arXiv subjects

Eduardo Bañados

Publications and source records attributed to Eduardo Bañados.

At least 73 records · Page 4Linked to original sources

arXiv2024

IGM damping wing constraints on the tail end of reionisation from the enlarged XQR-30 sample

The attenuation of Ly$α$ photons by neutral hydrogen in the intergalactic medium (IGM) at $z\gtrsim5$ continues to be a powerful probe for studying the epoch of reionisation. Given a framework to estimate the intrinsic (true) Ly$α$ emission of high-$z$ sources, one can infer the ionisation state of the IGM during reionisation. In this work, we use the enlarged XQR-30 sample of 42 high-resolution and high-SNR QSO spectra between $5.8\lesssim\,z\lesssim\,6.6$ obtained with VLT/X-Shooter to place constraints on the IGM neutral fraction. This is achieved using our existing Bayesian QSO reconstruction framework which accounts for uncertainties such as the: (i) posterior distribution of predicted intrinsic Ly$α$ emission profiles (obtained via covariance matrix reconstruction of the Ly$α$ and N V emission lines from unattenuated high-ionisation emission line profiles; C IV, Si IV + O IV] and C III]) and (ii) distribution of ionised regions within the IGM using synthetic damping wing profiles drawn from a $1.6^3$ Gpc$^3$ reionisation simulation. Following careful quality control, we used 23 of the 42 available QSOs to obtain constraints/limits on the IGM neutral fraction during the tail-end of reionisation. Our median and 68th percentile constraints on the IGM neutral fraction are: $0.20\substack{+0.14\\-0.12}$ and $0.29\substack{+0.14\\-0.13}$ at $z = 6.15$~and 6.35. Further, we also report 68th percentile upper-limits of $\bar{x}_{\mathrm{H\,{\scriptscriptstyle I}}} < 0.21$, 0.20, 0.21 and 0.18 at $z = 5.8, 5.95, 6.05$~and 6.55. These results imply reionisation is still ongoing at $5.8\lesssim\,z\lesssim\,6.55$, consistent with previous results from XQR-30 (dark fraction and Ly$α$ forest) along with other observational probes considered in the literature.

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arXiv2024

Exploring the Mpc Environment of the Quasar ULAS J1342+0928 at z = 7.54

Theoretical models predict that z~6 quasars are hosted in the most massive halos of the underlying dark matter distribution and thus would be immersed in protoclusters of galaxies. However, observations report inconclusive results. We investigate the 1.1 pMpc2 environment of the z = 7.54 luminous quasar ULAS J1342+0928. We search for Lyman-break galaxy candidates (LBG) using deep imaging from the Hubble Space Telescope (HST) in the ACS/F814W, WFC3/F105W/F125W bands, and Spitzer/IRAC at 3.6 $μ$m and 4.5 $μ$m. We report a zphot = $7.69^{+0.33}_{-0.23}$ LBG with magF125W = 26.41 at 223 projected-pkpc from the quasar. We find no HST counterpart to one [CII]-emitter previously found with ALMA at 27 projected-pkpc and $z[CII]=7.5341\pm0.0009$ (Venemans et al. 2020). We estimate the completeness of our LBG candidates using results from CANDELS/GOODS deep blank field searches sharing a similar filter setup. We find that >50% of the z~7.5 LBGs with magF125W >25.5 are missed due to the absence of a filter redward of the Lyman-break in F105W, hindering the UV color accuracy of the candidates. We conduct a QSO-LBG clustering analysis revealing a low LBG excess of $0.46^{+1.52}_{-0.08}$ in this quasar field, consistent with an average or low-density field. Consequently, this result does not present strong evidence of an LBG overdensity around ULAS J1342+0928. Furthermore, we identify two LBG candidates with a zphot matching a confirmed z=6.84 absorber along the line-of-sight to the quasar. All these galaxy candidates are excellent targets for follow-up observations with JWST and/or ALMA to confirm their redshift and physical properties.

