Active Galactic Nuclei (AGN) are highly luminous sources (L ∼ 1042 − 1048 erg/s), powered by accretion onto supermassive black holes (SMBHs) with masses ranging from MBH ∼ 106 − 109M⊙, located at the centers of galaxies. The observed tight correlations between SMBH properties and those of their host galaxies has led to the understanding that they co-evolve, influencing one another. Over the past few years, AGN research has increasingly focused on their role in galaxy evolution, although the physical processes underlying this interaction remain still unclear. Growing evidence indicates that AGN-driven outflows can deposit energy and momentum into the interstellar medium of their host galaxies, influencing star formation activity. This suggests these outflows as a potential feedback mechanism in regulating galaxy evolution. This feedback effect is expected to be particularly strong during the Cosmic Noon, at z = 2 − 3, when both AGN accretion and star formation activity are at their peak. As the efficiency in generating powerful outflows appears to scale with AGN luminosity, understanding the coupling between nuclear energy output and the host galaxy is an open issue particularly relevant for the most luminous AGN, known as quasars (hereafter QSOs). Another thriving field of study focuses on understanding the formation of these supermassive black holes. Indeed, due to their extreme luminosities, QSOs can be observed at very early cosmic epochs, reaching back to z = 6 − 7.5, when the Universe was less than 1 Gyr old. These extremely distant QSOs reveal the existence of fully formed SMBHs already in the early Universe, posing a significant challenge to our current models, which struggle to explain how such massive objects could form in such a short span of time. A direct approach to understanding the physical processes within AGN is through their spectral energy distri- bution (SED), which represents their emission across the electromagnetic spectrum. The AGN SED extends from the hard X-ray to the radio bands and is due to the sum of several contributions, arising from distinct regions and from different physical mechanisms. Specifically, gas inflow toward the SMBH via an accretion disk converts gravitational energy into thermal energy through viscous torques, producing the distinctive blue bump in UV and optical emission. This primary radiation is Compton up-scattered to X-ray energies in a region of hot electron gas called the corona. Additionally, a surrounding dusty torus absorbs photons emitted by the corona and the accretion disk, re-emitting them in the near and mid-IR. Dust at lower temperatures (T ≈ 20 − 100 K), located much farther from the nucleus and heated by both hot stars and partially by the AGN itself, generates far-IR emission. Finally, in approximately 10% of AGN, the presence of a relativistic jet accounts for their radio emission. This thesis focuses on high-redshift AGN, specifically on their SED. Initially, we concentrated on the WISSH sample, comprising 85 hyperluminous QSOs at redshifts 2–4. These QSOs, which can drive some of the fastest and most powerful outflows, are ideal targets for exploring the AGN-host galaxy connection. Then, we moved to even higher redshifts, studying the X-ray-to-NIR broadband emission of the HYPERION and E-XQR samples at z = 5.5 − 7.5. These quasars host the most massive SMBHs observed in this redshift range, challenging our current models of SMBH formation. Finally, we address preparatory work for the upcoming Vera Rubin Legacy Survey of Space and Time (LSST), which, with its wide and deep coverage of the southern sky, is expected to discover many new high-z QSOs, increasing the known population by at least an order of magnitude. This thesis is organized as follows: - Chapter 1 provides a brief introduction to AGN, covering their powering mechanism, structure and resulting spectral energy distribution. Sec. 1.5 highlights the particular interest in studying luminous QSOs within the broader AGN population. - Chapter 2 presents the derivation of a X-ray to FIR mean SED for the WISSH sample, which consists of 85 hyperluminous QSOs at Cosmic Noon, and its comparison with other averaged AGN SEDs from the literature. - Chapter 3 is dedicated to investigating the broadband emission of 54 QSOs at the Epoch of Reionization, drawn from the HYPERION and E-XQR-30 samples. 1 - Chapter 4 describes the AGILE pipeline, developed to produce a realistic photometric catalog that closely mirrors the data expected from LSST observations. This chapter particularly focuses on the modeling and assignment of multiwavelength SEDs for AGN in the catalog. - Chapter 5 presents the results of applying machine learning tools on datasets with LSST-like features to efficiently identify luminous, high-redshift QSOs within the vast amount of data that LSST will deliver, and, more generally, to distinguish AGN from inactive galaxies and stars. - Chapter 6 provides a brief summary of the results of the work and outlines future perspectives for continuing the analysis of high-z QSOs.
Saccheo, I. (2025). A Multi-band view of high-redshift QSOs and future outlook for LSST.
