Unresolved AGN Research Articles

The XXL Survey

Context. The XMM-XXL survey has used observations from the XMM-Newton observatory to detect clusters of galaxies over a wide range in mass and redshift. The moderate PSF (FWHM ~ 6″ on-axis) of XMM-Newton means that point sources within or projected onto a cluster may not be separated from the cluster emission, leading to enhanced luminosities and affecting the selection function of the cluster survey. Aims. We present the results of short Chandra observations of 21 galaxy clusters and cluster candidates at redshifts z > 1 detected in the XMM-XXL survey in X-rays or selected in the optical and infra-red. Methods. With the superior angular resolution of Chandra, we investigate whether there are any point sources within the cluster region that were not detected by the XMM-XXL analysis pipeline, and whether any point sources were misclassified as distant clusters. Results. Of the 14 X-ray selected clusters, 9 are free from significant point source contamination, either having no previously unresolved sources detected by Chandra or with less than about 10% of the reported XXL cluster flux being resolved into point sources. Of the other five sources, one is significantly contaminated by previously unresolved AGN, and four appear to be AGN misclassified as clusters. All but one of these cases are in the subset of less secure X-ray selected cluster detections and the false positive rate is consistent with that expected from the XXL selection function modelling. We also considered a further seven optically selected cluster candidates associated with faint XXL sources that were not classed as clusters. Of these, three were shown to be AGN by Chandra, one is a cluster whose XXL survey flux was highly contaminated by unresolved AGN, while three appear to be uncontaminated clusters. By decontaminating and vetting these distant clusters, we provide a pure sample of clusters at redshift z > 1 for deeper follow-up observations, and demonstrate the utility of using Chandra snapshots to test for AGN in surveys with high sensitivity but poor angular resolution.

Mass-Discrepancy Acceleration Relation: A Natural Outcome of Galaxy Formation in Cold Dark Matter Halos.

We analyze the total and baryonic acceleration profiles of a set of well-resolved galaxies identified in the eagle suite of hydrodynamic simulations. Our runs start from the same initial conditions but adopt different prescriptions for unresolved stellar and active galactic nuclei feedback, resulting in diverse populations of galaxies by the present day. Some of them reproduce observed galaxy scaling relations, while others do not. However, regardless of the feedback implementation, all of our galaxies follow closely a simple relationship between the total and baryonic acceleration profiles, consistent with recent observations of rotationally supported galaxies. The relation has small scatter: Different feedback implementations-which produce different galaxy populations-mainly shift galaxies along the relation rather than perpendicular to it. Furthermore, galaxies exhibit a characteristic acceleration g_{†}, above which baryons dominate the mass budget, as observed. These observations, consistent with simple modified Newtonian dynamics, can be accommodated within the standard cold dark matter paradigm.

AN XMM-NEWTON SURVEY OF THE SOFT X-RAY BACKGROUND. II. AN ALL-SKY CATALOG OF DIFFUSE O VII AND O VIII EMISSION INTENSITIES

We present an all-sky catalog of diffuse O VII and O VIII line intensities, extracted from archival XMM observations. The O VII and O VIII intensities are typically ~2-11 and <~3 ph/cm^2/s/sr (LU), respectively, although much brighter intensities were also recorded. Our data set includes 217 directions observed multiple times by XMM. The time variation of the intensities from such directions may be used to constrain SWCX models. The O VII and O VIII intensities typically vary by <~5 and <~2 LU between repeat observations, although several intensity enhancements of >10 LU were observed. We compared our measurements with SWCX models. The heliospheric SWCX intensity is expected to vary with ecliptic latitude and solar cycle. We found that the observed oxygen intensities generally decrease from solar maximum to solar minimum, both at high ecliptic latitudes (as expected) and at low ecliptic latitudes (not as expected). The geocoronal SWCX intensity is expected to depend on the solar wind proton flux and on the sightline's path through the magnetosheath. The intensity variations seen in directions that have been observed multiple times are in poor agreement with the predictions of a geocoronal SWCX model. The oxygen lines account for ~40-50% of the 3/4 keV X-ray background that is not due to unresolved AGN, in good agreement with a previous measurement. However, this fraction is not easily explained by a combination of SWCX emission and emission from hot plasma in the halo. The line intensities tend to increase with longitude toward the inner Galaxy, possibly due to an increase in the supernova rate in that direction or the presence of a halo of accreted material centered on the Galactic Center. The variation of intensity with Galactic latitude differs in different octants of the sky, and cannot be explained by a single simple plane-parallel or constant-intensity halo model. (Abridged)

