About

Nahum Arav is a faculty member at the Fralin Biomedical Research Institute at Virginia Tech, where his research involves the use of functional magnetic resonance imaging (fMRI) to study various aspects of neuroscience, psychology, political science, law, and economics. His laboratory contains research-dedicated Siemens fMRI devices in Roanoke and Blacksburg, enabling the simultaneous scanning of multiple participants during negotiation or game-based experiments using a novel technique called hyperscanning. Faculty across the institute collaborate with the Human Neuroimaging Laboratory to utilize fMRI technology in human research studies, particularly within the Addiction Recovery Research Center, the Center for Transformative Health Behaviors, and the Computational Psychiatry Unit. His work focuses on understanding brain function through advanced imaging techniques, with a particular emphasis on how brain activity correlates with decision-making and behavior. The laboratory's research includes detailed anatomical imaging of the brain using conventional MRI, as well as functional imaging to observe brain activation in response to various stimuli. The fMRI technology employed detects small changes in blood flow related to neural activity, allowing researchers to identify specific brain regions involved in different cognitive and behavioral processes. The Roanoke Brain Study, part of a large-scale research initiative, combines neuroimaging, genetics, and computational approaches to explore decision behavior and its changes over the lifespan, aiming to create a comprehensive resource for scientists, clinicians, and policymakers to better understand decision-making processes and promote healthier lives.

Research topics

• Astronomy
• Astrophysics
• Physics
• Optics

Selected publications

• Novel method to trace the dark matter density profile around supermassive black holes with AGN reverberation mapping

  Physical review. D/Physical review. D. · 2026-02-03 · 1 citations

  articleOpen access

  We propose a new method to determine the dark matter density profile in the vicinity of distant supermassive black holes (SMBH) using reverberation mapping (RM) measurements of active galactic nuclei (AGN). The mapping of multiple emission lines allows the measurement of the enclosed mass within different radii from the central SMBH, which can be used to infer or constrain the dark matter density profile on sub-parsec scales. We apply a toy model based on this method to a sample of fourteen AGN to test its feasibility based on current measurements. We find that for five objects, the observed enclosed mass does grow with radii, hinting towards the presence of a dark matter component at the 1-2 $σ$ level. For these sources, we find global evidence for a universal dark matter profile with a preferred radial steepness of index $γ\sim 1.6$, consistent with the scenario expected for a dark matter spike mildly relaxed by stellar heating processes. The enclosed dark matter mass, however, is found to be significantly larger than expected. We show that the current RM based mass measurements suffer from large systematic uncertainties, that limit the effectiveness of our method. Our work emphasizes the importance of applying the recent developments in mass determination techniques to target multiple emission lines with future RM and interferometry campaigns. This provides the most direct way of constraining the dark matter density in the sub-parsec regions around extragalactic SMBHs, which is crucial to our understanding of the dynamics and nature of dark matter.

  PublisherOA PDFDOI

• AGN STORM 2. XII. Ground-based Optical Photometry and Lag Measurements of Mrk 817

  The Astrophysical Journal · 2026-05-21

  articleOpen access

  Abstract We present the ground-based imaging campaign and light curves of Markarian 817 as part of the multiwavelength monitoring program AGN STORM 2. Observations were carried out over 1.4 yr in the uBgVriz filters, with a median cadence of 0.4 day in the g band. Reverberation lags are measured using three methods (interpolated cross-correlation function (ICCF), Just Another Vehicle for Estimating Lags In Nuclei, and PyROA ) with the Swift UVW2 band (1928 Å) as the reference light curve. The ICCF centroid lags range from 3.0 ± 0.8 days for the u band up to 7.9 ± 1.5 days for z , and are consistent with a τ ∝ λ 4/3 dependence, the relation expected for lamppost reprocessing by a Shakura–Sunyaev disk. Lags measured with the other methods are systematically shorter, and deviate from a λ 4/3 power-law spectrum at long wavelengths. The lags exceed thin-disk reprocessing predictions by factors of ∼3–6, similar to the “disk size discrepancy” seen in other Seyfert galaxies. We divide the campaign into three epochs with different levels of mean luminosity and X-ray obscuring column density and find that the lags vary by as much as a factor of 2 between epochs. The intrinsic spectral energy distribution is bluer and brighter during the first third of the campaign, and the longest continuum reverberation lags are obtained during that period. These results suggest that changes in ionizing luminosity can produce large variations in continuum lags on short timescales by altering the diffuse continuum luminosity emitted by the broad-line region (BLR) and/or obscuring outflow, although changes in obscuration between the central engine and BLR may also contribute to the lag variations.

