224 research outputs found
A high-resolution mm and cm study of the obscured LIRG NGC 4418 - A compact obscured nucleus fed by in-falling gas?
The aim of this study is to constrain the dynamics, structure and feeding of
the compact nucleous of NGC4418, and to reveal the nature of the main hidden
power source: starburst or AGN. We obtained high spatial resolution
observations of NGC4418 at 1.4 and 5 GHz with MERLIN, and at 230 and 270 GHz
with the SMA very extended configuration. We use the continuum morphology and
flux density to estimate the size of the emitting region, the star formation
rate and the dust temperature. Emission lines are used to study the kinematics
through position-velocity diagrams. Molecular emission is studied with
population diagrams and by fitting an LTE synthetic spectrum. We detect bright
1mm line emission from CO, HC3N, HNC and C34S, and 1.4 GHz absorption from HI.
The CO 2-1 emission and HI absorption can be fit by two velocity components at
2090 and 2180 km s-1. We detect vibrationally excited HC3N and HNC, with Tvib
300K. Molecular excitation is consistent with a layered temperature structure,
with three main components at 80, 160 and 300 K. For the hot component we
estimate a source size of less than 5 pc. The nuclear molecular gas surface
density of 1e4 Msun pc-2 is extremely high, and similar to that found in the
ultra-luminous infrared galaxy (ULIRG) Arp220. Our observations confirm the the
presence of a molecular and atomic in-flow, previously suggested by Herschel
observations, which is feeding the activity in the center of NGC4418. Molecular
excitation confirms the presence of a very compact, hot dusty core. If a
starburst is responsible for the observed IR flux, this has to be at least as
extreme as the one in Arp220, with an age of 3-10 Myr and a star formation rate
>10 Msun yr-1. If an AGN is present, it must be extremely Compton-thick.Comment: 18 pages, 11 figures, Accepted for publication by A&A on 10/6/201
The cosmic radio dipole: Bayesian estimators on new and old radio surveys
The cosmic radio dipole is an anisotropy in the number counts of radio
sources, analogous to the dipole seen in the cosmic microwave background (CMB).
Measurements of source counts of large radio surveys have shown that though the
radio dipole is generally consistent in direction with the CMB dipole, the
amplitudes are in tension. These observations present an intriguing puzzle as
to the cause of this discrepancy, with a true anisotropy breaking with the
assumptions of the cosmological principle, invalidating the most common
cosmological models that are built on these assumptions. We present a novel set
of Bayesian estimators to determine the cosmic radio dipole and compare the
results with commonly used methods on the Rapid ASKAP Continuum Survey (RACS)
and the NRAO VLA Sky Survey (NVSS) radio surveys. In addition, we adapt the
Bayesian estimators to take into account systematic effects known to affect
such large radio surveys, folding information such as the local noise floor or
array configuration directly into the parameter estimation. The enhancement of
these estimators allows us to greatly increase the amount of sources used in
the parameter estimation, yielding tighter constraints on the cosmic radio
dipole estimation than previously achieved with NVSS and RACS. We extend the
estimators further to work on multiple catalogues simultaneously, leading to a
combined parameter estimation using both NVSS and RACS. The result is a dipole
estimate that perfectly aligns with the CMB dipole in terms of direction but
with an amplitude that is three times as large, and a significance of
4.8. This new dipole measurement is made to an unprecedented level of
precision for radio sources, which is only matched by recent results using
infrared quasars.Comment: 14 pages, 11 figures. Accepted for publication in Astronomy &
Astrophysic
The OH Megamaser Emission in Arp\,220: the rest of the story
The OH Megamaser emission in the merging galaxy Arp220 has been re-observed
with the Multi-Element Radio Linked Interferometer Network (MERLIN) and the
European VLBI Network (EVN). Imaging results of the OH line emission at the two
nuclei are found to be consistent with earlier observations and confirm
additional extended emission structures surrounding the nuclei. Detailed
information about the distributed emission components around the two nuclei has
been obtained using a concatenated MERLIN and EVN database with intermediate
(40 mas) spatial resolution. Continuum imaging shows a relatively compact West
nucleus and a more extended East nucleus in addition to an extended continuum
ridge stretching below and beyond the two nuclei. Spectral line imaging show
extended emission regions at both nuclei together with compact components and
additional weaker components north and south of the West nucleus. Spectral line
analysis indicates that the dominant OH line emission originates in foreground
molecular material that is part of a large-scale molecular structure that
engulfs the whole nuclear region. Compact OH components are representative of
star formation regions within the two nearly edge-on nuclei and define the
systemic velocities of East and West as 5425 km/s and 5360 km/s.
