Explaining the existence of \(\gtrsim10^8\,\mathrm{M_\odot}\) SMBHs at \(z>6\) is a persistent challenge to modern astrophysics. Multi-wavelength observations of \(z\gtrsim6\) QSOs reveal that, on ...average, their accretion physics is similar to that of their counterparts at lower redshift. However, QSOs showing properties that deviate from the general behavior can provide useful insights into the physical processes responsible for the rapid growth of SMBHs in the early universe. We present X-ray (XMM-Newton, 100 ks) follow-up observations of a \(z\approx6\) QSO, J1641+3755, which was found to be remarkably X-ray bright in a 2018 Chandra dataset. J1641+3755 is not detected in the 2021 XMM-Newton observation, implying that its X-ray flux decreased by a factor \(\gtrsim7\) on a notably short timescale (i.e., \(\approx115\) rest-frame days), making it the \(z>4\) QSO with the largest variability amplitude. We also obtained rest-frame UV spectroscopic and photometric data with textit{LBT}, and compared them with archival datasets. Surprisingly, we found that J1641+3755 became brighter in the rest-frame UV band from 2003 to 2016, while no strong variation occurred from 2016 to 2021. Multiple narrow absorption features are detected in its rest-frame UV spectrum, and several of them can be associated with an intervening system at \(z=5.67\). The variability properties of J1641+3755 can be due to intrinsic variations of the accretion rate, a small-scale obscuration event, gravitational lensing due to an intervening object, or an unrelated X-ray transient in a foreground galaxy in 2018. Accounting for all of the \(z>6\) QSOs with multiple X-ray observations separated by \(>10\) rest-frame days, we found an enhancement of strongly (i.e., by a factor \(>3\)) X-ray variable objects compared to QSOs at later cosmic times. This finding may be related to the physics of fast accretion in high-redshift QSOs.
We present the radio properties of 66 spectroscopically-confirmed normal star-forming galaxies (SFGs) at \(4.4<z<5.9\) in the COSMOS field that were C II detected in the Atacama Large Millimeter ...Array (ALMA) Large Program to INvestigate C II at Early times (ALPINE). We separate these galaxies ("CII-detected-all") into lower redshift ("CII-detected-lz", \(\langle z\rangle=4.5\)) and higher redshift ("CII-detected-hz", \(\langle z\rangle=5.6\)) sub-samples and stack multi-wavelength imaging for each sub-sample from X-ray to radio bands. A radio signal is detected in the stacked 3 GHz image of CII-detected-all and -lz samples at \(\gtrsim3\sigma\). We find that the infrared-radio correlation of our sample, quantified by \(q_{\mathrm{TIR}}\), is lower than the local relation for normal SFGs at \(\sim\)3\(\sigma\) significance level, and is instead broadly consistent with that of bright sub-mm galaxies at \(2<z<5\). Neither of these samples show evidence of dominant AGN activity in their stacked Spectral Energy Distributions (SEDs), rest-frame UV spectra, or X-ray images. Although we cannot rule out the possible effect of the assumed spectral index and the applied infrared SED templates as at least partially causing these differences, the lower obscured fraction of star formation than at lower redshift can alleviate the tension between our stacked \(q_{\mathrm{TIR}}\)s and that of local normal SFGs. It is possible that the dust buildup, which primarily governs the IR emission in addition to older stellar populations, has not had enough time to occur fully in these galaxies, whereas the radio emission can respond on a more rapid timescale. Therefore, we might expect a lower \(q_{\mathrm{TIR}}\) to be a general property of high-redshift SFGs.
Radio emission at cm wavelengths from highly star-forming galaxies, such as SMGs, is dominated by synchrotron radiation arising from supernova activity. Using deep, high-resolution (\(1\sigma=2.3\) ...\(\mu\)Jy beam\(^{-1}\); \(0.75^{"}\)) cm radio-continuum observations taken by the VLA-COSMOS 3 GHz Large Project, we studied the radio-emitting sizes of a flux-limited sample of SMGs in the COSMOS field. Of the 39 SMGs studied here, 3 GHz emission was detected towards 18 of them (\(\sim46\pm11\%\)) with S/N ratios in the range of \({\rm S/N=4.2-37.4}\). Using 2D elliptical Gaussian fits, we derived a median deconvolved major axis FWHM size of \(0.54^{"}\pm 0.11^{"}\) for our 18 SMGs detected at 3 GHz. For the 15 SMGs with known redshift we derived a median linear major axis FWHM of \(4.2\pm0.9\) kpc. No clear correlation was found between the radio-emitting size and the 3 GHz or submm flux density, or the redshift of the SMG. However, there is a hint of larger radio sizes at \(z\sim2.5-5\) compared to lower redshifts. The sizes we derived are consistent with previous SMG sizes measured at 1.4 GHz and in mid-\(J\) CO emission, but significantly larger than those seen in the (sub)mm continuum emission. One possible scenario is that SMGs have i) an extended gas component with a low dust temperature, and which can be traced by low- to mid-\(J\) CO line emission and radio continuum emission, and ii) a warmer, compact starburst region giving rise to the high-\(J\) line emission of CO, which could dominate the dust continuum size measurements. Because of the rapid cooling of CR electrons in dense starburst galaxies (\(\sim10^4-10^5\) yr), the more extended synchrotron radio-emitting size being a result of CR diffusion seems unlikely. Instead, if SMGs are driven by galaxy mergers the radio synchrotron emission might arise from more extended magnetised ISM around the starburst region.