  PublisherDOI

• The Distance of Quasar Outflows from the Central Source: The First Consistent Values from Emission and Absorption Determinations

  The Astrophysical Journal · 2025-04-03 · 2 citations

  articleOpen access

  Abstract Measuring the distance of quasar outflows from the central source ( R ) is essential for determining their importance for active galactic nucleus feedback. There are two methods to measure R : (1) a direct determination using spatially resolved integral field spectroscopy (IFS) of the outflow in emission and (2) an indirect method that uses the absorption troughs from ionic excited states. The column density ratio between the excited and resonance states yields the outflow number density. Combined with a knowledge of the outflow’s ionization parameter, R can be determined. Generally, the IFS method probes an R range of several kiloparsecs or more, while the absorption method usually yields R values of less than 1 kpc. There is no inconsistency between the two methods as the determinations come from different objects. Here we report the results of applying both methods to the same quasar outflow, where we derive consistent determinations of R ≈ 5 kpc. This is the first time that the indirect absorption R determination is verified by a direct spatially resolved IFS observation. In addition, the velocities (and energetics) from the IFS and absorption data are found to be consistent. Therefore, these are two manifestations of the same outflow. In this paper we concentrate on the absorption R determination for the outflow seen in quasar 3C 191 using Very Large Telescope/X-shooter observations. We also reanalyze an older absorption determination for the outflow based on Keck/High Resolution Echelle Spectrometer data and find the revised measurement to be consistent with ours. Our companion paper details the IFS analysis of the same object.

  PublisherOA PDFDOI

• Quasar absorption outflows on galactic scales: Insights from DESI

  ArXiv.org · 2025-05-27

  preprintOpen access

  Absorption outflows in quasars play an important role in understanding active galactic nuclei (AGN) feedback and their influence on galaxy evolution. The unprecedented spectral data provided by the Dark Energy Spectroscopic Instrument (DESI) opens new avenues to explore these outflows. We analyze five low-ionization absorption outflow systems in four intermediate-redshift quasars ($2 < z < 3$) using the data obtained by DESI in order to characterize their physical properties and energetics, and also to assess their role in AGN feedback. We use the spectra from DESI's Early Data Release to determine the ionic column densities, total hydrogen column densities, electron number densities, and ionization parameters via photoionization modeling and absorption line analysis. We derive the outflows' distance from the AGN, and its kinematic properties: mass-flow rates, kinetic luminosity, and momentum flux. Our study identifies five distinct mini-broad absorption line outflow systems, hosted by four quasars. The identified outflows exhibit hydrogen column densities of $\log (N_H) = 20.0-20.7$[cm$^{-2}$], ionization parameters of $\log (U_H) = -2.7$ to $-2.1$, and electron number densities of $\log (n_e) =1.45-2.85$[cm$^{-2}$]. The distances of the outflows from the central source are between 4.5 to 31 kpc, and the kinetic luminosities range from $2\times10^{-6}$ to $5\times10^{-3}$ of the Eddington luminosity. The outflows in J1407 and J1032 show the largest kinetic luminosities, with momentum flux ratios ($\dot{p} / \dot{p}_{rad}$) of about 2 and 0.2, respectively. Our findings highlight the vital role of DESI data in uncovering the diversity and significance of quasar outflows in galaxy evolution.

  PublisherOA PDFDOI

• An energetic absorption outflow in QSO J1402+2330: Analysis of DESI observations

  ArXiv.org · 2025-01-29

  preprintOpen access

  Context. Quasar outflows play a significant role in the active galactic nucleus (AGN) feedback, impacting the interstellar medium and potentially influencing galaxy evolution. Characterizing these outflows is essential for understanding AGN-driven processes. Aims. We aim to analyze the physical properties of the mini-broad absorption line outflow in quasar J1402+2330 using data from the Dark Energy Spectroscopic Instrument (DESI) survey. We seek to measure the outflows location, energetics, and potential impact on AGN feedback processes. Methods. In the spectrum of J1402+2330, we identify multiple ionic absorption lines, including ground and excited states. We measure the ionic column densities and then use photoionization models to determine the total hydrogen column density and ionization parameter of the outflow. We utilized the population ratio of the excited state to the ground state of N iii and S iv to determine the electron number density. Results. The derived electron number density, combined with the ionization parameter, indicates an outflow distance of approximately 2200 pc from the central source. Having a mass outflow rate of more than one thousand solar masses per year and a kinetic energy output exceeding 5 percent of the Eddington luminosity, this outflow can significantly contribute to AGN feedback. Conclusions. Our findings suggest the absorption outflow in J1402+2330 plays a potentially significant role in AGN feedback processes. This study highlights the value of DESI data in exploring AGN feedback mechanisms.

