4,386 research outputs found
Spectral ageing: a new age perspective
We present an up-to-date critique of the physical basis for the spectral
ageing method. We find that the number of cases where this method may be
meaningfully applied to deduce the ages of classical double radio sources is
small indeed. This critique is much more than merely a re-expression of
anxieties about the calibration of spectral ageing (which have been articulated
by others in the past).Comment: 9 pages, 3 figures, published in `Particles & Fields in Radio
Galaxies', PASP, eds RA Laing & KM Blundel
Determining the cosmic ray ionization rate in dynamically evolving clouds
The ionization fraction is an important factor in determining the chemical
and physical evolution of star forming regions. In the dense, dark starless
cores of such objects, the ionization rate is dominated by cosmic rays; it is
therefore possible to use simple analytic estimators, based on the relative
abundances of different molecular tracers, to determine the cosmic ray
ionization rate.
This paper uses a simple model to investigate the accuracy of two well-known
estimators in dynamically evolving molecular clouds. It is found that, although
the analytical formulae based on the abundances of H3+,H2,CO,O,H2O and HCO+
give a reasonably accurate measure of the cosmic ray ionization rate in static,
quiescent clouds, significant discrepancies occur in rapidly evolving
(collapsing) clouds. As recent evidence suggests that molecular clouds may
consist of complex, dynamically evolving sub-structure, we conclude that simple
abundance ratios do not provide reliable estimates of the cosmic ray ionization
rate in dynamically active regions.Comment: Accepted by A&A. 17 pages, 4 figure
Mapping of interstellar clouds with infrared light scattered from dust: TMC-1N
Mapping of near-infrared (NIR) scattered light is a recent method for the
study of interstellar clouds, complementing other, more commonly used methods,
like dust emission and extinction. Our goal is to study the usability of this
method on larger scale, and compare the properties of a filament using NIR
scattering and other methods. We also study the radiation field and differences
in grain emissivity between diffuse and dense areas. We have used scattered J,
H, and K band surface brightness WFCAM-observations to map filament TMC-1N in
Taurus, covering an area of 1dx1d corresponding to ~(2.44 pc)^2. We have
converted the data into optical depth and compared the results with NIR
extinction and Herschel observations of submm dust emission. We see the
filament in scattered light in all three NIR bands. We note that our WFCAM
observations in TMC-1N show notably lower intensity than previous results in
Corona Australis using the same method. We show that 3D radiative transfer
simulations predict similar scattered surface brightness levels as seen in the
observations. However, changing the assumptions about the background can change
the results of simulations notably. We derive emissivity by using optical depth
in the J band as an independent tracer of column density. We obtain opacity
sigma(250um) values 1.7-2.4x10^-25 cm^2/H, depending on assumptions of the
extinction curve, which can change the results by over 40%. These values are
twice as high as obtained for diffuse areas, at the lower limit of earlier
results for denser areas. We show that NIR scattering can be a valuable tool in
making high resolution maps. We conclude, however, that NIR scattering
observations can be complicated, as the data can show relatively low-level
artefacts. This suggests caution when planning and interpreting the
observations.Comment: abstract shortened and figures reduced for astrop
Evaluating the impact of conditional cash transfer programs : lessons from Latin America
Unlike most development initiatives, conditional cash transfer programs recently introduced in the Latin America and the Caribbean region have been subject to rigorous evaluations of their effectiveness. These programs provide money to poor families, conditional on certain behavior, usually investments in human capital-such as sending children to school or bringing them to health centers on a regular basis. Rawlings and Rubio review the experience in evaluating the impact of these programs, exploring the application of experimental and quasi-experimental evaluation methods and summarizing results from programs launched in Brazil, Honduras, Jamaica, Mexico, and Nicaragua. Evaluation results from the first generation of programs in Brazil, Mexico, and Nicaragua show that conditional cash transfer programs are effective in promoting human capital accumulation among poor households. There is clear evidence of success in increasing enrollment rates, improving preventive health care, and raising household consumption. Despite this promising evidence, many questions remain unanswered about the impact of conditional cash transfer programs, including those concerning their effectiveness under different country conditions and the sustainability of the welfare impacts.Public Health Promotion,Poverty Monitoring&Analysis,Poverty Impact Evaluation,Health Monitoring&Evaluation,Health Systems Development&Reform,Poverty Monitoring&Analysis,Poverty Impact Evaluation,Health Monitoring&Evaluation,Poverty Assessment,Health Systems Development&Reform
Extremely red galaxy counterparts to 7C radio sources
We present RIJHK imaging of seven radio galaxies from the 7C Redshift Survey
(7CRS) which lack strong emission lines and we use these data to investigate
their spectral energy distributions (SEDs) with models which constrain their
redshifts. Six of these seven galaxies have extremely red colours (R-K>5.5) and
we find that almost all of them lie in the redshift range 1<z<2. We also
present near-infrared spectroscopy of these galaxies which demonstrate that
their SEDs are not dominated by emission lines, although tentative lines,
consistent with H-alpha at z=1.45 and z=1.61, are found in two objects.
