arXiv subjects

C. M. Urry

Publications and source records attributed to C. M. Urry.

At least 73 records · Page 4Linked to original sources

arXiv2012

SMARTS optical and infrared monitoring of 12 gamma-ray bright blazars

We present multiwavelength data for twelve blazars observed from 2008-2010 as part of an ongoing optical-infrared photometric monitoring project. Sources were selected to be bright, southern (dec < 20 deg) blazars observed by the Fermi Gamma-Ray Space Telescope, with daily and weekly gamma-ray fluxes made available from the start of the Fermi mission. Light curves are presented for the twelve blazars in BVRJK at near-daily cadence. We find that optical and infrared fluxes are well correlated in all sources. Gamma-ray bright flat spectrum radio quasars (FSRQs) in our sample have optical/infrared emission correlated with gamma-rays consistent with inverse Compton-scattering models for GeV emission. In FSRQs, the variability amplitude decreases towards optical/IR wavelengths, consistent with the presence of a thermal emission component from the accretion disk varying on significantly longer timescales than the jet synchrotron emission. In BL Lac objects, variability is mainly constant across wavelengths, consistent with a weak or radiatively inefficient disk. FSRQs have redder optical-infrared colors when they are brighter, while BL Lac objects show no such trend. Several objects show complicated color-magnitude behavior: AO 0235+164 appears in two different states depending on whether it is gamma-ray bright or not. OJ 287 and 3C 279 show some hysteresis tracks in their color-magnitude diagrams. Individual flares may be achromatic or otherwise depart from the trend, suggesting different jet components becoming important at different times. We present a time-dependent spectral energy distribution of the bright FSRQ 3C 454.3 during its December 2009 flare, which is well fit by an external Compton model in the bright state, although day to day changes pose challenges to a simple one-zone model. All data from the SMARTS monitoring program are publicly available on our website.

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arXiv2011

Obscured GOODS AGN and Their Host Galaxies at z < 1.25: The Slow Black Hole Growth Phase

We compute black hole masses and bolometric luminosities for 87 obscured AGN in the redshift range 0.25 < z < 1.25, selected from the GOODS deep multi-wavelength survey fields via their X-ray emission. We fit the optical images and obtain morphological parameters for the host galaxy, separating the galaxy from its central point source, thereby obtaining a four-band optical SED for each active nucleus. We calculate bolometric luminosities for these AGN by reddening a normalized mean SED of GOODS broad-line AGN to match the observed central point-source SED of each obscured AGN. This estimate of Lbol has a smaller spread than simple bolometric corrections to the X-ray luminosity or direct integration of the observed multi-wavelength SED, suggesting it is a better measure. We estimate central black hole masses from the bulge luminosities. The black hole masses span a wide range, 7 x 10^6 M_sun to 6 x 10^9 M_sun; the median black hole mass is 5 x 10^8 M_sun. The majority of these AGN have L/L_Edd < 0.01, and we detect no significant evolution of the mean Eddington ratio to z = 1.25. This implies that the bulk of black hole growth in these obscured AGN must have occurred at z > 1 and that we are observing these AGN in a slow- or no-growth state.

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arXiv2010

A Flare in the Jet of Pictor A

A Chandra X-ray imaging observation of the jet in Pictor A showed a feature that appears to be a flare that faded between 2000 and 2002. The feature was not detected in a follow-up observation in 2009. The jet itself is over 150 kpc long and a kpc wide, so finding year-long variability is surprising. Assuming a synchrotron origin of the observed high-energy photons and a minimum energy condition for the outflow, the synchrotron loss time of the X-ray emitting electrons is of order 1200 yr, which is much longer than the observed variability timescale. This leads to the possibility that the variable X-ray emission arises from a very small sub-volume of the jet, characterized by magnetic field that is substantially larger than the average over the jet.

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arXiv2009

The Space Density of Compton-thick AGN

We constrain the number density and evolution of Compton-thick Active Galactic Nuclei (AGN), and their contribution to the extragalactic X-ray background. In the local Universe we use the wide area surveys from the Swift and INTEGRAL satellites, while for high redshifts we explore candidate selections based on mid-IR parameters. We present the properties of a sample of 211 heavily-obscured AGN candidates in the Extended Chandra Deep Field-South (ECDF-S) selecting objects with f24/fR>1000 and R-K>4.5. The X-ray to mid-IR ratios for these sources are significantly larger than that of star-forming galaxies and ~2 orders of magnitude smaller than for the general AGN population, suggesting column densities of NH>5x10^24 cm^-2. The space density of CT AGN at z~2 derived from these observations is ~10^-5 Mpc^{-3}, finding a strong evolution in the number of LX>10^44 erg/s sources from z=1.5 to 2.5.

