403 research outputs found
GMRT detection of HI 21 cm associated absorption towards the z=1.2 red quasar 3C 190
We report the GMRT detection of associated HI 21 cm-line absorption in the
z=1.1946 red quasar 3C 190. Most of the absorption is blue-shifted with respect
to the systemic redshift. The absorption, at 647.7 MHz, is broad and
complex, spanning a velocity width of 600 \kms. Since the core is
self-absorbed at this frequency, the absorption is most likely towards the
hotspots. Comparison of the radio and deep optical images reveal linear
filaments in the optical which overlap with the brighter radio jet towards the
south-west. We therefore suggest that most of the HI 21 cm-line absorption
could be occurring in the atomic gas shocked by the south-west jet.Comment: 8 pages, 1 fugure. To appear in Journal of Astrophysics and Astronom
The Juno Magnetic Field Investigation
The Juno Magnetic Field investigation (MAG) characterizes Jupiter's planetary magnetic field and magnetosphere, providing the first globally distributed and proximate measurements of the magnetic field of Jupiter. The magnetic field instrumentation consists of two independent magnetometer sensor suites, each consisting of a tri-axial Fluxgate Magnetometer (FGM) sensor and a pair of co-located imaging sensors mounted on an ultra-stable optical bench. The imaging system sensors are part of a subsystem that provides accurate attitude information (to approx. 20 arcsec on a spinning spacecraft) near the point of measurement of the magnetic field. The two sensor suites are accommodated at 10 and 12 m from the body of the spacecraft on a 4 m long magnetometer boom affixed to the outer end of one of 's three solar array assemblies. The magnetometer sensors are controlled by independent and functionally identical electronics boards within the magnetometer electronics package mounted inside Juno's massive radiation shielded vault. The imaging sensors are controlled by a fully hardware redundant electronics package also mounted within the radiation vault. Each magnetometer sensor measures the vector magnetic field with 100 ppm absolute vector accuracy over a wide dynamic range (to 16 Gauss = 1.6 x 10(exp. 6) nT per axis) with a resolution of approx. 0.05 nT in the most sensitive dynamic range (+/-1600 nT per axis). Both magnetometers sample the magnetic field simultaneously at an intrinsic sample rate of 64 vector samples per second. The magnetic field instrumentation may be reconfigured in flight to meet unanticipated needs and is fully hardware redundant. The attitude determination system compares images with an on-board star catalog to provide attitude solutions (quaternions) at a rate of up to 4 solutions per second, and may be configured to acquire images of selected targets for science and engineering analysis. The system tracks and catalogs objects that pass through the imager field of view and also provides a continuous record of radiation exposure. A spacecraft magnetic control program was implemented to provide a magnetically clean environment for the magnetic sensors, and residual spacecraft fields andor sensor offsets are monitored in flight taking advantage of Juno's spin (nominally 2 rpm) to separate environmental fields from those that rotate with the spacecraft
A backward evolution model for infrared surveys: the role of AGN- and Color-L_TIR distributions
Empirical "backward" galaxy evolution models for infrared bright galaxies are
constrained using multi-band infrared surveys. We developed a new Monte-Carlo
algorithm for this task, implementing luminosity dependent distribution
functions for the galaxies' infrared spectral energy distributions (SEDs) and
for the AGN contribution, allowing for evolution of these quantities. The
adopted SEDs take into account the contributions of both starbursts and AGN to
the infrared emission, for the first time in a coherent treatment rather than
invoking separate AGN and star-forming populations. In the first part of the
paper we consider the quantification of the AGN contribution for local universe
galaxies, as a function of total infrared luminosity. It is made using a large
sample of LIRGs and ULIRGs for which mid-infrared spectra are available in the
Spitzer archive. In the second part we present the model. Our best-fit model
adopts very strong luminosity evolution, , up to , and
density evolution, , up to , for the population of
infrared galaxies. At higher , the evolution rates drop as and
respectively. To reproduce mid-infrared to submillimeter number
counts and redshift distributions, it is necessary to introduce both an
evolution in the AGN contribution and an evolution in the
luminosity-temperature relation. Our models are in plausible agreement with
current photometry-based estimates of the typical AGN contribution as a
function of mid-infrared flux, and well placed to be compared to upcoming
Spitzer spectroscopic results. As an example of future applications, we use our
best-fitting model to make predictions for surveys with Herschel.Comment: Model available at: (http://www.physics.ubc.ca/~valiante/model) ApJ
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A multi-wavelength survey of AGN in the XMM-LSS field: I. Quasar selection via the KX technique
AIMS: We present a sample of candidate quasars selected using the
KX-technique. The data cover 0.68 deg^2 of the X-ray Multi-Mirror (XMM)
Large-Scale Structure (LSS) survey area where overlapping multi-wavelength
imaging data permits an investigation of the physical nature of selected
sources. METHODS: The KX method identifies quasars on the basis of their
optical (R and z') to near-infrared (Ks) photometry and point-like morphology.
