1. An Arecibo HI 21-cm Absorption Survey of Rich Abell Clusters
H. Hernández, T. Ghosh, C. Salter, E. Momjian (NAIC/Arecibo Observatory)
The Sample and the Observations
Our sample of radio-loud Abell cluster galaxies was selected to include objects with redshifts, z < 0.29, 1400-MHz flux densities of > 50 mJy, and visible with the Arecibo telescope (-1o < Dec < +38o). It contains 85 galaxies that are not seriously affected by radio frequency interference (RFI) at the redshifted frequencies of either HI or the OH molecule main-lines. Out of these, 49 clusters are known to be X-Ray rich. To eliminate standing waves associated with the continuum emission of the galaxies, we used the double-position-switching observing method. A typical on-source integration time of ~15 min was obtained, with a Hanning-smoothed resolution of ~5 kms-1. This gave typical rms noise of ~0.4 mJy for sources detected in emission, and down to for the optical depths of lines seen in absorption.
Introduction
To investigate a number of puzzles concerning cluster of galaxies, we have undertaken a search for neutral hydrogen(HI) in 85 radio-loud galaxies within Abell clusters. One unsolved problem is as to where the inflowing gas in cooling-flow clusters (CFCs) ends up? It is known that the dominant cD galaxies in CFCs often emit blue light and exhibit massive star formation, although this cannot simply account for more than a few percent of the gas mass supplied by a CF. Part of this gas probably fuels massive black holes in the central galaxies, and triggers AGN activity.
Edge et al. (2002) have concluded that these central galaxies contain clouds hosting, (1) ionized atomic H, (2) warm H2 molecular gas, (3) cool H2 seen via CO, and (4) a “skin” of HI on these clouds. Other possibilities for the presence of detectable HI in clusters are (Dwarakanath & Owen, 2001), (1) in gas tori around AGNs in cluster galaxies, (2) in the ISM of (radio-loud) cluster galaxies, (3) in foreground galaxies to radio-loud galaxies, and (4) via tidal interactions with the cluster potential. There is also evidence for galaxy cannibalism in the central regions of clusters. This, and the gas inflowing from CFs, may contribute to the formation of central cD galaxies.
We have undertaken a large survey of HI associated with radio-loud galaxies in Abell clusters in the redshift range, z < 0.29 via observations of the 21-cm spectral line. The sample includes both X-Ray-rich and -quiet clusters.
Coming for free, we also included a search for the main-lines of the 18-cm transition of the OH molecule.
Results
From a preliminary analysis of our observation, we have detected 6 of our target galaxies in HI emission, and 10 through HI absorption. While 8 of these absorptions are measured with good signal-to-noise ratios, the other 2 require further observation to confirm their presence. In addition, OH absorption is seen against 2 objects, while a further galaxy displays a possible OH line in both emission and absorption, and requires further observation. The spectra for the detected galaxies, plus images at other wavelengths, are displayed in Fig. 1.
Discussion
We note that all galaxies seen in emission are, perforce, at low redshifts, while the sensitivity to absorption is effectively distance independent. Two of the galaxies seen in emission may be fore-ground objects.
One noticeable correlation is that while all galaxies seen in emission are in clusters showing a lack of X-ray emission, all targets detected in absorption lie in X-ray rich clusters. HI absorption is weighted towards cool gas. It is now important to image the objects that we have detected in absorption with high spatial resolution in order to investigate whether we are seeing the inflow of gas from a CF, or the result of simple galactic cannibalism.
Acknowledgments
This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.
The Arecibo Observatory is part of the National Astronomy and Ionosphere Center, which is operated by Cornell University under a cooperative agreement with the National Science Foundation.
Figure 1: All cases of detected HI spectral lines from our survey are displayed above in the leftmost panels of each image column. For the three cases in which OH features are seen, the relevant spectra are shown to the right of those for HI. To the right of our spectra are displayed optical, radio and infra-red images obtained from NED. Where an OH detection is shown, the corresponding infra-red image has been omitted. The single exception to the above is for Abell 2390, where we present just the Chandra X-ray and the HST optical images.