arXiv subjects

C. Norman

Publications and source records attributed to C. Norman.

At least 55 records · Page 3Linked to original sources

arXiv2007

COSMOS : Hubble Space Telescope Observations

The Cosmic Evolution Survey (COSMOS) was initiated with an extensive allocation (590 orbits in Cycles 12-13) using the Hubble Space Telescope (HST) for high resolution imaging. Here we review the characteristics of the HST imaging with the Advanced Camera for Surveys (ACS) and parallel observations with NICMOS and WFPC2. A square field (1.8$\sq$°) has been imaged with single-orbit ACS I-F814W exposures with 50% completeness for sources 0.5\arcsec in diameter at I$_{AB} $ = 26.0 mag. The ACS imaging is a key part of the COSMOS survey, providing very high sensitivity and high resolution (0.09\arcsec FWHM, 0.05\arcsec pixels) imaging and detecting 1.2 million objects to a limiting magnitude of 26.5 (AB). These images yield resolved morphologies for several hundred thousand galaxies. The small HST PSF also provides greatly enhanced sensitivity for weak lensing investigations of the dark matter distribution.

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arXiv2007

Tracing the evolution in the iron content of the ICM

We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies at z>0.3, which cover a temperature range 3 0.5. The emission weighted iron abundance is significantly higher (Z_{Fe}~0.4 Z_o) in the redshift range z~0.3-0.5, approaching the value measured locally in the inner 0.15 R_{vir} radii for a mix of cool-core and non cool-core clusters at 0.1<z<0.3. The decrease in metallicity with redshift can be parametrized by a power law of the form ~(1+z)^{-1.25}. The observed evolution implies that the average iron content of the ICM at present epoch is a factor of ~2 larger than at z~1.2. We confirm that the ICM is already significantly enriched (Z_{Fe} ~ 0.25 Z_o) at a look-back time of 9 Gyr. Our data provide significant constraints on the time scales and physical processes which drive the chemical enrichment of the ICM. (abridged)

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arXiv2006

Most hard X-ray selected quasars in Chandra Deep Fields are obscured

Measuring the population of obscured quasars is one of the key issues to understand the evolution of active galactic nuclei (AGNs). With a redshift completeness of 99%, the X-ray sources detected in Chandra Deep Field South (CDF-S) provide the best sample for this issue. In this letter we study the population of obscured quasars in CDF-S by choosing the 4 -- 7 keV selected sample, which is less biased by the intrinsic X-ray absorption. The 4 -- 7 keV band selected samples also filter out most of the X-ray faint sources with too few counts, for which the measurements of N_H and L_X have very large uncertainties. Simply adopting the best-fit L_2-10keV and N_H, we find 71% (20 out of 28) of the quasars (with intrinsic L_2-10keV > 10^44 erg/s) are obscured with N_H > 10^22 cm^-2. Taking account of the uncertainties in the measurements of both N_H and L_X, conservative lower and upper limits of the fraction are 54% (13 out 24) and 84% (31 out 37). In Chandra Deep Field North, the number is 29%, however, this is mainly due to the redshift incompleteness. We estimate a fraction of ~ 50% - 63% after correcting the redshift incompleteness with a straightforward approach. Our results robustly confirm the existence of a large population of obscured quasars.

