Giuseppe Cirimele

An X-Ray and Optical Study of Matter Distribution in Clusters of Galaxies

A sample of 12 Abell clusters of galaxies has been studied in the optical and X-ray bands. The optical data are derived from automatic photometry based on microdensitometric scans of Palomar Schmidt F-band plates. ROSAT PSPC data were obtained from the public archive. Galaxy density and X-ray luminosity profiles have been constructed and deprojected by parametric and nonparametric techniques to obtain volume density profiles. We find a relation between the gas density ρgas and the galaxy density ρgal consistent with the prediction of the hydrostatic isothermal model. We confirm that more than 70% of the cluster mass is dark within 1.5 Mpc h -->−150 from the cluster center. Outside 250 kpc we find a nearly constant M/LV ratio, which on average is 137 M☉/L☉, implying that the galaxy mass traces the total matter distribution. The dark mass-to-light ratio has a similar behavior. We also find a baryonic fraction fb ≈ 0.2, which, assumed as representative of the cosmic value and compared with nucleosynthesis calculations, constrains the cosmological parameter Ω0 to be smaller than 0.25.

The digitized first Byurakan survey - DFBS

Context. The First Byurakan Survey (FBS), also known as the Markarian Survey, is the largest low resolution spectroscopic survey of the sky and led to the discovery of 1500 UV-excess (UVX) galaxies and starburst galaxies. The FBS plates have also been used to search for UVX stellar objects, late-type stars, and for the identification of unusual infrared sources. Aims. The Digitized First Byurakan Survey (DFBS) provides the astronomical community with a digitized version of the FBS images and with the extracted spectra for the objects present in the plates. Methods. Nearly 2000 plates have been scanned and stored and programs were developed to compute the astrometric solution, extract the spectra, and apply wavelength and photometric calibration for the objects present in the plates. Results. The DFBS database and catalog of objects has been assembled. The DFBS database contains data for 20 000 000 objects present in the survey and provides tools for accessing the DFBS. Conclusions. New scientific projects as well as existing surveys will benefit by the digitized images and the ready-to-use extracted spectra which will allow an efficient computer-based analysis of the dataset.

The orientation of galaxies in clusters

We present a statistical analysis of directional data for the 10-20 brightest galaxies in a sample of 55 clusters. The data are derived from the digitization of Palomar 48 in. Schmidt telescope plates. We consider the orientation of clusters, of each galaxy, the direction of the line joining each galaxy with the cluster center or with the brightest galaxy, and all possible relative angles between these directions. We apply various statistical tests and we find no significant alignment for galaxies fainter than the two first ranked cluster members. We confirm a strong alignment of the brightest galaxy major axis with the long axis of the parent cluster

Matter Distribution in the Galaxy Clusters a 539 and A 2319

We performed a combined X-ray and optical analysis of the two clusters A539 and A2319, based on ROSAT PSPC 0.4-2.4 keV images of the public archive and F band photometry from microdensitometric scans of Palomar 48 inch plates (Trevese et al. 1992, A&AS, 94, 327). Assuming spherical symmetry and following the methods adopted in Cirimele, Nesci, and Trevese (1997, ApJ, 475, 11 (CNT97)) we derived the radial distribution of gas and galaxy densities p gas and p gal and we have computed the morphological parameter \( {{\beta }_{{xo}}} \equiv d\ln {{\rho }_{{gas}}}(r)/d\ln {{\rho }_{{gal}}}(r) \), introduced in CNT97. This allows to check the validity of the hydrostatic equilibrium condition, which reads \( {{\beta }_{{xo}}} \equiv d\ln {{\rho }_{{gas}}}(r)/d\ln {{\rho }_{{gal}}}(r) \), for an isotropic and uniform velocity distribution of r.m.s. dispersion σr. In the case of A539, adopting σR=629 km s−1 from Fadda et al. (1996, ApJ, 473, 670) and T=1.57 keV David et al. (1996, ApJ, 473, 692), we obtained marginally consistent values of βspec=1.54±0.50 and βxo=1.08±0.11. In the case of A2319 we took into account the presence of the secondary component A2319B (Oegerle et al. 1995, AJ, 110, 32) and the temperature gradient (Markevitch M. 1996, ApJ, 465, L1). The resulting radial increase of β spec is consistent with that of (βX0r) + d lnT(r)/d In pgal), suggesting that the hydrostatic equilibrium holds also in the presence of a temperature gradient. The radial distribution of the total binding mass, the mass in galaxies and intergalactic gas show that in both clusters the gas mass profile is steeper than galaxies and total masses consistently with our previous results (CNT97). Adopting a constant gas temperature, the relevant baryon fractions are larger than 20 %, adding new evidence to the “baryon catastrophe”. Taking into account the radial decrease of gas temperature, the baryon fraction is further increased. This implies that either Ω o < 0.25, or that large halos of dark matter surround galaxy clusters, as suggested by White & Fabian (1995, MNRAS, 273, 72).