A multi-wavelength study of the interstellar medium in active galactic nuclei at cosmic noon

One of the most remarkable discoveries of the last two decades is that all massive galaxies host a supermassive black hole (SMBH) at their center, which has gone through an active phase of growth known as active galactic nucleus (AGN). Although galaxy evolution models need AGN feedback to reproduce the observed properties of galaxies, direct observational evidence for the mechanisms through which SMBHs and galaxies co-evolve has still to be proven. The peak epoch of galaxy assembly and SMBH growth, the so-called "cosmic noon" (1<z<3), represents a key laboratory to understand how the connection between AGN and their host galaxies was established. In this thesis, I presented observational studies of the interstellar medium (ISM) of X-ray selected AGN at cosmic noon. The gas content is directly linked to the evolution of the AGN and its host galaxy. Indeed, it feeds both the formation of new stars in the galaxy and the growth of the central SMBH. The gas in the host along our line of sight may also have a pivotal role in obscuring the central nuclear source. Finally, the kinematics and composition of the gas could be affected by the energy released by the central AGN, through the so-called AGN feedback. To achieve a comprehensive study of the ISM, I adopted a multi-wavelength approach and exploited a variety of data, from the X-ray to the radio regime, by using techniques such as broad-band spectral energy distribution (SED) fitting, X-ray spectral analysis and submm spectroscopy. I first investigated whether the obscuration observed in the X-ray spectra of AGN can be produced by the ISM of the host galaxy. For a sample of far-IR detected AGN at z>2.5 I found that the total hydrogen column density along the line of sight, measured in the X-ray band, is comparable to the column density associated with the ISM of the host, derived through SED-fitting analysis and assuming galaxy sizes. Therefore, the ISM of the host appears to be capable of providing significant absorption on kpc scales. Such absorption adds to (or even replaces) that produced on pc scales by any circumnuclear material, challenging the view of the obscured/unobscured AGN dichotomy as due to inclination effects only. Then, I conducted two complementary studies to probe the ionized and molecular phases of the ISM in a blindly-selected sample of AGN at z~2 that covers a wide range in luminosities. Such studies are necessary to properly constrain the impact of AGN feedback on galaxy evolution. The first, called SUPER, traces ionized outflows through the [OIII] line by using high-resolution spatially-resolved integral field spectroscopic observations (SINFONI). In this work I laid the foundations of the survey, by performing a multi-wavelength characterization of AGN and host galaxy properties, and showed how comparing insights from different techniques/observations is crucial to confirm and provide extra confidence in SED-fitting as well as spectroscopic results. These multi-wavelength properties will then be connected with those of the outflows as traced by SINFONI. The second study used a set of ALMA observations of the CO(J=3-2) transition, to carry out a comparison of the CO properties of AGN with those of a control sample of inactive galaxies matched in redshift, stellar mass and star-formation rate. I found that the AGN sample appears to be underluminous in CO with respect to the control sample and the difference is especially significant at high masses, log(Mstar) > 11. These observations demonstrated that the AGN may have an effect on the ISM of the hosts, although the exact mechanisms in place require further observations to be understood. These projects set the scene for future investigations, which will significantly improve our understanding of the role of AGN in galaxy evolution

Heavily obscured AGN: an ideal laboratory to study the early co-evolution of galaxies and black holes

