Person:

Wilkes, Belinda

The unification model for powerful radio galaxies (RGs) and radio-loud quasars postulates that these objects are intrinsically the same but viewed along different angles. Herschel Space Observatory data permit the assessment of that model in the far-infrared spectral window. We analyze photometry from Spitzer and Herschel for the distant 3CR hosts, and find that RGs and quasars have different mid-infrared, but indistinguishable far-infrared colors. Both these properties, the former being orientation dependent and the latter orientation invariant, are in line with expectations from the unification model. Adding powerful radio-quiet active galaxies and typical massive star-forming (SF) galaxies to the analysis, we demonstrate that infrared colors not only provide an orientation indicator, but can also distinguish active from SF galaxies.

We have carried out a survey with the Chandra X-Ray Observatory of a sample of 10 bright broad absorption line (BAL) quasars (QSOs). Eight of 10 sources are detected. The six brightest sources have only high-ionization BALs (hiBALs), while the four faintest all show low-ionization BALs (loBALs). We perform a combined spectral fit for hiBAL QSOs (384 counts total; 0.5-6 keV) to determine the mean spectral parameters of this sample. We derive an underlying best-fit power-law slope Γ = 1.8 ± 0.35, which is consistent with the mean slope for radio-quiet QSOs from ASCA, but BAL QSOs require a (rest-frame) absorbing column of 6.5 × 1022 cm-2, with a partial covering fraction of ~80%. The optical-to-X-ray spectral slope (αox from 2500 Å to 2 keV) varies from 1.7 to 2.4 across the full sample, consistent with previous results that BAL QSOs appear to be weak soft X-ray emitters. Removing the absorption component from our best-fit spectral model yields a range of αox from 1.55 to 2.28. All six hiBAL QSOs have deabsorbed X-ray emission consistent with non-BAL QSOs of similar luminosity. The spectral energy distributions of the hiBAL QSOs—both the underlying power-law slope and αox—provide the first conclusive evidence that BAL QSOs have appeared to be X-ray weak because of intrinsic absorption and that their underlying emission is consistent with non-BAL QSOs. By contrast, the removal of the best-fit absorption column detected in the hiBAL QSOs still leaves the four loBAL QSOs with values of αox > 2 that are unusually X-ray faint for their optical luminosities, which is consistent with other evidence that loBALs have higher column density, dustier absorbers. Important questions of whether BAL QSOs represent a special line of sight toward a QSO nucleus or rather an early evolutionary or high-accretion phase in a QSO lifetime remain to be resolved, and the unique properties of loBAL QSOs will be an integral part of that investigation.

Using the large multiwavelength data set in the Chandra/SWIRE Survey (0.6 deg2 in the Lockman Hole), we show evidence for the existence of highly obscured (Compton-thick) AGNs, estimate a lower limit to their surface density, and characterize their multiwavelength properties. Two independent selection methods based on the X-ray and infrared spectral properties are presented. The two selected samples contain (1) five X-ray sources with hard X-ray spectra and column densities gsim1024 cm-2 and (2) 120 infrared sources with red and AGN-dominated infrared SEDs. We estimate a surface density of at least 25 Compton-thick AGNs deg-2 detected in the infrared in the Chandra/SWIRE field, of which ~40% show distinct AGN signatures in their optical/near-infrared SEDs, the remaining being dominated by the host galaxy emission. Only ~33% of all Compton-thick AGNs are detected in the X-rays at our depth [F(0.3-8 keV) > 10-15 ergs cm-2 s-1]. We report the discovery of two sources in our sample of Compton-thick AGNs, SWIRE J104409.95+585224.8 (z = 2.54) and SWIRE J104406.30+583954.1 (z = 2.43), which are the most luminous Compton-thick AGNs at high z currently known. The properties of these two sources are discussed in detail with an analysis of their spectra, SEDs, luminosities, and black hole masses.

