1,889 research outputs found
Mid--IR emission of galaxies in the Virgo cluster: II. Integrated properties
We analyse the integrated properties of the Mid-IR emission of a complete,
optically selected sample of galaxies in the Virgo cluster observed with the
ISOCAM instrument on board the ISO satellite. The analysis shows that the
Mid-IR emission up to 15 mic of optically-selected, normal early-type galaxies
(E, S0 and S0a) is dominated by the Rayleigh-Jeans tail of the cold stellar
component. The Mid-IR emission of late-type galaxies is instead dominated by
the thermal emission from dust. The small dust grains emitting in the Mid-IR
have an excess of emission if compared to big grains emitting in the Far-IR.
While the Far-IR emission increases with the intensity of the interstellar
radiation field, their Mid-IR emission is non--linearly related to the UV
radiation field. The spectral energy distributions of the target galaxies
indicate that there is a linear relationship between the UV radiation field and
the Mid-IR emission of galaxies for low or intermediate activities of star
formation, while the emission from the hot dust seems to drop for strong UV
fields. The Mid-IR colour of late-type galaxies is not related to their
activity of star formation. The properties of the dust emission in the Mid-IR
seem more related to the mass than to the morphological type of the target
galaxy. Since the activity of star formation is anticorrelated to the mass of
galaxies, this reflects a relationship between the emission of dust in the
Mid-IR and the UV radiation field: galaxies with the lowest Mid-IR emission for
a given UV field are low mass, dwarf galaxies. These observational evidences
are easily explained if the carriers of the Unidentified Infrared Bands that
dominate the 6.75 mic emission are destroyed by the intense UV radiation field
of dwarf galaxies, although abundance effects can also play a role.Comment: 17 pages, Latex, 7 figures; to be published in Astronomy &
Astrophysics, Main Journal; Figure legend should be corrected in: 1 - 1a; 2 -
1b; 3 - 2; 4 - 3a; 5 - 3b; 6 - 3c; 7 - 3d; 8 - 3e; 9 - 4; 10 - 5; 11 - 6; 12
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The tilt of the Fundamental Plane of Early-type galaxies: wavelength dependence
The photometric parameters R_e and mu_e of 74 early-type (E+S0+S0a) galaxies
in the Coma cluster are derived for the first time in the near IR H band. These
are used, coupled with measurements of the central velocity dispersion found in
the literature, to determine the H band Fundamental Plane (FP) relation of this
cluster. The same procedure is applied to previously available photometric data
in the B, V, r, I, and K bands, to perform a multi-wavelength study of the FP.
Because systematic uncertainties in the value of the FP parameters are
introduced both by the choice of the fitting algorithm, and by the presence of
statistical biases connected with the sample selection procedure, we emphasize
the importance of deriving the FP parameters in the six photometric bands using
an identical fitting algorithm, and appropriate corrections to eliminate the
effects of sample incompleteness. We find that the FP mu_e coefficient is
stable with wavelength, while the sigma coefficient increases significantly
with increasing wavelength, in agreement with an earlier result presented by
Pahre & Djorgovski. The slope of the FP relation, although changing with
wavelength, never approaches the virial theorem expectation. We also find that
the magnitude of the slope change can be entirely explained by the presence of
the well known relation between color and magnitude among early-type galaxies.
We conclude that the tilt of the Fundamental Plane is significant, and must be
due to some form of broken homology among early-type galaxies, while its
wavelength dependence derives from whatever mechanism (currently the preferred
one is the existence of a mass-metal content sequence) produces the
color-magnitude relation in those galaxies.Comment: 8 pages, 7 figures, 6 tables; table 3 should be printed in landscape
mode, and inserted into the text. Accepted for publication in MNRA
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
Higgs boson decay into four leptons at NLOPS electroweak accuracy
In view of precision studies of the Higgs sector at the Run II of the LHC,
the improvement of the accuracy of the theoretical prediction is becoming a
pressing issue. In this framework, we detail a calculation of the full
Next-to-Leading Order (NLO) electroweak corrections to Higgs boson decay into
four charged leptons, by considering the gold-plated channel H -> Z(*) Z(*) ->
2l 2l', l,l' = e, mu. We match the NLO corrections with a QED Parton Shower
(PS), in order to simulate exclusive multiple photon emission and provide novel
results at NLOPS electroweak accuracy. We compare our NLO predictions to those
of the program Prophecy4f and present NLOPS phenomenological results relevant
for Higgs physics studies, with particular attention to precision measurements
of the Higgs boson mass, spin-parity assignment and tests of the Standard
Model. Our calculation is implemented in a new code, Hto4l, which can be easily
interfaced to any generator describing Higgs boson production. As an example,
we provide illustrative results for Higgs production and decay in the process
gg -> H -> 4l using POWHEG with NLOPS accuracy in the production mode.Comment: 27 pages, 2 tables, 9 figures. New numerical results and plots for
dressed leptons. Conclusions unchanged. Version to appear in JHE