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arXiv2009

Heavily Obscured AGN in Star-Forming Galaxies at z~2

We study the properties of a sample of 211 heavily-obscured Active Galactic Nucleus (AGN) candidates in the Extended Chandra Deep Field-South selecting objects with f_24/f_R>1000 and R-K>4.5. Of these, 18 were detected in X-rays and found to be obscured AGN with neutral hydrogen column densities of ~10^23 cm^-2. In the X-ray undetected sample, the following evidence suggests a large fraction of heavily-obscured (Compton Thick) AGN: (i) The stacked X-ray signal of the sample is strong, with an observed ratio of soft to hard X-ray counts consistent with a population of ~90% heavily obscured AGN combined with 10% star-forming galaxies. (ii) The X-ray to mid-IR ratios for these sources are significantly larger than that of star-forming galaxies and ~2 orders of magnitude smaller than for the general AGN population, suggesting column densities of N_H>5x10^24 cm^-2. (iii) The Spitzer near- and mid-IR colors of these sources are consistent with those of the X-ray-detected sample if the effects of dust self-absorption are considered. Spectral fitting to the rest-frame UV/optical light (dominated by the host galaxy) returns stellar masses of ~10^11 M_sun and =0.5, and reveals evidence for a significant young stellar population, indicating that these sources are experiencing considerable star-formation. This sample of heavily-obscured AGN candidates implies a space density at z~2 of ~10^-5 Mpc^-3, finding a strong evolution in the number of L_X>10^44 erg/s sources from z=1.5 to 2.5, possibly consistent with a short-lived heavily-obscured phase before an unobscured quasar is visible.

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arXiv2009

Multiwavelength monitoring of the enigmatic Narrow-Line Seyfert 1 PMN J0948+0022 in March-July 2009

Following the recent discovery of gamma rays from the radio-loud narrow-line Seyfert 1 galaxy PMN J0948+0022 (z=0.5846), we started a multiwavelength campaign from radio to gamma rays, which was carried out between the end of March and the beginning of July 2009. The source displayed activity at all the observed wavelengths: a general decreasing trend from optical to gamma-ray frequencies was followed by an increase of radio emission after less than two months from the peak of the gamma-ray emission. The largest flux change, about a factor of about 4, occurred in the X-ray band. The smallest was at ultraviolet and near-infrared frequencies, where the rate of the detected photons dropped by a factor 1.6-1.9. At optical wavelengths, where the sampling rate was the highest, it was possible to observe day-scale variability, with flux variations up to a factor of about 3. The behavior of PMN J0948+0022 observed in this campaign and the calculated power carried out by its jet in the form of protons, electrons, radiation and magnetic field are quite similar to that of blazars, specifically of flat-spectrum radio quasars. These results confirm the idea that radio-loud narrow-line Seyfert 1 galaxies host relativistic jets with power similar to that of average blazars.

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arXiv2009

Polarimetry and the High-Energy Emission Mechanisms in Quasar Jets

The emission mechanisms in extragalactic jets include synchrotron and various inverse-Compton processes. At low (radio through infrared) energies, it is widely agreed that synchrotron emission dominates in both low-power (FR I) and high-power (FR II and quasar) jets, because of the power-law nature of the spectra observed and high polarizations. However, at higher energies, the emission mechanism for high-power jets at kpc scales is hotly debated. Two mechanisms have been proposed: either inverse-Compton of cosmic microwave background photons or synchrotron emission from a second, high-energy population of electrons. Here we discuss optical polarimetry as a method for diagnosing the mechanism for the high-energy emission in quasar jets, as well as revealing the jet's three-dimensional energetic and magnetic field structure. We then discuss high-energy emission mechanisms for powerful jets in the light of the HST polarimetry of PKS 1136-135.