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arXiv2006

Tracing the evolution in the iron content of the ICM

We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies at z>0.3, which cover a temperature range of 3>kT>15 keV. Our analysis is aimed at measuring the iron abundance in the ICM out to the highest redshift probed to date. We find that the emission-weighted iron abundance measured within (0.15-0.3)R_vir in clusters below 5 keV is, on average, a factor of ~2 higher than in hotter clusters, following Z(T)~0.88T^-(0.47)Z_o, which confirms the trend seen in local samples. We made use of combined spectral analysis performed over five redshift bins at 0.3>z>1.3 to estimate the average emission weighted iron abundance. We find a constant average iron abundance Z_Fe~0.25Z_o as a function of redshift, but only for clusters at z>0.5. The emission-weighted iron abundance is significantly higher (Z_Fe~0.4Z_o) in the redshift range z~0.3-0.5, approaching the value measured locally in the inner 0.15R_vir radii for a mix of cool-core and non cool-core clusters in the redshift range 0.1<z<0.3. The decrease in Z_Fe with redshift can be parametrized by a power law of the form ~(1+z)^(-1.25). The observed evolution implies that the average iron content of the ICM at the present epoch is a factor of ~2 larger than at z=1.2. We confirm that the ICM is already significantly enriched (Z_Fe~0.25Z_o) at a look-back time of 9 Gyr. Our data provide significant constraints on the time scales and physical processes that drive the chemical enrichment of the ICM.

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arXiv2006

Single star progenitors of long gamma-ray bursts I: Model grids and redshift dependent GRB rate

We present grids of massive star evolution models at four different metallicities (Z=0.004, 0.002, 0.001, 0.00001). The effects of rotation on the stellar structure and the transport of angular momentum and chemical elements through the Spruit-Tayler dynamo and rotationally induced instabilities are considered. After discussing uncertainties involved with the adopted physics, we elaborate the final fate of massive stars as a function of initial mass and spin rate, at each considered metallicity. In particular, we investigate for which initial conditions long gamma-ray bursts (GRBs) are expected to be produced in the frame of the collapsar model. Then, using an empirical spin distribution of young massive metal-poor stars and a specified metallicity-dependent history of star-formation, we compute the expected GRB rate as function of metallicity and redshift based on our stellar evolution models. The GRB production in our models is limited to metallicities of Z \lsim 0.004, with the consequence that about 50 % of all GRBs are predicted to be found at redshifts above z = 4, with most supernovae occurring at redshifts below z\simeq 2.2. The average GRB/SN ratio predicted by our model is about 1/200 globally, and 1/1250 at low redshift. Future strategies for testing the considered GRB progenitor scenario are briefly discussed.

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arXiv2006

X-ray spectral properties of AGN in the Chandra Deep Field South

We present a detailed X-ray spectral analysis of the sources in the 1Ms catalog of the Chandra Deep Field South (CDFS) taking advantage of optical spectroscopy and photometric redshifts for 321 sources. As a default spectral model, we adopt a power law with slope Gamma with an intrinsic redshifted absorption N_H, a fixed Galactic absorption and an unresolved Fe emission line. For 82 X-ray bright sources, we perform the X-ray spectral analysis leaving both Gamma and N_H free. The weighted mean value is ~ 1.75+-0.02, with an intrinsic dispersion of sigma~0.30. We do not find hints of a correlation between Gamma and the N_H. We detect the presence of a scattered component at soft energies in 8 sources, and a pure reflection spectrum, typical of Compton-thick AGN, in 14 sources (Compton-thick AGN candidates). The intrinsic N_H distribution shows a lognormal shape, peaking around log(N_H)~23.1 and with sigma~1.1. We find that the fraction of absorbed sources (with N_H>10^{22} cm^{-2}) in the sample is constant (at the level of about 75%) or moderately increasing with redshift. Finally, we compare the optical classification to the X-ray spectral properties, confirming that the correspondence of unabsorbed (absorbed) X-ray sources to optical Type I (Type II) AGN is accurate for at least 80% of the sources with spectral identification (1/3 of the total X-ray sample).

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arXiv2005

Relativistic outflow in CXO CDFS J033260.0-274748

In this letter we report the detection of a strong and extremely blueshifted X-ray absorption feature in the 1 Ms Chandra spectrum of CXO CDFS J033260.0-274748, a quasar at z = 2.579 with L_2-10keV ~ 4x10^44 ergs/s. The broad absorption feature at ~ 6.3 keV in the observed frame can be fitted either as an absorption edge at 20.9 keV or as a broad absorption line at 22.2 keV rest frame. The absorber has to be extremely ionized with an ionization parameter ξ~ 10^4, and a high column density N_H >5x10^23 cm^-2. We reject the possibility of a statistical or instrumental artifact. The most likely interpretation is an extremely blueshifted broad absorption line or absorption edge, due to H or He--like iron in a relativistic jet-like outflow with bulk velocity of ~ 0.7-0.8 c. Similar relativistic outflows have been reported in the X-ray spectra of several other AGNs in the past few years.

