103 research outputs found
A chemical model for the interstellar medium in galaxies
We present and test chemical models for three-dimensional hydrodynamical
simulations of galaxies. We explore the effect of changing key parameters such
as metallicity, radiation and non-equilibrium versus equilibrium metal cooling
approximations on the transition between the gas phases in the interstellar
medium. The microphysics is modelled by employing the public chemistry package
KROME and the chemical networks have been tested to work in a wide range of
densities and temperatures. We describe a simple H/He network following the
formation of H, and a more sophisticated network which includes metals.
Photochemistry, thermal processes, and different prescriptions for the H
catalysis on dust are presented and tested within a one-zone framework. The
resulting network is made publicly available on the KROME webpage. We find that
employing an accurate treatment of the dust-related processes induces a faster
HI--H transition. In addition, we show when the equilibrium assumption for
metal cooling holds, and how a non-equilibrium approach affects the thermal
evolution of the gas and the HII--HI transition. These models can be employed
in any hydrodynamical code via an interface to KROME and can be applied to
different problems including isolated galaxies, cosmological simulations of
galaxy formation and evolution, supernova explosions in molecular clouds, and
the modelling of star-forming regions. The metal network can be used for a
comparison with observational data of CII 158 m emission both for
high-redshift as well as for local galaxies.Comment: A&A accepte
Priorities in gravitational waveforms for future space-borne detectors: vacuum accuracy or environment?
In preparation for future space-borne gravitational-wave (GW) detectors,
should the modelling effort focus on high-precision vacuum templates or on the
astrophysical environment of the sources? We perform a systematic comparison of
the phase contributions caused by 1) known environmental effects in both
gaseous and stellar matter backgrounds, or 2) high-order post-Newtonian {(PN)}
terms in the evolution of mHz GW sources {during the inspiral stage of massive
binaries}. We use the accuracy of currently available analytical waveform
models as a benchmark {value, finding} the following trends: the largest
unmodelled phase contributions are likely environmental rather than PN for
binaries lighter than ~M, where is the
redshift. Binaries heavier than ~M do not require
more accurate {inspiral} waveforms due to low signal-to-noise ratios (SNRs).
For high-SNR sources, environmental {phase contributions} are relevant at low
redshift, while high-order vacuum templates are required at . Led by
these findings, we argue that including environmental effects in waveform
models should be prioritised in order to maximize the science yield of future
mHz detectors.Comment: Accepted in MNRA
Black hole accretion versus star formation rate: theory confronts observations
We use a suite of hydrodynamical simulations of galaxy mergers to compare
star formation rate (SFR) and black hole accretion rate (BHAR) for galaxies
before the interaction ('stochastic' phase), during the `merger' proper,
lasting ~0.2-0.3 Gyr, and in the `remnant' phase. We calculate the bi-variate
distribution of SFR and BHAR and define the regions in the SFR-BHAR plane that
the three phases occupy. No strong correlation between BHAR and galaxy-wide SFR
is found. A possible exception are galaxies with the highest SFR and the
highest BHAR. We also bin the data in the same way used in several
observational studies, by either measuring the mean SFR for AGN in different
luminosity bins, or the mean BHAR for galaxies in bins of SFR. We find that the
apparent contradiction or SFR versus BHAR for observed samples of AGN and star
forming galaxies is actually caused by binning effects. The two types of
samples use different projections of the full bi-variate distribution, and the
full information would lead to unambiguous interpretation. We also find that a
galaxy can be classified as AGN-dominated up to 1.5 Gyr after the merger-driven
starburst took place. Our study is consistent with the suggestion that most
low-luminosity AGN hosts do not show morphological disturbances.Comment: MNRAS Letters, in pres
Supermassive black hole pairs in clumpy galaxies at high redshift: delayed binary formation and concurrent mass growth
Massive gas-rich galaxy discs at host massive star-forming
clumps with typical baryonic masses in the range ~M which
can affect the orbital decay and concurrent growth of supermassive black hole
(BH) pairs. Using a set of high-resolution simulations of isolated clumpy
galaxies hosting a pair of unequal-mass BHs, we study the interaction between
massive clumps and a BH pair at kpc scales, during the early phase of the
orbital decay. We find that both the interaction with massive clumps and the
heating of the cold gas layer of the disc by BH feedback tend to delay
significantly the orbital decay of the secondary, which in many cases is
ejected and then hovers for a whole Gyr around a separation of 1--2 kpc. In the
envelope, dynamical friction is weak and there is no contribution of disc
torques: these lead to the fastest decay once the orbit of the secondary BH has
circularised in the disc midplane. In runs with larger eccentricities the delay
is stronger, although there are some exceptions. We also show that, even in
discs with very sporadic transient clump formation, a strong spiral pattern
affects the decay time-scale for BHs on eccentric orbits. We conclude that,
contrary to previous belief, a gas-rich background is not necessarily conducive
to a fast BH decay and binary formation, which prompts more extensive
investigations aimed at calibrating event-rate forecasts for ongoing and future
gravitational-wave searches, such as with Pulsar Timing Arrays and the future
evolved Laser Interferometer Space Antenna.Comment: Accepted by MNRA
Growing black holes and galaxies: black hole accretion versus star formation rate
We present a new suite of hydrodynamical simulations and use it to study, in
detail, black hole and galaxy properties. The high time, spatial and mass
resolution, and realistic orbits and mass ratios, down to 1:6 and 1:10, enable
us to meaningfully compare star formation rate (SFR) and BH accretion rate
(BHAR) timescales, temporal behaviour and relative magnitude. We find that (i)
BHAR and galaxy-wide SFR are typically temporally uncorrelated, and have
different variability timescales, except during the merger proper, lasting