X-ray emission from QSOs has been used to assess SMBH accretion properties up to \(z\)~6. However, at \(z>6\) only ~15 QSOs are covered by sensitive X-ray observations, preventing a statistically ...significant investigation of the X-ray properties of QSOs in the first Gyr of the Universe. We present new Chandra observations of 10 \(z>6\) QSOs, selected to have virial black-hole mass estimates from Mg II line spectroscopy. Adding archival X-ray data for an additional 15 \(z>6\) QSOs, we investigate the X-ray properties of the QSO population in the first Gyr of the Universe, focusing in particular on the \(L_{UV}-L_{X}\) relation, which is traced by the \(\alpha_{ox}\) parameter, and the shape of their X-ray spectra. We performed photometric analyses to derive estimates of the X-ray luminosities, and thus the \(\alpha_{ox}\) values and bolometric corrections (\(K_{bol}=L_{bol}/L_{X}\)). We compared the resulting \(\alpha_{ox}\) and \(K_{bol}\) distributions with the results found for QSO samples at lower redshift. Finally, we performed a basic X-ray spectral analysis of the brightest \(z>6\) QSOs to derive their individual photon indices, and joint spectral analysis of the whole sample to estimate the average photon index. We confirm a lack of significant evolution of \(\alpha_{ox}\) with redshift, extending the results from previous works up to \(z>6\), and the trend of an increasing bolometric correction with increasing luminosity found for QSOs at lower redshifts. The average power-law photon index of our sample (\(\Gamma=2.20_{-0.34}^{+0.39}\) and \(\Gamma=2.13_{-0.13}^{+0.13}\) for sources with \(<30\) and \(>30\) net counts, respectively) is slightly steeper than, but still consistent with, typical QSOs at \(z=1-6\). All these results point toward a lack of substantial evolution of the inner accretion-disk/hot-corona structure in QSOs from low redshift to \(z>6\). Our data hint at generally high Eddington ratios at \(z>6\).
The discovery of hundreds of QSOs in the first Gyr of the Universe powered by already grown SMBHs challenges our knowledge of SMBH formation. In particular, investigations of \(z>6\) QSOs presenting ...notable properties can provide unique information on the physics of fast SMBH growth in the early universe. We present the results of follow-up observations of the \(z=6.515\) radio-quiet QSO PSO167-13, which is interacting with a close companion galaxy. The PSO167-13 system has been recently proposed to host the first heavily obscured X-ray source at high redshift. We observed PSO167-13 with Chandra/ACIS-S (177 ks), and obtained new spectroscopic observations (7.2 h) with Magellan/FIRE. No significant X-ray emission is detected from the PSO167-13 system, suggesting that the obscured X-ray source previously tentatively detected was either due to a strong background fluctuation or is highly variable. The upper limit (90% confidence level) on the X-ray emission of PSO167-13 (\(L_{2-10\,\mathrm{keV}}<8.3\times10^{43}\,\mathrm{erg s^{-1}}\)) is the lowest available for a \(z>6\) QSO. The ratio between the X-ray and UV luminosity of \(\alpha_{ox}<-1.95\) makes PSO167-13 a strong outlier from the \(\alpha_{ox}-L_{UV}\) and \(L_X-L_{\mathrm{bol}}\) relations. In particular, its X-ray emission is \(>6\) times weaker than the expectation based on its UV luminosity. The new Magellan/FIRE spectrum of PSO167-13 is strongly affected by the unfavorable sky conditions, but the tentatively detected C IV and Mg II emission lines appear strongly blueshifted. The most plausible explanations for the X-ray weakness of PSO167-13 are intrinsic weakness or small-scale absorption by Compton-thick material. The possible strong blueshift of its emission lines hints at the presence of nuclear winds, which could be related to its X-ray weakness.