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arXiv2024

The cold molecular gas in z$\gtrsim$6 quasar host galaxies

Probing the molecular gas reservoirs of z>~6 quasar (QSO) host galaxies is fundamental to understanding the coevolution of star formation and black hole growth in these extreme systems. Yet, there is still an inhomogeneous coverage of molecular gas tracers. To measure the average excitation and mass of the molecular gas reservoirs in the brightest z>6.5 QSO hosts, we combined new observations of CO(2-1) emission with existing observations of CO(6-5), CO(7-6), [C I], [C II], and dust-continuum emission. We reduced and analysed the VLA observations of CO(2-1) in three z=6.5-6.9 QSO hosts -- the highest redshift observations of CO(2-1) to date. By combining these with the nine z=5.7-6.4 QSO hosts for which CO(2-1) has already been observed, we studied the spread in molecular gas masses and CO excitation. Two of our three QSOs, were undetected in CO(2-1), implying more highly excited CO than in the z=6.4 QSO J1148+5251. We detected CO(2-1) at $5.1σ$ for our highest-redshift target, J2348-3054, yielding a molecular gas mass of $(1.2\pm0.2)\times 10^{10}\, \mathrm{M}_\odot$. This molecular gas mass is equivalent to the lower limit on the dynamical mass measured from resolved [C II] observations, implying little mass in stars or neutral gas within the [C II]-emitting region. On average, these QSO hosts have far higher CO(6-5)-, CO(7-6)-, and [C II] vs CO(2-1) line ratios than local AGN hosts; with a mean CO(6-5)-to-CO(1-0) line luminosity ratio of $r_{6,1}=0.9\pm0.2$. Our new CO(2-1) observations show that even at 780 Myr after the Big Bang, QSO host galaxies can already have molecular gas masses of $10^{10}$ M$_\odot$, consistent with a picture in which these z>6 QSOs reside in massive starbursts that are coevolving with the supermassive black holes. Our results imply the presence of extremely dense and warm molecular gas reservoirs illuminated by strong interstellar radiation fields.

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arXiv2024

Quasar Island -- Three new $z\sim6$ quasars, including a lensed candidate, identified with contrastive learning

Of the hundreds of $z\gtrsim6$ quasars discovered to date, only one is known to be gravitationally lensed, despite the high lensing optical depth expected at $z\gtrsim6$. High-redshift quasars are typically identified in large-scale surveys by applying strict photometric selection criteria, in particular by imposing non-detections in bands blueward of the Lyman-$α$ line. Such procedures by design prohibit the discovery of lensed quasars, as the lensing foreground galaxy would contaminate the photometry of the quasar. We present a novel quasar selection methodology, applying contrastive learning (an unsupervised machine learning technique) to Dark Energy Survey imaging data. We describe the use of this technique to train a neural network which isolates an 'island' of 11 sources, of which 7 are known $z\sim6$ quasars. Of the remaining four, three are newly discovered quasars (J0109-5424, $z=6.07$; J0122-4609, $z=5.99$; J0603-3923, $z=5.94$), as confirmed by follow-up Gemini-South/GMOS and archival NTT/EFOSC2 spectroscopy, implying a 91 per cent efficiency for our novel selection method; the final object on the island is a brown dwarf. In one case (J0109-5424), emission below the Lyman limit unambiguously indicates the presence of a foreground source, though high-resolution optical/near-infrared imaging is still needed to confirm the quasar's lensed (multiply-imaged) nature. Detection in the g band has led this quasar to escape selection by traditional colour cuts. Our findings demonstrate that machine learning techniques can thus play a key role in unveiling populations of quasars missed by traditional methods.

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arXiv2024

A quasar-galaxy merger at $z\sim6.2$: black hole mass and quasar properties from the NIRSpec spectrum

