Emmanuel Momjian

A subset of active galactic nuclei (AGN) manifest as compact radio sources with peaked radio spectra, commonly referred to as peaked-spectrum (PS) sources. Generally, PS sources are interpreted as young radio jets, but the early evolution of the jet is rarely observed and not fully understood. Here, we present VT J182357-234511 (VT J1823-2345), a radio transient in the Very Large Array Sky Survey (VLASS) that resembles a PS source. Notable properties of VT J1823-2345 include (i) brightening by a factor of 3 between 1996 and 2021 at 1.4 GHz, (ii) peaked spectra with high-frequency peaks at ν _p,obs ≳ 6 GHz, (iii) a peaked 3 GHz light curve that has been fading since 2023, (iv) a resolved Gaussian (major-axis) size of 0.4 mas in 2024, and (v) a linear polarization fraction p _L ≃ 0.73% at 8.8 GHz with an extreme Faraday rotation measure RM _obs ∼ +12,000 rad m ⁻² . On the sky, VT J1823-2345 coincides with a supernova remnant and an F-type star, but no background galaxy is seen. Based on the multiwavelength properties, we disfavor Galactic transients, explosive extragalactic transients, and blazar variability as plausible explanations. We argue that VT J1823-2345 is consistent with a “short-lived” parsec-scale PS source with a decades-old jet that does not grow into a large-scale radio galaxy. From comparing the full VLASS light curves, we speculate that VT J1823-2345 is similar to many VLASS nuclear transients reported in the literature, which may represent an emerging class of transient radio jets, perhaps related to episodic AGN activity or jetted tidal disruption events.

Doppler boosting allows blazars to be detected out to high z , making them promising probes of the intergalactic medium through the 21 cm forest. We report 0.144−11 GHz observations of the most distant known blazar, VLASS J041009.05−013919.88 at z ∼ 7, obtained with the upgraded Giant Metrewave Radio Telescope (uGMRT), the LOw Frequency ARray (LOFAR), and the Very Large Array (VLA). The first uGMRT epoch (300−820 MHz, 2023 April) revealed an inverted radio spectrum that, combined with earlier (2021–2022) VLA data (1.5–11 GHz), unveiled a double-peaked spectrum potentially indicative of multiepoch jet activity. A second uGMRT campaign (2023 August), simultaneous with new VLA observations (1.5–11 GHz), instead revealed a flat low-frequency and a peaked high-frequency spectrum, ruling out this interpretation. While limited by the two uGMRT epochs, variability analysis favors intrinsic jet processes, indicating a highly relativistic, closely aligned jet ( θ < 3°, δ > 19.3, Γ > 9.7). The equipartition magnetic field (>1 mG) exceeds the equivalent cosmic microwave background field at z ∼ 7 (0.2 mG), indicating that synchrotron losses dominate. The inferred Doppler boosting ( δ > 19.3) implies that J0410−0139 is intrinsically radio-weak. As a blazar, it traces a much larger parent population of radio quasars at z ∼ 7, detectable only with deep (≤ μ Jy) next-generation radio observations. LOFAR 144 MHz observations (2024 April and July) yielded ∼2 mJy, well below the ∼8 mJy predicted from uGMRT epochs, confirming strong variability at rest-frame ∼1 GHz. Low-frequency monitoring will be crucial for identifying high radio intensity states suitable for future 21 cm forest studies.

We present 6 GHz radio continuum observations of the galaxy cluster A2744 ( z = 0.307) obtained with the Karl G. Jansky Very Large Array (VLA) as part of the VLA Frontier Fields program, the goal of which is to explore the radio continuum emission from high-redshift galaxies that are magnified by foreground massive galaxy clusters. With an rms noise of ≈1 μ Jy beam ⁻¹ , in the image plane, and subarcsec angular resolution ( θ 1 / 2 = 0 . ″ 82 ), this is the deepest and most detailed radio image of A2744 ever obtained. A total of 93 sources is detected with a peak signal-to-noise ratio ≥5, of which 46 have optical/near-infrared (IR) counterparts with available redshift, magnification ( μ ), and stellar mass ( M _* ) estimates. The radio sources are distributed over a redshift of 0.15–3.55, with a median redshift value of z ¯ = 0.9 3 − 0.63 + 1.48 and with a range mass from 5.5 × 10 ⁹ M _⊙ to 1.3 × 10 ¹¹ M _⊙ . A comparison between the radio-based star formation rates (SFRs) and those derived from UV-to-near IR data reveals that radio SFRs are typically an order of magnitude higher. This discrepancy is likely a result of strong dust obscuration affecting the UV-to-NIR tracers. We look for radio counterparts of the so-called “little red dots” (LRDs) galaxies at z ≈ 6 seen behind A2744, but find no significant detections. After stacking, we derive a 3 σ upper limit to the 6 GHz radio luminosity of LRDs of 4.1 × 10 ³⁹ erg s ⁻¹ . Finally, we present a sample of 22 moderately/strongly lensed galaxies ( μ ≳ 2) in the VLA Frontier Fields survey, which provides a zoomed view of the star formation processes within main-sequence galaxies at z ≈ 1–2.

