Many low-luminosity active galactic nuclei (AGNs) contain a compact radio core which can be observed with high angular resolution using very long baseline interferometry (VLBI). Combining arcsec-scale structural information with milliarcsec-resolution VLBI imaging is a useful way to characterise the objects and to find compact cores on parsec scales. VLBI imaging could also be employed to look for dual AGNs when the sources show kpc-scale double symmetric structure with flat or inverted radio spectra. We observed five such sources at redshifts 0.36 < z < 0.58 taken from an optically selected sample of Type 2 quasars with the European VLBI Network (EVN) at 1.7 and 5 GHz. Out of the five sources, only one (SDSS J1026-0042) shows a confidently detected compact VLBI core at both frequencies. The other four sources are marginally detected at 1.7 GHz only, indicating resolved-out radio structure and steep spectra. Using first-epoch data from the ongoing Karl G. Jansky Very Large Array Sky Survey, we confirm that indeed all four of these sources have steep radio spectra on arcsec scale, contrary to the inverted spectra reported earlier in the literature. However, the VLBI-detected source, SDSS J1026-0042, has a flat integrated spectrum. Radio AGNs that show kpc-scale symmetric structures with truly flat or inverted spectra could still be promising candidates of dual AGNs, to be targeted with VLBI observations in the future.
Optically bright, wide separation double (gravitationally lensed) quasars can be easily monitored, leading to light curves of great importance in determining the Hubble constant and other cosmological parameters, as well as the structure of active nuclei and halos of galaxies. Searching for new double quasars in the SDSS-III database, we discovered SDSS J1442+4055. This consists of two bright images (18-19 magnitudes in the r band) of the same distant quasar at redshift z = 2.575. The two quasar images are separated by about 2.1 arcsec, show significant parallel flux variations and can be moni…
The origin of the radio emission in Radio Quiet (RQ) quasars is not established yet. Important hints can be provided by the spectral slope, and its relation to other emission properties. We compiled the highest resolution 5 and 8.4 GHz VLA observations available of the RQ optically selected PG quasars at z<0.5. We derive the 5-8.4 GHz spectral slope, alpha_R, for 25 of the complete and well studied sample of 71 RQ PG quasars. We find a highly significant correlation of alpha_R with L/L_Edd, where high L/L_Edd (>0.3) quasars have a steep slope (alpha_R<-0.5), indicative of an optically thin syn…
We present the radio luminosity function (RLF) of optically-selected quasars below 1~mJy, constructed by applying a Bayesian-fitting stacking technique to objects well below the nominal radio flux-density limit. We test the technique using simulated data, confirming that we can reconstruct the RLF over three orders of magnitude below the typical $5σ$ detection threshold. We apply our method to 1.4-GHz flux-densities from the Faint Images of the Radio Sky at Twenty-cm survey (FIRST), extracted at the positions of optical quasars from the Sloan Digital Sky Survey (SDSS) over seven redshift bins …
We present the result of our spectroscopic follow-up observation for faint quasar candidates at z~5 in a part of the Canada-France-Hawaii Telescope Legacy Survey wide field. We select nine photometric candidates and identify three z~5 faint quasars, one z~4 faint quasar, and a late-type star. Since two faint quasar spectra show Civ emission line without suffering from a heavy atmospheric absorption, we estimate the black hole mass (M$_{BH}$) and Eddington ratio (L/L$_{Edd}$) of them. The inferred log M$_{BH}$ are 9.04+/-0.14 and 8.53+/-0.20, respectively. In addition, the inferred log (L/L$_{E…
