About

Aaron Evans is a Professor at the University of Virginia, serving on the GECO management committee within the Department of Astronomy. His research focuses on extragalactic studies, planetary science, X-ray astronomy, and clusters of galaxies. He is involved in initiatives such as the Galaxy Evolution and Cosmology Initiative (GECO) and the Interconnected Cosmos Initiative (ICI), contributing to the understanding of cosmic origins and galaxy evolution. Evans is based in Room 229 of the Astronomy Building at UVA, where he engages in both research and outreach activities, including supporting the Dark Skies, Bright Kids program and participating in the history and public engagement of the Leander McCormick Observatory.

Research topics

• Physics
• Astrophysics
• Astronomy
• Computer Science
• Artificial Intelligence
• Astrobiology

Selected publications

• Probing Heavily Obscured Active Galactic Nuclei in Major Galaxy Mergers Using the Millimeter–X-Ray Correlation

  The Astrophysical Journal Letters · 2026-02-24

  articleOpen access

  Abstract The study of heavily obscured supermassive black hole (SMBH) growth in late-stage galaxy mergers is challenging: column densities N H > 10 24 cm −2 can block most nuclear emission, leaving significant gaps in the SMBH growth census. Millimeter-wave continuum emission offers a potential window into this obscured phase, as it can trace active galactic nuclei (AGN) activity through mechanisms less affected by dust extinction. In this work, we test whether the observed correlation between millimeter (∼200 GHz) and hard X-ray (14–150 keV) luminosities can be used to plausibly identify hidden AGN in local ultraluminous infrared galaxies, including systems hosting confirmed dual AGN. We identify three sources—one confirmed AGN and two strong candidates—presenting significant evidence of AGN activity. The confirmed dual AGN lie within ∼3 σ of the millimeter–X-ray correlation, suggesting this relation can be used to identify hidden pairs. By combining the position of each source relative to this correlation with independent star formation rate constraints, we propose a method to disentangle AGN and star formation contributions for sources with measured column densities. While our analysis is based on a small, heterogeneous local sample and relies on empirical scaling relations, these results indicate that millimeter continuum emission may provide a useful complementary diagnostic for obscured SMBH growth. Atacama Large Millimeter/submillimeter Array observations at high angular resolutions are particularly valuable for this approach, while future facilities such as the ngVLA will be essential to test its robustness in larger and more distant samples.

  PublisherDOI

• Probing Active Galactic Nuclei-driven Feedback in Dwarf Galaxies with Spatially Resolved Near-infrared Coronal Lines from JWST

  The Astrophysical Journal · 2026-03-25

  articleOpen access

  Abstract We present the first spatially resolved investigation of near-infrared coronal lines in dwarf galaxies hosting active galactic nuclei (AGN), using JWST/NIRSpec integral field spectroscopy. Coronal lines (CLs), which are forbidden transitions from highly ionized species with ionization potentials up to 450 eV, act as sensitive tracers of the AGN ionizing continuum and feedback processes. Across four dwarf galaxies with ionized gas outflows traced by the optical [O III ] lines, we report the detection of 16 unique species of near-infrared CLs. Line ratio diagnostics indicate that photoionization from the AGN dominates the excitation of CLs. We find that the coronal line region in dwarf galaxies, traced by the various CLs, extends up to 0.5 kpc, and can constitute up to 10% of the size of the host galaxy. Correlations between CL luminosities and the properties of [O III ] ionized gas outflows are consistent with a scenario in which AGN-driven outflows likely facilitate the detection of CLs and contribute to their extent. Several CLs, including [Si VI ], [Si VII ], and [Mg VIII ], exhibit a secondary broad component with W 80 (the line width enclosing 80% of the total flux) >300 km s −1 . If we interpret this spatially compact gas as part of an outflow, this would indicate that the outflowing gas includes a wide range of ionization states. The estimated energetics imply this highly ionized component is compact yet powerful enough to perturb gas in the central regions of the host dwarfs. These results indicate that AGN in low-mass galaxies may produce outflows capable of influencing their structure and evolution.

