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1. The PG-RQS survey. Building the radio spectral distribution of radio-quiet quasars. I. The 45-GHz data

   https://doi.org/10.1093/mnras/stab3445  

   Baldi, R. D. ; Laor, A. ; Behar, E. ; Horesh, A. ; Panessa, F. ; McHardy, I. ; Kimball, A. ( November 2021 , Monthly Notices of the Royal Astronomical Society)

   The origin of the radio emission in radio-quiet quasars (RQQs) remains unclear. Radio emission may be produced by a scaled-down version of the relativistic jets observed in radio-loud (RL) AGN, an AGN-driven wind, the accretion disc corona, AGN photon-ionization of ambient gas (free–free emission), or star formation (SF). Here, we report a pilot study, part of a radio survey (‘PG-RQS’) aiming at exploring the spectral distributions of the 71 Palomar–Green (PG) RQQs: high angular resolution observations (∼50 mas) at 45 GHz (7 mm) with the Karl G. Jansky Very Large Array of 15 sources. Sub-mJy radio cores are detected in 13 sources on a typical scale of ∼100 pc, which excludes significant contribution from galaxy-scale SF. For 9 sources the 45-GHz luminosity is above the lower frequency (∼1–10 GHz) spectral extrapolation, indicating the emergence of an additional flatter-spectrum compact component at high frequencies. The X-ray luminosity and black hole (BH) mass, correlate more tightly with the 45-GHz luminosity than the 5-GHz. The 45 GHz-based radio-loudness increases with decreasing Eddington ratio and increasing BH mass MBH. These results suggest that the 45-GHz emission from PG RQQs nuclei originates from the innermost region of the core, probably from the accretion disc corona. Increasing contributions to 45-GHz emission from a jet at higher MBH and lower Eddington ratios and from a disc wind at large Eddington ratios are still consistent with our results. Future full radio spectral coverage of the sample will help us investigating the different physical mechanisms in place in RQQ cores.

    

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2. Is there a sub-parsec-scale jet base in the nearby dwarf galaxy NGC 4395?

   https://doi.org/10.1093/mnras/stac1753  

   Yang, Jun ; Yang, Xiaolong ; Wrobel, Joan M. ; Paragi, Zsolt ; Gurvits, Leonid I. ; Ho, Luis C. ; Nyland, Kristina ; Fan, Lulu ; Tafoya, Daniel ( June 2022 , Monthly Notices of the Royal Astronomical Society)

   NGC 4395 is a dwarf galaxy at a distance of about 4.3 Mpc (scale: ∼0.021 pc mas−1). It hosts an intermediate-mass black hole (IMBH) with a mass between ∼104 and ∼105 solar masses. The early radio observations of NGC 4395 with the very long baseline interferometry (VLBI) network, High Sensitivity Array (HSA), at 1.4 GHz in 2005 showed that its nucleus has a sub-mJy outflow-like feature (E) extending over 15 mas. To probe the possibility of the feature E as a continuous jet with a base physically coupled with the accretion disc, we performed deep VLBI observations with the European VLBI Network (EVN) at 5 GHz, and analysed the archival data obtained with the HSA at 1.4 GHz in 2008, NSF’s Karl G. Jansky Very Large Array (VLA) at 12–18 GHz and the Atacama Large Millimetre/submillimetre Array (ALMA) at 237 GHz. The feature E displays more diffuse structure in the HSA image of 2008 and has no compact substructure detected in the EVN image. Together with the optically thin steep spectrum and the extremely large angular offset (about 220 mas) from the accurate optical Gaia position, we explain the feature E as nuclear shocks likely formed by the IMBH’s episodic ejection or wide-angle outflow. The VLA and ALMA observations find a sub-mJy pc-scale diffuse feature, possibly tracing a thermal free–free emission region near the IMBH. There is no detection of a jet base at the IMBH position in the VLBI maps. The non-detections give an extremely low luminosity of ≤4.7 × 1033 erg s−1 at 5 GHz and indicate no evidence of a disc-jet coupling on sub-pc scales.

    

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3. Collimation of the Relativistic Jet in the Quasar 3C 273

   https://doi.org/10.3847/1538-4357/ac97e5  

   Okino, Hiroki ; Akiyama, Kazunori ; Asada, Keiichi ; Gómez, José L. ; Hada, Kazuhiro ; Honma, Mareki ; Krichbaum, Thomas P. ; Kino, Motoki ; Nagai, Hiroshi ; Bach, Uwe ; et al ( November 2022 , The Astrophysical Journal)

