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

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

A Three-Year Program of Micro- and Nano-System Technology Development for X-Ray Astronomy

For many years the work at MIT aimed at the development of new concepts and technologies for space experiments in high-energy astrophysics, but not explicitly supported by flight programs, has been supported. This work has yielded new devices and techniques for X-ray astronomy, primarily low-noise, deep-depletion charge-coupled devices (CCDS) for spectrally-resolved X-ray imaging, and high-performance transmission gratings for high-resolution X-ray spectroscopy. Among the most significant recent achievements have been the development by G. Ricker and associates of the X-ray CCD camera flying on ASCA, and currently in development for AXAF and Astro-E, and the development by C. Canizares and associates of thick, 200 nm-period transmission gratings employing the phenomenon of phase shifting for high-resolution X-ray spectroscopy up to energies of 8- 1 0 keV that is essential for the operation of the AXAF High Energy Transmission Grating Spectrometer (HETGS). Through the current SR&T grant, the latter technology is now being extended successfully to the fabrication of 100 nm-period transmission gratings, which have twice the dispersion of the AXAF gratings. We note that, among other outcomes, the modest investments of past SR&T Grants at MIT resulted in the development of the key technologies for fully one-half of the scientific instrumentation on AXAF. In addition, NASA flight programs that have benefited from previous SR&T support at MIT include the SAS 3 X-ray Observatory, which carried the first rotation modulation collimator, the Focal Plane Crystal Spectrometer (FPCS) on the Einstein Observatory, the CCD cameras on ASCA and planned for Astro-E, the High Energy Transient Experiment (HETE), the Solar EUV Monitor on the Solar and Heliospheric Observatory (SOHO), the Medium Energy Neutral Atom imager (MENA) on the Image for Magnetopause-to-aurora Global Exploration (IMAGE) mission, and the recently-approved Two Wide-Angle Imaging Neutral-atom Spectrometers (TWINS) Mission of Opportunity

Are the Effects of Structure Formation Seen in the Central Metallicity of Galaxy Clusters?

A sample of 46 nearby clusters observed with Chandra is analyzed to produce radial density, temperature, entropy and metallicity profiles, as well as other morphological measurements. The entropy profiles are computed to larger radial extents than in previous Chandra cluster sample analyses. We find that the iron mass fraction measured in the inner 0.15 R500 shows a larger dispersion across the sample of low-mass clusters, than it does for the sample of high-mass clusters. We interpret this finding as the result of the mixing of more haloes in large clusters than in small clusters, which leads to an averaging of the metal content in the large clusters, and thus less dispersion of metallicity for high-mass clusters. This interpretation lends support to the idea that the low-entropy, metal-rich gas of merging haloes reaches clusters' centers, which explains observations of Core-Collapse Supernova products metallicity peaks, and which is seen in hydrodynamical simulations. The gas in these merging haloes would have to reach the centers of clusters without mixing in the outer regions, in order to support our interpretation. On the other hand, metallicity dispersion does not change with mass in the outer regions of clusters, suggesting that most of the outer metals come from a source with a more uniform metallicity level, such as during pre-enrichment. We also measure a correlation between the metal content in low-mass clusters and the degree to which their Intra-Cluster Medium (ICM) is morphologically disturbed, as measured by centroid shift. This suggests an alternative interpretation of the large width of the metallicity distribution in low-mass clusters, whereby a metallicity boost in the center of low-mass clusters is induced as a transitional state, during mergers.Comment: Accepted in ApJ, March 9, 201

The Evolution of Cluster Substructure with Redshift

Using Chandra archival data, we quantify the evolution of cluster morphology with redshift. To quantify cluster morphology, we use the power ratio method developed by Buote and Tsai (1995). Power ratios are constructed from moments of the two-dimensional gravitational potential and are, therefore, related to a cluster's dynamical state. Our sample will include 40 clusters from the Chandra archive with redshifts between 0.11 and 0.89. These clusters were selected from two fairly complete flux-limited X-ray surveys (the ROSAT Bright Cluster Sample and the Einstein Medium Sensitivity Survey), and additional high-redshift clusters were selected from recent ROSAT flux-limited surveys. Here we present preliminary results from the first 28 clusters in this sample. Of these, 16 have redshifts below 0.5, and 12 have redshifts above 0.5.Comment: 5 pages, 1 figure, corrected a reference, to appear in the proceeding of Multiwavelength Cosmology, ed. M. Plioni