Anisotropies in the diffuse gamma-ray background measured by the Fermi-LAT

The small angular scale fluctuations of the (on large scale) isotropic gamma-ray background (IGRB) carry information about the presence of unresolved source classes. A guaranteed contribution to the IGRB is expected from the unresolved gamma-ray AGN while other extragalactic sources, Galactic gamma-ray source populations and dark matter Galactic and extragalactic structures (and sub-structures) are candidate contributors.The IGRB was measured with unprecedented precision by the Large Area Telescope (LAT) on-board of the Fermi gamma-ray observatory, and these data were used for measuring the IGRB angular power spectrum (APS). Detailed Monte Carlo simulations of Fermi-LAT all-sky observations were performed to provide a reference against which to compare the results obtained for the real data set. The Monte Carlo simulations are also a method for performing those detailed studies of the APS contributions of single source populations, which are required in order to identify the actual IGRB contributors.We present preliminary results of an anisotropy search in the IGRB. At angular scales <2° (e.g., above multipole 155), angular power above the photon noise level is detected, at energies between 1 and 10GeV in each energy bin, with statistical significance between 7.2 and 4.1σ. The obtained energy dependences point to the presence of one or more unclustered source populations with the components having an average photon index Γ=2.40±0.07.

Gamma-ray luminosity function of blazars and the cosmic gamma-ray background: evidence for the luminosity-dependent density evolution

We present a comprehensive study of the gamma-ray luminosity function (GLF) of blazars and their contribution to the extragalactic diffuse gamma-ray background (EGRB). Radio and gamma-ray luminosity correlation is introduced with a modest dispersion, consistent with observations, to take into account the radio detectability, which is important for blazar identification. Previous studies considered only pure luminosity evolution (PLE) or pure density evolution, but here we introduce the luminosity-dependent density evolution (LDDE) model, which is favored on the basis of the evolution of the X-ray luminosity function (XLF) of AGNs. The model parameters are constrained by likelihood analyses of the observed redshift and gamma-ray flux distributions of the EGRET blazars. Interestingly, we find that the LDDE model gives a better fit to the observed distributions than the PLE model, indicating that the LDDE model is also appropriate for gamma-ray blazars and that the jet activity is universally correlated with the accretion history of AGNs. We then find that only 25–50% of the EGRB can be explained by unresolved blazars with the best-fit LDDE parameters. Unresolved blazars can account for all the EGRB only with a steeper index of the faint-end slope of the GLF, which is marginally consistent with the EGRET data but inconsistent with XLF data. Therefore, unresolved AGNs cannot be the dominant source of the EGRB, unless there is a new population of gamma-ray emitting AGNs that evolves differently from the XLF of AGNs. Predictions for the GLAST mission are made, and we find that the best-fit LDDE model predicts about 3000 blazars in the entire sky, which is considerably fewer (by a factor of more than 3) than a previous estimate.

Gamma‐Ray Luminosity Function of Blazars and the Cosmic Gamma‐Ray Background: Evidence for the Luminosity‐Dependent Density Evolution

We present a comprehensive study for the gamma-ray luminosity function (GLF) of blazars and their contribution to the extragalactic diffuse gamma-ray background (EGRB). Radio and gamma-ray luminosity correlation is introduced to take into account the radio detectability which is important for the blazar identification. Previous studies considered only pure luminosity evolution (PLE) or pure density evolution, but we introduce the luminosity dependent density evolution (LDDE) model, which is favored from the evolution of X-ray luminosity function (XLF) of AGNs. The model parameters are constrained by likelihood analyses about the observed redshift and gamma-ray flux distributions of the EGRET blazars. We find that the LDDE model gives a better fit to the observed distributions than the PLE model, indicating that the LDDE model is also appropriate for gamma-ray blazars, and that the jet activity is universally correlated with the accretion history of AGNs. The normalization between the GLF and XLF is consistent with the unified picture of AGNs, when the beaming and a reasonable duty cycle of jet activity are taken into account. We then find that only 25--50% of the EGRB can be explained by unresolved blazars with the best-fit LDDE parameters. Unresolved blazars can account for all the EGRB only with a steeper index of the faint-end slope of the GLF, which is marginally consistent with the EGRET data but inconsistent with that of the XLF. Therefore unresolved AGNs cannot be the dominant source of the EGRB, unless there is a new population of gamma-ray emitting AGNs that evolves differently from the XLF of AGNs. Predictions for the GLAST mission are made, and we find that the best-fit LDDE model predicts about 3000 blazars in the entire sky, which is considerably fewer than a previous estimate.

