A MeerKAT view of the Neutral Atomic Gas in Stephan's Quintet

Abstract

We present new MeerKAT 21cm spectral line observations of the neutral hydrogen gas in the compact galaxy group Stephan's Quintet (HCG 92). These data provide a significantly improved view of the atomic gas distribution and kinematics in the group. New features include the first detections of HI associated with member galaxies NGC 7319 and NGC 7320C, the identification of an additional high-velocity HI component associated with SQ-A, and the detection of additional HI at low velocities filling much of the area of the NGC~7318B disk. We also find HI in the previously detected gas bridge linking NGC 7319 and NGC 7318B, and a new northern bridge linking NGC 7319 to the SQ-A star-formation region. We detect HI with a wide range of velocities along the line of sight through the northern half of the famous shock ridge, including in the 6200-6500$\rm\,km\,s^{-1}$ velocity range occupied by shocked H$α$ emission. We examine the morphology and velocity structure of the HI and consider the origins of different components, finding some evidence that while the gas associated with NGC 7318B has been disturbed by its collision with the group, it may still retain a component of disk rotation. We find no gaseous connection between the tidal tails and NGC 7320C, but reaffirm the close connection between the shocked gas in the ridge (traced by X-ray, radio continuum and warm H$_2$ emission) and the southern tidal tail. Based on the integrated spectrum, we find a total HI mass in the group of 3.5$\pm$0.4$\times$10$^{10}\,M_{\odot}$, higher than the VLA estimate and comparable to FAST.

Figures (16)

• Figure 1: Outline map of the galaxies, stellar tidal features and Hi distribution in Stephan's Quintet. The combined Hubble Space Telescope (HST) and Digitized Sky Survey (DSS) optical image is overlaid with a MeerKAT Hi contour drawn at $0.012 \rm \,Jy\,beam^{-1} km~s^{-1}$, corresponding to an Hi column density of $5.6\times10^{19}$ atoms cm$^{-2}$. Spiral arms and arm-like features in NGC 7318B are labeled in red. The 17$^\prime$$^\prime$$\times$14$^\prime$$^\prime$ synthesized beam size is indicated in the bottom left corner.
• Figure 2: MeerKAT moment-zero maps at medium and low resolutions showing the total Hi column density distribution in SQ. Left: the beam size is $17\arcsec\times14\arcsec$ and the contours are drawn at $5.6, 11.1, 22.3, 44.5, 89.1, 178 \times 10^{19}\rm \,atoms\,cm^{-2}$. Right: The beam size is $25\arcsec\times25\arcsec$ and the contours are drawn at $3.9, 7.8, 15.6, 31.1, 62.2, 124\times 10^{19}\rm \,atoms\,cm^{-2}$. The MeerKAT observations reveal previously undetected H$\,$i emission in the group core, NGC 7319 and NGC 7320C.
• Figure 3: Top: HST+DSS image overlaid with Hi column density and radio continuum contours. The Hi image beam size is $17\arcsec\times14\arcsec$ and the contours are drawn at $5.6, 11.1, 22.3, 44.5, 89.1, 178 \times 10^{19}\rm \,atoms\,cm^{-2}$ (left panel).
• Figure 4: Hi moment-one (Left) and moment-two (Right) maps of Stephan's Quintet obtained from SoFiA. The solid black outlines mark the D$_{25}$ ellipses of the group member galaxies, using parameters from the LEDA catalogue, while the dashed ellipse marks the foreground galaxy
• Figure 5: Spectra of the total Hi emission and of individual subregions in SQ. The brown vertical dashed lines mark the boundaries of the five kinematical components, namely 5700, 6000, 6300, 6600, and 6900. The dashed gray horizontal lines indicate the zero flux density level and red lines (with labels) mark the optical velocities of the major group member galaxies. Figure \ref{['fig::HI_regions']} shows the regions used to extract these. We note that the spectrum of the northern region includes the SQ-A region.