471 research outputs found
Global torques and stochasticity as the drivers of massive black hole pairing in the young Universe
The forthcoming Laser Interferometer Space Antenna (LISA) will probe the
population of coalescing massive black hole (MBH) binaries up to the onset of
structure formation. Here we simulate the galactic-scale pairing of MBHs in a typical, non-clumpy main-sequence galaxy embedded in a
cosmological environment at . In order to increase our statistical
sample, we adopt a strategy that allows us to follow the evolution of six
secondary MBHs concomitantly. We find that the magnitude of the
dynamical-friction induced torques is significantly smaller than that of the
large-scale, stochastic gravitational torques arising from the perturbed and
morphologically evolving galactic disc, suggesting that the standard dynamical
friction treatment is inadequate for realistic galaxies at high redshift. The
dynamical evolution of MBHs is very stochastic, and a variation in the initial
orbital phase can lead to a drastically different time-scale for the inspiral.
Most remarkably, the development of a galactic bar in the host system either
significantly accelerates the inspiral by dragging a secondary MBH into the
centre, or ultimately hinders the orbital decay by scattering the MBH in the
galaxy outskirts. The latter occurs more rarely, suggesting that galactic bars
overall promote MBH inspiral and binary coalescence. The orbital decay time can
be an order of magnitude shorter than what would be predicted relying on
dynamical friction alone. The stochasticity, and the important role of global
torques, have crucial implications for the rates of MBH coalescences in the
early Universe: both have to be accounted for when making predictions for the
upcoming LISA observatory.Comment: Accepted for publication in MNRAS; 15 pages, 10 Figures, 2 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
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
A definitive merger-AGN connection at z~0 with CFIS: mergers have an excess of AGN and AGN hosts are more frequently disturbed
The question of whether galaxy mergers are linked to the triggering of active
galactic nuclei (AGN) continues to be a topic of considerable debate. The issue
can be broken down into two distinct questions: 1) Can galaxy mergers trigger
AGN? 2) Are galaxy mergers the dominant AGN triggering mechanism? A complete
picture of the AGN-merger connection requires that both of these questions are
addressed with the same dataset. In previous work, we have shown that galaxy
mergers selected from the Sloan Digital Sky Survey (SDSS) show an excess of
both optically-selected, and mid-IR colour-selected AGN, demonstrating that the
answer to the first of the above questions is affirmative. Here, we use the
same optical and mid-IR AGN selection to address the second question, by
quantifying the frequency of morphological disturbances in low surface
brightness r-band images from the Canada France Imaging Survey (CFIS). Only ~30
per cent of optical AGN host galaxies are morphologically disturbed, indicating
that recent interactions are not the dominant trigger. However, almost 60 per
cent of mid-IR AGN hosts show signs of visual disturbance, indicating that
interactions play a more significant role in nuclear feeding. Both mid-IR and
optically selected AGN have interacting fractions that are a factor of two
greater than a mass and redshift matched non-AGN control sample, an excess that
increases with both AGN luminosity and host galaxy stellar mass.Comment: Accepted for publication in MNRA
Multiple regimes and coalescence timescales for massive black hole pairs ; the critical role of galaxy formation physics
We discuss the latest results of numerical simulations following the orbital
decay of massive black hole pairs in galaxy mergers. We highlight important
differences between gas-poor and gas-rich hosts, and between orbital evolution
taking place at high redshift as opposed to low redshift. Two effects have a
huge impact and are rather novel in the context of massive black hole binaries.
The first is the increase in characteristic density of galactic nuclei of
merger remnants as galaxies are more compact at high redshift due to the way
dark halo collapse depends on redshift. This leads naturally to hardening
timescales due to 3-body encounters that should decrease by two orders of
magnitude up to . It explains naturally the short binary coalescence
timescale, Myr, found in novel cosmological simulations that follow
binary evolution from galactic to milliparsec scales. The second one is the
inhomogeneity of the interstellar medium in massive gas-rich disks at high
redshift. In the latter star forming clumps 1-2 orders of magnitude more
massive than local Giant Molecular Clouds (GMCs) can scatter massive black
holes out of the disk plane via gravitational perturbations and direct
encounters. This renders the character of orbital decay inherently stochastic,
often increasing orbital decay timescales by as much as a Gyr. At low redshift
a similar regime is present at scales of pc inside Circumnuclear Gas
Disks (CNDs). In CNDs only massive black holes with masses below can be significantly perturbed. They decay to sub-pc separations in
up to yr rather than the in just a few million years as in a smooth
CND. Finally implications for building robust forecasts of LISA event rates are
discussedComment: 13 pages, 3 Figures, Invited Paper to appear in the Proceedings of
the 11th International LISA Symposium, IOP Journal of Physics: Conference
Serie
The role of bars on the dynamical-friction driven inspiral of massive perturbers
In this paper, we explore the impact of a galactic bar on the inspiral
time-scale of a massive perturber (MP) within a Milky Way-like galaxy. We
integrate the orbit of MPs in a multi-component galaxy model via a
semi-analytical approach including an accurate treatment for dynamical friction
generalized to rotationally supported backgrounds. We compare the MP evolution
in a galaxy featuring a Milky Way-like rotating bar to the evolution within an
analogous axisymmetric galaxy without the bar. We find that the bar presence
may significantly affect the inspiral, sometimes making it shorter by a factor
of a few, sometimes hindering it for a Hubble time, implying that dynamical
friction alone is greatly insufficient to fully characterize the orbital decay.
The effect of the bar is more prominent for initially in-plane, prograde MPs,
especially those crossing the bar co-rotation radius or outer Lindblad
resonance. In the barred galaxy, we find the sinking of the most massive MPs
(>~10^7.5 Msun) approaching the galaxy from large separations (>~8 kpc) to be
most efficiently hampered. Neglecting the effect of global torques associated
to the non-symmetric mass distribution is thus not advisable even within our
idealized, smooth Milky Way model, and it should be avoided when dealing with
more complex and realistic galaxy systems. This has important implications for
the orbital decay of massive black holes in late-type spirals, the natural
candidate sources to be detected with the Laser Interferometer Space Antenna
(LISA).Comment: 16 pages, 12 figures, 2 tables. Submitted to MNRAS. Comments welcome
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