Active galactic nuclei (AGN) that show strong rest-frame optical/UV variability in their blue continuum and broad line emission are classified as "changing-look" AGN, or at higher luminosities changing look quasars (CLQs). These surprisingly large and sometimes rapid transitions challenge accepted models of quasar physics and duty cycles, offer several new avenues for study of quasar host galaxies, and open a wider interpretation of the cause of differences between broad and narrow line AGN. To better characterize extreme quasar variability, we present follow-up spectroscopy as part of a compr…
The role of radio mode feedback in non radio-loud quasars needs to be explored in depth to determine its true importance. Its effects can be identified based on the evidence of interactions between the radio structures and the ambient ionised gas. We investigate this in a sample of 13 optically selected type-2 quasars (QSO2) at z<0.2 with FIRST radio detections. None are radio loud. All show complex optical morphologies, with signs of distortion across tens of kpc due to mergers/interactions. The radio luminosity has an AGN component in 11/13 QSO2. It is spatially extended in 9 of them (jets/l…
We carry out a series of deep Karl G. Jansky Very Large Array (VLA) S-band observations of a sample of 21 quasars at $z\sim6$. The new observations expand the searches of radio continuum emission to the optically faint quasar population at the highest redshift with rest-frame $4400 \rm Å$ luminosities down to $3 \times10^{11} \ L_{\odot}$. We report the detections of two new radio-loud quasars: CFHQS J2242+0334 (hereafter J2242+0334) at $z=5.88$ and CFHQS J0227$-$0605 (hereafter J0227$-$0605) at $z=6.20$, detected with 3 GHz flux densities of $87.0 \pm 6.3 \ μ\rm Jy$ and $55.4 \pm 6.7 \ μ\rm J…
The radio-loud/radio-quiet (RL/RQ) dichotomy in quasars is still an open question. Although it is thought that accretion onto supermassive black holes in the centre the host galaxies of quasars is responsible for some radio continuum emission, there is still a debate as to whether star formation or active galactic nuclei (AGN) activity dominate the radio continuum luminosity. To date, radio emission in quasars has been investigated almost exclusively using high-frequency observations in which the Doppler boosting might have an important effect on the measured radio luminosity, whereas extended…
We present preliminary results of the CIDA Equatorial Variability Survey (CEVS), looking for quasar (hereafter QSO) candidates near the Galactic plane. The CEVS contains photometric data from extended and adjacent regions of the Milky Way disk ($\sim$ 500 sq. deg.). In this work 2.5 square degrees with moderately high temporal sampling in the CEVS were analyzed. The selection of QSO candidates was based on the study of intrinsic optical photometric variability of 14,719 light curves. We studied samples defined by cuts in the variability index (Vindex $>$ 66.5), periodicity index (Q $>$ 2), and…
The observed optical colors of quasars are generally interpreted in one of two frameworks: unified models which attribute color to random orientation of the accretion disk along the line-of-sight, and evolutionary models which invoke connections between quasar systems and their environments. We test these schema by probing the dark matter halo environments of optically-selected quasars as a function of $g-i$ optical color by measuring the two-point correlation functions of $\sim$ 0.34 million eBOSS quasars as well as the gravitational deflection of cosmic microwave background photons around $\…
We study the environments of 49 WISE/NVSS-selected dusty, hyper-luminous, z~2 quasars using the Atacama Large Millimeter/Sub-millimeter Array (ALMA) 345GHz images. We find that 17 of the 49 WISE/NVSS sources show additional sub-mm galaxies within the ALMA primary beam, probing scales within ~150 kpc. We find a total of 23 additional sub-mm sources, four of which in the field of a single WISE/NVSS source. The measured 870 um source counts are ~10 times expectations for unbiased regions, suggesting such hyper-luminous dusty quasars are excellent at probing high-density peaks.
