Understanding Galaxy Clusters: Chandra Symposium 2006 Insights

1. C. Jones, W. Forman, L. Van Speybroeck The cluster scaling relations observed by Chandra Chandra Fellows Symposium 2006

2. Introduction • Galaxy clusters are powerful cosmological probes • They are dominated by dark matter and their properties are sensitive to dark energy • Measurements of e.g. mass function, gas fraction and apparent evolution can place useful cosmological constraints • Provide independent constraints to e.g. CMB, SNIa with different degeneracies and systematics • Best cosmology with clusters requires mass estimates Chandra Fellows Symposium 2006

3. Measuring Cluster Masses • Clusters are ~85% dark matter and ~15% baryons • Baryons are dominated by hot X-ray emitting gas • Very luminous so can be detected to z~1.5 • Emissivity 2 so projection effects minimised Chandra Fellows Symposium 2006

4. Measuring Cluster Masses • Clusters are ~85% dark matter and ~15% baryons • Baryons are dominated by hot X-ray emitting gas • Very luminous so can be detected to z~1.5 • Emissivity 2 so projection effects minimised • Use X-rays to measure radial profiles of gas  and kT • Under assumption of hydrostatic equilibrium, solve for total gravitating mass • However, such detailed measurements require LOTS of photons – hard for less luminous / more distant clusters Chandra Fellows Symposium 2006

5. Scaling Relations • Current and future cluster surveys will detect 1000’s of clusters out to high z • Need efficient method to estimate masses from simple properties measured in survey data (e.g. kT, Lx) • Simple self-similar models predict tight scaling relations between basic cluster properties (e.g. LxkT2, MkT3/2) • Observations find relations do exist, but differ from SS predictions (e.g. LxkT3) • Indicates importance of non-gravitational processes Chandra Fellows Symposium 2006

6. Non-Gravitational Processes • Dense gas in cluster cores radiates efficiently and cools • Bright, cool cores in many clusters • Scaled temperature profiles from Vikhlinin et. al. (2006) Chandra Fellows Symposium 2006

7. Non-Gravitational Processes • Dense gas in cluster cores radiates efficiently and cools • Bright, cool cores in many clusters • Large amounts of very cool gas not detected in cores • Cooling balanced by energy input • Prime candidate: AGN activity, details uncertain • Mergers also important to cluster energetics Chandra Fellows Symposium 2006

8. Non-Gravitational Processes • Dense gas in cluster cores radiates efficiently and cools • Bright, cool cores in many clusters • Large amounts of very cool gas not detected in cores • Cooling balanced by energy input • Prime candidate: AGN activity, details uncertain • Mergers also important to cluster energetics • Study of scaling relations gives insight into when, where and how these processes affect cluster properties Chandra Fellows Symposium 2006

9. The Sample • 128 clusters observed with Chandra ACIS-I • Includes all such clusters at z>0.1 with published z Chandra Fellows Symposium 2006

10. Analysis Methods • Use blank sky bg files for spectral and imaging analysis • Measure kT within R500 and iterate until stable • Estimate R500 using Vikhlinin et. al. (2006) MT relation • R500 radius within which mean  is 500crit(z) • Generally exclude central region for kT and Lx measurements • Fit surface brightness profile with projected 3D emission measure profile (Vikhlinin et. al. 2006) • Modified -model with core component and steeper slope at large R • Derive gas density profile Chandra Fellows Symposium 2006

11. Gratuitous Eye Candy adaptively smoothed, 3Mpc per side, in order of z Chandra Fellows Symposium 2006

12. Gratuitous Eye Candy Drop z<0.1 for the purposes discussed here (leaves 111) adaptively smoothed, 3Mpc per side, in order of z Chandra Fellows Symposium 2006

13. The L-T Relation • Lx and kT with no correction for cool cores compared with Markevitch (1998) relation for local clusters • Predicted SS evolution removed • CC clusters: kT in core at least 1 cooler than external kT • N.B. local relation corrected for cool cores Chandra Fellows Symposium 2006

14. The L-T Relation • Plot fractional residuals from local relation against redshift without evolution correction • Solid line marks locus of expected SS evolution • Points should scatter about that Chandra Fellows Symposium 2006

15. The L-T Relation • Now exclude central 70kpc from Lx and central 0.15 R500 from kT measurements • Consistent with method for local relation Chandra Fellows Symposium 2006

16. The L-T Relation • Finally exclude central 0.15 R500 for Lx too • Scatter dominated by core properties • Expected SS evolution generally ok • Still some significant deviations from relation Chandra Fellows Symposium 2006

17. Gas Density Profiles • Compare most significantly deviant clusters with the non-deviant clusters • Plot gas density normalised to crit(z) and radius normalised to R500 (from kT with 0.15 R500 excluded) • Deviant clusters have high gas densities out to ~0.5R500 Chandra Fellows Symposium 2006

18. Deviant Clusters • Two deviants with best data • Mass profiles consistent with normal systems • Both have striking cold fronts – are these responsible? R500 ZW3146 (z=0.29) MS1455 (z=0.26) Chandra Fellows Symposium 2006

19. The Lx-Yx Relation • Yx = product of gas mass and temperature (Kravtsov et al. 2006) • Has more robust, low-scatter relation with total mass than other X-ray observables • kT in Yx is with central 0.15 R500 excluded • Lx is not CC corrected Chandra Fellows Symposium 2006

20. The Lx-Yx Relation • Excluding central 0.15 R500 for Lx gives very tight relation • Lowest scatter of all relations studied here • If Yx good mass proxy, implies Lx outside core is too Chandra Fellows Symposium 2006

21. The Lx-Yx Relation • Plot fractional residuals from local relation • Predicted evolution is strong, but matches data • Data suggests slightly weaker evolution? Chandra Fellows Symposium 2006

22. Summary and conclusions • Looked at scaling relations in large sample of Chandra clusters • Deviations/scatter dominated by cluster cores • Lx-kT deviations also revealed subset of clusters with elevated gas density out to large radii • Related to cool cores & cold fronts? • Surprisingly tight Lx-Yx relation outside core • Lx outside core good mass proxy? • Lx-kT deviations outside core due to low kT for M? • SS evolution obeyed in all relations • Possible weaker evolution in Lx-Yx? Chandra Fellows Symposium 2006