37 research outputs found
Constraining Dynamical Dark Energy Models through the Abundance of High-Redshift Supermassive Black Holes
We compute the number density of massive Black Holes (BHs) at the centre of
galaxies at z=6 in different Dynamical Dark Energy (DDE) cosmologies, and
compare it with existing observational lower limits, to derive constraints on
the evolution of the Dark Energy equation of state parameter w. Our approach
only assumes the canonical scenario for structure formation from the collapse
of overdense regions of the Dark Matter dominated primordial density field on
progressively larger scales; the Black Hole accretion and merging rate have
been maximized in the computation so as to obtain robust constraints on w and
on its look-back time derivative w_a. Our results provide independent
constraints complementary to those obtained by combining Supernovae, Cosmic
Microwave Background and Baryonic Acoustic Oscillations; while the latter
concern combinations of w_0 and w_a leaving the time evolution of the state
parameter w_a highly unconstrained, the BH abundance mainly provide upper
limits on w_a, only weakly depending on w_0. Combined with the existing
constraints, our results significantly restrict the allowed region in DDE
parameter space, ruling out DDE models not providing cosmic time and fast
growth factor large enough to allow for the building up of the observed
abundance of BHs; in particular, models with -1.2 \leq w_0 \leq -1 and positive
redshift evolution w_a > 0.8 - completely consistent with previous constraints
- are strongly disfavoured by our independent constraints from BH abundance.
Such range of parameters corresponds to "Quintom" DDE models, with w crossing
-1 starting from larger values.Comment: 19 pages, 6 figures, accepted to MNRA
The deepest view of radio AGN in COSMOS: a twoÂfold population
The 3 GHz VLAÂCOSMOS surve
Why z > 1 radio-loud galaxies are commonly located in protoclusters
Distant powerful radio-loud active galactic nuclei (RLAGN) tend to reside in dense environments and are commonly found in protoclusters at z > 1.3. We examine whether this occurs because RLAGN are hosted by massive galaxies, which preferentially reside in rich environments. We compare the environments of powerful RLAGN at 1.3 1014M cluster having experienced powerful radio-loud feedback of duration ~60 Myr during 1.3<z<3.2. This feedback could heat the intracluster medium to the extent of 0.5â1 keV per gas particle, which could limit the amount
of gas available for further star formation in the protocluster galaxies
Black hole spin: theory and observation
In the standard paradigm, astrophysical black holes can be described solely
by their mass and angular momentum - commonly referred to as `spin' - resulting
from the process of their birth and subsequent growth via accretion. Whilst the
mass has a standard Newtonian interpretation, the spin does not, with the
effect of non-zero spin leaving an indelible imprint on the space-time closest
to the black hole. As a consequence of relativistic frame-dragging, particle
orbits are affected both in terms of stability and precession, which impacts on
the emission characteristics of accreting black holes both stellar mass in
black hole binaries (BHBs) and supermassive in active galactic nuclei (AGN).
Over the last 30 years, techniques have been developed that take into account
these changes to estimate the spin which can then be used to understand the
birth and growth of black holes and potentially the powering of powerful jets.
In this chapter we provide a broad overview of both the theoretical effects of
spin, the means by which it can be estimated and the results of ongoing
campaigns.Comment: 55 pages, 5 figures. Published in: "Astrophysics of Black Holes -
From fundamental aspects to latest developments", Ed. Cosimo Bambi, Springer:
Astrophysics and Space Science Library. Additional corrections mad
The star formation and AGN luminosity relation: predictions from a semi-analytical model
In a universe where active galactic nucleus (AGN) feedback regulates star formation in massive galaxies, a strong correlation between these two quantities is expected. If the gas causing star formation is also responsible for feeding the central black hole, then a positive correlation is expected. If powerful AGNs are responsible for the star formation quenching, then a negative correlation is expected. Observations so far have mainly found a mild correlation or no correlation at all [i.e. a flat relation between star formation rate (SFR) and AGN luminosity], raising questions about the whole paradigm of âAGN feedbackâ. In this paper, we report the predictions of the GALFORM GALFORM semi-analytical model, which has a very strong coupling between AGN activity and quenching of star formation. The predicted SFRâAGN luminosity correlation appears negative in the low AGN luminosity regime, where AGN feedback acts, but becomes strongly positive in the regime of the brightest AGN. Our predictions reproduce reasonably well recent observations by Rosario et al., yet there is some discrepancy in the normalization of the correlation at low luminosities and high redshifts. Though this regime could be strongly influenced by observational biases, we argue that the disagreement could be ascribed to the fact that GALFORM GALFORM neglects AGN variability effects. Interestingly, the galaxies that dominate the regime where the observations imply a weak correlation are massive early-type galaxies that are subject to AGN feedback. Nevertheless, these galaxies retain high enough molecular hydrogen contents to maintain relatively high SFRs and strong infrared emission
The cluster-scale environment of PKS 2155-304
PKS 2155â304 is one of the brightest extragalactic source in the X-ray and EUV bands, and is a prototype for the BL Lac class of objects. In this paper, we investigate the large-scale environment of this source using new multi-object as well as long-slit spectroscopy, together with archival spectra and optical images. We find clear evidence of a modest overdensity of galaxies at z = 0.116â10 ± 0.000â06, consistent with previous determinations of the BL Lac redshift. The galaxy group has a radial velocity dispersion of 250+80â40 km sâ1 and a virial radius of 0.22 Mpc, yielding a role-of-thumb estimate of the virial mass of Mvir ⌠1.5 Ă 1013 Mâ, i.e. one order of magnitude less than that observed in similar objects. This result hints towards a relatively wide diversity in the environmental properties of BL Lac objects