  PublisherDOI

• A precessing jet from an active galactic nucleus drives gas outflow from a disk galaxy

  Science · 2026-01-08 · 2 citations

  articleOpen access

  To reproduce observed galaxy properties, cosmological simulations require that massive galaxies experience feedback from active galactic nuclei, which regulates star formation within those galaxies. However, the energetics and timescales of these feedback processes are poorly constrained. We combined optical, infrared, submillimeter, and radio observations of the active galaxy VV 340a, which is hosting a low-power jet launched from a supermassive black hole at its center. We found that the jet undergoes precession, with a period of (8.2 ± 5.5) × 10 5 years, and drives an outflow of gas at a rate of 19.4 ± 7.9 solar masses per year. The jet shocks the gas, producing highly ionized plasma that extends several kiloparsecs from the nucleus. The outflow ejects sufficient gas from the galaxy to influence its star-formation rate.

  PublisherOA PDFDOI

• High-resolution ALMA observations of H₂S in LIRGs

  Astronomy and Astrophysics · 2025-09-10

  articleOpen access

  Context. Molecular gas plays a critical role in regulating star formation and nuclear activity in galaxies. Sulphur-bearing molecules, such as H 2 S, are sensitive to the physical and chemical environments in which they reside and are potential tracers of shocked, dense gas in galactic outflows and active galactic nuclei (AGNs). Aims. We aim to investigate the origin of H 2 S emission and its relation to dense gas and outflow activity in the central regions of nearby infrared-luminous galaxies. Methods. We present Atacama Large Millimeter/submillimeter Array (ALMA) Band 5 observations of the ortho-H 2 S 1 1, 0 − 1 0, 1 transition in three nearby galaxies: NGC 1377, NGC 4418, and NGC 1266. We performed radiative transfer modelling using RADEX to constrain the physical conditions of the H 2 S-emitting gas and compare the results to ancillary CO and continuum data. Results. We detect compact H 2 S emission in all three galaxies, arising from regions smaller than ∼150 pc. The H 2 S spectral profiles exhibit broad line wings, suggesting an association with outflowing or shocked gas. In NGC 4418, H 2 S also appears to be tracing gas that is counter-rotating. A peculiar redshifted emission feature may be inflowing gas, or possibly a slanted outflow. RADEX modelling indicates that the H 2 S-emitting gas has high densities ( n H 2 ≳ 10 7 cm −3 ) and moderately warm temperatures (40−200 K). The derived densities exceed those inferred from CO observations, implying that H 2 S traces denser regions of the ISM.

  PublisherDOI

• Hydrodynamic Predictions for the Next Outburst of T Coronae Borealis: It will be the Brightest Classical or Recurrent Nova Ever Observed in X-rays

  arXiv (Cornell University) · 2025-02-15

  preprintOpen access

  T Coronae Borealis (TCrB) is a recurrent nova (RN) with recorded outbursts in 1866, and 1946 and possible outbursts in 1217 and 1787. It is predicted to explode again in 2025 or 2026 based on multiple observational studies. The system consists of a massive (M$_{wd}$ $\gtrsim$ 1.35 M$_\odot$) white dwarf (WD) and a red giant (M3-M4 III). We have performed 1-D hydrodynamic simulations with NOVA to predict the behavior of the next outburst. These simulations consist of a range of mass accretion rates onto $\sim$1.35 M$_\odot$ WDs, designed to bound the conditions necessary to achieve ignition of an explosion after an $\approx$80 year inter-outburst period. We have used both carbon-oxygen and oxygen-neon initial compositions, in order to include the possible ejecta abundances to be measured in the observations of the next outburst. As the WD in the TCrB system is observed to be massive, theoretical predictions reported here imply that the WD is growing in mass as a consequence of the TNR. Therefore, the secular evolution of the WD may allow it to approach the Chandrasekhar limit and either explode as a Type Ia supernova or undergo accretion induced collapse, depending on its underlying composition. We have followed the evolution of just the WD, after removing the ejected matter from the surface layers. Our intent is to illuminate the mystery of the unique, second, maximum in the two well observed outbursts and we have found conditions that bracket the predictions.

