Testing the low-mass end of X-ray scaling relations with a sample ofChandragalaxy groups

Abstract

Context. Well-determined scaling relations between X-ray observables and cluster mass are essential for using large cluster samples to constrain fundamental cosmological parameters. Scaling relations between cluster masses and observables, such as the luminositytemperature, mass-temperature, luminosity-mass relations, have been investigated extensively, however the question of whether these relations hold true also for poor clusters and groups remains unsettled. Some evidence supports a "break" at the low end of the group/cluster mass range, possibly caused by the stronger influence of non-gravitational physics on low-mass systems. Aims. The main goal of this work is to test local scaling relations for the low-mass range in order to check whether or not there is a systematic difference between clusters and groups, and to thereby extend this method of reliable and convenient cluster mass determination for future large samples down to the group regime. Methods. We compiled a statistically complete sample of 112 X-ray galaxy groups, 26 of which have usable Chandra data. Temperature, metallicity, and surface brightness profiles were created for these 26 groups, and used to determine the main physical quantities and scaling relations. We then compared the group properties to those of the HIFLUGCS clusters, as well as several other group and cluster samples. Results. We present radial profiles for the individual objects and scaling relations of the whole sample (. Temperature and metallicity profiles behave universally, except for the core regions.and L x − Y x relations of the group sample are generally in good agreement with clusters. The L x − T relation steepens for T < 3 keV, which could point to a larger impact of heating mechanisms on cooler systems. We found a significant drop in the gas mass fraction below < ∼ 1 keV, as well as a correlation with radius, which indicates the ICM is less dominant in groups compared to clusters and the galaxies have a stronger influence on the global properties of the system. In all relations the intrinsic scatter for groups is larger than for clusters, which appears not to be correlated with merger activity but could be due to scatter caused by baryonic physics in the group cores. We also demonstrate the importance of selection effects. Conclusions. We have found some evidence for a similarity break between groups and clusters. However this does not have a strong effect on the scaling relations.

The 2XMMi/SDSS Galaxy Cluster Survey

“…We found that the median scaling factor of T ap and T 500 of the full sample, T ap /T 500 , was 1.2, where T 500 is the predicted temperature based on the L 500 − T 500 relation by Pratt et al (2009) using our value for L 500 . Eckmiller et al (2011) found that the slope of L X − T relation of galaxy groups (26 systems, L x ∼ 1−26 × 10 42 erg s −1 ) is slightly shallower than that derived for clusters (HIFLUGCS), but they are still consistent within the errors. These authors found no significant change either of the slope derived from a sample that combined groups and clusters to a sample consisting of clusters only, which is consistent with the results reported by Osmond & Ponman (2004).…”

“…Markevitch 1998;Pratt et al 2009;Mittal et al 2011;Eckmiller et al 2011;Reichert et al 2011;Takey et al 2011;Maughan et al 2012;Hilton et al 2012). These studies showed that the observed L X − T relation is much steeper than that predicted by self-similar evolution.…”

Scaling Relations for Galaxy Clusters: Properties and Evolution

“…In the last few years deep X-ray surveys have provided the first statistically significant samples of X-ray galaxy groups (e.g. Finoguenov et al 2007;Gastaldello et al 2007;Sun et al 2009;Eckmiller et al 2011). For example Pratt et al (2009) examined possible causes for the observed steeper slopes, concluding that it is mostly associated with the variation of the gas content with mass, while structural variations play only a minor role.…”

“…The deviation of the L X − T relation from the pure gravitational prediction can also be interpreted in terms of entropy variation (Evrard & Henry 1991;Bower 1997;Tozzi & Norman 2001;Borgani et al 2001;Voit et al 2002;Younger & Bryan 2007;Eckmiller et al 2011). One way to inhibit the gas from reaching the center of the potential well, thus changing the gas fraction, is to increase the entropy of the gas.…”

“…Gastaldello et al 2007;Sun et al 2009;Eckmiller et al 2011). Such deviations indicate that non-gravitational processes may be a significant contributor to the global energy budget in clusters.…”