5 research outputs found
Massive Black Hole Dynamics in Unequal Mass Galaxy Mergers.
Observed scaling relations suggest an evolutionary link between massive black holes (BHs) and their host galaxies. We investigate the coevolution of BH and host in galaxy mergers, which drive active galactic nuclei (AGN) activity, produce BH mergers, and lead to starbursts and morphological transitions in galaxies. Furthermore, we focus on the most cosmologically relevant galaxy mergers: unequal mass mergers at high redshift (z = 3). Using high resolution N-body SPH simulations, we track star formation, BH accretion, and associated feedback as the BHs move from separations of tens of kiloparsecs to tens of parsecs. We focus on the role of merger triggered gaseous inflows in driving both central starbursts and efficient BH accretion. We find that the efficiency of BH pairing depends sensitively on the strength of central star formation in the secondary galaxy. Strong gas inflows build up a central stellar cusp that is denser than the primary galaxy, leading the secondary's nucleus to disrupt the nucleus of the larger primary galaxy and resulting in a short timescale (10-20 Myr) for the formation of a BH binary. If the secondary instead experiences weak inflows and strong ram pressure from the primary's disk, the secondary's nucleus is disrupted due to tidal shocks and binary formation is delayed. We also consider simultaneous accretion onto both BHs, testing when and for how long accretion is triggered at a number of observability thresholds. We find that strong dual AGN activity occurs in the late stages of the mergers, at small separations of a few kiloparsecs. Most of the BH accretion is not simultaneous, limiting the observable dual AGN fraction. Finally, we consider the evolution of BHs in low mass systems with quiet merger histories, probing the distribution of BHs on the low mass end of the observed scaling relations. We evolve a population of seed BHs in a Milky Way halo and find that the BH population in dwarf galaxies has not grown much since formation, providing an indicator of the original BH seed formation mechanism. We derive the BH occupation fraction and mass distribution for a range of dwarf galaxies.PHDAstronomy and AstrophysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/97952/1/svanwas_1.pd
Observability of Dual Active Galactic Nuclei in Merging Galaxies
Supermassive black holes (SMBHs) have been detected in the centers of most
nearby massive galaxies. Galaxies today are the products of billions of years
of galaxy mergers, but also billions of years of SMBH activity as active
galactic nuclei (AGNs) that is connected to galaxy mergers. In this context,
detection of AGN pairs should be relatively common. Observationally, however,
dual AGN are scant, being just a few percent of all AGN. In this Letter we
investigate the triggering of AGN activity in merging galaxies via a suite of
high resolution hydrodynamical simulations. We follow the dynamics and
accretion onto the SMBHs as they move from separations of tens of kiloparsecs
to tens of parsecs. Our resolution, cooling and star formation implementation
produce an inhomogeneous, multi-phase interstellar medium, allowing us to
accurately trace star formation and accretion onto the SMBHs. We study the
impact of gas content, morphology, and mass ratio, allowing us to study AGN
activity and dynamics across a wide range of relevant conditions. We test when
the two AGN are simultaneously detectable, for how long and at which
separations. We find that strong dual AGN activity occurs during the late
phases of the mergers, at small separations (<1-10 kpc) below the resolution
limit of most surveys. Much of the SMBH accretion is not simultaneous, limiting
the dual AGN fraction detectable through imaging and spectroscopy to a few
percent, in agreement with observational samples.Comment: Published in ApJL; additional material available at
http://www.astro.lsa.umich.edu/~svanwas/dualAGN.htm
Nuclear coups: dynamics of black holes in galaxy mergers
We study the dynamical evolution of supermassive black holes (BHs) in merging
galaxies on scales of hundreds of kpc to 10 pc, to identify the physical
processes that aid or hinder the orbital decay of BHs. We present
hydrodynamical simulations of galaxy mergers with a resolution of 20 pc,
chosen to accurately track the motion of the nuclei and provide a realistic
environment for the evolution of the BHs. We find that, during the late stages
of the merger, tidal shocks inject energy in the nuclei, causing one or both
nuclei to be disrupted and leaving their BH `naked', without any bound gas or
stars. In many cases, the nucleus that is ultimately disrupted is that of the
larger galaxy (`nuclear coup'), as star formation grows a denser nuclear cusp
in the smaller galaxy. We supplement our simulations with an analytical
estimate of the orbital-decay time required for the BHs to form a binary at
unresolved scales, due to dynamical friction. We find that, when a nuclear coup
occurs, the time-scale is much shorter than when the secondary's nucleus is
disrupted, as the infalling BH is more massive, and it also finds itself in a
denser stellar environment.Comment: Accepted for publication in MNRAS, 16 pages, 13 figures, 2 table
Origin of intermittent accretion-powered X-ray oscillations in neutron stars with millisecond spin periods
We have shown previously that many of the properties of persistent
accretion-powered millisecond pulsars can be understood if their X-ray emitting
areas are near their spin axes and move as the accretion rate and structure of
the inner disk vary. Here we show that this "nearly aligned moving spot model"
may also explain the intermittent accretion-powered pulsations that have been
detected in three weakly magnetic accreting neutron stars. We show that
movement of the emitting area from very close to the spin axis to about 10
degrees away can increase the fractional rms amplitude from less than about 0.5
percent, which is usually undetectable with current instruments, to a few
percent, which is easily detectable. The second harmonic of the spin frequency
usually would not be detected, in agreement with observations. The model
produces intermittently detectable oscillations for a range of emitting area
sizes and beaming patterns, stellar masses and radii, and viewing directions.
Intermittent oscillations are more likely in stars that are more compact. In
addition to explaining the sudden appearance of accretion-powered millisecond
oscillations in some neutron stars with millisecond spin periods, the model
explains why accretion-powered millisecond oscillations are relatively rare and
predicts that the persistent accretion-powered millisecond oscillations of
other stars may become undetectable for brief intervals. It suggests why
millisecond oscillations are frequently detected during the X-ray bursts of
some neutron stars but not others and suggests mechanisms that could explain
the occasional temporal association of intermittent accretion-powered
oscillations with thermonuclear X-ray bursts.Comment: 5 pages, 1 figure; includes additional discussion and updated
references; accepted for publication in ApJ
A model for the waveform behavior of accreting millisecond pulsars: Nearly aligned magnetic fields and moving emission regions
We investigate further a model of the accreting millisecond X-ray pulsars we
proposed earlier. In this model, the X-ray-emitting regions of these pulsars
are near their spin axes but move. This is to be expected if the magnetic poles
of these stars are close to their spin axes, so that accreting gas is channeled
there. As the accretion rate and the structure of the inner disk vary, gas is
channeled along different field lines to different locations on the stellar
surface, causing the X-ray-emitting areas to move. We show that this "nearly
aligned moving spot model" can explain many properties of the accreting
millisecond X-ray pulsars, including their generally low oscillation amplitudes
and nearly sinusoidal waveforms; the variability of their pulse amplitudes,
shapes, and phases; the correlations in this variability; and the similarity of
the accretion- and nuclear-powered pulse shapes and phases in some. It may also
explain why accretion-powered millisecond pulsars are difficult to detect, why
some are intermittent, and why all detected so far are transients. This model
can be tested by comparing with observations the waveform changes it predicts,
including the changes with accretion rate.Comment: 21 pages, 6 figures; includes 3 new sections, 14 additional pages, 4
additional figures with 11 new plots, and additional references; accepted for
publication in Ap