Hot gas in galaxy groups: recent observations

Abstract

Galaxy groups are the least massive systems where the bulk of baryons begin to be accounted for. Not simply the scaled-down versions of rich clusters following self-similar relations, galaxy groups are ideal systems to study baryon physics, which is important for both cluster cosmology and galaxy formation. We review the recent observational results on the hot gas in galaxy groups. The first part of this paper is on the scaling relations, including x-ray luminosity, entropy, gas fraction, baryon fraction and metal abundance. Compared to clusters, groups have a lower fraction of hot gas around the center (e.g. r < r 2500 ), but may have a comparable gas fraction at large radii (e.g. r 2500 < r < r 500 ). Better constraints on the group gas and baryon fractions require sample studies with different selection functions and deep observations at r > r 500 regions. The hot gas in groups is also iron-poor at large radii (0.3r 500 -0.7 r 500 ). The iron content of the hot gas within the central regions (r < 0.3r 500 ) correlates with the group mass, in contrast to the trend of the stellar mass fraction. It remains to be seen where the missing iron in low-mass groups is. In the second part, we discuss several aspects of x-ray cool cores in galaxy groups, including their difference from cluster cool cores, radio AGN heating in groups and the cold gas in group cool cores. Because of the vulnerability of the group cool cores to radio AGN heating and the weak heat conduction in groups, group cool cores are important systems to test the AGN feedback models and the multiphase cool-core models. At the end of the paper, some outstanding questions are listed.

Galaxy formation in the Planck cosmology – I. Matching the observed evolution of star formation rates, colours and stellar masses

“…= 10 14 M⊙. In lower mass groups and clusters, X-ray data show significantly lower hot gas fractions, at least in the inner regions (see Sun 2012 for a review on the subject). Given that characteristic orbital velocities are also substantially lower in such systems, it is plausible that stripping effects should be less important there, and indeed Font et al (2008) already advocated such a reduction with respect to the findings of direct simulations of the stripping process, noting that this significantly improved the colours of dwarf galaxies in their own galaxy formation model.…”

Planckintermediate results

“…Recent X-ray observations based primarily on representative samples have returned a consistent picture of the scaling and structural properties of halos, from high mass clusters down to the low mass group regime (see, e.g., Böhringer et al 2007;Vikhlinin et al 2006;Croston et al 2008;Pratt et al 2009;Arnaud et al 2010;Sun et al 2011;Sun 2012). In parallel, SZ observations with instruments such as the SunyaevZeldovich Array (SZA, Muchovej et al 2007), the South Pole Telescope (SPT, Carlstrom et al 2011), the Atacama Cosmology Telescope (ACT, Swetz et al 2011), and Planck (Tauber et al 2010) 1 have recently started to deliver on the promise of SZ observations for cluster studies.…”

Far-ultraviolet morphology of star-forming filaments in cool core brightest cluster galaxies

“…The paradigm is motivated by strong circumstantial evidence, including nearly ubiquitous observations of radio-bright AGN outflows driving shocks and excavating kpc-scale buoyant cavities in the ambient X-ray gas, acting as lower limit calorimeters to the often extreme ( 10 46 erg s −1 ) AGN kinetic energy input (e.g. reviews by Sun 2012). Yet amid panoramic supporting evidence (reviewed by Fabian 2012), the physics that governs the spatial distribution and thermal coupling of AGN mechanical energy to the multiphase (10-10 7 K) gaseous environment remains poorly understood, and cooling flow alternatives invoking (e.g.)…”