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Open Access
Issue
A&A
Volume 681, January 2024
Article Number L12
Number of page(s) 5
Section Letters to the Editor
DOI https://doi.org/10.1051/0004-6361/202348602
Published online 11 January 2024

© The Authors 2024

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article is published in open access under the Subscribe to Open model. Subscribe to A&A to support open access publication.

1. Introduction

The most powerful active galactic nuclei (AGN) can be observed from great distances, at redshifts beyond z = 10 (Goulding et al. 2023; Bogdán et al. 2023), which correspond to about 3.5% of the present age of the Universe. These objects are powered by material accretion onto the central supermassive black hole (SMBH) of their host galaxy. The existence of black holes reaching billions of solar masses at z ∼ 6 − 7 poses a challenge to models describing the formation and early growth of SMBHs (e.g. Volonteri et al. 2021). Recent discoveries of AGN at extremely high redshifts (z ≈ 10) favour models with massive (∼104 − 105 M⊙) black hole seeds such as those resulting from direct collapse of gravitationally unstable gas clouds (e.g. Lodato & Natarajan 2006), the merger of or accretion onto primordial black holes (e.g. Cappelluti et al. 2022), and spherically symmetric accretion onto the combined potential of a stellar-mass seed black hole at the centre of a dark matter halo (Sharma & Sharma 2023).

According to the updated catalogue1 of Perger et al. (2017), the number of currently known z > 6 quasars with measured spectroscopic redshift is nearly 300. From these, only 15 sources have been individually detected in radio bands, although many of the remaining objects may have some level of weak AGN-related radio emission (Perger et al. 2019, 2024). Radio-emitting AGN are expected to be found up to z ≈ 15 in the coming decades thanks to new sensitive radio and optical instruments (Agudo et al. 2015; Ighina et al. 2023; Latif et al. 2024).

In general, only a minor fraction of quasars (i.e. about 10%; e.g. Ivezić et al. 2002) produce relativistic plasma jets that originate from the close vicinity of the central SMBH. These are strong radio emitters via the synchrotron process of charged particles accelerated to relativistic speeds and moving in magnetic fields. Such sources are important targets for very-long-baseline interferometric (VLBI) observations, which are capable of imaging the radio emission from compact jets down to parsec (pc) scales – more powerful than any other technique in astronomy at present. Radio measurements are not affected by dust obscuration in the young host galaxies and provide unique insight into jet launching and emission. These distant AGN are signposts of the earliest occurrence of SMBH activity in the Universe. Studies of these objects contribute to our understanding of the evolution of jetted AGN and the role jets play in the coevolution of SMBHs and their host galaxies, and may even help evaluate cosmological models.

Blazars form a special class of AGN; their jets are believed to be closely aligned with the line of sight, which means their emission is enhanced by Doppler boosting. About half of the currently known 21 distant radio quasars at z ≥ 5.8 have been observed (ten objects), and nine have been detected with VLBI (Frey et al. 2003, 2005, 2008, 2011; Momjian et al. 2008, 2018, 2021; Cao et al. 2014; Wang et al. 2017; Spingola et al. 2020; Zhang et al. 2022; Liu et al. 2022). Among the latter, the only blazar candidate is PSO J030947.49+271757.31 at z = 6.10 (Spingola et al. 2020), based on its prominent one-sided core–jet structure. The other typical z ≳ 6 VLBI radio sources known to date have a compact but somewhat resolved structure and a steep radio spectrum, indicating unbeamed jet emission and suggesting young age. The double structure of the quasar NDWFS J14276+3312 (z = 6.12), with components separated by ∼160 pc (Frey et al. 2008; Momjian et al. 2008), is reminiscent of compact symmetric objects known to be young (≲103 − 104 yr) radio sources.

