New Radio-Loud QSOs at the end of the Re-ionisation Epoch

We present the selection of high-redshift ($z\gtrsim5.7$) 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 $S_{\rm 888MHz}>1$ mJy beam$^{-1}$ and ${\rm mag}(z_\mathrm{{DES}})<21.3$ using the dropout technique (in the $i$-band). From this sample, we were able to confirm the high-$z$ nature ($z\sim6.1$) 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$\alpha$ 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 $5.9<z<6.4$ to be 0.13$^{+0.18}_{-0.09}$ and found it to be consistent with the expectations based on our current knowledge of the blazar population up to $z\sim5$.Comment: Accepted in MNRAS on 05 December 2022. Ten pages with five figures and three table

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&

Constraining the radio properties of the z = 6.44 QSO VIK J2318-3113

The recent detection of the quasi-stellar object (QSO) VIKING J231818.3−311346 (hereafter VIK J2318−3113) at redshift z = 6.44 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 J2318−3113 in the observed range ∼0.110 GHz. At high frequencies (0.8885.5 GHz in the observed frame) the QSO presents a steep spectrum (αr = 1.24, with Sν ∝ ν−αr), 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 J2318−3113 must be a few hundred years old in the case of a turnover, or less than a few × 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

X-ray properties of z > 4 blazars

We present the X-ray analysis of the largest flux-limited complete sample of blazar candidates at z &gt; 4 selected from the Cosmic Lens All Sky Survey (CLASS). After obtaining a nearly complete (24/25) X-ray coverage of the sample (from Swift-XRT, XMM-Newton, and Chandra), we analysed the spectra in order to identify the bona fide blazars.We classified the sources based on the shape of their Spectral Energy Distributions and, in particular, on the flatness of the X-ray emission and its intensity compared to the optical one.We then compared these high-z blazars with a blazar sample selected at lower redshifts (z 3c 1). We found a significant difference in the X-ray-to-radio luminosity ratios, with the CLASS blazars having a mean ratio 2.4 \ub1 0.5 times larger than low-z blazars. We tentatively interpret this evolution as due to the interaction of the electrons of the jet with the Cosmic Microwave Background photons, which is expected to boost the observed X-ray emission at high redshifts. Such a dependence has been already observed in highly radio loud AGNs in the recent literature. This is the first time it is observed using a statistically complete radio flux limited sample of blazars. We have then evaluated whether this effect could explain the differences in the cosmological evolution recently found between radio and X-ray selected samples of blazars. We found that the simple version of this model is not able to solve the tension between the two evolutionary results

The impact of the {CMB} on the evolution of high-z blazars

Different works have recently found an increase of the average X-ray-to-radio luminosity ratio with redshift in the blazar population. We evaluate here whether the inverse Compton interaction between the relativistic electrons within the jet and the photons of the cosmic microwave background (IC/CMB) can explain this trend. Moreover, we test whether the IC/CMB model can also be at the origin of the different space density evolutions found in X-ray and radio-selected blazar samples. By considering the best statistically complete samples of blazars selected in the radio or in the X-ray band and covering a large range of redshift (0.5 less than or similar to z less than or similar to 5.5), we evaluate the expected impact of the CMB on the observed X-ray emission on each sample and then we compare these predictions with the observations. We find that this model can satisfactorily explain both the observed trend of the X-ray-to-radio luminosity ratios with redshift and the different cosmological evolutions derived from the radio and X-ray band. Finally, we discuss how currently on-going X-ray missions, like extended ROentgen Survey with an Imaging Telescope Array, could help to further constrain the observed evolution at even higher redshifts (up to z similar to 6-7)

X-ray properties of z &gt; 4 blazars

We present the X-ray analysis of the largest flux-limited complete sample of blazar candidates at z &gt; 4 selected from the Cosmic Lens All Sky Survey (CLASS). After obtaining a nearly complete (24/25) X-ray coverage of the sample (from Swift-XRT, XMM-Newton, and Chandra), we analysed the spectra in order to identify the bona fide blazars.We classified the sources based on the shape of their Spectral Energy Distributions and, in particular, on the flatness of the X-ray emission and its intensity compared to the optical one.We then compared these high-z blazars with a blazar sample selected at lower redshifts (z 3c 1). We found a significant difference in the X-ray-to-radio luminosity ratios, with the CLASS blazars having a mean ratio 2.4 \ub1 0.5 times larger than low-z blazars. We tentatively interpret this evolution as due to the interaction of the electrons of the jet with the Cosmic Microwave Background photons, which is expected to boost the observed X-ray emission at high redshifts. Such a dependence has been already observed in highly radio loud AGNs in the recent literature. This is the first time it is observed using a statistically complete radio flux limited sample of blazars. We have then evaluated whether this effect could explain the differences in the cosmological evolution recently found between radio and X-ray selected samples of blazars. We found that the simple version of this model is not able to solve the tension between the two evolutionary results

