26 research outputs found
A powerful (and likely young) radio-loud quasar at z=5.3
We present the discovery of PSO J191.0569686.43172 (hereafter PSO
J19186), a new powerful radio-loud quasar (QSO) in the early Universe (z =
5.32). We discovered it by cross-matching the NRAO VLA Sky Survey (NVSS) radio
catalog at 1.4 GHz with the first data release of the Panoramic Survey
Telescope and Rapid Response System (Pan-STARRS PS1) in the optical. With a
NVSS flux density of 74.2 mJy, PSO J19186 is one of the brightest radio QSO
discovered at z5. The intensity of its radio emission is also confirmed
by the very high value of radio loudness (R>300). The observed radio spectrum
of PSO J19186 shows a possible turnover around 1 GHz (i.e., 6
GHz in the rest frame), making it a Gigahertz-Peaked Spectrum (GPS) source.
However, variability could affect the real shape of the radio spectrum, since
the data in hand have been taken 25 years apart. By assuming a peak of
the observed radio spectrum between 1 and 2 GHz (i.e. 6 and 13 GHz in
the rest-frame) we found a linear size of the source of 10-30 pc and a
corresponding kinetic age of 150-460 yr. This would make PSO J19186 a newly
born radio source. However, the large X-ray luminosity (5.310
erg s), the flat X-ray photon index (=1.32) and the
optical-X-ray spectral index (=1.329) are typical of
blazars. This could indicate that the non-thermal emission of PSO J19186 is
Doppler boosted. Further radio observations (both on arcsec and parsec scales)
are necessary to better investigate the nature of this powerful radio QSO.Comment: 10 pages, 9 figures, 5 tables, Accepted for publication in A&
Parsec-scale properties of the radio brightest jetted AGN at z > 6
We present Director's Discretionary Time multi-frequency observations
obtained with the Jansky Very Large Array (VLA) and the Very Long Baseline
Array (VLBA) of the blazar PSO J030947.49+271757.31 (hereafter PSO J0309+27) at
. The milliarcsecond angular resolution of our VLBA
observations at 1.5, 5 and 8.4 GHz unveils a bright one-sided jet extended for
parsecs in projection. This high-z radio-loud AGN is resolved into
multiple compact sub-components, embedded in a more diffuse and faint radio
emission, which enshrouds them in a continuous jet structure. We derive limits
on some physical parameters directly from the observable quantities, such as
viewing angle, Lorentz and Doppler factors. If PSO J0309+27 is a genuine
blazar, as suggested by its X-ray properties, then we find that its bulk
Lorentz factor must be relatively low (less than 5). Such value would be in
favour of a scenario currently proposed to reconcile the paucity of high-z
blazars with respect to current predictions. Nevertheless, we cannot exclude
that PSO J0309+27 is seen under a larger viewing angle, which would imply that
the X-ray emission must be enhanced, for example, by inverse Compton with the
Cosmic Microwave Background. More stringent constraints on the bulk Lorentz
factor in PSO J0309+27 and the other high-z blazars are necessary to test
whether their properties are intrinsically different with respect to the low-z
blazar population.Comment: 10 pages, 5 figures, accepted for publication in A&A Letter
Obscuration in high redshift jetted QSO
Obscuration in high-redshift quasi-stellar objects (QSO) has a profound
impact on our understanding of the evolution of supermassive black holes across
the cosmic time. An accurate quantification of its relevance is therefore
mandatory. We present a study aimed at evaluating the importance of obscuration
in high redshift jetted QSO, i.e. those active nuclei characterized by the
presence of powerful relativistic jets. We compare the observed number of radio
detected QSO at different radio flux density limits with the value predicted by
the beaming model on the basis of the number of oriented sources (blazars). Any
significant deficit of radio-detected QSO compared to the predictions can be
caused by the presence of obscuration along large angles from the jet
direction. We apply this method to two sizable samples characterized by the
same optical limit (mag=21) but significantly different radio density limits
(30 mJy and 1 mJy respectively) and containing a total of 87 independent
radio-loud 4<z<6.8 QSO, 31 of which classified as blazars. We find a general
good agreement between the numbers predicted by the model and those actually
observed, with only a marginal discrepancy at 0.5 mJy that could be caused by
the lack of completeness of the sample. We conclude that we have no evidence of
obscuration within angles 10-20deg from the relativistic jet direction. We also
show how the ongoing deep wide-angle radio surveys will be instrumental to test
the presence of obscuration at much larger angles, up to 30-35deg. We finally
suggest that, depending on the actual fraction of obscured QSO, relativistic
jets could be much more common at high redshifts compared to what is usually
observed in the local UniverseComment: 14 pages, 5 figures. Accepted for publication on A&
New Radio-Loud QSOs at the end of the Re-ionisation Epoch
We present the selection of high-redshift () radio-loud (RL)
quasi-stellar object (QSO) candidates from the combination of the radio Rapid
ASKAP Continuum Survey (RACS; at 888 MHz) and the optical/near-infrared Dark
Energy Survey (DES). In particular, we selected six candidates brighter than
mJy beam and using
the dropout technique (in the -band). From this sample, we were able to
confirm the high- nature () of two sources, which are now among