Although the red colours of the 7CRS galaxies can formally be explained by
stellar populations which are either very old or young and heavily reddened,
independent evidence favours the former hypothesis. At z~1.5 at least 1/4 of
powerful radio jets are triggered in massive (>L*) galaxies which formed the
bulk of their stars several Gyr earlier, that is at epochs corresponding to
redshifts z>5. If a similar fraction of all z~1.5 radio galaxies are old, then
extrapolation of the radio luminosity function shows that, depending on the
radio source lifetimes, between 10-100% of the near-IR selected extremely red
object (ERO) population undergo a radio outburst at epochs corresponding to
1<z<2. An ERO found serendipitously in the field of one of the 7CRS radio
sources appears to be a radio-quiet analogue of the 7CRS EROs with an emission
line likely to be [OII] at z=1.20. The implication is that some of the most
massive elliptical galaxies formed the bulk of their stars at z>5 and these
objects probably undergo at least two periods of AGN activity: one at high
redshift during which the black hole forms and another one at an epoch
corresponding to z~1.5.Comment: 20 pages, 10 figures, accepted for publication in MNRA
The Nature and Evolution of Classical Double Radio Sources from Complete Samples
We present a study of the trends in luminosity, linear size, spectral index,
and redshift of classical double radio sources from three complete samples
selected at successively fainter low radio-frequency flux-limits. We have been
able to decouple the effects of the tight correlation between redshift and
luminosity (inherent in any single flux-limited sample) which have hitherto
hindered interpretation of the relationships between these four source
properties. The major trends found are that (i) spectral indices increase with
linear size, (ii) rest-frame spectral indices have a stronger dependence on
luminosity than on redshift except at high (GHz) frequencies, and that (iii)
the linear sizes are smaller at higher redshifts. We reproduce the observed
dependences in a model for radio sources (born throughout cosmic time according
to a radio-source birth function) whose lobes are fed with a
synchrotron-emitting population (whose energy distribution is governed by
compact hotspots), and which suffer inverse Compton, synchrotron and adiabatic
expansion losses. In simulating the basic observed dependences, we find that
there is no need to invoke any systematic change in the environments of these
objects with redshift if the consequences of imposing a survey flux-limit on
our simulated datasets are properly included in the model. We present evidence
that for a radio survey there is an unavoidable `youth--redshift degeneracy',
even though radio sources are short-lived relative to the age of the Universe;
it is imperative to take this into account in studies which seemingly reveal
correlations of source properties with redshift such as the `alignment effect'.Comment: 48 pages, 19 figures, uses aas2pp4.sty. To appear in AJ. Also
available at http://www-astro.physics.ox.ac.uk/~kmb References updated and
minor typos correcte
The radio luminosity function of radio-loud quasars from the 7C Redshift Survey
We present a complete sample of 24 radio-loud quasars (RLQs) from the new 7C
Redshift Survey. Every quasar with a low-frequency (151 MHz) radio flux-density
S_151 > 0.5 Jy in two regions of the sky covering 0.013 sr is included; 23 of
these have sufficient extended flux to meet the selection criteria, 18 of these
have steep radio spectra (hereafter denoted as SSQs). The key advantage of this
sample over most samples of RLQs is the lack of an optical magnitude limit. By
combining the 7C and 3CRR samples, we have investigated the properties of RLQs
as a function of redshift z and radio luminosity L_151.
We derive the radio luminosity function (RLF) of RLQs and find that the data
are well fitted by a single power-law with slope alpha_1=1.9. We find that
there must be a break in the RLQ RLF at log_10(L_151 / W Hz^-1 sr^-1) < 27, in
order for the models to be consistent with the 7C and 6C source counts. The
z-dependence of the RLF follows a one-tailed gaussian which peaks at z=1.7. We
find no evidence for a decline in the co-moving space density of RLQs at higher
redshifts.
A positive correlation between the radio and optical luminosities of SSQs is
observed, confirming a result of Serjeant et al. (1998). We are able to rule
out this correlation being due to selection effects or biases in our combined
sample. The radio-optical correlation and best-fit model RLF enable us to
estimate the distribution of optical magnitudes of quasars in samples selected
at low radio frequencies. We conclude that for samples with S_151 < 1 Jy one
must use optical data significantly deeper than the POSS-I limit (R approx 20),
in order to avoid severe incompleteness.Comment: 28 pages with 13 figures. To appear in MNRA
Time and M-theory
We review our recent proposal for a background independent formulation of a
holographic theory of quantum gravity. The present review incorporates the
necessary background material on geometry of canonical quantum theory,
holography and spacetime thermodynamics, Matrix theory, as well as our specific
proposal for a dynamical theory of geometric quantum mechanics, as applied to
Matrix theory. At the heart of this review is a new analysis of the conceptual
problem of time and the closely related and phenomenologically relevant problem
of vacuum energy in quantum gravity. We also present a discussion of some
observational implications of this new viewpoint on the problem of vacuum
energy.Comment: 86 pages, 5 figures, LaTeX, typos fixed, references added, and Sec.
6.2 revised; invited review for Int. J. Mod. Phys.
Unified Models of Molecular Emission from Class 0 Protostellar Outflow Sources
Low mass star-forming regions are more complex than the simple spherically
symmetric approximation that is often assumed. We apply a more realistic
infall/outflow physical model to molecular/continuum observations of three late
Class 0 protostellar sources with the aims of (a) proving the applicability of
a single physical model for all three sources, and (b) deriving physical
parameters for the molecular gas component in each of the sources.
We have observed several molecular species in multiple rotational
transitions. The observed line profiles were modelled in the context of a
dynamical model which incorporates infall and bipolar outflows, using a three
dimensional radiative transfer code. This results in constraints on the
physical parameters and chemical abundances in each source.
Self-consistent fits to each source are obtained. We constrain the
characteristics of the molecular gas in the envelopes as well as in the
molecular outflows. We find that the molecular gas abundances in the infalling
envelope are reduced, presumably due to freeze-out, whilst the abundances in
the molecular outflows are enhanced, presumably due to dynamical activity.
Despite the fact that the line profiles show significant source-to-source
variation, which primarily derives from variations in the outflow viewing
angle, the physical parameters of the gas are found to be similar in each core.Comment: MNRAS 12 pages, 16 figure
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