Obscured AGN are a crucial ingredient to understand the full growth history of super massive black holes and the coevolution with their host galaxies, since they constitute the bulk of the BH accretion. In the distant Universe, many of them are hosted by submillimeter galaxies (SMGs), characterized by a high production of stars and a very fast consumption of gas. Therefore, the analysis of this class of objects is fundamental to investigate the role of the ISM in the early coevolution of galaxies and black holes. We present a multiwavelength study of a sample of six obscured X-ray selected AGN at z>2.5 in the CDF-S, detected in the far-IR/submm bands. We performed the X-ray spectral analysis based on the 7Ms Chandra dataset, which provides the best X-ray spectral information currently available for distant AGN. We were able to place constraints on the obscuring column densities and the intrinsic luminosities of our targets. Moreover, we built up the UV to FIR spectral energy distributions (SEDs) by combining the broad-band photometry from CANDELS and the Herschel catalogs, and analyzed them by means of an SED decomposition technique. Therefore, we derived important physical parameters of both the host galaxy and the AGN. In addition, we obtained, through an empirical calibration, the gas mass in the host galaxy and assessed the galaxy sizes in order to estimate the column density associated with the host ISM. The comparison of the ISM column densities with the values measured from the X-ray spectral analysis pointed out that the contribution of the host ISM to the obscuration of the AGN emission can be substantial, ranging from ~10% up to ~100% of the value derived from the X-ray spectra. The absorption may occur at different physical scales in these sources and, in particular, the medium in the host galaxy is an ingredient that should be taken into account, since it may have a relevant role in driving the early co-evolution of galaxies with their black holes

A multi-wavelength study of the interstellar medium in active galactic nuclei at cosmic noon

One of the most remarkable discoveries of the last two decades is that all massive galaxies host a supermassive black hole (SMBH) at their center, which has gone through an active phase of growth known as active galactic nucleus (AGN). Although galaxy evolution models need AGN feedback to reproduce the observed properties of galaxies, direct observational evidence for the mechanisms through which SMBHs and galaxies co-evolve has still to be proven. The peak epoch of galaxy assembly and SMBH growth, the so-called "cosmic noon" (1<z<3), represents a key laboratory to understand how the connection between AGN and their host galaxies was established. In this thesis, I presented observational studies of the interstellar medium (ISM) of X-ray selected AGN at cosmic noon. The gas content is directly linked to the evolution of the AGN and its host galaxy. Indeed, it feeds both the formation of new stars in the galaxy and the growth of the central SMBH. The gas in the host along our line of sight may also have a pivotal role in obscuring the central nuclear source. Finally, the kinematics and composition of the gas could be affected by the energy released by the central AGN, through the so-called AGN feedback. To achieve a comprehensive study of the ISM, I adopted a multi-wavelength approach and exploited a variety of data, from the X-ray to the radio regime, by using techniques such as broad-band spectral energy distribution (SED) fitting, X-ray spectral analysis and submm spectroscopy. I first investigated whether the obscuration observed in the X-ray spectra of AGN can be produced by the ISM of the host galaxy. For a sample of far-IR detected AGN at z>2.5 I found that the total hydrogen column density along the line of sight, measured in the X-ray band, is comparable to the column density associated with the ISM of the host, derived through SED-fitting analysis and assuming galaxy sizes. Therefore, the ISM of the host appears to be capable of providing significant absorption on kpc scales. Such absorption adds to (or even replaces) that produced on pc scales by any circumnuclear material, challenging the view of the obscured/unobscured AGN dichotomy as due to inclination effects only. Then, I conducted two complementary studies to probe the ionized and molecular phases of the ISM in a blindly-selected sample of AGN at z~2 that covers a wide range in luminosities. Such studies are necessary to properly constrain the impact of AGN feedback on galaxy evolution. The first, called SUPER, traces ionized outflows through the [OIII] line by using high-resolution spatially-resolved integral field spectroscopic observations (SINFONI). In this work I laid the foundations of the survey, by performing a multi-wavelength characterization of AGN and host galaxy properties, and showed how comparing insights from different techniques/observations is crucial to confirm and provide extra confidence in SED-fitting as well as spectroscopic results. These multi-wavelength properties will then be connected with those of the outflows as traced by SINFONI. The second study used a set of ALMA observations of the CO(J=3-2) transition, to carry out a comparison of the CO properties of AGN with those of a control sample of inactive galaxies matched in redshift, stellar mass and star-formation rate. I found that the AGN sample appears to be underluminous in CO with respect to the control sample and the difference is especially significant at high masses, log(Mstar) > 11. These observations demonstrated that the AGN may have an effect on the ISM of the hosts, although the exact mechanisms in place require further observations to be understood. These projects set the scene for future investigations, which will significantly improve our understanding of the role of AGN in galaxy evolution

