751 research outputs found
The Luminosity Distribution of Local Group Galaxies
From a rediscussion of Local Group membership, and of distances to individual
galaxies, we obtain values for 35 probable and possible Local Group
members. The luminosity function of these objects is well fitted by a Schechter
function with faint end slope . The probability that the
luminosity distribution of the Local Group is a single Schechter function with
steeper than -1.3 is less than 1 per cent. However, more complicated
luminosity functions, such as multi-component Schechter functions with steep
faint-end slopes, cannot be ruled out. There is some evidence that the
luminosity distribution of dwarf spheroidal galaxies in the Local Group is
steeper than that of dwarf irregular galaxies.Comment: 13 pages, 2 figures, accepted for publication in The Astronomical
Journal. Figure 2 replaced, conclusion based on this figure change
Solution of the Two-Channel Anderson Impurity Model - Implications for the Heavy Fermion UBe -
We solve the two-channel Anderson impurity model using the Bethe-Ansatz. We
determine the ground state and derive the thermodynamics, obtaining the
impurity entropy and specific heat over the full range of temperature. We show
that the low temperature physics is given by a line of fixed points decribing a
two-channel non Fermi liquid behavior in the integral valence regime associated
with moment formation as well as in the mixed valence regime where no moment
forms. We discuss relevance for the theory of UBe.Comment: 4 pages, 2 figures, (to be published in PRL
FORS spectroscopy of galaxies in the Hubble Deep Field South
We present low resolution multi-object spectroscopy of an I-band magnitude
limited (I_{AB} ~ 23--23.5) sample of galaxies located in an area centered on
the Hubble Deep Field-South (HDFS). The observations were obtained using the
Focal Reducer low dispersion Spectrograph (FORS) on the ESO Very Large
Telescope. Thirty-two primary spectroscopic targets in the HST-WFPC2 HDFS were
supplemented with galaxies detected in the Infrared Space Observatory's survey
of the HDFS and the ESO Imaging Deep Survey to comprise a sample of 100
galaxies for spectroscopic observations. Based on detections of several
emission lines, such as [OII]3727, H_beta and [OIII]5007, or other
spectroscopic features, we have measured accurate redshifts for 50 objects in
the central HDFS and flanking fields. The redshift range of the current sample
of galaxies is 0.6--1.2, with a median redshift of 1.13 (at I ~ 23.5 not
corrected for completeness). The sample is dominated by starburst galaxies with
only a small fraction of ellipticals (~10%). For the emission line objects, the
extinction corrected [OII]3727 line strengths yield estimates of star formation
rates in the range 0.5--30 M_solar/yr. We have used the present data to derive
the [OII]3727 luminosity function up to redshift of 1.2. When combined with
[OII]3727 luminosity densities for the local and high redshift Universe, our
results confirm the steep rise in the star formation rate (SFR) to z ~ 1.3.Comment: Tables 2 and 3 provided as separate files. Accepted for publication
by Astronomy and Astrophysic
Probing the phase diagram of CeRu_2Ge_2 by thermopower at high pressure
The temperature dependence of the thermoelectric power, S(T), and the
electrical resistivity of the magnetically ordered CeRu_2Ge_2 (T_N=8.55 K and
T_C=7.40 K) were measured for pressures p < 16 GPa in the temperature range 1.2
K < T < 300 K. Long-range magnetic order is suppressed at a p_c of
approximately 6.4 GPa. Pressure drives S(T) through a sequence of temperature
dependences, ranging from a behaviour characteristic for magnetically ordered
heavy fermion compounds to a typical behaviour of intermediate-valent systems.
At intermediate pressures a large positive maximum develops above 10 K in S(T).
