1,146 research outputs found
Optical spectroscopy of faint gigahertz peaked spectrum sources
We present spectroscopic observations of a sample of faint Gigahertz Peaked
Spectrum (GPS) radio sources drawn from the Westerbork Northern Sky Survey
(WENSS). Redshifts have been determined for 19 (40%) of the objects. The
optical spectra of the GPS sources identified with low redshift galaxies show
deep stellar absorption features. This confirms previous suggestions that their
optical light is not significantly contaminated by AGN-related emission, but is
dominated by a population of old (>9 Gyr) and metal-rich (>0.2 [Fe/H]) stars,
justifying the use of these (probably) young radio sources as probes of galaxy
evolution. The optical spectra of GPS sources identified with quasars are
indistinguishable from those of flat spectrum quasars, and clearly different
from the spectra of Compact Steep Spectrum (CSS) quasars. The redshift
distribution of the GPS quasars in our radio-faint sample is comparable to that
of the bright samples presented in the literature, peaking at z ~ 2-3. It is
unlikely that a significant population of low redshift GPS quasars is missed
due to selection effects in our sample. We therefore claim that there is a
genuine difference between the redshift distributions of GPS galaxies and
quasars, which, because it is present in both the radio-faint and bright
samples, can not be due to a redshift-luminosity degeneracy. It is therefore
unlikely that the GPS quasars and galaxies are unified by orientation, unless
the quasar opening angle is a strong function of redshift. We suggest that the
GPS quasars and galaxies are unrelated populations and just happen to have
identical observed radio-spectral properties, and hypothesise that GPS quasars
are a sub-class of flat spectrum quasars.Comment: LaTeX, 13 pages. Accepted by MNRAS. For related papers see
http://www.ast.cam.ac.uk/~snelle
High-resolution radio observations of Seyfert galaxies in the extended 12-micron sample - II. The properties of compact radio components
We discuss the properties of compact nuclear radio components in Seyfert
galaxies from the extended 12-micron AGN sample of Rush et al.(1993). Our main
results can be summarised as follows. Type 1 and type 2 Seyferts produce
compact radio components which are indistinguishable in strength and aspect,
indicating that their central engines are alike as proposed by the unification
model. Infrared IRAS fluxes are more closely correlated with low-resolution
radio fluxes than high-resolution radio fluxes, suggesting that they are
dominated by kiloparsec-scale, extra-nuclear emission regions; extra-nuclear
emission may be stronger in type 2 Seyferts. Early-type Seyfert galaxies tend
to have stronger nuclear radio emission than late-type Seyfert galaxies.
V-shaped extended emission-line regions, indicative of `ionisation cones', are
usually found in sources with large, collimated radio outflows. Hidden broad
lines are most likely to be found in sources with powerful nuclear radio
sources. Type 1 and type 2 Seyferts selected by their IRAS 12-micron flux
densities have well matched properties
The interplay between tissue growth and scaffold degradation in engineered tissue constructs
In vitro tissue engineering is emerging as a potential tool to meet the high demand for replacement tissue, caused by the increased incidence of tissue degeneration and damage. A key challenge in this field is ensuring that the mechanical properties of the engineered tissue are appropriate for the in vivo environment. Achieving this goal will require detailed understanding of the interplay between cell proliferation, extracellular matrix (ECM) deposition and scaffold degradation.\ud
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In this paper, we use a mathematical model (based upon a multiphase continuum framework) to investigate the interplay between tissue growth and scaffold degradation during tissue construct evolution in vitro. Our model accommodates a cell population and culture medium, modelled as viscous fluids, together with a porous scaffold and ECM deposited by the cells, represented as rigid porous materials. We focus on tissue growth within a perfusion bioreactor system, and investigate how the predicted tissue composition is altered under the influence of (i) differential interactions between cells and the supporting scaffold and their associated ECM, (ii) scaffold degradation, and (iii) mechanotransduction-regulated cell proliferation and ECM deposition.\ud
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Numerical simulation of the model equations reveals that scaffold heterogeneity typical of that obtained from μCT scans of tissue engineering scaffolds can lead to significant variation in the flow-induced mechanical stimuli experienced by cells seeded in the scaffold. This leads to strong heterogeneity in the deposition of ECM. Furthermore, preferential adherence of cells to the ECM in favour of the artificial scaffold appears to have no significant influence on the eventual construct composition; adherence of cells to these supporting structures does, however, lead to cell and ECM distributions which mimic and exaggerate the heterogeneity of the underlying scaffold. Such phenomena have important ramifications for the mechanical integrity of engineered tissue constructs and their suitability for implantation in vivo
Quasars, their host galaxies, and their central black holes
We present the final results from our deep HST imaging study of the hosts of
radio-quiet quasars (RQQs), radio-loud quasars (RLQs) and radio galaxies (RGs).