We have obtained very high angular resolution (a few milliarcseconds or sub-parsec-scale) Very Long Baseline Array (VLBA) and European Very Long Baseline Interferometry Network (EVN) radio continuum images of the nucleus in the barred spiral galaxy NGC 7479, to search for possible nuclear emission on parsec scales. The observations were taken using phase referencing. Previous Karl G. Jansky Very Large Array and Multi-Element Radio Linked Interferometer Network observations revealed a large jetlike structure, apparently emanating from the nucleus, and unresolved nuclear emission at 0 . ″ 1 (≈15 pc at the assumed distance of 32 Mpc) scale, respectively. Our sensitive new VLBA and EVN images resolve the previously unresolved nuclear source and reveal two distinct emission regions (very long baseline interferometry (VLBI) components) that are separated by about 30 mas. We also report an apparent change in separation of the two main emission regions over the 10 yr between EVN and VLBA observations, implying relativistic radio jet motion or changes in shock illumination of gas by a nuclear wind. We measure the spectral indices and brightness temperatures of the VLBI components and discuss possible physical causes of the observed emission.

We have observed six “dark” neutral hydrogen (H i ) clouds discovered in the Virgo Cluster by the Arecibo Galaxy Environment Survey (AGES) with the Karl G. Jansky Very Large Array (VLA), giving higher angular and velocity resolution than the original AGES observations. We detected compact H i emission in two of the sources, AGESVC1 231 and AGESVC1 274, allowing us to firmly associate them with faint ( m _g > 18.5), blue ( g − i < 0.1) optical counterparts with high M _{H I} / L _g ratios. In a further two sources, we detected low-column-density extended H i emission, consistent with these being dispersing clouds from ram pressure stripping or tidal interactions. The final two sources were not detected with the VLA, allowing us to set low column-density limits on the H i detected by AGES that are consistent with these clouds also being formed from H i that is dispersing into the intracluster medium. The four H i sources not associated with optical counterparts thus appear likely to be relatively short-lived objects. No evidence was found for either pressure-supported turbulent spheres or stable dark galaxies.

We present subarcsecond ( θ ∼ 0 . ″ 1) resolution VLA 1.3 cm continuum and 22.2 GHz H ₂ O maser observations toward 15 compact radio continuum sources with rising spectral index and eight string-like radio continuum structures in the V. Rosero et al. survey. Three different morphologies are observed: elongated or double-peak string-like structure (6 out of 23 cases), a collection of distinct continuum peaks (4 out of 23 cases), and single compact sources (13 out of 23 cases). The majority of H ₂ O maser spots detected are within a sky-projected distance of ∼5000 au from the radio continuum peaks and tend to be well aligned and distributed in an elongated structure when more than three spots are observed. We generally recover less emission than V. Rosero et al., which together with the fact that more than half of the jet candidates in our survey appear mostly compact, suggest core/halo shock structures even on small scales. We also detected proper motion in 10 cases and measured an average projected velocity of approximately 120 km s ⁻¹ . Radio brightness variability is detected in at least two cases, possibly due to weak accretion bursts. This work, together with our previous molecular jet study, provides further evidence that support the main source of ionization in the studied sources is shocks, yet collimation is only observed in 4 cases. We conclude that the available data supports the thermal jet classification of seven sources, and the ionized jet interpretation is further supported in 16 sources.

We report the results of a pilot study that searched for dual active galactic nuclei (AGN) in local ( z < 0.25) galaxies hosting double-peaked narrow emission lines in their optical spectra. We present high-resolution L -band (1.5 GHz or 18 cm) continuum images from the Very Long Baseline Array as well as WFC3/IR F160W images from the Hubble Space Telescope of two candidate dual AGN systems: J0948+6848 and J1223+5409. In both targets, we detected compact nonthermal radio emission that is approximately cospatial with the near-infrared AGN. Both systems host two high brightness temperature (>10 ⁸ K) radio sources that indicate the presence of either a parsec-scale-separation dual AGN ( d _sep ∼ 90 and ∼56 pc, respectively) or a radio jet. Matched-resolution multiband radio observations are necessary to further characterize the AGN activity in these systems.

We present the neutral gas morphology of four galaxies from z = 0.22 to 0.47 obtained with the COSMOS H i Large Extragalactic Survey (CHILES). The H i is resolved at the highest redshift with the 7 . ″ 5 beam of CHILES and 43 kpc linear scale, with all four galaxies having extended H I . Three are massive galaxies ( M _* > 3 × 10 ¹⁰ M _⊙ ), with H i masses of (1.6–6.7) × 10 ¹⁰ M _⊙ and active star formation (3–30 M _⊙ yr ⁻¹ ). The morphology and kinematics of the galaxies vary from regular to disturbed, including an asymmetric H i disk surrounding the fourth smaller galaxy ( M _* ∼ 10 ⁹ M _⊙ ). CO (1–0) observations of the sample, obtained with the Large Millimeter Telescope, confirm the redshifts of three of the four galaxies, and we derive H ₂ masses of (0.4–5.2) × 10 ¹⁰ M _⊙ . JWST imaging with four combined NIRCam filters reveals disturbed stellar components with compact knots in two of the galaxies. We combine our new higher-redshift galaxies with previously published observations to conduct a more complete study of H i and H ₂ evolution in the redshift range 0–0.5. With our H i flux-limited observations compared to similar lower-redshift galaxies with high stellar mass ( M _* > 10 ¹⁰ M _⊙ ), the results show that the mean H ₂ /H i ratio at the highest redshift is 10.3 ± 3.4, larger than the mean H ₂ /H i ratio in the local Universe.