High-z Quasars Research Articles

MOLECULAR GAS IN LENSEDz>2 QUASAR HOST GALAXIES AND THE STAR FORMATION LAW FOR GALAXIES WITH LUMINOUS ACTIVE GALACTIC NUCLEI

We report the detection of luminous CO(2-1), CO(3-2), and CO(4-3) emission in the strongly lensed high-redshift quasars B1938+666 (z=2.059), HE0230-2130 (z=2.166), HE1104-1805 (z=2.322), and B1359+154 (z=3.240), using the Combined Array for Research in Millimeter-wave Astronomy (CARMA). B1938+666 was identified in a `blind' CO redshift search, demonstrating the feasibility of such investigations with millimeter interferometers. These galaxies are lensing-amplified by factors of mu_L~11-170, and thus allow us to probe molecular gas in intrinsically fainter galaxies than currently possible without the aid of gravitational lensing. We report lensing-corrected intrinsic CO line luminosities of L'(CO) = 0.65-21 x 10^9 K km/s pc^2, translating to H2 masses of M(H2) = 0.52-17 x 10^9 (alpha_CO/0.8) M_sun. To investigate whether or not the AGN in luminous quasars substantially contribute to L_FIR, we study the L'(CO)-L_FIR relation for quasars relative to galaxies without a luminous AGN as a function of redshift. We find no substantial differences between submillimeter galaxies and high-z quasars, but marginal evidence for an excess in L_FIR in nearby low-L_FIR AGN galaxies. This may suggest that an AGN contribution to L_FIR is significant in systems with relatively low gas and dust content, but only minor in the most far-infrared-luminous galaxies (in which L_FIR is dominated by star formation).

Genesis and evolution of dust in galaxies in the early Universe

Aims. We intend to assess the most plausible scenarios for generating large amounts of dust in high-z quasars (QSOs) on the basis of observationally derived physical properties of QSOs at z ≳ 6.

The proximity effect around high redshift quasars

The term proximity effect refers to the relative lack of Lyα absorption in quasar spectra close to the ionizing source, produced by the enhanced photoionization rate due the local radiation field. This effect can be detected both by looking along the line of sight of the quasar, (proximity effect along the line of sight) and also by studying the effect produced in the Lyα forest of a bright quasar by a foreground source located close to the quasar line of sight (transverse proximity effect). The proximity effect has been appreciated as a tool to investigate on the ionization level of the Universe at redshifts approaching the reionization epoch, and to study the environment and properties of quasars. In this work we discuss the limits related to the use of the proximity effect along the lines of sight towards high-z quasars to measure the neutral hydrogen fraction at z ≈ 6. Moreover, we present the first-ever detection of the transverse proximity effect in the HI Lyα forest.

A Photo-Ionization Method for Black Hole Mass Estimation in Quasars

AbstractDetermining the masses of the central compact object believed to power all active galactic nuclei is relevant to our understanding of their evolution and of their inner workings. Keys to present-day mass estimates are: (1) the assumption of line broadening due to virial motion of the emitting gas, (2) an estimate of the distance of broad-line emitting gas from the central compact object, and (3) a measure of the AGN luminosity. We discuss the merits and the limitations of an alternative method based on estimates of physical conditions in the broad line emitting region derived from an appropriate multi-component analysis of emission line profiles. This ‘photo-ionization method’, applied to UV intermediate-ionization lines appears to be promising for at least a sizable population of high-z quasars.