Optically quiescent quasars (OQQs) represent a recently systematised class of infrared-luminous active galactic nuclei (AGN) which have galaxy-like optical continua. They may represent an interesting, brief phase in the AGN life cycle, e.g. either cocooned within high-covering-factor media or indicative of recent triggering, though their nature remains unclear. Here, we present the first targeted simultaneous X-ray observations of an OQQ, our previously identified prototype, SDSS J075139.06+402811.2 at $z$=0.587. The source is significantly detected over 0.5-16 keV with XMM-Newton and NuSTAR, …
Recent studies of luminous infrared-selected active galactic nuclei (AGN) suggest that the reddest, most obscured objects display a higher angular clustering amplitude, and thus reside in higher-mass dark matter halos. This is a direct contradiction to the prediction of the simplest unification-by-orientation models of AGN and quasars. However, clustering measurements depend strongly on the "mask" that removes low-quality data and describes the sky and selection function. We find that applying a robust, conservative mask to WISE-selected quasars yields a weaker but still significant difference…
Recent work has found evidence for a difference in the bias and dark matter halo masses of WISE-selected obscured and unobscured quasars, implying a distinction between these populations beyond random line-of-sight effects. However, the significance of this difference in the most up-to-date measurements is relatively weak, at $\sim$2$σ$ for individual measurements but bolstered by agreement from different techniques, including angular clustering and cross-correlations with cosmic microwave background (CMB) lensing maps. Here, we expand the footprint of previous work, aiming to improve the prec…
Recent measurements of the dark matter halo masses of infrared-selected obscured quasars are in tension --- some indicate that obscured quasars have higher halo mass compared to their unobscured counterparts, while others find no difference. The former result is inconsistent with the simplest models of quasar unification that rely solely on viewing angle, while the latter may support such models. Here, using empirical relationships between dark matter halo and supermassive black hole masses, we provide a simple evolutionary picture that naturally explains these findings and is motivated by mor…
We present the results of a new spectroscopic survey for dusty intervening absorption systems, particularly damped Ly$α$ absorbers (DLAs), towards reddened quasars. The candidate quasars are selected from mid-infrared photometry from the Wide-field Infrared Survey Explorer combined with optical and near-infrared photometry. Out of 1073 candidates, we secure low-resolution spectra for 108 using the Nordic Optical Telescope on La Palma, Spain. Based on the spectra, we are able to classify 100 of the 108 targets as quasars. A large fraction (50 %) is observed to have broad absorption lines (BALs)…
Quasi--stellar objects (quasars) located behind nearby galaxies provide an excellent absolute reference system for astrometric studies, but they are difficult to identify because of fore- and background contamination. Deep wide--field, high angular resolution surveys spanning the entire area of nearby galaxies are needed to obtain a complete census of such quasars. We embarked on a program to expand the quasar reference system behind the Large and the Small Magellanic Clouds, the Magellanic Bridge, and the Magellanic Stream, connecting the Clouds with the Milky Way. Hundreds of quasar candidat…
We present our ALMA Cycle 4 measurements of the [CII] emission line and the underlying far-infrared (FIR) continuum emission from four optically low-luminosity ($M_{\rm 1450} > -25$) quasars at $z \gtrsim 6$ discovered by the Subaru Hyper Suprime Cam (HSC) survey. The [CII] line and FIR continuum luminosities lie in the ranges $L_{\rm [CII]} = (3.8-10.2) \times 10^8~L_\odot$ and $L_{\rm FIR} = (1.2-2.0) \times 10^{11}~L_\odot$, which are at least one order of magnitude smaller than those of optically-luminous quasars at $z \gtrsim 6$. We estimate the star formation rates (SFR) of our targets a…
We exploit the spectral archive of the Sloan Digital Sky Survey (SDSS) Data Release 7 to select unusual quasar spectra. The selection method is based on a combination of the power of self-organising maps and the visual inspection of a huge number of spectra. Self-organising maps were applied to nearly 10^5 spectra classified as quasars by the SDSS pipeline. Particular attention was paid to minimise possible contamination by rare peculiar stellar spectral types. We present a catalogue of 1005 quasars with unusual spectra. This large sample provides a useful resource for both studying properties…