We present JWST/NIRSpec integral field data of the quasar PJ308-21 at $z=6.2342$. As shown by previous ALMA and HST imaging, the quasar has two companion sources, interacting with the quasar host galaxy. The high-resolution G395H/290LP NIRSpec spectrum covers the $2.87-5.27\ \rm μm$ wavelength range and shows the rest-frame optical emission of the quasar with exquisite quality ($S/N\sim 100-400$ per spectral element). Based on the H$β$ line from the broad line region, we obtain an estimate of the black hole mass $M_{\rm BH,Hβ}\sim 2.7\times 10^{9}\ \rm M_{\odot}$. This value is within a factor $\lesssim 1.5$ of the H$α$-based black hole mass from the same spectrum ($M_{\rm BH, Hα}\sim 1.93\times 10^{9}\ \rm M_{\odot}$) and is consistent with a previous estimate relying on the MgII $λ2799$ ($M_{\rm BH, MgII}\sim 2.65\times 10^{9}\ \rm M_{\odot}$). All these $M_{\rm BH}$ are within the $\sim 0.5$ dex intrinsic scatter of the adopted mass calibrations. The high Eddington ratio of PJ308-21 $λ_{\rm Edd,Hβ}\sim 0.67$ ($λ_{\rm Edd,Hα}\sim 0.96$) is in line with the overall quasar population at $z \gtrsim 6$. The relative strengths of the [OIII], FeII and H$β$ lines are consistent with the empirical "Eigenvector 1" correlations as observed for low redshift quasars. We find evidence for blueshifted [OIII] $λ5007$ emission with a velocity offset $Δv_{\rm [OIII]}=-1922\pm 39$ km s$^{-1}$ from the systemic velocity and a $\rm FWHM([OIII])=2776^{+75}_{-74}$ km s$^{-1}$. This may be the signature of an outflow from the nuclear region, despite the true values of $Δv_{\rm [OIII]}$ and $\rm FWHM([OIII])$ are likely more uncertain due to the blending with H$β$ and FeII lines. Our study demonstrates the unique capabilities of NIRSpec in capturing quasar spectra at cosmic dawn and studying their properties in unprecedented detail.

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arXiv2024

A Massive Protocluster Anchored by a Luminous Quasar at $z=6.63$

Protoclusters, the progenitors of galaxy clusters, trace large scale structures in the early Universe and are important to our understanding of structure formation and galaxy evolution. To date, only a handful of protoclusters have been identified in the Epoch of Reionization (EoR). As one of the rarest populations in the early Universe, distant quasars that host active supermassive black holes are thought to reside in the most massive dark matter halos at that cosmic epoch, and could thus potentially pinpoint some of the earliest protoclusters. In this letter, we report the discovery of a massive protocluster around a luminous quasar at $z=6.63$. This protocluster is anchored by the quasar, and includes three [CII] emitters at $z\sim6.63$, 12 spectroscopically confirmed Ly$α$ emitters (LAEs) at $6.54 30$ in its central region (i.e., $R\lesssim2$ cMpc). We estimate that this protocluster will collapse into a galaxy cluster with a mass of $6.9^{+1.2}_{-1.4}\times10^{15}~M_\odot$ at the current epoch, more massive than the most massive clusters known in the local Universe such as Coma. In the quasar vicinity, we discover a double-peaked LAE which implies that the quasar has a UV lifetime greater than 0.8 Myrs and has already ionized its surrounding intergalactic medium.

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arXiv2024

Diverse molecular gas excitations in quasar host galaxies at z \sim 6

We present observations using the NOrthern Extended Millimetre Array (NOEMA) of CO and $\rm H_{2}O$ emission lines, and the underlying dust continuum in two quasars at $z \sim 6$, i.e., P215-16 at $z$ = 5.78 and J1429+5447 at $z$ = 6.18. Notably, among all published CO SLEDs of quasars at $z \sim 6$, the two systems reveal the highest and the lowest CO level of excitation, respectively. Our radiative transfer modeling of the CO SLED of P215-16 suggests that the molecular gas heated by AGN could be a plausible origin for the high CO excitation. For J1429+5447, we obtain the first well-sampled CO SLED (from transitions from 2-1 to 10-9) of a radio-loud quasar at $z\gtrsim 6$. Analysis of the CO SLED suggests that a single photo-dissociation region (PDR) component could explain the CO excitation in the radio-loud quasar J1429+5447. This work highlights the utility of the CO SLED in uncovering the ISM properties in these young quasar-starburst systems at the highest redshift. The diversity of the CO SLEDs reveals the complexities in gas conditions and excitation mechanisms at their early evolutionary stage.

