We discuss selection of QSO candidates from the combined SDSS and GALEX catalogues. We discuss properties of QSOs within the combined sample, and note uncertainties in number counts and completeness, compared with other SDSS-based samples. We discuss colour and other properties with redshift within the sample and the SEDs for subsets. We estimate the numbers of faint QSOs that are classified as extended objects in the SDSS, and consequent uncertainties that follow.
We discuss the UV, optical, and IR properties of the SDSS sources detected by GALEX as part of its All-sky Imaging Survey Early Release Observations. Virtually all of the GALEX sources in the overlap region are detected by SDSS. GALEX sources represent ~2.5% of all SDSS sources within these fields and about half are optically unresolved. Most unresolved GALEX/SDSS sources are bright blue turn-off thick disk stars and are typically detected only in the GALEX near-UV band. The remaining unresolved sources include low-redshift quasars, white dwarfs, and white dwarf/M dwarf pairs, and these domina…
In the unusual intrinsic QSO redshift models, QSOs are ejected by active galaxies with periodic non-cosmological reshifts, thus QSOs are generally associated with active galaxies, and certain structures will be revealed in the QSO redshift distribution. As the largest homogeneous sample of QSOs and galaxies, SDSS data provide the best opportunity to examine this issue. We review the debates on this issue, focused on those based on SDSS and 2dF data, and conclude that there is no strong connection between foreground active galaxies and high-redshift QSOs. The existence of two dips in the SDSS Q…
We aim to obtain a complete sample of redshift > 3.6 radio QSOs from FIRST sources having star-like counterparts in the SDSS DR5 photometric survey (r<=20.2). We found that simple supervised neural networks, trained on sources with SDSS spectra, and using optical photometry and radio data, are very effective for identifying high-z QSOs without spectra. The technique yields a completeness of 96 per cent and an efficiency of 62 per cent. Applying the trained networks to 4415 sources without DR5 spectra we found 58 z>=3.6 QSO candidates. We obtained spectra of 27 of them, and 17 are confirmed as …
We selected from VLA-FIRST a sample of 94 objects starlike in SDSSS, and with APM colour O-E>2, i.e. consistent with their being high-z QSOs. 78 candidates were classified spectroscopically from published data (mainly SDSS) or observations reported here. The fractions of QSOs (51/78) and z > 3 QSOs (23/78) are comparable to those found in other photometric searches for high-z QSOs. We confirm that O-E>2 ensures inclusion of all QSOs with 3.7 < z < 4.4. The fraction of broad absorption line (BAL) QSOs for 2 < z < 4.4 is 27+-10 per cent (7/26), and the estimated BAL fraction for radio loud (RL) …
Investigations of He II Ly-alpha (304 A rest) absorption toward a half-dozen quasars at z~3-4 have demonstrated the great potential of helium studies of the IGM, but the current critically small sample size of clean sightlines for the He II Gunn-Peterson test limits confidence in cosmological inferences, and a larger sample is required. Although the unobscured quasar sightlines to high redshift are extremely rare, SDSS DR6 provides thousands of z>2.8 quasars. We have cross-correlated these SDSS quasars with GALEX GR2/GR3 to establish a catalog of 200 higher-confidence (~70% secure) cases of qu…
We use the GALEX (Galaxy Evolution Explorer) Medium Imaging Survey (MIS) and All-Sky Imaging Survey (AIS) data available in the first internal release, matched to the SDSS catalogs in the overlapping regions, to classify objects by comparing the multi-band photometry to model colors. We show an example of the advantage of such broad wavelength coverage (GALEX far-UV and near-UV, SDSS ugriz) in classifying objects and augmenting the existing samples and catalogs. From the MIS [AIS] sample over an area of 75 [92] square degrees, we select a total of 1736 [222] QSO candidates at redshift less tha…
The GALEX Arecibo SDSS Survey (GASS) is a large targeted survey that started at Arecibo in March 2008. GASS is designed to measure the neutral hydrogen content of ~1000 massive galaxies (with stellar mass Mstar > 10^10 Msun) at redshift 0.025<z<0.05, uniformly selected from the SDSS spectroscopic and GALEX imaging surveys. Our selected mass range straddles the recently identified "transition mass" (Mstar ~3x10^10 Msun) above which galaxies show a marked decrease in their present to past-averaged star formation rates. GASS will produce the first statistically significant sample of massive "tran…