Its origin is attributed to the Kondo effect and its position is assumed to
reflect the Kondo temperature T_K. The pressure dependence of T_K is discussed
in a revised and extended (T,p) phase diagram of CeRu_2Ge_2.Comment: 7 pages, 6 figure
Effect of four plant species on soil 15N-access and herbage yield in temporary agricultural grasslands
Positive plant diversity-productivity relationships have been reported for experimental semi-natural grasslands (Cardinale et al. 2006; Hector et al. 1999; Tilman et al. 1996) as well as temporary agricultural grasslands (Frankow-Lindberg et al. 2009; Kirwan et al. 2007; Nyfeler et al. 2009; Picasso et al. 2008). Generally, these relationships are explained, on the one hand, by niche differentiation and facilitation (Hector et al. 2002; Tilman et al. 2002) and, on the other hand, by greater probability of including a highly productive plant species in high diversity plots (Huston 1997). Both explanations accept that diversity is significant because species differ in characteristics, such as root architecture, nutrient acquisition and water use efficiency, to name a few, resulting in composition and diversity being important for improved productivity and resource use (Naeem et al. 1994; Tilman et al. 2002). Plant diversity is generally low in temporary agricultural grasslands grown for ruminant fodder production. Grass in pure stands is common, but requires high nitrogen (N) inputs. In terms of N input, two-species grass-legume mixtures are more sustainable than grass in pure stands and consequently dominate low N input grasslands (Crews and Peoples 2004; Nyfeler et al. 2009; Nyfeler et al. 2011).
In temperate grasslands, N is often the limiting factor for productivity (Whitehead 1995). Plant available soil N is generally concentrated in the upper soil layers, but may leach to deeper layers, especially in grasslands that include legumes (Scherer-Lorenzen et al. 2003) and under conditions with surplus precipitation (Thorup-Kristensen 2006). To improve soil N use efficiency in temporary grasslands, we propose the addition of deep-rooting plant species to a mixture of perennial ryegrass and white clover, which are the most widespread forage plant species in temporary grasslands in a temperate climate (Moore 2003). Perennial ryegrass and white clover possess relatively shallow root systems (Kutschera and Lichtenegger 1982; Kutschera and Lichtenegger 1992) with effective rooting depths of <0.7 m on a silt loamy site (Pollock and Mead 2008). Grassland species, such as lucerne and chicory, grow their tap-roots into deep soil layers and exploit soil nutrients and water in soil layers that the commonly grown shallow-rooting grassland species cannot reach (Braun et al. 2010; Skinner 2008). Chicory grown as a catch crop after barley reduced the inorganic soil N down to 2.5 m depth during the growing season, while perennial ryegrass affected the inorganic soil N only down to 1 m depth (Thorup-Kristensen 2006). Further, on a Wakanui silt loam in New Zealand chicory extracted water down to 1.9 m and lucerne down to 2.3 m soil depth, which resulted in greater herbage yields compared with a perennial ryegrass-white clover mixture, especially for dryland plots (Brown et al. 2005).
There is little information on both the ability of deep- and shallow-rooting grassland species to access soil N from different vertical soil layers and the relation of soil N-access and herbage yield in temporary agricultural grasslands. Therefore, the objective of the present work was to test the hypotheses 1) that a mixture comprising both shallow- and deep-rooting plant species has greater herbage yields than a shallow-rooting binary mixture and pure stands, 2) that deep-rooting plant species (chicory and lucerne) are superior in accessing soil N from 1.2 m soil depth compared with shallow-rooting plant species, 3) that shallow-rooting plant species (perennial ryegrass and white clover) are superior in accessing soil N from 0.4 m soil depth compared with deep-rooting plant species, 4) that a mixture of deep- and shallow-rooting plant species has greater access to soil N from three soil layers compared with a shallow-rooting two-species mixture and that 5) the leguminous grassland plants, lucerne and white clover, have a strong impact on grassland N acquisition, because of their ability to derive N from the soil and the atmosphere
Inverse Compton Contribution to the Star-Forming Extragalactic Gamma-Ray Background
Fermi has resolved several star-forming galaxies, but the vast majority of
the star-forming universe is unresolved and thus contributes to the
extragalactic gamma ray background (EGB). Here, we calculate the contribution
from star-forming galaxies to the EGB in the Fermi range from 100 MeV to 100
GeV, due to inverse-Compton (IC) scattering of the interstellar photon field by
cosmic-ray electrons. We first construct a one-zone model for a single
star-forming galaxy, assuming supernovae power the acceleration of cosmic rays.