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arXiv2023

Possible Contamination of the Intergalactic Medium Damping Wing in ULAS J1342+0928 by Proximate Damped Ly$α$ Absorption

The red damping wing from neutral hydrogen in the intergalactic medium is a smoking-gun signal of ongoing reionization. One potential contaminant of the intergalactic damping wing signal is dense gas associated with foreground galaxies, which can give rise to proximate damped Ly$α$ absorbers. The Ly$α$ imprint of such absorbers on background quasars is indistinguishable from the intergalactic medium within the uncertainty of the intrinsic quasar continuum, and their abundance at $z\gtrsim7$ is unknown. Here we show that the complex of low-ionization metal absorption systems recently discovered by deep JWST/NIRSpec observations in the foreground of the $z=7.54$ quasar ULAS~J1342$+$0928 can potentially reproduce the quasar's spectral profile close to rest-frame Ly$α$ without invoking a substantial contribution from the intergalactic medium, but only if the absorbing gas is extremely metal-poor ($[{\rm O}/{\rm H}]\sim-3.5$). Such a low oxygen abundance has never been observed in a damped Ly$α$ absorber at any redshift, but this possibility still complicates the interpretation of the spectrum. Our analysis highlights the need for deep spectroscopy of high-redshift quasars with JWST or ELT to "purify" damping wing quasar samples, an exercise which is impossible for much fainter objects like galaxies.

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arXiv2023

Uncovering the First AGN Jets with AXIS

Jets powered by AGN in the early Universe ($z \gtrsim 6$) have the potential to not only define the trajectories of the first-forming massive galaxies but to enable the accelerated growth of their associated SMBHs. Under typical assumptions, jets could even rectify observed quasars with light seed formation scenarios; however, not only are constraints on the parameters of the first jets lacking, observations of these objects are scarce. Owing to the significant energy density of the CMB at these epochs capable of quenching radio emission, observations will require powerful, high angular resolution X-ray imaging to map and characterize these jets. As such, \textit{AXIS} will be necessary to understand early SMBH growth and feedback.

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arXiv2023

A SPectroscopic survey of biased halos In the Reionization Era (ASPIRE): JWST Discovers an Overdensity around a Metal Absorption-selected Galaxy at $z\sim5.5$

The launch of ${\it JWST}$ opens a new window for studying the connection between metal-line absorbers and galaxies at the end of the Epoch of Reionization (EoR). Previous studies have detected absorber-galaxy pairs in limited quantities through ground-based observations. To enhance our understanding of the relationship between absorbers and their host galaxies at $z>5$, we utilized the NIRCam Wide Field Slitless Spectroscopy (WFSS) to search for absorber-associated galaxies by detecting their rest-frame optical emission lines (e.g., [OIII] + H$β$). We report the discovery of a MgII-associated galaxy at $z=5.428$ using data from the ${\it JWST}$ ASPIRE program. The MgII absorber is detected on the spectrum of quasar J0305--3150 with a rest-frame equivalent width of 0.74$\mathring{A}$. The associated galaxy has an [OIII] luminosity of $10^{42.5}\ {\rm erg\ s^{-1}}$ with an impact parameter of 24.9 proper kiloparsecs (pkpc). The joint ${\it HST}$-${\it JWST}$ spectral energy distribution (SED) implies a stellar mass and star-formation rate of ${\rm M_* \approx 10^{8.8}}$ ${\rm M_{\odot}}$, ${\rm SFR}\approx 10\ {\rm M_{\odot}\ yr^{-1}}$. Its [OIII] equivalent width and stellar mass are typical of [OIII] emitters at this redshift. Furthermore, connecting the outflow starting time to the SED-derived stellar age, the outflow velocity of this galaxy is $\sim300\ {\rm km\ s^{-1}}$, consistent with theoretical expectations. We identified six additional [OIII] emitters with impact parameters of up to $\sim300$ pkpc at similar redshifts ($|dv|<1000\ {\rm km\ s^{-1}}$). The observed number is consistent with that in cosmological simulations. This pilot study suggests that systematically investigating the absorber-galaxy connection within the ASPIRE program will provide insights into the metal-enrichment history in the early universe.