the highest-redshift RL QSOs currently known. Based on our Gemini-South/GMOS
observations, neither object shows a prominent Ly emission line. This
suggests that both sources are likely to be weak emission-line QSOs hosting
radio jets and would therefore further strengthen the potential increase of the
fraction of weak emission-line QSOs recently found in the literature. However,
further multiwavelength observations are needed to constrain the properties of
these QSOs and of their relativistic jets. From the discovery of these two
sources, we estimated the space density of RL QSOs in the redshift range
to be 0.13 and found it to be consistent with the
expectations based on our current knowledge of the blazar population up to
.Comment: Accepted in MNRAS on 05 December 2022. Ten pages with five figures
and three table
Very long baseline interferometry observations of the high-redshift blazar candidate J0141-5427
Active galactic nuclei (AGN) have been observed as far as redshift z~7. They
are crucial in investigating the early Universe as well as the growth of
supermassive black holes at their centres. Radio-loud AGN with their jets seen
at a small viewing angle are called blazars and show relativistic boosting of
their emission. Thus, their apparently brighter jets are easier to detect in
the high-redshift Universe. DES J014132.4-542749.9 is a radio-luminous but
X-ray weak blazar candidate at z = 5. We conducted high-resolution radio
interferometric observations of this source with the Australian Long Baseline
Array at 1.7 and 8.5 GHz. A single, compact radio emitting feature was detected
at both frequencies with a flat radio spectrum. We derived the
milliarcsecond-level accurate position of the object. The frequency dependence
of its brightness temperature is similar to that of blazar sources observed at
lower redshifts. Based on our observations, we can confirm its blazar nature.
We compared its radio properties with those of two other similarly X-ray-weak
and radio-bright AGN, and found that they show very different relativistic
boosting characteristics.Comment: 8 pages, 4 figures, accepted for publication in PAS
No strong radio absorption detected in the low-frequency spectra of radio-loud quasars at z > 5.6
© 2023 The Author(s). Published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY), https://creativecommons.org/licenses/by/4.0/We present the low-frequency radio spectra of 9 high-redshift quasars at using the Giant Metre Radio Telescope band-3, -4, and -5 observations (300-1200 MHz), archival Low Frequency Array (LOFAR; 144 MHz), and Very Large Array (VLA; 1.4 and 3 GHz) data. Five of the quasars in our sample have been discovered recently, representing some of the highest redshift radio bright quasars known at low-frequencies. We model their radio spectra to study their radio emission mechanism and age of the radio jets by constraining the spectral turnover caused by synchrotron self-absorption (SSA) or free-free absorption (FFA). Besides J0309+2717, a blazar at , our quasars show no sign of a spectral flattening between 144 MHz and a few GHz, indicating there is no strong SSA or FFA absorption in the observed frequency range. However, we find a wide range of spectral indices between and , including the discovery of 3 potential ultra-steep spectrum quasars. Using further archival VLBA data, we confirm that the radio SED of the blazar J0309+2717 likely turns over at a rest-frame frequency of 0.6-2.3 GHz (90-330 MHz observed frame), with a high-frequency break indicative of radiative ageing of the electron population in the radio lobes. Ultra-low frequency data below 50 MHz are necessary to constrain the absorption mechanism for J0309+2717 and the turnover frequencies for the other high- quasars in our sample. A relation between linear radio jet size and turnover frequency has been established at low redshifts. If this relation were to hold at high redshifts, the limits on the turnover frequency of our sample suggest the radio jet sizes must be more extended than the typical sizes observed in other radio-bright quasars at similar redshift. To confirm this deep radio follow-up observations with high spatial resolution are required.Peer reviewe
The first blazar observed at z>6
We present the discovery of PSO J030947.49+271757.31, the radio brightest (23.7 mJy at 1.4 GHz) active galactic nucleus (AGN) at z>6.0. It was selected by cross-matching the NRAO VLA Sky Survey and the Panoramic Survey Telescope and Rapid Response System PS1 databases and its high-z nature was confirmed by a dedicated spectroscopic observation at the Large Binocular Telescope. A pointed Neil Gehrels S wi f t Observatory XRT observation allowed us to measure a flux of 3c3.4
710 1214 erg s 121 cm 122 in the [0.5-10] keV energy band, which also makes this object the X-ray brightest AGN ever observed at z>6.0. Its flat radio spectrum (\u3b1\u3bdr<0.5), very high radio loudness (R>103), and strong X-ray emission, compared to the optical, support the hypothesis of the blazar nature of this source. Assuming that this is the only blazar at this redshift in the surveyed area of sky, we derive a space density of blazars at z 3c6 and with M1450\uc5 < -21.5 of 5.5+11.2 124.6
710 123 Gpc 123. From this number, and assuming a reasonable value of the bulk velocity of the jet (\u393=10), we can also infer a space density of the entire radio-loud AGN population at z 3c6 with the same optical/UV absolute magnitude of 1.10+2.53 120.91 Gpc 123. Larger samples of blazars will be necessary to better constrain these estimates