  PublisherOA PDFDOI

• Erratum: “The Farthest Quasar Mini-Broad Absorption Line Outflow from Its Central Source: Very Large Telescope/UVES Observation of SDSS J0242+0049” (2022, ApJ, 927, 176)

  The Astrophysical Journal · 2025-07-01

  erratumOpen access
  PublisherDOI

• Determining the absolute chemical abundance of nitrogen and sulfur in the quasar outflow of 3C298

  Astronomy and Astrophysics · 2025-01-01 · 2 citations

  articleOpen access

  Context. Quasar outflows are key players in the feedback processes that influence the evolution of galaxies and the intergalactic medium. The chemical abundance of these outflows provides crucial insights into their origin and impact. Aims. We determine the absolute abundances of nitrogen and sulfur and the physical conditions of the outflow seen in quasar 3C298. Methods. We analyzed archival spectral data from the Hubble Space Telescope for 3C298. We measured the ionic column densities from the absorption troughs and compared the results to photoionization predictions made with the Cloudy code for three different spectral energy distributions (SEDs), including MF87, UV-soft, and HE0238 SEDs. We also calculated the ionic column densities of the excited and ground states of N III to estimate the electron number density and location of the outflow using the Chianti atomic database. Results. The MF87, UV-soft, and HE0238 SEDs yield nitrogen and sulfur abundances at supersolar, solar, and subsolar values, respectively, with a spread of 0.4–3 times solar. Additionally, we determined an electron number density of log( n e )≥3.3 cm −3 , and the outflow might extend up to a maximum distance of 2.8 kpc. Conclusions. Our results indicate a solar metallicity within an uncertainty range of 60% that is driven by variations in the chosen SED and photoionization models. This study underscores the importance of the SED impact on determining chemical abundances in quasar outflows. These findings highlight the necessity of considering a wider range of possible abundances that span from subsolar to supersolar values.

  PublisherOA PDFDOI

• DESI survey of S IV absorption outflows in quasars: Contribution to AGN feedback and comparison with [O III] emission outflows

  Astronomy and Astrophysics · 2025-07-08 · 1 citations

  articleOpen access

  Aims. Quasar outflows play a crucial role in the evolution of their host galaxies through various feedback processes. This effect is thought to be particularly important when the Universe was only 2–3 billion years old, during the period known as cosmic noon. By utilizing existing observations from the Dark Energy Spectroscopy Instrument (DESI), we conducted a survey of high-ionization quasar outflows at cosmic noon, aiming to double the current sample of such outflows with distance and energetics determination. We also aimed to compare these properties with those derived from spatially resolved outflows in similar quasars probed through integral field spectroscopy. Methods. We performed Monte Carlo simulations on a sample of 130 quasars and detect signatures of high-ionization outflows in the form of S IV troughs in eight objects. The absorption features of each outflow were then individually analyzed to characterize their physical conditions by determining the total hydrogen column density ( N H ), the ionization parameter ( U H ), and the electron number density ( n e ). Through these parameters, we determined the distance of the outflows from their central source ( R ), their mass outflow rate, and their kinetic luminosity. Results. The detected outflows show complex kinematic structures with a wide range of blueshifted velocities (100–4600 km s −1 ). We locate five of the eight outflows at distances between 240–5500 pc from the central source. Only upper limits could be obtained for two outflows, placing them closer than 100 and 900 pc, respectively; for one outflow, the distance could not be determined. From the combined sample of 15 high-ionization S IV outflows at cosmic noon, we find that a high fraction (up to 46%) are powerful enough to contribute significantly to multistage active galactic nucleus feedback processes. Their energetics are also consistent with spatially resolved outflows in a luminosity- and redshift-matched sample of quasars. Comparison with previous spectra reveals interesting variations in some objects, including two cases of emerging high-velocity broad absorption line features with velocities of –8000 and –39 000 km s −1 . An impressive case of four line-locked Si IV outflow systems is also revealed in one object.

