The GEMS project: X-ray analysis and statistical properties of the group sample

By J. Osmond, Trevor J. Ponman. 2004

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The Group Evolution Multiwavelength Study (GEMS) involves a multiwavelength study of a sample of 60 galaxy groups, chosen to span a wide range of group properties. Substantial ROSAT Position Sensitive Proportional Counter (PSPC) observations, available for all of these groups, are used to characterize the state of the intergalactic medium in each. We present the results of a uniform analysis of these ROSAT data and a statistical investigation of the relationship between X-ray and optical properties across the sample. Our analysis improves in several respects on previous work: (i) we distinguish between systems in which the hot gas is a group-scale medium and those in which it appears to be just a hot halo associated with a central galaxy; (ii) we extrapolate X-ray luminosities to a fixed overdensity radius (r 500 ) using fitted surface brightness models, in order to avoid biases arising from the fact that cooler systems are detectable to smaller radii, and (iii) optical properties have been rederived in a uniform manner from the NASA Extragalactic Database, rather than relying on the data in the disparate collection of group catalogues from which our systems are drawn. The steepening of the L X -T X relation in the group regime reported previously is not seen in our sample, which fits well on to the cluster trend, albeit with large non-statistical scatter. A number of biases affect the fitting of regression lines under these circumstances, and until the impact of these has been thoroughly investigated it seems best to regard the slope of the group L X -T X relation as being poorly determined. A significant problem in comparing the properties of groups and clusters is the derivation of system radii, to allow different systems to be compared within regions having the same overdensity. We find evidence that group velocity dispersion (σ v ) provides a very unreliable measure of system mass (and hence radius), with a number of groups having remarkably low values of σ v , given that they appear from their X-ray properties to be collapsed systems. We confirm that the surface brightness profiles of groups are significantly flatter than those of clusters - the maximum value of the β fit parameter for our sample is 0.58, lower than the typical value of 0.67 seen in clusters - however, we find no significant tendency within our sample for cooler groups to show flatter profiles. This result is inconsistent with simple universal pre-heating models. The morphology of the galaxies in the GEMS groups is correlated to their X-ray properties in a number of ways: we confirm the very strong relationship between X-ray emission and a dominant early-type central galaxy, which has been noted since the early X-ray studies of groups, and also find that spiral fraction is correlated with the temperature of the hot gas and hence the depth of the gravitational potential. A class of spiral-rich groups with little or no X-ray emission probably corresponds to groups that have not yet fully collapsed.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics, Statistical and Nonlinear Physics

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Citations in the corpus

Properties of galaxy groups in the Sloan Digital Sky Survey – I. The dependence of colour, star formation and morphology on halo mass (2005, background): “For example, X-ray observations show that haloes with pronounced X-ray emission contain virtually always an early-type central galaxy (e.g. Ebeling, Voges & Böhringer 1994; Osmond & Ponman 2004).”
Modelling the galaxy bimodality: shutdown above a critical halo mass (2006, background): “…rate (Kauffmann et al. 2003), (d) the gasto-stellar mass ratio (Kannappan 2004), (e) the bulge-todisc ratio (Kauffmann et al. 2003), (f) the environment den- sity (Blanton et al. 2006). and (g) the presence of an X-ray halo (Helsdon & Ponman 2003; Mathews & Brighenti 2003; Osmond & Ponman 2004).”
Simulations of AGN Feedback in Galaxy Clusters and Groups: Impact on Gas Fractions and the LX-T Scaling Relation (2008, background): “Data from a number of observational X–ray studies is shown for comparison (Horner 2001; Helsdon & Ponman 2000; Osmond & Ponman 2004; Arnaud & Evrard 1999; Markevitch 1998).”
The BAHAMAS project: Calibrated hydrodynamical simulations for large-scale structure cosmology (2016, background): “16 we compare the predicted X-ray luminosity−halo mass (left panel) and X-ray luminosity−temperature (right panel) relations with that of local X-ray-selected groups and clusters (Osmond & Ponman 2004; Vikhlinin et al. 2009; Pratt et al. 2009; Lovisari, Reiprich, & Schellenberger 2015).”
Galaxy cluster X-ray luminosity scaling relations from a representative local sample (REXCESS) (2008, background): “Better constraints in the group regime are still required, especially in the light of the increased scatter apparently seen there (Osmond & Ponman 2004; but see Sun et al. 2008).”
Towards a realistic population of simulated galaxy groups and clusters (2013, methodology): “In particular, the Sun et al. (2009) X-ray-selected sample (black diamonds in the left panel) has a significantly higher mean luminosity than the Osmond & Ponman (2004) X-ray-selected sample (black semi-circles in the left panel), the Rozo et al. (2009) optically-selected sample (black lines in the…”
Galaxy bimodality due to cold flows and shock heating (2004, background): “Group properties have a transition near σv ∼ 140 kms−1 (Helsdon & Ponman 2003; Osmond & Ponman 2004).”
Simulating the effect of active galactic nuclei feedback on the metal enrichment of galaxy clusters (2009, background): “Its slope at the scaleof clusters is observed to beLX ∝ T α with α ≃ 2.5–3 (e.g., Horner 2001; Pratt et al. 2009), and possibly even steeper or with a larger scatter at the scale of galaxy groups (e.g., Osmond & Ponman 2004).”
Scaling Properties of a Complete X-ray Selected Galaxy Group Sample (2014, background): “The observed slope of our group sample is consistent within the errors with that of the clusters and in general with the results from other papers investigating galaxy groups (e.g., Osmond & Ponman 2004; Shang & Scharf 2009; Eckmiller et al. 2011).”
MASSIVE HALOS IN MILLENNIUM GAS SIMULATIONS: MULTIVARIATE SCALING RELATIONS (2009, result): “While adiabatic simulations match the self-similar prediction, L ∼ T 2, for the X-ray luminosity– temperature relation, observations show a steeper slope (Arnaud & Evrard 1999; Osmond & Ponman 2004).”
The case for AGN feedback in galaxy groups (2009, methodology): “For comparison, we show in the left panel the ROSAT measurements of Helsdon & Ponman (2000) and Mulchaey et al. (2003) (temperatures uncorrected) and in the right panel the ROSAT measurements of Osmond & Ponman (2004) (cool core-corrected temperatures).”
Gravitational quenching in massive galaxies and clusters by clumpy accretion (2007, background): “In particular, the observed scaling with dispersion velocity (Osmond & Ponman 2004) can be translated to Lx ≃ 2 × 1042 M0.”