Many EHB stars have been found in short-period binaries, where the companions in these post-common envelope systems are either white dwarfs or dM stars; these systems are catalogued as hot subdwarfs because the subdwarf is the more luminous component. Hypothesized Roche-lobe overflow systems (with more massive companions) may largely be uncatalogued, since the G band or Ca II K-line from the companion may have caused them to be overlooked or discarded. In particular, many candidate objects were excluded from the PG catalog because of such spectroscopic indicators. Could these rejects include l…
We present a catalog of 36,120 QSO candidates from the Galaxy Evolution Explorer (GALEX) Release Two (GR2) UV catalog and the USNO-A2.0 optical catalog. The selection criteria are established using known quasars from the Sloan Digital Sky Survey (SDSS). The SDSS sample is then used to assign individual probabilities to our GALEX-USNO candidates. The mean probability is ~50%, and would rise to ~65% if better morphological information than that from USNO were available to eliminate galaxies. The sample is ~40% complete for i<=19.1. Candidates are cross-identified in 2MASS, FIRST, SDSS, and XMM-N…
We use the Galaxy Evolution Explorer (GALEX) Medium and All-Sky-Imaging Survey (MIS & AIS) data from the first public data release (GR1), matched to the Sloan Digital Sky Survey (SDSS) DR3 catalog, to perform source classification. The GALEX surveys provide photometry in far- and near-UV bands and the SDSS in five optical bands (u,g,r,i,z). The GR1/DR3 overlapping areas are 363[83]deg^2 for the GALEX AIS[MIS], for sources within the 0.5deg central area of the GALEX fields. Our sample covers mostly |b|>30deg galactic latitudes. We present statistical properties of the GALEX/SDSS matched sources…
We present ultraviolet photometry for a sample of morphologically early-type galaxies selected by matching the Sloan Digital Sky Survey Data Release 1 with the GALEX Medium and All-sky Imaging Surveys. We obtain a working sample of 1032 early-type galaxies with GALEX FUV detections, SDSS spectroscopy, and $z<0.2$. Using the SDSS spectra to identify galaxies with even weak star formation or evidence of AGN, and further removing galaxies with any evidence of non early-type morphology, we derive a final sample of 172 red quiescent early-type galaxies. We find that the $FUV-r$ color has a full ran…
We discuss the panchromatic properties of 99,088 galaxies selected from the SDSS Data Release 1 spectroscopic sample (a flux-limited sample for 1360 deg^2). These galaxies are positionally matched to sources detected by ROSAT, GALEX, 2MASS, IRAS, GB6, FIRST, NVSS and WENSS. We find strong correlations between the detection fraction at other wavelengths and optical properties such as flux, colors, and emission-line strengths. Using GALEX, SDSS, and 2MASS data, we construct the UV-IR broad-band spectral energy distributions for various types of galaxies, and find that they form a nearly one-para…
We combine data from SDSS and the GALEX and Spitzer observatories to create a sample of galaxies observed homogeneously from the UV to the Far-IR. This sample, consisting of ~460 galaxies observed spectroscopically by SDSS provides a multiwavelength (0.15-24 micron) view of obscured and unobscured star formation in nearby (z<0.3) galaxies with SFRs ranging from 0.01 to 100 M_solar/yr. We calculate a robust dust measure from the infrared to UV ratio (IRX) and explore the influence of star formation history (SFH) on the dust-UV color relation (i.e. the IRX-beta relation). We find that the UV col…
Abridged) In the SDSS, several unusual QSOs have been discovered that have very blue rest-frame UV spectra but no discernible emission lines. Their UV spectra strongly resemble that of the newly discovered quasar PHL 1811 ($z=0.192$; $\rm M_{V}=-25.9$). With magnitudes of $B = 14.4$ and $R = 14.1$, PHL 1811 is the second brightest quasar known with $z>0.1$ after 3C 273. Optically it is classified as a Narrow-line Seyfert 1 galaxy (NLS1). Objects of this class are generally strong soft X-ray emitters, but a BeppoSAX observation of PHL 1811 showed that it is anomalously X-ray weak. The inferred …
Aims: In this work the luminosity function of QSOs is measured in the redshift range 3.5<z<5.2 for the absolute magnitude interval -21<M_{145}<-28. The determination of the faint end of the luminosity function at these redshifts provides important constraints on models of joint evolution of galaxies and AGNs. Methods: We have defined suitable criteria to select faint QSOs in the GOODS fields, checking in detail their effectiveness and completeness. Spectroscopic follow-up of the resulting QSO candidates has been carried out. The confirmed sample of faint QSOs is compared with a brighter one de…
