skip to main content


The NSF Public Access Repository (NSF-PAR) system and access will be unavailable from 5:00 PM ET until 11:00 PM ET on Friday, June 21 due to maintenance. We apologize for the inconvenience.

Search for: All records

Creators/Authors contains: "Brandt, W. N."

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.


    Quasar winds can shock and sweep up ambient interstellar medium (ISM) gas, contributing to galactic quenching. We combine and extend past models of energy-conserving shock bubbles around quasars, investigate model implications from an observational standpoint, and test model predictions using new high-resolution spectroscopic observations of the broad absorption-line quasar SDSS J030000.56+004828.0 (J0300). Even with constant energy input from the wind, a bubble’s expansion decelerates over time as more ISM gas is swept up. Our new observations enable a direct search for this deceleration. We obtain the tightest reported 3σ limit on the average rest-frame deceleration (or acceleration) of a quasar outflow: |a| < 0.1 km s−1 yr−1 (<3 × 10−4 cm s−2) in the relatively low-velocity Ca ii outflow of J0300 over 9.65 rest-frame years. We can satisfy these limits with certain parameter choices in our model, but the large velocity range of the Ca ii absorption in J0300 rules out the hypothesis that such gas shares the velocity of the swept-up ISM gas in a self-similar shock bubble. We investigate the possibility of ram-pressure acceleration of preexisting ISM clouds and conclude that the velocity range seen in Ca ii in J0300 is potentially consistent with such an explanation. The Ca ii-absorbing gas clouds in J0300 have been inferred to have high densities by Choi et al., in which case they can only have been accelerated to their current speeds if they were originally at least an order of magnitude less dense than they are today.

    more » « less
  2. Abstract

    A fundamental question in galaxy and black hole evolution remains how galaxies and their supermassive black holes have evolved together over cosmic time. Specifically, it is still unclear how the position of X-ray active galactic nucleus (AGN) host galaxies with respect to the star-forming main sequence (MS) may change with the X-ray luminosity (LX) of the AGN or the stellar mass (M) of the host galaxy. We use data from the XMM-Spitzer Extragalactic Representative Volume Survey (XMM-SERVS) to probe this issue. XMM-SERVS is covered by the largest medium-depth X-ray survey (with superb supporting multiwavelength data) and thus contains the largest sample to date for study. To ensure consistency, we locally derive the MS from a large reference galaxy sample. In our analysis, we demonstrate that the turnover of the galaxy MS does not allow reliable conclusions to be drawn for high-mass AGNs, and we establish a robust safe regime where the results do not depend upon the choice of MS definition. Under this framework, our results indicate that less massive AGN host galaxies (logM9.510.5M) generally possess enhanced star formation rates compared to their normal-galaxy counterparts while the more massive AGN host galaxies (logM10.511.5M) lie on or below the star-forming MS. Further, we propose an empirical model for how the placement of an AGN with respect to the MS (SFRnorm) evolves as a function of bothMandLX.

    more » « less
  3. Abstract

    Periodic signatures in time-domain observations of quasars have been used to search for binary supermassive black holes (SMBHs). These searches, across existing time-domain surveys, have produced several hundred candidates. The general stochastic variability of quasars, however, can masquerade as a false-positive periodic signal, especially when monitoring cadence and duration are limited. In this work, we predict the detectability of binary SMBHs in the upcoming Rubin Observatory Legacy Survey of Space and Time (LSST). We apply computationally inexpensive sinusoidal curve fits to millions of simulated LSST Deep Drilling Field light curves of both single, isolated quasars and binary quasars. The period and phase of simulated binary signals can generally be disentangled from quasar variability. Binary amplitude is overestimated and poorly recovered for two-thirds of potential binaries due to quasar accretion variability. Quasars with strong intrinsic variability can obscure a binary signal too much for recovery. We also find that the most luminous quasars mimic current binary candidate light curves and their properties: The false-positive rates are 60% for these quasars. The reliable recovery of binary period and phase for a wide range of input binary LSST light curves is promising for multi-messenger characterization of binary SMBHs. However, pure electromagnetic detections of binaries using photometric periodicity with amplitude greater than 0.1 mag will result in samples that are overwhelmed by false positives. This paper represents an important and computationally inexpensive way forward for understanding the true and false-positive rates for binary candidates identified by Rubin.