The Chandra Multiwavelength Project (ChaMP) is a wide-area (~14 deg2) survey of serendipitous Chandra X-ray sources, aiming to establish fair statistical samples covering a wide range of characteristics (such as absorbed active galactic nuclei [AGNs] and high-z clusters of galaxies) at flux levels (fX ~ 10-15 to 10-14 ergs s-1 cm-2) intermediate between the Chandra Deep Field surveys and previous missions. We present the first results of X-ray source properties obtained from the initial sample of 62 observations. The data have been uniformly reduced and analyzed with techniques specifically developed for the ChaMP and then validated by visual examination. Utilizing only near-on-axis X-ray-bright sources (to avoid problems caused by incompleteness and the Eddington bias), we derive the log N- log S relation in soft (0.5-2 keV) and hard (2-8 keV) energy bands. The ChaMP data are consistent with previous results of ROSAT, ASCA, and Chandra Deep Field surveys. In particular, our data nicely fill in the flux gap in the hard band between the Chandra Deep Field data and the previous ASCA data. We check whether there is any systematic difference in the source density between cluster and noncluster fields and also search for field-to-field variation, both of which have been previously reported. We found no significant field-to-field cosmic variation in either test within the statistics (~1 σ) across the flux levels included in our sample. In the X-ray color-color plot, most sources fall in the location characterized by photon index = 1.5-2 and NH = a few × 1020 cm2, suggesting that they are typical broadline AGNs. There also exist a considerable number of sources with peculiar X-ray colors (e.g., highly absorbed, very hard, very soft). We confirm a trend that on average the X-ray color hardens as the count rate decreases. Since the hardening is confined to the softest energy band (0.3-0.9 keV), we conclude that it is most likely due to absorption. We cross-correlate the X-ray sources with other catalogs and describe their properties in terms of optical color, X-ray-to-optical luminosity ratio, and X-ray colors.

We present the final installment of an intensive 13 year study of variations of the optical continuum and broad Hβ emission line in the Seyfert 1 galaxy NGC 5548. The database consists of 1530 optical continuum measurements and 1248 Hβ measurements. The Hβ variations follow the continuum variations closely, with a typical time delay of about 20 days. However, a year-by-year analysis shows that the magnitude of emission-line time delay is correlated with the mean continuum flux. We argue that the data are consistent with the simple model prediction between the size of the broad-line region and the ionizing luminosity, r ∝ Lion. Moreover, the apparently linear nature of the correlation between the Hβ response time and the nonstellar optical continuum Fopt arises as a consequence of the changing shape of the continuum as it varies, specifically Fopt ∝ FUV.

We report on Chandra observations of the three quasars SDSSp J083643+005453, SDSSp J103027+052455, and SDSSp J130608+035626 at redshifts 5.82, 6.28, and 5.99, respectively. All three sources are clearly detected in the X-ray band, up to rest-frame energies of ~55 keV. These observations demonstrate the unprecedented sensitivity of Chandra to detect faint sources in relatively short exposure times (5.7-8.2 ks). The broadband X-ray properties of these highest redshift quasars do not appear to be any different from their lower redshift cousins, implying little or no evolution for these few sources out to z ~ 6, at redshifts where the optical space density is falling. Spectra of the sources could not be determined with only a few counts detected. Observations with XMM-Newton will be able to constrain the spectral shapes, if they are simple. Determination of complex spectra in a reasonable amount of time, however, will have to await the next generation of X-ray missions.

An 8 month monitoring campaign on the Seyfert 1 galaxy Fairall 9 has been conducted with the International Ultraviolet Explorer in an attempt to obtain reliable estimates of continuum-continuum and continuum-emission-line delays for a high-luminosity active galactic nucleus (AGN). While the results of this campaign are more ambiguous than those of previous monitoring campaigns on lower luminosity sources, we find general agreement with the earlier results: (1) there is no measurable lag between ultraviolet continuum bands, and (2) the measured emission-line time lags are very short. It is especially notable that the Lyα + N V emission-line lag is about 1 order of magnitude smaller than determined from a previous campaign by Clavel, Wamsteker, & Glass (1989) when Fairall 9 was in a more luminous state. In other well-monitored sources, specifically NGC 5548 and NGC 3783, the highest ionization lines are found to respond to continuum variations more rapidly than the lower ionization lines, which suggests a radially ionization-stratified broad-line region. In this case, the results are less certain, since none of the emission-line lags are very well determined. The best-determined emission line lag is Lyα + N V, for which we find that the centroid of the continuum-emission-line cross-correlation function is τcent ≈ 14-20 days. We measure a lag τcent lesssim 4 days for He II λ1640; this result is consistent with the ionization-stratification pattern seen in lower luminosity sources, but the relatively large uncertainties in the emission-line lags measured here cannot rule out similar lags for Lyα + N V and He II λ1640 at a high level of significance. We are unable to determine a reliable lag for C IV λ1550, but we note that the profiles of the variable parts of Lyα and C IV λ1550 are not the same, which does not support the hypothesis that the strongest variations in these two lines arise in the same region.

