Physical and projected pairs of quasars.

Quasars are the most luminous type of active galactic nuclei and their study is fundamental to shed light on the high redshift Universe. In this work we take advantage of the rare cases of close projected and physical quasar pairs to detect the presence of the cool enriched gas and of the dark mass that surround quasars. This Thesis consists of two parts. In the first one we present optical spectroscopy of projected quasar pairs (i.e., two quasars that are close in the sky but have discordant redshifts) that represents a powerful tool to investigate the metal absorption lines imprinted on the spectrum of the background quasar by the gaseous halo surrounding the foreground one (Chapter 1). In the last decades, this technique has been extensively used to detect the haloes of non\u2013active galaxies, but to date no systematic study has been performed in the case of quasars. We selected a sample of 46 projected pairs, with separations ranging from 3c 50 kpc to 3c 200 kpc, aimed to assess the presence of Mg II and CIV absorptions systems associated to foreground quasars. In Chapter 2 we show the first outcomes of our programme, based on the spectra of 13 pairs partially collected by me at the ESO Very Large Telescope (VLT). These data reveal a high covering fraction for the Mg II absorbers and a non isotropical distribution of the gas. Our results are compared to those of similar works performed on non\u2013active galaxies available in the literature. We propose that the cool gaseous haloes of quasars and of normal galaxies are similar, once the stellar mass of the systems is taken into account. Observations of the remaining 33 pairs are ongoing at the Gran Telescopio Canarias (GTC). The preparatory analysis of the SDSS spectra of these systems show that a 10m class telescope is mandatory for this kind of studies (Chapter 3). The second part of the Thesis addresses with physical systems of quasars (i.e., two or more quasars that have small projected and redshift separations) that are an expected outcome of the current hierarchical framework of galaxy formation and evolution (Chapter 4). In Chapter 5 we accurately study the dynamics of six low redshift quasar pairs and we found that the dynamical mass required for this systems to be gravitationally bound is larger of than the stellar masses of the host galaxies. This suggests that these pairs are hosted by galaxies with massive dark haloes or that they reside in a group/cluster of galaxies. In Chapter 6, we present our discovery of the second physical quasar triplet known to date. We estimate that these systems are extremely rare in terms of simple accidental superposition of the three quasars, suggesting a possible common origin for their ignition. Finally, conclusions and future perspectives of this study are presented in Chapter 7. The present work principally deals with the analysis and studies presented in Farina et al. (2012b, on the gaseous haloes of quasars), in Farina et al. (2011, on the dynamical study of low redshift quasar pairs), and in Farina et al. (2012c, on the discovery of a new quasar triplet). A complete list of works published during my PhD is provided in Appendix C

Physical and projected pairs of quasars.