HAWAII QUASAR AND T DWARF SURVEY. I. METHOD AND DISCOVERY OF FAINT FIELD ULTRACOOL DWARFS,

The Hawaii Quasar and T dwarf survey (HQT Survey) is a wide-field, red optical survey carried out with the Suprime-Cam mosaic CCD camera on the 8.2 m Subaru telescope. The HQT survey is designed to search for low-luminosity (M_(AB1450) 5.7) as well as T dwarfs, both of which are selected by their very red I − z' colors. We use an optical narrowband filter NB816 to break a well-known I − z' color degeneracy between high-z quasars and foreground M and L dwarfs, which are more numerous than quasars. This paper is the first in a series of papers from the HQT survey and we report on the discovery of six faint (19 ≤ J ≤ 20) ultracool dwarfs found over a ~9.3 deg^2 area with a limiting magnitude of z'_(AB) ≤ 23.3. These dwarfs were confirmed by near-IR imaging and/or spectroscopy conducted at various facilities on Mauna Kea. With estimated distances of 60–170 pc, these are among the most distant spectroscopically confirmed field brown dwarfs to date. Limits on the proper motions of these ultracool dwarfs suggest that they are old members of the Galactic disk, though future follow-up observations are necessary to minimize errors. Our finding rate of ultracool dwarfs is within model predictions of Liu et al. However, the large brightening amplitude (~1 mag) previously reported for the L/T transition objects appears to overpredict the numbers. We also examine how the survey field latitude affects the survey sensitivity to the vertical scale height of ultracool dwarfs.

The earliest galaxies seen in 21 cm line absorption

We investigate the 21 cm absorption lines produced by non-linear structures during the early stage of reionization, i.e. the starless minihaloes and the dwarf galaxies. After a detailed modelling of their properties, with particular attention to the coupling physics, we determine their 21 cm absorption line profiles. The infalling gas velocity around minihaloes/dwarf galaxies strongly affects the line shape and, with the low spin temperatures outside the virial radii of the systems, gives rise to horn-like line profiles. The optical depth of a dwarf galaxy is reduced for lines of sight penetrating through its H ii region and, especially, a large H ii region created by a dwarf galaxy with higher stellar mass and/or a top-heavy initial mass function results in an optical depth trough rather than an absorption line. We compute synthetic spectra of 21 cm forest for both high-redshift quasars and radio afterglows of gamma-ray bursts (GRBs). Even with the planned Square Kilometre Array (SKA), radio afterglows of most if not all GRBs would still be too dim to be the background sources for high-resolution (1 kHz) observations, but absorption lines can be easily detected towards a high-z quasar. Broadband observation against GRB afterglows can also be used to reveal the evolving 21 cm signal from both minihaloes and dwarf galaxies if there was no X-ray background or it was extremely weak, but it becomes difficult if an early X-ray background existed. Hence, the 21 cm absorption could be a powerful probe of the presence/intensity of the X-ray background and the thermal history of the early Universe.

Evidence for supernova-synthesized dust from the rising afterglow of GRB 071025 at z∼ 5

We present observations and analysis of the broadband afterglow of Swift GRB 071025. Using optical and infrared (RIYJHK) photometry, we derive a photometric redshift of 4.4 < z < 5.2; at this redshift our simultaneous multicolour observations begin at ~30 s after the GRB trigger in the host frame and during the initial rising phase of the afterglow. We associate the light curve peak at 580 s in the observer frame with the formation of the forward shock, giving an estimate of the initial Lorentz factor Gamma_0 ~ 200. The red spectral energy distribution (even in regions not affected by the Lyman-alpha break) provides secure evidence of a large dust column. However, the inferred extinction curve shows a prominent flat component between 2000-3000 Angstroms in the rest-frame, inconsistent with any locally observed template but well-fit by models of dust formed by supernovae. Time-dependent fits to the extinction profile reveal no evidence of dust destruction and limit the decrease in the extinction column to Delta A_3000 < 0.54 mag after t = 50 s in the rest frame. Our observations provide evidence of a transition in dust properties at z~5, in agreement with studies of high-z quasars, and suggest that SN-formed dust continues to dominate the opacity of typical galaxies at this redshift.