Obscuration in quasars may arise from steep viewing angles along the dusty torus, or instead may represent a distinct phase of supermassive black hole growth. We test these scenarios by probing the host dark matter halo environments of $\sim 1.4$ million WISE-selected obscured and unobscured quasars at $\langle z \rangle = 1.4$ using angular clustering measurements as well as cross-correlation measurements of quasar positions with the gravitational lensing of the cosmic microwave background (CMB). We interpret these signals within a halo occupation distribution (HOD) framework to conclude that…
In the framework of the Gravitational LENses and DArk MAtter (GLENDAMA) project, we present a database of nine gravitationally lensed quasars (GLQs) that have two or four images brighter than $r$ = 20 mag and are located in the northern hemisphere. This new database consists of a rich variety of follow-up observations included in the GLENDAMA global archive, which is publicly available online and contains 6557 processed astronomical frames of the nine lens systems over the period 1999$-$2016. In addition to the GLQs, our archive also incorporates binary quasars, accretion-dominated radio-loud …
We present ten metallicity measurements for quasar absorbers near $z=1.2$ that were selected for having unusually significant ZnII absorption in their SDSS spectra. Follow-up UV space spectroscopy of the Ly$α$ region shows that all ten have damped Ly$α$ (DLA) absorption, corresponding to neutral hydrogen column densities in the range $2.4\times 10^{20}$ $\le$ N(HI) $\le$ $2.5\times 10^{21}$ atoms cm$^{-2}$, and indicating that the gas is very optically thick and essentially neutral. The sample is a very small subset of systems compiled by searching the University of Pittsburgh catalog of $\app…
We report the result of optical identifications of FIRST radio sources with the Hyper Suprime-Cam Subaru Strategic Program survey (HSC-SSP). The positional cross-match within 1" between the FIRST and HSC-SSP catalogs (i ~< 26) produced more than 3600 optical counterparts in the 156 deg^2 of the HSC-SSP field. The matched counterparts account for more than 50% of the FIRST sources in the search field, which substantially exceed previously reported fractions of SDSS counterparts (i ~< 22) of ~30%. Among the matched sample, 9% are optically unresolved sources such as radio-loud quasars. The optic…
Many Active Galactic Nuclei (AGN) surveys rely on optical emission line signatures for robust source classification. There are, however, examples of luminous AGN candidates lacking such signatures, including those from the narrow line region, which are expected to be less susceptible to classical nuclear (torus) obscuration. Here, we seek to formalise this subpopulation of AGN with a prototypical candidate, SDSS J075139.06+402810.9. This shows IR colours typical for AGN, an optical spectrum dominated by an early type galaxy continuum,an [OIII] 5007Å limiting flux about two dex below Type 2 qua…
Radio-loud quasars (RLQs) are more X-ray luminous than predicted by the X-ray-optical/UV relation (i.e. $L_\mathrm{x}\propto L_\mathrm{uv}^γ$) for radio-quiet quasars (RQQs). The excess X-ray emission depends on the radio-loudness parameter ($R$) and radio spectral slope ($α_\mathrm{r}$). We construct a uniform sample of 729 optically selected RLQs with high fractions of X-ray detections and $α_\mathrm{r}$ measurements.We find that steep-spectrum radio quasars (SSRQs; $α_\mathrm{r}\le-0.5$) follow a quantitatively similar $L_\mathrm{x}\propto L_\mathrm{uv}^γ$ relation as that for RQQs, suggest…
We present the luminosity function of z=4 quasars based on the Hyper Suprime-Cam Subaru Strategic Program Wide layer imaging data in the g, r, i, z, and y bands covering 339.8 deg^2. From stellar objects, 1666 z~4 quasar candidates are selected by the g-dropout selection down to i=24.0 mag. Their photometric redshifts cover the redshift range between 3.6 and 4.3 with an average of 3.9. In combination with the quasar sample from the Sloan Digital Sky Survey in the same redshift range, the quasar luminosity function covering the wide luminosity range of M1450=-22 to -29 mag is constructed. It is…