Quasars are the most luminous type of active galactic nuclei and their study is fundamental to shed light on the high redshift Universe. In this work we take advantage of the rare cases of close projected and physical quasar pairs to detect the presence of the cool enriched gas and of the dark mass that surround quasars. This Thesis consists of two parts. In the first one we present optical spectroscopy of projected quasar pairs (i.e., two quasars that are close in the sky but have discordant redshifts) that represents a powerful tool to investigate the metal absorption lines imprinted on the spectrum of the background quasar by the gaseous halo surrounding the foreground one (Chapter 1). In the last decades, this technique has been extensively used to detect the haloes of non–active galaxies, but to date no systematic study has been performed in the case of quasars. We selected a sample of 46 projected pairs, with separations ranging from ∼ 50 kpc to ∼ 200 kpc, aimed to assess the presence of Mg II and CIV absorptions systems associated to foreground quasars. In Chapter 2 we show the first outcomes of our programme, based on the spectra of 13 pairs partially collected by me at the ESO Very Large Telescope (VLT). These data reveal a high covering fraction for the Mg II absorbers and a non isotropical distribution of the gas. Our results are compared to those of similar works performed on non–active galaxies available in the literature. We propose that the cool gaseous haloes of quasars and of normal galaxies are similar, once the stellar mass of the systems is taken into account. Observations of the remaining 33 pairs are ongoing at the Gran Telescopio Canarias (GTC). The preparatory analysis of the SDSS spectra of these systems show that a 10m class telescope is mandatory for this kind of studies (Chapter 3). The second part of the Thesis addresses with physical systems of quasars (i.e., two or more quasars that have small projected and redshift separations) that are an expected outcome of the current hierarchical framework of galaxy formation and evolution (Chapter 4). In Chapter 5 we accurately study the dynamics of six low redshift quasar pairs and we found that the dynamical mass required for this systems to be gravitationally bound is larger of than the stellar masses of the host galaxies. This suggests that these pairs are hosted by galaxies with massive dark haloes or that they reside in a group/cluster of galaxies. In Chapter 6, we present our discovery of the second physical quasar triplet known to date. We estimate that these systems are extremely rare in terms of simple accidental superposition of the three quasars, suggesting a possible common origin for their ignition. Finally, conclusions and future perspectives of this study are presented in Chapter 7. The present work principally deals with the analysis and studies presented in Farina et al. (2012b, on the gaseous haloes of quasars), in Farina et al. (2011, on the dynamical study of low redshift quasar pairs), and in Farina et al. (2012c, on the discovery of a new quasar triplet). A complete list of works published during my PhD is provided in Appendix C

Lyα\alpha Halos Around z∼6z\sim6 Quasars

We present deep MUSE observations of five quasars within the first Gyr of the Universe ($z\gtrsim6$), four of which display extended Ly$\alpha$ halos. After PSF-subtraction, we reveal halos surrounding two quasars for the first time, as well as confirming the presence of two more halos for which tentative detections exist in long-slit spectroscopic observations and narrow-band imaging. The four Ly$\alpha$ halos presented here are diverse in morphology and size, they each display spatial asymmetry, and none are centred on the position of the quasar. Spectra of the diffuse halos demonstrate that none are dramatically offset in velocity from the systemic redshift of the quasars ($\Delta$ v $< 200$ kms$^{-1}$), however each halo shows a broad Ly$\alpha$ line, with a velocity width of order $\sim1000$ kms$^{-1}$. Total Ly$\alpha$ luminosities range between $\sim$ $2 \times 10^{43}$ erg s$^{-1}$ and $\sim$ $2 \times 10^{44}$ erg s$^{-1}$, reaching maximum radial extents of $13 - 30$ pkpc from the quasar positions. We find larger sizes and higher Ly$\alpha$ luminosities than previous literature results at this redshift, but find no correlation between the quasar properties and the Ly$\alpha$ halo, suggesting that the detected emission is most closely related to the physical properties of the circum-galactic mediumComment: 19 pages, Accepted for publication in Ap

X-ray Observations of a [C II]-bright, z=6.59 Quasar/Companion System

We present deep Chandra observations of PSO J231.6576$-$20.8335, a quasar at redshift z=6.59 with a nearby (${\sim}8$ proper kpc) companion galaxy. ALMA observed both the quasar and companion to be bright in [C II], and the system has significant extended Ly$\alpha$ emission around the quasar, suggesting that a galaxy merger is ongoing. Unlike previous studies of two similar systems, and despite observing the system with Chandra for 140 ks, we do not detect the companion in X-rays. The quasar itself is detected, but only $13.3^{+4.8}_{-3.7}$ net counts are observed. From a basic spectral analysis, the X-ray spectrum of the quasar is soft (hardness ratio of $\mathcal{HR} = -0.60_{-0.27}^{+0.17}$, power-law index of $\Gamma=2.6^{+1.0}_{-0.9}$), which results in a rest-frame X-ray luminosity comparable to other bright quasars ($L_{2-10} = 1.09^{+2.20}_{-0.70}\times 10^{45}\ \textrm{erg}\ \textrm{s}^{-1}$) despite the faint observed X-ray flux. We highlight two possible interpretations of this result: the quasar has a steep value of$\Gamma$-- potentially related to observed ongoing Eddington accretion -- thereby pushing much of the emission out of our observed band, or the quasar has a more normal spectrum ($\Gamma{\sim}2$) but is therefore less X-ray luminous ($L_{2-10} \sim 0.6 \times 10^{45}\ \textrm{ erg}\ \textrm{ s}^{-1}$).Comment: 11 pages, 4 figures. Accepted for publication in Ap

