122 research outputs found
Fragmentation dynamics of the ethyl bromide and ethyl iodide cations: a velocity-map imaging study
The photodissociation dynamics of ethyl bromide and ethyl iodide cations (C2H5Br+ and C2H5I+) have been studied. Ethyl halide cations were formed through vacuum ultraviolet (VUV) photoionization of the respective neutral parent molecules at 118.2 nm, and were photolysed at a number of ultraviolet (UV) photolysis wavelengths, including 355 nm and wavelengths in the range from 236 to 266 nm. Time-of-flight mass spectra and velocity-map images have been acquired for all fragment ions and for ground (Br) and spin–orbit excited (Br*) bromine atom products, allowing multiple fragmentation pathways to be investigated. The experimental studies are complemented by spin–orbit resolved ab initio calculations of cuts through the potential energy surfaces (along the RC–Br/I stretch coordinate) for the ground and first few excited states of the respective cations. Analysis of the velocity-map images indicates that photoexcited C2H5Br+ cations undergo prompt C–Br bond fission to form predominantly C2H5+ + Br* products with a near-limiting ‘parallel’ recoil velocity distribution. The observed C2H3+ + H2 + Br product channel is thought to arise via unimolecular decay of highly internally excited C2H5+ products formed following radiationless transfer from the initial excited state populated by photon absorption. Broadly similar behaviour is observed in the case of C2H5I+, along with an additional energetically accessible C–I bond fission channel to form C2H5 + I+ products. HX (X = Br, I) elimination from the highly internally excited C2H5X+ cation is deemed the most probable route to forming the C2H4+ fragment ions observed from both cations. Finally, both ethyl halide cations also show evidence of a minor C–C bond fission process to form CH2X+ + CH3 products
The Strongly Polarized Afterglow of GRB 020405
We report polarization measurements and photometry for the optical afterglow
of the gamma-ray burst GRB 020405. We measured a highly significant 9.9%
polarization (in V band) 1.3 days after the burst and argue that it is
intrinsic to the GRB. The light curve decay is well fitted by a
power-law; we do not see any evidence for a break between 1.24 and 4.3 days
after the burst. We discuss these measurements in the light of several models
of GRB afterglows.Comment: submitted to ApJ
Structure of the Galaxies in the NGC 80 Group
BV-bands photometric data obtained at the 6-m telescope of the Special
Astrophysical Observatory are used to analyze the structure of 13 large disk
galaxies in the NGC 80 group. Nine of the 13 galaxies under consideration are
classified by us as lenticular galaxies. The stellar populations in the
galaxies are very different, from old ones with ages of T>10 Gyrs (IC 1541) to
relatively young, with the ages of T<2-3 Gyr (IC 1548, NGC 85). In one case,
current star formation is known (UCM 0018+2216). In most of the galaxies, more
precisely in all of them more luminous than M(B) -18, two-tiered
(`antitruncated') stellar disks are detected, whose radial surface brightness
profiles can be fitted by two exponential segments with different scalelengths
-- shorter near the center and longer at the periphery. All dwarf S0 galaxies
with single-scalelength exponential disks are close companions to giant
galaxies. Except for this fact, no dependence of the properties of S0 galaxies
on distance from the center of the group is found. Morphological traces of
minor merger are found in the lenticular galaxy NGC 85. Basing on the last two
points, we conclude that the most probable mechanisms for the transformation of
spirals into lenticular galaxies in groups are gravitational ones, namely,
minor mergers and tidal interactions.Comment: 24 pages, 9 figures, slightly improved version of the paper published
in the December, 2009, issue of the Astronomy Report
Repeatability and reproducibility of multiparametric magnetic resonance imaging of the liver
As the burden of liver disease reaches epidemic levels, there is a high unmet medical need to develop robust, accurate and reproducible non-invasive methods to quantify liver tissue characteristics for use in clinical development and ultimately in clinical practice. This prospective cross-sectional study systematically examines the repeatability and reproducibility of iron-corrected T1 (cT1), T2*, and hepatic proton density fat fraction (PDFF) quantification with multiparametric MRI across different field strengths, scanner manufacturers and models. 61 adult participants with mixed liver disease aetiology and those without any history of liver disease underwent multiparametric MRI on combinations of 5 scanner models from two manufacturers (Siemens and Philips) at different field strengths (1.5T and 3T). We report high repeatability and reproducibility across different field strengths, manufacturers, and scanner models in standardized cT1 (repeatability CoV: 1.7%, bias -7.5ms, 95% LoA of -53.6 ms to 38.5 ms; reproducibility CoV 3.3%, bias 6.5 ms, 95% LoA of -76.3 to 89.2 ms) and T2* (repeatability CoV: 5.5%, bias -0.18 ms, 95% LoA -5.41 to 5.05 ms; reproducibility CoV 6.6%, bias -1.7 ms, 95% LoA -6.61 to 3.15 ms) in human measurements. PDFF repeatability (0.8%) and reproducibility (0.75%) coefficients showed high precision of this metric. Similar precision was observed in phantom measurements. Inspection of the ICC model indicated that most of the variance in cT1 could be accounted for by study participants (ICC = 0.91), with minimal contribution from technical differences. We demonstrate that multiparametric MRI is a non-invasive, repeatable and reproducible method for quantifying liver tissue characteristics across manufacturers (Philips and Siemens) and field strengths (1.5T and 3T)