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arXiv2005

The Chandra Deep Field South/GOODS survey. Optically faint X-ray sources

We provide important new constraints on the nature and redshift distribution of optically faint (R>25) X-ray sources in the Chandra Deep Field South Survey. We show that we can derive accurate photometric redshifts for the spectroscopically unidentified sources thus maximizing the redshift completeness for the whole X-ray sample. Our new redshift distribution for the X-ray source population is in better agreement with that predicted by X-ray background synthesis models; however, we still find an overdensity of low redshift (z<1) sources. The optically faint sources are mainly X-ray absorbed AGN, as determined from direct X-ray spectral analysis and other diagnostics. Many of these optically faint sources have high (>10) X-ray-to-optical flux ratios. We also find that ~71% of them are well fitted with the SED of an early-type galaxy with ~1.9 and the remaining 29% with irregular or starburst galaxies mainly at z_phot>3. We estimate that 23% of the optically faint sources are X-ray absorbed QSOs. The overall population of X-ray absorbed QSOs contributes a ~15% fraction of the [2-10] keV X-ray Background (XRB) whereas current XRB synthesis models predict a ~38% contribution.

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arXiv2004

The spatial clustering of X-ray selected AGN and galaxies in the Chandra Deep Field South and North

We investigate the spatial clustering of X-ray selected sources in the two deepest X-ray fields to date, namely the 2Msec Chandra Deep Field North (CDFN) and the 1Msec Chandra Deep Field South (CDFS). The projected correlation function w(r_p), measured on scales ~0.2-10 h^-1 Mpc for a sample of 240 sources with spectroscopic redshift in the CDFN and 124 sources in the CDFS at a median redshift of z~0.8, is used to constrain the amplitude and slope of the real space correlation function xi(r)=(r/r0)^-gamma. The clustering signal is detected at high confidence (>~ 7 sigma) in both fields. The amplitude of the correlation is found to be significantly different in the two fields, the correlation length r0 being 8.6 +- 1.2 h^-1 Mpc in the CDFS and 4.2 +- 0.4 h^-1 Mpc in the CDFN, while the correlation slope gamma is found to be flat in both fields: gamma=1.33 +- 0.11 in the CDFS and gamma=1.42 +- 0.07 in the CDFN (a flat Universe with Omega_m=0.3 and Omega_L=0.7 is assumed; 1 sigma Poisson error estimates are considered). The correlation function has been also measured separately for sources classified as AGN or galaxies. In both fields AGN have a median redshift of z~0.9 and a median 0.5-10 keV luminosity of L_x~10^43 erg s^-1, i.e. they are generally in the Seyfert luminosity regime. As in the case of the total samples, we found a significant difference in the AGN clustering amplitude between the two fields, the best fit correlation parameters being r0=10.3 +- 1.7 h^-1 Mpc, gamma=1.33 +- 0.14 in the CDFS, and r0=5.5 +- 0.6 h^-1 Mpc, gamma=1.50 +- 0.12 in the CDFN. Within each field no statistically significant difference is found between soft and hard X-ray selected sources or between type 1 and type 2 AGN. (abridged)

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arXiv2004

The Chandra Deep Field South: Optical Spectroscopy I

We present the results of our spectroscopic follow-up program of the X-ray sources detected in the 942 ks exposure of the Chandra Deep Field South (CDFS). 288 possible counterparts were observed at the VLT with the FORS1/FORS2 spectrographs for 251 of the 349 Chandra sources (including three additional faint X-ray sources). Spectra and R-band images are shown for all the observed sources and R-K colours are given for most of them. Spectroscopic redshifts were obtained for 168 X-ray sources, of which 137 have both reliable optical identification and redshift estimate (including 16 external identifications). The R<24 observed sample comprises 161 X-ray objects (181 optical counterparts) and 126 of them have unambiguous spectroscopic identification. We introduce a new classification based solely on X-ray properties, HR and X-ray luminosity, consistent with the unified AGN model. This X-ray classification uncovers a large fraction of optically obscured, X-ray luminous AGNs missed by the classical optical classification.

