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Register a new userRadio-loud high-redshift protogalaxy candidates in Bootes
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We used the Near Infrared Camera on Keck I to obtain Ks-band images of four candidate high-redshift radio galaxies selected using optical and radio data in the NOAO Deep Wide-Field Survey in Bootes. Our targets have 1.4 GHz radio flux densities greater than 1 mJy, but are undetected in the optical to fainter than 24 Vega mag. Spectral energy distribution fitting suggests that three of these objects are at z > 3, with radio luminosities near the FR-I / FR-II break. The other has photometric redshift 1.2, but may in fact be at higher redshift. Two of the four objects exhibit diffuse morphologies in Ks -band, suggesting that they are still in the process of forming.
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We report on Swift observations of four z>2 radio-loud quasars (0212+735, 0537-286, 0836+710, and 2149-307), classified as blazars. The sources, well-known emitters at soft-medium X-rays, were detected at >5sigma with the BAT experiment in 15-150 keV. No flux variability was detected within the XRT and BAT exposures, with the exception of 0836+710 which shows an increase of a factor 4 of the 15-150 keV flux on a timescale of one month. The 0.3-10 keV spectra are well fitted by power law models, with rather hard continua (photon indices Gamma_XRT ~1.3-1.5); similarly, the 15-150 keV spectra are described by power laws with Gamma_BAT ~1.3-1.8. The XRT data exhibit spectral curvature, which can be modeled either in terms of excess absorption along the line of sight, or a downward-curved broken power law. In the former case, if the excess N_H is at the rest-frame of the source, columns of N_H^z=(0.3-6)x10^22 cm^-2 are measured. Modeling of the SEDs of the four quasars shows that the emission at the higher frequencies, >~ 10^16 Hz, is dominated by the jet, while the steep optical-to-UV continua, observed with the UVOT, can be attributed to thermal emission from the accretion disk. The disk luminosity is between 1% and 10% the jet power, similar to other powerful blazars.
The very existence of more than a dozen of high-redshift (z>4) blazars indicates that a much larger population of misaligned powerful jetted AGN was already in place when the Universe was <1.5 Gyr old. Such parent population proved to be very elusive, and escaped direct detection in radio surveys so far. High redshift blazars themselves seem to be failing in producing extended radio-lobes, raising questions about the connection between such class and the vaster population of radio-galaxies. We show that the interaction of the jet electrons with the intense cosmic microwave background (CMB) radiation explains the lack of extended radio emission in high redshift blazars and in their parent population, helping to explain the apparently missing misaligned counterparts of high redshift blazars. On the other hand, the emission from the more compact and more magnetised hot spots are less affected by the enhanced CMB energy density. By modelling the spectral energy distribution of blazar lobes and hot spots we find that most of them should be detectable by low frequency deep radio observations, e.g., by LOw-Frequency ARray for radio astronomy (LOFAR) and by relatively deep X-ray observations with good angular resolution, e.g., by the Chandra satellite. At high redshifts, the emission of a misaligned relativistic jet, being de-beamed, is missed by current large sky area surveys. The isotropic flux produced in the hot spots can be below ~1 mJy and the isotropic lobe radio emission is quenched by the CMB cooling. Consequently, even sources with very powerful jets can go undetected in current radio surveys, and misclassified as radio-quiet AGNs.
We report spectral, imaging, and variability results from four new XMM-Newton observations and two new Chandra observations of high-redshift (z > 4) radio-loud quasars (RLQs). Our targets span lower, and more representative, values of radio loudness than those of past samples of high-redshift RLQs studied in the X-ray regime. Our spectral analyses show power-law X-ray continua with a mean photon index, Gamma =1.74 +/- 0.11, that is consistent with measurements of lower redshift RLQs. These continua are likely dominated by jet-linked X-ray emission, and they follow the expected anti-correlation between photon index and radio loudness. We find no evidence of iron Kalpha ~ emission lines or Compton-reflection continua. Our data also constrain intrinsic X-ray absorption in these RLQs. We find evidence for significant absorption (N_H ~ 10^22 cm^-2) in one RLQ of our sample (SDSS J0011+1446); the incidence of X-ray absorption in our sample appears plausibly consistent with that for high-redshift RLQs that have higher values of radio loudness. In the Chandra observation of PMN J221-2719 we detect apparent extended (~ 14 kpc) X-ray emission that is most likely due to a jet; the X-ray luminosity of this putative jet is ~2% that of the core. The analysis of a 4.9 GHz VLA image of PMN J221-2719 reveals a structure that matches the X-ray extension found in this source. We also find evidence for long-term (450-460 days) X-ray variability by 80-100% in two of our targets.
(abridged) We firstly present the results of X-ray spectroscopic observations with XMM-Newton for four high-redshift radio-loud quasars at z>4. These observations more than double the number of z>4 radio-loud quasars having X-ray spectroscopic data to seven, which compose a significant subset of a flux-limited sample of z>4 radio-loud quasars. Based on this subset we show some preliminary results on the overall X-ray spectral properties of the sample. Soft X-ray spectral flattening, which is thought to arise from intrinsic X-ray absorption, was found in about half of the sample. We give a preliminary distribution of the absorption column density NH. For those with detected X-ray absorption, the derived NH values fall into a very narrow range (around a few times 10^(22)cm^(-2) for `cold absorption), suggesting a possible common origin of the absorber. Compared to lower-redshift samples at z<2, there is an extension, or a systematic shift, toward higher values in the intrinsic NH distribution at z>4, and an increase of the fraction of radio-loud quasars showing X-ray absorption toward high redshifts. These results indicate a cosmic evolution effect, which seems to be the strongest at redshifts around 2. The rest frame 1-50keV continua have photon indices with a mean of 1.64 and a standard deviation of 0.11. Variability appears to be common on timescales from a few months to years in the quasar rest-frame, sometimes in both fluxes and spectral slopes.
The existence of BAL outflows in only radio-quiet QSOs was thought to be an important clue to mass ejection and the radio-loud - radio-quiet dichotomy. Recently a few radio-loud BAL QSOs have been discovered at high redshift. We present evidence that PKS 1004+13 is a radio-loud BAL QSO. It would be the first known at low-redshift (z = 0.24), and one of the most radio luminous. For PKS 1004+13, there appear to be broad absorption troughs of O VI, N V, Si IV, and C IV, indicating high-ionization outflows up to about 10,000 km/s. There are also two strong, broad (~500 km/s), high-ionization, associated absorption systems that show partial covering of the continuum source. The strong UV absorption we have detected suggests that the extreme soft-X-ray weakness of PKS 1004+13 is primarily the result of absorption. The large radio-lobe dominance indicates BAL and associated gas at high inclinations to the central engine axis, perhaps in a line-of-sight that passes through an accretion disk wind.
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