FIRST J233153.20+112952.11 (J2331+1129 hereafter) is a source from the Faint Images of the Radio Sky at Twenty centimeters (FIRST) survey (Becker et al. 1995) catalogue, with a flux density of S1.4 GHz = 1.85 mJy. This object was recently identified as a high-redshift AGN candidate by cross-matching zPS1-band dropouts from the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS1, Chambers et al. 2016) with FIRST radio sources (Koptelova & Hwang 2022). The source has counterparts in the near- and mid-infrared, as well as in other radio surveys. Its radio spectrum is flat within a relatively narrow range of frequencies, between the observed 888 MHz and 3 GHz (spectral index α = −0.01 ± 0.06, defined here as S ∝ να, where ν is the frequency), with flux densities around 2 mJy. The optical-to-radio spectral energy distribution of J2331+1129 is dominated by the synchrotron emission of the jet and closely resembles those of BL Lac objects. The non-thermal ultraviolet/optical continuum has a spectral index of αopt = 1.43 ± 0.23. The source appears variable in near-infrared – and possibly in radio as well – on timescales of months to years in the observer’s frame. The observed near-infrared spectrum shows no detected emission lines (Koptelova & Hwang 2022). Its redshift was estimated based on the Gunn–Peterson trough (Gunn & Peterson 1965) found at the wavelength λ = 0.921 μm, giving a lower redshift limit of z = 6.57. The observed properties together suggest that this AGN belongs to a subclass of blazars, BL Lac objects, and therefore might be the highest-redshift BL Lac discovered to date. Notably, no BL Lac objects have been found so far beyond z = 4 (Koptelova & Hwang 2022).

VLBI imaging observations with milliarcsec (mas) resolution are essential for verifying the truly compact radio structure and the high brightness temperature expected for a relativistically beamed jet of a BL Lac object. Here we report on our dual-frequency (1.6 and 4.9 GHz) observations of J2331+1129 with the U.S. National Radio Astronomy Observatory (NRAO) Very Long Baseline Array (VLBA). Throughout this Letter, we assume a flat Λ cold dark matter cosmological model with Hubble constant H0 = 70 km s−1 Mpc−1, matter density parameter Ωm = 0.3, and vacuum energy density parameter ΩΛ = 0.7. In this model, the luminosity distance at z = 6.57 is DL = 64.119 Gpc, the angular scale is 5.425 pc mas−1, and the age of the Universe is 813 Myr (Wright 2006).

2. Observations and data reduction

We conducted observations of J2331+1129 with the VLBA at the central frequencies of 1.6 and 4.9 GHz on 2022 Feb 1 and 4, respectively (project code: BF132, P.I.: S. Frey). The experiment was performed in phase-referencing mode (Beasley & Conway 1995) using J2330+1100 as a phase calibrator. This source is listed in the third edition of the International Celestial Reference Frame (ICRF3, Charlot et al. 2020) and its angular separation from the target is 0 . ° 59 $ 0{{\overset{\circ}{.}}}59 $. Both experiment segments lasted for 4 h and recorded data in left and right circular polarisations. The observing time spent on the target, J2331+1129, was 165 min in each segment. For the lower frequency band, the observations were conducted in two intermediate frequency channels (IFs) around 1.4 and 1.7 GHz. The data rate was 2 Gbps. At 4.9 GHz, the data rate was 4 Gbps with four IFs. In both experiment segments, the bandwidth was 128 MHz per IF and polarisation. The raw data recorded at each VLBA telescope were correlated at the DiFX correlator (Deller et al. 2011) in Socorro (New Mexico, USA) with 2 s integration time.

The VLBA data were calibrated in the standard way in the NRAO Astronomical Image Processing System package (AIPS, Greisen 2003). The appropriate tasks and procedures were used to correct for the dispersive ionospheric delays based on models from Global Navigation Satellite Systems data, and the accurate Earth orientation parameters. After digital sampling corrections, we performed manual phase calibration to remove instrumental delays using a short 1min scan spent on the secondary calibrator source J2327+0940 also scheduled in the experiment. Bandpass correction was performed using the same section of data. Amplitude calibration was then carried out using antenna-based system temperatures and gain curves supplied along with the correlated visibility data. Finally, global fringe-fitting was attempted for the bright sources observed, J2330+1100, J2327+0940, and the fringe-finder 3C 454.3. The phase, delay, and delay-rate solutions were applied to the respective source data.

The calibrated single-source visibility files were then exported to the Caltech DIFMAP program (Shepherd 1997) for hybrid mapping. The procedure started with several iterations of CLEAN image decomposition and phase self-calibration. To begin the amplitude and phase self-calibration, the overall antenna gain correction factors were determined. In a few cases, when gain corrections exceeded ±5%, those values were fed back into AIPS to refine the amplitude calibration.

