31 research outputs found
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 ( per cent) are
detected with flux densities exceeding 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 J14190838
The radio quasar NVSS J141922083830 (J14190838) was initially
classified as an uncategorised blazar-type object, following its detection in
the -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 J14190838 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 -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
individually radio-non-detected active galactic nuclei (AGNs) at , we
performed image stacking analysis to examine the sub-mJy emission at GHz.
We found characteristic monochromatic radio powers of W Hz,
W Hz, 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 , 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 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 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
, which is in agreement with the median spectral index
of the radio-detected 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 to uncover the hidden Jy radio emission
in these active galactic nuclei (AGN). The median stacking procedure for the
full sample of optically identified AGN uncovered an unresolved point
source with an integrated flux density of 52Jy, with a signal-to-noise
ratio . We co-added the individual image centre pixels to estimate the
characteristic monochromatic radio power at GHz considering various
values for the radio spectral index, revealing a radio population with
W Hz. 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, Myr. 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 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