1,060 research outputs found
Penetrating the air-liquid interface is the key to colonization and wrinkly spreader fitness
In radiating populations of Pseudomonas fluorescens SBW25, adaptive wrinkly spreader (WS) mutants are able to gain access to the air–liquid (A–L) interface of static liquid microcosms and achieve a significant competitive fitness advantage over other non-biofilm-forming competitors. Aerotaxis and flagella-based swimming allows SBW25 cells to move into the high-O2 region located at the top of the liquid column and maintain their position by countering the effects of random cell diffusion, convection and disturbance (i.e. physical displacement). However, wild-type cells showed significantly lower levels of enrichment in this region compared to the archetypal WS, indicating that WS cells employ an additional mechanism to transfer to the A–L interface where displacement is no longer an issue and a biofilm can develop at the top of the liquid column. Preliminary experiments suggest that this might be achieved through the expression of an as yet unidentified surface active agent that is weakly associated with WS cells and alters liquid surface tension, as determined by quantitative tensiometry. The effect of physical displacement on the colonization of the high-O2 region and A–L interface was reduced through the addition of agar or polyethylene glycol to increase liquid viscosity, and under these conditions the competitive fitness of the WS was significantly reduced. These observations suggest that the ability to transfer to the A–L interface from the high-O2 region and remain there without further expenditure of energy (through, for example, the deployment of flagella) is a key evolutionary innovation of the WS, as it allows subsequent biofilm development and significant population increase, thereby affording these adaptive mutants a competitive fitness advantage over non-biofilm-forming competitors located within the liquid column
Discovery of a Small Central Disk of CO and HI in the Merger Remnant NGC 34
We present CO(1-0) and HI(21-cm) observations of the central region of the
wet merger remnant NGC 34. The Combined Array for Research in Millimeter-wave
Astronomy (CARMA) observations detect a regularly rotating disk in CO with a
diameter of 2.1 kpc and a total molecular hydrogen mass of (. The rotation curve of this gas disk rises steeply,
reaching maximum velocities at 1" (410 pc) from the center. Interestingly, HI
observations done with the Karl G. Jansky Very Large Array show that the
absorption against the central continuum has the exact same velocity range as
the CO in emission. This strongly suggests that the absorbing HI also lies
within 1" from the center, is mixed in and corotates with the molecular gas. A
comparison of HI absorption profiles taken at different resolutions (5"-45")
shows that the spectra at lower resolutions are less deep at the systemic
velocity. This provides evidence for HI emission in the larger beams, covering
the region from 1 kpc to 9 kpc from the center. The central rapidly rotating
disk was likely formed either during the merger or from fall-back material.
Lastly, the radio continuum flux of the central source at mm wavelengths
( mJy) is significantly higher than expected from an extrapolation
of the synchrotron spectrum, indicating the contribution of thermal free-free
emission from the central starburst.Comment: Accepted for publication in A
Herschel Survey of the Palomar-Green QSOs at Low Redshift
We investigate the global cold dust properties of 85 nearby (z < 0.5) QSOs,
chosen from the Palomar-Green sample of optically luminous quasars. We
determine their infrared spectral energy distributions and estimate their
rest-frame luminosities by combining Herschel data from 70 to 500 microns with
near-infrared and mid-infrared measurements from the Two Micron All Sky Survey
(2MASS) and the Wide-Field Infrared Survey Explorer (WISE). In most sources the
far-infrared (FIR) emission can be attributed to thermally heated dust. Single
temperature modified black body fits to the FIR photometry give an average dust
temperature for the sample of 33~K, with a standard deviation of 8~K, and an
average dust mass of 7E6 Solar Masses with a standard deviation of 9E6 Solar
Masses. Estimates of star-formation that are based on the FIR continuum
emission correlate with those based on the 11.3 microns PAH feature, however,
the star-formation rates estimated from the FIR continuum are higher than those
estimated from the 11.3 microns PAH emission. We attribute this result to a
variety of factors including the possible destruction of the PAHs and that, in
some sources, a fraction of the FIR originates from dust heated by the active
galactic nucleus and by old stars.Comment: accepted for publication in ApJ
A Direct Upper Limit on the Density of Cosmological Dust from the Absence of an X-ray Scattering Halo around the z=4.3 QSO 1508+5714
We report on the results of a search for an intergalactic X-ray dust
scattering halo in a deep observation of the bright, high-redshift quasar QSO
1508+5714 with the Chandra X-ray Observatory. We do not detect such a halo. Our
result implies an upper limit on the density of diffuse, large-grained
intergalactic dust of Omega_ dust < 2 x 10^-6, assuming a characteristic grain
size of 1micron. The result demonstrates the sensitivity of this technique for
detecting very small amounts of intergalactic dust which are very hard to
detect otherwise. This will allow us to put important constraints on systematic
effects induced by extinction on the interpretation of the SN Ia Hubble
Diagram, as well as on the amount and properties of cosmological dust being
expelled into the intergalactic medium at early z~2 times.Comment: 14 pages, 2 figures. to appear in ApJ, vol. 651, Nov. 200
BugVis:Commit slicing for fault visualisation
In this paper we present BugVis, our tool which allows the visualisation of the lifetime of a code fault. The commit history of thefault from insertion to fix is visualised. Unlike previous similartools, BugVis visualises only the lines of each commit involved inthe fault. The visualisation creates a commit slice throughout thehistory of the fault which enables comprehension of the evolutionof the code involved in the fault
Process orientation of the world heritage city management system
Purpose
This article aims to explain how the world heritage city management system may be improved, and the structure of its processes enhanced by including relevant stakeholders to reach better results, i.e. outputs and outcomes. With this aim, process-based orientation is proposed as a new management approach.
