News from the collaboration

March 8, 2023: In a new paper published in MNRAS, we present evidence for rapid evolution in the CIV content of the early Universe which may be linked to hydrogen reionization. The work is lead by Postdoc researcher Rebecca Davies, working at Swinburne University of Technology in Australia.
 
Absoprtion lines due to the triply ionized carbon, C IV,  trace gas in the circumgalactic and intergalactic medium around galaxies. The C IV content of the Universe is sensitive to both the overall carbon content, which is regulated by chemical enrichment and outflows, and the ionization state of carbon, which depends sensitively on the shape and strength of the ultraviolet background. The ultraviolet background is expected to change rapidly near the end of the epoch of reionization, and therefore C IV absorbers are an important probe of this transformative period in the Universe’s history.
 
We analysed a sample of 260 C IV absorbers from the E-XQR-30 metal absorber catalog and found that the C IV content of the Universe increased by a factor of 5 between z~5.8 and z~4.7; a period of only 300 million years (see Figure). This is significantly more rapid than the growth observed at z < 4. The large sample size enabled us to constrain the evolution of the C IV column density distribution function and show that its slope does not change significantly as a function of redshift.
 
Assuming that the carbon content of galaxy haloes evolves as the integral of the cosmic star formation rate density (with some delay due to stellar lifetimes and outflow travel times), we show that chemical evolution alone could plausibly explain the fast decline in C IV over 4.3 < z < 6.3. However, the C IV/C II ratio decreases at the highest redshifts, so the accelerated decline in C IV at z < 5 may be more naturally explained by rapid changes in the gas ionization state driven by evolution of the UV background towards the end of hydrogen reionization.
November 28, 2022: Public release of the XQR-30 metal absorber catalog!
 
A catalog of 778 metal absorption systems in the E-XQR-30 spectra has been published in MNRAS. The absorbers cover a wide range of redshifts (2 < z < 6) and ionization states (including OI, CII, MgII, CIV, SiIV, and NV). The catalogue significantly expands on existing samples of z > 5 absorbers, especially for C IV and Si IV which are important probes of the ionizing photon background at high redshift. The catalog can be downloaded at this GitHub link: https://github.com/XQR-30/Metal-catalogue.
 
The size and diversity of the absorber sample is visualised in the animated figure below. Each frame of the animation adds data for one quasar in the sample. In the bottom panel, each row shows all of the absorption systems detected in one of the six primary ions across all of the E-XQR-30 spectra combined. For each system, the hue represents the energy required to ionize the relevant ion (according to the colour scale on the right). The top panel shows a zoom-in on the redshift range 5.77 < z < 6 and illustrates the range of different kinematics and ionization structures found within the absorber sample.
April 6, 2022: Huanqing Chen, a PhD student at the University of Chicago, is leading the work “Measuring the Density Fields around Bright Quasars at z~6 with XQR-30 Spectra”, which has been just accepted for publication in the Astrophysical Journal. 
 
In this paper, Huanqing and collaborators studied detailed features of Lyman-alpha absorption in the regions adjacent to z~6 quasars and used them to measure the density of the intergalactic medium (IGM). Measuring the density of IGM at z~6 is very challenging, due to the almost completely saturated absorption from neutral hydrogen in the Lyman-alpha forest. However, a bright quasar ionizes its surrounding reducing the neutral fraction of hydrogen near it, and creating a large zone where the Lyman-α absorption is not saturated (called “quasar proximity zone”). 
Applying a novel method she recently developed for quasar proximity zones and tested on simulated quasar spectra, Huanqing and collaborators recovered the continuous large-scale density field at z~6 along 10 quasar lines of sight for the first time. The recovered density fields contain rich information about cosmology and the environment of the first quasars. In particular, the recovered density fields in this study show that on average, bright quasars at z~6 live in overdense environments, with a typical quasar halo mass of the order of 10^12-10^13 solar masses. 

April 5, 2022: The paper “Chemical Abundance of z~6 Quasar Broad-Line Regions in the XQR-30 Sample” has been accepted for publication in MNRAS. This work is led by PhD student, Samuel Lai, working with Dr. Fuyan Bian and Dr. Chiara Mazzucchelli of European Southern Observatory, as well as Prof. Christian Wolf and Dr. Christopher Onken at Australian National University.

The brightest quasars at redshift z ~ 6 reflect some of the most massive and fastest growing black holes with likely the largest host galaxies within the first billion years of the universe since the big bang. The chemical abundance (or metallicity) of these earliest galaxies provides clues regarding their rates of star formation, supernovae frequency, and the resulting chemical evolution. In the low-redshift universe, it’s clear that properties of central black holes and host galaxies are closely intertwined. Though this relationship is not yet convincingly established at high redshift, studying metallicity in galactic nuclear material provides a window to probe the chemical properties of the reservoir of gas from which the black hole accretes.
Our study is made possible by the high signal and high resolution observations of the brightest z ~ 6 quasars by the XQR-30 survey. We were able to take advantage of the high quality spectra to measure the structure and properties of broad emission-lines originating from chemical elements at various stages of ionisation within the hot gas surrounding the black hole.
This research investigated emission lines originating from the quasar broad-line region (BLR) in close proximity to the black hole. For this study, we utilised relationships between metallicity and emission-line flux ratios in the rest-frame ultraviolet determined from photoionisation models. We measured average line ratios which are consistent with composites of low redshift quasar spectra from black holes with similar physical properties, but the line ratios are also strongly correlated with the blueshift of the CIV emission line, an indicator of emission from an outflowing component. If this outflowing contribution is not taken into account, it is possible to measure metallicity over 10 times the solar abundance at high redshift, which may indicate exotic chemical enrichment scenarios or enhanced supernova rates. Rather, our results found the metallicity of the BLR to be at least 2-4 times the solar abundance when the contribution of the blueshifted emission is minimised. While still super-solar, this lower metallicity is more readily reproduced in less extreme chemical evolution models in the galactic nuclear region. Overall, these results suggest that the BLRs of these quasars are already rapidly enriched to several times the solar abundance very early on in the universe. After which, there is no appreciable change in the metallicity for black holes with similar properties over vast swathes of cosmic time.

