Abstract
Motivated by evidences favoring a rapid and late hydrogen reionization process completing at
z
∼ 5.2–5.5 and mainly driven by rare and luminous sources, we have reassessed the estimate of ...the space density of ultra-luminous QSOs at
z
∼ 5 in the framework of the QUBRICS survey. A ∼ 90% complete sample of 14 spectroscopically confirmed QSOs at
M
1450
≤ −28.3 and 4.5 ≤
z
≤ 5.0 has been derived in an area of 12,400 deg
2
, thanks to multiwavelength selection and Gaia astrometry. The space density of
z
∼ 5 QSOs within −29.3 ≤
M
1450
≤ −28.3 is three times higher than previous determinations. Our results suggest a steep bright-end slope for the QSO luminosity function at
z
∼ 5 and a mild redshift evolution of the space density of ultrabright QSOs (
M
1450
∼ −28.5) at 3 <
z
< 5.5, in agreement with the redshift evolution of the much fainter active galactic nucleus (AGN) population at
M
1450
∼ −23. These findings are consistent with a pure density evolution for the AGN population at
z
> 3. Adopting our
z
∼ 4 QSO luminosity function and applying a mild density evolution in redshift, a photoionization rate of
Γ
HI
=
0.46
−
0.09
+
0.17
×
10
−
12
s
−
1
has been obtained at
z
= 4.75, assuming an escape fraction of ∼70% and a steep faint-end slope of the AGN luminosity function. The derived photoionization rate is ∼50–100% of the ionizing background measured at the end of the reionization epoch, suggesting that AGNs could play an important role in the cosmological reionization process.
The first high-redshift changing-look quasars Ross, Nicholas P; Graham, Matthew J; Calderone, Giorgio ...
Monthly notices of the Royal Astronomical Society,
10/2020, Letnik:
498, Številka:
2
Journal Article
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ABSTRACT
We report on three redshift z > 2 quasars with dramatic changes in their C iv emission lines, the first sample of changing-look quasars (CLQs) at high redshift. This is also the first time ...the changing-look behaviour has been seen in a high-ionization emission line. SDSS J1205+3422, J1638+2827, and J2228 + 2201 show interesting behaviour in their observed optical light curves, and subsequent spectroscopy shows significant changes in the C iv broad emission line, with both line collapse and emergence being displayed on rest-frame time-scales of ∼240–1640 d. These are rapid changes, especially when considering virial black hole mass estimates of MBH > 109M⊙ for all three quasars. Continuum and emission line measurements from the three quasars show changes in the continuum-equivalent width plane with the CLQs seen to be on the edge of the full population distribution, and showing indications of an intrinsic Baldwin effect. We put these observations in context with recent state-change models, and note that even in their observed low-state, the C iv CLQs are generally above ∼5 per cent in Eddington luminosity.
Abstract
Based on results by recent surveys, the number of bright quasars at redshifts
z
> 3 is being constantly revised upward. The current consensus is that at bright magnitudes (
M
1450
≤ −27) the ...number densities of such sources could have been underestimated by a factor of 30%–40%. In the framework of the QUBRICS survey, we identified 58 bright QSOs at 3.6 ≤
z
≤ 4.2, with magnitudes
i
psf
≤ 18, in an area of 12400 deg
2
. The uniqueness of our survey is underlined by the fact that it allows us, for the first time, to extend the sampled absolute magnitude range up to
M
1450
= −29.5. We derived a bright-end slope of
β
= −4.025 and a space density at 〈
M
1450
〉 = −28.75 of 2.61 × 10
−10
Mpc
−3
comoving, after taking into account the estimated incompleteness of our observations. Taking into account the results of fainter surveys, active galactic nuclei (AGNs) brighter than
M
1450
= −23 could produce at least half of the ionizing emissivity at
z
∼ 4. Considering a mean escape fraction of 0.7 for the QSO and AGN population, combined with a mean free path of 41.3 proper Mpc at
z
= 3.9, we derive a photoionization rate of
, produced by AGNs at
M
1450
< −18, that is, ∼100% of the measured ionizing background at
z
∼ 4.
Abstract
One of the key open questions in cosmology is the nature of sources that completed cosmological hydrogen reionization at
z
∼ 5.2. High-
z
primeval galaxies have been long considered the main ...drivers of reionization, with a minor role played by high-
z
active galactic nuclei (AGN). However, in order to confirm this scenario, it is fundamental to measure the photoionization rate produced by active SMBHs close to the Epoch of Reionization. Given the pivotal role played by spectroscopically complete observations of high-
z
QSOs, in this paper we present the first results of the RUBICON (Reionizing the Universe with Bright Cosmological Nuclei) survey. It consists of a color-selected sample of bona fide
z
∼ 5 QSO candidates from the Hyper Suprime-Cam Subaru Strategic Survey. Our QSO candidates have been validated both by photometric redshifts based on spectral energy distribution fitting and by spectroscopic redshifts, confirming that they lie at 4.5 <
z
spec
< 5.2. A relatively high space density of QSOs (Φ ∼ 1.4 × 10
−8
c Mpc
−3
) is thus confirmed at
z
∼ 5 and
M
1450
∼ −27, consistent with a pure density evolution of the AGN luminosity function from
z
= 4 to
z
= 5, with a mild density evolution rate of 0.25 dex. This indicates that AGN could play a nonnegligible role in cosmic reionization. The Rubicon of reionization has been crossed.