The Jet and Circumnuclear Environment of 3C 293

We present the new HST near-infrared polarimetry, broad and narrow-band imaging, and MERLIN 4.5GHz Multi-Frequency Synthesis radio imaging of 3C 293, a unique radio galaxy whose host is an obvious merger remnant, in an exceptionally under-dense region of space. We have discovered near-infrared, optical, and ultra-violet synchrotron emission from the jet. In the optical, the jet is mostly obscured by a dust lane, but three knots are clear in our HST NICMOS images at 1.6 and 2.0 microns, clearly aligning with features in the radio. The outer jet knot is highly polarized (~15%) at 2 microns, confirming the synchrotron emission mechanism. The radio-IR spectral index steepens significantly with distance from the nucleus, as in 3C 273 and in contrast to M 87. The inner knot is visible (with hindsight) on the WFPC2 and STIS images obtained for the earlier 3CR HST snapshot surveys. There is no [Fe II] emission seen associated with the jet, constraining the role of shock-induced ionisation by the jet. Overall there is a strong implication that the NIR jet emission is indeed synchrotron. From our NIR images, the core of the galaxy is clearly identifiable with the main feature in the western extension of the radio ``jet'' image, although no unresolved AGN component is identifiable even at K-band, consistent with an FRII-like nucleus obscured by an optically thick torus. The galaxy appears to have a single nucleus, with any multiple nuclei falling within the central </~100 pc.

Warm-hot intergalactic medium in the Sculptor supercluster

We have analyzed the soft X-ray emission in a wide area of the Sculptor supercluster by using overlapping ROSAT PSPC pointings. After subtraction of the point sources we have found evidence for extended, diffuse soft X-ray emission. We have investigated the nature of such extended emission through the cross-correlation with the density of galaxies as inferred from the Muenster Redshift Survey. In particular we have analyzed the correlation as a function of the temperature of the X-ray emitting gas. We have found a significant correlation of the galaxy distribution only with the softest X-ray emission (0.1--0.3 keV) and only for gas temperatures kT < 0.5 keV. We have excluded that this soft X-ray diffuse emission, and its correlation with the galaxy distribution, is significantly contributed by unresolved AGN, group of galaxies or individual galaxies. The most likely explanation is that the soft, diffuse X-ray emission is tracing Warm-Hot Intergalactic Medium, with temperatures below 0.5 keV, associated with the large-scale structures in the Sculptor supercluster.

Shower power: isolating the prompt atmospheric neutrino flux using electronneutrinos

At high energies, the very steep decrease of the conventional atmospheric component of theneutrino spectrum should allow the emergence of even small and isotropic components ofthe total spectrum, indicative of new physics, provided that they are less steeplydecreasing, as generically expected. One candidate is the prompt atmosphericneutrino flux, a probe of cosmic ray composition in the region of the knee as well assmall-x QCD, below the reach of collider experiments. A second is the diffuse extragalacticbackground due to distant and unresolved AGNs and GRBs, a key test of the nature of thehighest-energy sources in the universe. Separating these new physics components from theconventional atmospheric neutrino flux, as well as from each other, will be very challenging.We show that the charged-current electron neutrino ‘shower’ channel should be particularlyeffective for isolating the prompt atmospheric neutrino flux, and that it is moregenerally an important complement to the usually considered charged-currentmuon neutrino ‘track’ channel. These conclusions remain true even for the lowprompt atmospheric neutrino flux predicted in a realistic cosmic ray scenariowith heavy and varying composition across the knee (Candia and Roulet, 2003J. Cosmol. Astropart. Phys. JCAP09(2003)005). We also improve the correspondingcalculation of the neutrino flux induced by cosmic ray collisions with the interstellarmedium.