The History of Galaxy Formation in Groups: An Observational Perspective
We present a pedagogical review on the formation and evolution of galaxies in
groups, utilizing observational information from the Local Group to galaxies at
z~6. The majority of galaxies in the nearby universe are found in groups, and
galaxies at all redshifts up to z~6 tend to cluster on the scale of nearby
groups (~1 Mpc). This suggests that the group environment may play a role in
the formation of most galaxies. The Local Group, and other nearby groups,
display a diversity in star formation and morphological properties that puts
limits on how, and when, galaxies in groups formed. Effects that depend on an
intragroup medium, such as ram-pressure and strangulation, are likely not major
mechanisms driving group galaxy evolution. Simple dynamical friction arguments
however show that galaxy mergers should be common, and a dominant process for
driving evolution. While mergers between L_* galaxies are observed to be rare
at z < 1, they are much more common at earlier times. This is due to the
increased density of the universe, and to the fact that high mass galaxies are
highly clustered on the scale of groups. We furthermore discus why the local
number density environment of galaxies strongly correlates with galaxy
properties, and why the group environment may be the preferred method for
establishing the relationship between properties of galaxies and their local
density.Comment: Invited review, 16 pages, to be published in ESO Astrophysics
Symposia: "Groups of Galaxies in the Nearby Universe", eds. I. Saviane, V.
Ivanov, J. Borissov
Close companions to Brightest Cluster Galaxies: Support for minor mergers and downsizing
We identify close companions of Brightest Cluster Galaxies (BCGs) for the
purpose of quantifying the rate at which these galaxies grow via mergers. By
exploiting deep photometric data from the CFHTLS, we probe the number of
companions per BCG (Nc) with luminosity ratios down to those corresponding to
potential minor mergers of 20:1. We also measure the average luminosity in
companions per galaxy (Lc). We find that Nc and Lc rise steeply with luminosity
ratio for both the BCGs, and a control sample of other bright, red, cluster
galaxies. The trend for BCGs rises more steeply, resulting in a larger number
of close companions. For companions within 50kpc of a BCG, Nc= 1.38+/-0.14 and
Lc=(2.14+/-0.31)x10^(10)L_sun and for companions within 50kpc of a luminosity
matched control sample of non-BCGs, Nc=0.87+/-0.08 and
Lc=(1.48+/-0.20)x10^(10)L_sun. This suggests that the BCGs are likely to
undergo more mergers compared to otherwise comparable luminous galaxies.
Additionally, compared to a local sample of luminous red galaxies, the more
distant sample presented in this study (with redshifts between 0.15-0.39,)
shows a higher Nc, suggesting the younger and smaller BCGs are still undergoing
hierarchical formation. Using the Millennium Simulations we model and estimate
the level of contamination due to unrelated cluster galaxies. The contamination
by interloping galaxies is 50% within projected separations of 50kpc, but
within 30kpc, 60% of identified companions are real physical companions. We
conclude that the luminosity of bound merger candidates down to luminosity
ratios of 20:1 could be adding as much as 10% to the mass of a typical BCG over
0.5Gyr at redshifts of z~0.3.Comment: 10 pages, 7 figures. Accepted and to be published in MNRA
Deep Low-Frequency Radio Observations of the NOAO Bootes Field: I. Data Reduction and Catalog Construction
In this article we present deep, high-resolution radio interferometric
observations at 153 MHz to complement the extensively studied NOAO Bootes
field. We provide a description of the observations, data reduction and source
catalog construction. From our single pointing GMRT observation of ~12 hours we
obtain a high-resolution (26" x 22") image of ~11.3 square degrees, fully
covering the Bootes field region and beyond. The image has a central noise
level of ~1.0 mJy/beam, which rises to 2.0-2.5 mJy/beam at the field edge,
placing it amongst the deepest ~150 MHz surveys to date. The catalog of 598
extracted sources is estimated to be ~92 percent complete for >10 mJy sources,
while the estimated contamination with false detections is <1 percent. The low
RMS position uncertainty of 1.24" facilitates accurate matching against
catalogs at optical, infrared and other wavelengths. Differential source counts
are determined down to <~10 mJy. There is no evidence for flattening of the
counts towards lower flux densities as observed in deep radio surveys at higher
frequencies, suggesting that our catalog is dominated by the classical
radio-loud AGN population that explains the counts at higher flux densities.