We combine these data with optical (u*,g'r',i',z') and mid-infrared (3.6-24
micron) wavebands to reconstruct the spectral energy distributions (SEDs) of
candidate quasars. RESULTS: Of 93 sources selected as candidate quasars by the
KX method, 25 are classified as quasars by the subsequent SED analysis.
Spectroscopic observations are available for 12/25 of these sources and confirm
the quasar hypothesis in each case. Even more, 90% of the SED-classified
quasars show X-ray emission, a property not shared by any of the false
candidates in the KX-selected sample. Applying a photometric redshift analysis
to the sources without spectroscopy indicates that the 25 sources classified as
quasars occupy the interval 0.7 < z < 2.5. The remaining 68/93 sources are
classified as stars and unresolved galaxies.Comment: 13 pages, 9 figures, A&A 494, p. 579-589. Replaced with published
version. Fig. 9 in first astro-ph submission has been update
Massive stars exploding in a He-rich circumstellar medium. I. Type Ibn (SN 2006jc-like) events
We present new spectroscopic and photometric data of the type Ibn supernovae
2006jc, 2000er and 2002ao. We discuss the general properties of this recently
proposed supernova family, which also includes SN 1999cq. The early-time
monitoring of SN 2000er traces the evolution of this class of objects during
the first few days after the shock breakout. An overall similarity in the
photometric and spectroscopic evolution is found among the members of this
group, which would be unexpected if the energy in these core-collapse events
was dominated by the interaction between supernova ejecta and circumstellar
medium. Type Ibn supernovae appear to be rather normal type Ib/c supernova
explosions which occur within a He-rich circumstellar environment. SNe Ibn are
therefore likely produced by the explosion of Wolf-Rayet progenitors still
embedded in the He-rich material lost by the star in recent mass-loss episodes,
which resemble known luminous blue variable eruptions. The evolved Wolf-Rayet
star could either result from the evolution of a very massive star or be the
more evolved member of a massive binary system. We also suggest that there are
a number of arguments in favour of a type Ibn classification for the historical
SN 1885A (S-Andromedae), previously considered as an anomalous type Ia event
with some resemblance to SN 1991bg.Comment: 17 pages including 12 figures and 4 tables. Slightly revised version,
conclusions unchanged, 1 figure added. Accepted for publication in MNRA
Two Type Ic supernovae in low-metallicity, dwarf galaxies: diversity of explosions
We present BVRI photometry and optical spectroscopy of two Type Ic supernovae
SN 2007bg and SN 2007bi discovered in wide-field, non-targeted surveys and
associated with sub-luminous blue dwarf galaxies. Neither SNe 2007bg nor 2007bi
were found in association with an observed GRB, but are found to inhabit
similar low-metallicity environments as GRB associated supernovae. The
radio-bright SN 2007bg is hosted by an extremely sub-luminous galaxy of
magnitude MB = -12.4+/-0.6 mag with an estimated oxygen abundance of
12+log(O/H) = 8.18+/-0.17. The lightcurve of SN 2007bg displays one of the
fastest post-maximum decline rates of all broad-lined Type Ic supernovae known
to date and, when combined with its high expansion velocities, a high kinetic
energy to ejected mass ratio (E_K/Mej ~ 2.7). We show that SN 2007bi is
possibly the most luminous Type Ic known, reaching a peak magnitude of MR ~
21.3 mag and displays a remarkably slow decline, following the radioactive
decay rate of 56Co to 56Fe throughout the course of its observed lifetime. From
a simple model of the bolometric light curve of SN 2007bi we estimate a total
ejected 56Ni mass of M_Ni = 3.5 - 4.5 solar masses, the largest 56Ni mass
measured in the ejecta of a supernova to date. There are two models that could
explain the high luminosity and large ejected 56Ni mass. One is a
pair-instability supernova (PISN) which has been predicted to occur for massive
stars at low metallicities. We measure the host galaxy metallicity of SN 2007bi
to be 12 + log(O/H) = 8.15+/-0.15 which is somewhat high to be consistent with
the PISN model. An alternative is the core-collapse of a C+O star of 20 - 40
solar masses which is the core of a star of originally 50 - 100 solar masses.
