134 research outputs found
Effect of discontinuities in surface catalytic activity on laminar heat transfer in arc-heated nitrogen streams
Discontinuity effects in surface catalytic activity on laminar heat transfer in arc heated nitrogen stream
Compact Symmetric Objects -- I Towards a Comprehensive Bona Fide Catalog
Compact Symmetric Objects (CSOs) are jetted Active Galactic Nuclei (AGN) with
overall projected size <1 kpc. The classification was introduced to distinguish
these objects from the majority of compact jetted-AGN in centimeter wavelength
very long baseline interferometry observations, where the observed emission is
relativistically boosted towards the observer. The original classification
criteria for CSOs were: (i) evidence of emission on both sides of the center of
activity, and (ii) overall size <1 kpc. However some relativistically boosted
objects with jet axes close to the line of sight appear symmetric and have been
mis-classified as CSOs, thereby undermining the CSO classification. This is
because two essential CSO properties, pointed out in the original papers, have
been neglected: (iii) low variability, and (iv) low apparent speeds along the
jets. As a first step towards creating a comprehensive catalog of ``bona fide''
CSOs, we identify 79 bona fide CSOs, including 15 objects claimed as confirmed
CSOs here for the first time, that match the CSO selection criteria. This
sample of bona fide CSOs can be used for astrophysical studies of CSOs without
contamination by mis-classified CSOs. We show that the fraction of CSOs in
complete flux density limited AGN samples with S >700 mJy is
between % and %.Comment: 28 pages, 9 figures, 3 tables, accepted for publicatio
Compact Symmetric Objects -- II Confirmation of a Distinct Population of High-Luminosity Jetted Active Galaxies
Compact Symmetric Objects (CSOs) are compact (<1 kpc), jetted Active Galactic
Nuclei (AGN), whose jet axes are not aligned close to the line of sight, and
whose observed emission is not predominantly relativistically boosted towards
us. Two classes of CSOs have previously been identified: approximately one
fifth are edge-dimmed and designated as CSO 1s, while the rest are edge
brightened and designated as CSO 2s. This paper focuses almost exclusively on
CSO 2s. Using complete samples of CSO 2s we present three independent lines of
evidence, based on their relative numbers, redshift distributions, and size
distributions, which show conclusively that the vast majority (> 99%) of CSO 2s
do not evolve into larger-scale radio sources. These CSO 2s belong to a
distinct population of jetted-AGN, which should be characterized as
``short-lived'' compared to the classes of larger jetted-AGN, as opposed to
``young''. We show that there is a sharp upper cutoff in the CSO 2 size
distribution at pc. The distinct differences between most CSO 2s
and other jetted-AGN provides a crucial new time domain window on the formation
and evolution of relativistic jets in AGN and the supermassive black holes that
drive them.Comment: 29 pages, 10 figures, 7 tables, accepted for publicatio
High-resolution VLA low radio frequency observations of the Perseus cluster: radio lobes, mini-halo and bent-jet radio galaxies
We present the first high-resolution 230-470 MHz map of the Perseus cluster
obtained with the Karl G. Jansky Very Large Array. The high dynamic range and
resolution achieved has allowed the identification of previously-unknown
structures in this nearby galaxy cluster. New hints of sub-structures appear in
the inner radio lobes of the brightest cluster galaxy NGC 1275. The spurs of
radio emission extending into the outer X-ray cavities, inflated by past
nuclear outbursts, are seen for the first time at these frequencies, consistent
with spectral aging. Beyond NGC 1275, we also analyze complex radio sources
harbored in the cluster. Two new distinct, narrowly-collimated jets are visible
in IC 310, consistent with a highly-projected narrow-angle tail radio galaxy
infalling into the cluster. We show how this is in agreement with its
blazar-like behavior, implying that blazars and bent-jet radio galaxies are not
mutually exclusive. We report the presence of filamentary structures across the
entire tail of NGC 1265, including two new pairs of long filaments in the
faintest bent extension of the tail. Such filaments have been seen in other
cluster radio sources such as relics and radio lobes, indicating that there may
be a fundamental connection between all these radio structures. We resolve the
very narrow and straight tail of CR 15 without indication of double jets, so
that the interpretation of such head-tail sources is yet unclear. Finally, we
note that only the brightest western parts of the mini-halo remain, near NGC
1272 and its bent double jets.Comment: 17 pages, 12 figures, Accepted for publication in MNRA
Compact Symmetric Objects -- III Evolution of the High-Luminosity Branch and a Possible Connection with Tidal Disruption Events
We use a sample of 54 Compact Symmetric Objects (CSOs) to confirm that there