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arXiv2004

Photometric Redshift of X-Ray Sources in the Chandra Deep Field South

Based on the photometry of 10 near-UV, optical, and near-infrared bands of the Chandra Deep Field South, we estimate the photometric redshifts for 342 X-ray sources, which constitute ~99% of all the detected X-ray sources in the field. The models of spectral energy distribution are based on galaxies and a combination of power-law continuum and emission lines. Color information is useful for source classifications: Type-I AGN show non-thermal spectral features that are distinctive from galaxies and Type-II AGN. The hardness ratio in X-ray and the X-ray-to-optical flux ratio are also useful discriminators. Using rudimentary color separation techniques, we are able to further refine our photometric redshift estimations. Among these sources, 137 have reliable spectroscopic redshifts, which we use to verify the accuracy of photometric redshifts and to modify the model inputs. The average relative dispersion in redshift distribution is ~8%, among the most accurate for photometric surveys. The high reliability of our results is attributable to the high quality and broad coverage of data as well as the applications of several independent methods and a careful evaluation of every source. We apply our redshift estimations to study the effect of redshift on broadband colors and to study the redshift distribution of AGN. Our results show that both the hardness ratio and U-K color decline with redshift, which may be the result of a K-correction. The number of Type-II AGN declines significantly at z>2 and that of galaxies declines at z>1. However, the distribution of Type-I AGN exhibits less redshift dependence. As well, we observe a significant peak in the redshift distribution at z=0.6. We demonstrate that our photometric redshift estimation produces a reliable database for the study of X-ray luminosity of galaxies and AGN.

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arXiv2003

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.

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arXiv2003

Global asymptotic solutions for non-relativistic MHD jets and winds

We present general and global analytical solutions, valid from pole to equator, for the asymptotic structure of non-relativistic, rotating, stationary, axisymmetric, polytropic, unconfined, perfect MHD winds. The asymptotic structure of such winds consists of field-regions virtually devoid of poloidal current. We show that an Hamilton-Jacobi equation, or equivalently a Grad-Shafranov equation, gives the asymptotic structure in the field regions.These field regions are bordered by current-carrying boundary layers around the polar axis and near null magnetic surfaces. The solution is given in the form of matched asymptotics separately valid outside and inside these boundary layers. The polar boundary layer is pressure-supported against the pinching force exerted by the axial poloidal current and has the structure of a current pinch, while the null-surface boundary layers have the structure of current sheet pinches.For winds which are kinetic-energy dominated at infinity we derive WKBJ analytic solutions whose magnetic surfaces focus into paraboloids. The current slowly weakens as the inverse of the logarithm of the distance to the wind source while the axial plasma density falls-off as a negative power of this logarithm.For winds carrying Poynting flux at large distances the solutions asymptotically approach to nested cylindrical and conical magnetic surfaces.

