1,111 research outputs found
Daily changes in global cloud cover and Earth transits of the heliospheric current sheet
Changes in cloud cover are found to occur for periods of a few days following Earth transits of the heliospheric current sheet (HCS), provided also that the transits occur in years of high stratospheric aerosol loading. Using global cloud products from the International Satellite Cloud Climatology Project (ISCCP) D1 data series, epoch superposition analyses were made for various samples of HCS events. For the period August 1991 to June 1994 for the stratospheric aerosol loading due to the Pinatubo eruption, the analysis of the data in 30° geomagnetic latitude intervals revealed that cloud anomalies that were significant and negative were located in the Southern Hemisphere high and middle latitudes, and anomalies that were significant and positive were found in both hemispheres at low latitudes. When the key days in the superposed epoch analysis were determined by minima in the relativistic electron flux, rather than by the HCS crossings, the location of the significant negative anomalies was in the northern high latitudes, and the location of the significant positive anomalies was in middle latitudes in the Northern Hemisphere. The spatial and temporal patterns of these cloud cover variations are in broad agreement with the expected opposite variations at high and low latitudes of the current density Jz in the global electric circuit caused by the relativistic electron flux variations, during periods when the aerosol loading has made a large increase in stratospheric resistivity
The compositional and metabolic responses of gilthead seabream (Sparus aurata) to a gradient of dietary fish oil and associated n-3 long-chain PUFA content
The authors express their gratitude to the technical team at the BioMar Feed Trial Unit, Hirtshals, in particular, Svend JĂžrgen Steenfeldt for expert care of the experimental subjects, for training and supervision provided by laboratory staff at Nutrition Analytical Services and Molecular Biology at the Institute of Aquaculture, University of Stirling, UK. S. J. S. Hâs. PhD was co-funded by BioMar and the Marine Alliance for Science and Technology Scotland. BioMar provided the experimental feeds, trial facilities and fish, and covered travel expenses. V. K. and J. T. designed and executed the nutritional trial and all authors contributed to planning the analyses. V. K., J. T. and S. J. S. H. carried out the sampling. O. M., D. R. T and S. A. M. M. supervised the lead author. M. B. B. provided training in molecular biology to S. J. S. H. who carried out all analytical procedures. S. J. S. H. analysed all of the data and prepared the manuscript. Subsequently the manuscript was shared between all authors who made amendments, contributions and recommendations. The authors declare that there are no conflicts of interestPeer reviewedPublisher PD
Does the Number Density of Elliptical Galaxies Change at z<1?
We have performed a detailed V/Vmax test for a sample of the Canada-France
Redshift Survey (CFRS) for the purpose of examining whether the comoving number
density of field galaxies changes significantly at redshifts of z<1. Taking
into account the luminosity evolution of galaxies which depends on their
morphological type through different history of star formation, we obtain
\sim 0.5 in the range of 0.3<z<0.8, where reliable redshifts were
secured by spectroscopy of either absorption or emission lines for the CFRS
sample. This indicates that a picture of mild evolution of field galaxies
without significant mergers is consistent with the CFRS data. Early-type
galaxies, selected by their (V-I)_{AB} color, become unnaturally deficient in
number at z>0.8 due to the selection bias, thereby causing a fictitious
decrease of . We therefore conclude that a reasonable choice of upper
bound of redshift z \sim 0.8 in the V/Vmax test saves the picture of passive
evolution for field ellipticals in the CFRS sample, which was rejected by
Kauffman, Charlot, & White (1996) without confining the redshift range.