    more » « less
  4. Abstract

    “Changing-look” active galactic nuclei (CL-AGNs) challenge our basic ideas about the physics of accretion flows and circumnuclear gas around supermassive black holes. Using first-year Sloan Digital Sky Survey V (SDSS-V) repeated spectroscopy of nearly 29,000 previously known active galactic nuclei (AGNs), combined with dedicated follow-up spectroscopy, and publicly available optical light curves, we have identified 116 CL-AGNs where (at least) one broad emission line has essentially (dis-)appeared, as well as 88 other extremely variable systems. Our CL-AGN sample, with 107 newly identified cases, is the largest reported to date, and includes ∼0.4% of the AGNs reobserved in first-year SDSS-V operations. Among our CL-AGNs, 67% exhibit dimming while 33% exhibit brightening. Our sample probes extreme AGN spectral variability on months to decades timescales, including some cases of recurring transitions on surprisingly short timescales (≲2 months in the rest frame). We find that CL events are preferentially found in lower-Eddington-ratio (fEdd) systems: Our CL-AGNs have afEdddistribution that significantly differs from that of a carefully constructed, redshift- and luminosity-matched control sample (Anderson–Darling test yieldingpAD≈ 6 × 10−5; medianfEdd≈ 0.025 versus 0.043). This preference for lowfEddstrengthens previous findings of higher CL-AGN incidence at lowerfEdd, found in smaller samples. Finally, we show that the broad Mgiiemission line in our CL-AGN sample tends to vary significantly less than the broad Hβemission line. Our large CL-AGN sample demonstrates the advantages and challenges in using multi-epoch spectroscopy from large surveys to study extreme AGN variability and physics.

    more » « less
    Free, publicly-accessible full text available April 26, 2025
  5. Abstract

    We explore reprocessing models for a sample of 17 hypervariable quasars, taken from the Sloan Digital Sky Survey Reverberation Mapping project, which all show coordinated optical luminosity hypervariability with amplitudes of factors ≳2 between 2014 and 2020. We develop and apply reprocessing models for quasar light curves in simple geometries that are likely to be representative of quasar inner environments. In addition to the commonly investigated thin-disk model, we include the thick-disk and hemisphere geometries. The thick-disk geometry could, for instance, represent a magnetically elevated disk, whereas the hemisphere model can be interpreted as a first-order approximation for any optically thick out-of-plane material caused by outflows/winds, warped/tilted disks, and so on. Of the 17 quasars in our sample, 11 are best-fitted by a hemisphere geometry, five are classified as thick disks, and both models fail for just one object. We highlight the successes and shortcomings of our thermal reprocessing models in case studies of four quasars that are representative of the sample. While reprocessing is unlikely to explain all of the variability that we observe in quasars, we present our classification scheme as a starting point for revealing the likely geometries of reprocessing for quasars in our sample and hypervariable quasars in general.

    more » « less

    The Australia Telescope Large Area Survey (ATLAS) and the VLA survey in the XMM-LSS/VIDEO deep field provide deep (≈15 $\mu$ Jy beam−1) and high-resolution (≈4.5–8 arcsec) radio coverage of the three XMM-SERVS fields (W-CDF-S, ELAIS-S1, and XMM-LSS). These data cover a total sky area of 11.3 deg2 and contain ≈11 000 radio components. Furthermore, about 3 deg2 of the XMM-LSS field also has deeper MIGHTEE data that achieve a median RMS of 5.6 $\mu$ Jy beam−1 and detect more than 20 000 radio sources. We analyse all these radio data and find source counterparts at other wavebands utilizing deep optical and infrared (IR) surveys. The nature of these radio sources is studied using radio-band properties (spectral slope and morphology) and the IR–radio correlation. Radio AGNs are selected and compared with those selected using other methods (e.g. X-ray). We found 1656 new AGNs that were not selected using X-ray and/or MIR methods. We constrain the FIR-to-UV SEDs of radio AGNs using cigale and investigate the dependence of radio AGN fraction upon galaxy stellar mass and star formation rate.

