arXiv subjects

L. Ighina

Publications and source records attributed to L. Ighina.

13 recordsLinked to original sources

arXiv2026

The most extreme high-z radio quasars in the eRASS:1 X-ray survey

We present the selection and identification of high-z radio quasars from a combination of X-ray (eROSITA All-Sky Survey), optical/NIR (DECam Local Volume Exploration), and radio (Rapid ASKAP Continuum Survey) surveys. After building a sample of 68 new high-z quasar candidates, we performed follow-up spectroscopic observations on 46 sources, confirming 39 to have redshifts z>3.5. For the subset of radio quasars at z>4, the 14 new sources identified here represent an increase of $\sim$65% in the known population of objects above the same flux limits in the same area. Using X-ray-to-optical relative intensity, we identified blazars in our sample, which we then used to compare the total number of z>4, X-ray selected blazars to the predictions of a fractional IC/CMB model previously proposed in the literature. Overall we find a relatively good agreement, even considering the potential biases in the redshift distribution from the incompleteness of the spectroscopic follow-up and the blazar classification criteria. The sources presented in this work represent a sample well suited for investigating the evolution of the most extreme systems in the early Universe, including their relativistic jets and accretion properties

astro-ph.GA↗

arXiv2026

SKA$-$VLBI view of AGN jets in the early Universe

Active Galactic Nuclei (AGN) are among the brightest sources in the Universe, and those that are also jetted are uniquely valuable at the earliest epochs, because their relativistic outflows can regulate the gas supply of their host galaxies, potentially affecting both early star formation and the rapid growth of supermassive black holes (SMBHs). Their compact, high-brightness-temperature radio cores provide the sharpest beacons for very long baseline interferometry (VLBI), enabling direct constraints on Doppler boosting, jet duty cycles, and jet$-$environment coupling at extreme redshifts. In this White Paper, we discuss how the SKA-VLBI will provide sub-$\mu$Jy sensitivity together with milliarcsecond (mas) angular resolution to image and characterise jetted AGN at $z>6$ across SKA-Mid and SKA-Low frequencies. These observations can directly test SMBHs ($>10^6$ M$_{\odot}$) formation/evolution models (including jet-assisted super-Eddington phases) and infer the geometry of the Universe, directly probing the cosmological framework at high precision. Synergies with current and next-generation multi-band facilities will also be crucial to fully understand their host galaxies and their environment, providing an unprecedented panchromatic knowledge of the first jetted AGN.

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arXiv2025

X-ray investigation of possible super-Eddington accretion in a radio-loud quasar at $z=6.13$

We present radio and X-ray observations of the recently discovered $z=6.13$ radio-powerful quasar RACS J032021.44$-$352104.1 using uGMRT, ATCA, LBA, and Chandra. The observed radio properties are in line with what is typically observed in high-$z$ radio quasars ($\alpha_{\rm r}=0.72\pm 0.02$ and L$_{\rm 1.4GHz}=5.8 \pm 0.9 \times 10^{26}$ W Hz$^{-1}$). Despite the relatively low X-ray flux observed $F_{\rm 0.5-7.0 keV}=2.3\pm0.5 \times 10^{-14}$ erg sec$^{-1}$ cm$^{-2}$, the intrinsic luminosity in the 2-10 keV rest frame is markedly high, $L_{\rm 2-10 keV}=1.8^{+1.1}_{-0.7} \times 10^{46}$ erg sec$^{-1}$, making RACS J032021.44$-$352104.1 one of the most luminous quasars currently known at $z>5.5$. The high X-ray luminosity is largely driven by an extrapolation to energies below the observable X-ray window with Chandra and the slope derived in the 0.5-7 keV band (or 3.5--50 keV in the rest-frame; $\Gamma_{\rm X}=3.3\pm0.4$). By analysing the overall spectral energy distribution of the quasar we found that the remarkably soft X-ray emission: (1) cannot be produced by relativistic jets, even when relativistic boosting is considered; and (2) is consistent with expectations for a super-Eddington accreting SMBH. If such a high accretion rate was confirmed, this source would be a unique laboratory to study high accretion in the early Universe and could help resolve some challenges inherent in early black hole growth paradigms.

