171 research outputs found
Including Wrist Flexion in the Human Arm Model Changes Everything!
Does your introductory physics laboratory experiment that analyzes the human arm as a lever assume an inflexible wrist? If so, the analysis of the biceps force required to perform a biceps curl will lead to results that contradict experience: one does not expect the required biceps force to decrease as the mass is raised. We will show that allowing for wrist flexion leads to agreement with empirical data: that the required force does increase as the mass is raised if the wrist angle is allowed to be optimal
Quasars are more luminous than radio galaxies - so what?
Surveys to find high-redshift radio galaxies deliberately exclude
optically-bright objects, which may be distant radio-loud quasars. In order to
properly determine the space density of supermassive black holes, the fraction
of such objects missed must be determined within a quantitative framework for
AGN unification. I briefly describe the receding torus model, which predicts
that quasars should have more luminous ionizing continua than radio galaxies of
similar radio luminosity, and present evidence to support it. I also suggest
two further tests of the model which should constrain some of its parameters.Comment: 6 pages. To appear in "Radio galaxies: past, present and future", eds
M. Jarvis et al., Leiden, Nov 200
Implications for unified schemes from submillimetre and far-infrared follow-up of radio-selected samples
We extend our previous analysis which used generalized luminosity functions
(GLFs) to predict the number of quasars and galaxies in
low-radio-frequency-selected samples as a function of redshift, radio
luminosity, narrow-emission-line luminosity and type of unified scheme. Our
extended analysis incorporates the observed submillimetre (850 micron) flux
densities of radio sources, employs a new method which allows us to deal with
non detections, and focuses on the high-luminosity population. First, we
conclude that the submillimetre luminosity L_{850} of low-frequency-selected
radio sources is correlated with the bolometric luminosity L_{Bol} of their
quasar nuclei via an approximate scaling relation L_{850} \propto L_{Bol}^{0.7
\pm 0.2}. Second, we conclude that there is quantitative evidence for a
receding-torus-like physical process for the high-luminosity population within
a two-population unified scheme for radio sources; this evidence comes from the
fact that radio quasars are brighter in both narrow emission lines and
submillimetre luminosity than radio galaxies matched in radio luminosity and
redshift. Third, we note that the combination of a receding-torus-like scheme
and the assumption that the observed submillimetre emission is dominated by
quasar-heated dust yields a scaling relation L_{850} \propto L_{Bol}^{0.5}
which is within the errors of that determined here for radio-selected quasars,
and consistent with that inferred for radio-quiet quasars by Willott, Rawlings
& Grimes (2003).Comment: 13 pages (including an appendix), 5 figures, to appear in MNRA
A Wideband Polarization Study of Cygnus A with the JVLA. I: The Observations and Data
We present results from deep, wideband, high spatial and spectral resolution
observations of the nearby luminous radio galaxy Cygnus A with the Jansky Very
Large Array. The high surface brightness of this source enables detailed
polarimetric imaging, providing images at 0.75\arcsec, spanning 2 - 18 GHz,
and at 0.30\arcsec (6 - 18 GHz). The fractional polarization from 2000
independent lines of sight across the lobes decreases strongly with decreasing
frequency, with the eastern lobe depolarizing at higher frequencies than the
western lobe. The depolarization shows considerable structure, varying from
monotonic to strongly oscillatory. The fractional polarization in general
increases with increasing resolution at a given frequency, as expected.
However, there are numerous lines of sight with more complicated behavior. We
have fitted the 0.3\arcsec images with a simple model incorporating random,
unresolved fluctuations in the cluster magnetic field to determine the high
resolution, high-frequency properties of the source and the cluster. From these
derived properties, we generate predicted polarization images of the source at
lower frequencies, convolved to 0.75\arcsec. These predictions are remarkably
consistent with the observed emission. The observations are consistent with the
lower-frequency depolarization being due to unresolved fluctuations on scales
300 - 700 pc in the magnetic field and/or electron density superposed
on a partially ordered field component. There is no indication in our data of
the location of the depolarizing screen or the large-scale field, either, or
both of which could be located throughout the cluster, or in a boundary region
between the lobes and the cluster.Comment: 24 pages, 13 figures. The manuscript has been accepted for
publication in The Astrophysical Journa
The quasar fraction in low-frequency selected complete samples and implications for unified schemes
Low-frequency radio surveys are ideal for selecting orientation-independent
samples of extragalactic sources because the sample members are selected by
virtue of their isotropic steep-spectrum extended emission. We use the new 7C
Redshift Survey along with the brighter 3CRR and 6C samples to investigate the
fraction of objects with observed broad emission lines - the `quasar fraction'
- as a function of redshift and of radio and narrow emission line luminosity.