EDDINGTON-LIMITED ACCRETION AND THE BLACK HOLE MASS FUNCTION AT REDSHIFT 6

We present discovery observations of a quasar in the Canada-France High-z Quasar Survey (CFHQS) at redshift z=6.44. We also use near-IR spectroscopy of nine CFHQS quasars at z~6 to determine black hole masses. These are compared with similar estimates for more luminous Sloan Digital Sky Survey (SDSS) quasars to investigate the relationship between black hole mass and quasar luminosity. We find a strong correlation between MgII FWHM and UV luminosity and that most quasars at this early epoch are accreting close to the Eddington limit. Thus these quasars appear to be in an early stage of their life cycle where they are building up their black hole mass exponentially. Combining these results with the quasar luminosity function, we derive the black hole mass function at z=6. Our black hole mass function is ~10^4 times lower than at z=0 and substantially below estimates from previous studies. The main uncertainties which could increase the black hole mass function are a larger population of obscured quasars at high-redshift than is observed at low-redshift and/or a low quasar duty cycle at z=6. In comparison, the global stellar mass function is only ~10^2 times lower at z=6 than at z=0. The difference between the black hole and stellar mass function evolution is due to either rapid early star formation which is not limited by radiation pressure as is the case for black hole growth or inefficient black hole seeding. Our work predicts that the black hole mass - stellar mass relation for a volume-limited sample of galaxies declines rapidly at very high redshift. This is in contrast to the observed increase at 4<z<6 from the local relation if one just studies the most massive black holes.

Quasar candidate selection and photometric redshift estimation based on SDSS and UKIDSS data

We present a sample of 8498 quasars with both SDSS $ugriz$ optical and UKIDSS $YJHK$ near-IR photometric data. With this sample, we obtain the median colour-z relations based on 7400 quasars with magnitude uncertainties less than 0.1mag in all bands. By analyzing the quasar colours, we propose an empirical criterion in the $Y-K$ vs. $g-z$ colour-colour diagram to separate stars and quasars with redshift $z<4$, and two other criteria for selecting high-z quasars. Using the SDSS-UKIDSS colour-z relations, we estimate the photometric redshifts of 8498 SDSS-UKIDSS quasars, and find that 85.0% of them are consistent with the spectroscopic redshifts within $|\Delta z|<0.2$, which leads to a significant increase of the photometric redshift accuracy than that based on the SDSS colour-z relations only. We compare our colour selection criterion with a small UKIDSS/EDR quasar/star sample and a sample of 4671 variable sources in the SDSS Stripe 82 region with both SDSS and UKIDSS data, and find that they can be clearly divided into two classes (quasars and stars) by our criterion in the $Y-K$ vs. $g-z$ plot. We select 3834 quasar candidates from the variable sources with $g<20.5$ in Stripe 82, 826 of them being SDSS quasars and the rest without SDSS spectroscopy. We demonstrate that even at the same spectroscopy limit as SDSS, with our criterion we can at least partially recover the missing quasars with $z\sim2.7$ in SDSS. The SDSS identified quasars only take a small fraction (21.5%) of our quasar candidates selected from the variable sources in Stripe 82, indicating that a deeper spectroscopy is very promising in producing a larger sample of quasars than SDSS. The implications of our results to the future Chinese LAMOST quasar survey are also discussed.

VERY HIGH GAS FRACTIONS AND EXTENDED GAS RESERVOIRS INz= 1.5 DISK GALAXIES

We present evidence for very high gas fractions and extended molecular gas reservoirs in normal, near-infrared selected (BzK) galaxies at z~1.5, based on multi-configuration CO[2-1] observations obtained at the IRAM PdBI. Six of the six galaxies observed were securely detected. High resolution observations resolve the CO emission in four of them, implying sizes of order of 6-11 kpc and suggesting the presence of rotation. The UV morphologies are consistent with clumpy, unstable disks, and the UV sizes are consistent with the CO sizes. The star formation efficiencies are homogeneously low and similar to local spirals - the resulting gas depletion times are ~0.5 Gyr, much higher than what is seen in high-z submm galaxies and quasars. The CO luminosities can be predicted to within 0.15 dex from the star formation rates and stellar masses, implying a tight correlation of the gas mass with these quantities. We use dynamical models of clumpy disk galaxies to derive dynamical masses. These models are able to reproduce the peculiar spectral line shapes of the CO emission. After accounting for the stellar and dark matter masses we derive gas masses of 0.4-1.2x10^11 Msun. The conversion factor is very high: alpha_CO=3.6+-0.8, consistent with the Galaxy but four times higher than that of local ultra-luminous IR galaxies. The gas accounts for an impressive 50-65% of the baryons within the galaxies' half light radii. We are witnessing truly gas-dominated galaxies at z~1.5, a finding that explains the high specific SFRs observed for z>1 galaxies. The BzK galaxies can be viewed as scaled-up versions of local disk galaxies, with low efficiency star formation taking place inside extended, low excitation gas disks. They are markedly different than local ULIRGs and high-z submm galaxies, which have more excited and compact gas.