We performed a search for WLQs in the spectroscopic data from the Sloan Digital Sky Survey Data Release 7 based on Kohonen self-organising maps for nearly 10^5 quasar spectra. The final sample consists of 365 quasars and includes in particular a subsample of 46 WLQs with equivalent widths W(MgII) < 11 A and W(CIV) < 4.8 A. We compared various properties of the WLQs with those of control samples of ordinary quasars. Particular attention was paid to selection effects. The WLQs have, on average, significantly higher luminosities, Eddington ratios, and accretion rates. About half of the excess com…
We investigate the properties of HI-rich galaxies detected in blind radio surveys within the hierarchical structure formation scenario using a semi-analytic model of galaxy formation. By drawing a detailed comparison between the properties of HI-selected galaxies and HI absorption systems, we argue a link between the local galaxy population and quasar absorption systems, particularly for Damped Ly-alpha absorption (DLA) systems and sub-DLA systems. First, we evaluate how many HI-selected galaxies exhibit HI column densities as high as those of DLA systems. We find that HI-selected galaxies wit…
We studied a sample of 274 radio and X-ray selected quasars (XQSOs) detected in the COSMOS and XXL-S radio surveys at 3 GHz and 2.1 GHz, respectively. This sample was identified by adopting a conservative threshold in X-ray luminosity, Lx [2-10\ keV] >= 10^44 erg/s, selecting only the most powerful quasars. Using available multiwavelength data, we examined various criteria for the selection of radio-loud (RL) and radio-quiet (RQ) XQSOs, finding that the number of RL/RQ XQSOs changes significantly depending on the chosen criterion. This discrepancy arises due to the different criteria tracing d…
We model the projected angular two-point correlation function (2PCF) of obscured and unobscured quasars selected using the Wide-field Infrared Survey Explorer (WISE), at a median redshift of $z \sim 1$ using a five-parameter Halo Occupation Distribution (HOD) parameterization, derived from a cosmological hydrodynamic simulation by Chatterjee et al. The HOD parameterization was previously used to model the 2PCF of optically selected quasars and X-ray bright active galactic nuclei (AGN) at $z \sim 1$. The current work shows that a single HOD parameterization can be used to model the population o…
We present the results of an optical spectroscopic survey of 46 heavily obscured quasar candidates. Objects are selected using their mid-infrared (mid-IR) colours and magnitudes from the Wide-Field Infrared Survey Explorer (WISE) and their optical magnitudes from the Sloan Digital Sky Survey (SDSS). Candidate Active Galactic Nuclei (AGNs) are selected to have mid-IR colours indicative of quasar activity and lie in a region of mid-IR colour space outside previously published X-ray based selection regions. We obtain optical spectra for our sample using the Robert Stobie Spectrograph on the South…
We report a new changing-look quasar, WISE~J105203.55+151929.5 at $z=0.303$, found by identifying highly mid-IR variable quasars in the WISE/NEOWISE data stream. Compared to multi-epoch mid-IR photometry of a large sample of SDSS-confirmed quasars, WISE J1052+1519 is an extreme photometric outlier, fading by more than a factor of two at $3.4$ and $4.6 μ$m since 2009. Swift target-of-opportunity observations in 2017 show even stronger fading in the soft X-rays compared to the ROSAT detection of this source in 1995, with at least a factor of fifteen decrease. We obtained second-epoch spectroscop…
A minority of the optically selected quasar population are red at optical wavelengths due to the presence of dust along the line-of-sight. A key focus of many red quasar studies is to understand their relationship with the overall quasar population: are they blue quasars observed at a (slight) inclination angle or do they represent a transitional phase in the evolution of quasars? Identifying fundamental differences between red and blue quasars is key to discriminate between these two paradigms. To robustly explore this, we have uniformly selected quasars from the Sloan Digital Sky Survey with…
We provide a catalog of 391 mid-infrared-selected (MIR, 24$μ$m) broad-emission-line (BEL, type 1) quasars in the 22 deg$^2$ SWIRE Lockman Hole field. This quasar sample is selected in the MIR from Spitzer MIPS with $S_{\rm 24} > 400μ$Jy, jointly with an optical magnitude limit of r (AB) $<$ 22.5 for broad line identification. The catalog is based on MMT and SDSS spectroscopy to select BEL quasars, extends the SDSS coverage to fainter magnitudes and lower redshifts, and recovers a more complete quasar population. The MIR-selected quasar sample peaks at $z\sim$1.4, and recovers a significant and…
