394 research outputs found
Higher Resolution VLBI Imaging with Fast Frequency Switching
Millimetre-VLBI is an important tool in AGN astrophysics, but it is limited
by short atmospheric coherence times and poor receiver and antenna performance.
We demonstrate a new kind of phase referencing for the VLBA, enabling us to
increase the sensitivity in mm-VLBI by an order of magnitude. If a source is
observed in short cycles between the target frequency, nu_t, and a reference
frequency, nu_ref, the nu_t data can be calibrated using scaled-up phase
solutions from self-calibration at nu_ref. We have demonstrated the phase
transfer on 3C 279, where we were able to make an 86 GHz image with 90 %
coherence compared to self-calibration at nu_t. We have detected M81, our
science target in this project, at 86 GHz using the same technique. We describe
scheduling strategy and data reduction. The main impacts of fast frequency
switching are the ability to image some of the nearest, but relatively weak AGN
cores with unprecedented high angular resolution and to phase-reference the
nu_t data to the nu_ref core position, enabling the detection of possible core
shifts in jets due to optical depth effects. This ability will yield important
constraints on jet properties and might be able to discriminate between the two
competing emission models of Blandford-Konigl jets and spherical
advection-dominated accretion flows (ADAFs) in low-luminosity AGNs.Comment: 4 pages, 6 figures, appears in: Proceedings of the 6th European VLBI
Network Symposium held on June 25th-28th in Bonn, Germany. Edited by: E. Ros,
R.W. Porcas, A.P. Lobanov, and J.A. Zensu
A wider audience: Turning VLBI into a survey instrument
Radio observations using the Very Long Baseline Interferometry (VLBI)
technique typically have fields of view of only a few arcseconds, due to the
computational problems inherent in imaging larger fields. Furthermore,
sensitivity limitations restrict observations to very compact and bright
objects, which are few and far between on the sky. Thus, while most branches of
observational astronomy can carry out sensitive, wide-field surveys, VLBI
observations are limited to targeted observations of carefully selected
objects. However, recent advances in technology have made it possible to carry
out the computations required to target hundreds of sources simultaneously.
Furthermore, sensitivity upgrades have dramatically increased the number of
objects accessible to VLBI observations. The combination of these two
developments have enhanced the survey capabilities of VLBI observations such
that it is now possible to observe (almost) any point in the sky with
milli-arcsecond resolution. In this talk I review the development of wide-field
VLBI, which has made significant progress over the last three years.Comment: Invited review at the General Assembly of the Astronomische
Gesellschaf
Are the hosts of VLBI selected radio-AGN different to those of radio-loud AGN?
Recent studies have found that radio-AGN selected by radio-loudness show
little difference in terms of their host galaxy properties when compared to
non-AGN galaxies of similar stellar mass and redshift. Using new 1.4~GHz VLBI
observations of the COSMOS field we find that approximately 49\% of
high-mass (M 10 M), high luminosity (L
10 W~Hz) radio-AGN possess a VLBI detected counterpart. These
objects show no discernible bias towards specific stellar masses, redshifts or
host properties other than what is shown by the radio-AGN population in
general. Radio-AGN that are detected in VLBI observations are not special, but
form a representative sample of the radio-loud AGN population.Comment: 6 pages, 4 figures, lette
The first VLBI image of an Infrared-Faint Radio Source
Context: To investigate the joint evolution of active galactic nuclei and
star formation in the Universe. Aims: In the 1.4 GHz survey with the Australia
Telescope Compact Array of the Chandra Deep Field South and the European Large
Area ISO Survey - S1 we have identified a class of objects which are strong in
the radio but have no detectable infrared and optical counterparts. This class
has been called Infrared-Faint Radio Sources, or IFRS. 53 sources out of 2002
have been classified as IFRS. It is not known what these objects are. Methods:
To address the many possible explanations as to what the nature of these
objects is we have observed four sources with the Australian Long Baseline
Array. Results: We have detected and imaged one of the four sources observed.
Assuming that the source is at a high redshift, we find its properties in
agreement with properties of Compact Steep Spectrum sources. However, due to
the lack of optical and infrared data the constraints are not particularly
strong.Comment: Accepted for publication in Astronomy and Astrophysics, 5 pages,
needs aa.cl
Infrared-Faint Radio Sources are at high redshifts
Context: Infrared-Faint Radio Sources (IFRS) are characterised by relatively
high radio flux densities and associated faint or even absent infrared and
optical counterparts. The resulting extremely high radio-to-infrared flux
density ratios up to several thousands were previously known only for
High-redshift Radio Galaxies (HzRGs), suggesting a link between the two classes
of object. Prior to this work, no redshift was known for any IFRS in the
Australia Telescope Large Area Survey (ATLAS) fields which would help to put
IFRS in the context of other classes of object, especially of HzRGs. Aims: This
work aims at measuring the first redshifts of IFRS in the ATLAS fields.