THE CANADA-FRANCE HIGH-zQUASAR SURVEY: NINE NEW QUASARS AND THE LUMINOSITY FUNCTION AT REDSHIFT 6

We present discovery imaging and spectroscopy for nine new z ~ 6 quasars found in the Canada-France High-z Quasar Survey (CFHQS) bringing the total number of CFHQS quasars to 19. By combining the CFHQS with the more luminous SDSS sample we are able to derive the quasar luminosity function from a sample of 40 quasars at redshifts 5.74 < z < 6.42. Our binned luminosity function shows a slightly lower normalisation and flatter slope than found in previous work. The binned data also suggest a break in the luminosity function at M_1450 approx -25. A double power law maximum likelihood fit to the data is consistent with the binned results. The luminosity function is strongly constrained (1 sigma uncertainty < 0.1 dex) over the range -27.5 < M_1450 < -24.7. The best-fit parameters are Phi(M_1450^*) = 1.14 x 10^-8 Mpc^-3 mag^-1, break magnitude M_1450^* = -25.13 and bright end slope beta = -2.81. However the covariance between beta and M_1450^* prevents strong constraints being placed on either parameter. For a break magnitude in the range -26 < M_1450^* < -24 we find -3.8 < beta < -2.3 at 95% confidence. We calculate the z = 6 quasar intergalactic ionizing flux and show it is between 20 and 100 times lower than that necessary for reionization. Finally, we use the luminosity function to predict how many higher redshift quasars may be discovered in future near-IR imaging surveys.

PROBING THE WARM DARK MATTER WITH THE HIGH- z QUASAR LUMINOSITY FUNCTION

In a warm dark matter (WDM) cosmology, the first objects to form at z ⩾ 20 are one-dimensional filaments with mean length on the order of the WDM free-streaming scale. Gao & Theuns recently claimed by using high-resolution hydrodynamic simulations that the eventual collapse of these WDM filaments along their longest axes may seed the supermassive black holes that power high-z quasars. In this picture, it is supposed that the high-z quasar luminosity function should reflect how abundant the WDM filaments are in the early universe. We derive analytically the mass function of early-universe filaments with the help of the Zel'dovich approximation. Then, we determine the rate of its decrease in the mass section corresponding to the free-streaming scale of a WDM particle of mass mν. Adjusting the value of mν, we fit the slope of the analytic model to that of the high-z quasar luminosity function measured from the Sloan Digital Sky Survey DR3. A new WDM constraint from this feasibility study is found to be consistent with the lightest super-symmetric partner.

Interpreting the transmission windows of distant quasars

We propose the apparent shrinking criterion (ASC) to interpret the spatial extent, R W , of transmitted flux windows in the absorption spectra of high-z quasars. The ASC can discriminate between the two regimes in which R w corresponds either to the physical size, R HII , of the quasar H II region or to the distance, R max W , at which the transmitted flux drops to =0.1 and a Gunn-Peterson (GP) trough appears. In the first case [H II region (HR) regime], one can determine the intergalactic medium mean H I fraction, x HI ; it the second [proximity region (PR) regime], the value of R W allows one to measure the local photoionization rate and the local enhancement of the photoionization rate, Γ G , due to nearby/intervening galaxies. The ASC has been tested against radiative transfer+smoothed particle hydrodynamics numerical simulations, and applied to a sample of 15 high-z (z > 5.8) quasar spectra. All sample quasars are found to be in the PR regime; hence, their observed spectral properties (inner flux profile, extent of transmission window) cannot reliably constrain the value of x HI . Four sample quasars show evidence for a local enhancement (up to 50 per cent) in the local photoionization rate possibly produced by a galaxy overdensity. We discuss the possible interpretations and uncertainties of this result.