We find that the quasar fraction is more strongly dependent upon luminosity
(both narrow line and radio) than it is on redshift. Above a narrow [OII]
emission line luminosity of log L_[OII] > 35 W (or radio luminosity log L_151 >
26.5 W/Hz/sr), the quasar fraction is virtually independent of redshift and
luminosity; this is consistent with a simple unified scheme with an obscuring
torus with a half-opening angle theta_trans approx 53 degrees. For objects with
less luminous narrow lines, the quasar fraction is lower. We show that this is
not due to the difficulty of detecting lower-luminosity broad emission lines in
a less luminous, but otherwise similar, quasar population. We discuss evidence
which supports at least two probable physical causes for the drop in quasar
fraction at low luminosity: (i) a gradual decrease in theta_trans and/or a
gradual increase in the fraction of lightly-reddened (0 < A(V) < 5)
lines-of-sight with decreasing quasar luminosity; and (ii) the emergence of a
distinct second population of low luminosity radio sources which, like M87,
lack a well-fed quasar nucleus and may well lack a thick obscuring torus.Comment: 10 pages, 4 figures, accepted for publication in MNRA
Using seasonal rainfall clusters to explain the interannual variability of the rain belt over the Greater Horn of Africa
The seasonal cycle of rainfall over the Greater Horn of Africa (GHA) is dominated by the latitudinal migration and activity of the tropical rain belt (TRB). The TRB exhibits high interannual variability in the GHA and the reasons for the recent dry period in the Long Rains (MarchâMay) are poorly understood. In addition, few studies have addressed the rainfall fluctuations during the Msimu Rains (Dec.âMar.) in the southern GHA region. Interannual variations of the seasonal cycle of the TRB between 1981 and 2018 were analysed using two statistical indices. The Rainfall Cluster Index (RCI) describes the seasonal cycle as a succession of six characteristic rainfall patterns, while the Seasonal Location Index (SLI) captures the latitudinal location of the TRB. The SLI and RCI depict the full seasonal cycle of the TRB supporting interpretations of the interannual variations and trends. The Msimu Rains are dominated by two clusters with opposite rainfall characteristics between the Congo Basin and Tanzania. The associated anomalies in moisture flux and divergence indicate variations in the location of the TRB originating from an interplay between lowâlevel air flows from the Atlantic and Indian Oceans and tropical and subtropical teleconnections. The peak period of the Long Rains shows a complex composition of five clusters, which is tightly connected to intraseasonal and interannual variability of latitudinal locations of the TRB. A persistent location of the TRB near the equator, evidenced in a frequent occurrence of a cluster related to an anomalously weak Walker circulation, is associated with wet conditions over East Africa. Dry Long Rains are associated with strong and frequent latitudinal variations of the TRB position with a late onset and intermittent rainfall. These results offer new opportunities to understand recent variability and trends in the GHA region
Radio galaxy evolution: what you can learn from a Brief Encounter
We describe the pitfalls encountered in deducing from classical double radio
source observables (luminosity, spectral index, redshift and linear size) the
essential nature of how these objects evolve. We discuss the key role played by
hotspots in governing the energy distribution of the lobes they feed, and
subsequent spectral evolution. We present images obtained using the new 74 MHz
receivers on the VLA and discuss constraints which these enforce on models of
the backflow and ages in classical doubles.Comment: invited talk at `Lifecycles of Radio Galaxies' workshop; eds John
Biretta et a
Young stars and non-stellar emission in the aligned radio galaxy 3C 256
We present ground-based images of the z=1.824 radio galaxy 3C 256 in the
standard BVRIJHK filters and an interference filter centered at 8800A, a Hubble
Space Telescope image in a filter dominated by Ly-alpha emission (F336W), and
spectra covering rest-frame wavelengths from Ly-alpha to [O III] 5007. Together
with published polarimetry observations, we use these to decompose the overall
spectral energy distribution into nebular continuum emission, scattered quasar
light, and stellar emission. The nebular continuum and scattered light together
comprise half (one third) of the V-band (K-band) light within a 4-arcsec
aperture, and are responsible for the strong alignment between the
optical/near-infrared light and the radio emission. The stellar emission is
dominated by a population estimated to be 100-200 Myr old (assuming a Salpeter
IMF), and formed in a short burst with a peak star formation rate of 1-4x10^3
Msun/yr. The total stellar mass is estimated to be no more than 2x10^{11} Msun,
which is far less than other luminous radio galaxies at similar redshifts, and
suggests that 3C 256 will undergo further star formation or mergers.Comment: 35 pages including 10 figures; to appear in Nov 10 Ap
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