We report results from a pilot program to obtain spectroscopy for objects detected in the Galaxy Evolution Explorer (GALEX) Medium Imaging Survey (MIS). Our study examines the properties of galaxies detected by GALEX fainter than the Sloan Digital Sky Survey (SDSS) spectroscopic survey. This is the first study to extend the techinques of Salim et al. 2005 to estimate stellar masses, star formation rates (SFR) and the b (star formation history) parameter for star-forming galaxies out to z~0.7. We obtain redshifts for 50 GALEX MIS sources reaching NUV=23.9 (AB mag), having counterparts in the SD…
We apply machine learning in the form of a nearest neighbor instance-based algorithm (NN) to generate full photometric redshift probability density functions (PDFs) for objects in the Fifth Data Release of the Sloan Digital Sky Survey (SDSS DR5). We use a conceptually simple but novel application of NN to generate the PDFs - perturbing the object colors by their measurement error - and using the resulting instances of nearest neighbor distributions to generate numerous individual redshifts. When the redshifts are compared to existing SDSS spectroscopic data, we find that the mean value of each…
We investigate the relationship between black hole mass and host galaxy velocity dispersion for QSOs in Data Release 3 of the Sloan Digital Sky Survey. We derive black hole mass from the broad Hbeta line width and continuum luminosity, and the bulge stellar velocity dispersion from the [OIII] narrow line width. At higher redshifts, we use MgII and [OII] in place of Hbeta and [OIII]. For redshifts z < 0.5, our results agree with the black hole mass - bulge velocity dispersion relationship for nearby galaxies. For 0.5 < z < 1.2, this relationship appears to show evolution with redshift in the se…
Four large-area Sunyaev-Zeldovich (SZ) experiments -- APEX-SZ, SPT, ACT, and Planck -- promise to detect clusters of galaxies through the distortion of Cosmic Microwave Background photons by hot (> 10^6 K) cluster gas (the SZ effect) over thousands of square degrees. A large observational follow-up effort to obtain redshifts for these SZ-detected clusters is under way. Given the large area covered by these surveys, most of the redshifts will be obtained via the photometric redshift (photo-z) technique. Here we demonstrate, in an application using ~3000 SDSS stripe 82 galaxies with r<20, how th…
We present catalogs of QSO candidates selected using photometry from GALEX combined with SDSS in the Stripe 82 region and Blanco Cosmology Survey (BCS) near declination -55 degrees. The SDSS region contains ~700 objects with magnitude i < 20 and ~3600 objects with i < 21.5 in a ~60 square degree sky region, while the BCS region contains ~280 objects with magnitude i < 20 and ~2000 objects with i < 21.5 for a 11 square degree sky region that is being observed by three current microwave Sunyaev-Zeldovich surveys. Our QSO catalog is the first one in the BCS region. Deep GALEX exposures (~2000 sec…
We present a deep XMM-Newton observation of the z=6.30 QSO SDSS J1030+0524, the second most distant quasar currently known. The data contain sufficient counts for spectral analysis, demonstrating the ability of XMM-Newton to measure X-ray spectral shapes of z~6 QSOs with integration times >100ks. The X-ray spectrum is well fit by a power law with index Gamma=2.12 +/- 0.11, an optical-X-ray spectral slope of a_{ox}=-1.80, and no absorption excess to the Galactic value, though our data are also consistent with a power law index in the range 2.02 < Gamma < 2.5 and excess absorption in the range 0…
We present a sample of 33 damped Lyman alpha systems (DLAs) discovered in the Sloan Digital Sky Survey (SDSS) whose absorption redshifts (z_abs) are within 6000 km/s of the QSO's systemic redshift (z_sys). Our sample is based on 731 2.5 < z_sys < 4.5 non-broad-absorption-line (non-BAL) QSOs from Data Release 3 (DR3) of the SDSS. We estimate that our search is ~100 % complete for absorbers with N(HI) >= 2e20 cm^-2. The derived number density of DLAs per unit redshift, n(z), within v < 6000 km/s is higher (3.5 sigma significance) by almost a factor of 2 than that of intervening absorbers observe…
We investigate the production of nitrogen in star forming galaxies with ultraviolet (UV) radiation detected by the Galaxy Evolution Explorer Satellite (GALEX). We use a sample of 8,745 GALEX emission line galaxies matched to the Sloan Digital Sky Survey (SDSS) spectroscopic sample. We derive both gas-phase oxygen and nitrogen abundances for the sample, and apply stellar population synthesis models to derive stellar masses and star formation histories of the galaxies. We compare oxygen abundances derived using three different diagnostics. We derive the specific star formation rates of the galax…
