Gordon T Richards

We use microlensing to probe the inner broad line region (BLR) of the lensed quasar SDSS J1004+4112, finding evidence of substructure. We study the recurrent microlensing events observed in the blue wing of the C IV emission line in image A of this lensed quasar from a series of 20 spectra taken over 15 yr. We obtain a microlensing light curve and confirm the presence of three high magnification events (Δm > –0.7 mag). A caustic crossing is a natural explanation for each one of the events. The fast rising and fading of the events imply that the width of the region scanned by the caustic in each event, ≲0.1 μas (≲0.93 ± 0.36 light days), is much smaller than the BLR size. However, the large range of velocities involved implies significant overlapping with the inner BLR velocity field. An elongated thin substructure in the BLR fulfills both requirements at once. A sequence of caustics crossing a single elongated substructure may be a possible explanation of the observed recurrence. However, this hypothesis requires some ad hoc assumptions about the microlens population. Alternatively, a single caustic encountering several narrow-stripped or bow-shaped substructures in the approaching part of the BLR could explain the variability. We discuss the possible identification of these elongated substructures with ripples or spiral structure on the inner BLR. Simulations of caustic crossings of a rippled disk statistically support this interpretation. The study of the C IV emission-line variability in SDSS J1004+4112 illustrates the incomparable scanning power of microlensing in both velocity and space.

We present the first infrared (IR) spectral predictions from a self-consistent simulation of the formation of a quasar in a starburst galaxy, spanning the cosmological environment to scales well below the dust sublimation region. The IR emission is dominated by a torus-like dust structure composed of a highly magnetized, turbulence-supported outer accretion disk and of accreting gas tidally torn from the interstellar medium (ISM). At these early stages, the active galactic nucleus is buried and Compton thick. The near- to mid-IR escaping luminosity varies by almost an order of magnitude across sight lines, largely due to extinction from the inflowing stream of cold dust. Self-absorption within the torus suppresses silicate emission features, and further reprocessing by the ambient ISM leads to prominent silicate absorption and colder IR emission. The sublimation structure is stratified by composition and size, producing sight-line-dependent extinction curves that intrinsically vary in shape. However, after repeated scattering in the optically thick dusty medium, these curves emerge substantially grayed. We also demonstrate that bipolar outflows from the central black hole, which carves biconical cavities and reveals the central engine in later stages, can preserve IR anisotropy and silicate features. These results suggest that dusty starburst quasars can undergo a buried, IR-bright phase early in their evolution.

Strong outflows from active galactic nuclei are frequently observed in objects with lower coronal X-ray luminosity. This intrinsic X-ray weakness is considered a requirement for the formation of radiatively driven winds. To obtain an unbiased view on the connection between X-ray emission and the presence of powerful winds in the most luminous quasar phase, we present an X-ray analysis of a sample of extremely luminous, radio-quiet quasars with signatures of strong outflows in their rest-frame ultraviolet (UV) emission spectra. We study the Chandra iv emission line blueshifts, comparing them to typical optically blue quasars. iv emission-line outflows might emerge at wind velocities greater than 3,000 km/s. Our study provides additional evidence that the relationship between X-ray emission and the presence of winds is intricate. Our findings emphasise the need for X-ray observations of a larger sample of UV-selected quasars with confirmed strong emission-line outflows to unravel the nuanced interplay between winds and X-ray emission.

We present a multiwavelength spectroscopic survey of 23 luminous mid-infrared–selected Type-2 quasars at the redshifts of z = 0.88–3.49. The targets were selected in the SDSS Stripe 82 field based on their bright WISE 22 μm detections and extremely faint or red optical counterparts. Near-infrared (Gemini/GNIRS) and optical (Keck/LRIS and KCWI) spectroscopy confirm 23 out of 24 candidates as Type-2 quasars, including 12 objects at z > 2. The spectra exhibit strong rest-frame UV and optical emission lines (Lyα, C IV, [O III], Hα) with a wide range of line widths, indicating significant spectral diversity. Approximately one-third of the sample (8 of 23) shows broad Hα emission (FWHM >2000 km s−1) despite their Type-2 classification, while the rest have only narrow lines (FWHM <2000 km s−1) characteristic of classical obscured quasars. Notably, these broad-line Type-2 quasars share similar spectral energy distributions with the JWST-discovered “little red dot” (LRD), suggesting that our sample could be lower-redshift analogues of the heavily obscured broad-line AGNs uncovered by JWST. We also find that the [O III] λ5007 emission is relatively weak for their high bolometric luminosities, deviating from trends seen in lower-z Type-2 QSOs. A new composite spectrum for Type-2 QSOs is built using our sample. Overall, our results demonstrate that mid-IR selection efficiently uncovers diverse populations of obscured quasars and that spectroscopic follow-up is crucial for revealing their true nature. This study provides new insights into heavily obscured SMBH growth at cosmic noon and bridges the gap to the obscured AGN populations being revealed by JWST.