(Abridged)Comment: Minor changes. 19 pages, 21 Figures. Accepted by A&
SN 2008S: an electron capture SN from a super-AGB progenitor?
We present comprehensive photometric and spectroscopic observations of the
faint transient SN 2008S discovered in NGC 6946. SN 2008S exhibited slow
photometric evolution and almost no spectral variability during the first nine
months, implying a high density CS medium. The light curve is similar in shape
to that of SN 1998S and SN 1979C, although significantly fainter at maximum
light. Our quasi-bolometric lightcurve extends to 300 days and shows a tail
phase decay rate consistent with that of ^{56}Co. We propose that this is
evidence for an explosion and formation of ^{56}Ni (0.0015 +/- 0.0004 M_Sun).
The large MIR flux detected shortly after explosion can be explained by a light
echo from pre-exisiting dust. The late NIR flux excess is plausibly due to a
combination of warm newly-formed ejecta dust together with shock-heated dust in
the CS environment. We reassess the progenitor object detected previously in
Spitzer archive images, supplementing this discussion with a model of the MIR
spectral energy distribution. This supports the idea of a dusty, optically
thick shell around SN 2008S with an inner radius of nearly 90AU and outer
radius of 450AU, and an inferred heating source of 3000 K and luminosity of L ~
10^{4.6} L_Sun. The combination of our monitoring data and the evidence from
the progenitor analysis leads us to support the scenario of a weak electron
capture supernova explosion in a super-AGB progenitor star (of initial mass 6-8
M_sun) embedded within a thick CS gaseous envelope. We suggest that all of main
properties of the electron capture SN phenomenon are observed in SN 2008S and
future observations may allow a definitive answer.Comment: accepted for publication in MNRAS (2009 May 7
Deathscapes of Settler Colonialism : The necro-settlement of Stoney Creek, Ontario, Canada
This is the Accepted Manuscript of an article published by Taylor & Francis Group in annals of Association of American Geographers on 23 January 2018, available online https://doi.org/10.1080/24694452.2017.1406327. The Accepted Manuscript is under embargo until 23 January 2019.This article considers the influence of burials and memorials to colonial soldiers from an earlier era on contemporary social and cultural landscapes in Canada. Through the example of a landscape centered on Smith’s Knoll, a burial ground for war dead from the British-American War of 1812, it explores the process of necro-settlement: the strengthening of settler colonial claims to land based on the development of complex, meaningladen landscapes of dead and memory. This article consists of three parts: The first situates geographical studies of deathscapes alongside theories about settler colonialism through intersecting discourses of land use. The second includes a settler colonial microhistorical geography of Smith’s Knoll and the local deathscape that surrounds it. The third section draws on this case study to reveal new perspectives on the role of burial and memorial in settler colonial place-making and the erasure of Indigenous histories and peoples.Peer reviewedFinal Accepted Versio
Instrumental Variable Estimation of the Causal Effect of Plasma 25-Hydroxy-Vitamin D on Colorectal Cancer Risk: A Mendelian Randomization Analysis
