23 research outputs found
222Rn concentration in the atmosphere in Milan and in the plain around
Radon measurements at ground level are a useful tool for the study on atmospheric stability or mixing conditions. Radon concentration in the atmosphere was measured in Milan over a continuous four-year period from 1996 to 1999. In the town area pollutants emitted during late evening and night hours by motor traffic as well as by industrial plants accumulate at low height concurring with radon accumulation in the Nocturnal Stable Layer. In years
1998-1999 radon measurements were taken in Landriano, an area located 30 km south of the town. In Lombard plain there are numerous huge plants, thermoelectric power stations burning by methane and carbon, oil refineries; moreover other plants are being carried out. With the aim to control the contribute of these plants to air pollution, it is important to know atmospheric stability and turbulence conditions. Landriano is only few kilometres from the thermoelectric power station in Tavazzano. In this
paper the results of measuring campaigns as well as the comparison of radon concentrations between the two sites surveyed are reported. The general criteria of the
measurements, the interpretative models of radon concentration in the atmosphere are discussed
Post-Newtonian evolution of massive black hole triplets in galactic nuclei - III. A robust lower limit to the nHz stochastic background of gravitational waves
Inspiraling massive black hole binaries (MBHBs) forming in the aftermath of galaxy mergers are expected to be the loudest gravitational-wave (GW) sources relevant for pulsar-timing arrays (PTAs) at nHz frequencies. The incoherent overlap of signals from a cosmic population of MBHBs gives rise to a stochastic GW background (GWB) with characteristic strain around h c ~ 10 -15 at a reference frequency of 1 yr -1 , although uncertainties around this value are large. Current PTAs are piercing into theGWamplitude range predicted by MBHB-population models, but no detection has been reported so far. To assess the future success prospects of PTA experiments, it is therefore important to estimate the minimum GWB level consistent with our current understanding of the formation and evolution of galaxies and massive black holes (MBHs). To this purpose, we couple a semi-analytic model of galaxy evolution and an extensive study of the statistical outcome of triple MBH interactions. We show that even in the most pessimistic scenario where all MBHBs stall before entering the GW-dominated regime, triple interactions resulting from subsequent galaxy mergers inevitably drive a considerable fraction of the MBHB population to coalescence. At frequencies relevant for PTA, the resulting GWB is only a factor of 2-3 suppressed compared to a fiducial model where binaries are allowed to merge over Gyr time-scales. Coupled with current estimates of the expected GWB amplitude range, our findings suggest that the minimum GWB from cosmic MBHBs is unlikely to be lower than h c ~ 10 -16 (at f = 1 yr -1 ), well within the expected sensitivity of projected PTAs based on future observations with FAST, MeerKAT, and SKA. \ua9 2018 The Author(s)
A Radio Census of Binary Supermassive Black Holes
Using archival VLBI data for 3114 radio-luminous active galactic nuclei, we
searched for binary supermassive black holes using a radio spectral index
mapping technique which targets spatially resolved, double radio-emitting
nuclei. Only one source was detected as a double nucleus. This result is
compared with a cosmological merger rate model and interpreted in terms of (1)
implications for post-merger timescales for centralisation of the two black
holes, (2) implications for the possibility of "stalled" systems, and (3) the
relationship of radio activity in nuclei to mergers. Our analysis suggests that
the binary evolution of paired supermassive black holes (both of masses >= 1e8
Msun) spends less than 500 Myr in progression from the merging of galactic
stellar cores to within the purported stalling radius for supermassive black
hole pairs. The data show no evidence for an excess of stalled binary systems
at small separations. We see circumstantial evidence that the relative state of
radio emission between paired supermassive black holes is correlated within
orbital separations of 2.5 kpc.Comment: 11 Pages, 7 figures, accepted for publication in MNRA
Modeling the cosmological co-evolution of supermassive black holes and galaxies: II. The clustering of quasars and their dark environment
We use semi-analytic modeling on top of the Millennium simulation to study
the joint formation of galaxies and their embedded supermassive black holes.
Our goal is to test scenarios in which black hole accretion and quasar
activity are triggered by galaxy mergers, and to constrain different models for
the lightcurves associated with individual quasar events. In the present work
we focus on studying the spatial distribution of simulated quasars. At all
luminosities, we find that the simulated quasar two-point correlation function
is fit well by a single power-law in the range 0.5 < r < 20 h^{-1} Mpc, but its
normalization is a strong function of redshift. When we select only quasars
with luminosities within the range typically accessible by today's quasar
surveys, their clustering strength depends only weakly on luminosity, in
agreement with observations. This holds independently of the assumed lightcurve
model, since bright quasars are black holes accreting close to the Eddington
limit, and are hosted by dark matter haloes with a narrow mass range of a few
10^12 h^{-1} M_sun. Therefore the clustering of bright quasars cannot be used
to disentangle lightcurve models, but such a discrimination would become
possible if the observational samples can be pushed to significantly fainter
limits.
Overall, our clustering results for the simulated quasar population agree
rather well with observations, lending support to the conjecture that galaxy
mergers could be the main physical process responsible for triggering black
hole accretion and quasar activity.Comment: 17 pages, 16 figures, to be published on MNRA
The Demography of Super-Massive Black Holes: Growing Monsters at the Heart of Galaxies
Supermassive black holes (BHs) appear to be ubiquitous at the center of all
galaxies which have been observed at high enough sensitivities and resolution
with the Hubble Space Telescope. Their masses are found to be tightly linked
with the masses and velocity dispersions of their host galaxies. On the other
hand, BHs are widely held to constitute the central engines of quasars and
active galactic nuclei (AGN) in general. It is however still unclear how BHs
have grown, and whether they have co-evolved with their hosts. In this Review I
discuss how, in ways independent of specific models, constraints on the growth
history of BHs and their host galaxies have been set by matching the statistics
of local BHs to the emissivity, number density, and clustering properties of
AGNs at different cosmological epochs. I also present some new results obtained
through a novel numerical code which evolves the BH mass function and
clustering adopting broad distributions of Eddington ratios. I finally review
BH evolution in a wider cosmological context, connecting BH growth to galaxy
evolution.Comment: 70 pages. New Astronomy Reviews, in pres
Galaxy Formation Theory
We review the current theory of how galaxies form within the cosmological
framework provided by the cold dark matter paradigm for structure formation.
Beginning with the pre-galactic evolution of baryonic material we describe the
analytical and numerical understanding of how baryons condense into galaxies,
what determines the structure of those galaxies and how internal and external
processes (including star formation, merging, active galactic nuclei etc.)
determine their gross properties and evolution. Throughout, we highlight
successes and failings of current galaxy formation theory. We include a review
of computational implementations of galaxy formation theory and assess their
ability to provide reliable modelling of this complex phenomenon. We finish
with a discussion of several "hot topics" in contemporary galaxy formation
theory and assess future directions for this field.Comment: 58 pages, to appear in Physics Reports. This version includes minor
corrections and a handful of additional reference