1,672 research outputs found
Constraining the Nature of X-ray Cavities in Clusters and Galaxies
We present results from an extensive survey of 64 cavities in the X-ray halos
of clusters, groups and normal elliptical galaxies. We show that the evolution
of the size of the cavities as they rise in the X-ray atmosphere is
inconsistent with the standard model of adiabatic expansion of purely
hydrodynamic models. We also note that the majority of the observed bubbles
should have already been shredded apart by Rayleigh-Taylor and
Richtmyer-Meshkov instabilities if they were of purely hydrodynamic nature.
Instead we find that the data agrees much better with a model where the
cavities are magnetically dominated and inflated by a current-dominated
magneto-hydrodynamic jet model, recently developed by Li et al. (2006) and
Nakamura et al. (2006). We conduct complex Monte-Carlo simulations of the
cavity detection process including incompleteness effects to reproduce the
cavity sample's characteristics. We find that the current-dominated model
agrees within 1sigma, whereas the other models can be excluded at >5sigma
confidence. To bring hydrodynamic models into better agreement, cavities would
have to be continuously inflated. However, these assessments are dependent on
our correct understanding of the detectability of cavities in X-ray
atmospheres, and will await confirmation when automated cavity detection tools
become available in the future. Our results have considerable impact on the
energy budget associated with active galactic nucleus feedback.Comment: 21 pages, 12 figures, emulateapj, accepted for publication in ApJ,
responded to referee's comments and added a new model, conclusions unchange
The StarScan plate measuring machine: overview and calibrations
The StarScan machine at the U.S. Naval Observatory (USNO) completed measuring
photographic astrograph plates to allow determination of proper motions for the
USNO CCD Astrograph Catalog (UCAC) program. All applicable 1940 AGK2 plates,
about 2200 Hamburg Zone Astrograph plates, 900 Black Birch (USNO Twin
Astrograph) plates, and 300 Lick Astrograph plates have been measured. StarScan
comprises of a CCD camera, telecentric lens, air-bearing granite table, stepper
motor screws, and Heidenhain scales to operate in a step-stare mode. The
repeatability of StarScan measures is about 0.2 micrometer. The CCD mapping as
well as the global table coordinate system has been calibrated using a special
dot calibration plate and the overall accuracy of StarScan x,y data is derived
to be 0.5 micrometer. Application to real photographic plate data shows that
position information of at least 0.65 micrometer accuracy can be extracted from
course grain 103a-type emulsion astrometric plates. Transformations between
"direct" and "reverse" measures of fine grain emulsion plate measures are
obtained on the 0.3 micrometer level per well exposed stellar image and
coordinate, which is at the limit of the StarScan machine.Comment: 24 pages, 8 figures, accepted for PAS
First LOFAR results on galaxy clusters
Deep radio observations of galaxy clusters have revealed the existence of
diffuse radio sources related to the presence of relativistic electrons and
weak magnetic fields in the intracluster volume. The role played by this
non-thermal intracluster component on the thermodynamical evolution of galaxy
clusters is debated, with important implications for cosmological and
astrophysical studies of the largest gravitationally bound structures of the
Universe. The low surface brightness and steep spectra of diffuse cluster radio
sources make them more easily detectable at low-frequencies. LOFAR is the first
instrument able to detect diffuse radio emission in hundreds of massive galaxy
clusters up to their formation epoch. We present the first observations of
clusters imaged by LOFAR and the huge perspectives opened by this instrument
for non-thermal cluster studies.Comment: Proceedings of the 2012 week of the French Society of Astronomy and
Astrophysics (SF2A) held in Nice, June 5th-8t
Sniff nasal inspiratory pressure in patients with moderate-to-severe chronic obstructive pulmonary disease: learning effect and short-term between-session repeatability
Background: Sniff nasal inspiratory pressure (SNIP) is a non-invasive measure of inspiratory muscle function often used as an outcome measure in clinical studies. An initial period of familiarisation with the test is recommended to minimise the learning effect. The repeatability of SNIP in patients with chronic obstructive pulmonary disease (COPD) is currently unknown. Objectives: The aim of this study was to assess the between-session repeatability of SNIP over a 3-week period in moderate-to-severe COPD patients and compare it with that of maximal inspiratory (PImax) and expiratory pressure (PEmax). Methods: Twenty-one patients (13 males) with a mean forced expiratory volume in 1 s (FEV1) of 38% of predicted (SD: 15) and FEV1/forced vital capacity of 34.3% (SD: 10.4) performed SNIP and PImax and PEmax manoeuvres on 3 different sessions (S1, S2 and S3) 3-7 days apart. SNIP was performed at functional residual capacity (FRC), and PImax was performed at FRC and at residual volume (RV) to explore volume-dependent differences in the learning effect between sessions and PEmax from total lung capacity. Results: The intra-class correlation coefficient (ICC) for SNIP was the highest of the three measures: S1-S3 ICC (95% CI) SNIP: 0.96 (0.88-0.94); PImax at FRC 0.82 (0.63-0.92); PImax at RV: 0.89 (0.78-0.95), and PEmax: 0.96 (0.92-0.98), and had the lowest mean change between sessions [mean S2 - S1: 2.1(p = 0.4) and S3 - S2: -0.3 (p = 0.9)]. Conclusions: SNIP is repeatable over a period of 3 weeks in medically stable, moderate-to-severe COPD patients. In our study, 2 sessions were adequate to learn how to perform the test
