958 research outputs found
Protein refolding in an oscillatory flow reactor
We demonstrate that an oscillatory flow reactor is a viable reactor for protein refolding via direct dilution. The
mixing characteristics of the oscillatory flow reactor are well described and controllable and, importantly, can be
scaled-up to process scale without a loss of mixing efficiency. This makes the oscillatory flow reactor an attractive
alternative to conventional stirred-tank reactors for process-scale renaturation
Gas Around Active Galactic Nuclei and New Phase Calibration Strategies for High-Frequency VLBI
We have a project to measure magnetic fields in the obscuring tori of AGNs.
We observed five free-free absorbed jets in radio galaxies with polarimetric
VLBI at 15 GHz to determine Faraday rotation measures. Surprisingly, all
sources are unpolarized, putting severe constraints on the degree of magnetic
field turbulence and the gas distribution in these objects. We have further
carried out multi-frequency VLBI monitoring of the Seyfert 2 galaxy NGC 3079
and find unusually steep and inverted spectra in the radio components, and
derive general properties of Seyfert galaxies using published VLBI observations
of Seyfert galaxies.
We have developed a new phase-referencing technique for high frequency VLBI
observations with the VLBA. Instead of inserting short scans on a calibrator
into the target source observations, the target source is continuously observed
while rapidly switching between the target frequency and a lower reference
frequency. We demonstrate that the technique allows phase calibration almost
reaching the thermal noise limit and present the first detection of the AGN in
NGC 4261 at 86 GHz. This is the weakest source ever detected with VLBI at that
frequency.Comment: PhD thesis, 188 page
VLBI detection of an Infrared-Faint Radio Source
Infrared-Faint Radio Sources represent a new and unexpected class of object
which is bright at radio wavelengths but unusually faint at infrared
wavelengths. If, like most mJy radio sources, they were either conventional
active or star-forming galaxies in the local Universe, we would expect them to
be detectable at infrared wavelengths, and so their non-detection by the
Spitzer Space Telescope is surprising. Here we report the detection of one of
these sources using Very Long Baseline Interferometry, from which we conclude
that the sources are driven by Active Galactic Nuclei. We suggest that these
sources are either normal radio-loud quasars at high redshift or abnormally
obscured radio galaxies.Comment: accepted by MNRA
Automated Editing of Radio Interferometer Data with Pieflag
Editing of radio interferometer data, a process commonly known as
``flagging'', can be laborious and time-consuming. One quickly tends to flag
more data than actually required, sacrificing sensitivity and image fidelity in
the process. I describe a program, Pieflag, which can analyse radio
interferometer data to filter out measurements which are likely to be affected
by interference. Pieflag uses two algorithms to allow for data sets which are
either dominated by receiver noise or by source structure. Together, the
algorithms detect essentially all affected data whilst the amount of data which
is not affected by interference but falsely marked as such is kept to a
minimum. The sections marked by Pieflag are very similar to what would be
deemed affected by the observer in a visual inspection of the data. Pieflag
displays its results concisely and allows the user to add and remove flags
interactively. It is written in Python, is easy to install and use, and has a
variety of options to adjust its algorithms to a particular observing
situation. I describe how Pieflag works and illustrate its effect using data
from typical observations.Comment: 6 pages, 5 figures, requires apj.bst, accepted for publication in
Publications of the Astronomical Society of Australi
Unveiling the origin of the radio emission in radio-quiet quasars
The origin of the radio emission in radio-quiet quasars (RQQs) has been a
matter of debate for a long time. It is not well understood whether the
emission is caused by star formation in the host galaxy or by black hole
activity of the active galactic nuclei (AGN). We shed some light on these
questions using the Very Long Baseline Interferometry (VLBI) technique to
search for RQQs in the field of the Cosmological Evolution Survey (COSMOS). The
extensive multi-wavelength coverage of the field (from radio to X-rays) was
used to classify RQQs, and the milli-arcsecond resolution of VLBI provides a
direct way to identify AGNs. In a sample of 18 RQQs we detected 3 using the
Very Long Baseline Array (VLBA) at 1.4 GHz. In this letter we report for the
first time on a sample of RQQs with a measured lower limit on the fraction of
radio emission coming from the AGN, thus demonstrating that the radio emission
of at least some RQQs is dominated by an AGN.Comment: 4 pages, letter to the editor, accepted for publication in A&
Infrared-Faint Radio Sources: A Cosmological View - AGN Number Counts, the Cosmic X-Ray Background and SMBH Formation
Context. Infrared Faint Radio Sources (IFRS) are extragalactic emitters
clearly detected at radio wavelengths but barely detected or undetected at
optical and infrared wavelengths, with 5 sigma sensitivities as low as 1 uJy.
Aims. Recent SED-modelling and analysis of their radio properties shows that
IFRS are consistent with a population of (potentially extremely obscured)
high-redshift AGN at 3<z<6. We demonstrate some astrophysical implications of
this population and compare them to predictions from models of galaxy evolution
and structure formation.
Methods. We compiled a list of IFRS from four deep extragalactic surveys and
extrapolated the IFRS number density to a survey-independent value of (30.8 +-
15.0) per square degree. We computed the IFRS contribution to the total number
of AGN in the Universe to account for the Cosmic X-ray Background. By
estimating the black hole mass contained in IFRS, we present conclusions for
the SMBH mass density in the early universe and compare it to relevant
simulations of structure formation after the Big Bang.
Results. The number density of AGN derived from the IFRS density was found to
be about 310 deg^-2, which is equivalent to a SMBH mass density of the order of
10^3 M_sun Mpc^-3 in the redshift range 3<z<6. This produces an X-ray flux of 9
10^-16 W m^-2 deg^-2 in the 0.5-2.0 keV band and 3 10^-15 W m^-2 deg^-2 in the
2.0-10 keV band, in agreement with the missing unresolved components of the
Cosmic X-ray Background. Concerning the problem of SMBH formation after the Big
Bang we find evidence for a scenario involving both halo gas accretion and
major mergers.Comment: 8 pages, 4 figures, accepted for publication in A&
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