Ultraviolet Light from Young Stars in GEMS Quasar Host Galaxies at 1.8 &lt; z &lt; 2.75

We have performed HST imaging of a sample of 23 high-redshift (1.8<z<2.75) AGN, drawn from the COMBO-17 survey. The sample contains moderately luminous quasars (M_B ~ -23). The data are part of the GEMS imaging survey that provides high resolution optical images obtained with ACS in two bands (F606W and F850LP), sampling the rest-frame UV flux of the targets. To deblend the AGN images into nuclear and resolved (host galaxy) components we use a PSF subtraction technique that is strictly conservative with respect to the flux of the host galaxy. We resolve the host galaxies in both filter bands in 9 of the 23 AGN, whereas the remaining 14 objects are considered non-detections, with upper limits of less than 5% of the nuclear flux. However, when we coadd the unresolved AGN images into a single high signal-to-noise composite image we find again an unambiguously resolved host galaxy. The recovered host galaxies have apparent magnitudes of 23.0<F606W<26.0 and 22.5<F850LP<24.5 with rest-frame UV colours in the range -0.2<(F606W-F850LP)_obs<2.3. The rest-frame absolute magnitudes at 200 nm are -20.0<M_200nm<-22.2. The photometric properties of the composite host are consistent with the individual resolved host galaxies. We find that the UV colors of all host galaxies are substantially bluer than expected from an old population of stars with formation redshift z=5, independent of the assumed metallicities. These UV colours and luminosities range up to the values found for Lyman-break galaxies (LBGs) at z=3. Our results agree with the recent discovery of enhanced blue stellar light in AGN hosts at lower redshifts. [abridged]

1.2 mm emission from Cen A

Centaurus A is the closest active radio galaxy at a distance of around 3.5 Mpc. New 1.2 mm imaging performed with SIMBA at the Swedish ESO Submillimetre Telescope (SEST) in Chile gives an insight into the distribution of thermal and non-thermal emission. Apart from extended synchrotron emission in the radio lobes we have detected the unresolved AGN core and the characteristic dust lane.

Deep Submillimeter Imaging of Dust Structures in Centaurus A

Images covering the central 450'' × 100'' (~8.0 kpc × 2.0 kpc) of NGC 5128 (Centaurus A) obtained using SCUBA at 850 and 450 μm with beam sizes of 145 and 8'', respectively, are presented. These data are compared with those obtained at other wavelengths, in particular the optical, mid-infrared, and far-infrared continuum. The sensitive 850 and 450 μm images show that the submillimeter (submm) continuum morphology and spectral index distribution of Centaurus A comprise four regions: an unresolved AGN core, an inner jet interacting with gas in the dust lane, an inner disk of radius ~90'', and colder outer dust. The inner disk has a high surface brightness, reverse-S-shaped feature in the 850 and 450 μm images that coincides with the regions of intense 7 and 15 μm continuum and a region of active star formation. The infrared (IR) and submm images seem to reveal the same material as predicted by a geometric warped-disk model consisting of tilted rings. We suggest that this scenario is more plausible than that recently proposed in literature, in which the mid-IR emission in Centaurus A is primarily from a bar with a structure that is different from the extended warped disk alone. A dust mass total of 2.2 × 106 M☉ has been calculated within a radius of 225'', 45% of which is in the star-forming region of radius ~90'' about the nucleus.