Fringe-fitting was then repeated for the phase-reference calibrator J2330+1100 in AIPS. Now the CLEAN image of the source obtained in DIFMAP was also used as an input to account for a potential small phase contribution of the source structure. The solutions obtained for the calibrator were then interpolated to the target source.

During imaging of J2331+1129 in DIFMAP, we did not attempt any self-calibration because of the weakness of the target source. Radio emission was clearly detected both at 1.6 and 4.9 GHz. However, the source position was offset by ∼0.1″ in both right ascension (RA) and declination (Dec) with respect to the a priori pointing position. To minimise smearing effects, we repeated the calibration steps in AIPS but with the target source coordinates shifted to the newly determined accurate position.

3. Results and discussion

Results of the CLEAN imaging of J2331+1129 are displayed in Fig. 1. We find that the source is compact but slightly resolved at both observing frequencies. To characterise its brightness distribution quantitatively, we fitted circular Gaussian model components directly to the interferometric visibility data in DIFMAP. At 1.6 GHz, the flux density is S1.6 GHz = (1.8 ± 0.2) mJy and the full width at half maximum (FWHM) of the component diameter is θ1.6 GHz = (3.67 ± 0.05) mas. At 4.9 GHz, the parameters of the circular Gaussian component that adequately describe the source are S4.9 GHz = (1.6 ± 0.2) mJy and θ4.9 GHz = (0.68 ± 0.01) mas. The fitted component sizes exceed the minimum resolvable angular size of the interferometer (Kovalev et al. 2005). These angular sizes correspond to a linear extent of about (4 − 20) pc if we assume that the source is at z = 6.57.

thumbnail Fig. 1.

Naturally weighted VLBA images of J2331+1129 at 1.6 GHz (left) and 4.9 GHz (right). The intensity colour scales are displayed on the right-hand side of the panels. The lowest contours start at ±3σ rms noise, and the positive levels increase by a factor of 2. Image rms and peak intensity values are shown at the bottom of the panels, and the elliptical Gaussian restoring beams are shown in the lower left corners. The beam sizes are 11.7 mas × 5.7 mas and 4.0 mas × 1.8 mas (FWHM), with major axis position angles of 3 . ° 2 $ 3{{\overset{\circ}{.}}}2 $ and − 1 . ° 0 $ -1{{\overset{\circ}{.}}}0 $ (measured from north through east) at 1.6 and 4.9 GHz, respectively.

The accurate coordinates of the target source calculated by finding the 4.9 GHz image brightness peak position with the AIPS command MAXFIT are RA = 23 h 31 m 53 . s 21007 $ \mathrm{RA}=23^{\mathrm{h}} 31^{\mathrm{m}} 53{{\overset{\text{ s}}{.}}}21007 $ and Dec = 11 ° 29 ′ 52 . ″ 0089 $ \mathrm{Dec}= 11^{\circ}29^{\prime}52{{\overset{\prime\prime}{.}}}0089 $. The estimated uncertainty of 0.2 mas in both coordinates is dominated by the effect arising from the angular distance of the phase-reference calibrator source (cf. Chatterjee et al. 2004; Rioja & Dodson 2020). The position determined at 1.6 GHz is slightly less accurate but the coordinates are consistent with the above values within the errors.

Based on the fitted flux densities, the radio spectrum of the ∼10 pc scale structure is flat (αpc = −0.11), similar to what is known from the total flux density measurements (Koptelova & Hwang 2022). Irrespective of whether or not we consider a ∼10 − 20% coherence loss (Martí-Vidal et al. 2010) in phase-referenced observations, the measured flux densities are a good match to those obtained with lower-resolution connected-element interferometers, indicating that the total radio emission of J2331+1129 is mostly confined to the region imaged with VLBI.

For comparison with other high-redshift radio AGN, we calculated the 1.4 GHz rest-frame radio power as P=4π S 1.4 GHz D L 2 (1+z) −α−1 $ P=4\pi S_\mathrm{1.4\,GHz} D_\mathrm{L}^2(1+z)^{-\alpha-1} $. The high value of P = 1.4 × 1026 W Hz−1 suggests that the emission originates from a powerful AGN (see Perger et al. 2019, and references therein), provided that the source is indeed at such a high redshift.