The same IC interactions leading to gamma rays also substantially contribute to
the energy loss of the high-energy cosmic-ray electrons. Consequently, a
galaxy's IC emission is determined by the relative importance of IC losses in
the cosmic-ray electron energy budget ("partial calorimetry"). We use our
template for galactic IC luminosity to find the cosmological contribution of
star-forming galaxies to the EGB. For all of our models, we find the IC EGB
contribution is almost an order of magnitude less than the peak of the emission
due to cosmic-ray ion interactions (mostly pionic p_cr p_ism \rightarrow \pi_0
\rightarrow \gamma \gamma); even at the highest Fermi energies, IC is
subdominant. Moreover, the flatter IC spectrum increases the high-energy signal
of the pionic+IC sum, bringing it into better agreement with the EGB spectral
index observed by Fermi . Partial calorimetry ensures that the overall IC
signal is well constrained, with only modest uncertainties in the amplitude and
spectral shape for plausible model choices. Partial calorimetry of cosmic-ray
electrons should hold true in both normal and starburst galaxies, and thus we
include starbursts in our calculation. We conclude with a brief discussion on
how the pionic spectral feature and other methods can be used to measure the
star-forming component of the EGB.Comment: 32 pages, 4 figures. Interested readers may wish to consult Lacki,
Horiuchi and Beacom (2012), which focusses on the MeV background from
galaxie
The inverse-Compton ghost HDF 130 and the giant radio galaxy 6C 0905+3955: matching an analytic model for double radio source evolution
We present new GMRT observations of HDF 130, an inverse-Compton (IC) ghost of
a giant radio source that is no longer being powered by jets. We compare the
properties of HDF 130 with the new and important constraint of the upper limit
of the radio flux density at 240 MHz to an analytic model. We learn what values
of physical parameters in the model for the dynamics and evolution of the radio
luminosity and X-ray luminosity (due to IC scattering of the cosmic microwave
background (CMB)) of a Fanaroff-Riley II (FR II) source are able to describe a
source with features (lobe length, axial ratio, X-ray luminosity, photon index
and upper limit of radio luminosity) similar to the observations. HDF 130 is
found to agree with the interpretation that it is an IC ghost of a powerful
double-lobed radio source, and we are observing it at least a few Myr after jet
activity (which lasted 5--100 Myr) has ceased. The minimum Lorentz factor of
injected particles into the lobes from the hotspot is preferred to be
for the model to describe the observed quantities well,
assuming that the magnetic energy density, electron energy density, and lobe
pressure at time of injection into the lobe are linked by constant factors
according to a minimum energy argument, so that the minimum Lorentz factor is
constrained by the lobe pressure. We also apply the model to match the features
of 6C 0905+3955, a classical double FR II galaxy thought to have a low-energy
cutoff of in the hotspot due to a lack of hotspot
inverse-Compton X-ray emission. The models suggest that the low-energy cutoff
in the hotspots of 6C 0905+3955 is , just slightly above
the particles required for X-ray emission.Comment: 9 pages, 3 figure
Squeezing MOND into a Cosmological Scenario
Explaining the effects of dark matter using modified gravitational dynamics
(MOND) has for decades been both an intriguing and controversial possibility.
By insisting that the gravitational interaction that accounts for the Newtonian
force also drives cosmic expansion, one may kinematically identify which
cosmologies are compatible with MOND, without explicit reference to the
underlying theory so long as the theory obeys Birkhoff's law. Using this
technique, we are able to self-consistently compute a number of quantities of
cosmological interest. We find that the critical acceleration a_0 must have a
slight source-mass dependence (a_0 ~ M^(1/3)) and that MOND cosmologies are
naturally compatible with observed late-time expansion history and the
contemporary cosmic acceleration. However, cosmologies that can produce enough
density perturbations to account for structure formation are contrived and
fine-tuned. Even then, they may be marginally ruled out by evidence of early (z
\~ 20) reionization.Comment: 11 pages revtex, 2 figure
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