consistent with large surveys and that the velocity dispersion correlates with star-formation-rate surface density, best described by $\sigma \propto \epsilon \Sigma_{\mathrm{SFR}}^{\alpha}$ with $\epsilon = 80$ and $\alpha = 0.285$, indicating turbulence driven largely by stellar feedback. Inner-ring regions tend to be more luminous and exhibit higher dispersions, suggesting additional bulk motions contribute near the starburst ring. The work validates CH$\alpha$S as a powerful tool for detailed kinematic studies of galaxy disks and lays groundwork for a larger survey of nearby galaxies to further constrain turbulence drivers in diverse environments.

Abstract

We investigate the ionized gas kinematics of HII regions in the disk of NGC 7331 using integral field unit data collected with the Circumgalactic H$α$ Spectrograph (CH$α$S). NGC 7331 is a well-studied nearby galaxy with HII regions resolved by seeing-limited observations, making it ideally suited for this work. The galaxy disk features vigorous star formation, especially in the central ring of starburst activity. We present a catalog of 136 HII regions detected in the SIRTF Nearby Galaxies Survey (SINGS) H$α$ image. Using this refined catalog, we perform aperture photometry on the SINGS narrowband H$α$ images of NGC 7331, extracting the H$α$ luminosity L(H$α$) of these regions. We present corresponding measurements of the average line-of-sight ionized gas velocity dispersion $σ$ in these HII regions with CH$α$S. High-resolution velocity and dispersion maps of the galactic disk are produced from the CH$α$S spectral imaging, selecting spaxels with high signal to noise in order to measure velocity dispersions as low as 12 km s$^{-1}$. Our measurements of the L(H$α$), $\rm Σ_{SFR}$ and $σ$ in NGC 7331 are consistent with spatially resolved observations of HII regions in large surveys of nearby galaxies. We explore the L(H$α$)$- σ$ relationship, identifying turbulent HII regions with nonthermal dispersions likely driven by stellar feedback. The dispersion is correlated with the star formation rate surface density, and using the relation $\rm σ\propto εΣ_{SFR}^α$, HII regions in NGC 7331 are best fit by $ε= 80$ , $α=0.285$.

Figures (6)

• Figure 1: Comparison of CH$\alpha$S (left) and SINGS (right) images. The SINGS data is overlaid with our catalog of 136 H ii regions represented by circles with diameters corresponding to their measured FWHM. Purple regions are defined as residing in the inner ring selected based on their overlap with the CO ring Helfer2003. All other regions shown in green are defined as residing in the outer galactic disk. Regions that have been rejected based on size or contamination are shown as orange point sources.
• Figure 2: We show that H$\alpha$ emission across the velocity profile in the disk of NGC 7331 is well-covered by the filter, while contamination from [N ii] emission is largely rejected. The approximate filter response curve (pink) has a central wavelength of 6582 $\rm \AA$ and a bandpass of 20 $\rm \AA$ FWHM. Black horizontal lines centered on H$\alpha$ and [N ii] emission have a width equal to the H i 21 cm W$_{20}$ measurement. The sky spectrum (gray) shows the strong Telluric lines; only one bright sky line remains in the filter FWHM Osterbrock1996.
• Figure 3: CH$\alpha$S instrument line spread function derived from the stacked sky background line in more than $3000$ spectra. The instrument dispersion measured from the Gaussian fit shown here is convolved with the (much narrower) intrinsic width of the OH 6-1P1e,1f (4.5) telluric line.
• Figure 4: Kinematic measurements derived from the CH$\alpha$S IFS. In all panels, the coordinate system has been rotated ($15^{\circ}$) in order to align the hexagonally packed spectra along the Cartesian y-axis. The transformed coordinates can be compared directly with Figure \ref{['fig:general']}. We apply an S/N cut on the observed flux, only displaying detections in lenslets with S/N $\geq 3$. The X marker denotes the galactic center coordinates. The individual panel descriptions are as follows: (left) absolute velocity measured in each CH$\alpha$S lenslet, (right) line-of-sight ionized gas velocity dispersion measured in each CH$\alpha$S lenslet.
• Figure 5: Kinematic measurements in NGC 7331. In all panels, the coordinate system has been rotated ($15^{\circ}$) in order to align the hexagonally packed spectra along the Cartesian y-axis. The transformed coordinates can be compared directly with Figure \ref{['fig:general']}. We apply an S/N cut, only displaying detections in lenslets with S/N $\geq 3$. The X marker denotes the galactic center coordinates. The individual panel descriptions are as follows: (left) H i 21cm moment 1 velocity field from THINGS Walter2008. This data has been extracted in regions corresponding to each CH$\alpha$S lenslet. The beam size (robust weighting; $\rm B_{maj} = 4.94"$, $B_{min} = 4.60"$Walter2008) is shown in the lower right corner. (right) Residual offset between the H$\alpha$ velocity map and the H i velocity map. The gray contours correspond to the inner gas ring seen in the BIMA SONG CO (1$-$0) intensity map.