SIX MORE QUASARS AT REDSHIFT 6 DISCOVERED BY THE CANADA-FRANCE HIGH-zQUASAR SURVEY

We present imaging and spectroscopic observations for six quasars at z ? 5.9 discovered by the Canada-France High-z Quasar Survey (CFHQS). The CFHQS contains subsurveys with a range of flux and area combinations to sample a wide range of quasar luminosities at z ~ 6. The new quasars have luminosities 10-75 times lower than the most luminous Sloan Digital Sky Survey quasars at this redshift. The least luminous quasar, CFHQS?J0216?0455 at z = 6.01, has absolute magnitude M 1450 = ?22.21, well below the likely break in the luminosity function. This quasar is not detected in a deep XMM-Newton survey showing that optical selection is still a very efficient tool for finding high-redshift quasars.

GISMO, a 2 mm Bolometer Camera Optimized for the Study of High Redshift Galaxies

The 2 mm spectral range provides a unique terrestrial window enabling ground based observations of the earliest active dusty galaxies in the universe and thereby allowing a better constraint on the star formation rate in these objects. We have built GISMO (the Goddard-IRAM Superconducting 2-Millimeter Observer), a 2 mm, 128 element superconducting Transition Edge Sensor (TES) based bolometer camera for the IRAM 30 m telescope in Spain. The camera uses an 8×16 planar array of multiplexed TES bolometers, which incorporates our recently designed Backshort Under Grid (BUG) architecture, described elsewhere. The optical design incorporates a 100 mm (4 inches) diameter silicon lens cooled to 4 K, which provides the required fast beam of 0.9 λ/D. With this spatial sampling, GISMO will be very efficient at detecting sources serendipitously in large sky surveys, while the capability for diffraction-limited observations is preserved. With the background limited performance of the detectors, the camera provides significantly greater imaging sensitivity and mapping speed at this wavelength than has previously been possible. The major scientific driver for the instrument is to provide the IRAM 30 m telescope with the capability to rapidly observe galactic and extragalactic dust emission, in particular from high-z Ultra Luminous Infrared Galaxies (ULIRGs) and quasars, even in the summer season. The instrument will fill in the SEDs of high redshift galaxies at the Rayleigh-Jeans part of the dust emission spectrum, even at the highest redshifts. Our source count models predict that GISMO will serendipitously detect one galaxy every four hours on the blank sky, and that one quarter of these galaxies will be at a redshift of z 6.5. We expect to install GISMO at the 30 m telescope in the second half of 2007.

Four Quasars above Redshift 6 Discovered by the Canada-France High-zQuasar Survey

The Canada-France High-z Quasar Survey (CFHQS) is an optical survey designed to locate quasars during the epoch of reionization. In this paper we present the discovery of the first four CFHQS quasars at redshifts greater than 6, including the most distant known quasar, CFHQS J2329-0301 at z = 6.43. We describe the observational method used to identify the quasars and present optical, infrared, and millimeter photometry and optical and near-infrared spectroscopy. We investigate the dust properties of these quasars, finding an unusual dust extinction curve for one quasar and a high far-infrared luminosity due to dust emission for another. The mean millimeter continuum flux for CFHQS quasars is substantially lower than that for SDSS quasars at the same redshift, likely due to a correlation with quasar UV luminosity. For two quasars with sufficiently high signal-to-noise ratio optical spectra, we use the spectra to investigate the ionization state of hydrogen at z > 5. For CFHQS J1509-1749 at z = 6.12 we find significant evolution (beyond a simple extrapolation of lower redshift data) in the Gunn-Peterson optical depth at z > 5.4. The line of sight to this quasar has one of the highest known optical depths at z ≈ 5.8. An analysis of the sizes of the highly ionized near-zones in the spectra of two quasars at z = 6.12 and 6.43 suggest that the intergalactic medium surrounding these quasars was substantially ionized before these quasars turned on. Together, these observations point toward an extended reionization process, but we caution that cosmic variance is still a major limitation in z > 6 quasar observations.