The foreground material at East and West has a 100 km/s lower velocity at
5314 and 5254 km/s. These emission results confirm a maser amplification
scenario where the background continuum and the line emission of the star
formation regions are amplified by foreground masering material that is excited
by the FIR radiation field originating in the two nuclear regions.Comment: 17 pages, 18 figure
A close-pair binary in a distant triple supermassive black-hole system
Galaxies are believed to evolve through merging, which should lead to
multiple supermassive black holes in some. There are four known triple black
hole systems, with the closest pair being 2.4 kiloparsecs apart (the third
component is more distant at 3 kiloparsecs), which is far from the
gravitational sphere of influence of a black hole with mass 10
M (about 100 parsecs). Previous searches for compact black hole systems
concluded that they were rare, with the tightest binary system having a
separation of 7 parsecs. Here we report observations of a triple black hole
system at redshift z=0.39, with the closest pair separated by 140
parsecs. The presence of the tight pair is imprinted onto the properties of the
large-scale radio jets, as a rotationally-symmetric helical modulation, which
provides a useful way to search for other tight pairs without needing extremely
high resolution observations. As we found this tight pair after searching only
six galaxies, we conclude that tight pairs are more common than hitherto
believed, which is an important observational constraint for low-frequency
gravitational wave experiments.Comment: 21 pages, 6 figures. Published online by Nature on 25 June 2014.
Extremely minor differences with published version exis
The SKA and "High-Resolution" Science
"High-resolution", or "long-baseline", science with the SKA and its
precursors covers a broad range of topics in astrophysics. In several research
areas, the coupling between improved brightness sensitivity of the SKA and a
sub-arcsecond resolution would uncover truly unique avenues and opportunities
for studying extreme states of matter, vicinity of compact relativistic
objects, and complex processes in astrophysical plasmas. At the same time, long
baselines would secure excellent positional and astrometric measurements with
the SKA and critically enhance SKA image fidelity at all scales. The latter
aspect may also have a substantial impact on the survey speed of the SKA, thus
affecting several key science projects of the instrument.Comment: JENAM-2010: Invited talk at JENAM session S7: The Square Kilometre
Array: Paving the way for the new 21st century radio astronomy paradigm; 9
page
Optical Control of Field-Emission Sites by Femtosecond Laser Pulses
We have investigated field emission patterns from a clean tungsten tip apex
induced by femtosecond laser pulses. Strongly asymmetric modulations of the
field emission intensity distributions are observed depending on the
polarization of the light and the laser incidence direction relative to the
azimuthal orientation of tip apex. In effect, we have realized an ultrafast
pulsed field-emission source with site selectivity on the 10 nm scale.
Simulations of local fields on the tip apex and of electron emission patterns
based on photo-excited nonequilibrium electron distributions explain our
observations quantitatively.Comment: 4 pages, submitted to Physical Review Letter
The preferentially magnified active nucleus in IRAS F10214+4724 - III. VLBI observations of the radio core
We report 1.7GHz very long baseline interferometry (VLBI) observations of IRAS F10214+4724, a lensed z = 2.3 obscured quasar with prodigious star formation. We detect what we argue to be the obscured active nucleus with an effective angular resolution of 1dex) and therefore distort our view of high-redshift, gravitationally lensed galaxies.Department of HE and Training approved lis
A Virtual Sky with Extragalactic HI and CO Lines for the SKA and ALMA
We present a sky simulation of the atomic HI emission line and the first ten
CO rotational emission lines of molecular gas in galaxies beyond the Milky Way.
The simulated sky field has a comoving diameter of 500/h Mpc, hence the actual
field-of-view depends on the (user-defined) maximal redshift zmax; e.g. for
zmax=10, the field of view yields ~4x4 sqdeg. For all galaxies, we estimate the
line fluxes, line profiles, and angular sizes of the HI and CO emission lines.
The galaxy sample is complete for galaxies with cold hydrogen masses above 10^8
Msun. This sky simulation builds on a semi-analytic model of the cosmic
evolution of galaxies in a Lambda-cold dark matter (LCDM) cosmology. The
evolving CDM-distribution was adopted from the Millennium Simulation, an N-body
CDM-simulation in a cubic box with a side length of 500/h Mpc. This side length
limits the coherence scale of our sky simulation: it is long enough to allow
the extraction of the baryon acoustic oscillations (BAOs) in the galaxy power
spectrum, yet the position and amplitude of the first acoustic peak will be
imperfectly defined. This sky simulation is a tangible aid to the design and
operation of future telescopes, such the SKA, the LMT, and ALMA. The results
presented in this paper have been restricted to a graphical representation of
the simulated sky and fundamental dN/dz-analyzes for peak flux density limited
and total flux limited surveys of HI and CO. A key prediction is that HI will
be harder to detect at redshifts z>2 than predicted by a no-evolution model.
The future verification or falsification of this prediction will allow us to
qualify the semi-analytic models.Comment: 16 pages, 9 figures, 1 tabl
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