24,785 research outputs found
The Spectral Energy Distribution of Normal, Starburst and Active Galaxies
We present the results of an extensive literature search of multiwavelength
data for a sample of 59 galaxies, consisting of 26 Starbursts, 15 Seyfert 2's,
5 LINER's, 6 normal spirals and 7 normal elliptical galaxies. The data include
soft X-ray fluxes, ultraviolet and optical spectra, near, mid/far infrared
photometry and radio measurements, selected to match as closely as possible the
IUE aperture (10" X 20"). The galaxies are separated into 6 groups with similar
characteristics, namely, Ellipticals, Spirals, LINER's, Seyfert 2's, Starbursts
of Low and High reddening, for which we create average spectral energy
distributions (SED). The individual groups SED's are normalized to the
7000\AA flux and compared, looking for similarities and differences
among them.The bolometric fluxes of different types of galaxies were calculated
integrating their SED's. These values are compared with individual waveband
flux densities, in order to determine the wavebands which contribute most to
the bolometric flux. Linear regressions were performed between the bolometric
and individual band fluxes for each kind of galaxy. These fits can be used in
the calculation of the bolometric flux for other objects of similar activity
type, but with reduced waveband information. We have also collected
multiwavelength data for 4 HII regions, a thermal supernova remnant, and a
non-thermal supernova remnant (SNR), which are compared with the Starburst
SED's.Comment: 29 pages, 13 postscript figures and 10 tables. To appear in The
Astronomical Journa
Population synthesis of HII galaxies
We study the stellar population of galaxies with active star formation,
determining ages of the stellar components by means of spectral population
synthesis of their absorption spectra. The data consist of optical spectra of
185 nearby () emission line galaxies. They are mostly HII
galaxies, but we also include some Starbursts and Seyfert 2s, for comparison
purposes. They were grouped into 19 high signal-to-noise ratio template
spectra, according to their continuum distribution, absorption and emission
line characteristics. The templates were then synthesized with a star cluster
spectral base. The synthesis results indicate that HII galaxies are typically
age-composite stellar systems, presenting important contribution from
generations up to as old as 500 Myr. We detect a significant contribution of
populations with ages older than 1 Gyr in two groups of HII galaxies. The age
distributions of stellar populations among Starbursts can vary considerably
despite similarities in the emission line spectra. In the case of Seyfert 2
groups we obtain important contributions of old population, consistent with a
bulge. From the diversity of star formation histories, we conclude that typical
HII galaxies in the local universe are not systems presently forming their
first stellar generation.Comment: 12 pages, 4 figures, MNRAS in pres
Stellar population gradients in Seyfert 2 galaxies. Northern sample
We use high signal-to-noise ratio long-slit spectra in the 3600-4700A range
of the twenty brightest northern Seyfert 2 galaxies to study the variation of
the stellar population properties as a function of distance from the nucleus.
In order to characterize the stellar population and other continuum sources
(e.g. featureless continuum FC) we have measured equivalent widths Ws of six
absorption features, four continuum colours and their radial variations, and
performed spectral population synthesis as a function of distance from the
nucleus. About half the sample has CaIIK and G-band W values smaller at the
nucleus than at 1 kpc from it, due to a younger population and/or FC. The
stellar population synthesis shows that, while at the nucleus, 75% of the
galaxies present contribution > 20% of ages younger or equal than 100Myr and/or
of a FC, this proportion decreases to 45% at 3 kpc. In particular, 55% of the
galaxies have contribution > 10% of the 3 Myr/FC component (a degenerate
component in which one cannot separate what is due to a FC or to a 3 Myr
stellar population) at the nucleus, but only 25% of them have this contribution
at 3 kpc. As reference, the stellar population of 10 non-Seyfert galaxies,
spanning the Hubble types of the Seyfert (from S0 to Sc) was also studied. A
comparison between the stellar population of the Seyferts and that of the
non-Seyferts shows systematic differences: the contribution of ages younger
than 1 Gyr is in most cases larger in the Seyfert galaxies than in
non-Seyferts, not only at the nucleus but up to 1 kpc from it.Comment: 23 pages, 18 figures, MNRAS in pres
Intensity Thresholds and the Statistics of the Temporal Occurrence of Solar Flares
Introducing thresholds to analyze time series of emission from the Sun
enables a new and simple definition of solar flare events, and their
interoccurrence times. Rescaling time by the rate of events, the waiting and
quiet time distributions both conform to scaling functions that are independent
of the intensity threshold over a wide range. The scaling functions are well
described by a two parameter function, with parameters that depend on the phase
of the solar cycle. For flares identified according to the current, standard
definition, similar behavior is found.Comment: 5 pages, 4 figures, revtex
Moments of the latitudinal dependence of the sunspot cycle : a new diagnostic of dynamo models
All A&A articles are in open access ONE year after their publication date (August 2013)Aims: The latitude-distribution of solar activity as represented by sunspots is studied.
Methods: We first determined the latitudinal distribution of a sunspot cycle by integrating the butterfly diagram at each latitude over the length of each cycle. We then formed the five lowest moments of the latitudinal distribution of all complete sunspot cycles since 1874 and compared these moments with each other.
Results: The three lowest moments correlate remarkably well with each other. For example, the mean latitude of the sunspots during a cycle and the latitude range are correlated at the 0.96 level. A clear asymmetry is seen between the two hemispheres, with the southern solar hemisphere showing consistently stronger and more positive correlations than the northern hemisphere. When applied to different simple dynamo models, the same analysis reveals significant differences between the models and demonstrates that such moments are a useful diagnostic in distinguishing between dynamo models. Remarkably, dynamos without a meridional flow provide results closer to those of the Sunâs northern hemisphere, while a dynamo with a meridional flow produces fields more like those in the Sunâs southern hemisphere. This may provide a clue to the cause of the well-known north-south asymmetry of solar activity
Large N limit of SO(N) gauge theory of fermions and bosons
In this paper we study the large N_c limit of SO(N_c) gauge theory coupled to
a Majorana field and a real scalar field in 1+1 dimensions extending ideas of
Rajeev. We show that the phase space of the resulting classical theory of
bilinears, which are the mesonic operators of this theory, is OSp_1(H|H
)/U(H_+|H_+), where H|H refers to the underlying complex graded space of
combined one-particle states of fermions and bosons and H_+|H_+ corresponds to
the positive frequency subspace. In the begining to simplify our presentation
we discuss in detail the case with Majorana fermions only (the purely bosonic
case is treated in our earlier work). In the Majorana fermion case the phase
space is given by O_1(H)/U(H_+), where H refers to the complex one-particle
states and H_+ to its positive frequency subspace. The meson spectrum in the
linear approximation again obeys a variant of the 't Hooft equation. The linear
approximation to the boson/fermion coupled case brings an additonal bound state
equation for mesons, which consists of one fermion and one boson, again of the
same form as the well-known 't Hooft equation.Comment: 27 pages, no figure
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