Constraining the radio properties of the z = 6.44 QSO VIK J2318-3113
The recent detection of the quasi-stellar object (QSO) VIKING J231818.3â311346 (hereafter VIK J2318â3113) at redshift zâ=â6.44 in the Rapid ASKAP Continuum Survey (RACS) uncovered its radio-loud nature, making it one of the most distant known to date in this class. By using data from several radio surveys of the Galaxy And Mass Assembly 23h field and from a dedicated follow-up, we were able to constrain the radio spectrum of VIK J2318â3113 in the observed range âŒ0.110 GHz. At high frequencies (0.8885.5 GHz in the observed frame) the QSO presents a steep spectrum (αrâ=â1.24, with SÎœâââÎœâαr), while at lower frequencies (0.40.888 GHz in the observed frame) it is nearly flat. The overall spectrum can be modelled by either a curved function with a rest-frame turnover around 5 GHz, or with a smoothly varying double power law that is flat below a rest-frame break frequency of about 20 GHz and above which it significantly steepens. Based on the model adopted, we estimated that the radio jets of VIK J2318â3113 must be a few hundred years old in the case of a turnover, or less than a few Ă 104 years in the case of a break in the spectrum. Having multiple observations at two frequencies (888 MHz and 5.5 GHz), we further investigated the radio variability previously reported for this source. We found that the marginally significant flux density variations are consistent with the expectations from refractive interstellar scintillation, even though relativistic effects related to the orientation of the source may still play a non-negligible role. Further radio and X-ray observations are required to conclusively discern the nature of this variation
Nuclear versus integrated spectroscopy of galaxies in the Herschel Reference Survey
21 pagesInternational audienceContext. The determination of the relative frequency of active galactic nuclei (AGN) versus other spectral classes, for example, HII region-like (HII), transition objects (TRAN), passive (PAS), and retired (RET), in a complete set of galaxies in the local Universe is of primary importance to discriminate the source of ionization in the nuclear region of galaxies. Aims. Here we aim to provide a spectroscopic characterization of the nuclei of galaxies belonging to the Herschel Reference Survey (HRS), a volume and magnitude limited sample representative of the local Universe, which has become a benchmark for local and high-z studies, for semianalytical models and cosmological simulations. The comparison between the nuclear spectral classification and the one determined on the global galactic scale provides information about how galaxy properties change from the nuclear to the outer regions. Moreover, the extrapolation of the global star formation (SF) properties from the SDSS fiber spectroscopy compared to the one computed by Halpha photometry can be useful for testing the method based on aperture correction for determining the global star formation rate (SFR) for local galaxies. Methods. By collecting the existing nuclear spectroscopy available from the literature, complemented with new observations obtained using the Loiano 1.52m telescope, we analyze the 322 nuclear spectra of HRS galaxies. Results. Using two diagnostic diagrams (the BPT and the WHAN) we provide a nuclear and an integrated spectral classification for the HRS galaxies. Conclusions. We find that the fraction of AGNs increases with stellar mass, such that at logM > 10.0 M\odot or 66% of the LTGs are AGNs or TRAN
Multi-scale VLBI observations of the candidate host galaxy of GRB 200716C
We present the discovery and the subsequent follow up of radio emission from SDSS J130402.36+293840.6 (J1304+2938), the candidate host galaxy of the gamma-ray burst (GRB) GRB 200716C. The galaxy is detected in the RACS (0.89 GHz), the NVSS, the Apertif imaging survey, and the FIRST (1.4 GHz), the VLASS (3 GHz), and in public LOFAR (130â170 MHz), WISE (3.4â22 ÎŒm), and SDSS (z,âi,âr,âg,âu filters) data. The luminosity inferred at 1.4 GHz is (5.1â
屉
0.2)â
Ăâ
1030 erg sâ1 Hzâ1. To characterise the emission and distinguish between different components within the galaxy, we performed dedicated, high-sensitivity and high-resolution observations with the European VLBI Network (EVN) + e-MERLIN at 1.6 and 5 GHz. We did not detect any emission from a compact core, suggesting that the presence of a radio-loud active galactic nucleus (AGN) is unlikely, and therefore we ascribe the emission observed in the public surveys to star-forming regions within the galaxy. We confirm and refine the redshift estimate, zâ=â0.341â
屉
0.004, with a dedicated Telescopio Nazionale Galileo (TNG) spectroscopic observation. Finally, we compiled a list of all the known hosts of GRB afterglows detected in radio and computed the corresponding radio luminosity: if GRB 200716C belongs to J1304+2938, this is the third most radio-luminous host of a GRB, implying one of the highest star-formation rates (SFRs) currently known, namely SFR ⌠324±61 Mâ yrâ1. On the other hand, through the analysis of the prompt emission light curve, recent works suggest that GRB 200716C might be a short-duration GRB located beyond J1304+2938 and gravitationally lensed by an intermediate-mass black hole (IMBH) hosted by the galaxy. Neither the public data nor our Very Long Baseline Interferometry (VLBI) observations can confirm or rule out the presence of an IMBH acting as a (milli-)lens hosted by the galaxy, a scenario still compatible with the set of radio observations presented in this work