We measure the average deflection of cosmic microwave background photons by quasars at $\langle z \rangle =1.7$. Our sample is selected from the Sloan Digital Sky Survey to cover the redshift range $0.9\leq z\leq2.2$ with absolute i-band magnitudes of $M_i\leq-24$ (K-corrected to z=2). A stack of nearly 200,000 targets reveals an 8$σ$ detection of Planck's estimate of the lensing convergence towards the quasars. We fit the signal with a model comprising a Navarro-Frenk-White density profile and a 2-halo term accounting for correlated large scale structure, which dominates the observed signal. …
We present an estimate of the optical luminosity function (OLF) of LOFAR radio-selected quasars (RSQs) at $1.4<z<5.0$ in the $9.3\:\textrm{deg}^{2}$ NOAO Deep Wide-field survey (NDWFS) of the Boötes field. The selection was based on optical/mid-ir photometry used to train three different machine learning (ML) algorithms. Objects taken as quasars by the ML algorithms are required to be detected at $5σ$ significance in deep radio maps to be classified as candidate quasars. The optical imaging came from the SDSS and the PS1 $3π$ survey; mid-ir photometry was taken from the SDWFS survey; and radio…
We perform a Halo Occupation Distribution (HOD) modeling of the projected two-point correlation function (2PCF) of quasars that are observed in the Wide-field Infrared Survey Explorer (WISE) telescope with counter-parts in the Sloan Digital Sky Survey (SDSS) Data Release (DR)-8 quasar catalog at a median redshift of $z\sim 1.04 (\pm 0.58)$. Using a four parameter HOD model we derive the host mass scales of WISE selected quasars. Our results show that the median halo masses of central and satellite quasars lie in the range $M_{\mathrm{cen}} = (5 \pm 1.0) \times 10^{12} M_{\odot}$ and $M_{\mathr…
Thanks to incredible advances in instrumentation, surveys like the Sloan Digital Sky Survey have been able to find and catalog billions of objects, ranging from local M dwarfs to distant quasars. Machine learning algorithms have greatly aided in the effort to classify these objects; however, there are regimes where these algorithms fail, where interesting oddities may be found. We present here an X-ray bright quasar misidentified as a red supergiant/X-ray binary, and a subsequent search of the SDSS quasar catalog for X-ray bright stars misidentified as quasars.
Many brightest cluster galaxies (BCGs) at the centers of X-ray selected clusters exhibit clear evidence for recent star formation. However, studies of BCGs in optically-selected clusters show that star formation is not enhanced when compared to control samples of non-BCGs of similar stellar mass. Here we analyze a sample of 113 BCGs in low redshift (z<0.1), optically-selected clusters, a matched control sample of non-BCGs, and a smaller sample of BCGs in X-ray selected clusters. We convolve the SDSS images of the BCGs to match the resolution of the GALEX data and we measure UV-optical colours …
The radiative and jet power in active galactic nuclei is generated by accretion of material on to supermassive galactic-centre black holes. For quasars, where the radiative power is by definition very high, objects with high radio luminosities form around 10 per cent of the population, although it is not clear whether this is a stable phase. Traditionally, quasars with high radio luminosities have been thought to present jets with edge-brightened morphology (Fanaroff-Riley II - FR II) due to the limitations of previous radio surveys (i.e., FRIs were not observed as part of the quasar populatio…
Quasars are very important in materializing the reference frame. The excess emission of AGNs (active galactic nuclei) in the mid infrared band can be used to identify quasar candidates. As extremely distant and point like objects, quasars also could be further selected by astrometry method. Increasing the number of reliable quasar candidates is necessary in characterizing the properties of Gaia astrometric solution and evaluating the reliability of Gaia own quasars classification. We identify quasars by using appropriate AllWISE [W1-W2] color and different combinations of astrometric criteria.…
Periodic quasars have been suggested as candidates for hosting binary supermassive black holes (SMBHs), although alternative scenarios remain possible to explain the optical light curve periodicity. To test the alternative hypothesis of precessing radio jet, we present deep 6 GHz radio imaging conducted with NSF's Karl G. Jansky Very Large Array (VLA) in its C configuration for the three candidate periodic quasars, DES J024703.24$-$010032.0, DES J024944.66$-$000036.8, and DES J025214.67$-$002813.7. Our targets were selected based on their optical variability using 20-yr long multi-color light …