1.65mic (H-band) surface photometry of galaxies. VI: The history of star formation in normal late-type galaxies
We have collected a large body of NIR (H band), UV (2000 A) and Halpha
measurements of late-type galaxies. These are used, jointly with spectral
evolutionary synthesis models, to study the initial mass function (IMF) in the
mass range m > 2 Mo. For spirals (Sa-Sd), Magellanic irregulars (Im) and blue
compact dwarfs (BCD), our determination is consistent with a Salpeter IMF with
an upper mass cutoff M_up = 80 Mo. The history of star formation and the amount
of total gas (per unit mass) of galaxies are found to depend primarily on their
total masses (as traced by the H band luminosities) and only secondarily on
morphological type. The present star formation activity of massive spirals is
up to 100 times smaller than that average over their lifetime, while in low
mass galaxies it is comparable to or higher than that at earlier epochs. Dwarf
galaxies have presently larger gas reservoirs per unit mass than massive
spirals. The efficiency in transforming gas into stars and the time scale for
gas depletion (10 Gyrs) are independent of the luminosity and/or of the
morphological type. These evidences are consistent with the idea that galaxies
are coeval systems,that they evolved as closed-boxes forming stars following a
simple, universal star formation law whose characteristic time scale is small
(1 Gyr) in massive spirals and large (10 Gyr) in low mass galaxies. A similar
conclusion was drawn by Gavazzi and Scodeggio (1996) to explain the
colour-magnitude relation of late-type galaxies. The consequences of this
interpretation on the evolution of the star formation rate and of the gas
density per comoving volume of the Universe with look-back time are discussed.Comment: LaTex, 24 pages, 12 figures, accepted for publication on Astronomical
Journa
Ram Pressure Stripping in High-Density Environments
Galaxies living in rich environments are suffering different perturbations
able to drastically affect their evolution. Among these, ram pressure
stripping, i.e. the pressure exerted by the hot and dense intracluster medium
(ICM) on galaxies moving at high velocity within the cluster gravitational
potential well, is a key process able to remove their interstellar medium (ISM)
and quench their activity of star formation. This review is aimed at describing
this physical mechanism in different environments, from rich clusters of
galaxies to loose and compact groups. We summarise the effects of this
perturbing process on the baryonic components of galaxies, from the different
gas phases (cold atomic and molecular, ionised, hot) to magnetic fields and
cosmic rays, and describe their induced effects on the different stellar
populations, with a particular attention to its role in the quenching episode
generally observed in high density environments. We also discuss on the
possible fate of the stripped material once removed from the perturbed galaxies
and mixed with the ICM, and we try to estimate its contribution to the
pollution of the surrounding environment. Finally, combining the results of
local and high redshift observations with the prediction of tuned models and
simulations, we try to quantify the importance of this process on the evolution
of galaxies of different mass, from dwarfs to giants, in various environments
and at different epochs.Comment: Invited review for The Astronomy and Astrophysics Revie
High mass star formation in normal late-type galaxies: observational constraints to the IMF
We use Halpha and FUV GALEX data for a large sample of nearby objects to
study the high mass star formation activity of normal late-type galaxies. The
data are corrected for dust attenuation using the most accurate techniques at
present available, namely the Balmer decrement and the total far-infrared to
FUV flux ratio. The sample shows a highly dispersed distribution in the Halpha
to FUV flux ratio indicating that two of the most commonly used star formation
tracers give star formation rates with uncertainties up to a factor of 2-3. The
high dispersion is due to the presence of AGN, where the UV and the Halpha
emission can be contaminated by nuclear activity, highly inclined galaxies, for
which the applied extinction corrections are probably inaccurate, or starburst
galaxies, where the stationarity in the star formation history required for
transforming Halpha and UV luminosities into star formation rates is not
satisfied. Excluding these objects we reach an uncertainty of ~50% on the SFR.
The Halpha to FUV flux ratio increases with their total stellar mass. If
limited to normal star forming galaxies, however, this relationship reduces to
a weak trend that might be totally removed using different extinction
correction recipes. In these objects the Halpha to FUV flux ratio seems also
barely related with the FUV-H colour, the H band effective surface brightness,
the total star formation activity and the gas fraction. The data are consistent
with a Kroupa and Salpeter initial mass function in the high mass stellar range
and imply, for a Salpeter IMF, that the variations of the slope cannot exceed
0.25, from g=2.35 for massive galaxies to g=2.60 in low luminosity systems. We
show however that these observed trends, if real, can be due to the different
micro history of star formation in massive galaxies with respect to dwarf.Comment: Accepted for publication on Ap
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
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