Astronomy and Astrophysics, volume 303, 361-378 (1995/11-2)
The CO and HI emission of spiral and lenticular galaxies in the Fornax cluster.
HORELLOU C., CASOLI F. and DUPRAZ C.
Abstract (from CDS):
We present a 12CO(1-0) and HI survey of the spirals and lenticulars in the nearby southern cluster of galaxies Fornax. We have not found any evidence for a strong HI deficiency in this cluster, which shows weak X-ray emission. However, three lenticular galaxies located near the cluster centre, NGC 1380, NGC 1386 and NGC 1387, present some HI deficiency with an HI content respectively 50, 25 and 16 times times lower than expected. On the other hand, the CO emission of Fornax galaxies is weak in general and we have clear detections of only 11 galaxies out of 21 observed. We have found that on the average, less than 10 percent of the gas in these galaxies is in the molecular phase, if the CO to H2 conversion factor has the same value as in our galaxy. We have compared these results with those of other extragalactic surveys and found that such low H2 contents are characteristic of samples that are not selected on a far-infrared criterion. In particular the nearby galaxies surveyed by Sage (1993a, b) also have a low CO emission, although to a lesser level. The low molecular gas content of Fornax galaxies is consistent with their low star formation activity suggested by their low far-infrared and nonthermal radiocontinuum emissions. We have made optical spectroscopical observations of the two CO-poor HI-rich spirals NGC 1350 and 1425 and have found that those galaxies have weak Hα emission. We have also mapped the CO(1-0) and (2-1) emission of the interacting spiral NGC 1532. The CO distribution shows two maxima at 7kpc radius, compatible with the presence of a molecular ring. As most of Fornax spirals NGC 1532 is very poor in molecular gas since it contains about 30 times less H2 than HI.