We present new upper limits on the volume-weighted neutral hydrogen fraction,
$\bar{x}_{\rm H\,\small {i}}$
, at z ∼ 5–6 derived from spectroscopy of bright quasars. The fraction of the Lyα and Lyβ ...forests that is ‘dark’ (with zero flux) provides the only model-independent upper limit on
$\bar{x}_{\rm H\,\small {i}}$
, requiring no assumptions about the physical conditions in the intergalactic medium or the quasar's unabsorbed UV continuum. In this work, we update our previous results using a larger sample (22 objects) of medium-depth (∼ few hours) spectra of high-redshift quasars obtained with the Magellan, MMT, and VLT. This significantly improves the upper bound on
$\bar{x}_{\rm H\,\small {i}}$
derived from dark pixel analysis to
$\bar{x}_{\rm H\,\small {i}}\le 0.06{+0.05 (1\sigma )}$
at z = 5.9 and
$\bar{x}_{\rm H\,\small {i}}\le 0.04{+0.05 (1\sigma )}$
at z = 5.6. These results provide robust constraints for theoretical models of reionization, and provide the strongest available evidence that reionization has completed (or is very nearly complete) by z ≈ 6.
We present results from a spectroscopic survey of z ∼ 5 quasars in the CFHT Legacy Survey. Using both optical color selection and a likelihood method, we select 97 candidates over an area of 105 deg2 ...to a limit of iAB < 23.2, and 7 candidates in the range 23.2 < iAB < 23.7 over an area of 18.5 deg2. Spectroscopic observations for 43 candidates were obtained with Gemini, MMT, and Large Binocular Telescope, of which 37 are z > 4 quasars. This sample extends measurements of the quasar luminosity function ∼1.5 mag fainter than our previous work in Sloan Digital Sky Survey Stripe 82. The resulting luminosity function is in good agreement with our previous results, and suggests that the faint end slope is not steep. We perform a detailed examination of our survey completeness, particularly the impact of the Ly emission assumed in our quasar spectral models, and find hints that the observed Ly emission from faint z ∼ 5 quasars is weaker than for z ∼ 3 quasars at a similar luminosity. Our results strongly disfavor a significant contribution of faint quasars to the hydrogen-ionizing background at z = 5.
ABSTRACT We present the discovery of nine quasars at identified in the Sloan Digital Sky Survey (SDSS) imaging data. This completes our survey of quasars in the SDSS footprint. Our final sample ...consists of 52 quasars at , including 29 quasars with mag selected from 11,240 deg2 of the SDSS single-epoch imaging survey (the main survey), 10 quasars with selected from 4223 deg2 of the SDSS overlap regions (regions with two or more imaging scans), and 13 quasars down to mag from the 277 deg2 in Stripe 82. They span a wide luminosity range of . This well-defined sample is used to derive the quasar luminosity function (QLF) at . After combining our SDSS sample with two faint ( mag) quasars from the literature, we obtain the parameters for a double power-law fit to the QLF. The bright-end slope β of the QLF is well constrained to be . Due to the small number of low-luminosity quasars, the faint-end slope and the characteristic magnitude are less well constrained, with and mag. The spatial density of luminous quasars, parametrized as , drops rapidly from to 6, with . Based on our fitted QLF and assuming an intergalactic medium (IGM) clumping factor of C = 3, we find that the observed quasar population cannot provide enough photons to ionize the IGM at ∼90% confidence. Quasars may still provide a significant fraction of the required photons, although much larger samples of faint quasars are needed for more stringent constraints on the quasar contribution to reionization.