  PublisherOA PDFDOI

• Molecular Gas in Major Mergers Hosting Dual and Single AGNs at <10 kpc Nuclear Separations

  The Astrophysical Journal · 2025-12-09 · 1 citations

  articleOpen access

  Abstract We present high-resolution (∼50–100 pc) Atacama Large Millimeter/submillimeter Array observations of 12 CO(2–1) or 12 CO(1–0) emission in seven local ( z ≲ 0.05) major mergers—five of which are dual active galactic nucleus (AGN) systems, and two of which are single AGN systems. We model the molecular gas kinematics through rotating disk profiles using a Bayesian Markov Chain Monte Carlo approach. The residuals were then used to isolate nonrotating components of the molecular gas—the most likely contributor to future supermassive black hole (SMBH) growth. We find that more-massive SMBHs have higher surface densities of nonrotating molecular gas within their sphere of influence. This potential molecular gas supply, however, does not correlate with the current accretion efficiency of the SMBHs, suggesting that only a fraction of the observed nonrotating gas is currently reaching the SMBH. Finally, we tentatively find no significant differences in the nuclear molecular gas masses of single-AGN and dual-AGN hosts, both within the SMBH sphere of influence and within the central kiloparsec. Our results indicate that the probability of occurrence of the dual AGN phenomenon is likely dependent on AGN variability and/or obscuration rather than the availability of molecular gas in the nuclear regions.

  PublisherDOI

• The Arp 240 Galaxy Merger: A Detailed Look at the Molecular Kennicutt–Schmidt Star Formation Law on Subkiloparsec Scales

  The Astrophysical Journal · 2025-01-28 · 2 citations

  articleOpen access

  Abstract The molecular Kennicutt–Schmidt Law has been key for understanding star formation (SF) in galaxies across all redshifts. However, recent subkiloparsec observations of nearby galaxies reveal deviations from the nearly unity slope ( N ) obtained with disk-averaged measurements. We study SF and molecular gas (MG) distribution in the early-stage luminous infrared galaxy merger Arp 240 (NGC 5257-8). Using Very Large Array radio continuum (RC) and Atacama Large Millimeter/submillimeter Array CO(2–1) observations at 500 pc scale, with a uniform grid analysis, we estimate SF rates and MG surface densities (Σ SFR and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mo mathvariant="normal">Σ</mml:mo> </mml:mrow> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> , respectively). In Arp 240, N is sublinear at 0.52 ± 0.17. For NGC 5257 and NGC 5258, N is 0.52 ± 0.16 and 0.75 ± 0.15, respectively. We identify two SF regimes: high surface brightness (HSB) regions in RC with N ~ 1, and low surface brightness (LSB) regions with shallow N (ranging 0.15 ± 0.09–0.48 ± 0.04). Median CO(2–1) linewidth and MG turbulent pressure ( P turb ) are 25 km s −1 and 9 × 10 5 K cm −3 . No significant correlation was found between Σ SFR and CO(2–1) linewidth. However, Σ SFR correlates with P turb , particularly in HSB regions ( ρ &gt; 0.60). In contrast, SF efficiency moderately anticorrelates with P turb in LSB regions but shows no correlation in HSB regions. Additionally, we identify regions where peaks in SF and MG are decoupled, yielding a shallow N (≤0.28 ± 0.18). Overall, the range of N reflects distinct physical properties and distribution of both the SF and MG, which can be masked by disk-averaged measurements.

