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Register a new userHigh-Resolution Radio Image of a Candidate Radio Galaxy at z=5.72
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Added by
Krisztina Eva Gabanyi
Publication date
2018
fields
Physics
and research's language is
English
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Recently, Saxena et al. (2018) reported the discovery of a possible radio galaxy, J1530$+$1049 at a redshift of z=5.72. We observed the source with the European Very Long Baseline Interferometry Network at $1.7$ GHz. We detected two faint radio features with a separation of $sim 400$ mas. The radio power calculated from the VLA flux density by Saxena et al. (2018), and the projected source size derived from our EVN data place J1530$+$1049 among the medium-sized symmetric objects (MSOs) which are thought to be young counterparts of radio galaxies (An and Baan 2012). Thus, our finding is consistent with a radio galaxy in an early phase of its evolution as proposed by Saxena et al. (2018).
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Active galactic nuclei are the most luminous persistent (non-transient, even if often variable) objects in the Universe. They are bright in the entire electromagnetic spectrum. Blazars are a special class where the jets point nearly to our line of sight. Because of this special geometry and the bulk relativistic motion of the plasma in the jet, their radiation is enhanced by relativistic beaming. The majority of extragalactic objects detected in gamma-rays are blazars. However, finding their counterparts in other wavebands could be challenging. Here we present the results of our 5-GHz European VLBI Network (EVN) observation of the radio source J1331+2932, a candidate blazar found while searching for possible gamma-ray emission from the stellar binary system DG CVn (Loh et al. 2017). The highest-resolution radio interferometric measurements provide the ultimate tool to confirm the blazar nature of a radio source by imaging compact radio jet structure with Doppler-boosted radio emission, and give the most accurate celestial coordinates as well.
Radio sources with steep spectra are preferentially associated with the most distant galaxies, the $alpha-z$ relation, but the reason for this relation is an open question. The spatial distribution of spectra in high-z radio sources can be used to study this relation, and low-frequency observations are particularly important in understanding the particle acceleration and injection mechanisms. However, the small angular sizes of high-z sources together with the inherently low resolution of low-frequency radio telescopes until now has prevented high angular resolution low-frequency observations of distant objects. Here we present subarcsecond observations of a $z = 2.4$ radio galaxy at frequencies between $121$ MHz and $166$ MHz. We measure the spatial distribution of spectra, and discuss the implications for models of the $alpha-z$ relation. We targeted 4C 43.15 with the High Band Antennas (HBAs) of the textit{International LOFAR Telescope} (ILT) with a range of baselines up to $1300 mathrm{km}$. At the central frequency of $143$ MHz we achieve an angular resolution of $sim 0.3$. By complementing our data with archival textit{Very Large Array} (VLA) data we study the spectral index distribution across 4C 43.15 between $55 mathrm{MHz}$ and $8.4 mathrm{GHz}$ at resolutions of $0.4$ and $0.9$. With a magnetic field strength of $B = 5.2$ nT and fitted injection indices of $alpha^mathrm{north}_mathrm{inj} = -0.8$ and $alpha^mathrm{south}_mathrm{inj} = -0.6$, fitting a Tribble spectral ageing model results in a spectral age of $tau_mathrm{spec} = 1.1 pm 0.1$ Myr. We conclude that our data on 4C 43.15 indicates that inverse Compton losses could become comparable to or exceed synchrotron losses at higher redshifts and that inverse Compton losses could be a viable explanation for the $alpha-z$ relation.
We report the discovery of a relic Giant Radio Galaxy (GRG) J021659-044920 at redshift $z sim 1.3$ that exhibits large-scale extended, nearly co-spatial, radio and X-ray emission from radio lobes, but no detection of Active Galactic Nuclei core, jets and hotspots. The total angular extent of the GRG at the observed frame 0.325 GHz, using Giant Metrewave Radio Telescope observations is found to be ${sim}$ 2.4 arcmin, that corresponds to a total projected linear size of $sim$ 1.2 Mpc. The integrated radio spectrum between 0.240 and 1.4 GHz shows high spectral curvature (${alpha}_{rm 0.610~GHz}^{rm 1.4~GHz} - {alpha}_{rm 0.240~GHz}^{rm 0.325~GHz}$ $>$ 1.19) with sharp steepening above 0.325 GHz, consistent with relic radio emission that is $sim$ 8 $times$ 10$^{6}$ yr old. The radio spectral index map between observed frame 0.325 and 1.4~GHz for the two lobes varies from 1.4 to 2.5 with the steepening trend from outer-end to inner-end, indicating backflow of plasma in the lobes. The extended X-ray emission characterized by an absorbed power-law with photon index $sim$ 1.86 favours inverse-Compton scattering of the Cosmic Microwave Background (ICCMB) photons as the plausible origin. Using both X-ray and radio fluxes under the assumption of ICCMB we estimate the magnetic field in the lobes to be 3.3 $mu$G. The magnetic field estimate based on energy equipartition is $sim$ 3.5 $mu$G. Our work presents a case study of a rare example of a GRG caught in dying phase in the distant Universe.
