912 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
The interaction-driven starburst contribution to the cosmic star formation rate density
An increasing amount of observational evidence supports the notion that there
are two modes of star formation: a quiescent mode in disk-like galaxies, and a
starburst mode, which is generally interpreted as driven by merging. Using a
semi-analytic model of galaxy formation, we derive the relative contribution to
the cosmic star formation rate density of quiescently starforming and starburst
galaxies, predicted under the assumption that starburst events are triggered by
galaxy encounters (merging and fly-by kind) during their merging histories. We
show that, within this framework, quiescently starforming galaxies dominate the
cosmic star formation rate density at all redshifts. The contribution of the
burst-dominated starforming galaxies increases with redshift, rising from <5%
at low redshift (z5. We estimated that the fraction of the
final (z=0) galaxy stellar mass which is formed through the burst component of
star formation is ~10% for 10^10 M_\odot<M_*<10^11.5 M_\odot. Starburst
galaxies, selected according to their distance from the galaxy main sequence,
account for ~10% of the star formation rate density in the redshift interval
1.5<z<2.5, i.e. at the cosmic peak of the star formation activity.Comment: 11 pages, 8 figures. Accepted for publication in A&
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
Is Open Source about innovation? How interactions with the Open Source community impact on the innovative performances of entrepreneurial ventures
Practitioners generally assert that collaboration with the Open Source software (OSS) community enables young software firms to achieve superior innovation performance. Nonetheless, to the best of our knowledge, scholars have never extensively speculated about this assertion or rigorously tested it. In this paper, we attempt to do so. First, we root on the entrepreneurship literature and on the OSS research stream to discuss and empirically investigate whether entrepreneurial ventures collaborating with the OSS community (OSS EVs) achieve innovation performance superior to that of their non-collaborating peers. Then, we refer to the concept of absorptive capacity to determine which factors make OSS EVs better able to leverage their collaboration with the OSS community for innovation purposes. Our econometric estimates use a sample of 230 firms and indicate that OSS EVs collaborating with the OSS community achieve superior innovation. At the same time, the impact of community collaborations on innovation is stronger for EVs that are endowed with more skilled human capital, have experience with firm- OSS community collaboration, and actively contribute to the community.Entrepreneurial ventures, Open Source, firm-community collaboration, innovation performance
On the determinants of the degree of openness of Open Source firms: An entry model
This paper examines the relationship between the degree of openness that software start-ups choose and some of the main industrial features faced by new entrants. Hypotheses derived from a formal model are tested through the implementation of econometric techniques and information provided by a novel database (ELISS). Theoretical predictions and empirical results indicate that the choice by start-ups of the degree of openness is negatively influenced by the sensitivity of consumers to price and is positively related both to the strength of network externalities their products exhibit and to the competitive advantage of the incumbent.open-source software; network effects; entry
The Building Up of the Black Hole Mass - Stellar Mass Relation
We derive the growth of SMBHs relative to the stellar content of their host
galaxy predicted under the assumption of BH accretion triggered by galaxy
encounters occurring during their merging histories. We show that, within this
framework, the ratio Gamma=(M_BH/M_*)(z)/(M_BH/M_*)(z=0) between the Black Hole
mass and the galactic stellar mass (normalized to the local value) depends on
both BH mass and redshift. While the average value and the spread of Gamma(z)
increase with z, such an effect is larger for massive BHs, reaching values
Gamma=5 for massive Black Holes (M>10^9 M_{\odot}) at z>4, in agreement with
recent observations of high-redshift QSOs; this is due to the effectiveness of
interactions in triggering BH accretion in high-density environments at high
redshifts. To test such a model against observations, we worked out specific
predictions for sub-samples of the simulated galaxies corresponding to the
different observational samples for which measurements of Gamma have been
obtained. We found that for Broad Line AGNs at 1<z<2 values of Gamma=2 are
expected, with a mild trend toward larger value for increasing BH mass.
Instead, when we select from our Monte Carlo simulations only extremely gas
rich, rapidly star forming galaxies at 2<z<3, we find low values 0.3<Gamma<1.5,
consistent with recent observational findings on samples of sub-mm galaxies; in
the framework of our model, these objects end up at z=0 in low-to-intermediate
mass BHs (M<10^9 M_{\odot}), and they do not represent typical paths leading to
local massive galaxies. The latter have formed preferentially through paths
passing above the local M_*-M_BH relation. We discuss how the global picture
emerging from the model is consistent with a downsizing scenario, where massive
BHs accrete a larger fraction of their final mass at high redshifts z>4.Comment: 13 pages, 9 figures. Accepted to MNRA
The Holistic Ministry of the Anointed One: An Interpretive Analysis of Isaiah 61:1-7
Isaiah 61:1-7 is a passage that is important to both the Jewish and Christian faiths due to the clear Messianic implications that it contains. The first two verses are especially familiar to the Christian community due to its quotation by Jesus in Luke 4:18-19 as one of only three direct quotes of the Old Testament made by Jesus in Luke’s Gospel. Isaiah 61:1-7 prophecies on Judah’s upcoming destruction, exile, and ultimate Messianic deliverance, containing references to both the near and distant future; all of these conditions must be considered when approaching this passage in order to arrive at an accurate understanding of its original meaning. While Jesus’ ministry is sometimes presented in Christendom almost exclusively as a means to allow fallen man to escape hell and enter heaven, the Bible presents a much more holistic view of Jesus’ ministry in which he was concerned not only with future eternal realities but also the needs that people had in the here and now. Isaiah 61:1 - 7 has been noted as one of the marquee Old Testament prophecies supporting such a holistic view of Jesus’ ministry
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&
- …