Radio emission from dust-obscured galaxies

The coevolution of galaxies and their central supermassive black holes is a subject of intense research. A class of objects, the dust-obscured galaxies (DOGs) are particularly interesting in this respect as they are thought to represent a short evolutionary phase when violent star formation activity in the host galaxy may coexist with matter accretion onto the black hole powering the active nucleus. Here we investigate different types of DOGs classified by their mid-infrared spectral energy distributions to reveal whether they can be distinguished by their arcsec-scale radio properties. Radio emission is unaffected by dust obscuration and may originate from both star formation and an active nucleus. We analyse a large sample of 661 DOGs complied from the literature and find that only a small fraction of them ($\sim 2$ per cent) are detected with flux densities exceeding $\sim 1$ mJy in the Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey. These radio-detected objects are almost exclusively `power-law' DOGs. Stacking analysis of the FIRST image cutouts centred on the positions of individually radio-undetected sources suggests that weak radio emission is present in `power-law' DOGs. On the other hand, radio emission from `bump' DOGs is only marginally detected in the median-stacked FIRST image.Comment: 7 pages, 3 figures, accepted to MNRA

Radio and mid-infrared properties of the blazar J1419−-0838

The radio quasar NVSS J141922$-$083830 (J1419$-$0838) was initially classified as an uncategorised blazar-type object, following its detection in the $\gamma$-ray band with the Fermi space telescope. Later, using multi-waveband observations and modeling, its was found to be a flat-spectrum radio quasar (FSRQ). However, its radio emission has never been discussed in depth in the literature. Here we present a detailed analysis on the radio properties of J1419$-$0838 using archival interferometric imaging data at pc and kpc scales. We conclude that the flux density variations, the flat radio spectrum, the compact nature of the quasar structure at all scales, and the relativistic Doppler enhancement of the radio emission all support the previous classification as an FSRQ. We also investigated the short- and long-term mid-infrared (MIR) light curve of the quasar based on observations by the Wide-field Infrared Survey Explorer satellite, and found that there is significant variability on time-scales of days as well as years. Comparison of the MIR light curve to the times of previously reported $\gamma$-ray and optical flares shows no clear correlation between the events at different wavebands.Comment: 15 pages, 5 figures, 3 tables, accepted for publication in Astrophysics and Space Scienc

Sub-mJy radio emission from high-redshift active galactic nuclei in the footprint of the VLA Sky Survey

Using empty-field `Quick Look' images from the first two epochs of the VLA Sky Survey (VLASS) observations, centred on the positions of $\sim3700$ individually radio-non-detected active galactic nuclei (AGNs) at $z\ge4$, we performed image stacking analysis to examine the sub-mJy emission at $3$ GHz. We found characteristic monochromatic radio powers of $P_\mathrm{char}=(2-13) \times 10^{24}$ W Hz$^{-1}$, $P_\mathrm{char}=2\times10^{24}-1.3\times10^{25}$ W Hz$^{-1}$, indicating that AGN-related radio emission is widespread in the sample. The signal-to-noise ratios of the redshift-binned median stacked maps are between $4-6$, and we expect that with the inclusion of the yet to be completed third-epoch VLASS observations, the detection limit defined as signal-to-noise ratio $\mathrm{SNR}\ge6$ could be reached, and the redshift dependence can be determined. To obtain information on the general spectral properties of the faint radio emission in high-redshift AGNs, we confined the sample to $\sim3000$ objects covered by both the VLASS and the Faint Images of the Radio Sky at Twenty-centimeters (FIRST) survey. We found that the flux densities from the median stacked maps show a characteristic spectral index of $\alpha^*=-0.30\pm0.15$, which is in agreement with the median spectral index of the radio-detected $z\ge4$ AGNs from our high-redshift AGN catalogue. The three-band mid-infrared colour--colour diagram based on Wide-field Infrared Survey Explorer observations provides further support regarding the AGN contribution to the radio emission in the sub-mJy sample.Comment: 9 pages,7 figures, 3 tables. Accepted for publication in MNRA