The contribution of unresolved radio-loud AGN to the extragalactic diffuse gamma-ray background

We present a calculation of the blazar contribution to the extragalactic diffuse γ-ray background (EGRB) in the EGRET energy range. Our model is based on inverse-Compton scattering as the dominant γ-ray production process in the jets of flat spectrum radio quasars (FSRQs) and BL Lac objects, and on the unification scheme of radio-loud AGN. According to this picture, blazars represent the beamed fraction of the Fanaroff–Riley radio galaxies (FR galaxies). The observed log N–log S distribution and redshift distribution of both FSRQs and BL Lacs constrain our model. Depending slightly on the evolutionary behaviour of blazars, we find that unresolved AGN underproduce the intensity of the extragalactic background radiation. With our model only 20–40 per cent of the extragalactic background emission can be explained by unresolved blazars if we integrate to a maximum redshift of Zmax=3. For Zmax=5, blazars could account for 40–80 per cent of the EGRB. Roughly 70–90 per cent of the AGN contribution to the EGRB would result from BL Lacs. While the systematic uncertainties in our estimate for the FSRQ contribution appear small, in the case of BL Lacs our model parameters are not consistent with the results from studies in other wavelength regimes, and therefore may have larger systematic uncertainties. Thus we end up with two possibilities, depending on whether we underpredict or overpredict the BL Lac contribution: either unresolved AGN cannot account for the entire EGRB, or unresolved BL Lacs produce the observed background. We predict a significant flattening of the γ-ray log N–log S function in the next two decades of flux below the EGRET threshold.

Quasar Candidates in the Hubble Deep Field

We focus on the search for unresolved faint quasars and AGN in the crude combine images using a multicolor imaging analysis that has proven very successful in recent years. Quasar selection was carried out both in multicolor space and in profile space, defined as the multi-parameter space formed by the radial profiles of the objects in the different images. By combining the dither frames available for each filter, we were able to obtain well-sampled radial profiles of the objects and measure their deviation from that of a stellar source. We also generated synthetic quasar spectra in the range 1.0 3.5. We find more quasar candidates than previous work because of our more extensive modeling and use of all of the available color information. (abridged)

Neutrinos from transient central engine shocks in AGN

If shock acceleration of protons is responsible for an appreciable fraction of the non-thermal emission from AGN central engines, they are probably also powerful sources of high energy neutrinos. The neutrino background intensity at the Earth has been predicted by several authors on that basis. This paper describes a calculation of the neutrino spectrum, starting from assumptions about the state of accreting plasma near the central black hole. Our assumptions are closest to those of Stecker and Salamon. where the protons cool by photo-pion production, and neutrino emission is normalised to X-ray emission. However, rather than using the non-relativistic test-particle analytic results for proton acceleration rate and spectral index, we use the results of a numerical code developed at Imperial College, which takes into account aspects of the non-linear shock regime, and shows that relativistic shocks produce a harder spectrum with a faster acceleration time. Also, because of the narrow ranges of plasma properties which permit steady shocks, and doubts about shock stability, we believe that particle acceleration in AGN central engines occurs at transient shocks, small compared with the overall source size, and at a wide range of radii, rather than a single steady shock at a fixed distance from the central black hole, as is usually assumed. Observations of rapid X-ray variability in Seyferts are also suggestive of such a picture. We calculate neutrino spectra for 3C 273, and more importantly, overall background spectra from unresolved AGN, which are compared with other model predictions. We show that with a realistic correction to the Bohm diffusion limit for protons, parallel shocks are unlikely to produce an observable neutrino flux, and quasi-perpendicular relativistic shocks are necessary.

On Compton reflection in the sources of the cosmic X-ray background

Consideration is given to recent models for the cosmic X-ray background that assume that it originates from unresolved AGN emitting spectra due to enhanced Compton reflection of a power-law photon spectrum incident on cold matter. The parameter space of the Compton reflection model is studied, and the allowed parameter space is found to be severely constrained by physical and cosmological effects. For an incident power-law energy index alpha is greater than about 1, the X-ray peak in the observed spectrum from a population of AGN is necessarily at an energy less than that of the observed peak. Two examples of improved fits to the X-ray background are shown. It is concluded that the Compton reflection models proposed to date do not provide a straightforward explanation of the X-ray background spectrum.

Constraints on possible precursor AGN sources of the cosmic X-ray background

New limits on small-scale surface brightness fluctuations of the residual cosmic X-ray background are used to reevaluate the conjecture that unresolved precursor AGN sources could be the origin of the flux. The large number of these predominantly X-ray-emitting objects thereby required imposes that their lifetime be longer than previously considered. Attention is drawn to the fact that the reduction in the effective Eddington luminosity limit for such sources due to electron-positron pair opacity could have the effect of increasing their lifetime to an appropriate level.