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Register a new userInto the central 10 pc of the most distant known radio quasar. VLBI imaging observations of J1429+5447 at z=6.21
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Authors
S. Frey
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Context: There are about 60 quasars known at redshifts z>5.7 to date. Only three of them are detected in the radio above 1 mJy flux density at 1.4 GHz frequency. Among them, J1429+5447 (z=6.21) is the highest-redshift radio quasar known at present. These rare, distant, and powerful objects provide important insight into the activity of the supermassive black holes in the Universe at early cosmological epochs, and on the physical conditions in their environment. Aims: We studied the compact radio structure of J1429+5447 on the milli-arcsecond (mas) angular scale, in order to compare the structural and spectral properties with those of other two z~6 radio-loud quasars, J0836+0054 (z=5.77) and J1427+3312 (z=6.12). Methods: We performed Very Long Baseline Interferometry (VLBI) imaging observations of J1429+5447 with the European VLBI Network (EVN) at 1.6 GHz on 2010 June 8, and at 5 GHz on 2010 May 27. Results: Based on its observed radio properties, the compact but somewhat resolved structure on linear scales of <100 pc, and the steep spectrum, the quasar J1429+5447 is remarkably similar to J0836+0054 and J1427+3312. To answer the question whether the compact steep-spectrum radio emission is a universal feature of the most distant radio quasars, it is essential to study more, yet to be discovered radio-loud active galactic nuclei at z>6.
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Context: The highest redshift quasars at z>~6 receive considerable attention since they provide strong constraints on the growth of the earliest supermassive black holes. They also probe the epoch of reionisation and serve as lighthouses to illuminate the space between them and the observer. The source J1427+3312 (z=6.12) has recently been identified as the first and so far the only known radio-loud quasar at z>6. Aims: We investigated the compact radio structure of J1427+3312 on milli-arcsecond (mas) angular scales, to compare it with that of the second most distant radio-loud quasar J0836+0054 (z=5.77) and with lower-redshift radio quasars in general. Methods: We observed J1427+3312 in phase-reference mode with ten antennas of the European Very Long Baseline Interferometry (VLBI) Network (EVN) at 1.6 GHz on 11 March 2007 and at 5 GHz on 3 March 2007. Results: The source was clearly detected at both frequencies. At 1.6 GHz, it shows a prominent double structure. The two components are separated by 8.3 mas, corresponding to a projected linear distance of ~160 pc. Both components with sub-mJy flux densities appear resolved. In the position of the brightest component at 1.6 GHz, we detected mas-scale radio emission at 5 GHz as well. The radio spectrum of this feature is steep. The double structure and the separation of the components of J1427+3312 are similar to those of the young (<~10^4 yr) compact symmetric objects (CSOs).
Using the Australia Telescope Compact Array we have detected CO(1-0) and CO(5-4) from TNJ0924-2201 at z=5.2, the most distant radio galaxy known to date. This is the second highest redshift detection of CO published so far. The CO(1-0) is 250-400 km/sec wide with a peak flux density of 520 +- 115 microJy/beam whilst the CO(5-4) line emission is 200-300 km/sec wide with a peak flux density of 7.8 +- 2.7 mJy/beam. Both transitions are spatially unresolved but there is marginal evidence for spatial offsets between the CO and the host galaxy; the CO(1-0) is located 28 +- 11 kpc (4.5 +- 1.7 arcsec) north of the radio galaxy whilst the CO(5-4) is located 18 +- 8 kpc (2.8 +- 1.2 arcsec) south of the radio galaxy. Higher spatial resolution observations are required to determine the reality of these offsets. Our result is the second detection of CO in a high redshift galaxy without pre-selection based on a massive dust content.
The highest-redshift quasars are still rare and valuable objects for observational astrophysics and cosmology. They provide important constraints on the growth of the earliest supermassive black holes in the Universe, and information on the physical conditions in their environment. Among the nearly 60 quasars currently known at redshifts z>5.7, only a handful are strong emitters in radio continuum. These can be targets of sensitive high-resolution Very Long Baseline Interferometry (VLBI) observations to reveal their innermost structure, down to ~10 pc linear scales. We review the results of our earlier European VLBI Network (EVN) experiments on three of the most distant radio quasars known to date, and give a preliminary report on the EVN detection of a fourth one. The results obtained so far suggest that we see really young active galactic nuclei - not just in a cosmological sense but also in terms of their active life in radio.
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.
We present the discovery of a Type Ia supernova (SN) at redshift $z = 1.914$ from the CANDELS multi-cycle treasury program on the textit{Hubble Space Telescope (HST)}. This SN was discovered in the infrared using the Wide-Field Camera 3, and it is the highest-redshift Type Ia SN yet observed. We classify this object as a SN,Ia by comparing its light curve and spectrum with those of a large sample of Type Ia and core-collapse supernovae (SNe). Its apparent magnitude is consistent with that expected from the $Lambda$CDM concordance cosmology. We discuss the use of spectral evidence for classification of $z > 1.5$ SNe,Ia using {it HST} grism simulations, finding that spectral data alone can frequently rule out SNe,II, but distinguishing between SNe,Ia and SNe,Ib/c can require prohibitively long exposures. In such cases, a quantitative analysis of the light curve may be necessary for classification. Our photometric and spectroscopic classification methods can aid the determination of SN rates and cosmological parameters from the full high-redshift CANDELS SN sample.
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