We combine GALEX near-UV photometry with a volume-limited sample of local (0.005<z<0.037) SDSS DR4 galaxies to examine the composition and the environmental dependencies of the optical and UV-optical colour-magnitude (C-M) diagrams. We find that ~30% of red sequence galaxies in the optical C-M diagram show signs of ongoing star-formation from their spectra having EW(Halpha)>2A. This contamination is greatest at faint magnitudes (Mr>-19) and in field regions where as many as three-quarters of red sequence galaxies are star-forming, and as such has important consequences for following the build-…
We use photometric data from Spitzer to explore the mid- and far-IR properties of 10 red QSOs (J-K>2, R-K>5) selected by combining the 2MASS in the NIR with the SDSS at optical wavelengths. Optical and/or near-infrared spectra are available for 8/10 sources. Modeling the SED from UV to far-IR shows that moderate dust reddening (A_V=1.3-3.2) can explain the red optical and near-IR colours of the sources in the sample. There is also evidence that red QSOs have 60/12micron luminosity ratio higher than PG QSOs (97% significance). This can be interpreted as a higher level of star-formation in these…
We present sensitive imaging at 1.4 GHz of the two highest redshift far-infrared (FIR) luminous QSOs SDSS J114816.65+525150.2 (z=6.42) and SDSS J104845.05+463718.3 (z=6.2). Radio continuum emission is detected from J1148+5251 with S_{1.4} = 55 \pm 12 uJy, while J1048+4637 is marginally detected with S_{1.4} = 26 \pm 12 uJy. Comparison of the radio and FIR luminosities shows that both sources follow the radio-FIR correlation for star forming galaxies, with implied (massive) star formation rates \sim 10^3 M_sun year^{-1}, although we cannot rule-out as much as 50% of the FIR luminosity being pow…
We present measurements of the UV galaxy luminosity function and the evolution of luminosity density from GALEX observations matched to the Sloan Digital Sky Survey (SDSS). We analyze galaxies in the Medium Imaging Survey overlapping the SDSS DR1 with a total coverage of 44 deg^2. Using the combined GALEX+SDSS photometry, we compute photometric redshifts and study the LF in three redshift shells between z=0.07 and 0.25. The Schechter function fits indicate that the faint-end slope alpha is consistent with -1.1 at all redshifts but the characteristic UV luminosity M* brightens by 0.2 mag from z…
From SDSS commissioning photometric and spectroscopic data, we investigate the utility of photometric redshift techniques to the task of estimating QSO redshifts. We consider empirical methods (e.g. nearest-neighbor searches and polynomial fitting), standard spectral template fitting and hybrid approaches (i.e. training spectral templates from spectroscopic and photometric observations of QSOs). We find that in all cases, due to the presence of strong emission-lines within the QSO spectra, the nearest-neighbor and template fitting methods are superior to the polynomial fitting approach. Applyi…
We derive a variety of physical parameters including star formation rates (SFRs), dust attenuation and burst mass fractions for 6472 galaxies observed by the Galaxy Evolution Explorer (GALEX) and present in the SDSS DR1 main spectroscopic sample. Parameters are estimated in a statistical way by comparing each observed broad-band SED (two GALEX and five SDSS bands) with an extensive library of model galaxy SEDs, which cover a wide range of star formation histories and include stochastic starbursts. We compare the constraints derived using SDSS bands only with those derived using the combination…
We analyzed the spectroscopic and photometric database of the 5th data release of the Sloan Digital Sky Survey (SDSS) to search for evidence of the quasar reddening produced by dust embedded in intervening damped Ly alpha (DLA) systems. From a list of 5164 quasars in the interval of emission redshift 2.25 </= z_e </= 3.5 and SDSS spectra with signal-to-noise ratio SNR >/= 4, we built up an "absorption sample" of 248 QSOs with a single DLA system in the interval of absorption redshift 2.2 < z_a </= 3.5 and a "pool" of 1959 control QSOs without DLA systems or strong metal systems. For each QSO o…
We use Sloan Digital Sky Survey Data Release 2 QSO spectra to constrain the dust-reddening caused by intervening damped Lyman-alpha systems (DLAs). Comparing the spectral index distribution of a 70 sight-line DLA sample with that of a large control sample reveals no evidence for dust-reddening at z~3. Our limit on the shift in spectral index, |Delta(alpha)| < 0.19 (3-sigma), corresponds to a limit on the colour excess due to SMC-like dust-reddening, E(B-V) < 0.02 mag (3-sigma). This is inconsistent with the early studies of Fall, Pei and collaborators who used the small QSO and DLA samples ava…
