279 research outputs found
The History of Cosmic Baryons: X-ray Emission vs. Star Formation Rate
We relate the star formation from cold baryons in virialized structures to
the X-ray properties of the associated diffuse, hot baryonic component. Our
computations use the standard ``semi-analytic'' models to describe i) the
evolution of dark matter halos through merging after the hierarchical
clustering, ii) the star formation governed by radiative cooling and by
supernova feedback, iii) the hydro- and thermodynamics of the hot gas, rendered
with our Punctuated Equilibria model. So we relate the X-ray observables
concerning the intra-cluster medium to the thermal energy of the gas pre-heated
and expelled by supernovae following star formation, and then accreted during
the subsequent merging events. We show that at fluxes fainter than erg/cm s (well within the reach of next generation X-ray
observatories) the X-ray counts of extended extragalactic sources (as well as
the faint end of the luminosity function, the contribution to the soft X-ray
background, and the correlation at the group scales) increase
considerably when the star formation rate is enhanced for z>1 as indicated by
growing optical/infrared evidence. Specifically, the counts in the range 0.5-2
keV are increased by factors when the the feedback is decreased and
star formation is enhanced as to yield a flat shape of the star formation rate
for 2<z<4.Comment: 11 pages, 8 figures, accepted for publication in MNRA
The SZ Effect as a Probe of Non-Gravitational Entropy in Groups and Clusters of Galaxies
We investigate how strongly and at what scales the Sunyaev- Zel'dovich effect
reflects the shifting balance between the two processes that compete for
governing the density and the thermodynamic state of the hot intra-cluster
medium pervading clusters and groups of galaxies: the hierarchical clustering
of the DM; the non-gravitational energy and momentum fed back into the ICM by
the condensing baryons. We base on a SAM of galaxy formation and clustering to
describe how the baryons are partitioned among the hot, the cool and the
stellar phase; the partition shifts as the galaxies cluster hierarchically, and
as the feedback by stellar winds and SN explosions follows the star formation.
Their impact is amplified by the same large scale accretion shocks that
thermalize the gravitational energy of gas falling into the growing potential
wells. We compute the Compton parameter , and find a relation of with
the ICM temperature, the relation, which departs from the self-similar
scaling and bends down at temperatures typical of galaxy groups. We
model-independently relate this with the analogous behaviour of the L_x - T
relation, and discuss to what extent our results are generic of the
hierarchical models of galaxy formation and clustering.Comment: 24 pages, 6 figures, submitted to MNRAS; typos correcte
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
Constraining the Warm Dark Matter Particle Mass through Ultra-Deep UV Luminosity Functions at z=2
We compute the mass function of galactic dark matter halos for different
values of the Warm Dark Matter (WDM) particle mass m_X and compare it with the
abundance of ultra-faint galaxies derived from the deepest UV luminosity
function available so far at redshift z~2. The magnitude limit M_UV=-13 reached
by such observations allows us to probe the WDM mass functions down to scales
close to or smaller than the half-mass mode mass scale ~10^9 M_sun. This
allowed for an efficient discrimination among predictions for different m_X
which turn out to be independent of the star formation efficiency adopted to
associate the observed UV luminosities of galaxies to the corresponding dark
matter masses. Adopting a conservative approach to take into account the
existing theoretical uncertainties in the galaxy halo mass function, we derive
a robust limit m_X>1.8 keV for the mass of thermal relic WDM particles when
comparing with the measured abundance of the faintest galaxies, while m_X>1.5
keV is obtained when we compare with the Schechter fit to the observed
luminosity function. The corresponding lower limit for sterile neutrinos
depends on the modeling of the production mechanism; for instance m_sterile > 4
keV holds for the Shi-Fuller mechanism. We discuss the impact of observational
uncertainties on the above bound on m_X. As a baseline for comparison with
forthcoming observations from the HST Frontier Field, we provide predictions
for the abundance of faint galaxies with M_UV=-13 for different values of m_X
and of the star formation efficiency, valid up to z~4.Comment: 14 pages, 3 figures. Accepted for publication in The Astrophysical
Journa
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
The Cosmological Mass Distribution from Cayley Trees with Disorder
We present a new approach to the statistics of the cosmic density field and
to the mass distribution of high-contrast structures, based on the formalism of
Cayley trees. Our approach includes in one random process both fluctuations and
interactions of the density perturbations. We connect tree-related quantities,
like the partition function or its generating function, to the mass
distribution. The Press \& Schechter mass function and the Smoluchowski kinetic
equation are naturally recovered as two limiting cases corresponding to
independent Gaussian fluctuations, and to aggregation of high-contrast
condensations, respectively. Numerical realizations of the complete random
process on the tree yield an excess of large-mass objects relative to the Press
\& Schechter function. When interactions are fully effective, a power-law
distribution with logarithmic slope -2 is generated.Comment: Plain TEX, compressed and uuencoded, 30 pages, 4 figs upon request
from [email protected]
- …