159 research outputs found
Size and Orientation of the `Z' in ZRGs
Some X-shaped radio galaxies (XRGs) show a Z-symmetric morphology in the less
luminous secondary lobes. Our geometrical arguments strongly support a merger
of two galaxies as mechanism for the formation of these sources (ZRG). They
also strengthen the conjecture that a jet is aligned with the spin of the BH at
its base and that the jet flips into the direction of the orbital angular
momentum of the pre-merger binary black hole (BHB). We could also restrict the
distance where the pre-merger jet is bent into Z-shape by the inspiralling
galaxy to the range of 30-100 kpc. One of three possible orientations of the
jet relative to our line of sight is more likely than the others and allows us
to deduce the direction of the spin of the merged BH. The existence of XRGs and
ZRGs proves that the binary has merged, contrary to previous speculations that
after a merger of two galaxies the decay of the BHB stalls due to loss cone
depletion. In ZRGs the black holes probably merge on timescales of some 10^8 yr
after the bending of the jet in a distance of about 50 kpc. Thus, in a way, the
bending starts a stop watch for the rest of the merger.Comment: 2 pages, to appear in the proceedings of ESO Astrophysics Symposia:
Groups of Galaxies in the Nearby Universe, Santiago de Chile, Dec. 200
Non-ideal magnetohydrodynamics on a moving mesh I: Ohmic and ambipolar diffusion
Especially in cold and high-density regions, the assumptions of ideal
magnetohydrodynamics (MHD) can break down, making first order non-ideal terms
such as Ohmic and ambipolar diffusion as well as the Hall effect important. In
this study we present a new numerical scheme for the first two resistive terms,
which we implement in the moving-mesh code AREPO using the single-fluid
approximation combined with a new gradient estimation technique based on a
least-squares fit per interface. Through various test calculations including
the diffusion of a magnetic peak, the structure of a magnetic C-shock, and the
damping of an Alfv\'en wave, we show that we can achieve an accuracy comparable
to the state-of-the-art code ATHENA++. We apply the scheme to the linear growth
of the magnetorotational instability and find good agreement with the
analytical growth rates. By simulating the collapse of a magnetised cloud with
constant magnetic diffusion, we show that the new scheme is stable even for
large density contrasts. Thanks to the Lagrangian nature of the moving mesh
method the new scheme is thus well suited for intended future applications
where a high resolution in the dense cores of collapsing protostellar clouds
needs to be achieved. In a forthcoming work we will extend the scheme to the
Hall effect.Comment: 17 pages, 18 figure
Merging of a massive binary due to ejection of bound stars
From the results of numerical scattering experiments and simulations of a
massive black hole binary in spherically symmetric and shallow cores it has
been deduced that most likely the shrinking process stalls due to loss-cone
depletion before emission of gravitational radiation becomes important. Here we
follow a different approach and focus on the population of stars which is bound
to the binary and so far has not received much attention. With simple
assumptions which should not be sensitive to initial conditions we derive a
lower limit for the mass of stars which needs to be ejected by the binary in
order to coalesce. We also compute this mass in dependence on the steepness of
the density profile according to which the stars are distributed. Our results
are not as pessimistic as earlier conclusions and actually suggest that the BHs
merge.Comment: 6 pages, 2 figures, minor corrections, references added. Accepted by
MNRAS Letter
Orientation and size of the `Z' in `X'-shaped radio galaxies
Some X-shaped radio galaxies show a Z-symmetric morphology in the less
luminous secondary lobes. Within the scenario of a merger between two galaxies,
each hosting a supermassive black hole in its center, this structure has been
explained before: As the smaller galaxy spirals towards the common center, it
releases gas to the ISM of the larger active galaxy. The ram pressure of this
streaming gas will bend the lobes of the pre-merger jet into a Z-shape. After
the black holes have merged the jet propagates in a new direction that is
aligned with the angular momentum of the binary black hole. In this article we
deproject the pre- and post-merger jets. Taking into account the expected
angles between the jet pairs and with the assumption that their directions are
uncorrelated, we show that one of three possible orientations of the jets with
respect to the line of sight is more likely than the others. This actually
depends on the distance where the bending occurs. Another result of our
deprojection is that the streaming gas bends the jet into a Z-shape in a range
between about 30 and 100 kpc distance to the center of the primary galaxy. We
