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Register a new userQuasars as probes of the submillimetre cosmos at z>5: I. Preliminary SCUBA photometry
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Robert S. Priddey
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We present submillimetre continuum observations of a sample of some of the highest redshift quasars currently known, made with the SCUBA bolometer array on the JCMT. The median redshift of the sample is 5.3. Two z>5 objects are strong (6 sigma) detections, with S(850um)>10mJy. A firm (5 sigma) detection is obtained for the z=5.7 quasar SDSS J1044-0125; and SDSS J1306+0356, at z=6.0, is detected with a signal-to-noise approx. 4. For the remainder of the sample we have obtained sensitive (sigma=1.5mJy) upper limits. Submm spectral indices measured for two of the sources are consistent with thermal reradiation from dust, rather than from synchrotron emission. Sensitive 450um upper limits imply that the dust is cool,suggesting large dust masses only 1Gyr after the Big Bang.
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The most massive haloes at high redshift are expected, according to hierarchical cosmologies, to reside in the most biased density fields. If powerful active galactic nuclei (AGN) are expected to exist anywhere in the early Universe (z>5), it is within these massive haloes. The most luminous of these AGN, powered by supermassive black holes (SMBHs) ~10^9Msun, thereby present an opportunity to test models of galaxy formation. Here, we present submillimetre (submm) continuum images of the fields of three luminous quasars at z>5, obtained at 850 and 450um using the Submm Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). N-body simulations predict that such quasars evolve to become the central dominant galaxies of massive clusters at z=0, but at z=5-6 they are actively forming stars and surrounded by a rich proto-filamentary structure of young galaxies. Our images show evidence of extended emission on a scale of ~100kpc from at least one quasar - indicative of a partially resolved merger or a colossal host galaxy. In addition, at >3sigma significance we detect 12 (5) submm galaxies (SMGs) at 850um (450um) in the surrounding fields. Number counts of these SMGs are systematically overabundant relative to blank-field submm surveys. Whilst the redshift-sensitive 850um/450um and 850um/1.4GHz flux density ratios indicate that some of these SMGs are likely foreground objects, the counts suggest that many probably lie in the same large-scale structures as the quasars.
We present statistically significant detections at 850um of the Lyman Break Galaxy (LBG) population at z=3, 4, and 5 using data from the Submillimetre Common User Bolometer Array 2 (SCUBA-2) Cosmology Legacy Survey (S2CLS) in the United Kingdom Infrared Deep Sky Survey Ultra Deep Survey (UKIDSS-UDS) field. We employ a stacking technique to probe beneath the survey limit to measure the average 850um flux density of LBGs at z=3, 4, and 5 with typical ultraviolet luminosities of L(1700A)~10^29 erg/s/Hz. We measure 850um flux densities of (0.25 +/- 0.03, (0.41 +/- 0.06), and (0.88 +/- 0.23) mJy respectively, and find that they contribute at most 20 per cent to the cosmic far-infrared background at 850um. Fitting an appropriate range of spectral energy distributions to the z=3, 4, and 5 LBG stacked 24-850um fluxes, we derive infrared (IR) luminosities of L(8-1000um)~3.2, 5.5, and 11.0x10^11 Lsun (corresponding to star formation rates of ~50-200 Msun/yr) respectively. We find that the evolution in the IR luminosity density of LBGs is broadly consistent with model predictions for the expected contribution of luminous IR galaxy (LIRG) to ultraluminous IR galaxy (ULIRG) type systems at these epochs. We also see a strong positive correlation between stellar mass and IR luminosity. Our data are consistent with the main sequence of star formation showing little or no evolution from z=3 to 5. We have also confirmed that, for a fixed mass, the reddest LBGs (UV slope Beta -> 0) are indeed redder due to dust extinction, with SFR(IR)/SFR(UV) increasing by approximately an order of magnitude over -2<Beta<0 such that SFR(IR)/SFR(UV)~20 for the reddest LBGs. Furthermore, the most massive LBGs also tend to have higher obscured-to-unobscured ratio, hinting at a variation in the obscuration properties across the mass range.
We report on submillimetre observations of three high redshift (z>6) quasars, made using the SCUBA camera on the JCMT. Only one of the sample was detected at 850um-- SDSS J1148+5251 (z=6.43). It was also detected (>3 sigma significance) at 450um, one of the few z>4 quasars for which this is the case. In combination with existing millimetric data, the submm detections allow us to place limits on the temperature of the submm-emitting dust, hence on the mass of dust to be synthesized under the time constraint imposed by the quasars redshift.
The faint-end slope of the quasar luminosity function at z~6 and its implication on the role of quasars in reionizing the intergalactic medium at early times has been an outstanding problem for some time. The identification of faint high-redshift quasars with luminosities of <1e44.5 erg/s is challenging. They are rare (few per square degree) and the separation of these unresolved quasars from late-type stars and compact star-forming galaxies is difficult from ground-based observations alone. In addition, source confusion becomes significant at >25mag, with ~30% of sources having their flux contaminated by foreground objects when the seeing resolution is ~0.7. We mitigate these issues by performing a pixel-level joint processing of ground and space-based data from Subaru/HSC and HST/ACS. We create a deconfused catalog over the 1.64 square-degrees of the COSMOS field, after accounting for spatial varying PSFs and astrometric differences between the two datasets. We identify twelve low-luminosity (M_UV ~ -21 mag) z>6 quasar candidates through (i) their red color measured between ACS/F814W and HSC/i-band and (ii) their compactness in the space-based data. We estimate that late-type stars could contribute up to 50% to our sample. Our constraints on the faint end of the quasar luminosity function at z~6.4 suggests a negligibly small contribution to reionization compared to the star-forming galaxy population. The confirmation of our candidates and the evolution of number density with redshift could provide better insights into how supermassive galaxies grew in the first billion years of cosmic time.
We report Chandra detection of three UV bright radio quiet quasars at $zgtrsim5$. We have collected a sufficient number of photons to extract an X-ray spectrum of each quasar to measure their basic X-ray properties, such as the X-ray flux, power law photon index ($Gamma$), and optical-to-X-ray spectral slope ($alpha_{rm OX}$). J074749+115352 at $z=5.26$ is the X-ray brightest radio-quiet quasar at $z>5$. It may have a short timescale variation (on a timescale of $sim3800rm~s$ in the observers frame, or $sim600rm~s$ in the rest frame) which is however largely embedded in the statistical noise. We extract phase folded spectra of this quasar. There are two distinguishable states: a high soft state with an average X-ray flux $sim2.7$ times of the low hard state, and a significantly steeper X-ray spectral slope ($Gamma=2.40_{-0.32}^{+0.33}$ vs $1.78_{-0.24}^{+0.25}$). We also compare the three quasars detected in this paper to other quasar samples. We find that J074749+115352, with a SMBH mass of $M_{rm SMBH}approx1.8times10^9rm~M_odot$ and an Eddington ratio of $lambda_{rm Edd}approx2.3$, is extraordinarily X-ray bright. It has an average $alpha_{rm OX}=-1.46pm0.02$ and a 2-10 keV bolometric correction factor of $L_{rm bol}/L_{rm2-10keV}=42.4pm5.8$, both significantly depart from some well defined scaling relations. We compare $Gamma$ of the three quasars to other samples at different redshifts, and do not find any significant redshift evolution based on the limited sample of $z>5$ quasars with reliable measurements of the X-ray spectral properties.
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