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arXiv2009

Galaxy Zoo Green Peas: Discovery of A Class of Compact Extremely Star-Forming Galaxies

We investigate a class of rapidly growing emission line galaxies, known as "Green Peas", first noted by volunteers in the Galaxy Zoo project because of their peculiar bright green colour and small size, unresolved in SDSS imaging. Their appearance is due to very strong optical emission lines, namely [O III] 5007 A, with an unusually large equivalent width of up to ~1000 A. We discuss a well-defined sample of 251 colour-selected objects, most of which are strongly star forming, although there are some AGN interlopers including 8 newly discovered narrow Line Seyfert 1 galaxies. The star-forming Peas are low mass galaxies (M~10^8.5 - 10^10 M_sun) with high star formation rates (~10 M_sun/yr), low metallicities (log[O/H] + 12 ~ 8.7) and low reddening (E(B-V) < 0.25) and they reside in low density environments. They have some of the highest specific star formation rates (up to ~10^{-8} yr^{-1}) seen in the local Universe, yielding doubling times for their stellar mass of hundreds of Myrs. The few star-forming Peas with HST imaging appear to have several clumps of bright star-forming regions and low surface density features that may indicate recent or ongoing mergers. The Peas are similar in size, mass, luminosity and metallicity to Luminous Blue Compact Galaxies. They are also similar to high redshift UV-luminous galaxies, e.g., Lyman-break galaxies and Lyman-alpha emitters, and therefore provide a local laboratory with which to study the extreme star formation processes that occur in high-redshift galaxies. Studying starbursting galaxies as a function of redshift is essential to understanding the build up of stellar mass in the Universe.

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arXiv2009

The Chandra COSMOS Survey, I: Overview and Point Source Catalog

The Chandra COSMOS Survey (C-COSMOS) is a large, 1.8 Ms, Chandra} program that has imaged the central 0.5 sq.deg of the COSMOS field (centered at 10h, +02deg) with an effective exposure of ~160ksec, and an outer 0.4sq.deg. area with an effective exposure of ~80ksec. The limiting source detection depths are 1.9e-16 erg cm(-2) s(-1) in the Soft (0.5-2 keV) band, 7.3e(-16) erg cm^-2 s^-1 in the Hard (2-10 keV) band, and 5.7e(-16) erg cm(-2) s(-1) in the Full (0.5-10 keV) band. Here we describe the strategy, design and execution of the C-COSMOS survey, and present the catalog of 1761 point sources detected at a probability of being spurious of <2e(-5) (1655 in the Full, 1340 in the Soft, and 1017 in the Hard bands). By using a grid of 36 heavily (~50%) overlapping pointing positions with the ACIS-I imager, a remarkably uniform (to 12%) exposure across the inner 0.5 sq.deg field was obtained, leading to a sharply defined lower flux limit. The widely different PSFs obtained in each exposure at each point in the field required a novel source detection method, because of the overlapping tiling strategy, which is described in a companion paper. (Puccetti et al. Paper II). This method produced reliable sources down to a 7-12 counts, as verified by the resulting logN-logS curve, with sub-arcsecond positions, enabling optical and infrared identifications of virtually all sources, as reported in a second companion paper (Civano et al. Paper III). The full catalog is described here in detail, and is available on-line.

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arXiv2009

Mapping the Dark Matter From UV Light at High Redshift: An Empirical Approach to Understand Galaxy Statistics

We present a simple formalism to interpret two galaxy statistics, the UV luminosity function and two-point correlation functions for star-forming galaxies at z~4, 5, 6 in the context of LCDM cosmology. Both statistics are the result of how star formation takes place in DM halos, and thus are used to constrain how UV light depends on halo properties such as mass. The two measures were taken from the GOODS data, thus ideal for joint analysis. The two physical quantities we explore are the SF duty cycle, and the range of L_UV that a halo of mass M can have (mean and variance). The former addresses the typical duration of SF activity in halos while the latter addresses the averaged SF history and regularity of gas inflow into these systems. We explore various physical models consistent with data, and find the following: 1) the typical duration of SF observed in the data is <0.4 Gyr (1 sig), 2) the inferred scaling law between L_UV and halo mass M from the observed slope of the LFs is roughly linear at all redshifts, and 3) L_UV for a fixed halo mass decreases with time, implying that the SF efficiency (after dust extinction) is higher at earlier times. We explore several physical scenarios relating star formation to halo mass, but find that these scenarios are indistinguishable due to the limited range of halo mass probed by our data. In order to discriminate between different scenarios, we discuss constraining the bright-faint galaxy cross-correlation functions and luminosity-dependence of galaxy bias. (Abridged)

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arXiv2009

The XMM-Newton wide-field survey in the COSMOS field. The point-like X-ray source catalogue

