Abstract
Observations with the Chandra X-ray Observatory are used to examine the hot gas properties within a sample of 10 galaxy groups selected from the Galaxy And Mass Assembly survey's optical Friends-of-Friends group catalogue. Our groups have been screened to eliminate spurious and unrelaxed systems, and the effectiveness of this procedure is demonstrated by the detection of intergalactic hot gas in 80 per cent of our sample. However, we find that 9 of the 10 are X-ray underluminous by a mean factor of $\sim$4 compared to typical X-ray-selected samples. Consistent with this, the majority of our groups have gas fractions that are lower and gas entropies somewhat higher than those seen in typical X-ray-selected samples. Two groups, which have high 2{\sigma} lower limits on their gas entropy, are candidates for the population of high-entropy groups predicted by some active galactic nucleus feedback models.
Highlights
Studies of the hot gas in groups and clusters of galaxies have demonstrated that this gas exhibits entropies in excess of self-similar expectations (e.g. Ponman, Cannon & Navarro 1999; Lloyd-Davies, Ponman & Cannon 2000)
This study has investigated the hot gas properties of a small sample of galaxy groups selected to have good galaxy membership data from the Galaxy and Mass Assembly (GAMA) survey and to show little optical substructure
Two of our groups are high-entropy candidates, with 2σ lower limits on central gas entropy that lie at the upper edge of the 1σ range predicted by the Overwhelmingly Large Simulations Project (OWLS) AGN feedback model
Summary
Studies of the hot gas in groups and clusters of galaxies have demonstrated that this gas exhibits entropies in excess of self-similar expectations (e.g. Ponman, Cannon & Navarro 1999; Lloyd-Davies, Ponman & Cannon 2000). Due to the smaller potential wells of groups compared to massive clusters, the entropy injected into gas at group scales can be significant compared to the entropy accrued during halo assembly. Alternatives, such as the action of cooling flows, which preferentially cool low-entropy gas into stars, raising the mean ICM entropy It is widely recognized that feedback from AGNs reduces the cooling rate within cluster cores, preventing massive cooling flows. Such feedback after cluster collapse provides another potential mechanism for raising the entropy of the ICM (Voit 2005)
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