The L_X-T relation and intracluster gas fractions of X-ray clusters
Abstract
We re-examine the X-ray luminosity-temperature relation using a nearly homogeneous data set of 24 clusters selected for statistically accurate temperature measurements and absence of strong cooling flows. The data exhibit a remarkably tight power-law relation between bolometric luminosity and temperature with a slope 2.88 +/- 0.15. With reasonable assumptions regarding cluster structure, we infer an upper limit on fractional variations in the intracluster gas fraction <(delta f_gas f_gas)^2 >^1/2 <= 15 per cent. A strictly homogeneous Ginga subset of 18 clusters places a more stringent limit of 9 per cent. Imaging data from the literature are employed to determine absolute values of f_gas within spheres encompassing density contrasts delta_c=500 and 200 with respect to the critical density. Comparing binding mass estimates based on the virial theorem (VT) and the hydrostatic beta-model (BM), we find a temperature-dependent discrepancy in f_gas between the two methods caused by systematic variation of the outer slope parameter beta with temperature. Mean values (for H_0=50 km s^-1 Mpc^-1) range from f_gas=0.10 for cool (T<4 keV) clusters using the VT at delta_c=500 to 0.22 for hot (T>4 keV) clusters using the BM at delta_c=200. There is evidence that cool clusters have a lower mean gas fraction than hot clusters, but it is not possible to assess the statistical significance of this effect in the present data set. The T dependence of the intracluster medium (ICM) density structure, coupled with the increase of the gas fraction with T in the VT approach, explains the steepening of the L_X-T relation. The small variation about the mean gas fraction within this majority subpopulation of clusters presents an important constraint for theories of galaxy formation and supports arguments against an Einstein-de Sitter universe based on the population mean gas fraction and conventional, primordial nucleosynthesis. The apparent trend of lower gas fractions and more extended atmospheres in low-temperature systems is consistent with expectations of models incorporating the effects of galactic winds on the ICM.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- May 1999
- DOI:
- 10.1046/j.1365-8711.1999.02442.x
- arXiv:
- arXiv:astro-ph/9806353
- Bibcode:
- 1999MNRAS.305..631A
- Keywords:
-
- GALAXIES: CLUSTERS: GENERAL;
- INTERGALACTIC MEDIUM;
- COSMOLOGY: OBSERVATIONS;
- COSMOLOGY: THEORY;
- DARK MATTER;
- X-RAYS: GALAXIES;
- Astrophysics
- E-Print:
- 11 pages, 4 figures, uses mn.sty and epsf.sty, accepted for publication in MNRAS