confirm this finding by comparing our predictions for the properties of the
rotational velocity field and its radius with observations and numerical
simulations of merging galaxies. Thus our results support the merger scenario
as explanation for X- and Z-shaped radio galaxies with the jet pointing along
the former axis of orbital angular momentum of the binary.Comment: 11 pages, 6 figures, minor changes in final version. Accepted for
publication in MNRA
Long-Term Evolution of Massive Black Hole Binaries. III. Binary Evolution in Collisional Nuclei
[Abridged] In galactic nuclei with sufficiently short relaxation times,
binary supermassive black holes can evolve beyond their stalling radii via
continued interaction with stars. We study this "collisional" evolutionary
regime using both fully self-consistent N-body integrations and approximate
Fokker-Planck models. The N-body integrations employ particle numbers up to
0.26M and a direct-summation potential solver; close interactions involving the
binary are treated using a new implementation of the Mikkola-Aarseth chain
regularization algorithm. Even at these large values of N, two-body scattering
occurs at high enough rates in the simulations that they can not be simply
scaled to the large-N regime of real galaxies. The Fokker-Planck model is used
to bridge this gap; it includes, for the first time, binary-induced changes in
the stellar density and potential. The Fokker-Planck model is shown to
accurately reproduce the results of the N-body integrations, and is then
extended to the much larger N regime of real galaxies. Analytic expressions are
derived that accurately reproduce the time dependence of the binary semi-major
axis as predicted by the Fokker-Planck model. Gravitational wave coalescence is
shown to occur in <10 Gyr in nuclei with velocity dispersions below about 80
km/s. Formation of a core results from a competition between ejection of stars
by the binary and re-supply of depleted orbits via two-body scattering. Mass
deficits as large as ~4 times the binary mass are produced before coalescence.
After the two black holes coalesce, a Bahcall-Wolf cusp appears around the
single hole in one relaxation time, resulting in a nuclear density profile
consisting of a flat core with an inner, compact cluster, similar to what is
observed at the centers of low-luminosity spheroids.Comment: 21 page
Massive perturbers and the efficient merger of binary massive black holes
We show that dynamical relaxation in the aftermath of a galactic merger and
the ensuing formation and decay of a binary massive black hole (MBH), are
dominated by massive perturbers (MPs) such as giant molecular clouds or
clusters. MPs accelerate relaxation by orders of magnitude relative to 2-body
stellar relaxation alone, and efficiently scatter stars into the binary MBH's
orbit. The 3-body star-binary MBH interactions shrink the binary MBH to the
point where energy losses from the emission of gravitational waves (GW) lead to
rapid coalescence. We model this process based on observed and simulated MP
distributions and take into account the decreased efficiency of the star-binary
MBH interaction due to acceleration in the galactic potential. We show that
mergers of gas-rich galactic nuclei lead to binary MBH coalescence well within
the Hubble time. Moreover, lower-mass binary MBHs (<10^8 Msun) require only a
few percent of the typical gas mass in a post-merger nucleus to coalesce in a
Hubble time. The fate of a binary MBH in a gas poor galactic merger is less
certain, although massive stellar structures (e.g. clusters, stellar rings)
could likewise lead to efficient coalescence. These coalescence events are
observable by their strong GW emission. MPs thus increase the cosmic rate of
such GW events, lead to a higher mass deficit in the merged galactic core and
suppress the formation of triple MBH systems and the resulting ejection of MBHs
into intergalactic space.Comment: 14 pages, 4 figures, 3 tables. More detailed explanations and changes
in structure. Section on hypervelocity stars moved to another paper (in
preparation). Results and conclusions unchanged. Accepted to Ap
Tidal Disruption of Stellar Objects by Hard Supermassive Black Hole Binaries
Supermassive black hole binaries (SMBHBs) are expected by the hierarchical
galaxy formation model in CDM cosmology. There is some evidence in the
literature for SMBHBs in AGNs, but there are few observational constraints on
the evolution of SMBHBs in inactive galaxies and gas-poor mergers. On the
theoretical front, it is unclear how long is needed for a SMBHB in a typical
galaxy to coalesce. In this paper we investigate the tidal interaction between
stars and binary BHs and calculate the tidal disruption rates of stellar
objects by the BH components of binary. We derive the interaction cross
sections between SMBHBs and stars from intensive numerical scattering
experiments with particle number and calculate the tidal disruption