We present the results of an intermediate-resolution (1.5\AA) spectroscopic study of 17 X-ray luminous narrow emission-line galaxies previously identified in the Cambridge-Cambridge {\it ROSAT} Serendipity Survey and the {\it Einstein} Extended Medium Sensitivity Survey. Emission-line ratios reveal that the sample is composed of ten Seyfert and seven starburst galaxies. Measured linewidths for the narrow Hα emission lines lie in the range 170−460kms−1. Five of the objects show clear evidence for asymmetry in the [OIII]λ5007 emission-line profile. Broad Hα emission is detected in six of the Seyfert galaxies, which range in type from Seyfert 1.5 to 2. Broad Hβ emission is only detected in one Seyfert galaxy. The mean full width at half maximum for the broad lines in the Seyfert galaxies is FWHM=3900±1750kms−1. Broad (FWHM=2200±600kms−1) Hα emission is also detected in three of the starburst galaxies, which could originate from stellar winds or supernovae remnants. The mean Balmer decrement for the sample is Hα/Hβ=3, consistent with little or no reddening for the bulk of the sample. There is no evidence for any trend with X-ray luminosity in the ratio of starburst galaxies to Seyfert galaxies. Based on our previous observations, it is therefore likely that both classes of object comprise ∼10per cent of the 2keV X-ray background.

We provide a catalog of 391 mid-infrared-selected (MIR; 24 μm) broad-emission-line (BEL; type 1) quasars in the 22 deg2 SWIRE Lockman Hole field. This quasar sample is selected in the MIR from Spitzer MIPS with S 24 > 400 μJy, jointly with an optical magnitude limit of r (AB) < 22.5 for broad line identification. The catalog is based on MMT and Sloan Digital Sky Survey (SDSS) spectroscopy to select BEL quasars, extending the SDSS coverage to fainter magnitudes and lower redshifts, and recovers a more complete quasar population. The MIR-selected quasar sample peaks at z ~ 1.4 and recovers a significant and constant (20%) fraction of extended objects with SDSS photometry across magnitudes, which were not included in the SDSS quasar survey dominated by point sources. This sample also recovers a significant population of z < 3 quasars at i > 19.1. We then investigate the continuum luminosity and line profiles of these MIR quasars, and estimate their virial black hole masses and the Eddington ratios. The supermassive black hole mass shows evidence of downsizing, although the Eddington ratios remain constant at 1 < z < 4. Compared to point sources in the same redshift range, extended sources at z < 1 show systematically lower Eddington ratios. The catalog and spectra are publicly available online.

We present results from a Chandra X-Ray Observatory study of the field X-ray source populations in four different observations: two high-redshift (z ~ 0.5) clusters of galaxies 3C 295 and RX J003033.2+261819; and two noncluster fields with similar exposure time. Surprisingly, the 0.5-2 keV source surface densities (~900-1200 sources deg-2 at a flux limit of 1.5 × 10-15 ergs cm-2 s-1) measured in an ~8' × 8' area surrounding each cluster exceed by a factor of ~2 the value expected on the basis of the ROSAT and Chandra log N- log S, with a significance of ~2 σ each, or ~3.5 σ when the two fields are combined (i.e., a probability to be a statistical fluctuation of <1% and <0.04%, respectively). The same analysis performed on the noncluster fields and on the outer chips of the cluster fields does not show evidence of such an excess. In both cluster fields, the summed 0.5-10 keV spectrum of the detected objects is well fitted by a power law with Γ ~ 1.7 similar to active galactic nuclei (AGNs) and shows no sign of intrinsic absorption. The few (~10 of 35) optical identifications available to date confirm that most of them are, as expected, AGNs, but the number of redshifts available is too small to allow conclusions on their nature. We discuss possible interpretations of the overdensity in terms of a statistical variation of cosmic background sources; a concentration of AGNs and/or powerful starburst galaxies associated with the clusters; and gravitational lensing of background QSOs by the galaxy clusters. All explanations, however, are difficult to reconcile with the large number of excess sources detected. Deeper X-ray observations and more redshifts measurements are clearly required to settle the issue.