March 9, 2022: XSHOOTER observations of the XQR-30 Large Programme have been completed!

The last OB (observing block) was observed in the night between 8 and 9 of March 2022. Observations of our Large Programme had started the 7th of April 2019. Due to the COVID pandemic there had been an interruption of operations at the ESO observatories from March 23 to October 21 2020. We are grateful to the ESO telescope operators and night astronomers  for the time they have dedicated to our program!

 

March 3, 2022: Discovery of strong black-hole outflows suppressing black-hole growth in quasars at redshifts 5.8-6.6, based on XQR-30 quasar spectra. Results have been published in Nature (Bischetti M., Feruglio C., D’Odorico V., et al. Suppression of black-hole growth by strong outflows at redshifts 5.8-6.6, 2022, Nature 605, 244–247 (2022). https://doi.org/10.1038/s41586-022-04608-1). This work is led by Postdoc researcher Manuela Bischetti, working at INAF Observatory of Trieste.

Bright quasars, powered by accretion onto billion-solar-mass black holes, are among the most luminous astronomical sources. While we observe that in the nearby Universe black holes and galaxies grow together, we do not know when this common evolution started. In this work, we observe black holes when the Universe was only 0.5-1 Gyr old, and we find that about half of them drive winds with extreme velocities, up to 17% of the speed of light, injecting large amounts of energy into the galaxy medium and slowing down black-hole growth by preventing gas accretion. This is different from what is observed at later epochs, when black-hole winds are neither this frequent nor this energetic. The higher fraction and the extreme speed of black hole winds that we observe indicate that black hole feedback is strong in the young Universe, and may play an important role in shaping the early growth phases of both black holes and galaxies. 
The large investment of time dedicated by the XQR-30 survey to observing z~6 bright quasars and the unique capacities of XSHOOTER in terms of efficiency, wavelength coverage and resolving power have allowed us to obtain very good quality spectra which enabled the interesting result obtained by Dr. Bischetti. 
This research investigated black hole outflows as traced by broad absorption line (BAL) features in the XSHOOTER spectra of XQR-30 quasars.  We found that ~40-50% of the XQR-30 quasars shows the presence of BAL systems against a ~20% observed in SDSS quasars at redshift z~2-3. Furthermore, XQR-30 BAL features have much higher velocities with respect to lower redshift BAL systems implying an injection of energy in their host galaxies which is  ≳20 times higher than the energy produced by quasars observed at later epochs. This will likely inhibit further gas accretion and slow down black-hole growth. BAL systems at these early epochs are dusty and may be caught during an initial quenching phase of obscured accretion. Quasars at the reionization epoch may thus be witnessing the onset of significant black-hole feedback and marking the transition from a black-hole dominated growth phase to the symbiotic growth of black-hole and host-galaxy, as observed in the nearby Universe. 
 
Sketch of a BAL QSOs, showing dust clouds in the BAL outflow, in the nuclear dusty torus and in the host-galaxy
 

September 14, 2021: The first paper based on the XQR-30 quasar spectra has been accepted for publication in the Astrophysical Journal, it is led by a young PhD student, Yongda Zhu, working in the group of Prof. George Becker at the University of California Riverside.

In this paper, Yongda exploits the “dark gap statistics” that is the presence of large portions of the spectrum where the flux is completely absorbed to constrain the properties of the Reionization process that occurred when the Universe was less than 1 Gyr old.
Reionization is the last major phase transition in the history of our Universe, during which most neutral hydrogen atoms in the intergalactic medium (IGM) were ionized by ionizing UV photons from e.g., galaxies. Determining when and how reionization occurred is thus essential for understanding the formation and evolution of galaxies in the early Universe.

This work is based on 55 high-quality spectra of quasars at redshifts 5.5<z<6.5, including new data from the XQR-30 program. The presence of long dark gaps (L≧30 comoving Mpc/h) in the Lyman-alpha forest may indicate neutral hydrogen islands, which are highly opaque to Lyman-alpha photons.
 
The results of the work by Zhu and collaborators are that about 90%, 60%, and 15% of quasar spectra exhibit long dark gaps at z=6.0, 5.8, and 5.6, respectively. The last long dark gaps persist down to z~5.3.
The comparison between these results and the predictions from simulations shows that a fully ionized IGM by z=6 with a homogeneous UV background is disfavored and overall, suggests  that the signature of reionization in the form of islands of neutral hydrogen and/or large-scale fluctuations in the ionizing background remain present in the IGM until at least z ≃ 5.3.
 
dark gaps = contiguous regions of the spectrum where the flux is completely absorbed
Lyman-alpha forest = region of the quasar spectrum at shorter wavelengths with respect to the Lyman-alpha emission line  at the redshift of the quasar populated by a “forest” of HI Lyman-alpha  absorption lines due to intervening material along the line of sight.  
 

 

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