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arXiv2023

First measurement of the Mg II forest correlation function in the Epoch of Reionization

In the process of producing the roughly three ionizing photons per atom required to reionize the IGM, the same massive stars explode and eject metals into their surroundings. While the overly sensitive Lya transition makes Gunn-Peterson absorption of background quasar light an ineffective probe of reionization at z > 6, strong low-ionization transitions like the MgII doublet will give rise to a detectable "metal-line forest", if metals pollute the neutral IGM. We measure the auto-correlation of the MgII forest transmission using a sample of ten ground based z >= 6.80 quasar spectra probing the redshift range 5.96 < z_MgII < 7.42 (z_MgII,median = 6.47). The correlation function exhibits strong small-scale clustering and a pronounced peak at the doublet velocity (768 km/s) arising from strong absorbers in the CGM of galaxies. After these strong absorbers are identified and masked the signal is consistent with noise. Our measurements are compared to a suite of models generated by combining a large hydrodynamical simulation with a semi-numerical reionization topology, assuming a simple uniform enrichment model. We obtain a 95% credibility upper limit of [Mg/H] < -3.73 at z_MgII,median = 6.47, assuming uninformative priors on [Mg/H] and the IGM neutral fraction x_HI. Splitting the data into low-z (5.96 < z_MgII < 6.47; z_MgII,median = 6.235) and high-z (6.47 < z_MgII < 7.42; z_MgII,median = 6.72) subsamples again yields null-detections and 95% upper limits of [Mg/H] < -3.75 and [Mg/H] < -3.45, respectively. These first measurements set the stage for an approved JWST Cycle 2 program (GO 3526) targeting a similar number of quasars that will be an order of magnitude more sensitive, making the Mgii forest an emerging powerful tool to deliver precision constraints on the reionization and enrichment history of the Universe.

astro-ph.CO↗

arXiv2023

New quasar proximity zone size measurements at $z\sim 6$ using the enlarged XQR-30 sample

Proximity zones of high-redshift quasars are unique probes of their central supermassive black holes as well as the intergalactic medium in the last stages of reionization. We present 22 new measurements of proximity zones of quasars with redshifts between 5.8 and 6.6, using the enlarged XQR-30 sample of high-resolution, high-SNR quasar spectra. The quasars in our sample have UV magnitudes of $M_{1450}\sim -27$ and black hole masses of $10^9$$\unicode{x2013}$$10^{10}$ M$_\odot$. Our inferred proximity zone sizes are 2$\unicode{x2013}$7 physical Mpc, with a typical uncertainty of less than 0.5 physical Mpc, which, for the first time, also includes uncertainty in the quasar continuum. We find that the correlation between proximity zone sizes and the quasar redshift, luminosity, or black hole mass, indicates a large diversity of quasar lifetimes. Two of our proximity zone sizes are exceptionally small. The spectrum of one of these quasars, with $z=6.02$, displays, unusually for this redshift, damping wing absorption without any detectable metal lines, which could potentially originate from the IGM. The other quasar has a high-ionization absorber $\sim$0.5 pMpc from the edge of the proximity zone. This work increases the number of proximity zone measurements available in the last stages of cosmic reionization to 87. This data will lead to better constraints on quasar lifetimes and obscuration fractions at high redshift, which in turn will help probe the seed mass and formation redshift of supermassive black holes.

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arXiv2023

A SPectroscopic survey of biased halos In the Reionization Era (ASPIRE): A First Look at the Rest-frame Optical Spectra of $z > 6.5$ Quasars Using JWST

Studies of rest-frame optical emission in quasars at $z>6$ have historically been limited by the wavelengths accessible by ground-based telescopes. The James Webb Space Telescope (JWST) now offers the opportunity to probe this emission deep into the reionization epoch. We report the observations of eight quasars at $z>6.5$ using the JWST/NIRCam Wide Field Slitless Spectroscopy, as a part of the ''A SPectroscopic survey of biased halos In the Reionization Era (ASPIRE)" program. Our JWST spectra cover the quasars' emission between rest frame $\sim$ 4100 and 5100 Å. The profiles of these quasars' broad H$β$ emission lines span a FWHM from 3000 to 6000 $\rm{km~s^{-1}}$. The H$β$-based virial black hole (BH) masses, ranging from 0.6 to 2.1 billion solar masses, are generally consistent with their MgII-based BH masses. The new measurements based on the more reliable H$β$ tracer thus confirm the existence of billion solar-mass BHs in the reionization epoch. In the observed [OIII] $λλ$4960,5008 doublets of these luminous quasars, broad components are more common than narrow core components ($\le~1200~\rm{km~s^{-1}}$), and only one quasar shows stronger narrow components than broad. Two quasars exhibit significantly broad and blueshifted [OIII] emission, thought to trace galactic-scale outflows, with median velocities of $-610~\rm{km~s^{-1}}$ and $-1430~\rm{km~s^{-1}}$ relative to the [CII] $158\,μ$m line. All eight quasars show strong optical FeII emission, and follow the Eigenvector 1 relations defined by low-redshift quasars. The entire ASPIRE program will eventually cover 25 quasars and provide a statistical sample for the studies of the BHs and quasar spectral properties.