rates by both single and binary BHs for a sample of realistic galaxy models,
taking into account the general relativistic effect and the loss cone refilling
because of two-body interaction. We estimate the frequency of tidal flares for
different types of galaxies using the BH mass function in the literature. We
find that because of the three-body slingshot effect, the tidal disruption rate
in SMBHB system is more than one order of magnitude smaller than that in single
SMBH system. The difference is more significant in less massive galaxies and
does not depend on detailed stellar dynamical processes. Our calculations
suggest that comparisons of the calculated tidal disruption rates for both
single and binary BHs and the surveys of X-ray or UV flares at galactic centers
could tell us whether most SMBHs in nearby galaxies are single and whether the
SMBHBs formed in gas-poor galaxy mergers coalesce rapidly.Comment: 43 pages, 12 figures, 2 tables; accepted for publication in Ap
Review: innovation through research in the North American pork industry
This article involved a broad search of applied sciences for milestone technologies we deem to be the most significant innovations applied by the North American pork industry, during the past 10 to 12 years. Several innovations shifted the trajectory of improvement or resolved significant production limitations. Each is being integrated into practice, with the exception being gene editing technology, which is undergoing the federal approval process. Advances in molecular genomics have been applied to gene editing for control of porcine reproductive and respiratory syndrome and to identify piglet genome contributions from each parent. Post-cervical artificial insemination technology is not novel, but this technology is now used extensively to accelerate the rate of genetic progress. A milestone was achieved with the discovery that dietary essential fatty acids, during lactation, were limiting reproduction. Their provision resulted in a dose-related response for pregnancy, pregnancy maintenance and litter size, especially in maturing sows and ultimately resolved seasonal infertility. The benefit of segregated early weaning (12 to 14 days of age) was realized for specific pathogen removal for genetic nucleus and multiplication. Application was premature for commercial practice, as piglet mortality and morbidity increased. Early weaning impairs intestinal barrier and mucosal innate immune development, which coincides with diminished resilience to pathogens and viability later in life. Two important milestones were achieved to improve precision nutrition for growing pigs. The first involved the updated publication of the National Research Council nutrient requirements for pigs, a collaboration between scientists from America and Canada. Precision nutrition advanced further when ingredient description, for metabolically available amino acids and net energy (by source plant), became a private sector nutrition product. The past decade also led to fortuitous discoveries of health-improving components in ingredients (xylanase, soybeans). Finally, two technologies converged to facilitate timely detection of multiple pathogens in a population: oral fluids sampling and polymerase chain reaction (PCR) for pathogen analysis. Most critical diseases in North America are now routinely monitored by oral fluid sampling and prepared for analysis using PCR methods
FIRST `Winged' and `X'-shaped Radio Source Candidates
A small number of double-lobed radio galaxies (17 from our own census of the
literature) show an additional pair of low surface brightness `wings', thus
forming an overall `X'-shaped appearance. The origin of the wings in these
radio sources is unclear. They may be the result of back-flowing plasma from
the currently active radio lobes into an asymmetric medium surrounding the
active nucleus, which would make these ideal systems in which to study
thermal/non-thermal plasma interactions in extragalactic radio sources. Another
possibility is that the wings are the aging radio lobes left over after a
(rapid) realignment of the central supermassive black-hole/accretion disk
system due perhaps to a merger. Generally, these models are not well tested;
with the small number of known examples, previous works focused on detailed
case studies of selected sources with little attempt at a systematic study of a
large sample. Using the VLA-FIRST survey database, we are compiling a large
sample of winged and X-shaped radio sources for such studies. As a first step
toward this goal, an initial sample of 100 new candidate objects of this type
are presented in this paper. ...[abridged]Comment: (all comments same as v2 except updated external link) AJ, accepted.
36 pgs. Added low resolution figures but table 2 still incomplete. Otherwise,
unchanged from previous astro-ph version. For complete version and table 2:
http://www.slac.stanford.edu/~teddy/Preprints/xshaped.pdf.g
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