We describe new WFPC2 R-band observations for 14 objects and model these images
in conjunction with the data already reported in McLure et al (1999). We find
that spheroidal hosts become more prevalent with increasing nuclear luminosity
such that, for nuclear luminosities M_V < -23.5, the hosts of both radio-loud
and radio-quiet AGN are virtually all massive ellipticals. Moreover we
demonstrate that the basic properties of these hosts are indistinguishable from
those of quiescent, evolved, low-redshift ellipticals of comparable mass. This
result kills any lingering notion that radio-loudness is determined by
host-galaxy morphology, and also sets severe constraints on evolutionary
schemes which attempt to link low-z ULIRGs with RQQs. Instead, we show that our
results are as expected given the relationship between black-hole and spheroid
mass established for nearby galaxies, and apply this relation to estimate the
mass of the black hole in each object. The results agree very well with
completely-independent estimates based on nuclear emission-line widths; all the
quasars in our sample have M(bh) > 5 x 10^8 solar masses, while the radio-loud
objects are confined to M(bh) > 10^9 solar masses. This apparent mass-threshold
difference, which provides a natural explanation for why RQQs outnumber RLQs by
a factor of 10, appears to reflect the existence of a minimum and maximum level
of black-hole radio output which is a strong function of black-hole mass.
Finally, we use our results to estimate the fraction of massive
spheroids/black-holes which produce quasar-level activity. This fraction is
\~0.1% at the present day, rising to > 10% at z = 2-3.Comment: Revised version accepted for publication in Monthly Notices of the
Royal Astronomical Society. 46 pages, the final 19 of which comprise an
Appendix. 15 figures in main text. A further 14 4-panel greyscale plots and
14 line plots which appear in the Appendix have been reproduced here with
reduced quality due to space limitations. A full resolution copy of the
manuscript can be obtained via ftp://ftp.roe.ac.uk/pub/jsd/dunlop2002.ps.g
Spreading dynamics on spatially constrained complex brain networks
The study of dynamical systems defined on complex networks provides a natural framework with which to investigate myriad features of neural dynamics and has been widely undertaken. Typically, however, networks employed in theoretical studies bear little relation to the spatial embedding or connectivity of the neural networks that they attempt to replicate. Here, we employ detailed neuroimaging data to define a network whose spatial embedding represents accurately the folded structure of the cortical surface of a rat brain and investigate the propagation of activity over this network under simple spreading and connectivity rules. By comparison with standard network models with the same coarse statistics, we show that the cortical geometry influences profoundly the speed of propagation of activation through the network. Our conclusions are of high relevance to the theoretical modelling of epileptic seizure events and indicate that such studies which omit physiological network structure risk simplifying the dynamics in a potentially significant way
Antibody-based detection of protein phosphorylation status to track the efficacy of novel therapies using nanogram protein quantities from stem cells and cell lines
This protocol describes a highly reproducible antibody-based method that provides protein level and phosphorylation status information from nanogram quantities of protein cell lysate. Nanocapillary isoelectric focusing (cIEF) combines with UV-activated linking chemistry to detect changes in phosphorylation status. As an example application, we describe how to detect changes in response to tyrosine kinase inhibitors (TKIs) in the phosphorylation status of the adaptor protein CrkL, a major substrate of the oncogenic tyrosine kinase BCR-ABL in chronic myeloid leukemia (CML), using highly enriched CML stem cells and mature cell populations in vitro. This protocol provides a 2.5 pg/nl limit of protein detection (<0.2% of a stem cell sample containing <104 cells). Additional assays are described for phosphorylated tyrosine 207 (pTyr207)-CrkL and the protein tyrosine phosphatase PTPRC/CD45; these assays were developed using this protocol and applied to CML patient samples. This method is of high throughput, and it can act as a screen for in vitro cancer stem cell response to drugs and novel agents
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