1,330 research outputs found
Star formation and dust extinction in nearby star forming and starburst galaxies
We study the star formation rate and dust extinction properties of a sample
of nearby star forming galaxies as derived from Halpha and UV (2000 A)
observations and we compare them to those of a sample of starburst galaxies.
The dust extinction in Halpha is estimated from the Balmer decrement and the
extinction in UV using the FIR to UV flux ratio or the attenuation law for
starburst galaxies of Calzetti et al. The Halpha and UV emissions are strongly
correlated with a very low scatter for the star forming objects and with a much
higher scatter for the starburst galaxies. The Halpha to UV flux ratio is found
larger by a factor ~ 2 for the starburst galaxies. We compare both samples with
a purely UV selected sample of galaxies and we conclude that the mean Halpha
and UV properties of nearby star forming galaxies are more representative of UV
selected galaxies than starburst galaxies.
We emphasize that the Halpha to UV flux ratio is strongly dependent on the
dust extinction: the positive correlation found between F{Halpha}/F{UV}$ and
F{FIR}/F{UV} vanishes when the Halpha and UV flux are corrected for dust
extinction. The Halpha to UV flux ratios converted into star formation rate and
combined with the Balmer decrement measurements are tentatively used to
estimate the dust extinction in UV.Comment: accepted for publication in Astronomy & Astrophysic
Magnetic ordering in GaAlAs:Mn double well structure
The magnetic order in the diluted magnetic semiconductor barrier of double
AlAs/GaAs: Mn quantum well structures is investigated by Monte Carlo
simulations. A confinement adapted RKKY mechanism is implemented for indirect
exchange between Mn ions mediated by holes. It is shown that, depending on the
barrier width and the hole concentration a ferromagnetic or a spin-glass order
can be established.Comment: 3 figure
The GALEX Ultraviolet Virgo Cluster Survey (GUViCS). II. Constraints on star formation in ram-pressure stripped gas
Context: Several galaxies in the Virgo cluster are known to have large HI gas
tails related to a recent ram-pressure stripping event. The Virgo cluster has
been extensively observed at 1539 A in the far-ultraviolet for the GALEX
Ultraviolet Virgo Cluster Survey (GUViCS), and in the optical for the Next
Generation Virgo Survey (NGVS), allowing a study of the stellar emission
potentially associated with the gas tails of 8 cluster members. On the
theoretical side, models of ram-pressure stripping events have started to
include the physics of star formation. Aim: We aim to provide quantitative
constraints on the amount of star formation taking place in the ram-pressure
stripped gas, mainly on the basis of the far-UV emission found in the GUViCS
images in relation with the gas content of the tails. Methods: We have
performed three comparisons of the young stars emission with the gas column
density: visual, pixel-by-pixel and global. We have compared our results to
other observational and theoretical studies. Results: We find that the level of
star formation taking place in the gas stripped from galaxies by ram-pressure
is low with respect to the available amount of gas. Star formation is lower by
at least a factor 10 compared to the predictions of the Schmidt Law as
determined in regular spiral galaxy disks. It is also lower than measured in
dwarfs galaxies and the outer regions of spirals, and than predicted by some
numerical simulations. We provide constraints on the star formation efficiency
in the ram-pressure stripped gas tails, and compare these with current models.Comment: Accepted in A&A, 17 pages (including the appendix and "on-line"
figures of the paper
Higgs decay into four charged leptons in the presence of dimension-six operators
We study the indirect effects of New Physics in the Higgs decay into four
charged leptons, using an Effective Field Theory (EFT) approach to Higgs
interactions. We evaluate the deviations induced by the EFT dimension-six
operators in observables like partial decay width and various kinematic
distributions, including angular observables, and compare them with the
contribution of the full SM electroweak corrections. The calculation is
implemented in an improved version of the event generator Hto4l, which can
provide predictions in terms of different EFT-bases and is available for data
analysis at the LHC. We also perform a phenomenological study in order to
assess the benefits coming from the inclusion of differential information in
the future analyses of very precise data which will be collected during the
high luminosity phase of the LHC.Comment: 31 pages, 8 figures, 1 table. Version to appear on JHEP, expanded
phenomenological section including an analysis for HL-LH
A semiquantitative approach to the impurity-band-related transport properties of GaMnAs nanolayers
We investigate the spin-polarized transport of GaMnAs nanolayers in which a
ferromagnetic order exists below a certain transition temperature. Our
calculation for the self-averaged resistivity takes into account the existence
of an impurity band determining the extended ("metallic" transport) or
localized (hopping by thermal excitation) nature of the states at and near the
Fermi level. Magnetic order and resistivity are inter-related due to the
influence of the spin polarization of the impurity band and the effect of the
Zeeman splitting on the mobility edge. We obtain, for a given range of Mn
concentration and carrier density, a "metallic" behavior in which the transport
by extended carriers dominates at low temperature, and is dominated by the
thermally excited localized carriers near and above the transition temperature.
This gives rise to a conspicuous hump of the resistivity which has been
experimentally observed and brings light onto the relationship between
transport and magnetic properties of this material
1.65 micrometers (H-band) surface photometry of galaxies. III: observations of 558 galaxies with the TIRGO 1.5m telescope
We present near-infrared H-band (1.65 micron) surface photometry of 558
galaxies in the Coma Supercluster and in the Virgo cluster. This data set,
obtained with the Arcetri NICMOS3 camera ARNICA mounted on the Gornergrat
Infrared Telescope, is aimed at complementing, with observations of mostly
early-type objects, our NIR survey of spiral galaxies in these regions,
presented in previous papers of this series. Magnitudes at the optical radius,
total magnitudes, isophotal radii and light concentration indices are derived.
We confirm the existence of a positive correlation between the near-infrared
concentration index and the galaxy H-band luminosity. (Tables 1 and 2 are only
available in electronic form upon request to [email protected])Comment: 12 pages, 6 figures. Accepted for publication in A&A
On the nature of the spin-polarized hole states in a quasi-two-dimensional GaMnAs ferromagnetic layer
A self-consistent calculation of the density of states and the spectral
density function is performed in a two-dimensional spin-polarized hole system
based on a multiple-scattering approximation. Using parameters corresponding to
GaMnAs thin layers, a wide range of Mn concentrations and hole densities have
been explored to understand the nature, localized or extended, of the
spin-polarized holes at the Fermi level for several values of the average
magnetization of the Mn ystem. We show that, for a certain interval of Mn and
hole densities, an increase on the magnetic order of the Mn ions come together
with a change of the nature of the states at the Fermi level. This fact
provides a delocalization of spin-polarized extended states anti-aligned to the
average Mn magnetization, and a higher spin-polarization of the hole gas. These
results are consistent with the occurrence of ferromagnetism with relatively
high transition temperatures observed in some thin film samples and
multilayered structures of this material.Comment: 3 page
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