Ground-based monitoring of comet 67P/Churyumov-Gerasimenko gas activity throughout the <i>Rosetta</i> mission

Simultaneously to the ESA Rosetta mission, a world-wide ground-based campaign provided measurements of the large scale activity of comet 67P/Churyumov-Gerasimenko through measurement of optically active gas species and imaging of the overall dust coma. We present more than two years of observations performed with the FORS2 low resolution spectrograph at the VLT, TRAPPIST, and ACAM at the WHT. We focus on the evolution of the CN production, as a tracer of the comet activity. We find that it is asymmetric with respect to perihelion and different from that of the dust. The CN emission is detected for the first time at 1.34 au pre-perihelion and production rates then increase steeply to peak about two weeks after perihelion at (1.00±0.10) ×1025 molecules s−1, while the post-perihelion decrease is more shallow. The evolution of the comet activity is strongly influenced by seasonal effects, with enhanced CN production when the Southern hemisphere is illuminated

The Intracluster Medium in z > 1 Galaxy Clusters

The Chandra X-ray Observatory was used to obtain a 190 ks image of three high redshift galaxy clusters in one observation. The results of our analysis of these data are reported for the two z > 1 clusters in this Lynx field, including the most distant known X-ray selected cluster. Spatially-extended X-ray emission was detected from both these clusters, indicating the presence of hot gas in their intracluster media. A fit to the X-ray spectrum of RX J0849+4452, at z=1.26, yields a temperature of kT = 5.8^{+2.8}_{-1.7} keV. Using this temperature and the assumption of an isothermal sphere, the total mass of RX J0849+4452 is found to be 4.0^{+2.4}_{-1.9} X 10^{14} h_{65}^{-1} M_{\sun} within r = 1 h_{65}^{-1} Mpc. The T_x for RX J0849+4452 approximately agrees with the expectation based on its L_{bol} = 3.3^{+0.9}_{-0.5} X 10^{44}$erg s$^{-1} according to the low redshift L_x - T_x relation. The very different distributions of X-ray emitting gas and of the red member galaxies in the two z > 1 clusters, in contrast to the similarity of the optical/IR colors of those galaxies, suggests that the early-type galaxies mostly formed before their host clusters.Comment: 4 pages in emulateapj style plus 2 color jpegs for Figure 3. Accepted by The Astrophysical Journa

Hubble Space Telescope Imaging in the Chandra Deep Field South: III. Quantitative Morphology of the 1Ms Chandra Counterparts and Comparison with the Field Population

We present quantitative morphological analyses of 37 HST/WFPC2 counterparts of X-ray sources in the 1 Ms Chandra Deep Field-South (CDFS). We investigate: 1) 1-D surface brightness profiles via isophotal ellipse fitting; 2) 2-D, PSF- convolved, bulge+disk+nucleus profile-fitting; 3) asymmetry and concentration indices compared with all ~3000 sources in our three WFPC2 fields; and 4) near- neighbor analyses comparing local environments of X-ray sources versus the field control sample. Significant nuclear point-source optical components appear in roughly half of the resolved HST/WFPC2 counterparts, showing a narrow range of F_X/F_{opt,nuc} consistent with the several HST-unresolved X-ray sources (putative type-1 AGN) in our fields. We infer roughly half of the HST/WFPC2 counterparts host unobscured AGN, which suggests no steep decline in the type-1/type-2 ratio out to the redshifts z~0.5-1 typical of our sources. The concentration indices of the CDFS counterparts are clearly larger on average than those of the field distribution, at 5-sigma, suggesting that the strong correlation between central black hole mass and host galaxy properties (including concentration index) observed in nearby galaxies is already evident by z~0.5-1. By contrast, the asymmetry index distribution of the 21 resolved CDFS sources at I<23 is indistinguishable from the I<23 field. Moreover, the frequency of I<23 near neighbors around the CDFS counterparts is not significantly different from the field sample. These results, combined with previous similar findings for local samples, suggest that recent merger/ interaction history is not a good indicator of AGN activity over a substantial range of look-back time.Comment: 30 pages, incl. 8 figures; accepted for publication in the Astrophysical Journa

Radio Signatures of HI at High Redshift: Mapping the End of the ``Dark Ages''