Combination with available deep 1.4 GHz observations yields an accurate
determination of spectral indices for 417 sources down to the lowest 153 MHz
flux densities, of which 16 have ultra-steep spectra with spectral indices
below -1.3. We confirm that flattening of the median spectral index towards low
flux densities also occurs at this frequency. The detection fraction of the
radio sources in NIR Ks-band is found to drop with radio spectral index, which
is in agreement with the known correlation between spectral index and redshift
for brighter radio sources.Comment: 14 pages, 7 figures. Accepted for publication by A&A. Source catalog
will be available from CDS soo
Multiwavelength characterization of faint ultra steep spectrum radio sources: a search for high-redshift radio galaxies
Context. Ultra steep spectrum (USS) radio sources are one of the efficient tracers of powerful high-z radio galaxies (HzRGs). In contrast to searches for powerful HzRGs from radio surveys of moderate depths, fainter USS samples derived from deeper radio surveys can be useful in finding HzRGs at even higher redshifts and in unveiling a population of obscured weaker radio-loud AGN at moderate redshifts.
Aims. Using our 325 MHz GMRT observations (5σ ∼ 800 μJy) and 1.4 GHz VLA observations (5σ ∼ 80−100 μJy) available in two subfields (VLA-VIMOS VLT Deep Survey (VLA-VVDS) and Subaru X-ray Deep Field (SXDF)) of the XMM-LSS field, we derive a large sample of 160 faint USS radio sources and characterize their nature.
Methods. The optical and IR counterparts of our USS sample sources are searched using existing deep surveys, at respective wavelengths. We attempt to unveil the nature of our faint USS sources using diagnostic techniques based on mid-IR colors, flux ratios of radio to mid-IR, and radio luminosities. Results. Redshift estimates are available for 86/116 (∼74%) USS sources in the VLA-VVDS field and for 39/44 (∼87%) USS sources in the SXDF fields with median values (zmedian) ∼1.18 and ∼1.57, respectively, which are higher than estimates for non-USS radio sources (zmedian non−USS ∼ 0.99 and ∼0.96), in the two subfields. The MIR color–color diagnostic and radio luminosities are consistent with most of our USS sample sources at higher redshifts (z > 0.5) being AGN. The flux ratio of radio to mid-IR (S 1.4 GHz/S 3.6 μm) versus redshift diagnostic plot suggests that more than half of our USS sample sources distributed over z ∼ 0.5 to 3.8 are likely to be hosted in obscured environments. A significant fraction (∼26% in the VLA-VVDS and ∼13% in the SXDF) of our USS sources without redshift estimates mostly remain unidentified in the existing optical, IR surveys, and exhibit high radio to mid-IR flux ratio limits similar to HzRGs, and so, can be considered as potential HzRG candidates.
Conclusions. Our study shows that the criterion of ultra steep spectral index remains a reasonably efficient method to select high-z sources even at sub-mJy flux densities. In addition to powerful HzRG candidates, our faint USS sample also contains populations of weaker radio-loud AGNs potentially hosted in obscured environments
Women's views and experiences of two alternative consent pathways for participation in a preterm intrapartum trial: a qualitative study
BACKGROUND: The Cord Pilot Trial compared alternative policies for timing of cord clamping at very preterm birth at eight UK hospitals. In addition to standard written consent, an oral assent pathway was developed for use when birth was imminent. The aim of this study was to explore women's views and experiences of two alternative consent pathways to participate in the Cord Pilot Trial.
METHODS: We conducted a qualitative study using semi-structured interviews. A total of 179 participants in the Cord Pilot Trial were sent a postal invitation to take part in interviews. Women who agreed were interviewed in person or by telephone to explore their experiences of two consent pathways for a preterm intrapartum trial. Data were analysed using inductive systematic thematic analysis.
RESULTS: Twenty-three women who gave either written consent (n = 18) or oral assent followed by written consent (n = 5) to participate in the trial were interviewed. Five themes were identified: (1) understanding of the implications of randomisation, (2) importance of staff offering participation, (3) information about the trial and time to consider participation, (4) trial secondary in women's minds and (5) reasons for agreeing to take part in the trial. Experiences were similar for the two consent pathways. Women recruited by the oral assent pathway reported being given less information about the trial but felt it was sufficient to make a decision regarding participation. There were gaps in women's understanding of the trial and intervention, regardless of the consent pathway.
CONCLUSIONS: Overall, women were positive about their experiences of being invited to participate in the trial. The oral assent pathway seems an acceptable option for women if the intervention is low-risk and time is limited.
TRIAL REGISTRATION: ISRCTN Registry, ISRCTN21456601 . Registered on 28 February 2013
Highly ionized gas on galaxy scales: mapping the interacting Seyfert galaxy LEDA 135736
We have used the VIMOS IFU to map the properties of the Seyfert 1.9 galaxy
LEDA 135736. These maps reveal a number of interesting features including: an
Extended Narrow Line Region detectable out to 9 kpc, an area of intense star
formation located at a projected distance of 12 kpc from the centre, an
elliptical companion galaxy, and kinematic features, aligned along the
long-axis of the ENLR, that are consistent with radio jet-driven mass outflow.
We propose that the ENLR results from extra-planar gas ionized by the AGN, and
that the AGN in turn might be triggered by interaction with the companion
galaxy, which can also explain the burst of star formation and morphological
features. Only about two percent of the ENLR's kinetic energy is in the mass
outflow. We infer from this that the bulk of mechanical energy imparted by the
jet is used to heat this gas.Comment: Accepted for publication in MNRAS Letter
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