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arXiv2025

The Centre of Attention: a Powerful Radio Galaxy Pinpoints a NIR-Dark Protocluster at z~3.9

We report the discovery of a $z\sim3.9$ protocluster identified from Atacama Large Millimetre/sub-millimetre Array Band 3 spectral scans of a bright radio source selected from the GaLactic and Extra-galactic All-sky Murchison Widefield Array (GLEAM) survey. Extended CO(4-3) and [CI](1-0) line emission was detected in GLEAM J005332$-$325630 confirming it to be a $z=3.879$ powerful radio galaxy with luminosity, $L_{500 MHz}=1.3\times10^{28}$ ${W Hz}^{-1}$. This source is part of a sample of candidate high redshift radio galaxies with bright radio fluxes, $S_{150MHz}>0.1$ Jy, but host galaxies with $K_s({AB})\gtrsim23$ mag. The molecular gas associated with the radio galaxy host has two kinematically separate components, likely in-falling and indicative of a recent interaction or merger with another galaxy. One 100-GHz continuum source $\sim120$ pkpc away is found to have both CO(4-3) and [CI](1-0) emission lines and a further five protocluster members are identified from CO(4-3) emission alone, all at similar redshift ($\Delta v<700$ km s$^{-1}$) and within a radius of $1.1^{\prime}$. Using photometry from the High Acuity Widefield K-band Imager $K_s$-band and the Dark Energy Survey $g, r, i, z$ and $Y$ bands, we find this protocluster harbours a rare, optically-dark, very massive $M_*\sim10^{12}$ ${M}_\odot$ galaxy. Comparisons with the TNG300 cosmological simulation puts this galaxy in a dark matter halo of $M_{DM}\sim3\times10^{13}$ ${M}_\odot$ which will evolve into a Coma-like DM halo ($M_{DM}\sim10^{15}$ ${M}_\odot$) by the present day.

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arXiv2025

High-$z$ radio Quasars in RACS I: Selection, identification, and multi-wavelength properties

Radio-bright, jetted quasars at $z>5$ serve as unique laboratories for studying supermassive black hole activity in the early Universe. In this work, we present a sample of high-$z$ jetted quasars selected from the combination of the radio Rapid ASKAP Continuum Survey (RACS) with deep wide-area optical/near-infrared surveys. From this cross-match we selected 45 new high-$z$ radio quasar candidates with S$_{888MHz}>1$ mJy and mag$z<21.3$ over an area of 16000deg$^2$. Using spectroscopic observations, we confirmed the high-$z$ nature of 24 new quasars, 13 at $4.5 5$. If we also consider similar, in terms of radio/optical fluxes and sky position, quasars at $z>5$ already reported in the literature, the overall $z>5$ RACS sample is composed by 33 powerful quasars, expected to be ~90% complete at mag$z<21.3$ and S$_{888MHz}>1$ mJy. Having rest-frame radio luminosities in the range $\nu L_{1.4GHz}=10^{41.5}-10^{44.4}$ erg s$^{-1}$, this sample contains the most extreme radio quasars currently known in the early Universe. We also present all X-ray and radio data currently available for the sample, including new, dedicated {\it Chandra}, uGMRT, MeerKAT and ATCA observations for a sub-set of the sources. from the modelling of their radio emission, either with a single power law or a broken power law, we found that these systems have a wide variety of spectral shapes with most quasars (22) having a flat radio emission (i.e., $-0.5<\alpha_{r}<0.5$). At the same time, the majority of the sources with X-ray coverage present a high-energy luminosity larger than the one expected from the X-ray corona only. Both the radio and X-ray properties of the high-$z$ RACS sample suggest that many of these sources have relativistic jets oriented close to our line of sight. (i.e., blazars) and can therefore be used to perform statistical studies on the entire jetted population at high redshift.

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arXiv2024

Multi-wavelength properties of three new radio-powerful $z\sim5.6$ quasi-stellar objects discovered from RACS