The Decoupled Kinematics of High- z QSO Host Galaxies and Their Ly α Halos

© 2022. The Author(s). Published by the American Astronomical Society. This is an open access article distributed under the Creative Commons Attribution License, to view a copy of the license, see: https://creativecommons.org/licenses/by/4.0/We present a comparison of the interstellar medium traced by [C ii] (Atacama Large Millimeter/submillimeter Array), and ionized halo gas traced by Lyα (Multi Unit Spectroscopic Explorer), in and around QSO host galaxies at z ∼ 6. To date, 18 QSOs at this redshift have been studied with both MUSE and high-resolution ALMA imaging; of these, 8 objects display a Lyα halo. Using data cubes matched in velocity resolution, we compare and contrast the spatial and kinematic information of the Lyα halos and the host galaxies’ [C ii] (and dust-continuum) emission. We find that the Lyα halos extend typically 3−30 times beyond the interstellar medium of the host galaxies. The majority of the Lyα halos do not show ordered motion in their velocity fields, whereas most of the [C ii] velocity fields do. In those cases where a velocity gradient can be measured in Lyα, the kinematics do not align with those derived from the [C ii] emission. This implies that the Lyα emission is not tracing the outskirts of a large rotating disk, which is a simple extension of the central galaxy seen in [C ii] emission. It rather suggests that the kinematics of the halo gas are decoupled from those of the central galaxy. Given the scattering nature of Lyα, these results need to be confirmed with James Webb Space Telescope Integral Field Unit observations that can constrain the halo kinematics further using the nonresonant Hα line.Peer reviewe

X-Ray Observations of a z ∼ 6.2 Quasar/Galaxy Merger

Quasars at early redshifts (z > 6) with companion galaxies offer unique insights into the growth and evolution of the first supermassive black holes. Here, we report on a 150 ks Chandra observation of PSO.J308.0416-21.2339, a z = 6.23 quasar with a merging companion galaxy identified in [C II] and rest-frame UV emission. With 72.3(-8.6)(+9.6) net counts, we find that PSO.J308.0416-21.2339 is powerful (L-X = 2.31(-0.76)(+1.14) x 10(45) erg s(-1) cm(-2) in rest-frame 2.0-10.0 keV) yet soft (spectral power-law index Gamma = 2.39(-0.36)(+0.37) and optical-to-X-ray slope alpha(OX) = -1.41 +/- 0.11). In addition, we detect three hard-energy photons 2 ''.0 to the west of the main quasar, cospatial with the brightest UV emission of the merging companion. As no soft-energy photons are detected in the same area, this is potentially indicative of a highly obscured source. With conservative assumptions, and accounting for both background fluctuations and the extended wings of the quasar's emission, these photons only have a probability P = 0.021 of happening by chance. If confirmed by deeper observations, this system is the first high-redshift quasar and companion individually detected in X-rays and is likely a dual active galactic nucleus.STSCI/NASAThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Enhanced X-ray Emission from the Most Radio-Powerful Quasar in the Universe's First Billion Years