While C iv is the most common absorption line in broad absorption line quasar spectra, Balmer absorption lines (BALs) are among the rarest. We present analysis of Balmer absorption in a sample of 14 iron low-ionization BAL quasars (FeLoBALQs); eight are new identifications. We measured velocity offset, width, and apparent optical depth. The partial covering that is ubiquitous in BAL quasar spectra alters the measured Balmer optical depth ratios; accounting for this, we estimated the true H(n = 2) column density. We found the anticipated correlation between Eddington ratio and outflow speed, but it is weak in this sample because nearly all of the objects have the low outflow speeds characterizing loitering outflow FeLoBAL quasars, objects that are also found to have low accretion rates. Measurements of dN/dv, the differential column density with respect to the outflow speed, are anticorrelated with the luminosity and Eddington ratio: the strongest absorption is observed at the lowest speeds in the lowest-luminosity objects. The absorption line width is correlated with αoi, the Fλ point-to-point slope between 5100 Å and 3 μm. This parameter is strongly correlated with the Eddington ratio among low-redshift quasars. BALs have been recently found in the spectra of little red dots (LRDs), a class of high-redshift objects discovered by JWST. We note suggestive similarities between LRDs and FeLoBAL quasars in the emission-line shape, the presence of steep reddening and a scattered blue continuum, the lack of hot dust emission, and X-ray weakness.

We present Chandra observations of 63 sources from the Gemini Near Infrared Spectrograph−Distant Quasar Survey, of which 54 were targeted by snapshot observations in Cycle 24. A total of 55 sources are clearly detected in at least one X-ray band, and we set stringent upper limits on the X-ray fluxes of the remaining eight sources. In combination with rest-frame ultraviolet–optical spectroscopic data for these sources, we assess whether X-rays can provide a robust accretion rate indicator for quasars, particularly at the highest accessible redshifts. We utilize a recently modified Hβ-based Eddington luminosity ratio estimator, as well as the C iv λ1549 emission-line parameter space, to investigate trends and correlations with the optical–X-ray spectral slope (αox) and the effective hard X-ray power-law photon index (Γ). We find that αox does not improve current accretion rate estimates based on Hβ or C iv. Instead, within the limitations of our sample, we confirm previous findings that the C iv parameter space may be a better indicator of the accretion rate up to z ∼ 3.5. We also find that the average Γ values for a small subset of our sources, as well as the average Γ value in different groupings of our sources, are consistent with their respective relatively high Eddington luminosity ratios. Deeper X-ray observations of our X-ray-detected sources are needed for measuring Γ accurately and testing whether this parameter can serve as a robust, unbiased accretion rate diagnostic.

We present the multiwavelength spectral energy distributions (SEDs) for 65 luminous broad absorption line (BAL) quasars with redshifts 1.55 ≲ z ≲ 3.50 from the Gemini Near Infrared Spectrograph–Distant Quasar Survey (GNIRS-DQS). We integrate data from a variety of ground- and space-based observatories to construct a comprehensive spectral profile of these objects from radio through X-rays. In addition, we present a mid-infrared to X-ray composite SED of these sources. Our data set represents the most uniform sample of BAL quasars, providing a statistically robust set of SEDs. Our findings indicate that the BAL quasars in the GNIRS-DQS sample exhibit significant reddening in the ultraviolet-optical continuum relative to their non-BAL counterparts, consistent with previous studies. Notably, our analysis reveals no significant differences in the mid- or near-infrared spectral regime between BAL and non-BAL quasars. In line with previous work, we find no strong evidence that BAL and non-BAL quasars possess fundamentally different SEDs, also consistent with recent findings that both groups display similar rest-frame optical emission-line properties.