~0.2-0.3 Gyr. BHAR and nuclear (<100 pc) SFR are better correlated, and their
variability are similar. Averaging over time, the merger phase leads typically
to an increase by a factor of a few in the BHAR/SFR ratio. (ii) BHAR and
nuclear SFR are intrinsically proportional, but the correlation lessens if the
long-term SFR is measured. (iii) Galaxies in the remnant phase are the ones
most likely to be selected as systems dominated by an active galactic nucleus
(AGN), because of the long time spent in this phase. (iv) The timescale over
which a given diagnostic probes the SFR has a profound impact on the recovered
correlations with BHAR, and on the interpretation of observational data.Comment: Accepted for publication in MNRA
Growth and activity of black holes in galaxy mergers with varying mass ratios
We study supermassive black holes (BHs) in merging galaxies, using a suite of
hydrodynamical simulations with very high spatial (~10 pc) and temporal (~1
Myr) resolution, where we vary the initial mass ratio, the orbital
configuration, and the gas fraction. (i) We address the question of when and
why, during a merger, increased BH accretion occurs, quantifying gas inflows
and BH accretion rates. (ii) We also quantify the relative effectiveness in
inducing AGN activity of merger-related versus secular-related causes, by
studying different stages of the encounter: the stochastic (or early) stage,
the (proper) merger stage, and the remnant (or late) stage. (iii) We assess
which galaxy mergers preferentially enhance BH accretion, finding that the
initial mass ratio is the most important factor. (iv) We study the evolution of
the BH masses, finding that the BH mass contrast tends to decrease in minor
mergers and to increase in major mergers. This effect hints at the existence of
a preferential range of mass ratios for BHs in the final pairing stages. (v) In
both merging and dynamically quiescent galaxies, the gas accreted by the BH is
not necessarily the gas with angular momentum, but the gas that
angular momentum.Comment: Accepted for publication in MNRAS, 23 pages, 22 figures, 3 table
The birth of a supermassive black hole binary
We study the dynamical evolution of supermassive black holes, in the late
stage of galaxy mergers, from kpc to pc scales. In particular, we capture the
formation of the binary, a necessary step before the final coalescence, and
trace back the main processes causing the decay of the orbit. We use
hydrodynamical simulations of galaxy mergers with different resolutions, from
down to , in order to study the effects of the
resolution on our results, remove numerical effects, and assess that resolving
the influence radius of the orbiting black hole is a minimum condition to fully
capture the formation of the binary. Our simulations include the relevant
physical processes, namely star formation, supernova feedback, accretion onto
the black holes and the ensuing feedback. We find that, in these mergers,
dynamical friction from the smooth stellar component of the nucleus is the main
process that drives black holes from kpc to pc scales. Gas does not play a
crucial role and even clumps do not induce scattering or perturb the orbits. We
compare the time needed for the formation of the binary to analytical
predictions and suggest how to apply such analytical formalism to obtain
estimates of binary formation times in lower resolution simulations.Comment: 12 pages, 12 Figures, submitted to MNRA
Improved constraints from ultra-faint dwarf galaxies on primordial black holes as dark matter
Soon after the recent first ever detection of gravitational waves from
merging black holes it has been suggested that their origin is primordial.
Appealingly, a sufficient number of primordial black holes (PBHs) could also
partially or entirely constitute the dark matter (DM) in the Universe. However,
recent studies on PBHs in ultra-faint dwarf galaxies (UFDGs) suggest that they
would dynamically heat up the stellar component due to two-body relaxation
processes. From the comparison with the observed stellar velocity dispersions
and the stellar half-light radii it was claimed that only PBHs with masses
can significantly contribute to the DM. In this work, we
improve the latter constraints by considering the largest observational sample
of UFDGs and by allowing the PBH masses to follow an extended (log-normal)
distribution. By means of collisional Fokker-Planck simulations, we explore a
wide parameter space of UFDGs containing PBHs. The analysis of the half-light
radii and velocity dispersions resulting from the simulations leads to three
general findings that exclude PBHs with masses
- from constituting all of the DM: (i) We
identify a critical sub-sample of UFDGs that only allows for
PBH masses; (ii) for any PBH mass, there is an
UFDG in our sample that disfavours it; (iii) the spatial extensions of a
majority of simulated UFDGs containing PBHs are too large to match the
observed
Generation of gravitational waves and tidal disruptions in clumpy galaxies
Obtaining a better understanding of intermediate-mass black holes (IMBHs) is crucial, as their properties could shed light on the origin and growth of their supermassive counterparts. Massive star-forming clumps, which are present in a large fraction of massive galaxies at z ∼ 1–3, are among the venues wherein IMBHs could reside. We perform a series of Fokker–Planck simulations to explore the occurrence of tidal disruption (TD) and gravitational wave (GW) events about an IMBH in a massive star-forming clump, modelling the latter so that its mass (108M⊙) and effective radius (100 pc) are consistent with the properties of both observed and simulated clumps. We find that the TD and GW event rates are in the ranges of 10−6 to 10−5 and 10−8 to 10−7 yr−1, respectively, depending on the assumptions for the initial inner density profile of the system (ρ ∝ r−2 or ∝ r−1) and the initial mass of the central IMBH (105 or 103M⊙). By integrating the GW event rate over z = 1–3, we expect that the Laser Interferometer Space Antenna will be able to detect ∼2 GW events per year coming from these massive clumps; the intrinsic rate of TD events from these systems amounts instead to a few 103 per year, a fraction of which will be observable by e.g. the Square Kilometre Array and the Advanced Telescope for High Energy Astrophysics. In conclusion, our results support the idea that the forthcoming GW and electromagnetic facilities may have the unprecedented opportunity of unveiling the lurking population of IMBHs
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