    more » « less
  7. Abstract We present a catalog of multi-band forced photometry in the CDFS and XMM-LSS fields. We used The Tractor image-modeling software to produce de-blended photometry across 13 to 15 optical/infrared bands and determine photometric redshifts. Our catalog, which is publicly available on IRSA, contains ∼1.5 million sources and covers a total area of ∼9 deg 2 . 
    more » « less
  8. Abstract

    We measure the correlation between black hole massMBHand host stellar massM*for a sample of 38 broad-line quasars at 0.2 ≲z≲ 0.8 (median redshiftzmed= 0.5). The black hole masses are derived from a dedicated reverberation mapping program for distant quasars, and the stellar masses are derived from two-band optical+IR Hubble Space Telescope imaging. Most of these quasars are well centered within ≲1 kpc from the host galaxy centroid, with only a few cases in merging/disturbed systems showing larger spatial offsets. Our sample spans two orders of magnitude in stellar mass (∼109–1011M) and black hole mass (∼107–109M) and reveals a significant correlation between the two quantities. We find a best-fit intrinsic (i.e., selection effects corrected)MBHM*,hostrelation oflog(MBH/M)=7.010.33+0.23+1.740.64+0.64log(M*,host/1010M), with an intrinsic scatter of0.470.17+0.24dex. Decomposing our quasar hosts into bulges and disks, there is a similarMBHM*,bulgerelation with slightly larger scatter, likely caused by systematic uncertainties in the bulge–disk decomposition. TheMBHM*,hostrelation atzmed= 0.5 is similar to that in local quiescent galaxies, with negligible evolution over the redshift range probed by our sample. With direct black hole masses from reverberation mapping and the large dynamical range of the sample, selection biases do not appear to affect our conclusions significantly. Our results, along with other samples in the literature, suggest that the locally measured black hole mass–host stellar mass relation is already in place atz∼ 1.

    more » « less
  9. Abstract

    This work studies the relationship between accretion-disk size and quasar properties, using a sample of 95 quasars from the Sloan Digital Sky Survey Reverberation Mapping Project with measured lags between thegandiphotometric bands. Our sample includes disk lags that are both longer and shorter than predicted by the Shakura and Sunyaev model, requiring explanations that satisfy both cases. Although our quasars each have one lag measurement, we explore the wavelength-dependent effects of diffuse broad-line region (BLR) contamination through our sample’s broad redshift range, 0.1 <z< 1.2. We do not find significant evidence of variable diffuse Feiiand Balmer nebular emission in the rms spectra, nor from Anderson–Darling tests of quasars in redshift ranges with and without diffuse nebular emission falling in the observed-frame filters. Contrary to previous work, we do not detect a significant correlation between the measured continuum and BLR lags in our luminous quasar sample, similarly suggesting that our continuum lags are not dominated by diffuse nebular emission. Similar to other studies, we find that quasars with larger-than-expected continuum lags have lower 3000 Å luminosities, and we additionally find longer continuum lags with lower X-ray luminosities and black hole masses. Our lack of evidence for diffuse BLR contribution to the lags indicates that the anticorrelation between continuum lag and luminosity is not likely to be due to the Baldwin effect. Instead, these anticorrelations favor models in which the continuum lag increases in lower-luminosity active galactic nuclei, including scenarios featuring magnetic coupling between the accretion disk and X-ray corona, and/or ripples or rims in the disk.

    more » « less
  10. Aims. We study the ensemble X-ray variability properties of active galactic nuclei (AGN) over large ranges of timescale (20 ks ≤  T  ≤ 14 yr), redshift (0 ≤  z  ≲ 3), luminosity (10 40  erg s −1  ≤  L X  ≤ 10 46  erg s −1 ), and black hole (BH) mass (10 6  ≤  M ⊙  ≤ 10 9 ). Methods. We propose the use of the variance-frequency diagram as a viable alternative to the study of the power spectral density (PSD), which is not yet accessible for distant, faint, and/or sparsely sampled AGN. Results. We show that the data collected from archival observations and previous literature studies are fully consistent with a universal PSD form, which does not show any evidence for systematic evolution of shape or amplitude with redshift or luminosity, even if there may be differences between individual AGN at a given redshift or luminosity. We find new evidence that the PSD bend frequency depends on BH mass and possibly on accretion rate. We finally discuss the implications for current and future AGN population and cosmological studies. 
    more » « less