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arXiv2023

A SPectroscopic survey of biased halos In the Reionization Era (ASPIRE): JWST Reveals a Filamentary Structure around a z=6.61 Quasar

We present the first results from the JWST ASPIRE program (A SPectroscopic survey of biased halos In the Reionization Era). This program represents an imaging and spectroscopic survey of 25 reionization-era quasars and their environments by utilizing the unprecedented capabilities of NIRCam Wide Field Slitless Spectroscopy (WFSS) mode. ASPIRE will deliver the largest ($\sim280~{\rm arcmin}^2$) galaxy redshift survey at 3-4 $μ$m among JWST Cycle-1 programs and provide extensive legacy values for studying the formation of the earliest supermassive black holes (SMBHs), the assembly of galaxies, early metal enrichment, and cosmic reionization. In this first ASPIRE paper, we report the discovery of a filamentary structure traced by the luminous quasar J0305-3150 and ten [OIII] emitters at $z=6.6$. This structure has a 3D galaxy overdensity of $δ_{\rm gal}=12.6$ over 637 cMpc$^3$, one of the most overdense structures known in the early universe, and could eventually evolve into a massive galaxy cluster. Together with existing VLT/MUSE and ALMA observations of this field, our JWST observations reveal that J0305-3150 traces a complex environment where both UV-bright and dusty galaxies are present, and indicate that the early evolution of galaxies around the quasar is not simultaneous. In addition, we discovered 31 [OIII] emitters in this field at other redshifts, $5.3<z<6.7$, with half of them situated at $z\sim5.4$ and $z\sim6.2$. This indicates that star-forming galaxies, such as [OIII] emitters, are generally clustered at high redshifts. These discoveries demonstrate the unparalleled redshift survey capabilities of NIRCam WFSS and the potential of the full ASPIRE survey dataset.

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arXiv2023

On the recent discovery claim of a new $z>7$ quasar

Koptelova et al. 2022 (K22) recently claimed a new quasar discovery at $z=7.46$. After careful consideration of the publicly-available data underlying K22's claim, we find that the observations were contaminated by a moving Solar System object, likely a main-belt asteroid. In the absence of the contaminated photometry, there is no evidence for the nearby, persistent WISE source being a high-redshift object; in fact, a detection of the source in DELS $z$-band rules out a redshift $z>7.3$. We present our findings as a cautionary tale of the dangers of passing asteroids for photometric selections.

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arXiv2023

The Pan-STARRS1 $\mathbf{z>5.6}$ Quasar Survey: III. The $\mathbf{z\approx6}$ Quasar Luminosity Function