So far, roughly 40 quasars with redshifts greater than z = 6 have been discovered. Each quasar contains a black hole with a mass of about one billion solar masses (10(9) M Sun symbol). The existence ...of such black holes when the Universe was less than one billion years old presents substantial challenges to theories of the formation and growth of black holes and the coevolution of black holes and galaxies. Here we report the discovery of an ultraluminous quasar, SDSS J010013.02+280225.8, at redshift z = 6.30. It has an optical and near-infrared luminosity a few times greater than those of previously known z > 6 quasars. On the basis of the deep absorption trough on the blue side of the Lyman-α emission line in the spectrum, we estimate the proper size of the ionized proximity zone associated with the quasar to be about 26 million light years, larger than found with other z > 6.1 quasars with lower luminosities. We estimate (on the basis of a near-infrared spectrum) that the black hole has a mass of ∼1.2 × 10(10) M Sun symbol, which is consistent with the 1.3 × 10(10) M Sun symbol derived by assuming an Eddington-limited accretion rate.
We present the HST WFC3/F275W UV imaging observations of A2218-Flanking, a lensed compact dwarf galaxy at redshift . The stellar mass of A2218-Flanking is and SFR is yr−1 after correcting the ...magnification. This galaxy has a young galaxy age of 127 Myr and a compact galaxy size of . The HST UV imaging observations cover the rest-frame Lyman continuum (LyC) emission (∼800 ) from A2218-Flanking. We firmly detect ( ) the LyC emission in A2218-Flanking in the F275W image. Together with the HST F606W images, we find that the absolute escape fraction of LyC is based on the flux density ratio between 1700 and 800 ( ). The morphology of the LyC emission in the F275W images is extended and follows the morphology of the UV continuum morphology in the F606W images, suggesting that the f800 is not from foreground contaminants. We find that the region with a high star formation rate surface density has a lower (higher ) ratio than the diffused regions, suggesting that LyC photons are more likely to escape from the region with the intensive star-forming process. We compare the properties of galaxies with and without LyC detections and find that LyC photons are easier to escape in low-mass galaxies.
Strong gravitational lensing provides a powerful probe of the physical properties of quasars and their host galaxies. A high fraction of the most luminous high-redshift quasars was predicted to be ...lensed due to magnification bias. However, no multiple imaged quasar was found at z > 5 in previous surveys. We report the discovery of J043947.08+163415.7, a strongly lensed quasar at z = 6.51, the first such object detected at the epoch of reionization, and the brightest quasar yet known at z > 5. High-resolution Hubble Space Telescope imaging reveals a multiple imaged system with a maximum image separation θ ∼ 0 2, best explained by a model of three quasar images lensed by a low-luminosity galaxy at z ∼ 0.7, with a magnification factor of ∼50. The existence of this source suggests that a significant population of strongly lensed, high-redshift quasars could have been missed by previous surveys, as standard color selection techniques would fail when the quasar color is contaminated by the lensing galaxy.
Despite extensive efforts, to date only two quasars have been found at z > 7, due to a combination of low spatial density and high contamination from more ubiquitous Galactic cool dwarfs in quasar ...selection. This limits our current knowledge of the super-massive black hole growth mechanism and reionization history. In this Letter, we report the discovery of a luminous quasar at z = 7.021, DELS J003836.10-152723.6 (hereafter J0038-1527), selected using photometric data from Dark Energy Spectroscopic Instrument Legacy Imaging Survey, Pan-STARRS1 (PS1) imaging Survey, as well as Wide-field Infrared Survey Explore mid-infrared all-sky survey. With an absolute magnitude of M1450 = -27.1 and bolometric luminosity of LBol = 5.6 × 1013 L , J0038-1527 is the most luminous quasar known at z > 7. Deep optical to near-infrared spectroscopic observations suggest that J0038-1527 hosts a 1.3 billion solar mass black hole accreting at the Eddington limit, with an Eddington ratio of 1.25 0.19. The C iv broad emission line of J0038-1527 is blueshifted by more than 3000 km s−1 relative to the quasar systemic redshift. More detailed investigations of the high-quality spectra reveal three extremely high-velocity C iv broad absorption lines with velocity from 0.08 to 0.14 times the speed of light and total "balnicity" index of more than 5000 km s−1, suggesting the presence of relativistic outflows. J0038-1527 is the first quasar found at the epoch of reionization with such strong outflows, and therefore provides a unique laboratory to investigate active galactic nuclei feedback on the formation and growth of the most massive galaxies in the early universe.
