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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - CACHEM (CACHEM: Probing Clustering of X-ray AGN using Chandra and Semi-Empirical Models)

Teaser

Summary

Almost every galaxy in the Universe hosts a super massive black hole (BH) at its center. During
active phases, when the BH growth is powered by matter accretion, the galaxy is observed as an
Active Galactic Nucleus (AGN). How BH growth and AGN formation are related to the host galaxy
evolution, and to the cosmic large-scale structure are among the most outstanding problems in
Cosmology. These questions also hold the key for a full understanding of galaxy formation.
AGN clustering provides a unique way to probe the link between BHs and host galaxies through their
connection with host dark matter halos. Clustering describes the distribution of AGN pairs as a
function of their spatial separation, provides an indirect measurement of hosting dark matter halo
masses, and quantifies how BH populate halos. Such information is a cogent benchmark for BH
triggering scenarios, and AGN evolutionary models.
The aim of this project is to clarify the interplay among BHs, host galaxies and dark matter halos by (i)
performing clustering measurements of X-ray selected AGN as a function of spatial scale, obscuration
and host galaxy properties; (ii) creating AGN mock catalogs based on Semi Empirical Models which
serve to (iii) interpret the clustering of AGN selected via X-ray and multi-wavelength current/future
surveys.

Work performed

I worked with the exquisite Chandra COSMOS Legacy data, to study how the BH activity depends on the host galaxy properties,
such as galaxy stellar mass, star formation rate and BH accretion rate over a broad redshift range.
The results have been published in Viitanen, Allevato et al. 2019, in Astronomy & Astrophysics Journal.
In particular, I investigated the relation between the stellar mass content of a galaxy and the mass of its dark matter halo
for the first time for obscured AGN at z > 1, results published in Allevato et al. 2019, in Astronomy & Astrophysics Journal,
showing that the stellar-to-halo mass relation is different for active and non-active BHs with similar host galaxy properties.
I extended my research interest towards semi-empirical models, with the specific aim of fully addressing the statistical
properties and spatial distribution of active BHs. By combining semi-empirical models and clustering measurements, I put
strong constraints on the BH â€“ host galaxy scaling relations in the local universe in a Nature Astronomy paper Shankar,
Allevato et al. 2019 as shown in the joint INAF-SNS Press Release in 2019
(https://normalenews.sns.it/new-linkfound-between-mass-of-supermassive-black-holes-and-their-host-galaxies).

I\'ve co-advised 1 PhD student from the Southampton University, C. Marsden who published 3 papers and is expected
to successfully defend in 2021 and Akke Viitanen, from the University of Helsinki, who published 3 papers
and is expected to successfully graduate in 2020.

As a result of my leadership qualities and my involvement in Euclid Consortium activities, in 2019 I was
appointed as co-leader of the Euclid Work Package â€œType 1 and 2 AGNâ€ that focuses on Euclid AGN
identification methods and AGN related science. In particular, Iâ€™ve coordinated the forecast identification
of AGN in the Euclid surveys based on imaging and/or spectroscopy, and definition of the AGN optimal
selection criteria. Iâ€™m also coordinating the team creating the official AGN Flagship mock catalogs. In
January 2020, I organized and hosted at Scuola Normale Superiore the first international Euclid Type 1
and 2 AGN workshop (https://www.sns.it/it/evento/euclid-agn).

Final results

Making full sense of the different clustering measurements available for X-ray selected AGN, obscured objects
and multi-wavelength surveys is challenging. Selection biases in terms of AGN luminosity, host galaxy
properties, and redshift might play a relevant role in determining AGN clustering. DARK addressed
the above issues both from an observational and theoretical point of view by using:
1) The new Chandra COSMOS-Legacy (CCL) catalog: this is the largest available sample of X-ray
selected AGN for clustering studies. This unique catalog allowed a proper understanding of the
evolution of the BH-host relation across cosmic time providing: 1) large samples of AGN, including
obscured sources, over a wide range of both redshifts and luminosities; 2) different AGN selections, in terms of
Eddington ratio, host-galaxy properties such as stellar mass, morphology and star formation rate.
AGN clustering properties as a function of this broad parameter space has explored for the first
time at z>1.
2) Semi-empirical models (SEMs): a methodology based on the generation of mock catalogs of
galaxies and their BHs assigned via semi-empirical relations to large samples of dark matter halos. The
comparison of the predictions from these mocks with our AGN clustering measurements at different
redshifts, scales and host galaxy properties has revealed invaluable information on the BH/galaxy key
parameters that determine the clustering measurements of CCL AGN, in particular of obscured
sources.
This in turn has been crucial to understand the AGN/galaxy co-evolution and to test different BH
triggering scenarios at different epochs. The proposed research has then simultaneously probed
AGN clustering and BH triggering mechanisms. Addressing these issues is of capital importance
at this time with the advent of major European surveys specifically dedicated to the observations
of AGN at different wavelengths, epochs, and environments (e.g., eRosita, EUCLID, Athena).