70 research outputs found
A relation between circumnuclear HI, dust, and optical cores in low-power radio galaxies
From new observations and literature data we investigate the presence of HI,
dust, and optical cores in the central kiloparsec of low-power radio galaxies.
The goal of this pilot study is to identify physical relations between these
components, which can help us to study kinematics and feeding mechanisms in
future samples of active galaxies. Our results are consistent with neutral gas
being associated with dust on sub-kiloparsec scales. Objects that have HI
absorption always have significant amounts of dust in their host galaxy. If
there is no visible dust in the host galaxy, there is also no HI absorption.
The presence of an unresolved optical core correlates with the HI column
density, with the core being absent in high column density sources. This work
opens a path for studying the kinematics of cold material in the central
regions of active galaxies by combining information of HI absorption and
molecular lines. Consistent with previous work, we find no evidence for a
compact, parsec-scale obscuring torus in low-power radio galaxies.Comment: Accepted for publication in A&
Mid-frequency aperture arrays: the future of radio astronomy
Aperture array (AA) technology is at the forefront of new developments and
discoveries in radio astronomy. Currently LOFAR is successfully demonstrating
the capabilities of dense and sparse AA's at low frequencies. For the
mid-frequencies, from 450 to 1450MHz, AA's still have to prove their scientific
value with respect to the existing dish technology. Their large field-of-view
and high flexibility puts them in an excellent position to do so. The Aperture
Array Verification Program is dedicated to demonstrate the feasibility of AA's
for science in general and SKA in particular. For the mid-frequency range this
has lead to the development of EMBRACE, which has already demonstrated the
enormous flexibility of AA systems by observing HI and a pulsar simultaneously.
It also serves as a testbed to demonstrate the technological reliability and
stability of AA's. The next step will put AA technology at a level where it can
be used for cutting-edge science. In this paper we discuss the developments to
move AA technology from an engineering activity to a fully science capable
instrument. We present current results from EMBRACE, ongoing tests of the
system, and plans for EMMA, the next step in mid-frequency AA technology.Comment: 8 pages, 7 figures, proceedings of Resolving The Sky - Radio
Astronomy: Past, Present and Future (RTS2012), April 17-20, 2012, Manchester,
U
Polarization and kinematics in Cygnus A
From optical spectropolarimetry of Cygnus A we conclude that the scattering
medium in the ionization cones in Cygnus A is moving outward at a speed of
170+-34 km/s, and that the required momentum can be supplied by the radiation
pressure of an average quasar. Such a process could produce a structure
resembling the observed ionization cones, which are thought to result from
shadowing by a circumnuclear dust torus. We detect a polarized red wing in the
[O III] emission lines arising from the central kiloparsec of Cygnus A. This
wing is consistent with line emission created close to the boundary of the
broad-line region.Comment: 5 pages, accepted for publication in MNRAS letter
A new period of activity in the core of NGC 660
The core of the nearby galaxy NGC 660 has recently undergone a spectacular radio outburst; using a combination of archival radio and Chandra X-ray data, together with new observations, the nature of this event is investigated. Radio observations made using e-MERLIN in mid-2013 show a new compact and extremely bright continuum source at the centre of the galaxy. High angular resolution observations carried out with the European VLBI Network show an obvious jet-like feature to the north east and evidence of a weak extension to the west, possibly a counter-jet. We also examine high angular resolution Hi spectra of these new sources, and the radio spectral energy distribution using the new wide-band capabilities of e-MERLIN. We compare the properties of the new object with possible explanations, concluding that we are seeing a period of new AGN activity in the core of this polar ring galaxy
A compact core-jet structure in the changing-look Seyfert NGC 2617
The nearby face-on spiral galaxy NGC 2617 underwent an unambiguous
'inside-out' multi-wavelength outburst in Spring 2013, and a dramatic Seyfert
type change probably between 2010 and 2012, with the emergence of broad optical
emission lines. To search for the jet activity associated with this variable
accretion activity, we carried out multi-resolution and multi-wavelength radio
observations. Using the very long baseline interferometric (VLBI) observations
with the European VLBI Network (EVN) at 1.7 and 5.0 GHz, we find that NGC 2617
shows a partially synchrotron self-absorbed compact radio core with a
significant core shift, and an optically thin steep-spectrum jet extending
towards the north up to about two parsecs in projection. We also observed NGC
2617 with the electronic Multi-Element Remotely Linked Interferometer Network
(e-MERLIN) at 1.5 and 5.5 GHz, and revisited the archival data of the Very
Large Array (VLA) and the Very Long Baseline Array (VLBA). The radio core had a
