How are AGN Found?

Mushotzky, R. F.

doi:10.1007/978-1-4020-2471-9_2

Cited by 72 publications

(29 citation statements)

References 128 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…X-ray observations are claimed to be a very direct tracer of AGN activity (e.g., Mushotzky 2004). The radiation originates very close to the supermassive black hole, it can leave these regions rather unabsorbed, in particular at energies of a few keV, and there is little contamination from other sources, such as stars.…”

Section: Co-located X-ray Observationsmentioning

confidence: 99%

See 1 more Smart Citation

Wide-field VLBA observations of theChandradeep field South

Middelberg

Deller

Morgan

et al. 2010

A&A

View full text Add to dashboard Cite

Context. Wide-field surveys are a commonly-used method for studying thousands of objects simultaneously, to investigate, e.g., the joint evolution of star-forming galaxies and active galactic nuclei. Very long baseline interferometry (VLBI) observations can yield valuable input for such studies because they are able to identify AGN unambiguously in the moderate-to-high-redshift Universe. However, VLBI observations of large swaths of the sky are impractical using standard methods, because the fields of view of VLBI observations are on the order of 10 or less, and have therefore so far only played a minor role in galaxy evolution studies. Aims. We have embarked on a project to carry out Very Long Baseline Array (VLBA) observations of all 96 known radio sources in one of the best-studied areas in the sky, the Chandra deep field South (CDFS). The challenge was to develop methods that could significantly reduce the amount of observing (and post-processing) time, making such a project feasible. Methods. We developed an extension to the DiFX software correlator that allows one to efficiently correlate up to hundreds of positions within the primary beams of the interferometer antennas. This extension enabled us to target many sources simultaneously, at full resolution and high sensitivity, using only a small amount of observing time. The combination of wide fields-of-view and high sensitivity across the field in this survey is unprecedented. Results. We observed a single pointing containing the CDFS with the VLBA, in which 96 radio sources were known from previous observations with the Australia Telescope Compact Array (ATCA). From our input sample of 96 sources, 20 were detected with the VLBA, and one more source was tentatively detected. The majority of these objects have flux densities in agreement with arcsec-scale observations, implying that their radio emission comes from very small regions. Two objects are visibly resolved. One VLBI-detected object had been classified earlier as a star-forming galaxy. After comparing the VLBI detections to sources found in sensitive, colocated X-ray observations, we find that X-ray detections are not a good indicator for VLBI detections. Conclusions. We have successfully demonstrated a new extension to the DiFX software correlator, allowing one to observe hundreds of fields of view simultaneously. In a sensitive observation of the CDFS we detect 21% of the sources and were able to reclassify 7 sources as AGN, which had not been identified as such before. Wide-field VLBI survey science is now coming of age.

show abstract

Section: Co-located X-ray Observationsmentioning

confidence: 99%

“…Since X-ray observations have been claimed to be a very good tracer of AGN activity (e.g., Mushotzky 2004;Brandt & Hasinger 2005), the lack of X-ray counterparts to almost 1/3 of the VLBI-detected sources deserves a closer look.…”

Section: Potential Causes Of the X-ray Non-detectionsmentioning

confidence: 99%

Wide-field VLBA observations of theChandradeep field South

Middelberg

Deller

Morgan

et al. 2010

A&A

View full text Add to dashboard Cite

show abstract

“…The importance of X-ray observations for constraining these properties are twofold. Firstly, selection at high energies yields AGN samples that are least biased in terms of either line-of-sight obscuration or dilution by stellar light from the host galaxy (Comastri et al 2002;Severgnini et al 2003;Mushotzky 2004;Georgantopoulos & Georgakakis 2005). Secondly, because the X-ray flux emitted by AGN is believed to originate at small scales close to the SMBH, spectroscopy at high energies is a unique diagnostic of the accretion properties and the geometry of the material in the vicinity of the central engine.…”