are two unrelated CSO classes: an edge-dimmed, low-luminosity class (CSO~1),
and an edge-brightened, high-luminosity class (CSO~2). Using blind tests, we
show that CSO~2s consist of three sub-classes: CSO 2.0, having prominent
hot-spots at the leading edges of narrow jets and/or narrow lobes; CSO~2.2,
without prominent hot-spots, and with broad jets and/or lobes; and CSO~2.1,
which exhibit mixed properties. Most CSO 2s do not evolve into larger
jetted-AGN, but spend their whole life-cycle as CSOs of size 500 pc
and age 5000 yr. The minimum energies needed to produce the radio
luminosity and structure in CSO~2s range from to
. We show that the transient nature of most CSO~2s, and
their birthrate, can be explained through ignition in the tidal disruption
events of giant stars. We also consider possibilities of tapping the spin
energy of the supermassive black hole, and tapping the energy of the accretion
disk. Our results demonstrate that CSOs constitute a large family of AGN in
which we have thus far studied only the brightest. More comprehensive CSO
studies, with higher sensitivity, resolution, and dynamic range, will
revolutionize our understanding of AGN and the central engines that power them.Comment: 44 pages, 16 figures, 9 tables, accepted for publicatio
Reorganisation of Wnt-response pathways in colorectal tumorigenesis
In most colorectal tumours, APC mutation stabilises β-catenin and mimics elements of Wnt growth factor signalling, but the high frequency of epigenetic loss of Wnt antagonists indicates an additional role for ligand-mediated Wnt signalling. Here, we have investigated the expression of key components of β-catenin-independent Wnt response pathways to determine whether their profiles change during the transition from normal mucosa to colorectal adenomas. Transcription of the Wnt/planar cell polarity pathway determinant NKD1 (naked cuticle homologue 1) was induced in adenomas by a median 135-fold and in cancers by 7.4-fold. While some Frizzleds (FZDs) were downregulated in adenomas, the Wnt/Ca2+ receptors FZD3 and FZD6 were induced by a median factor of 6.5 and 4.6, respectively. Naked cuticle homologue 1, FZD3 and FZD6 expression were coordinated in pre-malignant disease, but this relationship was lost in invasive cancers, where FZD induction was seen less frequently. Naked cuticle homologue 1 expression was associated with nuclear localisation of phospho-c-Jun in adenomas. In cultured cells, NKD1 transcription was induced by lithium chloride but FZD3 expression required Wnt growth factor treatment. These data show that Wnt responses are consistently directed towards both β-catenin-independent routes in early colorectal tumorigenesis and elements of this are retained in more advanced cancers. These β-catenin-independent Wnt signalling pathways may provide novel targets for chemoprevention of early colorectal tumours
StearoylCoA Desaturase-5: A Novel Regulator of Neuronal Cell Proliferation and Differentiation
Recent studies have demonstrated that human stearoylCoA desaturase-1 (SCD1), a Δ9-desaturase that converts saturated fatty acids (SFA) into monounsaturated fatty acids, controls the rate of lipogenesis, cell proliferation and tumorigenic capacity in cancer cells. However, the biological function of stearoylCoA desaturase-5 (SCD5), a second isoform of human SCD that is highly expressed in brain, as well as its potential role in human disease, remains unknown. In this study we report that the constitutive overexpression of human SCD5 in mouse Neuro2a cells, a widely used cell model of neuronal growth and differentiation, displayed a greater n-7 MUFA-to-SFA ratio in cell lipids compared to empty-vector transfected cells (controls). De novo synthesis of phosphatidylcholine and cholesterolesters was increased whereas phosphatidylethanolamine and triacylglycerol formation was reduced in SCD5-expressing cells with respect to their controls, suggesting a differential use of SCD5 products for lipogenic reactions. We also observed that SCD5 expression markedly accelerated the rate of cell proliferation and suppressed the induction of neurite outgrowth, a typical marker of neuronal differentiation, by retinoic acid indicating that the desaturase plays a key role in the mechanisms of cell division and differentiation. Critical signal transduction pathways that are known to modulate these processes, such epidermal growth factor receptor (EGFR)Akt/ERK and Wnt, were affected by SCD5 expression. Epidermal growth factor-induced phosphorylation of EGFR, Akt and ERK was markedly blunted in SCD5-expressing cells. Furthermore, the activity of canonical Wnt was reduced whereas the non-canonical Wnt was increased by the presence of SCD5 activity. Finally, SCD5 expression increased the secretion of recombinant Wnt5a, a non-canonical Wnt, whereas it reduced the cellular and secreted levels of canonical Wnt7b. Our data suggest that, by a coordinated modulation of key lipogenic pathways and transduction signaling cascades, SCD5 participates in the regulation of neuronal cell growth and differentiation
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