Cool Core Clusters at Redshift z > 0.7

Panoramic Views of Galaxy Formation and Evolution c 2008 ASP Conference Series, Vol. 399, T. Kodama, T. Yamada, and K. Aoki, eds.

Cool Core Clusters at Redshift z > 0.7 Joana S. Santos,1 Piero Rosati,2 Paolo Tozzi,3 Hans B¨ohringer,1 Stefano Ettori,4 and Andrea Bignamini3 Abstract. We present a study which aims at measuring the fraction of Cool Core (CC) Clusters in the distant galaxy cluster population, i.e., out to redshift 0.7 < z < 1.4. Using a morphologically heterogeneous sample of nearby (0.15 < z < 0.3) clusters drawn from the Chandra archive, we define criteria to characterize cool cores that are applicable to a high-z sample. We made a global surface brightness (SB) analysis, measuring radial SB profiles and we defined a surface brightness concentration, cSB , which is sensitive to the core properties. In addition, we measured the cooling time in inner regions with 20 kpc radius, thus evaluating central cooling in a more physical way. The overall SB analysis of the low-z sample indicates 3 categories of clusters: non-CC, moderate and strong CC, whereas in the high-z clusters we find only the first two regimes. The cooling time distribution supports this result, showing a strong negative correlation with cSB . Our analysis indicates a significant fraction of distant clusters harboring a moderate CC, similar to those found in the local sample. Nevertheless, strong CCs are not common at high-z, which could be related with the shorter age of these clusters.

1.

Introduction

The study of the formation and evolution of cool core clusters is important to decipher the long standing ”cooling flow” problem and also bears implications on the use of galaxy clusters as cosmological probes. In the local Universe the abundance of cool cores is observed to be in the range 50-70% (Peres et al. 1998). Using spatially resolved spectroscopy, Bauer et al. (2005) found no signs of evolution of the cool core fraction in the redshift range z ∼ 0.15−0.4: the cool cores at intermediate redshift show the same temperature decrement, T /Tcentral ∼ 3-4, as the nearby CC’s, and have a frequency rate similar to the local one. On the other hand, using pure imaging data, Vikhlinin et al. (2006) have recently claimed a lack of CCs at z > 0.5. In this contribution we investigate the fraction of cool core clusters in a statistically complete sample with 0.7 < z < 1.4 imaged with Chandra (Santos et al. 2008), based on the Rosat Distant Cluster Sample (Rosati et al. 1998).

1 Max-Planck-Institut f¨ ur extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany 2

European Southern Observatory, Karl-Schwarzchild Strasse 2, 85748 Garching, Germany

3

INAF, Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131, Trieste, Italy

4

INAF, Osservatorio Astronomico di Bologna, via Ranzani 1, 40127, Bologna, Italy

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376 2.

Santos et al. Surface Brightness Analysis and Cooling Time Results

A central surface brightness excess is a primary indicator of the presence of a cool core. We therefore analyse the clusters’ global SB properties using the empirical scaling (Fig.1), and evaluate the core SB excess by defining a concentration parameter, cSB = SB(r < 40kpc)/SB(r < 400kpc). The surface brightness diagnostics were developed based on the nearby sample and were validated to high-z using simulated distant clusters, obtained with a cloning technique. Central cooling times were measured in both samples (Fig.2).

Figure 1. Scaled SB profiles stacked in cSB bins: the nearby sample (blue) presents 3 bins: non-CC (solid), mild CC (dash) and strong CC (dash-dot); the distant sample (red) shows 2 bins: non-CC (solid) and mild CC (dash).

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Figure 2. Distribution of the cooling time, tcool (r = 20kpc) in the low-z (blue) and high-z (red) samples.

Discussion and Conclusions

The overall surface brightness analysis reveals that the majority (10/15) of the high-z clusters presents mild cool cores. The cooling time measurements of the reference, low-z sample, indicate that strong CC have tcool < 3 Gyr, moderate CC have 3 < tcool < 10 Gyr and non-CC have tcool > 10 Gyr. The bulk (11/15) of the high-z clusters are characterized by 3 < tcool < 10 Gyr, with only 4 clusters showing tcool > 10 Gyr. We found no distant cluster with tcool < 3 Gyr. The established scenario of hierarchical structure formation, in which galaxy clusters develop through mergers and by accretion of neigbouhring smaller objects, provides a possible mechanism for preventing the formation of prominent cool cores at high-z. References Bauer, F. E., Fabian, A. C., Sanders, J. S., et al., 2005, MNRAS, 359, 1481 Peres, C. B., Fabian, A. C., Edge, A. C. et al., 1998, MNRAS, 298, 416 Rosati, P., Della Ceca, R., Norman, C., Giacconi, R., 1998, ApJ, 492, 21

Cool Core Clusters at Redshift z ≥ 0.7

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Santos, J. S., Rosati, P., Tozzi, P., et al., 2008, astro-ph/08021445 Vikhlinin, A., Burenin, R., Forman, W. et al., 2006, in Proceedings of “Heating vs. Cooling in Galaxies and Clusters of Galaxies,” astro-ph/0611438

A panoramic view towards Mt. Fuji from the Venue.