Design/methodology
The article is divided into two parts. In the first part, theoretical background of the world heritage city management system is given, and process orientation of heritage city management explained. In the second part theoretical concepts were applied to the cases of the world heritage cities (sites) of Split (Croatia) and Venice (Italy). A brief description of the basic characteristics (elements, processes) and of the main problems related to the results (outputs and outcomes) is followed by the application of the Unified Modelling Language as the method. The process architecture of the new management meta-model is presented, processes designed and documented, and activities and responsibilities among stakeholders, both existing as well as potential, within the proposed hierarchical structure shared.
Findings
By proposing the process-oriented approach to the management of world heritage cities, the paper sets out to contribute to the growing body of knowledge related to cultural heritage management (especially world heritage cities). By applying knowledge from different theories into a process-based approach to the management of world heritage cities, it enables UNESCO's directives to be implemented into the actual managerial system of a World Heritage Site (WHS). We find it potentially useful not only to WHS managers, but also to scholars and other experts who have managerial responsibilities but limited knowledge in this area
The Spitzer mid-infrared AGN survey. II-the demographics and cosmic evolution of the AGN population
We present luminosity functions derived from a spectroscopic survey of AGN
selected from Spitzer Space Telescope imaging surveys. Selection in the
mid-infrared is significantly less affected by dust obscuration. We can thus
compare the luminosity functions of the obscured and unobscured AGN in a more
reliable fashion than by using optical or X-ray data alone. We find that the
AGN luminosity function can be well described by a broken power-law model in
which the break luminosity decreases with redshift. At high redshifts
(), we find significantly more AGN at a given bolometric luminosity than
found by either optical quasar surveys or hard X-ray surveys. The fraction of
obscured AGN decreases rapidly with increasing AGN luminosity, but, at least at
high redshifts, appears to remain at \% even at bolometric
luminosities . The data support a picture in which the
obscured and unobscured populations evolve differently, with some evidence that
high luminosity obscured quasars peak in space density at a higher redshift
than their unobscured counterparts. The amount of accretion energy in the
Universe estimated from this work suggests that AGN contribute about 12\% to
the total radiation intensity of the Universe, and a high radiative accretion
efficiency is required to match current
estimates of the local mass density in black holes.Comment: 14 pages, accepted by Ap
Large amounts of optically-obscured star formation in the host galaxies of some type-2 quasars
We present Hubble Space Telescope images, and spectral energy distributions
from optical to infrared wavelengths for a sample of six 0.3<z<0.8 type-2
quasars selected in the mid-infrared using data from the Spitzer Space
Telescope. All the host galaxies show some signs of disturbance. Most seem to
possess dusty, star-forming disks. The disk inclination, estimated from the
axial ratio of the hosts, correlates with the depth of the silicate feature in
the mid-infrared spectra, implying that at least some of the reddening towards
the AGN arises in the host galaxy. The star formation rates in these objects,
as inferred from the strengths of the PAH features and far-infrared continuum,
range from 3-90 Msun/yr, but are mostly much larger than those inferred from
the [OII]3727 emission line luminosity, due to obscuration. Taken together with
studies of type-2 quasar hosts from samples selected in the optical and X-ray,
this is consistent with previous suggestions that two types of extinction
processes operate within the type-2 quasar population, namely a component due
to the dusty torus in the immediate environment of the AGN, and a more extended
component due to a dusty, star forming disk.Comment: 5 pages, 3 figures. Accepted by ApJ Letter
The location of an active nucleus and a shadow of a tidal tail in the ULIRG Mrk 273
Analysis of data from the Chandra X-ray Observatory for the double nucleus ULIRG Mrk 273 reveals an absorbed hard X-ray source coincident with the southwest nucleus, implying that this unresolved, near-infrared source is where an active nucleus resides, while the northern nuclear region contains a powerful starburst that dominates the far infrared luminosity. There is evidence of a slight image extension in the 6–7 keV band, where an Fe K line is present, towards the northern nucleus. A large-scale, diffuse emission nebula detected in soft X-rays contains a dark lane that spatially coincides with a high surface-brightness tidal tail extending ~50 arcsec (40 kpc) to the south. The soft X-ray source is likely located behind the tidal tail, which absorbs X-ray photons along the line of sight. The estimated column density of cold gas in the tidal tail responsible for shadowing the soft X-rays is N_H ≥ 6 × 10^(21) cm^(-2), consistent with the tidal tail having an edge-on orientation
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