Vitamin D deficiency has been associated with several common diseases, including cancer and is being investigated as a possible risk factor for these conditions. We reported the striking prevalence of vitamin D deficiency in Scotland. Previous epidemiological studies have reported an association between low dietary vitamin D and colorectal cancer (CRC). Using a case-control study design, we tested the association between plasma 25-hydroxy-vitamin D (25-OHD) and CRC (2,001 cases, 2,237 controls). To determine whether plasma 25-OHD levels are causally linked to CRC risk, we applied the control function instrumental variable (IV) method of the Mendelian randomization (MR) approach using four single nucleotide polymorphisms (rs2282679, rs12785878, rs10741657, rs6013897) previously shown to be associated with plasma 25-OHD. Low plasma 25-OHD levels were associated with CRC risk in the crude model (odds ratio (OR): 0.76, 95% Confidence Interval (CI): 0.71, 0.81, p: 1.4×10−14) and after adjusting for age, sex and other confounding factors. Using an allele score that combined all four SNPs as the IV, the estimated causal effect was OR 1.16 (95% CI 0.60, 2.23), whilst it was 0.94 (95% CI 0.46, 1.91) and 0.93 (0.53, 1.63) when using an upstream (rs12785878, rs10741657) and a downstream allele score (rs2282679, rs6013897), respectively. 25-OHD levels were inversely associated with CRC risk, in agreement with recent meta-analyses. The fact that this finding was not replicated when the MR approach was employed might be due to weak instruments, giving low power to demonstrate an effect (<0.35). The prevalence and degree of vitamin D deficiency amongst individuals living in northerly latitudes is of considerable importance because of its relationship to disease. To elucidate the effect of vitamin D on CRC cancer risk, additional large studies of vitamin D and CRC risk are required and/or the application of alternative methods that are less sensitive to weak instrument restrictions
Evidence of marine ice-cliff instability in Pine Island Bay from iceberg-keel plough marks.
Marine ice-cliff instability (MICI) processes could accelerate future retreat of the Antarctic Ice Sheet if ice shelves that buttress grounding lines more than 800 metres below sea level are lost. The present-day grounding zones of the Pine Island and Thwaites glaciers in West Antarctica need to retreat only short distances before they reach extensive retrograde slopes. When grounding zones of glaciers retreat onto such slopes, theoretical considerations and modelling results indicate that the retreat becomes unstable (marine ice-sheet instability) and thus accelerates. It is thought that MICI is triggered when this retreat produces ice cliffs above the water line with heights approaching about 90 metres. However, observational evidence confirming the action of MICI has not previously been reported. Here we present observational evidence that rapid deglacial ice-sheet retreat into Pine Island Bay proceeded in a similar manner to that simulated in a recent modelling study, driven by MICI. Iceberg-keel plough marks on the sea-floor provide geological evidence of past and present iceberg morphology, keel depth and drift direction. From the planform shape and cross-sectional morphologies of iceberg-keel plough marks, we find that iceberg calving during the most recent deglaciation was not characterized by small numbers of large, tabular icebergs as is observed today, which would produce wide, flat-based plough marks or toothcomb-like multi-keeled plough marks. Instead, it was characterized by large numbers of smaller icebergs with V-shaped keels. Geological evidence of the form and water-depth distribution of the plough marks indicates calving-margin thicknesses equivalent to the threshold that is predicted to trigger ice-cliff structural collapse as a result of MICI. We infer rapid and sustained ice-sheet retreat driven by MICI, commencing around 12,300 years ago and terminating before about 11,200 years ago, which produced large numbers of icebergs smaller than the typical tabular icebergs produced today. Our findings demonstrate the effective operation of MICI in the past, and highlight its potential contribution to accelerated future retreat of the Antarctic Ice Sheet
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