A fast-rotator post-starburst galaxy quenched by supermassive black-hole feedback at z=3

There is compelling evidence that the most massive galaxies in the Universe stopped forming stars due to the time-integrated feedback from their central super-massive black holes (SMBHs). However, the exact quenching mechanism is not yet understood, because local massive galaxies were quenched billions of years ago. We present JWST/NIRSpec integral-field spectroscopy observations of GS-10578, a massive, quiescent galaxy at redshift z=3.064. From the spectrum we infer that the galaxy has a stellar mass of $M_*=1.6\pm0.2 \times 10^{11}$ MSun and a dynamical mass $M_{\rm dyn}=2.0\pm0.5 \times 10^{11}$ MSun. Half of its stellar mass formed at z=3.7-4.6, and the system is now quiescent, with the current star-formation rate SFR<9 MSun/yr. We detect ionised- and neutral-gas outflows traced by [OIII] emission and NaI absorption. Outflow velocities reach $v_{\rm out}\approx$1,000 km/s, comparable to the galaxy escape velocity and too high to be explained by star formation alone. GS-10578 hosts an Active Galactic Nucleus (AGN), evidence that these outflows are due to SMBH feedback. The outflow rates are 0.14-2.9 and 30-300 MSun/yr for the ionised and neutral phases, respectively. The neutral outflow rate is ten times higher than the SFR, hence this is direct evidence for ejective SMBH feedback, with mass-loading capable of interrupting star formation by rapidly removing its fuel. Stellar kinematics show ordered rotation, with spin parameter $\lambda_{Re}=0.62\pm0.07$, meaning GS-10578 is rotation supported. This study shows direct evidence for ejective AGN feedback in a massive, recently quenched galaxy, thus clarifying how SMBHs quench their hosts. Quenching can occur without destroying the stellar disc.Comment: 40 pages, 9 figures, submitted to Nat. Ast., comments welcome

Detecting and Characterizing Mg II absorption in DESI Survey Validation Quasar Spectra

In this paper we will present findings on the detection of Magnesium II (MgII, lambda = 2796 {\AA}, 2803 {\AA}) absorption systems observed in data from the Early Data Release (EDR) of the Dark Energy Spectroscopic Instrument (DESI). DESI is projected to obtain spectroscopy of approximately 3 million quasars (QSOs), of which over 99% are anticipated to be found at redshifts greater than z < 0.3, such that DESI would be able to observe an associated or intervening Mg II absorber illuminated by the background QSO. We have developed an autonomous supplementary spectral pipeline that detects such systems through an initial line-fitting process and then confirms line properties using a Markov Chain Monte Carlo (MCMC) sampler. Based upon both a visual inspection and the reanalysis of coadded observations, we estimate this sample of absorption systems to have a completeness of 82.56% and purity of 99.08%. As the spectra in which Mg II systems are detected are the result of coadding multiple observations, we can determine the sensitivity, and therefore completeness, of the sample by searching for known Mg II systems in coadded data with fewer observations (and therefore lower signal-to-noise). From a parent catalog containing 83,207 quasars, we detect a total of 23,921 Mg II absorption systems following a series of quality cuts. Extrapolating from this occurrence rate of 28.75% implies a catalog at the completion of the five-year DESI survey that contains over eight hundred thousand Mg II absorbers. The cataloging of these systems will enable significant further research as they carry information regarding circumgalactic medium (CGM) environments, the distribution of intervening galaxies, and the growth of metallicity across the redshift range 0.3 < z < 2.5.Comment: 12 pages, 7 figure