We analyze 23 spectroscopically confirmed Type-2 quasars (QSOs) selected from WISE 22$\hbox{$\mu $m}$ band in the SDSS Stripe 82 region, focusing on their multi-band photometry and spectral energy distributions (SEDs). The 24 candidates were selected to be IR-luminous ($\rm flux_{W4} > 5mJy$), optically faint (r > 23) or with red color (r − W4 > 8.38). Gemini/GNIRS and Keck/LRIS observations confirm 23 to be Type-2 QSOs at z = 0.88 − 3.49. Multi-band photometry are used for SED fitting, covering 0.1$\rm \mu$m to 10$\rm \mu$m in the rest frame. The IR emission is dominated by the dust torus, with an average luminosity of $L_{\rm torus} = 10^{46.84} \rm erg\, s^{-1}$. We present three possibilities for the origin of the rest-UV/optical: scattered light, stellar emission, and the reddened accretion disk. Assuming an obscured:unobscured ratio of 1:1, the targets have bolometric luminosities of $L_{\rm bol} = 10^{46.28} - 10^{48.08} \rm erg\, s^{-1}$ and SMBH masses of $\rm 10^{8.18} {\rm M}_{\odot } - 10^{9.98} {\rm M}_{\odot }$, averaging $L_{\rm bol} = 10^{47.04}~{\rm {erg/s}}$ and $M_{\rm BH} = 10^{8.94} \rm {\rm M}_{\odot }$, assuming the Eddington limit. Compared to previous Type-2 AGN SEDs, our targets have a brighter dust torus and redder optical-IR color. By comparing the SED to JWST ‘little red dots’ (LRDs), we find that these IR-selected Type-2 QSOs have similar SED shapes to the LRDs. This survey demonstrates mid-IR selection as an efficient method to find luminous Type-2 QSOs and the composite photometry generated by this sample provide a guide for finding more Type-2 QSOs at higher redshift in the future.

A damped random walk (DRW) process is often used to describe the temporal UV/optical continuum variability of active galactic nuclei (AGN). However, recent investigations have shown that this model fails to capture the full spectrum of AGN variability. In this work, we model the 22 yr long light curves of 21,767 quasars, spanning the redshift range 0.28 <  z  < 2.71, as a noise-driven damped harmonic oscillator (DHO) process. The light curves, in the optical g and r bands, are collected and combined from the Sloan Digital Sky Survey, the Panoramic Survey Telescope and Rapid Response System, and the Zwicky Transient Facility. A DHO process can be defined using four parameters, two for describing its long-term behavior/variability, and the other two for describing its short-term behavior/variability. We find that the best-fit DHO model describes the observed variability of our quasar light curves better than the best-fit DRW model. Furthermore, the best-fit DHO parameters exhibit correlations with the rest-frame wavelength, the Eddington ratio, and the black hole mass of our quasars. Based on the power spectral density shape of the best-fit DHOs and these correlations, we suggest that the observed long-term variability of our quasars can be best explained by accretion rate or thermal fluctuations originating from the accretion disk, and the observed short-term variability can be best explained by reprocessing of X-ray variability originating from the corona. The additional information revealed by DHO modeling emphasizes the need to go beyond DRW when analyzing AGN light curves delivered by next-generation wide-field time-domain surveys.

Understanding the links between different phases of outflows from active galactic nuclei is a key goal in extragalactic astrophysics. Here, we compare [O III] lambda lambda 4960, 5008 outflow signatures in quasars with and without broad absorption lines (BALs), aiming to test how the broad absorption troughs seen in the rest-frame ultraviolet are linked to the narrow line region outflows seen in the rest-frame optical. We present new near-infrared spectra from Magellan/FIRE that cover [O III] in 12 quasars with 2.1 < z < 2.3, selected to have strong outflow signatures in CIV lambda 1550. Combining with data from the literature, we build a sample of 73 BAL, 115 miniBAL, and 125 non-BAL quasars with 1.5 < z < 2.6. The strength and velocity width of [O iii] correlate strongly with the C iv emission properties, but no significant difference is seen in the [O III] emission-line properties between the BALs, non-BALs, and miniBALs once the dependence on CIV emission is taken into account. A weak correlation is observed between the velocities of C iv BALs and [O III] emission, which is accounted for by the fact that both outflow signatures correlate with the underlying CIV emission properties. Our results add to the growing evidence that BALs and non-BALs are drawn from the same parent population and are consistent with a scenario wherein BAL troughs are intermittent tracers of persistent quasar outflows, with a part of such outflow becoming optically thick along our line of sight for sporadic periods of time within which BALs are observed.