X-Ray Evidence for Multiphase Hot Gas with Nearly Solar Fe Abundances in the Brightest Groups of Galaxies

We analyze the ASCA spectra accumulated within ~100 kpc radii of 12 of the brightest groups of galaxies. Upon fitting isothermal models (1T) jointly to the ASCA SIS and GIS spectra we obtain fits for most groups that are of poor or at best marginal quality and give very sub-solar metallicities similar to previous studies, = 0.29 +/- 0.12 Z_sun. Two-temperature models (2T) provide significantly better fits for 11 out of the 12 groups and in every case have metallicities that are substantially larger than obtained for the 1T models, = 0.75 +/- 0.24 Z_sun. Although not very well constrained, for most of the groups absorption in excess of the Galactic value is indicated for the cooler temperature component of the 2T models. A simple multiphase cooling flow model gives results analogous to the 2T models including large metallicities, = 0.65 +/- 0.17 Z_sun. The nearly solar Fe abundances and also solar alpha/Fe ratios indicated by the 2T and cooling flow models are consistent with models of the chemical enrichment of ellipticals, groups, and clusters which assume ratios of Type Ia to Type II supernova and an IMF similar to those of the Milky Way. Thus, we have shown that the very sub-solar Fe abundances and Si/Fe enhancements obtained from most previous studies within r ~100 kpc of galaxy groups are an artifact of their fitting isothermal models to the X-ray spectra which also has been recently demonstrated for the brightest elliptical galaxies. Owing to the importance of these results for interpreting X-ray spectra, in an appendix we use simulated ASCA observations to examine in detail the ``Fe bias'' and ``Si bias'' associated with the spectral fitting of ellipticals, groups, and clusters of galaxies.Comment: 26 pages (6 figures), To Appear in MNRAS. Revised version contains more discussion of abundance gradients (see new section 4.1

Omega_0 and Substructure in Galaxy Clusters

(Abridged) We examine the theoretical relationship between Omega_0 and substructure in galaxy clusters which are formed by the collapse of high density peaks in a gaussian random field. The radial mass distributions of the clusters are computed from the spherical accretion model using the adiabatic approximation following Ryden & Gunn. For a cluster of mass, M(r,t), we compute the quantity dM/M_bar at a cosmic time t and within a radius r, where dM is the accreted mass and M_bar is the average mass of the cluster during the previous relaxation time, which is computed individually for each cluster. For a real cluster in three dimensions we argue that dM/M_bar should be strongly correlated with the low order multipole ratios, Phi^{int}_l/Phi^{int}_0, of the potential due to matter interior to r. It is shown that the expected correlation between dM/M_bar and Phi^{int}_l/Phi^{int}_0 extends to the two-dimensional multipole ratios, Psi^{int}_m/Psi^{int}_0, which are well defined observables of the cluster density distribution. The strongest dependence of dM/M_bar on Omega_0 (lambda_0=0) occurs at z=0 where dM/M_bar propto Omega_0^{1/2} for relaxation times ~1-2 crossing times and only very weakly depends on mass and radius. The fractional accreted mass in CDM models with Omega_0+lambda_0=1 depends very weakly on Omega_0 and has a magnitude similar to the Omega_0=1 value. dM/M_bar evolves more rapidly with redshift in low-density universes and decreases significantly with radius for Omega_0=1 models for z > ~0.5. We discuss how to optimize constraints on Omega_0 and lambda_0 using cluster morphologies.Comment: 18 pages (11 figures), Accepted for publication in MNRAS. In revised version a new section 2.2 describes how to infer the fractional accreted mass (and hence Omega_0) from observation