The CO Tully‐Fisher Relation and Implications for the Host Galaxies of High‐Redshift Quasars

The integrated line width derived from CO spectroscopy provides a powerful tool to study the internal kinematics of extragalactic objects, including quasars at high redshift, provided that the observed line width can be properly translated to more conventionally used kinematical parameters of galaxies. We show, through construction of a K-band CO Tully-Fisher relation for nearby galaxies spanning a wide range in infrared luminosity, that the CO line width measured at 20% of the peak intensity, when corrected for inclination and other effects, successfully recovers the maximum rotation velocity of the disk. The line width at 50% of the peak intensity performs much more poorly, in large part because CO lines have a wide range of profiles, which are shown to vary systematically with infrared luminosity. We present a practical prescription for converting observed CO line widths into the stellar velocity dispersion of the bulge (sigma), and then apply it to a sample of low-redshift (z < 0.2) and high-redshift (1.4 < z < 6.4) quasars to study their host galaxies. Nearby quasars roughly fall on the correlation between black hole mass and bulge stellar velocity dispersion established for inactive galaxies, but the host galaxies of the high-z quasars systematically deviate from the local M_BH-sigma relation. At a given sigma, high-z quasars have black hole masses larger by a factor of 4 relative to local galaxies, suggesting that early in the life-cycle of galaxies the development of the bulge lags behind the growth of the central black hole. An alternative explanation for these observations, which currently cannot be ruled out rigorously, is that high-redshift quasars are preferentially viewed at face-on orientations.

Is the dependence of spectral index on luminosity real in optically selected AGN samples?

We critically examine the dependence of spectral index on luminosity in optically selected AGN samples. An analysis of optically selected high-z quasars showed an anticorrelation of alpha(OX), the spectral index between the rest-frame 2500 angstrom and 2 keV, with optical luminosity. We examine this relationship by means of Monte Carlo simulations and conclude that a constant alpha(OX) independent of optical luminosity is still consistent with this high-z sample. We further find that contributions of large dispersions and narrow range of optical luminosity are most important for the apparent, yet artificial, alpha(OX)-l(o) correlation reported. We also examine another, but more complete, low-z optical selected AGN sub-sample from Steffen et al., and our analysis shows that a constant alpha(OX) independent of optical luminosity is also consistent with the data. By comparing X-ray and optical luminosity functions, we find that a luminosity-independent alpha(OX) is in fact more preferred than the luminosity-dependent alpha(OX) model. We also discuss the selection effects caused by flux limits, which might systematically bias the l(X)-l(o) relation and cause discrepancy in optically selected and X-ray selected AGN samples. To correctly establish a dependence of alpha(OX) of AGNs on their luminosity, a larger and more complete sample is needed and consequences of luminosity dispersions and selection effects in flux-limited samples must be taken into account properly.

New submillimeter diagnostics of physical properties of ISM in high redshift galaxies

AbstractWe present a new diagnosis method for determining physical properties of star-forming gas in high-z galaxies. In this method, we employed three key observational quantities, [CI], CO, and FIR luminosities, including our new detections of CO J = 4–3 emission from the pure-starburst (non-AGN) submm galaxy SMM J14011+0252 (z = 2.6) and the type-2 AGN IRAS FSC 10214+4724 (z = 2.3) obtained with the Nobeyama Millimeter Array (NMA) at the Nobeyama Radio Observatory. These two sources have extremely high star formation rate, and exhibit strong emission of CO and [CI] 609 μm lines. We determined ISM physical conditions for the two objects and another three high-z quasars in order to investigate the relationship between their ISM and power sources (i.e., massive star formation or AGN). A new PDR analysis (Wolfire et al. 2005, private communication) using CO, [CI], and FIR on five high-z sources provides new evidence that AGN host galaxies harbor denser (log nH ~ 5–6) ISM exposed to stronger far-UV fluxes of log G0 ~ 3.5–4 than the non-AGN submm galaxy. Volume filling factors of the star-forming dense gas in the AGN hosts are an order of magnitude smaller than that of the pure-starburst submm galaxy. This suggests that, in these AGN hosts, dense molecular clouds are dominating the central kpc around AGN, triggering extensive circumnuclear starbursts, and possibly feeding their central supermassive black hole simultaneously.

On the size of H II regions around high-redshift quasars

AbstractWe investigate the possibility of constraining the ionization state of the Intergalactic Medium (IGM) close to the end of re-ionization (z≈6) by measuring the size of the H II regions in high-z quasars spectra, via a combination of SPH and 3D radiative transfer (RT) simulations and a statistical analysis of mock quasar spectra through the simulated cosmological volume.