We present the $z\!\approx\!6$ type-1 quasar luminosity function (QLF) based on the Pan-STARRS1 (PS1) quasar survey. The PS1 sample includes 125 quasars at $z\approx5.7-6.2$ with $-28\lesssim M_{1450}\lesssim-25$. Complemented by 48 fainter quasars from the SHELLQs survey, we evaluate the $z\approx6$ QLF over $-28\lesssim M_{1450}\lesssim-22$. Adopting a double power law with an exponential evolution of the quasar density ($Φ(z)\propto10^{k(z-6)}$; $k=-0.7$), we use a maximum likelihood method to model our data. We find a break magnitude of $M^*=-26.38_{-0.60}^{+0.79}\,\text{mag}$, a faint end slope of $α=-1.70_{-0.19}^{+0.29}$, and a steep bright end slope with $β=-3.84_{-1.21}^{+0.63}$. Based on our new QLF model we determine the quasar comoving spatial density at $z\!\approx\!6$ to be $n( M_{1450}<-26)=1.16_{-0.12}^{+0.13}\,\text{cGpc}^{-3}$. In comparison with the literature, we find the quasar density to evolve with a constant value of $k\approx-0.7$ from $z\approx7$ to $z\approx4$. Additionally, we derive an ionizing emissivity of $ε_{912}(z=6) =7.23_{-1.02}^{+1.65}\times 10^{22}\,\text{erg}\,\text{s}^{-1}\text{Hz}^{-1}\text{cMpc}^{-3}$ based on the QLF measurement. Given standard assumptions and the recent measurement of the mean free path of Becker et al. (2021) at $z\approx6$ we calculate an HI photoionizing rate of $Γ_{\text{HI}}(z{=}6) \approx 6 \times10^{-16}\,\text{s}^{-1}$, strongly disfavoring a dominant role of quasars in hydrogen reionization.

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arXiv2023

When Spectral Modeling Meets Convolutional Networks: A Method for Discovering Reionization-era Lensed Quasars in Multi-band Imaging Data

Over the last two decades, around 300 quasars have been discovered at $z\gtrsim6$, yet only one has identified as being strongly gravitationally lensed. We explore a new approach -- enlarging the permitted spectral parameter space, while introducing a new spatial geometry veto criterion -- which is implemented via image-based deep learning. We first apply this approach to a systematic search for reionization-era lensed quasars, using data from the Dark Energy Survey, the Visible and Infrared Survey Telescope for Astronomy Hemisphere Survey, and the Wide-field Infrared Survey Explorer.Our search method consists of two main parts: (i) the preselection of the candidates based on their spectral energy distributions (SEDs) using catalog-level photometry and (ii) relative probabilities calculation of the candidates being a lens or some contaminant, utilizing a convolutional neural network (CNN) classification. The training data sets are constructed by painting deflected point-source lights over actual galaxy images, to generate realistic galaxy-quasar lens models, optimized to find systems with small image separations, i.e., Einstein radii of $θ_\mathrm{E} \leq 1$ arcsec. Visual inspection is then performed for sources with CNN scores of $P_\mathrm{lens} > 0.1$, which leads us to obtain 36 newly selected lens candidates, which are awaiting spectroscopic confirmation. These findings show that automated SED modeling and deep learning pipelines, supported by modest human input, are a promising route for detecting strong lenses from large catalogs that can overcome the veto limitations of primarily dropout-based SED selection approaches.

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arXiv2022

High-$z$ Quasar Candidate Archive: A Spectroscopic Catalog of Quasars and Contaminants in Various Quasar Searches

We present the high-$z$ quasar candidate archive (HzQCA), summarizing the spectroscopic observations of 174 $z\gtrsim5$ quasar candidates using Keck/LRIS, Keck/MOSFIRE, and Keck/NIRES. We identify 7 candidates as $z\sim 6$ quasars 3 of them newly reported here, and 51 candidates as brown dwarfs. In the remaining sources, 74 candidates are unlikely to be quasars; 2 sources are inconclusive; the others could not be fully reduced or extracted. Based on the classifications we investigate the distributions of quasars and contaminants in color space with photometry measurements from DELS ($z$), VIKING/UKIDSS ($YJHK_s$/$YJHK$), and un\textit{WISE} ($W1W2$). We find that the identified brown dwarfs are not fully consistent with the empirical brown dwarf model that is commonly used in quasar candidate selection methods. To refine spectroscopic confirmation strategies, we simulate synthetic spectroscopy of high-$z$ quasars and contaminants for all three instruments. The simulations utilize the spectroscopic data in HzQCA. We predict the required exposure times for quasar confirmation and propose and optimal strategy for spectroscopic follow-up observations. For example, we demonstrate that we can identify a $m_J=21.5$ at $z=7.6$ or a $m_J=23.0$ at $z=7.0$ within 15\,min of exposure time with LRIS. With the publication of the HzQCA we aim to provide guidance for future quasar surveys and candidate classification.

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