The COSMOS survey is a multiwavelength survey aimed to study the evolution of galaxies, AGN and the large scale structure. The XMM-COSMOS is a deep X-ray survey over the full 2 deg2 of the COSMOS area. It consists of 55 XMM-Newton pointings for a total exposure of ~1.5 Ms with an average vignetting corrected depth of 40 ks across the field of view and a sky coverage of 2.13 deg2. We present the catalogue of point-like X-ray sources detected with the EPIC CCD cameras, the logN-logS relations and the X-ray colour-colour diagrams. The analysis was performed in the 0.5-2 keV, 2-10 keV and 5-10 keV energy bands. The completeness of the catalogue as well as logN-logS have been calibrated using Monte Carlo simulations. The catalogs contains a total of 1887 unique sources detected in at least one band. The survey, that shows unprecedented homogeneity, has a flux limit of ~1.7x10-15 erg cm-2 s-1, ~9.3x10-15 erg cm-2 s-1 and ~1.3x10-14 erg cm-2 s-1 over 90% of the area (1.92 deg2) in the 0.5-2 keV, 2-10 keV and 5-10 keV energy band, respectively. Thanks to the rather homogeneous exposure over a large area, the derived logN-logS relations are very well determined over the flux range sampled by XMM-COSMOS. These relations have been compared with XRB synthesis models, which reproduce the observations with an agreement of ~10% in the 5-10 keV and 2-10 keV band, while in the 0.5-2 keV band the agreement is of the order of ~20%. The hard X-ray colors confirmed that the majority of the extragalactic sources, in a bright subsample, are actually Type I or Type II AGN. About 20% of the sources have X-ray luminosity typical of AGN (L_X >1042 erg/s) although they do not show any clear signature of nuclear activity in the optical spectrum.

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arXiv2008

Correlated variability in the blazar 3C 454.3

The blazar 3C 454.3 was revealed by the Fermi Gamma-ray Space Telescope to be in an exceptionally high flux state in July 2008. Accordingly, we performed a multi-wavelength monitoring campaign on this blazar using IR and optical observations from the SMARTS telescopes, optical, UV and X-ray data from the Swift satellite, and public-release gamma-ray data from Fermi. We find an excellent correlation between the IR, optical, UV and gamma-ray light curves, with a time lag of less than one day. The amplitude of the infrared variability is comparable to that in gamma-rays, and larger than at optical or UV wavelengths. The X-ray flux is not strongly correlated with either the gamma-rays or longer wavelength data. These variability characteristics find a natural explanation in the external Compton model, in which electrons with Lorentz factor gamma~10^(3-4) radiate synchrotron emission in the infrared-optical and also scatter accretion disk or emission line photons to gamma-ray energies, while much cooler electrons (gamma~10^(1-2)) produce X-rays by scattering synchrotron or other ambient photons.

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arXiv2008

Optical Spectroscopy of X-ray sources in the Extended Chandra Deep Field South

We present the first results of our optical spectroscopy program aimed to provide redshifts and identifications for the X-ray sources in the Extended Chandra Deep Field South. A total of 339 sources were targeted using the IMACS spectrograph at the Magellan telescopes and the VIMOS spectrograph at the VLT. We measured redshifts for 186 X-ray sources, including archival data and a literature search. We find that the AGN host galaxies have on average redder rest-frame optical colors than non-active galaxies, and that they live mostly in the "green valley". The dependence of the fraction of AGN that are obscured on both luminosity and redshift is confirmed at high significance and the observed AGN space density is compared with the expectations from existing luminosity functions. These AGN show a significant difference in the mid-IR to X-ray flux ratio for obscured and unobscured AGN, which can be explained by the effects of dust self-absorption on the former. This difference is larger for lower luminosity sources, which is consistent with the dust opening angle depending on AGN luminosity.

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arXiv2008

The Accuracy of Morphological Decomposition of Active Galactic Nucleus Host Galaxies

In order to assess the accuracy with which we can determine the morphologies of AGN host galaxies, we have simulated more than 50,000 ACS images of galaxies with z < 1.25, using image and noise properties appropriate for the GOODS survey. We test the effect of central point-source brightness on host galaxy parameter recovery with a set of simulated AGN host galaxies made by adding point sources to the centers of normal galaxies. We extend this analysis and also quantify the recovery of intrinsic morphological parameters of AGN host galaxies with a set of fully simulated inactive and AGN host galaxies. We can reliably separate good from poor fit results using a combination of reasonable error cuts, in the regime where L_{host}:L_{PS} > 1:4. We give quantitative estimates of parameter errors as a function of host-to-point-source ratio. In general, we separate host and point-source magnitudes reliably at all redshifts; point sources are well recovered more than 90% of the time, although spurious detection of central point sources can be as high as 25% for bulge-dominated sources. We find a general correlation between Sersic index and intrinsic bulge-to-total ratio, such that a host galaxy with Sersic n < 1.5 generally has at least 80% of its light from a disk component. Likewise, "bulge-dominated" galaxies with n > 4 typically derive at least 70% of their total host galaxy light from a bulge, but this number can be as low as 55%. Single-component Sersic fits to an AGN host galaxy are statistically very reliable to z < 1.25 (for ACS survey data like ours). In contrast, two-component fits involving separate bulge and disk components tend to over-estimate the bulge fraction by ~10%, with uncertainty of order 50%.