stable flux density of about 1.4 mJy at 5.0 GHz between 2013 June and 2014
January, in agreement with the expectation of a supermassive black hole in the
low accretion rate state. The northern jet component is unlikely to be
associated with the 'inside-out' outburst of 2013. Moreover, we report that
most optically selected changing-look AGN at z<0.83 are sub-mJy radio sources
in the existing VLA surveys at 1.4 GHz, and it is unlikely that they are more
active than normal AGN at radio frequencies.Comment: 10 pages, 7 figures, accepted for publication in MNRA
MeqSilhouette v2: spectrally resolved polarimetric synthetic data generation for the event horizon telescope
We present MeqSilhouette v2.0 (MeqSv2), a fully polarimetric, time-and frequency-resolved synthetic data generation software for simulating millimetre (mm) wavelength very long baseline interferometry (VLBI) observations with heterogeneous arrays. Synthetic data are a critical component in understanding real observations, testing calibration and imaging algorithms, and predicting performance metrics of existing or proposed sites. MeqSv2 applies physics-based instrumental and atmospheric signal corruptions constrained by empirically derived site and station parameters to the data. The new version is capable of applying instrumental polarization effects and various other spectrally resolved effects using the Radio Interferometry Measurement Equation (RIME) formalism and produces synthetic data compatible with calibration pipelines designed to process real data. We demonstrate the various corruption capabilities of MeqSv2 using different arrays, with a focus on the effect of complex bandpass gains on closure quantities for the EHT at 230 GHz. We validate the frequency-dependent polarization leakage implementation by performing polarization self-calibration of synthetic EHT data using PolSolve. We also note the potential applications for cm-wavelength VLBI array analysis and design and future directions.http://mnras.oxfordjournals.orghj2022Physic
First M87 Event Horizon Telescope Results and the Role of ALMA
In April 2019, the Event Horizon Telescope (EHT) collaboration revealed the
first image of the candidate super-massive black hole (SMBH) at the centre of
the giant elliptical galaxy Messier 87 (M87). This event-horizon-scale image
shows a ring of glowing plasma with a dark patch at the centre, which is
interpreted as the shadow of the black hole. This breakthrough result, which
represents a powerful confirmation of Einstein's theory of gravity, or general
relativity, was made possible by assembling a global network of radio
telescopes operating at millimetre wavelengths that for the first time included
the Atacama Large Millimeter/ submillimeter Array (ALMA). The addition of ALMA
as an anchor station has enabled a giant leap forward by increasing the
sensitivity limits of the EHT by an order of magnitude, effectively turning it
into an imaging array. The published image demonstrates that it is now possible
to directly study the event horizon shadows of SMBHs via electromagnetic
radiation, thereby transforming this elusive frontier from a mathematical
concept into an astrophysical reality. The expansion of the array over the next
few years will include new stations on different continents - and eventually
satellites in space. This will provide progressively sharper and
higher-fidelity images of SMBH candidates, and potentially even movies of the
hot plasma orbiting around SMBHs. These improvements will shed light on the
processes of black hole accretion and jet formation on event-horizon scales,
thereby enabling more precise tests of general relativity in the truly strong
field regime.Comment: 11 pages + cover page, 6 figure
Monitoring the Morphology of M87* in 2009-2017 with the Event Horizon Telescope
The Event Horizon Telescope (EHT) has recently delivered the first resolved images of M87*, the supermassive black hole in the center of the M87 galaxy. These images were produced using 230 GHz observations performed in 2017 April. Additional observations are required to investigate the persistence of the primary image feature—a ring with azimuthal brightness asymmetry—and to quantify the image variability on event horizon scales. To address this need, we analyze M87* data collected with prototype EHT arrays in 2009, 2011, 2012, and 2013. While these observations do not contain enough information to produce images, they are sufficient to constrain simple geometric models. We develop a modeling approach based on the framework utilized for the 2017 EHT data analysis and validate our procedures using synthetic data. Applying the same approach to the observational data sets, we find the M87* morphology in 2009-2017 to be consistent with a persistent asymmetric ring of ∼40 μas diameter. The position angle of the peak intensity varies in time. In particular, we find a significant difference between the position angle measured in 2013 and 2017. These variations are in broad agreement with predictions of a subset of general relativistic magnetohydrodynamic simulations. We show that quantifying the variability across multiple observational epochs has the potential to constrain the physical properties of the source, such as the accretion state or the black hole spin
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