   Abstract The collimation of relativistic jets launched from the vicinity of supermassive black holes (SMBHs) at the centers of active galactic nuclei (AGNs) is one of the key questions to understand the nature of AGN jets. However, little is known about the detailed jet structure for AGN like quasars since very high angular resolutions are required to resolve these objects. We present very long baseline interferometry (VLBI) observations of the archetypical quasar 3C 273 at 86 GHz, performed with the Global Millimeter VLBI Array, for the first time including the Atacama Large Millimeter/submillimeter Array. Our observations achieve a high angular resolution down to ∼60 μ as, resolving the innermost part of the jet ever on scales of ∼10 5 Schwarzschild radii. Our observations, including close-in-time High Sensitivity Array observations of 3C 273 at 15, 22, and 43 GHz, suggest that the inner jet collimates parabolically, while the outer jet expands conically, similar to jets from other nearby low-luminosity AGNs. We discovered the jet collimation break around 10 7 Schwarzschild radii, providing the first compelling evidence for structural transition in a quasar jet. The location of the collimation break for 3C 273 is farther downstream from the sphere of gravitational influence (SGI) from the central SMBH. With the results for other AGN jets, our results show that the end of the collimation zone in AGN jets is governed not only by the SGI of the SMBH but also by the more diverse properties of the central nuclei. 

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4. Prevalence of Compact Nuclear Radio Emission in Post-merger Galaxies and Its Origin

   https://doi.org/10.3847/1538-4357/ad50a1  

   Walsh, Gregory ; Burke-Spolaor, Sarah ( August 2024 , The Astrophysical Journal)

   Post-merger galaxies are unique laboratories to study the triggering and interplay of star formation and active galactic nucleus (AGN) activity. Combining new, high-resolution Jansky Very Large Array (VLA) observations with archival radio surveys, we have examined the radio properties of 28 spheroidal post-merger galaxies. We detect 18 radio sources in our post-merger sample and find a general lack of extended emission at (sub)kiloparsec scales, indicating the prevalence of compact, nuclear radio emission in these post-merger galaxies, with the majority (16/18; 89%) characterized as low luminosity. Using multiwavelength data, we determine the origin of the radio emission, discovering 15 new radio AGNs and three radio sources likely associated with star-forming (SF) processes. Among the radio AGNs, almost all are low luminosity (13/15; 87%), inconsistent with a relativistic jet origin. We discover a new dual AGN (DAGN) candidate, J1511+0417, and investigate the radio properties of the DAGN candidate J0843+3549. Five of these radio AGNs are hosted by a SF or SF-AGN composite emission-line galaxy, suggesting that radio AGN activity may be present during periods of SF activity in post-mergers. The low-power jets and compact morphologies of these radio AGNs also point to a scenario in which AGN feedback may be efficient in this sample of post-mergers. Lastly, we present simulated, multifrequency observations of the 15 radio AGNs with the Very Long Baseline Array and the very-long-baseline interferometry capabilities of the Next-Generation VLA to assess the feasibility of these instruments in searches for supermassive black hole binaries.

    

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5. Proper motion of the radio jets in two blazars at redshift above 3

   https://doi.org/10.1093/mnras/stae533  

   Krezinger, Máté ; Frey, Sándor ; Perger, Krisztina ; Gabányi, Krisztina É. ; An, Tao ; Zhang, Yingkang ; Gurvits, Leonid I. ; Titov, Oleg ; Melnikov, Alexey ; Paragi, Zsolt ( February 2024 , Monthly Notices of the Royal Astronomical Society)

   There is still a limited number of high-redshift (z > 3) active galactic nuclei (AGNs) whose jet kinematics have been studied with very long baseline interferometry (VLBI). Without a dedicated proper motion survey, regularly conducted astrometric VLBI observations of bright radio-emitting AGN with sensitive arrays can be utilized to follow changes in the jets, by means of high-resolution imaging and brightness distribution modelling. Here, we present a first-time VLBI jet kinematic study of NVSS J080518 + 614423 (z = 3.033) and NVSS J165844 − 073918 (z = 3.742), two flat-spectrum radio quasars that display milliarcsecond-scale jet morphology. Archival astrometric observations carried out mainly with the Very Long Baseline Array, supplemented by recent data taken with the European VLBI Network, allowed us to monitor changes in their radio structure in the 7.6−8.6 GHz frequency band, covering almost two decades. By identifying individual jet components at each epoch, we were able to determine the apparent proper motion for multiple features in both sources. Apparent superluminal motions range $(1-14)\, c$, and are found to be consistent with studies of other high-redshift AGN targets. Using the physical parameters derived from the brightness distribution modelling, we estimate the Doppler-boosting factors (δ ≈ 11.2 and δ ≈ 2.7), the Lorentz factors (Γ ≈ 7.4 and Γ ≈ 36.6), and the jet viewing angles (θ ≈ 4.4° and θ ≈ 8.0°), for NVSS J080518 + 614423 and NVSS J165844 − 073918, respectively. The data revealed a stationary jet component with negligible apparent proper motion in NVSS J165844 − 073918.

    

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