Groups and Clusters of Galaxies


New to the field? Here is a very brief introduction...

X-ray cluster surveys since 1990
(ok, the EMSS was conducted earlier but it was published only in 1990!)


 


(Bias disclosure: I am involved in the WARPS, BCS and MACS surveys... And then there is of course also XBACs!)

The surveys on the left half of this figure  (SHARC-S to Bright SHARC) paint a consistent picture of very little, if any, cluster evolution at low to intermediate luminosities (LX < 4 x 1044 erg/s) out to z ~ 0.85. The cluster X-ray luminosity functions obtained from these surveys agree with the z < 0.3 measurement to within a factor of two.

But what about the more luminous systems, the ones that really have to evolve on these redshifts scale unless Omega is zero after all?

The solid red line in this plot gives the minimal solid angle required for a survey to (statistically) detect 10 clusters of X-ray luminosity greater than 5 x 1044 erg/s based on the local cluster X-ray luminosity function (as determined by the BCS ).That's X-ray luminous clusters at any redshift, though. In order to be able to make a statement about the evolution of X-ray luminous (and hence, by inference, massive) clusters, a survey has to be able to find these systems at high redshift. This is what the dotted lines show: surveys falling on the upper dotted line will
(statistically, and under the no-evolution assumption) detect 10 clusters above the quoted luminosity threshold and at z > 0.3. Note that essentially none of the ROSAT serendipitous cluster surveys meet this criterion. Some are hopelessly off (SHARC-S, Bright SHARC), others come close to being able to address the evolution of massive clusters (160 deg survey, WARPS), but the serendipitous survey that fares best here is still the EMSS - more than 10 years after...

The sole contender among the ROSAT pointed surveys that might be able to find at least moderately X-ray luminous clusters at very high redshift is the RDCS as it goes deeper in flux than any other large-scale X-ray survey conducted to date. The crux of this quest lies in the follow-up work. The RDCS would have to break through the barrier that stopped WARPS at z ~ 1. At these redshifts, even deep optical imaging is not good enough to reliably confirm distant clusters and the follow-up has to move to NIR wavelengths. Measuring redshifts at z > 0.85 where all strong absorption features are buried in OH emission lines is another minor obstacle. But Piero may pull it off anyway...

But back to the really luminous systems:

Can the existing ROSAT All-Sky Survey (RASS) cluster surveys address the issue of evolution at LX > 5 x 1044 erg/s ? The NEP survey lacks the solid angle to compile a sizable sample of the rare, massive systems. The RASS1-BS (see DeGrandi et al 1999 ) does not quite have the depth (or solid angle) to reach that dotted line and, by design, is limited to redshifts less than about z = 0.25.  The same holds for the REFLEX survey (sky coverage estimated in the above plot). The BCS (see, e.g., Ebeling et al. 1998 ) and in particular its extended version (BCS-E) have the potential to probe beyond z = 0.3; however, the BCS is, by choice, limited to z < 0.3 although a few clusters were discovered at higher redshift.

Which leaves MACS, the MAssive Cluster Survey, and the second dotted red line in the above figure. It marks the loci corresponding to an expectation of 100 X-ray luminous clusters at z > 0.3. Does MACS actually find anywhere near this many clusters? Read our press release to find out...