A Multi-band view of high-redshift QSOs and future outlook for LSST
Saccheo Ivano
2025-03-06
Abstract
Active Galactic Nuclei (AGN) are highly luminous sources (L ∼ 1042 − 1048 erg/s), powered by accretion onto supermassive black holes (SMBHs) with masses ranging from MBH ∼ 106 − 109M⊙, located at the centers of galaxies. The observed tight correlations between SMBH properties and those of their host galaxies has led to the understanding that they co-evolve, influencing one another. Over the past few years, AGN research has increasingly focused on their role in galaxy evolution, although the physical processes underlying this interaction remain still unclear. Growing evidence indicates that AGN-driven outflows can deposit energy and momentum into the interstellar medium of their host galaxies, influencing star formation activity. This suggests these outflows as a potential feedback mechanism in regulating galaxy evolution. This feedback effect is expected to be particularly strong during the Cosmic Noon, at z = 2 − 3, when both AGN accretion and star formation activity are at their peak. As the efficiency in generating powerful outflows appears to scale with AGN luminosity, understanding the coupling between nuclear energy output and the host galaxy is an open issue particularly relevant for the most luminous AGN, known as quasars (hereafter QSOs). Another thriving field of study focuses on understanding the formation of these supermassive black holes. Indeed, due to their extreme luminosities, QSOs can be observed at very early cosmic epochs, reaching back to z = 6 − 7.5, when the Universe was less than 1 Gyr old. These extremely distant QSOs reveal the existence of fully formed SMBHs already in the early Universe, posing a significant challenge to our current models, which struggle to explain how such massive objects could form in such a short span of time. A direct approach to understanding the physical processes within AGN is through their spectral energy distri- bution (SED), which represents their emission across the electromagnetic spectrum. The AGN SED extends from the hard X-ray to the radio bands and is due to the sum of several contributions, arising from distinct regions and from different physical mechanisms. Specifically, gas inflow toward the SMBH via an accretion disk converts gravitational energy into thermal energy through viscous torques, producing the distinctive blue bump in UV and optical emission. This primary radiation is Compton up-scattered to X-ray energies in a region of hot electron gas called the corona. Additionally, a surrounding dusty torus absorbs photons emitted by the corona and the accretion disk, re-emitting them in the near and mid-IR. Dust at lower temperatures (T ≈ 20 − 100 K), located much farther from the nucleus and heated by both hot stars and partially by the AGN itself, generates far-IR emission. Finally, in approximately 10% of AGN, the presence of a relativistic jet accounts for their radio emission. This thesis focuses on high-redshift AGN, specifically on their SED. Initially, we concentrated on the WISSH sample, comprising 85 hyperluminous QSOs at redshifts 2–4. These QSOs, which can drive some of the fastest and most powerful outflows, are ideal targets for exploring the AGN-host galaxy connection. Then, we moved to even higher redshifts, studying the X-ray-to-NIR broadband emission of the HYPERION and E-XQR samples at z = 5.5 − 7.5. These quasars host the most massive SMBHs observed in this redshift range, challenging our current models of SMBH formation. Finally, we address preparatory work for the upcoming Vera Rubin Legacy Survey of Space and Time (LSST), which, with its wide and deep coverage of the southern sky, is expected to discover many new high-z QSOs, increasing the known population by at least an order of magnitude. This thesis is organized as follows: - Chapter 1 provides a brief introduction to AGN, covering their powering mechanism, structure and resulting spectral energy distribution. Sec. 1.5 highlights the particular interest in studying luminous QSOs within the broader AGN population. - Chapter 2 presents the derivation of a X-ray to FIR mean SED for the WISSH sample, which consists of 85 hyperluminous QSOs at Cosmic Noon, and its comparison with other averaged AGN SEDs from the literature. - Chapter 3 is dedicated to investigating the broadband emission of 54 QSOs at the Epoch of Reionization, drawn from the HYPERION and E-XQR-30 samples. 1 - Chapter 4 describes the AGILE pipeline, developed to produce a realistic photometric catalog that closely mirrors the data expected from LSST observations. This chapter particularly focuses on the modeling and assignment of multiwavelength SEDs for AGN in the catalog. - Chapter 5 presents the results of applying machine learning tools on datasets with LSST-like features to efficiently identify luminous, high-redshift QSOs within the vast amount of data that LSST will deliver, and, more generally, to distinguish AGN from inactive galaxies and stars. - Chapter 6 provides a brief summary of the results of the work and outlines future perspectives for continuing the analysis of high-z QSOs.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