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arXiv2003

Global asymptotic solutions for relativistic MHD jets and winds

We consider relativistic, stationary, axisymmetric, polytropic, unconfined, perfect MHD winds, assuming their five lagrangian first integrals to be known. The asymptotic structure consists of field-regions bordered by boundary layers along the polar axis and at null surfaces, such as the equatorial plane, which have the structure of charged column or sheet pinches supported by plasma or magnetic poloidal pressure. In each field-region cell, the proper current (defined here as the ratio of the asymptotic poloidal current to the asymptotic Lorentz factor) remains constant. Our solution is given in the form of matched asymptotic solutions separately valid outside and inside the boundary layers. An Hamilton-Jacobi equation, or equivalently a Grad-Shafranov equation, gives the asymptotic structure in the field-regions of winds which carry Poynting flux to infinity. An important consistency relation is found to exist between axial pressure, axial current and asymptotic Lorentz factor. We similarly derive WKB-type analytic solutions for winds which are kinetic-energy dominated at infinity and whose magnetic surfaces focus to paraboloids. The density on the axis in the polar boundary column is shown to slowly fall off as a negative power of the logarithm of the distance to the wind source. The geometry of magnetic surfaces in all parts of the asymptotic domain, including boundary layers, is explicitly deduced in terms of the first-integrals.

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arXiv2003

Kinetic energy flux versus Poynting flux in MHD winds and jets: the intermediate regime

We show that the formal asymptotic limit for all rotating polytropic axisymmetric perfect MHD flows is a kinetic energy dominated wind which collimates to paraboloids around the symmetry axis. We reach this result by showing that another, a priori possible, solution with finite Poynting flux can be excluded on the following physical grounds: (1) the wind velocity does not exceed the fast mode speed everywhere and (2) the circumpolar current increases with distance from the source.We show that asymptotic hoop stress collimation is mathematically robust and we give strong arguments why recent anti-collimation claims are not correct. However, in practice, due to the very slow logarithmic decline of the circumpolar current with increasing distance from the source, there is a broad intermediate regime with significant Poynting flux. This intermediate regime, rather than the mathematically exact asymptotic regime, may well apply along the finite length of the jet. We briefly discuss peculiarities that would be associated with Poynting jets in the intermediate regime. Force-free initial conditions in the near field are most likely to produce such jets, in which most of the energy flux is electromagnetic.

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arXiv2003

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.

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arXiv2003

A Puzzling X-Ray Source Found in the chandra Deep Field South

In this letter we report the detection of an extremely strong X-ray emission line in the 940ks chandra ACIS-I spectrum of CXO CDFS J033225.3-274219. The source was identified as a Type1 AGN at redshift of z = 1.617, with 2.0 -- 10.0 keV rest frame X-ray luminosity of ~ 10^44 ergs s^-1. The emission line was detected at 6.2^{+0.2}_{-0.1} keV, with an equivalent width (EW) of 4.4^{+3.2}_{-1.4} keV, both quantities referring to the observed frame. In the rest frame, the line is at 16.2^{+0.4}_{-0.3} keV with an EW of 11.5^{+8.3}_{-3.7} keV. An X-ray emission line at similar energy (~ 17 keV, rest frame) in QSO PKS 2149-306 was discovered before using ASCA data. We reject the possibility that the line is due to a statistical or instrumental artifact. The line is most likely due to blueshifted Fe-K emission from an relativistic outflow, probably an inner X-ray jet, with velocities of the order of ~ 0.6-0.7c. Other possible explanations are also discussed.

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arXiv2003

Iron abundance in the ICM at high redshift

We present the analysis of the X-ray spectra of 18 distant clusters of galaxies with redshift 0.3 < z < 1.3. Most of them were observed with the Chandra satellite in long exposures ranging from 36 ks to 180 ks. For two of the z>1 clusters we also use deep XMM-Newton observations. Overall, these clusters probe the temperature range 3 =0.8 is Z =0.25 +0.04 -0.06 Zo, consistent with the local canonical metallicity value, Z ~0.3 Zo, within 1sigma confidence level. Medium and low temperature clusters (kT <5 keV) tend to have larger iron abundances than hot clusters. At redshift ~1.2 (4 clusters at z>1) we obtain a statistically significant detection of the Fe-K line only in one cluster (Z>0.10 Zo at the 90% c.l.). Combining all the current data set from Chandra and XMM at z>1, the average metallicity is measured to be = 0.21 +0.10 -0.05 Zo (1sigma error), thus suggesting no evolution of the mean iron abundance out to z~1.2.

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