6,171 research outputs found
The effects of ground hydrology on climate sensitivity to solar constant variations
The effects of two different evaporation parameterizations on the climate sensitivity to solar constant variations are investigated by using a zonally averaged climate model. The model is based on a two-level quasi-geostrophic zonally averaged annual mean model. One of the evaporation parameterizations tested is a nonlinear formulation with the Bowen ratio determined by the predicted vertical temperature and humidity gradients near the earth's surface. The other is the linear formulation with the Bowen ratio essentially determined by the prescribed linear coefficient
Further Observational Evidence for a Critical Ionising Luminosity in Active Galaxies
We report the results of a survey for HI 21-cm absorption at redshifts of z >
2.6 in a new sample of radio sources with the Green Bank and Giant Metrewave
Radio Telescopes. From a total of 25 targets, we report zero detections in the
16 for which optical depth limits could be obtained. Based upon the detection
rate for z > 0.1 associated absorption, we would expect approximately four
detections. Of the 11 which have previously not been searched, there is
sufficient source-frame optical/ultra-violet photometry to determine the
ionising photon rate for four. Adding these to the literature, the hypothesis
that there is a critical rate of logQ = 56 ionising photons per second is now
significant at ~7 sigma. This reaffirms our assertion that searching z > 3
active galaxies for which optical redshifts are available selects sources in
which the ultra-violet luminosity is sufficient to ionise all of the neutral
gas in the host galaxy.Comment: Accepted by MNRA
On finite -groups whose automorphisms are all central
An automorphism of a group is said to be central if
commutes with every inner automorphism of . We construct a family of
non-special finite -groups having abelian automorphism groups. These groups
provide counter examples to a conjecture of A. Mahalanobis [Israel J. Math.,
{\bf 165} (2008), 161 - 187]. We also construct a family of finite -groups
having non-abelian automorphism groups and all automorphisms central. This
solves a problem of I. Malinowska [Advances in group theory, Aracne Editrice,
Rome 2002, 111-127].Comment: 11 pages, Counter examples to a conjecture from [Israel J. Math.,
{\bf 165} (2008), 161 - 187]; This paper will appear in Israel J. Math. in
201
HI emission and absorption in nearby, gas-rich galaxies II. -- sample completion and detection of intervening absorption in NGC 5156
We present the results of a survey for intervening 21cm HI absorption in a
sample of 10 nearby, gas-rich galaxies selected from the HI Parkes All-Sky
Survey (HIPASS). This follows the six HIPASS galaxies searched in previous work
and completes our full sample. In this paper we searched for absorption along
17 sightlines with impact parameters between 6 and 46 kpc, making one new
detection. We also obtained simultaneous HI emission-line data, allowing us to
directly relate the absorption-line detection rate to the HI distribution. From
this we find the majority of the non-detections in the current sample are
because sightline does not intersect the HI disc of the galaxy at sufficiently
high column density, but that source structure is also an important factor.
The detected absorption-line arises in the galaxy NGC 5156 () at an
impact parameter of 19 kpc. The line is deep and narrow with an integrated
optical depth of 0.82 km s. High resolution Australia Telescope Compact
Array (ATCA) images at 5 and 8 GHz reveal that the background source is
resolved into two components with a separation of 2.6 arcsec (500 pc at the
redshift of the galaxy), with the absorption likely occurring against a single
component. We estimate that the ratio of the spin temperature and covering
factor, , is approximately 950 K in the outer disc of NGC
5156, but further observations using VLBI would allow us to accurately measure
the covering factor and spin temperature of the gas.Comment: 31 pages, 11 figure
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