We investigate the global properties of the radiation emitted by the accretion disk around Kerr black holes. Using the Kerr blackbody numerical model, we build an analytic approximation of the disk ...emission features focusing on the pattern of the produced radiation as a function of the black hole spin, mass, accretion rate and viewing angle. The assumption of having a geometrically thin disk limits our analysis to systems emitting below ~0.3 of the Eddington luminosity. We apply this analytical model to four blazars (whose jets are pointing at us) at high redshift that show clear signatures of disk emission. For them, we derive the black hole masses as a function of spin. If these jetted sources are powered by the black hole rotation, they must have high spin values, further constraining their masses.
ABSTRACT
The number of known, bright (i < 18), high-redshift (z > 2.5) QSOs in the Southern hemisphere is considerably lower than the corresponding number in the Northern hemisphere due to the lack ...of multiwavelength surveys at δ < 0. Recent works, such as the QUBRICS survey, successfully identified new, high-redshift QSOs in the South by means of a machine-learning approach applied on a large photometric data-set. Building on the success of QUBRICS, we present a new QSO selection method based on the Probabilistic Random Forest (PRF), an improvement of the classic Random Forest algorithm. The PRF takes into account measurement errors, treating input data as probability distribution functions: this allows us to obtain better accuracy and a robust predictive model. We applied the PRF to the same photometric data-set used in QUBRICS, based on the SkyMapper DR1, Gaia DR2, 2MASS, WISE, and GALEX databases. The resulting candidate list includes 626 sources with i < 18. We estimate for our proposed algorithm a completeness of ∼84 per cent and a purity of ${\sim}78{{\ \rm per\ cent}}$ on the test data-sets. Preliminary spectroscopic campaigns allowed us to observe 41 candidates, of which 29 turned out to be z > 2.5 QSOs. The performances of the PRF, currently comparable to those of the CCA, are expected to improve as the number of high-z QSOs available for the training sample grows: results are however already promising, despite this being one of the first applications of this method to an astrophysical context.
Virial–based methods for estimating active supermassive black hole masses are now commonly used on extremely large spectroscopic quasar catalogs. Most spectral analyses, though, do not pay enough ...attention to the detailed continuum decomposition. To understand how this affects virial mass estimates, we test the influence of host galaxy light on them, along with a Balmer continuum component. A detailed fit with the new spectroscopic analysis software QSFIT demonstrates that the presence or absence of continuum components does not significantly affect the virial-based results for our sample. Taking a host galaxy component into consideration or not, instead, affects the emission line fitting in a more pronounced way at lower redshifts, where in fact we observe dimmer quasars and more visible host galaxies.
ABSTRACT
The observation of a population of massive black hole binaries (MBHBs) is key for our complete understanding of galaxy mergers and for the characterization of the expected gravitational ...waves (GWs) signal. However, MBHBs still remain elusive with only a few candidates proposed to date. Among these, SDSSJ143016.05 + 230344.4 (’tick-tock’ hereafter) is the only candidate with a remarkably well sampled light curve showing a clear reduction of the modulation period and amplitude over three years of observations. This particular feature has been recently claimed to be the signature of a MBHB that is about to merge. In this paper, we provide an optical follow-up of the tick-tock source using the Rapid Eye Mount (REM) telescope. The decreasing luminosity observed in our follow up is hardly explained within the binary scenario. We speculate about an alternative scenario that might explain the observed light curve through relativistic Lense-Thirring precession of an accretion disc around a single massive black hole.
The study of absorptions along the lines of sight to bright high-z QSOs is an invaluable cosmological tool that provides a wealth of information on the inter-/circum-galactic medium, dark matter, ...primordial elements, reionization, fundamental constants, and general relativity. Unfortunately, the number of bright (i 18) QSOs at z 2 in the southern hemisphere is much lower than that in the north, due to the lack of wide multiwavelength surveys at decl. δ < 0°, hampering the effectiveness of observations from southern observatories. In this work we present a new method based on Canonical Correlation Analysis to identify such objects, taking advantage of a number of available databases: Skymapper, Gaia DR2, WISE, and 2MASS. Our QSO candidate sample lists 1476 sources with i < 18 over 12,400 square degrees in the southern hemisphere. With a preliminary campaign we observed spectroscopically 69 of them, confirming 54 new bright QSOs at z > 2.5, corresponding to a success rate of our method of ∼80%. Furthermore, we estimate a completeness of ∼90% of our sample at completion of our observation campaign. The new QSOs confirmed by this first and the forthcoming campaigns (QUBRICS10 survey) will be the targets of subsequent studies using higher resolution spectrographs, like ESPRESSO, UVES, and (in the long term) ELT/HIRES.
Any project dealing with large astronomical datasets should consider the use of a relational database server (RDBS). Queries requiring quick selections on sky regions, objects cross-matching and ...other high-level data investigations involving sky coordinates could be unfeasible if tables are missing an effective indexing scheme. In this paper we present the Dynamic Index Facility (DIF) software package. By using the HTM and HEALPix sky pixelization schema, it allows a very efficient indexing and management of spherical data stored into MySQL tables. Any table hosting spherical coordinates can be automatically managed by DIF using any number of sky resolutions at the same time. DIF comprises a set of facilities among which SQL callable functions to perform queries on circular and rectangular regions. Moreover, by removing the limitations and difficulties of 2-d data indexing, DIF allows the full exploitation of the RDBS capabilities. Performance tests on Giga-entries tables are reported together with some practical usage of the package.
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