We use a simple optical/infrared (IR) photometric selection of high-redshift QSOs that identifies a Lyman Break in the optical photometry and requires a red IR color to distinguish QSOs from common interlopers. The search yields 100 z~3 (U-dropout) QSO candidates with 19<r'<22 over 11.7 deg^2 in the ELAIS-N1 (EN1) and ELAIS-N2 (EN2) fields of the Spitzer Wide-area Infrared Extragalactic (SWIRE) Legacy Survey. The z~3 selection is reliable, with spectroscopic follow-up of 10 candidates confirming they are all QSOs at 2.83<z<3.44. We find that our z~4$ (g'-dropout) sample suffers from both unrel…
The lensing of background QSOs by foreground galaxies is a powerful probe of the mass density of the Universe and the power spectrum of mass clustering. However, the 2dF QSO survey suggested that a strong anticorrelation effect at g<21 was seen for both galaxies and clusters which implied that galaxies are anti-biased (b~0.1) on small scales at a higher level than predicted by the standard cosmology (Myers et al., 2003, 2005) whereas results from SDSS suggested that the effect was much smaller (b~0.6) and in line with standard expectations (Scranton et al., 2005). We first cross-correlate the …
We combine the QSO samples from the 2dF QSO Redshift Survey (2QZ) and the 2dF-SDSS LRG and QSO Survey (2SLAQ) in order to investigate the clustering of z~1.4 QSOs and measure the correlation function. The clustering signal in z-space, projected along the sky direction, is similar to that previously obtained from 2QZ alone. By fitting the z-space correlation function and lifting the degeneracy between beta and Omega_m_0 by using linear theory predictions, we obtain beta(z=1.4) = 0.60+-0.12 and Omega_m_0=0.25+-0.08, implying a value for the QSO bias, b(z=1.4)=1.5+-0.2. We further find that QSO c…
We analyze archived Chandra and XMM-Newton X-ray observations of 536 Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5) quasars (QSOs) at 1.7 <= z <= 2.7 in order to characterize the relative UV and X-ray spectral properties of QSOs that do not have broad UV absorption lines (BALs). We constrain the fraction of X-ray weak, non-BAL QSOs and find that such objects are rare; for example, sources underluminous by a factor of 10 comprise $\la$2% of optically-selected SDSS QSOs. X-ray luminosities vary with respect to UV emission by a factor of $\la$2 over several years for most sources. UV contin…
We present the final spectroscopic QSO catalogue from the 2dF-SDSS LRG and QSO (2SLAQ) Survey. This is a deep, 18<g<21.85 (extinction corrected), sample aimed at probing in detail the faint end of the broad line AGN luminosity distribution at z<2.6. The candidate QSOs were selected from SDSS photometry and observed spectroscopically with the 2dF spectrograph on the Anglo-Australian Telescope. This sample covers an area of 191.9 deg^2 and contains new spectra of 16326 objects, of which 8764 are QSOs, and 7623 are newly discovered (the remainder were previously identified by the 2QZ and SDSS sur…
Using a large sample of 26623 quasars with redshifts in the range $1.5 \le z\le 5.1$ with \civ $λ$1549 Åemission line in Fifth Data Release of the Sloan Digital Sky Survey (SDSS), we investigate the cosmological evolution of the Baldwin Effect, i.e. the relation between the equivalent width (EW) of the \civ emission line and continuum luminosity. We confirm the earlier result that there exists a strong correlation between the \civ EW and the continuum luminosity, and we find that, up to $z\approx 5$, the slope of the Baldwin Effect seems to have no effect of cosmological evolution. A sub-sampl…
We present an analysis of the broadband UV/optical properties of z<3.4 quasars matched in the Galaxy Evolution Explorer (GALEX) General Data Release 1 (GR1) and the Sloan Digital Sky Survey Data Release 3 (SDSS DR3). Of the 6371 DR3 quasars covered by 204 GR1 tiles, 5380 have near-UV detections, while 3034 have both near-UV and far-UV detections using a matching radius of 7". Most of the DR3 sample quasars are detected in the near-UV until z~1.7, with the near-UV detection fraction dropping to ~50% by z~2. Statistical tests performed on the distributions of non-detections indicate that the opt…
We have constructed samples of low-ionization metal absorption-line systems selected for the strength of their MgII lambda2796 lines from the SDSS EDR. There are 640 absorption systems in our unbiased sample with lambda2796 REW > 1.0 A detected at 5 sigma and with redshifts 0.37 < z < 2.27. This sample is an order of magnitude larger than previous similarly selected samples. In this contribution, we present preliminary results on the statistical properties of the sample.