We present a detailed study of the kinematic, chemical and excitation properties of the giant Ly$alpha$ emitting nebula and the giant ion{H}{I} absorber associated with the $z = 2.92$ radio galaxy MRC 0943--242, using spectroscopic observations from VLT/MUSE, VLT/X-SHOOTER and other instruments. Together, these data provide a wide range of rest-frame wavelength (765 AA$,$ -- 6378 AA$,$ at $z = 2.92$) and 2D spatial information. We find clear evidence for jet gas interactions affecting the kinematic properties of the nebula, with evidence for both outflows and inflows being induced by radio-mode feedback. We suggest that the regions of relatively lower ionization level, spatially correlated with the radio hotspots, may be due to localised compression of photoionized gas by the expanding radio source, thereby lowering the ionization parameter, or due to a contribution from shock-heating. We find that photoionization of super-solar metallicity gas ($Z/Z_{odot}$ = 2.1) by an AGN-like continuum ($alpha$=--1.0) at a moderate ionization parameter ($U$ = 0.018) gives the best overall fit to the complete X-SHOOTER emission line spectrum. We identify a strong degeneracy between column density and Doppler parameter such that it is possible to obtain a reasonable fit to the ion{H}{I} absorption feature across the range log N(ion{H}{I}/cm$^{-2}$) = 15.20 and 19.63, with the two best-fitting occurring near the extreme ends of this range. The extended ion{H}{I} absorber is blueshifted relative to the emission line gas, but shows a systematic decrease in blueshift towards larger radii, consistent with a large scale expanding shell.
We report the identification of radio (1.4 and 3 GHz) and mid-infrared, far-infrared, and sub-mm (24-850$mu$m) emission at the position of one of 41 UV-bright ($mathrm{M_{UV}^{}}lesssim-21.25$) $zsimeq6.6-6.9$ Lyman-break galaxy candidates in the 1.5 deg$^2$ COSMOS field. This source, COS-87259, exhibits a sharp flux discontinuity (factor $>$3) between two narrow/intermediate bands at 9450 $mathring{A}$ and 9700 $mathring{A}$ and is undetected in all nine bands blueward of 9600 $mathring{A}$, as expected from a Lyman-alpha break at $zsimeq6.8$. The full multi-wavelength (X-ray through radio) data of COS-87529 can be self-consistently explained by a very massive (M$_{ast}=10^{10.8}$ M$_{odot}$) and extremely red (rest-UV slope $beta=-0.59$) $zsimeq6.8$ galaxy with hyperluminous infrared emission (L$_{mathrm{IR}}=10^{13.6}$ L$_{odot}$) powered by both an intense burst of highly-obscured star formation (SFR$approx$1800 M$_{odot}$ yr$^{-1}$) and an obscured ($tau_{mathrm{9.7mu m}}=7.7pm2.5$) radio-loud (L$_{mathrm{1.4 GHz}}sim10^{25.5}$ W Hz$^{-1}$) AGN. The radio emission is compact (1.04$pm$0.12 arcsec) and exhibits an ultra-steep spectrum between 1.4-3 GHz ($alpha=-2.06^{+0.27}_{-0.25}$) with evidence of spectral flattening at lower frequencies, consistent with known $z>4$ radio galaxies. We also demonstrate that COS-87259 may reside in a significant (11$times$) galaxy overdensity at $zsimeq6.6-6.9$, as common for systems hosting radio-loud AGN. Nonetheless, a spectroscopic redshift will ultimately be required to establish the true nature of COS-87259 as we cannot yet completely rule out low-redshift solutions. If confirmed to lie at $zsimeq6.8$, the properties of COS-87259 would be consistent with a picture wherein AGN and highly-obscured star formation activity are fairly common among very massive (M$_{ast}>10^{10}$ M$_{odot}$) reionization-era galaxies.
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