For the first time, a team from Paris Observatory detects the cold phase in the centre of the galaxy cluster Abell 1795, and demonstrates that the cold gas is associated to the cooling wake seen through X-ray emission and delineated by Halpha emission. The cold ( 20K) gas, detected through its CO rotational lines at 2.6 and 1.3mm wavelength, is clearly associated with the cooling flow and not with the central galaxies. A mass of 4.8 109 Mo of molecular hydrogen is derived, from standard CO/H2 conversion ratio. The cold gas is probably fueling the active nucleus at the center, and the consequent ejection of mass and energy in the radio jets provides re-heating of the gas, regulating the cooling flow. A large fraction of the mass in rich clusters of galaxies is under the form of diffuse hot gas (10 millions of degrees) observed in X-ray thermal emission. The density of this gas is increasing towards the center, and since its cooling time is inversely proportional to the density, it becomes much smaller than the Hubble time towards the center : over a region of hundreds of kpc wide, the gas should be able to cool down, and flow towards the center by lack of pressure support. Some evidence of cooling is observed through the drop in temperature of the X-ray gas, but it has been very difficult to trace the gas down to the very low temperatures expected.
Recent results with the X-ray satellites Chandra and XMM-Newton have led to a radical revision of the traditional view of cooling flow. The hot gas images from Chandra in cooling flow clusters reveal cavities, bubbles, very inhomogeneous temperatures, suggesting intermittent cooling and re-heating of the gas by the central active nucleus and associated jets. The simple spherically-symmetric model of regular cooling is to be abandonned, and the derived gas deposition rates are now lower. However, the evidence of large quantities of cold gas was up to now still missing in the scenario, questioning the very existence of cooling.Within a couple of years, this situation has changed rapidly : cold molecular gas has been detected in large quantities through the rotational lines of the CO molecule, tracer of the H2 molecule, in 23 cooling flow clusters (Edge 2001, Salomé & Combes 2003). But these detections were global, with not enough spatial resolution to actually prove that a cooling flow was at the origin of the gas. The present mapping of the CO emission in Abell 1795 brings the first proof of association of CO emission and the cooling gas.
The rich cluster of galaxies Abell 1795 has been known for a long time to host a cooling flow ; the gas temperature measured from X-ray emission is dropping by a factor 3 towards the core, at about 140kpc from the centre. The image from the Chandra satellite (cf Figure 1, Fabian et al 2001) reveals a 40arcsec long ( 60 kpc) X-ray filament in its core, coinciding with an Halpha +NII filament previously found by Cowie et al. (1983). This filament is also conspicuous in the U-band (McNamara et al 1996), and the site of star formation, at a rate of 20 Mo yr-1. The central cD galaxy, a little South of the Northern peak of the filament, has a peculiar velocity of +350km/s with respect to the rest of the cluster galaxies, and is oscillating back and forth within the cluster core. Most of the Halpha gas in the filament has a velocity centred on the cluster mean velocity, and may also be sloshing in the cluster potential. The radiative cooling time of the X-ray emitting gas in the filament is about 300 Myr, quite similar to the dynamical age of the filament (ratio of length to velocity). Therefore the filament morphology of the X-ray gas corresponds to a cooling wake, the cooling gas following the motion of the cD in the cluster.
The present observations of CO emission in the two first rotational lines (CO(1-0) and CO(2-1)) have been done with the IRAM Plateau de Bure interferometer. It shows that the CO emission is extended along the filament, and closely associated to the X-ray cooling gas. The association of the CO(2-1) emission with the Halpha emitting gas in Figure 2 (middle and right) is particularly striking. The cooling filament is intimately related to the radio jets and lobes from the radiosource 4C+26.42 (van Breugel et al. 1984) : the jet appears to flow within a hole, or in other terms, the cooling gas, the cold molecular gas and the young stars formed out of the cold gas, are all confined at the boarder of the radio lobes.The kinematics of the molecular gas is also very similar to that of the Halpha gas. Figure 3 shows the position-velocity diagrams for the two CO lines, along the axis of maximum emission (PA=27°) through the centre of the cD galaxy. Most of the CO emission is not associated to the central galaxy, which is at zero velocity in this frame. But the gas is at -350km/s, which is the mean velocity of the galaxy cluster.
In summary, the peculiar morphology of the cooling gas (X-ray, Halpha, blue continuum, molecular gas) appears to avoid the radio lobes, and is associated kinematically to the cluster as a whole, and not to the central cD galaxy. The more realistic scenario is that the plasma of the radio jets displaces and compress the hot X-ray cluster gas, and in these denser regions, the cooling is accelerated. This produces cold molecular gas, that further forms stars, explaining the coincidence of CO with X-ray and Halpha emissions.