15,316 research outputs found
Ecophysiology of phototrophic sulfur bacteria in lakes: Vertical distribution of planktonic populations
The study of purple and green sulfur bacterial populations in nature is of interest for the following reasons: (1) high quantities of biomass, with low species diversity can be collected; (2) study of planktonic life permits one to understand the mechanisms, structural as well as physiological, used to maintain their vertical position without sinking; and (3) they are capable of sulfur oxidations and reductions that act as important intermediates in the global sulfur cycle. Purple and green photosynthetic bacteria, moreover, may be responsible for certain geological deposits. Planktonic phototrophic sulfur bacteria were analyzed in relation to their vertical distribution in the water column. Factors, including competition for light, that determine their sedimentation rates and the numerical changes in species and populations were assessed
Superwind evolution: the young starburst-driven wind galaxy NGC 2782
We present results from a 30 ksec Chandra observation of the important
starburst galaxy NGC 2782, covering the 0.3-10keV energy band. We find evidence
of a superwind of small extent, that is likely in an early stage of
development. We find a total of 27 X-ray point sources within a region of
radius 2 of the galaxy centre and which are likely associated with the
galaxy. Of these, 13 are ULXs () and a number
have likely counterparts. The X-ray luminosities of the ULX candidates are
. NGC2782 seems to have an unusually large
number of ULXs. Central diffuse X-ray emission extending to ~ 3kpc from the
nuclear region has been detected. We also find an X-ray structure to the south
of the nucleus, coincident with H{\alpha} filaments and with a 5 GHz radio
source. We interpret this as a blow-out region of a forming superwind. This
X-ray bubble has a total luminosity (0.3-10 keV) of
(around of the total luminosity of the extended emission), and an
inferred wind mass of M. We also discuss the nature
of the central X-ray source in NGC2782, and conclude that it is likely a
low-luminosity AGN (LLAGN), with a total X-ray luminosity of
with strong Fe line emission at 6.4 keV.Comment: 14 pages, 14 figures, 3 tables. Accepted for publication in MNRA
Newtonian gravity and resonance on de-Sitter branes
A dS brane on the boundary between two five-dimensional spacetimes is
determined. We consider asymmetric scenarios with AdS vacua at each side
of the dS brane; and as a result, a resonant mode inside of the spectrum of the
gravitational fluctuations is found. We analyze the deviations to the Newton
potential generated by the gravitational excitations, finding that, for
scenarios with large values of the cosmological constants, the contribution of
the resonant mode is exponentially suppressed. However, when one of the vacua
is null, the resonant mode belongs to the light states set of the gravitational
fluctuations and five-dimensional gravity is recovered on the dS brane.Comment: 6 pages, 2 figure
Sulfur cycling and metabolism of phototrophic and filamentous sulfur bacteria
Phototrophic sulfur bacteria taken from different habitate (Alum Rock State Park, Palo Alto salt marsh, and Big Soda Lake) were grown on selective media, characterized by morphological and pigment analysis, and compared with bacteria maintained in pure culture. A study was made of the anaerobic reduction of intracellular sulfur globules by a phototrophic sulfur bacterium (Chromatium vinosum) and a filamentous aerobic sulfur bacterium (Beggiatoa alba). Buoyant densities of different bacteria were measured in Percoll gradients. This method was also used to separate different chlorobia in mixed cultures and to assess the relative homogeneity of cultures taken directly or enriched from natural samples (including the purple bacterial layer found at a depth of 20 meters at Big Soda Lake.) Interactions between sulfide oxidizing bacteria were studied
Close to the edge: hierarchy in a double braneworld
We show that the hierarchy between the Planck and the weak scales can follow
from the tendency of gravitons and fermions to localize at different edges of a
thick double wall embedded in an spacetime without reflection symmetry.
This double wall is a stable BPS thick-wall solution with two sub-walls located
at its edges; fermions are coupled to the scalar field through Yukawa
interactions, but the the lack of reflection symmetry forces them to be
localized in one of the sub-walls. We show that the graviton zero-mode
wavefunction is suppressed in the fermion edge by an exponential function of
the distance between the sub-walls, and that the massive modes decouple so that
Newtonian gravity is recuperated.Comment: Some typos corrected, references adde
Gauge field localization on brane worlds
We consider the effects of spacetime curvature and brane thickness on the
localization of gauge fields on a brane via kinetic terms induced by localized
fermions. We find that in a warped geometry with and infinitely thin brane,
both the infrared and the ultraviolet behavior of the electromagnetic
propagator are affected, providing a more stringent bound on the brane's
tension than that coming from the requirement of four-dimensional gravity on
the brane. On the other hand, for a thick wall in a flat spacetime, where the
fermions are localized by means of a Yukawa coupling, we find that
4-dimensional electromagnetism is recovered in a region bounded from above by
the same critical distance appearing in the thin case, but also from below by a
new scale related to the brane's thickness and the electromagnetic couplings.Comment: 7 page
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