1,052 research outputs found
Muscle fibrils: Solubilization and gel electrophoresis
In a three-year longitudinal intervention study developing an understanding of statisticalliteracy with a cohort of students from Years 4 to 6, teachers delivered lessons fromprovided materials, including scripts and prompts. Besides completing seven majorinvestigations, workbooks, and several in-class assessments, the students undertook foursurveys, from which their overall progress was determined, as well as individual learningprogressions. Findings indicated that despite experiencing the same lessons delivered insimilar ways, students had very different learning progressions. The implications of thesefindings are explored
Extragalactic megahertz-peaked spectrum radio sources at milliarcsecond scales
Extragalactic peaked-spectrum radio sources are thought to be the progenitors
of larger, radio-loud active galactic nuclei (AGN). Synchrotron self-absorption
(SSA) has often been identified as the cause of their spectral peak. The
identification of new megahertz-peaked spectrum sources from the GaLactic and
Extragalactic All-sky Murchison Widefield Array (GLEAM) survey provides an
opportunity to test how radio sources with spectral peaks below 1 GHz fit
within this evolutionary picture. We observed six peaked-spectrum sources
selected from the GLEAM survey, three that have spectral characteristics which
violate SSA and three that have spectral peaks below 230 MHz, with the Very
Long Baseline Array at 1.55 and 4.96 GHz. We present milliarcsecond resolution
images of each source and constrain their morphology, linear size, luminosity,
and magnetic field strength. Of the sources that are resolved by our study, the
sources that violate SSA appear to be compact doubles, while the sources with
peak frequencies below 230 MHz have core-jet features. We find that all of our
sources are smaller than expected from SSA by factors of >20. We also find that
component magnetic field strengths calculated from SSA are likely inaccurate,
differing by factors of >5 from equipartition estimates. The calculated
equipartition magnetic field strengths more closely resemble estimates from
previously studied gigahertz-peaked spectrum sources. Exploring a model of the
interaction between jets and the interstellar medium, we demonstrate that
free-free absorption (FFA) can accurately describe the linear sizes and peak
frequencies of our sources. Our findings support the theory that there is a
fraction of peaked-spectrum sources whose spectral peaks are best modelled by
FFA, implying our understanding of the early stages of radio AGN is incomplete.Comment: Accepted for publication in Astronomy & Astrophysics (A&A) on 16 July
2019. 13 pages, 6 figure
Milliarcsecond Structures of Variable Peaked-Spectrum Sources
Spectral variability offers a new technique to identify small scale
structures from scintillation, as well as determining the absorption mechanism
for peaked-spectrum (PS) radio sources. In this paper, we present very long
baseline interferometry (VLBI) imaging using the Long Baseline Array (LBA) of
two PS sources, MRC0225-065 and PMNJ0322-4820, identified as spectrally
variable from observations with the Murchison Widefield Array (MWA). We compare
expected milliarcsecond structures based on the detected spectral variability
with direct LBA imaging. We find MRC0225-065 is resolved into three components,
a bright core and two fainter lobes, roughly 430pc projected separation. A
comprehensive analysis of the magnetic field, host galaxy properties, and
spectral analysis implies that MRC0225-065 is a young radio source with recent
jet activity over the last 10^2-10^3years. We find PMNJ0322-4820 is unresolved
on milliarcsecond scales. We conclude PMNJ0322-4820 is a blazar with flaring
activity detected in 2014 with the MWA. We use spectral variability to predict
morphology and find these predictions consistent with the structures revealed
by our LBA images.Comment: Accepted for publication in PASA. 11 pages, 4 figure
Variability of M giant stars based on Kepler photometry: general characteristics
M giants are among the longest-period pulsating stars which is why their
studies were traditionally restricted to analyses of low-precision visual
observations, and more recently, accurate ground-based data. Here we present an
overview of M giant variability on a wide range of time-scales (hours to
years), based on analysis of thirteen quarters of Kepler long-cadence
observations (one point per every 29.4 minutes), with a total time-span of over
1000 days. About two-thirds of the sample stars have been selected from the
ASAS-North survey of the Kepler field, with the rest supplemented from a
randomly chosen M giant control sample.
We first describe the correction of the light curves from different quarters,
which was found to be essential. We use Fourier analysis to calculate multiple
frequencies for all stars in the sample. Over 50 stars show a relatively strong
signal with a period equal to the Kepler-year and a characteristic phase
dependence across the whole field-of-view. We interpret this as a so far
unidentified systematic effect in the Kepler data. We discuss the presence of
regular patterns in the distribution of multiple periodicities and amplitudes.