The evolution of the heaviest supermassive black holes in jetted {AGNs}

We present the space density evolution, from z=1.5 up to z=5.5, of the most massive (M$\geq10^9$M$_{\odot}$) black holes hosted in jetted Active Galactic Nuclei(AGNs). The analysis is based on a sample of 380 luminosity-selected ($\lambda$L$_{1350}\geq10^{46}$ erg s$^{-1}$ and P$_{5\text{GHz}}\geq10^{27}$ W Hz$^{-1}$) Flat Spectrum Radio Quasars (FSRQs) obtained from the Cosmic Lens All Sky Survey (CLASS). These sources are known to be face-on jetted AGNs (i.e. blazars) and can be exploited to infer the abundance of all the (misaligned) jetted AGNs, using a geometrical argument. We then compare the space density of the most massive SMBHs hosted in jetted AGNs with those present in the total population (mostly composed by non-jetted AGNs). We find that the space density has a peak at $z\sim3$, which is significantly larger than the value observed in the total AGN population with similar optical/UV luminosities ($z\sim2.2$), but not as extreme as the value previously inferred from X-ray selected blazars ($z\gtrsim4$). The jetted fraction (jetted AGNs/total AGNs) is overall consistent with the estimates in the local Universe (10--20\%) and at high redshift, assuming Lorentz bulk factors $\Gamma\approx5$. Finally, we find a marginal decrease in the jetted fraction at high redshifts (by a factor of $\sim2$). All these evidences point toward a different evolutionary path in the jetted AGNs compared to the total AGN population.Comment: This article has been accepted for publication in MNRA

Radio detection of VIK J2318-3113, the most distant radio-loud quasar (z = 6.44)

We report the 888 MHz radio detection in the Rapid ASKAP Continuum Survey (RACS) of VIK J2318-3113, a z = 6.44 quasar. Its radio luminosity (1.2 7 1026 W Hz-1 at 5 GHz) compared to the optical luminosity (1.8 7 1024 W Hz-1 at 4400 A) makes it the most distant radio-loud quasar observed so far, with a radio loudness R 3c 70 (R = L5 GHz/L4400A). Moreover, the high bolometric luminosity of the source (Lbol = 7.4 7 1046 erg s-1) suggests the presence of a supermassive black hole with a high mass (6 7 108 Mpdbl) at a time when the Universe was younger than a billion years. Combining the new radio data from RACS with previous ASKAP observations at the same frequency, we found that the flux density of the source may have varied by a factor of 3c2, which could suggest the presence of a relativistic jet oriented towards the line of sight, that is, a blazar nature. However, currently available radio data do not allow us to firmly characterise the orientation of the source. Further radio and X-ray observations are needed

An extremely X-ray weak blazar at

We present the discovery and properties of DES J014132.4−542749.9 (DES0141−54), a new powerful radio-loud active galactic nucleus (AGN) in the early Universe (z = 5.0). It was discovered by cross-matching the first data release of the Dark Energy Survey (DES DR1) with the Sidney University Molonglo Survey (SUMSS) radio catalog at 0.843 GHz. This object is the first radio-loud AGN at high redshift discovered in the DES. The radio properties of DES0141−54, namely its very large radio-loudness (R >  104), the high radio luminosity (L0.8 GHz = 1.73 × 1028 W Hz−1), and the flatness of the radio spectrum (α = 0.35) up to very high frequencies (120 GHz in the source’s rest frame), classify this object as a blazar, meaning, a radio-loud AGN observed along the relativistic jet axis. However, the X-ray luminosity of DES0141−54 is much lower compared to those of the high redshift (z ≥ 4.5) blazars discovered so far. Moreover its X-ray-to-radio luminosity ratio (log( L[0.5-10] keV / L1.4 GHz) = 9.96±0.30 Hz) is small also when compared to lower redshift blazars: only 2% of the low-z population has a similar ratio. By modeling the spectral energy distribution we found that this peculiar X-ray weakness and the powerful radio emission could be related to a particularly high value of the magnetic field. Finally, the mass of the central black hole is relatively small (MBH = 3−8 × 108 M⊙) compared to other confirmed blazars at similar redshift, making DES0141−54 the radio-loud AGN that host the smallest supermassive black hole ever discovered at z ≥ 5