  PublisherOA PDFDOI

• Galactic-scale emission-line outflow from the radio-loud quasar 3C 191

  ArXiv.org · 2025-02-26

  preprintOpen access

  Quasar feedback is routinely invoked as an indispensable ingredient in galaxy formation models. Galactic outflows are a crucial agent of quasar feedback that frequently manifest themselves in absorption and emission lines. Measuring the size and energetics of outflows based on absorption lines remains a challenge, and integral-field spectroscopy (IFS) mapping in emission lines is complementary. We present a VLT/SINFONI IFS mapping of quasar 3C 191 at $z \sim 2$, in which the outflow has been analyzed in absorption line spectroscopy. Three components are found based on the morphology and kinetics of [OIII]-emitting gas: a unshifted component which consistent with the systemic redshift and the location of the nucleus, a blueshifted in the north, and a redshifted in the south. The latter two components have velocities $\sim$ 600 km s$^{-1}$ and projected extents of 5 and 11 kpc, respectively, suggesting a biconical outflow structure. The blueshifted component's velocity is consistent with that derived from absorption lines. Using the electron density measured by the absorption lines and the luminosity and velocity of [OIII] outflow, we derive the mass outflow rate to be $\dot{M} \sim $ 9.5-13.4 M$_\odot$ yr$^{-1}$ and kinetic luminosity $\dot{E}_{\rm kin}$ ~ 2.5-3.7 $\times 10^{42}$ erg s$^{-1}$, consistent with absorption line analyses with VLT/Xshooter spectrum. The kinetic luminosity is only 0.01% of the bolometric luminosity, rendering a relatively weak outflow compared to typical expectation for effective feedback.

  PublisherOA PDFDOI

• Quasar absorption outflows on galactic scales: insights from DESI

  Monthly Notices of the Royal Astronomical Society · 2025-05-30 · 1 citations

  articleOpen access

  ABSTRACT Absorption outflows in quasars play an important role in understanding active galactic nuclei (AGNs) feedback and their influence on galaxy evolution. The unprecedented spectral data provided by the Dark Energy Spectroscopic Instrument (DESI) open new avenues to explore these outflows. We analyse five low-ionization absorption outflow systems in four intermediate-redshift quasars ($2 < z < 3$) using the data obtained by DESI in order to characterize their physical properties and energetics, and also to assess their role in AGNs feedback. We use the spectra from DESI’s Early Data Release to determine the ionic column densities, total hydrogen column densities, electron number densities, and ionization parameters via photoionization modelling and absorption line analysis. We derive the outflows’ distance from the AGN, and their kinematic properties: mass-flow rates, kinetic luminosity, and momentum flux. Our study identifies five distinct minibroad absorption line outflow systems, hosted by four quasars. The identified outflows exhibit hydrogen column densities of $\log (N_{\rm H}) = 20.0\!-\!20.7$ cm−2, ionization parameters of $\log (U_{\rm H}) = -2.7$ to $-2.1$, and electron number densities of $\log (n_{\rm e}) =1.45\!-\!2.85$ cm−2. The distances of the outflows from the central source are between 4.5 and 31 kpc, and the kinetic luminosities range from $2\times 10^{-6}$ to $5\times 10^{-3}$ of the Eddington luminosity. The outflows in J1407 and J1032 show the largest kinetic luminosities, with momentum flux ratios ($\dot{p} / \dot{p}_{\rm rad}$) of about 2 and 0.2, respectively. Our findings highlight the vital role of DESI data in uncovering the diversity and significance of quasar outflows in galaxy evolution.

  PublisherOA PDFDOI

Recent grants

• The Impact of Observed Quasar Outflows on Cosmological Structure Formation

  NSF · $342k · 2008–2011

• Where are the Broad Absorption Line Outflows Situated and How Important are they to Active Galactic Nuclei Feedback?

  NSF · $497k · 2014–2018

• QUASAR ABSORPTION OUTFLOWS, THE MAIN AGENT OF AGN FEEDBACK

  NSF · $500k · 2021–2026

Frequent coauthors

• J. S. Kaastra

  SRON Netherlands Institute for Space Research

  203 shared

• G. A. Kriss

  Space Telescope Science Institute

  186 shared

• P.‐O. Petrucci

  Institut de Planétologie et d'Astrophysique de Grenoble

  167 shared

• M. Mehdipour

  Space Telescope Science Institute

  110 shared

• E. Costantini

  The Netherlands Cancer Institute

  108 shared

• K. C. Steenbrugge

  Centre National de la Recherche Scientifique

  102 shared

• J. Ebrero

  European Space Agency

  86 shared

• G. Ponti

  85 shared