Ph.D. DISSERTATION:

Université Denis Diderot (Paris VII) -
Institut d'Astrophysique de Paris (IAP) -
European Southern Observatory (ESO)
December 11th, 2003, Paris, France

Author: Ricardo Demarco
Advisors: Dr. Florence Durret (IAP) & Dr. Piero Rosati (ESO)

Physical properties and evolution of baryons in nearby and distant clusters of galaxies

ABSTRACT:   My Ph.D. work was concentrated on two topics. One is devoted to the study of thermodynamical properties and scaling relations in nearby clusters (Demarco et al. 2003a) and the other one is aimed at carrying out an unprecedented study on the cluster internal dynamics and spectro-photometric properties of galaxies in the two most distant massive clusters of galaxies known today in the southern sky (Demarco et al. 2003b).  Clusters of galaxies offer unique possibilities to understand the physics involved during the formation and later evolution of self-gravitating structures down to the present epoch. Moreover, they are ideal laboratories to study the effects of environment on the formation and evolution of galaxies. By pushing these studies to large redshifts, one can obtain essential information to understand the mode and epoch of formation of elliptical galaxies, which dominate present-day clusters. In what follows, I summarize the main conclusions of the two topics I have worked on.
   Nearby clusters (in collaboration with Dr. F. Durret): In this work I analyzed a sample of 24 galaxy clusters with 0.01 < z < 0.3, observed with the ROSAT PSPC. The main goal of this work was to test whether clusters of galaxies, considered as self-gravitating structures, share similar thermodynamical properties and scaling relations as those already observed in cluster ellipticals (Márquez et al. 2001).  By running a dedicated code (Magnard 2002) to model the X-ray surface brightness of clusters I was able to fit a Sérsic profile to the ICM gas distribution and derive temperature and dynamical mass profiles, as well as the total potential energy, specific entropy and mass for the gas and DM components. We found that: 1) the Sérsic law parameters (intensity, shape and scale) describing the X-ray gas emission are correlated two by two, with a strong correlation between the shape and scale parameters; 2) the hot gas in all these clusters roughly has the same integrated specific entropy, although a second order correlation between this integrated specific entropy and both the gas mass and the dynamical mass is observed; 3) a scaling law links the cluster potential energy to its total mass, with the same slope as that derived for elliptical galaxies (Márquez et al. 2001) and for dark matter halo simulations (Lanzoni 2000; Jang-Condell & Hernquist 2001). Comparable relations are obtained for the dark matter component. All these correlations are probably the consequence of the formation and evolution processes undergone by clusters of galaxies and suggest that ellipticals can be considered as scaled down versions of clusters.
   Distant clusters (in collaboration with Dr. P. Rosati): The aim of this ESO Large Programme (PI: P. Rosati) is to make the most extensive spectro-photometric survey of galaxies in the two distant massive clusters RXJ0152 at z=0.837 and RDCS1252 at z=1.237, selected from the ROSAT Deep Cluster Survey (Rosati et al. 1998), in order to provide stronger constraints on the epoch at and the way in which early type galaxies were formed, only possible by studying clusters at redshifts similar or greater than unity, and to firmly characterize the dynamical state of the clusters. Very deep wide-area near-IR imaging with the VLT and NTT (e.g. Lidman et al. 2003) and optical Keck imaging have been obtained, observations which are also supported by Chandra (e.g. Rosati et al. 2003) and HST+ACS data on both clusters. My work was mainly concentrated on the spectroscopic survey of these clusters carried out with FORS1/2 at the VLT. I took an active part on the target selection and mask design process as well as in some of the observing runs as visiting astronomer. The spectral data reduction was accomplished with the assistance of a dedicated software (Demarco 2003), specially developed to reduce FORS2 MOS data. The spectroscopic work yielded 78 secure cluster members for RJX0152 (Demarco et al., in prep.)  and 36 secure members for RDCS1252, the latter being the survey with the largest number of spectroscopically confirmed members in a z > 1 cluster. We observe significant substructure in RXJ0152, and the filamentary structure observed in its galaxy distribution agrees very well with the X-ray distribution (Maughan et al. 2003) of the ICM, which supports the picture of a cluster in a merging phase. Moreover, we observe, for the first time, red galaxies with on-going star formation in RXJ0152. This discovery points toward a complex star formation history of galaxies in clusters. By studying the CM diagram in RDCS1252 (Lidman et al. 2003) and its Luminosity Function (Toft et al. 2003), we set new constraints on the formation redshift of the early-type galaxies. Moreover, spectroscopy of the 10 brightest members shows for the first time a clear signature of a young (~ 1 Gyr) stellar population living in early-type galaxies in that cluster. The data reduction phase is completed and I am now actively involved in the interpretation of this large imaging and spectroscopic data set to test models of galaxy formation and evolution. Recent spectacular HST+ACS imaging of these two clusters will be crucial in this endeavor.

The French agency ABES keeps a record of my Ph.D. thesis that can be obtained through the French library system.