We present deep (265 ks) Chandra X-ray observations of PSO J352.4034$-$15.3373, a quasar at z=5.831 that, with a radio-to-optical flux ratio of R>1000, is one of the radio-loudest quasars in the early universe and is the only quasar with observed extended radio jets of kpc-scale at $z \gtrsim 6$. Modeling the X-ray spectrum of the quasar with a power law, we find a best fit of $\Gamma = 1.99^{+0.29}_{-0.28}$, leading to an X-ray luminosity of $L_{2-10} = 1.26^{+0.45}_{-0.33} \times 10^{45}\ {\rm erg}\ {\rm s}^{-1}$ and an X-ray to UV brightness ratio of $\alpha_{\rm OX} = -1.36 \pm 0.11$. We identify a diffuse structure 50 kpc (${\sim}8^{\prime\prime}$) to the NW of the quasar along the jet axis that corresponds to a $3\sigma$ enhancement in the angular density of emission and can be ruled out as a background fluctuation with a probability of P=0.9985. While with few detected photons the spectral fit of the structure is uncertain, we find that it has a luminosity of $L_{2-10}\sim10^{44}\ {\rm erg}\ {\rm s}^{-1}$. These observations therefore potentially represent the most distant quasar jet yet seen in X-rays. We find no evidence for excess X-ray emission where the previously-reported radio jets are seen (which have an overall linear extent of $0.^{\prime\prime}28$), and a bright X-ray point source located along the jet axis to the SE is revealed by optical and NIR imaging to not be associated with the quasar.Comment: 16 pages, 7 Figures. Accepted for publication the Astrophysical Journa

Circum-Galactic Medium in the Halo of Quasars

The properties of circum-galactic gas in the halo of quasar host galaxies are investigated analyzing Mg II 2800 and C IV 1540 absorption-line systems along the line of sight close to quasars. We used optical spectroscopy of closely aligned pairs of quasars (projected distance = 0.3 associated with the foreground quasars are revealed up to 200 kpc from the centre of the host galaxy, showing that the structure of the absorbing gas is patchy with a covering fraction quickly decreasing beyond 100 kpc. In this contribution we use optical and near-IR images obtained at VLT to investigate the relations between the properties of the circum-galactic medium of the host galaxies and of the large scale galaxy environments of the foreground quasars

Staring at the Shadows of Archaic Galaxies: Damped Lyα\alpha and Metal Absorbers toward a Young z∼6z \sim 6 Weak-line Quasar

We characterize the Ly$\alpha$ halo and absorption systems toward PSO J083+11, a unique $z=6.3401$ weak-line quasar, using Gemini/GNIRS, Magellan/FIRE, and VLT/MUSE data. Strong absorptions by hydrogen and several metal lines (e.g., CII, MgII, and OI) are discovered in the spectrum, which indicates the presence of: (i) a proximate sub-damped Ly$\alpha$ (sub-DLA) system at $z=6.314$ and (ii) a MgII absorber at $z=2.2305$. To describe the observed damping wing signal, we model the Ly$\alpha$ absorption with a combination of a sub-DLA with the neutral hydrogen column density of $\log N_\mathrm{HI} = 20.03 \pm 0.30$ cm$^{-2}$ and absorption from the intergalactic medium with a neutral fraction of around 10 percent. The sub-DLA toward PSO J083+11 has an abundance ratio of [C/O] $=-0.04 \pm 0.33$ and metallicity of [O/H] $=-2.19 \pm 0.44$, similar to those of low-redshift metal-poor DLAs. These measurements suggest that the sub-DLA might truncate PSO J083+11's proximity zone size and complicate the quasar lifetime measurement. However, this quasar shows no sign of a Ly$\alpha$ halo in the MUSE datacube, where the estimated $1\sigma$ limit of surface brightness is $2.76 \times 10^{-18}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ at aperture size of 1 arcsecond, or equivalent to a Ly$\alpha$ luminosity of $\leq 43.46$ erg s$^{-1}$. This non-detection, while being only weak independent evidence on its own, is at least consistent with a young quasar scenario, as expected for a quasar with a short accretion timescale.Comment: 22 pages, 12 figures, and 1 table. Accepted for publication in The Astronomical Journal. We welcome the comments from the reader. Related paper: arXiv:2009.0778