The emission of 21-cm radiation from a neutral intergalactic medium (IGM) at high redshift is discussed in connection with the thermal and ionization history of the universe. The physical mechanisms that make such radiation detectable against the cosmic microwave background include Ly_alpha coupling of the hydrogen spin temperature to the kinetic temperature of the gas and preheating of the IGM by the first generation of stars and quasars. Three different signatures are investigated in detail: (a) the fluctuations in the redshifted 21-cm emission induced by the gas density inhomogeneities that develop at early times in cold dark matter (CDM) dominated cosmologies; (b) the sharp absorption feature in the radio sky due to the rapid rise of the Ly_alpha continuum background that marks the birth of the first UV sources in the universe; and (c) the 21-cm emission and absorption shells that are generated on several Mpc scales around the first bright quasars. Future radio observations with projected facilities like the Giant Metrewave Radio Telescope and the Square Kilometer Array may shed light on the power spectrum of density fluctuations at z>5, and map the end of the "dark ages", i.e. the transition from the post-recombination universe to one populated with radiation sources.Comment: LateX, 19 pages, 5 figures, significantly revised version to be published in the Ap

Detection of the Entropy of the Intergalactic Medium: Accretion Shocks in Clusters, Adiabatic Cores in Groups

The thermodynamics of the diffuse, X-ray emitting gas in clusters of galaxies is linked to the entropy level of the intra cluster medium. In particular, models that successfully reproduce the properties of local X-ray clusters and groups require the presence of a minimum value for the entropy in the center of X-ray halos. Such a minimum entropy is most likely generated by non-gravitational processes, in order to produce the observed break in self-similarity of the scaling relations of X-ray halos. At present there is no consensus on the level, the source or the time evolution of this excess entropy. In this paper we describe a strategy to investigate the physics of the heating processes acting in groups and clusters. We show that the best way to extract information from the local data is the observation of the entropy profile at large radii in nearby X-ray halos (z~0.1), both at the upper and lower extremes of the cluster mass scale. The spatially and spectrally resolved observation of such X-ray halos provides information on the mechanism of the heating. We demonstrate how measurements of the size of constant entropy (adiabatic) cores in clusters and groups can directly constrain heating models, and the minimum entropy value. We also consider two specific experiments: the detection of the shock fronts expected at the virial boundary of rich clusters, and the detection of the isentropic, low surface-brightness emission extending to radii larger than the virial ones in low mass clusters and groups. Such observations will be a crucial probe of both the physics of clusters and the relationship of non-gravitational processes to the thermodynamics of the intergalactic medium.Comment: ApJ accepted, 31 pages including 8 figures. Important material added; references update

Demography of obscured and unobscured AGN: prospects for a Wide Field X-ray Telescope

We discuss some of the main open issues in the evolution of Active Galactic Nuclei which can be solved by the sensitive, wide area surveys to be performed by the proposed Wide Field X-ray Telescope mission.Comment: Proceedings of "The Wide Field X-ray Telescope Workshop", held in Bologna, Italy, Nov. 25-26 2009. To appear in Memorie della Societa' Astronomica Italiana 2010 (arXiv:1010.5889

Chandra and XMM-Newton Observations of RDCS1252.9-2927, A Massive Cluster at z=1.24

We present deep Chandra and XMM obervations of the galaxy cluster RDCS1252.9-2927, which was selected from the ROSAT Deep Cluster Survey (RDCS) and confirmed by extensive spectroscopy with the VLT at redshift z=1.237. With the Chandra data, the X-ray emission from the intra-cluster medium is well resolved and traced out to 500 kpc, thus allowing a measurement of the physical properties of the gas with unprecedented accuracy at this redshift. We detect a clear 6.7 keV Iron K line in the Chandra spectrum providing a redshift within 1% of the spectroscopic one. By augmenting our spectroscopic analysis with the XMM data (MOS detectors only), we significantly narrow down the 1 sigma error bar to 10% for the temperature and 30% for the metallicity, with best fit values kT = 6.0(+0.7,-0.5) keV, Z = 0.36(+0.12,-0.10) Z_sun. In the likely hypothesis of hydrostatic equilibrium, we measure a total mass of M_{500} = (1.9+-0.3)10^14 h_70^{-1}M_sun within R_{Delta=500} = 536 kpc. Overall, these observations imply that RDCS1252.9-2927 is the most X-ray luminous and likely the most massive bona-fide cluster discovered to date at z>1. When combined with current samples of distant clusters, these data lend further support to a mild evolution of the cluster scaling relations, as well the metallicity of the intra-cluster gas. Inspection of the cluster mass function in the current cosmological concordance model (h,Omega_m,Omega_Lambda)=(0.7,0.3,0.7) and sigma_8=0.7-0.8 shows that RDCS1252.9-2927 is an M* cluster at z=1.24, in keeping with number density expectations in the RDCS survey volume.Comment: 9 pages, 1 color figure (fig6.jpg). The Astronomical Journal in press (Jan 2004). Full resolution preprint available at http://www.eso.org/~prosati/RDCS1252