| RESEARCH : |
An HI survey of Clusters in the Local Universe Virgo: a Laboratory for Studying Galaxy Evolution The Fate of Gas in Merger Remnants |
| This image show the HI emission (contours) from selected galaxies in the Coma cluster overlaid on an optical image (greyscale). The central contours show the X ray emission from the hot ICM. The image shows that the HI properties of galaxies close to the cluster center are very different from those further out. In the center the HI disks are smaller than the stellar disks and sometimes they are very asymmetrical. This is probably due to ram pressure stripping by the hot ICM. ( Bravo-Alfaro et al 2000 ) Many different processes can affect the environmental evolution, such as harassment, tidal interactions and merging, starvation, and ICM-ISM interactions. To distinguish which processes are most important we have obtained HI data with the VLA for 6 clusters at redshift from z=0 to z=0.2. For each cluster the entire volume was observed out to two Abell radii. These data will be combined with deep optical imaging and spectroscopy. The survey team:Jacqueline van Gorkom, Hector Bravo-Alfaro, K. Dwarakanath, Puragra Guhathakurta, Bianca Poggianti, David Schiminovich, Monica Valluri, Marc Verheijen, Eric Wilcots, Ann Zabludoff. ( VLA Large Proposals). | |
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This is an image of the HI emission (contours) overlaid on an R band image of NGC 4522. This galaxy is located in the outskirts of Virgo. The stellar disk looks very smooth and undisturbed, but the gas has been completely removed from the outer disk. In the inner part we see extraplanar gas being acccelerated toward the cluster mean velocity. Based on the HI kinematics we conclude (surprisingly) that ram pressure stripping is ongoing at this large distance from the center. Perhaps this is due to subclump merging and a first hint of "preprocessing" before the galaxy falls into the cluster. (Kenney, van Gorkom and Vollmer 2004). This illustrates that detailed observations of galaxies in a nearby cluster can constrain the mechanisms that affect the evolution of the galaxy. We are now doing a large survey of 40 galaxies in the Virgo cluster, located in a range of different local densities. We have already obtained deep optical photometry and H alpha imaging and we have been allocated (GALEX ) time to observe the sample in UV as well. The goal is to constrain for each galaxy various forces at work that cause trauma and study in detail the effects on the star formation history. From our detailed knowledge of the mechanisms affecting these Virgo galaxies, we can learn how the results can be generalized to understand galaxy evolution in richer and more distant clusters. The survey team: Jeff Kenney, Hugh Crowl, Bernd Vollmer, Aeree Chung and Jacqueline van Gorkom ( VLA Large Proposals). |
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In his thesis on (
The fate of Gas in Merging Disk Galaxies ) John Hibbard showed that
in merging disk galaxies a significant fraction of gas and stars
from the disk are thrown out to large distances forming tidal features.
Most of this material remains bound to the system and will eventually
fall back to the remnant. The timescales can be many Gyrs. I am now
addressing the question what is the fate of this tidal material.
In the picture at left, of the nearest radio galaxy NGC 5128, we
see HI (contours) associated with the shells in the outer parts of the
galaxy. We think that here we see the tidal debris, left over from the
merger that formed the galaxy NGC 5128 ( Schiminovich et al 1994 ) If enough gas is present and the geometry is right, fall back may eventually lead to the growth of another disk ( van Gorkom and Schiminovich 1997 ); ( Barnes 2002) ). An example of this is shown at right. It shows the HI emission (contours) overlaid on an optical image of the polar ring, shell galaxy MCG -5-7-1. To further investigate the fate of gas in merger remnants I am involved in a number of projects. We have collected images of all galaxies known to us of HI in Weird Galaxies and Weird HI in otherwise Normal Galaxies in an ( HI Rogues Gallery ). We are currently analyzing HI observations of HI in shell galaxies and HI in elliptical galaxies without any optical fine structure. Last but not least we have just been allocated ( GALEX ) time to observe UV in systems known to have extended high column density gas far outside the opticals disks. The goal is to probe the star formation history far outside the optical images. The team for the Galex survey: John Hibbard, Jacqueline van Gorkom, Tom Oosterloo, David Schiminovich, Paolo Serra, Scott Trager and Thijs van der Hulst. | |