The Evolution of Active Galactic Nuclei in Warm Dark Matter Cosmology

Recent measurements of the abundance of AGN with low-luminosities (L_X< 10^44 erg/s in the 2-10 keV energy band) at high redshifts z>4 provide a serious challenge for Cold Dark Matter (CDM) models based on interaction-driven fueling of AGN. Using a semi-analytic model of galaxy formation we investigate how such observations fit in a Warm Dark Matter (WDM) scenario of galaxy formation, and compare the results with those obtained in the standard CDM scenario with different efficiencies for the stellar feedback. Taking on our previous exploration of galaxy formation in WDM cosmology, we assume as a reference case a spectrum which is suppressed - compared to the standard CDM case - below a cut-off scale ~ 0.2$ Mpc corresponding (for thermal relic WDM particles) to a mass m_X=0.75 keV. We run our fiducial semi-analytic model with such a WDM spectrum to derive AGN luminosity functions from z~6 to the present over a wide range of luminosities (10^43< L_X/erg/s < 10^46 in the 2-10 keV X-ray band), to compare with recent observations and with the results in the CDM case. When compared with the standard CDM case, the luminosity distributions we obtain assuming a WDM spectrum are characterized by a similar behaviour at low redshift, and by a flatter slope at faint magnitudes for z>3, which provide an excellent fit to present observations. We discuss how such a result compares with CDM models with maximized feedback efficiency, and how future deep AGN surveys will allow for a better discrimination between feedback and cosmological effects on the evolution of AGN in interaction-driven models for AGN fueling.Comment: Accepted for publication in The Astrophysical Journal; typos and references correcte

Triggering Active Galactic Nuclei in Hierarchical Galaxy Formation: Disk instability vs. Interactions

Using a semi analytic model for galaxy formation we investigate the effects of Black Hole accretion triggered by disk instabilities (DI) in isolated galaxies on the evolution of AGN. Specifically, we took on, developed and expanded the Hopkins & Quataert (2011) model for the mass inflow following disk perturbations, and compare the corresponding evolution of the AGN population with that arising in a scenario where galaxy interactions trigger AGN (IT mode). We extended and developed the DI model by including different disk surface density profiles, to study the maximal contribution of DI to the evolution of the AGN population. We obtained the following results: i) for luminosities corresponding to $M_{1450}\gtrsim -26$ the DI mode can provide the BH accretion needed to match the observed AGN luminosity functions up to $z \approx 4.5$; in such a luminosity range and redshift, it can compete with the IT scenario as the main driver of cosmological evolution of AGN; ii) The DI scenario cannot provide the observed abundance of high-luminosity QSO with $M_{1450}\lesssim -26$ AGN, as well as the abundance of high-redhshift $z \approx 4.5$ QSOs with $M_{1450}\lesssim -24$, while the IT scenario provides an acceptable match up to $z \approx 6$, as found in our earliest works; iii) The dispersion of the distributions of Eddington ratio for low- and intermediate-luminosity AGN (bolometric $L_{AGN}$ = $10^{43}$ - $10^{45}$ erg/s) is predicted to be much smaller in the DI scenario compared to the IT mode; iv) The above conclusions are robust with respect to the explored variants of the Hopkins & Quataert (2011) model. We discuss the physical origin of our findings, and how it is possible to pin down the dominant fueling mechanism in the low-intermediate luminosity range $M_{1450}\gtrsim -26$ where both the DI and the IT modes are viable candidates as drivers for the AGN evolution.Comment: Accepted for publication in Astronomy & Astrophysics, 24 pages, 8 figures; updated reference

Extragalactic gamma-ray background from AGN winds and star-forming galaxies in cosmological galaxy formation models

We derive the contribution to the extragalactic gamma-ray background (EGB) from AGN winds and star-forming galaxies by including a physical model for the gamma-ray emission produced by relativistic protons accelerated by AGN-driven and supernova-driven shocks into a state-of-the-art semi-analytic model of galaxy formation. This is based on galaxy interactions as triggers of AGN accretion and starburst activity and on expanding blast wave as the mechanism to communicate outwards the energy injected into the interstellar medium by the active nucleus. We compare the model predictions with the latest measurement of the EGB spectrum performed by the Fermi-LAT in the range between 100 MeV and 820 GeV. We find that AGN winds can provide ~35$\pm$15% of the observed EGB in the energy interval E_{\gamma}=0.1-1 GeV, for ~73$\pm$15% at E_{\gamma}=1-10 GeV, and for ~60$\pm$20% at E_{\gamma}>10 GeV. The AGN wind contribution to the EGB is predicted to be larger by a factor of 3-5 than that provided by star-forming galaxies (quiescent plus starburst) in the hierarchical clustering scenario. The cumulative gamma-ray emission from AGN winds and blazars can account for the amplitude and spectral shape of the EGB, assuming the standard acceleration theory, and AGN wind parameters that agree with observations. We also compare the model prediction for the cumulative neutrino background from AGN winds with the most recent IceCube data. We find that for AGN winds with accelerated proton spectral index p=2.2-2.3, and taking into account internal absorption of gamma-rays, the Fermi-LAT and IceCube data could be reproduced simultaneously.Comment: 12 pages, 8 figures, accepted for publication in A&

Outflows in the Gaseous Discs of Active Galaxies and their impact on Black Hole Scaling Relations