However, about 10% of the CFRS sample consists of galaxies having colors much
bluer than predicted for irregular galaxies, and their \avmax is significantly
larger than 0.5. We discuss this population of extremely blue galaxies in terms
of starburst that has just turned on at their observed redshifts.Comment: 11 pages including 3 figures, to appear in ApJ Letter
A backwards approach to the formation of disk galaxies I. Stellar and gas content
A simple chemical enrichment code is described where the two basic mechanisms
driving the evolution of the ages and metallicities of the stellar populations
are the star formation efficiency and the fraction of gas ejected from the
galaxy. Using the observed Tully-Fisher relation in different passbands as a
constraint, it is found that a steep correlation between the maximum disk
rotational velocity and star formation efficiency must exist either for a
linear or a quadratic Schmidt law. Outflows do not play a major role. The
redshift evolution of disk galaxies is explored, showing that a significant
change in the slope of the Tully-Fisher relation is expected because of the
different age distributions of the stellar components in high and low-mass disk
galaxies. The slope measured in the rest frame B,K bands is found to change
from 3(B); 4(K) at z=0 up to 4.5(B); 5(K) at z~1, with a slight dependence on
formation redshift.Comment: Accepted for publication in ApJ. Uses emulateapj.sty. 12 pages with
10 embedded EPS figure
Testing the Relation Between the Local and Cosmic Star Formation Histories
Recently, there has been great progress toward observationally determining
the mean star formation history of the universe. When accurately known, the
cosmic star formation rate could provide much information about Galactic
evolution, if the Milky Way's star formation rate is representative of the
average cosmic star formation history. A simple hypothesis is that our local
star formation rate is proportional to the cosmic mean. In addition, to specify
a star formation history, one must also adopt an initial mass function (IMF);
typically it is assumed that the IMF is a smooth function which is constant in
time. We show how to test directly the compatibility of all these assumptions,
by making use of the local (solar neighborhood) star formation record encoded
in the present-day stellar mass function. Present data suggests that at least
one of the following is false: (1) the local IMF is constant in time; (2) the
local IMF is a smooth (unimodal) function; and/or (3) star formation in the
Galactic disk was representative of the cosmic mean. We briefly discuss how to
determine which of these assumptions fail, and improvements in observations
which will sharpen this test.Comment: 14 pages in LaTeX (uses aaspp4.sty). 5 postscript figures. To appear
in the Astrophysical Journa
Halos of Spiral Galaxies. III. Metallicity Distributions
(Abriged) We report results of a campaign to image the stellar populations in
the halos of highly inclined spiral galaxies, with the fields roughly 10 kpc
(projected) from the nuclei. We use the F814W (I) and F606W (V) filters in the
Wide Field Planetary Camera 2, on board the Hubble Space telescope. Extended
halo populations are detected in all galaxies. The color-magnitude diagrams
appear to be completely dominated by giant-branch stars, with no evidence for
the presence of young stellar populations in any of the fields. We find that
the metallicity distribution functions are dominated by metal-rich populations,
with a tail extending toward the metal poor end. To first order, the overall
shapes of the metallicity distribution functions are similar to what is
predicted by simple, single-component model of chemical evolution with the
effective yields increasing with galaxy luminosity. However, metallicity
distributions significantly narrower than the simple model are observed for a
few of the most luminous galaxies in the sample. It appears clear that more
luminous spiral galaxies also have more metal-rich stellar halos. The
increasingly significant departures from the closed-box model for the more
luminous galaxies indicate that a parameter in addition to a single yield is
required to describe chemical evolution. This parameter, which could be related
to gas infall or outflow either in situ or in progenitor dwarf galaxies that
later merge to form the stellar halo, tends to act to make the metallicity
distributions narrower at high metallicity.Comment: 20 pages, 8 figures (ApJ, in press
Consequences of a Change in the Galactic Environment of the Sun
The interaction of the heliosphere with interstellar clouds has attracted
interest since the late 1920's, both with a view to explaining apparent
quasi-periodic climate "catastrophes" as well as periodic mass extinctions.