X-ray Constraints on the Intrinsic Shape of the Lenticular Galaxy NGC 1332

We have analyzed ROSAT PSPC X-ray data of the optically elongated S0 galaxy NGC 1332 with the purposes of constraining the intrinsic shape of its underlying mass and presenting a detailed investigation of the uncertainties resulting from the assumptions underlying this type of analysis. The X-ray isophotes are elongated with ellipticity $0.10 - 0.27$ (90% confidence) for semi-major axes 75\arcsec -90\arcsec and have orientations consistent with the optical isophotes (ellipticity $\sim 0.43$). The spectrum is poorly constrained by the PSPC data and cannot rule out sizeable radial temperature gradients or an emission component due to discrete sources equal in magnitude to the hot gas. Using (and clarifying) the "geometric test" for dark matter, we determined that the hypothesis that mass-traces-light is not consistent with the X-ray data at 68% confidence and marginally consistent at 90% confidence independent of the gas temperature profile. Detailed modeling gives constraints on the ellipticity of the underlying mass of \epsilon_{mass} = 0.47 - 0.72 (0.31 - 0.83) at 68% (90%) confidence for isothermal and polytropic models. The total mass of the isothermal models within a=43.6 kpc (D = 20h^{-1}_{80} Mpc) is M_{tot} = (0.38 - 1.7) \times 10^{12}M_{\sun} (90% confidence) corresponding to total blue mass-to-light ratio \Upsilon_B = (31.9 - 143) \Upsilon_{\sun}. Similar results are obtained when the dark matter is fit directly using the known distributions of the stars and gas. When possible rotation of the gas and emission from discrete sources are included flattened mass distributions are still required, although the constraints on \epsilon_{mass}$, but not the total mass, are substantially weakened.Comment: 45 pages (figures missing), PostScript, to appear in ApJ on January 20, 199

X-ray Isophote Shapes and the Mass of NGC 3923

We present analysis of the shape and radial mass distribution of the E4 galaxy NGC 3923 using archival X-ray data from the ROSAT PSPC and HRI. The X-ray isophotes are significantly elongated with ellipticity e_x=0.15 (0.09-0.21) (90% confidence) for semi-major axis a\sim 10h^{-1}_70 kpc and have position angles aligned with the optical isophotes within the estimated uncertainties. Applying the Geometric Test for dark matter, which is independent of the gas temperature profile, we find that the ellipticities of the PSPC isophotes exceed those predicted if M propto L at a marginal significance level of 85% (80%) for oblate (prolate) symmetry. Detailed hydrostatic models of an isothermal gas yield ellipticities for the gravitating matter, e_mass=0.35-0.66 (90% confidence), which exceed the intensity weighted ellipticity of the R-band optical light, = 0.30 (e_R^max=0.39). We conclude that mass density profiles with rho\sim r^{-2} are favored over steeper profiles if the gas is essentially isothermal (which is suggested by the PSPC spectrum) and the surface brightness in the central regions (r<~15") is not modified substantially by a multi-phase cooling flow, magnetic fields, or discrete sources. We argue that these effects are unlikely to be important for NGC 3923. (The derived e_{mass} range is very insensitive to these issues.) Our spatial analysis also indicates that the allowed contribution to the ROSAT emission from a population of discrete sources with Sigma_x propto Sigma_R is significantly less than that indicated by the hard spectral component measured by ASCA.Comment: 14 pages (6 figures), To Appear in MNRA

Quantifying the Morphologies and Dynamical Evolution of Galaxy Clusters. I. The Method

We describe and test a method to quantitatively classify clusters of galaxies according to their projected morphologies. This method will be subsequently used to place constraints on cosmological parameters ($\Omega$ and the power spectrum of primordial fluctuations on scales at or slightly smaller than that of clusters) and to test theories of cluster formation. We specifically address structure that is easily discernible in projection and dynamically important to the cluster. The method is derived from the two-dimensional multipole expansion of the projected gravitational potential and yields dimensionless {\it power ratios} as morphological statistics. If the projected mass profile is used to characterize the cluster morphology, the power ratios are directly related to the cluster potential. However, since detailed mass profiles currently exist for only a few clusters, we use the X-ray--emitting gas as an alternative tracer of cluster morphology. In this case, the relation of the power ratios to the potential is qualitatively preserved. We demonstrate the feasibility of the method by analyzing simulated observations of simple models of X-ray clusters using the instrument parameters of the ROSAT PSPC. For illustrative purposes, we apply the method to ROSAT PSPC images of A85, A514, A1750, and A2029. These clusters, which differ substantially in their X-ray morphologies, are easily distinguished by their respective power ratios. We discuss the suitability of this method to address the connection between cluster morphology and cosmology and to assess whether an individual cluster is sufficiently relaxed for analysis of its intrinsic shape using hydrostatic methods. Approximately 50 X-ray observations of Abell clusters with the PSPC will be amenable to morphological analysis using the method of this paper.Comment: To appear in ApJ October 20, 1995. 29 pages (7 figures missing), PostScrip