We present a multi-wavelength study of three new $z\sim5.6$ quasi-stellar objects (QSOs) identified from dedicated spectroscopic observations. The three sources were selected as high-$z$ candidates based on their radio and optical/near-infrared properties as reported in the Rapid ASKAP Continuum Survey (RACS), the Dark Energy Survey (DES), and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) survey. These are among the most radio-bright QSOs currently known at $z>5.5$, relative to their optical luminosity, having $\rm R=S_{\rm 5GHz}/S_{\rm 4400A}>100$. In this work, we present their identification, and we also discuss their multi-wavelength properties (from the radio to the X-ray band) based on detections in public surveys as well as in dedicated radio and X-ray observations. The three sources present a wide range of properties in terms of relative intensity and spectral shape, highlighting the importance of multi-wavelength observations in accurately characterising these high-$z$ objects. In particular, from our analysis we found one source at $z=5.61$ that presents clear blazar properties (strong radio and X-ray emission), making it one of the most distant currently known in this class. Moreover, from the fit of the optical/near-infrared photometric measurements with an accretion disc model as well as the analysis of the CIV broad emission line in one case, we were able to estimate the mass and accretion rate of the central black holes in these systems, finding $\rm M_{\rm BH}\sim1-10\times10^9$~M$_\odot$ accreting at a rate $λ_{\rm Edd}\sim0.1-0.4$. The multi-wavelength characterisation of radio QSOs at $z>5.5$, such as the ones reported here, is essential to constraining the evolution of relativistic jets and supermassive black holes hosted in this class of objects.

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arXiv2024

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 Universe

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arXiv2022

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$α$ 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 $z\sim5$.

astro-ph.GA↗

arXiv2022

Direct observation of an extended X-ray jet at $z$=6.1

We report on the direct observation of an extended X-ray jet in the $z$=6.1 radio-loud Active Galactic Nucleus PSO J030947.49+271757.31 from a deep Chandra X-ray observation (128 ksec). This detection represents the most distant kpc off-nuclear emission resolved in the X-rays to date. The angular distance of the emission is $\sim$4" (corresponding to $\sim$20 kpc at $z$=6.1), along the same direction of the jet observed at parsec scales in previous VLBA high-resolution radio observations. Moreover, the 0.5-7.0 keV isophotes coincide with the extended radio emission as imaged by the VLA Sky Survey at 3 GHz. The rest-frame 2-10 keV luminosity of the extended component is L$_{2-10keV}$=5.9$\times$10$^{44}$ erg s$^{-1}$, about 8% of the core: this makes it one of the most luminous jets resolved in the X-rays so far. Through Spectral Energy Distribution modelling we find that this emission can be explained by the Inverse Compton interaction with the photons of the Cosmic Microwave Background assuming that the jet's physical parameters are similar to those in the local Universe. At the same time, we find that the radiation produced by a putative population of high-energetic electrons through the synchrotron process observed at low redshift is quenched at high redshift, hence becoming negligible.

astro-ph.HE↗

arXiv2021

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$\lesssim$ z$\lesssim$ 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 eROSITA, could help to further constrain the observed evolution at even higher redshifts (up to z$\sim$6-7).

astro-ph.GA↗

arXiv2021

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 $\times 10^{26}$ W Hz$^{-1}$ at 5 GHz) compared to the optical luminosity (1.8 $\times 10^{24}$ W Hz$^{-1}$ at 4400 A) makes it the most distant radio-loud quasar observed so far, with a radio loudness R$\sim$70 (R$=L_\mathrm{5GHz}/L_\mathrm{4400A}$). Moreover, the high bolometric luminosity of the source (L$_\mathrm{bol}$=7.4 $\times 10^{46}$ erg s$^{-1}$) suggests the presence of a supermassive black hole with a high mass ($\gtrsim$6 $\times$10$^8$ M$_\odot$) 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 $\sim$2, 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.

astro-ph.GA↗

arXiv2019

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>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 (SEDs) 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~1). We found a significant difference in the X-ray-to-radio luminosity ratios, with the CLASS blazars having a mean ratio 2.4+/-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 (CMB) 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.

astro-ph.GA↗

arXiv2019

An extremely X--ray weak blazar at z=5

We present the discovery and properties of DESJ014132.4-542749.9 (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>10$^{4}$), the high radio luminosity (L$_{0.8 GHz}$=1.73$\times$10$^{28}$ 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 DESJ0141-54 is much lower compared to those of the high redshift (z$\geq$4.5) blazars discovered so far. Moreover its X-ray-to-radio luminosity ratio (log($\frac{L_{[0.5-10]keV}}{L_{1.4GHz}}$)=9.96$\pm$0.30 Hz) is small also when compared to lower redshift blazars: only 2\% of the low-z population has a similar ratio. By modeling the spectral energy distribution we found that this peculiar X--ray weakness and the powerful radio emission could be related to a particularly high value of the magnetic field. Finally, the mass of the central black hole is relatively small (M$_{BH}$ = 3-8 $\times$10$^8$ M$_{\odot}$) 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$\geq$5.

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