590 research outputs found
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 the DI mode can provide the
BH accretion needed to match the observed AGN luminosity functions up to ; 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
AGN, as well as the abundance of high-redhshift QSOs with , while the IT scenario provides
an acceptable match up to , as found in our earliest works; iii)
The dispersion of the distributions of Eddington ratio for low- and
intermediate-luminosity AGN (bolometric = -
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 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 ~3515% of the observed EGB in
the energy interval E_{\gamma}=0.1-1 GeV, for ~7315% at E_{\gamma}=1-10
GeV, and for ~6020% 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&
Who talks about collaborative spaces, how, and why
Communities in urban contexts and firms in corporate offices have recently started to implement collaborative spaces. Several authors from different disciplines are currently advancing knowledge in this realm. Systematizing this diverse knowledge base helps to advance our understanding of this novel phenomenon. To this end, the present work reviews 29 papers focusing on collaborative spaces. We analyse these papers in terms of contents, research methods, fields of study, authors’ background, and impact on the academic community. Grounding on this analysis, we outline new relevant research questions and opportunities for future investigations
Open Business Models and Venture Capital Finance
We investigate the differences in venture capital (VC) governance of investee firms with Open Business Models, specifically Open Source Software (OSS), versus closed business models. Due to OSS’s pronounced complexity and uncertainty, we conjecture that VC-backed OSS firms are more frequently staged and syndicated. We present robust empirical evidence from the United States that OSS ventures have more financing rounds and are more likely to be syndicated, and mixed evidence that OSS ventures have a larger number of syndicated investors
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 , 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 Gyr, much longer with respect to the IT
scenario.v) The scatter of the 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
AGN counts at 15um. XMM observations of the ELAIS-S1-5 sample
Context: The counts of galaxies and AGN in the mid infra-red (MIR) bands are
important instruments for studying their cosmological evolution. However, the
classic spectral line ratios techniques can become misleading when trying to
properly separate AGN from starbursts or even from apparently normal galaxies.
Aims: We use X-ray band observations to discriminate AGN activity in
previously classified MIR-selected starburst galaxies and to derive updated
AGN1 and (Compton thin) AGN2 counts at 15 um.
Methods: XMM observations of the ELAIS-S1 15um sample down to flux limits
~2x10^-15 erg cm^-2 s^-1 (2-10 keV band) were used. We classified as AGN all
those MIR sources with a unabsorbed 2-10 keV X-ray luminosity higher that
~10^42 erg/s.
Results: We find that at least about 13(+/-6) per cent of the previously
classified starburst galaxies harbor an AGN. According to these figures, we
provide an updated estimate of the counts of AGN1 and (Compton thin) AGN2 at 15
um. It turns out that at least 24% of the extragalactic sources brighter than
0.6 my at 15 um are AGN (~13% contribution to the extragalactic background
produced at fluxes brighter than 0.6 mJy).Comment: Accepted for publication on A&
Galaxy Formation in Sterile Neutrino Dark Matter Models
We investigate galaxy formation in models with dark matter (DM) constituted
by sterile neutrinos. Given their large parameter space, defined by the
combinations of sterile neutrino mass and mixing parameter
with active neutrinos, we focus on models with
keV, consistent with the tentative 3.5 keV line detected in several X-ray
spectra of clusters and galaxies. We consider i) two resonant production models
with and , to cover
the range of mixing parameter consistent with the 3.5 keV line; ii) two
scalar-decay models, representative of the two possible cases characterizing
such a scenario: a freeze-in and a freeze-out case. We also consider thermal
Warm Dark Matter with particle mass keV. Using a semi-analytic model,
we compare the predictions for the different DM scenarios with a wide set of
observables. We find that comparing the predicted evolution of the stellar mass
function, the abundance of satellites of Milky Way-like galaxies, and the
global star formation history of galaxies with observations does not allow to
disentangle the effects of the baryonic physics from those related to the
different DM models. On the other hand, the distribution of the stellar-to-halo
mass ratios, the abundance of faint galaxies in the UV luminosity function at
, and the specific star formation and age distribution of local,
low-mass galaxies constitute potential probes for the considered DM scenarios.
We discuss how next observations with upcoming facilities will enable to rule
out or to strongly support DM models based on sterile neutrinos.Comment: 21 pages, accepted for publication in The Astrophysical Journa
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