Three in One: The VLBI Radio View of the X-ray Quasar RX J1456.0+5048

RX J1456.0+5048 is a prominent X-ray source detected by ROSAT. There is ~100-mJy level radio emission associated with the X-ray source. However, interferometric observations with increasing angular resolution revealed that three distinct objects located within 2 arcmin are responsible for the measured total flux density. Whether these radio sources lining up in the sky are physically associated or just seen close to each other in projection is not immediately clear. In fact, incorrect cross-identification of the X-ray, optical and radio sources can already be found in the literature. Here we summarise the current knowledge about this intriguing group of objects, where two of the three sources show compact radio emission detected with very long baseline interferometry (VLBI). We present a VLBI image of one of them for the first time, based on archival European VLBI Network (EVN) data taken at 5 GHz.Comment: 5 pages, 2 figures. To appear in the proceedings of the European VLBI Network Mini-Symposium and Users' Meeting 2021, Proceedings of Science, PoS(EVN2021)00

Unveiling the weak radio quasar population at z≥4

We applied image stacking on empty-field Faint Images of the Radio Sky at Twenty-Centimeters (FIRST) survey maps centred on optically identified high-redshift quasars at $z\geq4$ to uncover the hidden $\mu$Jy radio emission in these active galactic nuclei (AGN). The median stacking procedure for the full sample of $2229$ optically identified AGN uncovered an unresolved point source with an integrated flux density of 52$~\mu$Jy, with a signal-to-noise ratio $\sim10$. We co-added the individual image centre pixels to estimate the characteristic monochromatic radio power at $1.4~$GHz considering various values for the radio spectral index, revealing a radio population with $P_\mathrm{1.4GHz}\sim10^{24}~$W Hz$^{-1}$. Assuming that the entire radio emission originates from star-forming (SF) activity in the nuclear region of the host galaxy, we obtained an upper limit on the characteristic star formation rate, $\sim4200~$M$_\odot~$yr$^{-1}$. The angular resolution of FIRST images is insufficient to distinguish between the SF and AGN origin of radio emission at these redshifts. However, a comparison with properties of individual sources from the literature indicates that a mixed nature is likely. Future very long baseline interferometry radio observations and ultra-deep Square Kilometre Array surveys are expected to be sensitive enough to detect and resolve the central $1-10~$kpc region in the host galaxies, and thus discriminate between SF and AGN related emission.Comment: 9 pages, 3 figures, 3 tables, accepted for publication in MNRA

Is There a Blazar Nested in the Core of the Radio Galaxy 3C411?

Previous spectral energy distribution modeling based on XMM-Newton X-ray observation of the classical double-lobed radio galaxy 3C 411 left the possibility open for the presence of a blazar-like core. We investigated this scenario by characterizing the radio brightness distribution in the inner ~10-pc region of the source. We applied the very long baseline interferometry (VLBI) technique at four different frequencies from 1.7 to 7.6 GHz. We analyzed archival data from the Very Long Baseline Array (VLBA) taken in 2014, and data from the European VLBI Network (EVN) obtained in 2017. The VLBI images reveal pc-scale extended structure in the core of 3C 411 that can be modeled with multiple jet components. The measured core brightness temperatures indicate no Doppler enhancement that would be expected from a blazar jet pointing close to the line of sight. While there is no blazar-type core in 3C 411, we found indication of flux density variability. The overall morphology of the source is consistent with a straight jet with ~50{\deg} inclination angle at all scales from pc to kpc.Comment: 9 pages, 6 figures, 2 tables, accepted for publication in Ap

Constraining the radio jet proper motion of the high-redshift quasar J2134-0419 at z = 4.3

To date, PMN J2134-0419 (at a redshift z = 4.33) is the second most distant quasar known with a milliarcsecond-scale morphology permitting direct estimates of the jet proper motion. Based on two-epoch observations, we constrained its radio jet proper motion using the very long baseline interferometry (VLBI) technique. The observations were conducted with the European VLBI Network (EVN) at 5 GHz on 1999 November 26 and 2015 October 6. We imaged the central 10-pc scale radio jet emission and modeled its brightness distribution. By identifying a jet component at both epochs separated by 15.86 yr, a proper motion of μ = 0.035 ± 0.023 mas yr-1 is found. It corresponds to an apparent superluminal speed of βa = 4.1 ± 2.7 c. Relativistic beaming at both epochs suggests that the jet viewing angle with respect to the line of sight is smaller than 20°, with a minimum bulk Lorentz factor Γ = 4.3. The small value of the proper motion is in good agreement with the expectations from the cosmological interpretation of the redshift and the current cosmological model. Additionally we analyzed archival Very Large Array observations of J2143-0419 and found indication of a bent jet extending to ˜30 kpc