  PublisherDOI

• Near-Infrared Spectroscopy of the Recurrent Nova M31N 2008-12a

  ArXiv.org · 2025-05-15

  preprintOpen access

  Near infrared (NIR) 0.9--2.5$μ$m spectra of the remarkable recurrent nova M31N 2008-12a were obtained on days 6.3 and 10.3 after discovery of its 2024 outburst, and are the first NIR spectra of this object. The only prominent line seen in the spectra is that of HeI 1.083$μ$m, on day 6.3. Apart from this HeI line, there are only two other weak emission features: one at 1.0786$μ$m, suggested to be the [FeXIII] 1.075$μ$m coronal line, and one unidentified feature at 1.0969$μ$m. The observed full width at half maximum of the HeI line on day 6.3 (1350 km s$^{-1}$) is consistent with the behaviour of optical HeI lines during earlier eruptions of this RN, which show that the nova ejecta decelerate as they interact with the secondary's wind. The HeI 1.083$μ$m line faded rapidly, and was absent in the day 10.3 spectrum, along with any other emission lines. We use the relative strengths of optical He and H lines in previous eruptions to estimate the expected strengths of the HeI 1.083$μ$m line and of other infrared (including coronal) lines at 6.3 days after eruption. Our findings are consistent with the infrared spectra we observed during the 2024 eruption. We apply our analysis to account for the relative weakness of NIR coronal emission in known Galactic recurrent novae with giant secondaries.

  PublisherOA PDFDOI

• The Relationship Between Eddington Ratio and Column Density in U/LIRG AGN

  Research Notes of the AAS · 2025-05-30

  articleOpen access

  Abstract The local X-ray Active Galactic Nuclei (AGN) population appears to follow a growth cycle regulated by the AGN’s own radiation, marked by changes in their obscuration and Eddington ratio during accretion events. Because AGN in infrared-selected galaxies are more likely to be Compton-thick and have evidence for over-massive black holes, we explore whether infrared-selected AGN follow the radiation-regulated AGN growth scheme. We calculate the Eddington ratios of nine (Ultra)Luminous InfraRed Galaxy AGN with dynamical BH mass measurements, finding that though the number of objects is limited, AGN in IR-selected galaxies appear consistent with radiation pressure-regulated growth. We suggest that enlarging the sample of dynamical BH mass measurements in IR-selected systems will provide more stringent tests of whether their AGN are primarily regulated by radiation pressure.

  PublisherDOI

• A Spectroscopically Calibrated Prescription for Extracting Polycyclic Aromatic Hydrocarbon Flux from JWST MIRI Imaging

  The Astrophysical Journal · 2025-04-08 · 4 citations

  articleOpen access

  Abstract We introduce a prescription for estimating the flux of the 7.7 μ m and 11.3 μ m polycyclic aromatic hydrocarbon (PAH) features from broadband JWST/MIRI images. Probing PAH flux with MIRI imaging data has advantages in field of view, spatial resolution, and sensitivity compared with MIRI spectral maps, but comparisons with spectra are needed to calibrate these flux estimations over a wide variety of environments. For 267 MIRI/MRS spectra from independent regions in the four luminous infrared galaxies (LIRGs) in the Great Observatories All-sky LIRG Survey Early Release Science program, we derive synthetic filter photometry and directly compare estimated PAH fluxes to those measured from detailed spectral fits. We find that for probing PAH 7.7 μ m, the best combination of filters is F560W, F770W, and either F1500W or F2100W, and the best for PAH 11.3 μ m is F560W, F1000W, F1130W, and F1500W. The prescription with these combinations yields predicted flux densities that typically agree with values from spectral decomposition within ∼7% and ∼5% for PAH 7.7 and 11.3 μ m, respectively.

  PublisherOA PDFDOI

Recent grants

• The ISM of Luminous Infrared Galaxies: Tracing Gas Properties, Dynamics and Starburst/AGN Activity

  NSF · $325k · 2011–2015

• A Census of Starburst Regions in Luminous Infrared Galaxies as Unveiled by the Jansky VLA

  NSF · $343k · 2018–2022

Frequent coauthors

• V. Charmandaris

  375 shared

• L. Armus

  Infrared Processing and Analysis Center

  364 shared

• J. M. Mazzarella

  Infrared Processing and Analysis Center

  359 shared

• George C. Privon

  National Radio Astronomy Observatory

  307 shared

• T. Díaz-Santos

  304 shared

• D. B. Sanders

  260 shared

• L. Barcos-Muñoz

  National Radio Astronomy Observatory

  258 shared

• J. Surace

  Infrared Processing and Analysis Center

  252 shared

Labs

• X-ray Astronomy and Clusters of Galaxies GroupPI

Education

• PhD, Physics and Astronomy

  University of Hawaii at Manoa

  1996

• B.S., Astronomy, Physics

  University of Michigan

  1990