We present the variability study of broad absorption lines (BALs) in a uniformly radio-selected sample of 28 BAL quasars using the archival data from the FIRST Bright Quasar Survey (FBQS) and the Sloan Digital Sky Survey (SDSS), as well as those obtained by ourselves, covering time scales $\sim 1-10$ years in the quasar's rest-frame. To our surprise, 5 quasars showing strong variations are all belong to a special subclass of overlapping iron low ionization BAL (OFeLoBAL) quasars, however, other 4 non-overlapping FeLoBALs (non-OFeLoBALs) are invariable except one case with weak optical depth ch…
We use data from the All Wavelength Extended Groth Strip International Survey to construct stacked X-ray maps of optically bright active galaxies (AGN) and an associated control sample of galaxies at high redshift (z less than 0.6). From our analysis of the surface brightness profiles obtained from these X-ray maps, we find evidence of feedback from the active nuclei. We find that excluding galaxies and AGN, residing in group environments, from our samples enhances the significance of our detection. Our results support the tentative findings of Chatterjee et al. who use X-ray selected AGN for …
A systematic analysis of the X-ray emission in optically selected tidal disruption events: observational evidence for the unification of the optically and X-ray selected populations
A primary aim of the Nuclear Spectroscopic Telescope Array (NuSTAR) mission is to find and characterize heavily obscured Active Galactic Nuclei (AGNs). Based on mid-infrared photometry from the Wide-Field Infrared Survey Explorer (WISE) and optical photometry from the Sloan Digital Sky Surveys, we have selected a large population of luminous obscured AGN (i.e., obscured quasars). Here we report NuSTAR observations of four WISE-selected heavily obscured quasars for which we have optical spectroscopy from the Southern African Large Telescope and W. M. Keck Observatory. Optical diagnostics confir…
We have compiled a catalog of 903 candidates for type 1 quasars at redshifts 3<z<5.5 selected among the X-ray sources of the serendipitous XMM-Newton survey presented in the 3XMM-DR4 catalog (the median X-ray flux is 5x10^{-15} erg/s/cm^2 the 0.5-2 keV energy band) and located at high Galactic latitudes >20 deg in Sloan Digital Sky Survey (SDSS) fields with a total area of about 300 deg^2. Photometric SDSS data as well infrared 2MASS and WISE data were used to select the objects. We selected the point sources from the photometric SDSS catalog with a magnitude error Delta z<0.2 and a color i-z<…
The Baryon Oscillation Spectroscopic Survey (BOSS) has collected more than 150,000 $2.1 \leq z \leq 3.5$ quasar spectra since 2009. Using this unprecedented sample, we create a composite spectrum in the rest-frame of 102,150 quasar spectra from 800 Å to 3300 Å at a signal-to-noise ratio close to 1000 per pixel ($Δv$ of 69 km~s$^{-1}$). Included in this analysis is a correction to account for flux calibration residuals in the BOSS spectrophotometry. We determine the spectral index as a function of redshift of the full sample, warp the composite spectrum to match the median spectral index, and c…
We explore the connection between absorption by neutral gas and extinction by dust in mid-infrared (IR) selected luminous quasars. We use a sample of 33 quasars at redshifts 0.7 < z < 3 in the 9 deg^2 Boötes multiwavelength survey field that are selected using Spitzer Space Telescope Infrared Array Camera colors and are well-detected as luminous X-ray sources (with >150 counts) in Chandra observations. We divide the quasars into dust-obscured and unobscured samples based on their optical to mid-IR color, and measure the neutral hydrogen column density N_H through fitting of the X-ray spectra. …
The Time-Domain Spectroscopic Survey (TDSS) is an SDSS-IV eBOSS subproject primarily aimed at obtaining identification spectra of ~220,000 optically-variable objects systematically selected from SDSS/Pan-STARRS1 multi-epoch imaging. We present a preview of the science enabled by TDSS, based on TDSS spectra taken over ~320 deg^2 of sky as part of the SEQUELS survey in SDSS-III, which is in part a pilot survey for eBOSS in SDSS-IV. Using the 15,746 TDSS-selected single-epoch spectra of photometrically variable objects in SEQUELS, we determine the demographics of our variability-selected sample, …