Section: Introductionmentioning

confidence: 99%

X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue

Buchner

Georgakakis

Nandra

et al. 2014

A&A

1,448

1,263

View full text Add to dashboard Cite

Aims. Active galactic nuclei are known to have complex X-ray spectra that depend on both the properties of the accreting super-massive black hole (e.g. mass, accretion rate) and the distribution of obscuring material in its vicinity (i.e. the "torus"). Often however, simple and even unphysical models are adopted to represent the X-ray spectra of AGN, which do not capture the complexity and diversity of the observations. In the case of blank field surveys in particular, this should have an impact on e.g. the determination of the AGN luminosity function, the inferred accretion history of the Universe and also on our understanding of the relation between AGN and their host galaxies. Methods. We develop a Bayesian framework for model comparison and parameter estimation of X-ray spectra. We take into account uncertainties associated with both the Poisson nature of X-ray data and the determination of source redshift using photometric methods. We also demonstrate how Bayesian model comparison can be used to select among ten different physically motivated X-ray spectral models the one that provides a better representation of the observations. This methodology is applied to X-ray AGN in the 4 Ms Chandra Deep Field South. Results. For the ∼350 AGN in that field, our analysis identifies four components needed to represent the diversity of the observed X-ray spectra: (1) an intrinsic power law; (2) a cold obscurer which reprocesses the radiation due to photo-electric absorption, Compton scattering and Fe-K fluorescence; (3) an unabsorbed power law associated with Thomson scattering off ionised clouds; and (4) Compton reflection, most noticeable from a stronger-than-expected Fe-K line. Simpler models, such as a photo-electrically absorbed power law with a Thomson scattering component, are ruled out with decisive evidence (B > 100). We also find that ignoring the Thomson scattering component results in underestimation of the inferred column density, N H , of the obscurer. Regarding the geometry of the obscurer, there is strong evidence against both a completely closed (e.g. sphere), or entirely open (e.g. blob of material along the line of sight), toroidal geometry in favour of an intermediate case.Conclusions. Despite the use of low-count spectra, our methodology is able to draw strong inferences on the geometry of the torus. Simpler models are ruled out in favour of a geometrically extended structure with significant Compton scattering. We confirm the presence of a soft component, possibly associated with Thomson scattering off ionised clouds in the opening angle of the torus. The additional Compton reflection required by data over that predicted by toroidal geometry models, may be a sign of a density gradient in the torus or reflection off the accretion disk. Finally, we release a catalogue of AGN in the CDFS with estimated parameters such as the accretion luminosity in the 2−10 keV band and the column density, N H , of the obscurer.

show abstract

“…Hard X-rays can directly probe AGN activity, are uncontaminated by star formation processes at the X-ray luminosities of interest, and detect all but the most absorbed sources. Thus, hard X-ray surveys provide as complete and unbiased a sample of AGNs as is presently possible (e.g., Mushotzky 2004), although they will still miss Compton-thick sources.…”

Section: Introductionmentioning

confidence: 99%

The Cosmic Evolution of Hard X-Ray-selected Active Galactic Nuclei

et al. 2005

View full text Add to dashboard Cite

We use highly spectroscopically complete deep and wide-area Chandra surveys to determine the cosmic evolution of hard X-ray-selected active galactic nuclei (AGNs). For the deep fields, we supplement the spectroscopic redshifts with photometric redshifts to assess where the unidentified sources are likely to lie. We find that the median redshifts are fairly constant with X-ray flux at z $ 1.We classify the optical spectra and measure the FWHM line widths. Most of the broad-line AGNs show essentially no visible absorption in X-rays, whereas the sources without broad lines (FWHM < 2000 km s À1 ; ''optically narrow'' AGNs) show a wide range of absorbing column densities. We determine hard X-ray luminosity functions for all spectral types with L X ! 10 42 ergs s À1 and for broad-line AGNs alone. At z < 1:2, both are well described by pure luminosity evolution, with L * evolving as (1 þ z) 3:2AE0:8 for all spectral types and as (1 þ z) 3:0AE1:0 for broad-line AGNs alone. Thus, all AGNs drop in luminosity by almost an order of magnitude over this redshift range. We show that this observed drop is due to AGN downsizing rather than to an evolution in the accretion rates onto the supermassive black holes.We directly compare our broad-line AGN hard X-ray luminosity functions with the optical QSO luminosity functions and find that at the bright end they agree extremely well at all redshifts. However, the optical QSO luminosity functions do not probe faint enough to see the downturn in the broad-line AGN hard X-ray luminosity functions and even appear to be missing some sources at the lowest luminosities they probe.We find that broad-line AGNs dominate the number densities at the higher X-ray luminosities, while optically narrow AGNs dominate at the lower X-ray luminosities. We rule out galaxy dilution as a partial explanation for this effect by measuring the nuclear UV/optical properties of the Chandra sources using the Hubble Space Telescope Advanced Camera for Surveys GOODS-North data. The UV/optical nuclei of the optically narrow AGNs are much weaker than expected if the optically narrow AGNs were similar to the broad-line AGNs. We therefore postulate the need for a luminosity-dependent unified model. An alternative possibility is that the broad-line AGNs and the optically narrow AGNs are intrinsically different source populations. We cover both interpretations by constructing composite spectral energy distributions-including long-wavelength data from the mid-infrared to the submillimeterby spectral type and by X-ray luminosity. We use these spectral energy distributions to infer the bolometric corrections (from hard X-ray luminosities to bolometric luminosities) needed to map the accretion history.We determine the accreted supermassive black hole mass density for all spectral types and for broad-line AGNs alone, using the observed evolution of the hard X-ray energy density production rate and our inferred bolometric corrections. We find that only about one-half to one-quarter of the supermassive black hole mass ...

show abstract

How are AGN Found?

Cited by 72 publications

References 128 publications

Wide-field VLBA observations of theChandradeep field South

Wide-field VLBA observations of theChandradeep field South

X-ray spectral modelling of the AGN obscuring region in the CDFS: Bayesian model selection and catalogue

The Cosmic Evolution of Hard X-Ray-selected Active Galactic Nuclei

Contact Info

Product

Resources

About