To tackle the still unsolved and fundamental problem of the role of Active Galactic Nuclei (AGN) feedback in shaping galaxies, in this work we implement a new physical treatment of AGN-driven winds into our semi-analytic model of galaxy formation. To each galaxy in our model, we associate solutions for the outflow expansion and the mass outflow rates in different directions, depending on the AGN luminosity, on the circular velocity of the host halo, and on gas content of the considered galaxy. To each galaxy we also assign an effective radius derived from energy conservation during merger events, and a stellar velocity dispersion self-consistently computed via Jeans modelling. We derive all the main scaling relations between Black hole (BH) mass and total/bulge stellar mass, velocity dispersion, host halo dark matter mass, and star formation efficiency. We find that our improved AGN feedback mostly controls the dispersion around the relations but plays a subdominant role in shaping slopes and/or normalizations of the scaling relations. Including possible limited-resolution selection biases in the model provides better agreement with the available data. The model does not point to any more fundamental galactic property linked to BH mass, with velocity dispersion playing a similar role with respect to stellar mass, in tension with present data. In line with other independent studies carried out on comprehensive semi-analytic and hydrodynamic galaxy-BH evolution models, our current results signal either an inadequacy of present cosmological models of galaxy formation in fully reproducing the local scaling relations, in terms of both shape and residuals, and/or point to an incompleteness issue affecting the local sample of dynamically-measured BHs.Comment: 21 pages, accepted for publications in Astronomy & Astrophysic

Comparing the Evolution of the Galaxy Disk Sizes with CDM Models: The Hubble Deep Field

The intrinsic sizes of the field galaxies with I<26 in the Hubble and ESO-NTT Deep Fields are shown as a function of their redshifts and absolute magnitudes using photometric redshifts derived from the multicolor catalogs and are compared with the CDM predictions. Extending to lower luminosities and to higher z our previous analysis performed on the NTT field alone, we find that the distribution of the galaxy disk sizes at different cosmic epochs is within the range predicted by typical CDM models. However, the observed size distribution of faint (M_B>-19) galaxies is skewed with respect to the CDM predictions and an excess of small-size disks (R_d<2 kpc) is already present at z~ 0.5. The excess persists up to z~3 and involves brighter galaxies . Such an excess may be reduced if luminosity-dependent effects, like starburst activity in interacting galaxies, are included in the physical mechanisms governing the star formation history in CDM models.Comment: 9 pages, 3 figures, ApJ Letters in pres

Physical properties of AGN host galaxies as a probe of SMBH feeding mechanisms

Using an advanced semi analytic model (SAM) for galaxy formation, we have investigated the statistical effects of assuming two different mechanisms for triggering AGN activity on the properties of AGN host galaxies. We have considered a first accretion mode where AGN activity is triggered by disk instabilities (DI) in isolated galaxies, and a second feeding mode where such an activity is triggered by galaxy mergers and fly-by events (interactions, IT). We obtained the following results:i) for hosts with $M_* \lesssim 10^{11} M_{\bigodot}$, both DI and IT modes are able to account for the observed AGN hosts stellar mass function; for more massive hosts, the DI scenario predicts a lower space density than the IT model, lying below the observational estimates for z>0.8.ii) The analysis of the color-magnitude diagram (CMD) of AGN hosts for redshift z < 1.5 can provide a good observational test to effectively discriminate between the DI and IT mode, since DIs are expected to yield AGN host galaxy colors skewed towards bluer colors, while in the IT scenario the majority of hosts are expected to reside in the red sequence.iii) While both IT and DI scenarios can account for AGN triggered in main sequence or starburst galaxies, DIs fail in triggering AGN activity in passive galaxies.iv) The two modes are characterized by a different duration of the AGN phase, with DIs lasting even on time scales $\sim$ Gyr, much longer with respect to the IT scenario.v) The scatter of the $SFR-L_{bol}$ relation could represent another crucial diagnostics to discriminate between the two triggering modes, since the DI scenario predicts an appreciably lower scatter of the relation than the IT scenario. vi) Disk instabilities are not able to account for the observed fraction of AGN in groups for z < 1 and clusters for z < 0.7, while the IT scenario provides a good match to observational data.Comment: Paper accepted for publication in section 4. Extragalactic astronomy of Astronomy and Astrophysic

Quasar Evolution Driven by Galaxy Encounters in Hierarchical Structures

We link the evolution of the galaxies in the hierarchical clustering scenario with the changing accretion rates of cold gas onto the central massive black holes that power the quasars. We base on galaxy interactions as main triggers of accretion; the related scaling laws are taken up from Cavaliere & Vittorini (2000), and grafted to a semi-analytic code for galaxy formation. As a result, at high $z$ the protogalaxies grow rapidly by hierarchical merging; meanwhile, much fresh gas is imported and also destabilized, so the holes are fueled at their full Eddington rates. At lower $z$ the galactic dynamical events are mostly encounters in hierarchically growing groups; now the refueling peters out, as the residual gas is exhausted while the destabilizing encounters dwindle. So, with no parameter tuning other than needed for stellar observables, our model uniquely produces at $z>3$ a rise, and at $z\lesssim 2.5$ a decline of the bright quasar population as steep as observed. In addition, our results closely fit the observed luminosity functions of quasars, their space density at different magnitudes from $z\approx 5$ to $z\approx 0$, and the local $m_{BH}-\sigma$ relation.Comment: 5 pages. Accepted for publication in ApJ Letter