Isabella M. Gioia

Italian Laurea in Physics, University of Bologna, Italy, 1970

Research Interests

I work in extragalactic observational astronomy with particular interest in clusters of galaxies which, as the largest gravitationally bound structures known, provide clear constraints on the formation of structure and on the composition of the universe. My research is not confined to one wavelength but builds on observations in several domains from radio to optical and X-rays. I have been involved for many years in the construction and identification of complete samples of radio and X-ray sources to study the evolutionary properties of the different classes of astronomical objects (from AGN to clusters of galaxies) using their luminosity functions, source counts, etc. In the past I also investigated the role of distant clusters as gravitational lenses .

The best known result from my work is the discovery of the "negative" evolution of X-ray selected galaxy clusters (Gioia et al. 1990a) in the Einstein Extended Medium Sensitivity Survey (EMSS: Gioia et al. 1990b), recently confirmed by the ROSAT North Ecliptic Pole (NEP) survey data (Gioia et al. 2001) and by other independent complete samples of galaxy clusters (Mullis et al. 2004a). See the NEP SURVEY web page of Chris Mullis for a full description of the survey (also Mullis 2001 and Henry et al. 2006 ). Since the ROSAT NEP sample comes from a contiguous survey region it was possible also to characterize the three-dimensional large-scale structure of the universe by studying the cluster-cluster correlation function (Mullis et al. 2001) and the X-ray selected AGN spatial correlation function (Mullis et al. 2004b). Using telescopes on Mauna Kea, we have made spectroscopic observations to determine the nature and the redshifts of the ROSAT sources in the NEP region. Among the discoveries there are two clusters at z = 0.81, RXJ 1716+6708 which has a filamentary optical structure (Gioia et al. 1999), and RXJ 1821.6+6827, which was followed up with XMM-Newton to perform a detailed X-ray study reported in (Gioia et al. 2004). The most distant NEP quasar, RX J1759.4+6638, at a redshift of 4.320 was studied by (Henry et al. 1994) . The catalog of ROSAT NEP survey sources can be found in Gioia et al. 2003 and as an ascii data table online here: the ROSAT NEP source catalog . For updates and errata of the catalog click the documentation page of Chris Mullis here.

I continue to work on follow-up studies of EMSS clusters. Observations with XMM-Newton of the z = 0.83 cluster MS1054-0321 allowed us to correct previous determinations of the X-ray temperature for this object (Gioia et al. 2004).

As part of the PhD thesis of Marica Branchesi we studied the Radio Luminosity Function (RLF) of 18 NEP distant galaxy clusters with redshift betweeen z=0.3 and z=0.8 and found a change in the NEP RLF, both in slope and amplitude with respect to the local cluster samples, and also the absence of a break at log P_(1.4GHz) = 24.8 observed in nearby rich Abell clusters (Branchesi et al. 2006) . More recent work involves the analysis of Chandra archival observations to quantify the role of X-ray point sources in the inner regions of galaxy clusters Branchesi et al. 2007 , and how they affect X-ray observable relations like for instance the Lx-T. A different project on the changes of the Lx-T relationship with redshift led to an interesting and unexpected result. We adopted different statistical approaches to analyze the L_X-T relation for a sample of 40 clusters. The slope of the high-z cluster L_X-T is steeper (>= 3) than expected from the self-similar model predictions and steeper, even though still compatible within the errors, than the local L_X-T slope. The distant cluster L_X-T shows a significant evolution with respect to the local Universe: high-z clusters are more luminous than the local ones by a factor ~2 at any given temperature. The evolution with redshift of the L_X-T relation cannot be described by a single power law nor by the evolution predicted by the self-similar model in the full redshift range explored (0 < z < 1.3). We find a strong evolution, similar or stronger than the self-similar model, from z = 0 to z = 0.3 followed by a much weaker, if any, evolution at higher redshift. The weaker evolution is compatible with an increasing importance at high redshift of non-gravitational effects in the structure formation process (see Branchesi et al. 2007a and Branchesi et al. 2007b ).

Other past research projects focussed on the search for X-ray-selected Active Galactic Nuclei (Caccianiga et al. 2000) and BL Lac objects (Caccianiga et al. 2002) in pointed images of the ROSAT satellite (the REXs SURVEY or Radio-Emitting X-ray source survey). The resulting samples are instrumental in studying the statistical properties, the luminosity functions, and the cosmological evolution of BL Lac objects and AGNs, and will place new observational constraints on the current theoretical models of emission.

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Updated: Dec 2007