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arXiv2007

Clustering of Intermediate Luminosity X-ray selected AGN at z~3

We present the first clustering results of X-ray selected AGN at z~3. Using Chandra X-ray imaging and UVR optical colors from MUSYC photometry in the ECDF-S field, we selected a sample of 58 z~3 AGN candidates. From the optical data we also selected 1385 LBG at 2.8<z< 3.8 with R<25.5. We performed auto-correlation and cross-correlation analyses, and here we present results for the clustering amplitudes and dark matter halo masses of each sample. For the LBG we find a correlation length of r_0,LBG = 6.7 +/- 0.5 Mpc, implying a bias value of 3.5 +/- 0.3 and dark matter (DM) halo masses of log(Mmin/Msun) = 11.8 +/- 0.1. The AGN-LBG cross-correlation yields r_0,AGN-LBG = 8.7 +/- 1.9 Mpc, implying for AGN at 2.8<z<3.8 a bias value of 5.5 +/- 2.0 and DM halo masses of log(Mmin/Msun) = 12.6 +0.5/-0.8. Evolution of dark matter halos in the Lambda CDM cosmology implies that today these z~3 AGN are found in high mass galaxies with a typical luminosity of 7+4/-2 L*.

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arXiv2007

The UV colours of high-redshift early-type galaxies: evidence for recent star formation and stellar mass assembly over the last 8 billion years

We combine deep UBVRIzJK photometry from the Multiwavelength Survey by Yale-Chile (MUSYC) with redshifts from the COMBO-17 survey to perform a large-scale study of the rest-frame ultraviolet (UV) properties of 674 high-redshift (0.5 -20.5) counterparts potentially forming 30-60 percent of their mass in the same redshift range. (abridged)

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arXiv2007

Deep Chandra and multicolor HST observations of the jets of 3C 371 and PKS 2201+044

This paper presents multiwavelength imaging and broad-band spectroscopy of the relativistic jets in the two nearby radio galaxies 3C 371 and PKS 2201+044, acquired with Chandra, HST, VLA, and Merlin. Radio polarization images are also available. The two sources stand out as "intermediate'' between FRIs and FRIIs; their cores are classified as BL Lacs, although broad and narrow optical emission lines were detected at times. The multiwavelength images show jet morphologies with the X-ray emission peaking closer to the nucleus than the longer wavelengths. The jets are resolved at all wavelengths in a direction perpendicular to the jet axis. The jets SEDs are consistent with a single spectral component from radio to X-rays, interpreted as synchrotron emission. The SEDs show a progressive softening from the inner to the outer regions of the jet, indicating that the electron break energy moves to lower energies with distance from the core. Overall, the X-ray and multiwavelength properties of the jets of 3C 371 and PKS 2201+044 appear intermediate between those of FRIs and FRIIs.

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arXiv2007

Ly-alpha Emission-Line Galaxies at z = 3.1 in the Extended Chandra Deep Field South

We describe the results of an extremely deep, 0.28 deg^2 survey for z = 3.1 Ly-alpha emission-line galaxies in the Extended Chandra Deep Field South. By using a narrow-band 5000 Anstrom filter and complementary broadband photometry from the MUSYC survey, we identify a statistically complete sample of 162 galaxies with monochromatic fluxes brighter than 1.5 x 10^-17 ergs cm^-2 s^-1 and observers frame equivalent widths greater than 80 Angstroms. We show that the equivalent width distribution of these objects follows an exponential with a rest-frame scale length of w_0 = 76 +/- 10 Angstroms. In addition, we show that in the emission line, the luminosity function of Ly-alpha galaxies has a faint-end power-law slope of alpha = -1.49 +/- 0.4, a bright-end cutoff of log L^* = 42.64 +/- 0.2, and a space density above our detection thresholds of 1.46 +/- 0.12 x 10^-3 h70^3 galaxies Mpc^-3. Finally, by comparing the emission-line and continuum properties of the LAEs, we show that the star-formation rates derived from Ly-alpha are ~3 times lower than those inferred from the rest-frame UV continuum. We use this offset to deduce the existence of a small amount of internal extinction within the host galaxies. This extinction, coupled with the lack of extremely-high equivalent width emitters, argues that these galaxies are not primordial Pop III objects, though they are young and relatively chemically unevolved.

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