We obtained spectra of 60 red, starlike objects (E< 18.8) identified with FIRST radio sources, S_{1.4GHz} > 1 mJy. Eight are QSOs with redshift z> 3.6.Combined with our pilot search (Benn et al 2002), our sample of 121 candidates yields a total of 18 z > 3.6 QSOs (10 of these with z > 4.0). 8% of candidates with S_{1.4GHz}< 10 mJy, and 37% of candidates with S_{1.4GHz}> 10 mJy are QSOs with z > 3.6. The surface density of E < 18.8, S_{1.4GHz} > 1mJy, z> 4 QSOs is 0.003 deg^{-2}. This is currently the only well-defined sample of radio-loud QSOs at z ~ 4 selected independently of radio spectral …
We analyze a volume-limited sample of massive bulge-dominated galaxies with data from both the Sloan Digital Sky Survey and the Galaxy Evolution Explorer (GALEX) satellite. The galaxies have central velocity dispersions greater than 100 km/s and stellar surface mass densities that lie above the value where galaxies transition from actively star forming to passive systems. The sample is limited to redshifts 0.03<z<0.07. At these distances, the SDSS spectra sample the light from the bulge-dominated central regions of the galaxies. The GALEX NUV data provide high sensitivity to low rates of globa…
We present the first measurements of the angular correlation function of galaxies selected in the far (1530 A) and near (2310 A) Ultraviolet from the GALEX survey fields overlapping SDSS DR5 in low galactic extinction regions. The area used covers 120 sqdeg (GALEX - MIS) down to magnitude AB = 22, yielding a total of 100,000 galaxies. The mean correlation length is ~ 3.7 \pm 0.6 Mpc and no significant trend is seen for this value as a function of the limiting apparent magnitude or between the GALEX bands. This estimate is close to that found from samples of blue galaxies in the local universe …
Using a sample of nearly 700 quasars with strong (W_0(2796)>0.8 Angstrom) MgII absorption lines detected in the Early Data Release of the SDSS, we demonstrate the feasibility of measuring the photometric properties of the absorber systems by stacking SDSS imaging data. As MgII lines can be observed in the range 0.37<z_abs<2.2, the absorbing galaxies are in general not identified in SDSS images, but they produce systematic light excesses around QSOs which can be detected with a statistical analysis. In this Letter we present a 6-sigma detection of this effect over the whole sample in i-band, ri…
We study the Distant Red Galaxy (DRG, J-K_s > 2.3) neighbour population of Quasi Stellar Objects (QSOs) selected from the Sloan Digital Sky Survey (SDSS) in the redshift range 1 < z < 2. We perform a similar analysis for optically obscured AGNs (i.e. with a limiting magnitude I > 24) detected in the mid-infrared (24 $μ$m) with the Spitzer Space Telescope and a mean redshift $z\sim 2.2$ in the Flamingos Extragalactic Survey (FLAMEX). Both QSOs and obscured AGN target samples cover 4.7 deg$^2$ in the same region of the sky. We find a significant difference in the environment of these two target …
Observations of quasars (QSOs) shining through or close to galaxies offer a way to probe the properties of the foreground matter through dust extinction and gravitational lensing. In this paper the feasibility of measuring the dust extinction properties is investigated using the backlitting of QSOs. We test our method to search for QSOs affected by intervening extinction, by matching the coordinates in the SDSS QSO DR3 catalogue with the New York University Value-Added Galaxy Catalog. In total, 164 QSO-galaxy pairs were found with a distance of less than 30 kpc between the galaxy centre and th…
We study the spectral energy distributions and evolution of a large sample of optically selected quasars from the Sloan Digital Sky Survey (SDSS) that were observed in 323 Chandra images analyzed by the Chandra Multiwavelength Project (ChaMP). Our highest-confidence matched sample includes 1135 X-ray detected quasars in the redshift range 0.2<z<5.4, representing some 36Msec of effective exposure. Spectroscopic redshifts are available for about 1/3 of the detected sample; elsewhere, redshifts are estimated photometrically. With 56 z>3 QSOs detected, we find no evidence for evolution out to z~5 …