Ionized nebulae surrounding brightest cluster galaxies
We present IFU observations of six emission-line nebulae that surround the
central galaxy of cool core clusters. Qualitatively similar nebulae are
observed in cool core clusters even when the dynamics and possibly formation
and excitation source are different. Evidence for a nearby secondary galaxy
disturbing a nebula, as well as AGN and starburst driven outflows are presented
as possible formation mechanisms. One nebula has a rotation velocity of the
same amplitude as the underlying molecular reservoir, which implies that the
excitation or formation of a nebula does not require any disturbance of the
molecular reservoir within the central galaxy. Bulk flows and velocity shears
of a few hundred km/s are seen across all nebulae. The majority lack any
ordered rotation, their configurations are not stable so the nebulae must be
constantly reshaping, dispersing and reforming. The dimmer nebulae are
co-spatial with dust features whilst the more luminous are not. Significant
variation in the ionization state of the gas is seen in all nebulae through the
non-uniform [NII]/H_alpha ratio. There is no correlation between the line ratio
and H_alpha surface brightness, but regions with excess blue or UV light have
lower line ratios. This implies that UV from massive, young stars act in
combination with an underlying heating source that produces the observed
low-ionization spectra.Comment: 12 pages, accepted for publication in MNRA
Inclusive growth in English cities: mainstreamed or sidelined?
<p>The concept of inclusive growth is increasingly presented as offering prospects for more equitable social outcomes. However, inclusive growth is subject to a variety of interpretations and lacks definitional clarity. In England, via devolution, cities are taking on new powers for policy domains that can influence inclusive growth outcomes. This opens up opportunities for innovation to address central issues of low pay and poverty. This paper examines the extent to which inclusive growth concerns form a central or peripheral aspect in this new devolution through the content analysis of devolution agreements. It concludes that inclusive growth concerns appear to be largely sidelined.</p
Holographic Roberge-Weiss Transitions
We investigate N=4 SYM coupled to fundamental flavours at nonzero imaginary
quark chemical potential in the strong coupling and large N limit, using
gauge/gravity duality applied to the D3-D7 system, treating flavours in the
probe approximation. The interplay between Z(N) symmetry and the imaginary
chemical potential yields a series of first-order Roberge-Weiss transitions. An
additional thermal transition separates phases where quarks are bound/unbound
into mesons. This results in a set of Roberge-Weiss endpoints: we establish
that these are triple points, determine the Roberge-Weiss temperature, give the
curvature of the phase boundaries and confirm that the theory is analytic in
mu^2 when mu^2~0.Comment: 37 pages, 13 figures; minor comments added, to appear in JHE
Leadership and decision-making practices in public versus private universities in Pakistan
The goal of this study is to examine differences in leadership and decision-making practices in public and private universities in Pakistan, with a focus on transformational leadership (TL) and participative decision-making (PDM). We conducted semi-structured interviews with 46 deans and heads of department from two public and two private universities in Pakistan. Our findings indicate that leadership and decision-making practices are different in public and private universities. While differences were observed in all six types of TL-behaviour, the following three approaches emerged to be crucial in both public and private universities: (1) articulating a vision, (2) fostering the acceptance of group goals, and (3) high-performance expectations. In terms of PDM, deans and heads of department in public and private universities adopt a collaborative approach. However, on a practical level this approach is limited to teacher- and student-related matters. Overall, our findings suggest that the leadership and decision-making practices in Pakistani public and private universities are transformational and participative in nature
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