We investigate the dichotomy in the radio loudness distribution of quasars by modelling their radio emission and various selection effects using a Monte Carlo approach. The existence of two ...physically distinct quasar populations, the radio-loud and radio-quiet quasars, is controversial and over the last decade a bimodal distribution of radio loudness of quasars has been both affirmed and disputed. We model the quasar radio luminosity distribution with simple unimodal and bimodal distribution functions. The resulting simulated samples are compared to a fiducial sample of 8,300 quasars drawn from the SDSS DR7 Quasar Catalog and combined with radio observations from the FIRST survey. Our results indicate that the SDSS-FIRST sample is best described by a radio loudness distribution which consists of two components, with 12+/-1 % of sources in the radio-loud component. On the other hand, the evidence for a local minimum in the loudness distribution (bimodality) is not strong and we find that previous claims for its existence were probably affected by the incompleteness of the FIRST survey close to its faint limit. We also investigate the redshift and luminosity dependence of the radio loudness distribution and find tentative evidence that at high redshift radio-loud quasars were rarer, on average "louder", and exhibited a smaller range in radio loudness. In agreement with other recent work, we conclude that the SDSS-FIRST sample strongly suggests that the radio loudness distribution of quasars is not a universal function, and that more complex models than presented here are needed to fully explain available observations.
While theoretical arguments predict that most of the early growth of supermassive black holes (SMBHs) happened during heavily obscured phases of accretion, current methods used for selecting \(z>6\) ...quasars (QSOs) are strongly biased against obscured QSOs, thus considerably limiting our understanding of accreting SMBHs during the first Gyr of the Universe from an observational point of view. We report the \(Chandra\) discovery of the first heavily obscured QSO candidate in the early universe, hosted by a close (\(\approx5\) kpc) galaxy pair at \(z=6.515\). One of the members is an optically classified type 1 QSO, PSO167-13. The companion galaxy was first detected as a C II emitter by ALMA. An X-ray source is significantly (\(P=0.9996\)) detected by \(Chandra\) in the 2-5 keV band, with \(<1.14\) net counts in the 0.5-2 keV band, although the current positional uncertainty does not allow a conclusive association with either PSO167-13 or its companion galaxy. From X-ray photometry and hardness-ratio arguments, we estimated an obscuring column density of \(N_H>2\times10^{24}\,\mathrm{cm^{-2}}\) and \(N_H>6\times10^{23}\,\mathrm{cm^{-2}}\) at \(68\%\) and \(90\%\) confidence levels, respectively. Thus, regardless of which of the two galaxies is associated with the X-ray emission, this source is the first heavily obscured QSO candidate at \(z>6\).
We determine the radio size distribution of a large sample of 152 SMGs in COSMOS that were detected with ALMA at 1.3 mm. For this purpose, we used the observations taken by the VLA-COSMOS 3 GHz Large ...Project. One hundred and fifteen of the 152 target SMGs were found to have a 3 GHz counterpart. The median value of the major axis FWHM at 3 GHz is derived to be \(4.6\pm0.4\) kpc. The radio sizes show no evolutionary trend with redshift, or difference between different galaxy morphologies. We also derived the spectral indices between 1.4 and 3 GHz, and 3 GHz brightness temperatures for the sources, and the median values were found to be \(\alpha=-0.67\) and \(T_{\rm B}=12.6\pm2\) K. Three of the target SMGs, which are also detected with the VLBA, show clearly higher brightness temperatures than the typical values. Although the observed radio emission appears to be predominantly powered by star formation and supernova activity, our results provide a strong indication of the presence of an AGN in the VLBA and X-ray-detected SMG AzTEC/C61. The median radio-emitting size we have derived is 1.5-3 times larger than the typical FIR dust-emitting sizes of SMGs, but similar to that of the SMGs' molecular gas component traced through mid-\(J\) line emission of CO. The physical conditions of SMGs probably render the diffusion of cosmic-ray electrons inefficient, and hence an unlikely process to lead to the observed extended radio sizes. Instead, our results point towards a scenario where SMGs are driven by galaxy interactions and mergers. Besides triggering vigorous starbursts, galaxy collisions can also pull out the magnetised fluids from the interacting disks, and give rise to a taffy-like synchrotron-emitting bridge. This provides an explanation for the spatially extended radio emission of SMGs, and can also cause a deviation from the well-known IR-radio correlation.
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