Until recently, however, models describing the solar wind - local interstellar
medium (LISM) interaction self-consistently had not been developed. Here, we
describe the results of a two-dimensional (2D) simulation of the interaction
between the heliosphere and an interstellar cloud with the same properties as
currently, except that the neutral H density is increased from the present
value of n(H) ~ 0.2 cm^-3 to 10 cm^-3. The mutual interaction of interstellar
neutral hydrogen and plasma is included. The heliospheric cavity is reduced
considerably in size (approximately 10 - 14 AU to the termination shock in the
upstream direction) and is highly dynamical. The interplanetary environment at
the orbit of the Earth changes markedly, with the density of interstellar H
increasing to ~2 cm^-3. The termination shock itself experiences periods where
it disappears, reforms and disappears again. Considerable mixing of the shocked
solar wind and LISM occurs due to Rayleigh-Taylor-like instabilities at the
nose, driven by ion-neutral friction. Implications for two anomalously high
concentrations of 10Be found in Antarctic ice cores 33 kya and 60 kya, and the
absence of prior similar events, are discussed in terms of density enhancements
in the surrounding interstellar cloud. The calculation presented here supports
past speculation that the galactic environment of the Sun moderates the
interplanetary environment at the orbit of the Earth, and possibly also the
terrestrial climate.Comment: 23 pages, 2 color plates (jpg), 3 figures (eps
Constraints from Al Measurements on the Galaxy's Recent Global Star Formation Rate and Core Collapse Supernovae Rate
Gamma-rays from the decay of Al offer a stringent constraint on the
Galaxy's global star formation rate over the past million years, supplementing
other methods for quantifying the recent Galactic star formation rate, such as
equivalent widths of H emission. Advantages and disadvantages of using
Al gamma-ray measurements as a tracer of the massive star formation rate
are analyzed. Estimates of the Galactic Al mass derived from COMPTEL
measurements are coupled with a simple, analytical model of the Al
injection rate from massive stars and restrict the Galaxy's recent star
formation rate to \hbox{5 4 M\sun yr}. In addition, we show that
the derived Al mass implies a present day \hbox{Type II + Ib} supernovae
rate of 3.4 2.8 per century, which seems consistent with other
independent estimates of the Galactic core collapse supernova rate. If some
independent measure of the massive star initial mass function or star formation
rate or \hbox{Type II + Ib} supernovae rate were to become available (perhaps
through estimates of the Galactic Fe mass), then a convenient way to
restrain, or possibly determine, the other parameters is presented.Comment: 11 pages including 1 figure, ApJ in pres
Metallicities of 0.3<z<1.0 Galaxies in the GOODS-North Field
We measure nebular oxygen abundances for 204 emission-line galaxies with
redshifts 0.3<z<1.0 in the Great Observatories Origins Deep Survey North
(GOODS-N) field using spectra from the Team Keck Redshift Survey (TKRS). We
also provide an updated analytic prescription for estimating oxygen abundances
using the traditional strong emission line ratio, R_{23}, based on the
photoionization models of Kewley & Dopita (2003). We include an analytic
formula for very crude metallicity estimates using the [NII]6584/Halpha ratio.
Oxygen abundances for GOODS-N galaxies range from 8.2< 12+log(O/H)< 9.1
corresponding to metallicities between 0.3 and 2.5 times the solar value. This
sample of galaxies exhibits a correlation between rest-frame blue luminosity
and gas-phase metallicity (i.e., an L-Z relation), consistent with L-Z
correlations of previously-studied intermediate-redshift samples. The zero
point of the L-Z relation evolves with redshift in the sense that galaxies of a
given luminosity become more metal poor at higher redshift. Galaxies in
luminosity bins -18.5<M_B<-21.5 exhibit a decrease in average oxygen abundance
by 0.14\pm0.05 dex from z=0 to z=1. This rate of metal enrichment means that
28\pm0.07% of metals in local galaxies have been synthesized since z=1, in
reasonable agreement with the predictions based on published star formation
rate densities which show that ~38% of stars in the universe have formed during
the same interval. (Abridged)Comment: AASTeX, 49 pages, 16 figures, accepted for publication in The
Astrophysical Journa
Low Mass Stars and the He3 Problem
The prediction of standard chemical evolution models of higher abundances of
He3 at the solar and present-day epochs than are observed indicates a possible
problem with the yield of He3 for stars in the range of 1-3 solar masses.
Because He3 is one of the nuclei produced in Big Bang Nucleosynthesis (BBN), it
is noted that galactic and stellar evolution uncertainties necessarily relax
constraints based on He3. We incorporate into chemical evolution models which
include outflow, the new yields for He3 of Boothroyd & Malaney (1995) which
predict that low mass stars are net destroyers of He3. Since these yields do
not account for the high \he3/H ratio observed in some planetary nebulae, we
also consider the possibility that some fraction of stars in the 1 - 3 solar
mass range do not destroy their He3 in theirpost main-sequence phase. We also
consider the possibility that the gas expelled by stars in these mass ranges
does not mix with the ISM instantaneously thus delaying the He3 produced in
these stars, according to standard yields, from reaching the ISM. In general,
we find that the Galactic D and He3 abundances can be fit regardless of whether
the primordial D/H value is high (2 x 10^{-4}) or low (2.5 x 10^{-5}).Comment: 20 pages, latex, 9 ps figure
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