Further, we test the hypothesis that IFRS are similar to HzRGs, as
higher-redshift or dust-obscured versions of these massive galaxies. Methods: A
sample of IFRS was spectroscopically observed using the Focal Reducer and Low
Dispersion Spectrograph 2 (FORS2) at the Very Large Telescope (VLT). The data
were calibrated based on the Image Reduction and Analysis Facility (IRAF) and
redshifts extracted. This information was then used to calculate rest-frame
luminosities, and to perform the first spectral energy distribution modelling
of IFRS based on redshifts. Results: We found redshifts of 1.84, 2.13, and
2.76, for three IFRS, confirming the suggested high-redshift character of this
class of object. These redshifts as well as the resulting luminosities show
IFRS to be similar to HzRGs. We found further evidence that fainter IFRS are at
even higher redshifts. Conclusions: Considering the similarities between IFRS
and HzRGs substantiated in this work, the detection of IFRS, which have a
significantly higher sky density than HzRGs, increases the number of Active
Galactic Nuclei in the early universe and adds to the problems of explaining
the formation of supermassive black holes shortly after the Big Bang.Comment: 7 pages, 4 figures; version in prin
Spectral Properties of the Core and the VLBI-Jets of Cygnus A
We present a detailed VLBI study of the spectral properties of the inner core
region of the radio galaxy Cygnus A at 5 GHz, 15 GHz, 22 GHz, 43 GHz and 86
GHz. Our observations include an epoch using phase-referencing at 15 GHz and 22
GHz and the first successful VLBI observations of Cygnus A at 86 GHz. We find a
pronounced two-sided jet structure, with a steep spectrum along the jet and an
inverted spectrum towards the counter-jet. The inverted spectrum and the
frequency-dependent jet-to-counter-jet ratio suggest that the inner counter-jet
is covered by a circum-nuclear absorber as it is proposed by the unified
scheme.Comment: 2 pages, 2 figures, Proceedings of the 7th EVN Symposium held in
Toledo, Spain in October 2004, needs evn2004.cl
Multi-source self-calibration: Unveiling the microJy population of compact radio sources
Context. Very Long Baseline Interferometry (VLBI) data are extremely
sensitive to the phase stability of the VLBI array. This is especially
important when we reach {\mu}Jy r.m.s. sensitivities. Calibration using
standard phase referencing techniques is often used to improve the phase
stability of VLBI data but the results are often not optimal. This is evident
in blank fields that do not have in-beam calibrators. Aims. We present a
calibration algorithm termed Multi-Source Self-Calibration (MSSC) which can be
used after standard phase referencing on wide-field VLBI observations. This is
tested on a 1.6 GHz wide-field VLBI data set of the Hubble Deep Field-North and
the Hubble Flanking Fields. Methods. MSSC uses multiple target sources detected
in the field via standard phase referencing techniques and modifies the
visibili- ties so that each data set approximates to a point source. These are
combined to increase the signal to noise and permit self-calibration. In
principle, this should allow residual phase changes caused by the troposphere
and ionosphere to be corrected. By means of faceting, the technique can also be
used for direction dependent calibration. Results. Phase corrections, derived
using MSSC, were applied to a wide-field VLBI data set of the HDF-N comprising
of 699 phase centres. MSSC was found to perform considerably better than
standard phase referencing and single source self-calibration. All detected
sources exhibited dramatic improvements in dynamic range. Using MSSC, one
source reached the detection threshold taking the total detected sources to
twenty. 60% of these sources can now be imaged with uniform weighting compared
to just 45% with standard phase referencing. The Parseltongue code which
implements MSSC has been released and made publicly available to the
astronomical community (https://github.com/jradcliffe5/multi_self_cal).Comment: 7 pages, 4 figures, accepted to A&
The star catalogues of Ptolemaios and Ulugh Beg: Machine-readable versions and comparison with the modern Hipparcos Catalogue
In late antiquity and throughout the middle ages, the positions of stars on
the celestial sphere were obtained from the star catalogue of Ptolemaios. A
catalogue based on new measurements appeared in 1437, with positions by Ulugh
Beg, and magnitudes from the 10th-century astronomer al-Sufi. We provide
machine-readable versions of these two star catalogues, based on the editions
by Toomer (1998) and Knobel (1917), and determine their accuracies by
comparison with the modern Hipparcos Catalogue. The magnitudes in the
catalogues correlate well with modern visual magnitudes; the indication `faint'
by Ptolemaios is found to correspond to his magnitudes 5 and 6. Gaussian fits
to the error distributions in longitude / latitude give widths sigma ~ 27
arcmin / 23 arcmin in the range |Delta lambda, Delta beta|<50 arcmin for
Ptolemaios and sigma ~ 22 arcmin /18 arcmin in Ulugh Beg. Fits to the range
|Delta lambda, Delta beta|<100 arcmin gives 10-15 per cent larger widths,
showing that the error distributions are broader than gaussians. The fraction
of stars with positions wrong by more than 150 arcmin is about 2 per cent for
Ptolemaios and 0.1 per cent in Ulugh Beg; the numbers of unidentified stars are
1 in Ptolemaios and 3 in Ulugh Beg. These numbers testify to the excellent
quality of both star catalogues (as edited by Toomer and Knobel).Comment: to be published in Astronomy and Astrophysics; 34 pages with 57
Figures. Note changed address and email address of first autho
Infrared-faint Radio Sources Remain Undetected at Far-infrared Wavelengths
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