Merging clusters in the core of superclusters

People involved at OAB: Bardelli, Cappi, Giacintucci, Marini, Zucca.

A long term project in which the extragalactic group is involved is the multiwavelength study of the Shapley Concentration, the richest supercluster in the nearby Universe. This study is devoted to investigating the effects of the environment and of the merging phenomena on the physics of clusters and on their galaxy population. In particular, the central parts of superclusters are ideal laboratories in which to study dynamical processes, given the high peculiar velocities induced by the density excess.
We focused our attention on the three structures formed by interacting clusters (the A3558 complex, the A3528 complex and the A3571 complex) which dominate the core of the Shapley Concentration. In particular from all our data and analyses we concluded that these complexes are part of an evolutionary sequence: the A3528 complex is at the very beginning of a merger event, where the two merging entities have just started ``to feel each other''; the A3558 complex is thought to be an advanced merger, where two clusters of similar mass have already undergone the first core-core encounter; the A3571 complex represents the final stage of a merger event, where A3571 itself is the resulting cluster after virialization of the merger.

The main results of this work in the year 2003 were the following:

$\bullet$ In the last years we performed an extensive radio survey in the region of the three cluster complexes, in order to find the effects of major merging events on the radio emission of the galaxy population. The most remarkable result was a significant deficiency of radio galaxies in the A3558 complex (Venturi et al. 2000), consistent with an ongoing merge. In order to better investigate this point we performed a deeper survey in the region of A3562 and SC1329-131 at 1.4 GHz, in the A3558 complex (Giacintucci et al. 2004). We confirmed the presence of a radio halo and of a head-tail radio galaxy at the centre of the cluster A3562. We performed a detailed radio multifrequency study of the head-tail galaxy, which is completely embedded in the halo emission (Venturi et al. 2003). The radio halo has an irregular shape, and a largest linear size of $\sim 620$ kpc, which is among the smallest found in the literature. The source has a steep spectrum, i.e. $\alpha_{843\,\rm MHz}^{1.4\,\rm GHz} \sim 2$, and its total radio power, P $_{1.4\,\rm GHz} \sim 2 \times10^{23}$ W Hz$^{-1}$, is the lowest known to date. The radio power of the halo and the X-ray parameters of the cluster, such as $L_X$ and $kT$, nicely fit the correlations found in the literature for the other halo clusters, extending them to low radio powers. We found that the total number of electrons injected in the cluster environment by the head-tail source is enough to feed the halo, if we assume that the galaxy has been radio active over a large fraction of its crossing time. Given the high statistics, we computed the radio luminosity function for both early and late type galaxies in this region, finding for elliptical and S0 galaxies consistency with Ledlow & Owen (1999). Given the lack of bright radiosources that we found analysing the whole A3558 complex, this result suggests that the deficit is entirely due to the A3558 cluster. Moreover, a population of faint ( $\log P_{\rm 1.4 GHz}[{\rm W\ Hz}^{-1}]<22$) radiogalaxies has been found: half of these objects are blue, suggesting that starburst is the driving radio emission mechanism. Finally, we found 14 spiral galaxies, whose ratio between radio and optical emission is similar to that of galaxies located in rich and dynamically evolved clusters.

$\bullet$ It is known that cluster spiral galaxies tend to have less HI with respect to field objects. It has been proposed that the dynamical events acting during a merge could be responsible for the HI depletion. In order to check this hypothesis, we started a project aimed at observing the neutral hydrogen content of spirals in the A3558 complex. As pilot observations, we obtained time at the ATCA telescope for the coverage of three fields with exposure time of 12 hours each in a band corresponding to the velocity interval [10700-18000] km s$^{-1}$. The data are currently in the reduction phase.

This work is carried out in collaboration with T. Venturi, G. Brunetti (IRA-CNR, Bologna), D. Dallacasa (Astronomy Dept., Univ. of Bologna), R. Morganti (ASTRON, Dwingeloo) and R.W. Hunstead (Sydney Univ.).

$\bullet$ A3528, the dominant cluster of the complex, is a double cluster formed by two twin subclumps separated by 0.9 h$^{-1}$ Mpc, and the other two clusters of the complex (A3530 and A3532) are a close pair, separated by $\sim 1$ Mpc. Gastaldello, ... Bardelli, ... Zucca (2004) studied A3528 with XMM-Newton observations, obtaining surface brightness, temperature and abundance maps (see Fig. 11).

Figure 11:

XMM-Newton observations of A3528. Upper panel: X-ray surface brightness distribution; note the hot bridge connecting the two subclumps. Middle panel: temperature map. Lower panel: surface brightness residuals, obtained after having subtracted a smoothed distribution.

Although a bridge of hot gas connecting the two clumps has been found, no shock is detected: this fact is unexpected, given the estimated masses of the clumps ( $\sim 8\times 10^{13}\ M_\odot$ each) and their relative distance. The most reasonable explanation is that the merging was not head-on but off-axis. After having subtracted a $\beta$ model from the surface brightness of the two subclumps, we found emission excesses which can be used to determine the infalling direction. The conclusion is that this system is in an off-axis post-merging phase, with the closest core encounter happening $\sim 1$-2 Gyrs ago. The interesting point is that the optical blue luminosities of the two sub-clumps, which are twins with regard to the X-ray properties, differ by an order of magnitude. This could indicate that one of the two clumps suffered more than the other from the galaxy ``peeling" process, probably induced by a larger path through the large-scale environment. XMM-Newton data on the couple A3530/A3532 are at present in the reduction phase. Our general conclusion is that, although the two single pairs of clusters (the two clumps of A3528 and A3530/A3532) are mergers in an advanced state, the A3528 complex as a whole is at an earlier moment of collapse with respect to the A3558 complex, and the masses involved here are probably lower.

This work is carried out in collaboration with S. Ettori (ESO), S. DeGrandi (INAF-Milan Obs.), S. Molendi, F. Gastaldello (IASF-CNR, Milan) and T. Venturi (IRA-CNR, Bologna).

Further information about this project can be found on the WEB
S. Bardelli, A. Cappi, F. Marini and E. Zucca, in collaboration with L. Moscardini (Astronomy Dept., University of Bologna), S. De Grandi (INAF-Milano Obs.) and S. Ettori (ESO) are studying the two clusters A2061 and A2067 in the central region of the Corona Borealis supercluster. These two clusters appear to be separated by 1.8 h$^{-1}$ Mpc on the plane of the sky, suggesting the possible presence of interaction. From two BeppoSAX observations (50 ksec each), we estimated the global temperatures, the temperature profiles and maps for A2061 and A2067. We did not find evidence of interaction between these two clusters. However, from an analysis of the bi-dimensional distribution of the hot gas, A2061 turned out to be elongated along the axis connecting its two dominant galaxies. Moreover, in between these two galaxies we found evidence of a significant increase of temperature, due to a shock with a Mach number of 2-3. We speculated that a group of galaxies merged in A2061 and is now near the center of the cluster (Marini, Bardelli, Zucca, ... Cappi et al. 2003). The derived infall velocity is about 2000 km s$^{-1}$, similar to other cases found in literature.