101 research outputs found
On the semi-dynamical reflection equation: solutions and structure matrices
Explicit solutions of the non-constant semi-dynamical reflection equation are
constructed, together with suitable parametrizations of their structure
matrices. Considering the semi-dynamical reflection equation with rational
non-constant Arutyunov-Chekhov-Frolov structure matrices, and a specific
meromorphic ansatz, it is found that only two sets of the previously found
constant solutions are extendible to the non-constant case. In order to
simplify future constructions of spin-chain Hamiltonians, a parametrization
procedure is applied explicitly to all elements of the semi-dynamical
reflection equation available. Interesting expressions for `twists' and
R-matrices entering the parametrization procedure are found. In particular,
some expressions for the R-matrices seem to appear here for the first time. In
addition, a new set of consistent structure matrices for the semi-dynamical
reflection equation is obtained.Comment: typos corrected and some comments adde
Emission Line Properties of Seyfert 2 Nuclei
We present a study of the global properties of Joguet's sample of 65 Seyfert
2 galaxies. We use the population synthesis models of our Paper II to derive
`pure' emission-line spectra, and thus explore the statistical properties of
the nuclear nebular components and their relation to the stellar populations.
We find that the emission line clouds suffer substantially more extinction than
the starlight, and confirm the correlations between stellar and nebular
velocity dispersions and between emission line luminosity and velocity
dispersions. Nuclear luminosities correlate with stellar velocity dispersions.
Removing the contribution of young stars to the optical continuum produces a
tighter and steeper relation, , consistent with the
Faber-Jackson law. Emission line ratios indicative of the gas excitation such
as [OIII]/H and [OIII]/[OII] are statistically smaller for Seyferts with
significant star-formation, implying that ionization by massive stars is
responsible for a substantial, and sometimes even a dominant, fraction of the
H and [OII] fluxes. We use our models to constrain the maximum fraction
of the ionizing power that can be generated by a hidden AGN. We correlate this
fraction with classical indicators of AGN photo-ionization: X-ray luminosity
and nebular excitation, but find no significant correlations. Thus, while there
is a strong contribution of starbursts to the excitation of the nuclear nebular
emission in low-luminosity Seyferts, the contribution of the hidden AGN remains
elusive even in hard X-rays. (abridged)Comment: 12 pages, 13 figures. Final version accepted for publication in MNRA
The circumstellar medium of the peculiar supernova SN1997ab
We report the detection of the slow moving wind into which the compact
supernova remnant SN 1997ab is expanding. Echelle spectroscopy provides clear
evidence for a well resolved narrow (Full Width at Zero Intensity, FWZI ~ 180
km/s) P-Cygni profile, both in Ha and Hb, superimposed on the broad emission
lines of this compact supernova remnant. From theoretical arguments we know
that the broad and strong emission lines imply a circumstellar density (n ~
10^7 cm^-3). This, together with our detection, implies a massive and slow
stellar wind experienced by the progenitor star shortly prior to the explosion.Comment: 10 pages, 2 figures, acepted for publication in MNRAS. Uses
referee.sty, psfig.sty and mn.sty. A postscript file can also be retrieved at
http://www.strw.LeidenUniv.nl/~salamanc/latest.htm
Semi-empirical analysis of Sloan Digital Sky Survey galaxies: II. The bimodality of the galaxy population revisited
We revisit the bimodal distribution of the galaxy population commonly seen in
the local universe. Here we address the bimodality observed in galaxy
properties in terms of spectral synthesis products, such as mean stellar ages
and stellar masses, derived from the application of this powerful method to a
volume-limited sample, with magnitude limit cutoff M_r = -20.5, containing
about 50 thousand luminous galaxies from the SDSS Data Release 2. In addition,
galaxies are classified according to their emission line properties in three
distinct spectral classes: star-forming galaxies, with young stellar
populations; passive galaxies, dominated by old stellar populations; and, hosts
of active nuclei, which comprise a mix of young and old stellar populations. We
show that the extremes of the distribution of some galaxy properties,
essentially galaxy colours, 4000 A break index, and mean stellar ages, are
associated to star-forming galaxies at one side, and passive galaxies at
another. We find that the mean light-weighted stellar age of galaxies is the
direct responsible for the bimodality seen in the galaxy population. The
stellar mass, in this view, has an additional role since most of the
star-forming galaxies present in the local universe are low-mass galaxies. Our
results also give support to the existence of a 'downsizing' in galaxy
formation, where massive galaxies seen nowadays have stellar populations formed
at early times.Comment: 18 pages, 19 figures, accepted for publication in MNRA
A probabilistic formulation for Empirical Population Synthesis: Sampling methods and tests
We present a probabilistic formulation of the classical problem of
synthesizing spectral properties of a galaxy using a base of star clusters. The
problem consists of estimating the population vector x, composed by the
contributions of n_star base elements to the integrated spectrum of a galaxy,
and the extinction A_V, given a set of absorption line equivalent widths and
continuum colors. The formalism is applied to the n_star = 12 base defined by
Schmidt etal and subsequently used in several studies. The 13-D parameter space
is explored with a Markov chain Monte Carlo sampling scheme based on the
Metropolis algorithm, which produces a smooth and efficient mapping of the
P(x,A_V) probability distribution.
This version of Empirical Population Synthesis is used to investigate the
ability to recover the detailed history of star-formation and chemical
evolution using this spectral base. This is studied as a function of (1) the
magnitude of the measurement errors and (2) the set of observables used in the
synthesis. Only for extremely high S/N all 12 base proportions can be
accurately recovered, though the observables are very precisely reproduced for
any S/N. Furthermore, the individual mean x components are biased in the sense
that components which carry a large fraction of the light tend to share their
contribution preferably among components of same age. This compensation effect
is linked to noise-induced linear dependences in the base, which very
effectively redistribute the likelihood in x-space. The age distribution,
however, can be satisfactorily recovered for realistic data quality. (abridged)Comment: 22 pages, 12 figs. MNRAS accepte
The Stellar Content of Active Galaxies
We present results of a long-slit spectroscopic study of 39 active and 3
normal galaxies. Stellar absorption features, continuum colors and their radial
variations are analyzed in an effort to characterize the stellar population in
these galaxies and detect the presence of a featureless continuum underlying
the starlight spectral component. Spatial variations of the equivalent widths
of conspicuous absorption lines and continuum colors are detected in most
galaxies. Star-forming rings, in particular, leave clear fingerprints in the
equivalent widths and color profiles. We find that the stellar populations in
the inner regions of active galaxies present a variety of characteristics, and
cannot be represented by a single starlight template. Dilution of the stellar
lines by an underlying featureless continuum is detected in most broad-lined
objects, but little or no dilution is found for the most of the 20 type 2
Seyferts in the sample. Color gradients are also ubiquitous. In particular, all
but one of the observed Seyfert 2s are redder at the nucleus than in its
immediate vicinity. Possible consequences of these findings are outlined.Comment: MNRAS, accepted. Uses mn and epsf.sty. [29 pages, 3 jpg + 44 ps
figures]. also available at http://www.fsc.ufsc.br/~ci
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