Nicole Vogt » Spiral Infall into Local Clusters
The diagram shown below was created by examining H-alpha optical rotation curves and HI 21 cm. line profiles for ~ 300 local galaxies, drawn from x-ray hot, evolved clusters, from cooler clusters, from loose groups, and from the isolated field in the local Universe. The galaxies shown below were selected because, when we examine major axis longslit spectra, they exhibit an asymmetric distribution of H-alpha flux along the two sides of the disk. The extent of the H-alpha flux from side to side about the nucleus differs by more than 5 kpc/h and/or by 50%. They are all found to lie within 1 Mpc/h of a cluster center, though we have sampled the clusters to beyond 3 Mpc/h, and most are found within the hottest clusters A1656, A426, A1367 and A2151.
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A representative cluster is delineated by circles at 1 Mpc/h and 600 kpc/h, with the hot cluster X-ray gas illustrated by green contours inside 600 kpc/h. The galaxies are drawn relative to their individual cluster centers, in blue if they have a normal amount of HI gas and in red if they are strongly HI deficient (or in black if the gas content is unknown). Individual galaxy glyphs show the extent of the H-alpha flux along the major axis of the disk (dashed in one case to indicate H-alpha absorption). The truncated sides of the disks tends to point towards the center of the clusters, as expected for infalling spirals in predominantly radial orbits on a first pass through the hot intracluster medium.
The side panels show the H-alpha and [NII] rotation curves for the galaxies, where the x-axis shows the spatial extent along the major axis of the disk and the y-axis shows the distribution in circular velocity. HI line profiles, showing the distribution of the flux along the x-axis and, as before, the distribution in circular velocity on the y-axis, are drawn when available. Each rotation curve has been centered about the continuum flux, and the panels extend a minimum of 5 kpc/h to each side. Blue points denote the distribution of H-alpha flux, and green points that of [NII]. There is no H-alpha emission flux on the inner side of one galaxy (lower right, second from bottom), and the H-alpha absorption feature which is revealed is traced in magenta. The sense of the H-alpha flux truncation is typically matched by a decrease in HI gas on the same side of the galaxy disk.