The brightness temperatures were calculated as

T b = 1.22 × 10 12 ( 1 + z ) S ν ν 2 θ 2 K, $$ \begin{aligned} T_{\mathrm{b} } =1.22 \times 10^{12} (1 + z) \frac{S_{\nu }}{\nu ^2 \theta ^2} \,\text{ K,} \end{aligned} $$(1)

where the flux density Sν is measured in Jy, the observing frequency ν in GHz, and the circular Gaussian component diameter θ (FWHM) in mas. The values (Tb ≈ 5 × 108 K at 1.6 GHz and Tb ≈ 1.5 × 109 K at 4.9 GHz) also confirm the non-thermal nature of the radio emission that originates from AGN activity. This conclusion is so robust that it would hold true even if z = 0, because brightness temperatures exceeding Tb ∼ 105 K cannot be attributed to star-forming activity in the host galaxy (Condon 1992).

Even the higher measured brightness temperature for J2331+1129 is an order of magnitude below the equipartition limit of Tb, eq ≈ 5 × 1010 K (Readhead 1994) when the energy density in the radiating particles matches that of the magnetic field. This is conventionally assumed as the intrinsic value in the core region of the jet. In our case, the Doppler boosting factor, that is, the ratio of measured and intrinsic brightness temperatures, δ = Tb/Tb, int = Tb/Tb, eq, is smaller than 1. This would indicate that the radio emission is not relativistically boosted, contrary to what one would expect for a powerful blazar jet pointing nearly to the line of sight. Even considering a somewhat lower intrinsic brightness temperature of Tb, int ≈ 3 × 1010 K (Homan et al. 2006), which is the average value found in samples of powerful jetted AGN in their median-low state (i.e. not in outburst when brightness temperatures can reach order-of-magnitude-higher values), δ remains well below unity.

Based on a large sample of jetted AGN (mostly blazars) studied with VLBI, Cheng et al. (2020) found that brightness temperatures are generally frequency dependent, reaching their maximum at the emitted (rest-frame) frequency νem ≈ 6.8 GHz. If the redshift of J2331+1129 is indeed extremely high, the observed ν = 4.9 GHz frequency corresponds to νem = ν (1 + z) = 37 GHz in the expanding Universe. According to the phenomenological dependence derived by Cheng et al. (2020), the maximum brightness temperature of the source could then be a factor of ∼4 − 5 higher than what we measured at ν = 4.9 GHz, which would equate to ∼5 × 109 K. In any case, this is still not as high as expected having observed Doppler-boosted emission in the J2331+1129 jet. However, we note that the core brightness temperatures (or lower limits) of the only other claimed blazar candidate known to date at z > 6, namely PSO J030947.49+271757.31, are also on the order of Tb ∼ 108 K (Spingola et al. 2020). On the other hand, for the previously known second-most distant blazar, J0906+6930 (z = 5.47), the brightness temperature measured with VLBI (3 × 1011 K) clearly exceeds the equipartition limit (An et al. 2020).

The flat section of the radio spectrum of J2331+1129 around ν ≳ 1 GHz (see also Koptelova & Hwang 2022), although not yet sampled in a sufficiently broad range of frequencies, could possibly be interpreted as similar to those of gigahertz-peaked spectrum (GPS) radio sources (e.g. O’Dea & Saikia 2021, and references therein). However, GPS sources typically have more extended mas-scale radio structure, that is, within about 500 pc in size (O’Dea & Saikia 2021). Indeed, a high-redshift GPS source recently studied with VLBI, J1606+3124 (z = 4.56), shows a relatively compact (∼70 pc) triple structure (An et al. 2022), much different from the single component (< 20 pc) we see in J2331+1129.

We note that, assuming z = 6.57 for our source, the observed ∼1 GHz frequency range translates to above 5 GHz, where turnover frequencies are characteristic of high-frequency peaker (HFP) sources (O’Dea & Saikia 2021). Typically both HFPs and GPS sources have less compact structures that blazar core–jets. However, HFP and blazar properties are not necessarily mutually exclusive. A good example is J0906+6930 (z = 5.47), which has a peaked broad-band radio spectrum with a turnover frequency of νem ≈ 40 GHz (Coppejans et al. 2017; Zhang et al. 2017) but also very compact Doppler-boosted jet emission (An et al. 2020). Future flux density measurements of J2331+1129 in a frequency range wider than available at present (as in the sample of z > 5 radio AGN by Shao et al. 2022) would provide decisive information about the possible HFP nature of this source. At present, the previous low-resolution data and our recent VLBI data indicate that the radio spectrum of J2331+1129 is likely flat between the observed 888 MHz and 4.9 GHz, as expected for blazars. The currently available data do not show evidence for a spectral turnover at gigahertz frequencies.