In the period-amplitude plane we find that it is possible to distinguish
between solar-like oscillations and larger amplitude pulsations which are
characteristic for Mira/SR stars. This may indicate the region of the
transition between two types of oscillations as we move upward along the giant
branch.Comment: 12 pages, 13 figures, accepted for publication in MNRAS. The
normalized light curves are available upon reques
The SAMI Galaxy Survey: The Low-Redshift Stellar Mass Tully-Fisher Relation
We investigate the Tully-Fisher Relation (TFR) for a morphologically and
kine- matically diverse sample of galaxies from the SAMI Galaxy Survey using 2
dimensional spatially resolved Halpha velocity maps and find a well defined
relation across the stellar mass range of 8.0 < log(M*) < 11.5. We use an
adaptation of kinemetry to parametrise the kinematic Halpha asymmetry of all
galaxies in the sample, and find a correlation between scatter (i.e. residuals
off the TFR) and asymmetry. This effect is pronounced at low stellar mass,
corresponding to the inverse relationship between stellar mass and kinematic
asymmetry found in previous work. For galaxies with log(M*) < 9.5, 25 +/- 3%
are scattered below the root mean square (RMS) of the TFR, whereas for galaxies
with log(M*) > 9.5 the fraction is 10 +/- 1% We use 'simulated slits' to
directly compare our results with those from long slit spectroscopy and find
that aligning slits with the photometric, rather than the kinematic, position
angle, increases global scatter below the TFR. Further, kinematic asymmetry is
correlated with misalignment between the photometric and kinematic position
angles. This work demonstrates the value of 2D spatially resolved kinematics
for accurate TFR studies; integral field spectroscopy reduces the
underestimation of rotation velocity that can occur from slit positioning off
the kinematic axis
The Spectral Energy Distribution of Powerful Starburst Galaxies I: Modelling the Radio Continuum
We have acquired radio continuum data between 70\,MHz and 48\,GHz for a
sample of 19 southern starburst galaxies at moderate redshifts () with the aim of separating synchrotron and free-free emission
components. Using a Bayesian framework we find the radio continuum is rarely
characterised well by a single power law, instead often exhibiting low
frequency turnovers below 500\,MHz, steepening at mid-to-high frequencies, and
a flattening at high frequencies where free-free emission begins to dominate
over the synchrotron emission. These higher order curvature components may be
attributed to free-free absorption across multiple regions of star formation
with varying optical depths. The decomposed synchrotron and free-free emission
components in our sample of galaxies form strong correlations with the
total-infrared bolometric luminosities. Finally, we find that without
accounting for free-free absorption with turnovers between 90 to 500\,MHz the
radio-continuum at low frequency (\,MHz) could be overestimated by
upwards of a factor of twelve if a simple power law extrapolation is used from
higher frequencies. The mean synchrotron spectral index of our sample is
constrained to be , which is steeper then the canonical value of
for normal galaxies. We suggest this may be caused by an intrinsically
steeper cosmic ray distribution
LOFAR MSSS: Flattening low-frequency radio continuum spectra of nearby galaxies
Accepted for publication in Astronomy and AstrophysicsAims. The shape of low-frequency radio continuum spectra of normal galaxies is not well understood, the key question being the role of physical processes such as thermal absorption in shaping them. In this work we take advantage of the LOFAR Multifrequency Snapshot Sky Survey (MSSS) to investigate such spectra for a large sample of nearby star-forming galaxies. Methods. Using the measured 150 MHz flux densities from the LOFAR MSSS survey and literature flux densities at various frequencies we have obtained integrated radio spectra for 106 galaxies characterised by different morphology and star formation rate. The spectra are explained through the use of a three-dimensional model of galaxy radio emission, and radiation transfer dependent on the galaxy viewing angle and absorption processes. Results. Our galaxies' spectra are generally flatter at lower compared to higher frequencies: the median spectral index α low measured between ≈ 50 MHz and 1.5 GHz is -0.57 ± 0.01 while the high-frequency one α high, calculated between 1.3 GHz and 5 GHz, is -0.77 ± 0.03. As there is no tendency for the highly inclined galaxies to have more flattened low-frequency spectra, we argue that the observed flattening is not due to thermal absorption, contradicting the suggestion of Israel & Mahoney (1990, ApJ, 352, 30). According to our modelled radio maps for M 51-like galaxies, the free-free absorption effects can be seen only below 30 MHz and in the global spectra just below 20 MHz, while in the spectra of starburst galaxies, like M 82, the flattening due to absorption is instead visible up to higher frequencies of about 150 MHz. Starbursts are however scarce in the local Universe, in accordance with the weak spectral curvature seen in the galaxies of our sample. Locally, within galactic disks, the absorption effects are distinctly visible in M 51-like galaxies as spectral flattening around 100-200 MHz in the face-on objects, and as turnovers in the edge-on ones, while in M 82-like galaxies there are strong turnovers at frequencies above 700 MHz, regardless of viewing angle. Conclusions. Our modelling of galaxy spectra suggests that the weak spectral flattening observed in the nearby galaxies studied here results principally from synchrotron spectral curvature due to cosmic ray energy losses and propagation effects. We predict much stronger effects of thermal absorption in more distant galaxies with high star formation rates. Some influence exerted by the Milky Way's foreground on the spectra of all external galaxies is also expected at very low frequencies.Peer reviewedFinal Accepted Versio
First look at the giant radio galaxy 3C 236 with LOFAR
We have examined the giant radio galaxy 3C 236 using LOFAR at 143 MHz down to an angular resolution of 7″, in combination with observations at higher frequencies. We used the low frequency data to derive spectral index maps with the highest resolution yet at these low frequencies. We confirm a previous detection of an inner hotspot in the north-west lobe and for the first time observe that the south-east lobe hotspot is in fact a triple hotspot, which may point to an intermittent source activity. Also, the spectral index map of 3C 236 shows that the spectral steepening at the inner region of the northern lobe is prominent at low frequencies. The outer regions of both lobes show spectral flattening, in contrast with previous high frequency studies. We derive spectral age estimates for the lobes, as well as particle densities of the IGM at various locations. We propose that the morphological differences between the lobes are driven by variations in the ambient medium density as well as the source activity history
Catalytic enantioselective arylative cyclizations of alkynyl 1,3-diketones by 1,4-rhodium(i) migration
The enantioselective synthesis of densely functionalized polycarbocycles by the rhodium(I)-catalyzed reaction of arylboronic acids with 1,3-diketones is described. The key step in these desymmetrizing domino addition–cyclization reactions is an alkenyl-to-aryl 1,4-Rh(I) migration, which enables arylboronic acids to function effectively as 1,2-dimetalloarene surrogates
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