We present reverberation-mapping (RM) lags and black hole mass measurements using the C ivλ1549 broad emission line from a sample of 348 quasars monitored as a part of the Sloan Digital Sky Survey RM ...Project. Our data span four years of spectroscopic and photometric monitoring for a total baseline of 1300 days, allowing us to measure lags up to ∼750 days in the observed frame (this corresponds to a rest-frame lag of ∼300 days in a quasar at z = 1.5 and ∼190 days at z = 3). We report significant time delays between the continuum and the C ivλ1549 emission line in 48 quasars, with an estimated false-positive detection rate of 10%. Our analysis of marginal lag measurements indicates that there are on the order of ∼100 additional lags that should be recoverable by adding more years of data from the program. We use our measurements to calculate black hole masses and fit an updated C iv radius-luminosity relationship. Our results significantly increase the sample of quasars with C iv RM results, with the quasars spanning two orders of magnitude in luminosity toward the high-luminosity end of the C iv radius-luminosity relation. In addition, these quasars are located at some of the highest redshifts (z 1.4-2.8) of quasars with black hole masses measured with RM. This work constitutes the first large sample of C iv RM measurements in more than a dozen quasars, demonstrating the utility of multiobject RM campaigns.
The rare case of changing-look (CL) active galactic nuclei (AGNs), with the appearance or disappearance of broad Balmer emission lines within a few years, challenges our understanding of the AGN ...unified model. We present a sample of 21 new CL AGNs at 0.08 < z < 0.58 , which doubles the number of such objects known to date. These new CL AGNs were discovered in various ways, from (1) repeat spectra in the SDSS, (2) repeat spectra in the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) and SDSS, and (3) photometric variability and new spectroscopic observations. We use the photometric data from surveys, including the SDSS imaging survey, the Pan-STARRS1, the DESI Legacy imaging survey, the Wide-field Infrared Survey Explorer (WISE), the Catalina Real-time Transient Survey, and the Palomar Transient Factory. The estimated upper limits of the transition timescale of the CL AGNs in this sample spans from 0.9 to 13 years in the rest-frame. The continuum flux in the optical and mid-infrared becomes brighter when the CL AGNs turn on, or vice versa. Variations of more than 0.2 mag in the W1 band were detected in 15 CL AGNs during the transition. The optical and mid-infrared variability is not consistent with the scenario of variable obscuration in 10 CL AGNs at more than the 3 confidence level. We confirm a bluer-when-brighter trend in the optical. However, the mid-infrared WISE colors W1-W2 become redder when the objects become brighter in the W1 band, possibly due to a stronger hot dust contribution in the W2 band when the AGN activity becomes stronger. The physical mechanism of type transition is important for understanding the evolution of AGNs.
We present accretion disk structure measurements from continuum lags in the Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. Lags are measured using the JAVELIN software from the ...first-year SDSS-RM g and i photometry, resulting in well-defined lags for 95 quasars, 33 of which have lag S/N > 2 . We also estimate lags using the ICCF software and find consistent results, though with larger uncertainties. Accretion disk structure is fit using a Markov chain Monte Carlo approach, parameterizing the measured continuum lags as a function of disk size normalization, wavelength, black hole mass, and luminosity. In contrast with previous observations, our best-fit disk sizes and color profiles are consistent (within 1.5 ) with the Shakura & Sunyaev analytic solution. We also find that more massive quasars have larger accretion disks, similarly consistent with the analytic accretion disk model. The data are inconclusive on a correlation between disk size and continuum luminosity, with results that are consistent with both no correlation and the Shakura & Sunyaev expectation. The continuum lag fits have a large excess dispersion, indicating that our measured lag errors are underestimated and/or our best-fit model may be missing the effects of orientation, spin, and/or radiative efficiency. We demonstrate that fitting disk parameters using only the highest-S/N lag measurements biases best-fit disk sizes to be larger than the disk sizes recovered using a Bayesian approach on the full sample of well-defined lags.
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