4. Summary and conclusions

The redshift of z = 6.57 for the radio-emitting AGN J2331+1129 was inferred from the shape of the near-infrared continuum spectrum (Koptelova & Hwang 2022). The lack of identifiable emission lines, as well as the flat radio spectrum in the observed GHz range, the spectral energy distribution, and the indication of variability, led Koptelova & Hwang (2022) to classify the source as a candidate BL Lac object, by far the most distant known to date. However, as the redshift determination is based on the location of the break in the continuum spectrum and not on emission lines, the value should be considered approximate.

We performed high-resolution radio interferometric observations with the VLBA to reveal whether or not there is a compact core–jet structure in J2331+1129. Nearly simultaneous dual-frequency (1.6- and 4.9-GHz) imaging shows a compact, flat-spectrum radio core whose flux density matches the total flux density, indicating that the entire radio emission is confined within ∼20 pc. Via traditional nodding-style phase referencing to a nearby ICRF3 radio quasar, we precisely determined the astrometric position of the source with sub-mas accuracy.

The inferred maximum brightness temperature is about 5 × 109 K, confirming that this is an AGN source but providing no clear evidence for Doppler-boosted radio jet emission. This leaves the BL Lac identification of the source unconfirmed. Comparing the brightness temperature of J2331+1129 with those of other high-redshift (z ≳ 6) blazars may allow confirmation of the nature of this object. However, currently the sample of known sources with similar properties is very small. It is possible that the observed jet properties in extremely distant radio sources differ from those at lower redshifts because of yet poorly understood evolutionary and environmental effects. The rarity of z ≳ 6 blazars means that each new discovery and study significantly adds to our understanding of the behaviour of jetted SMBHs in the early Universe. Observational limitations could also play a role, as phase-referenced VLBI observations of weak radio sources may only provide an upper limit on the resolved source size, and therefore a lower limit to the brightness temperature.

To verify the BL Lac nature of J2331+1129, sensitive VLBI observations at higher frequencies and therefore higher resolution might prove useful. Also, densely time-sampled total flux density monitoring could possibly reveal rapid variations indicative of high brightness temperature. Finally, it cannot be excluded that the source is a misidentified BL Lac object and/or is not at extremely high redshift. This should be tested in the future with sensitive spectroscopic observations to look for emission lines and possibly re-evaluate the redshift.

Acknowledgments

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work made use of the Swinburne University of Technology software correlator (Deller et al. 2011), developed as part of the Australian Major National Research Facilities Programme and operated under licence. This work was supported by the Hungarian National Research, Development and Innovation Office (OTKA K134213 & PD146947). This project has received funding from the HUN-REN Hungarian Research Network. T.A. thanks for the financial support from the Pinghu Laboratory. E.K. thanks for support from Ministry of Science and Technology of Taiwan grants MOST 109-2112-M-008-021-MY3 and 112-2112-M-008-017-MY3. Y.Z. is supported by the National SKA Programme of China (grant no. 2022SKA0120102), Shanghai Sailing Program (grant no. 22YF1456100).

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All Figures

thumbnail Fig. 1.

Naturally weighted VLBA images of J2331+1129 at 1.6 GHz (left) and 4.9 GHz (right). The intensity colour scales are displayed on the right-hand side of the panels. The lowest contours start at ±3σ rms noise, and the positive levels increase by a factor of 2. Image rms and peak intensity values are shown at the bottom of the panels, and the elliptical Gaussian restoring beams are shown in the lower left corners. The beam sizes are 11.7 mas × 5.7 mas and 4.0 mas × 1.8 mas (FWHM), with major axis position angles of 3 . ° 2 $ 3{{\overset{\circ}{.}}}2 $ and − 1 . ° 0 $ -1{{\overset{\circ}{.}}}0 $ (measured from north through east) at 1.6 and 4.9 GHz, respectively.
In the text

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