141 research outputs found
On the deduction of galaxy abundances with evolutionary neural networks
A growing number of indicators are now being used with some confidence to
measure the metallicity(Z) of photoionisation regions in planetary nebulae,
galactic HII regions(GHIIRs), extra-galactic HII regions(EGHIIRs) and HII
galaxies(HIIGs). However, a universal indicator valid also at high
metallicities has yet to be found. Here, we report on a new artificial
intelligence-based approach to determine metallicity indicators that shows
promise for the provision of improved empirical fits. The method hinges on the
application of an evolutionary neural network to observational emission line
data. The network's DNA, encoded in its architecture, weights and neuron
transfer functions, is evolved using a genetic algorithm. Furthermore,
selection, operating on a set of 10 distinct neuron transfer functions, means
that the empirical relation encoded in the network solution architecture is in
functional rather than numerical form. Thus the network solutions provide an
equation for the metallicity in terms of line ratios without a priori
assumptions. Tapping into the mathematical power offered by this approach, we
applied the network to detailed observations of both nebula and auroral
emission lines in the optical for a sample of 96 HII-type regions and we were
able to obtain an empirical relation between Z and S23 with a dispersion of
only 0.16 dex. We show how the method can be used to identify new diagnostics
as well as the nonlinear relationship supposed to exist between the metallicity
Z, ionisation parameter U and effective (or equivalent) temperature T*.Comment: Accepted for publication in PASP. 6 pages, 2 figure
A grid of chemical evolution models along the Hubble Sequence
We have computed a grid of multiphase chemical evolution models whose results
are valid for any spiral galaxy, using as input the maximum rotation velocity
and the morphological type or index T.Comment: 2 pag., contribution to the conference Cosmic Evolution (Paris, Nov.
2000
The continuous star formation history of a giant HII region in M101
We present results about the star formation process in the giant HII region
NGC 5471 in the outskirts of M101. From resolved HST/WPFC2 photometry we find
that star formation has been going for the last 70 Myr. We further compare
previous results from integrated infrared-optical photometry with the stellar
resolved CMD and we discuss the star formation properties of this region and
its individual knots, as well as characterizing the different stellar content.
This result has very important consequences in our understanding of the burst
versus continuous star formation activity in spiral galaxies.Comment: 2 pages, 2 figures. Proceeding of the conference From Stars to
Galaxies: Building the pieces to build up the Universe (Venice, Italy
The metal abundance of circumnuclear star forming regions in early type spirals. Spectrophotometric observations
We have obtained long-slit observations in the optical and near infrared of
12 circumnuclear HII regions (CNSFR) in the early type spiral galaxies NGC
2903, NGC 3351 and NGC 3504 with the aim of deriving their chemical abundances.
Only for one of the regions, the [SIII] 6312 \AA was detected
providing, together with the nebular [SIII] lines at 9069,
9532 \AA, a value of the electron temperature of T([SIII])= 8400K. A semi-empirical method for the derivation of abundances in
the high metallicity regime is presented.
We obtain abundances which are comparable to those found in high metallicity
disc HII regions from direct measurements of electron temperatures and
consistent with solar values within the errors. The region with the highest
oxygen abundance is R3+R4 in NGC 3504, 12+log(O/H) = 8.85, about 1.5 solar if
the solar oxygen abundance is set at the value derived by Asplund et al.
(2005), 12+log(O/H) = 8.660.05. Region R7 in NGC 3351 has the
lowest oxygen abundance of the sample, about 0.6 times solar. In all the
observed CNSFR the O/H abundance is dominated by the O/H contribution,
as is also the case for high metallicity disc HII regions. For our observed
regions, however, also the S/S ratio is larger than one, contrary to
what is found in high metallicity disc HII regions for which, in general, the
sulphur abundances are dominated by S/H...Comment: 24 pages, 19 figures, accepted by MNRA
Properties of the ionized gas of circumnuclear star-forming regions in early type spirals
(Abbr.) A study of cicumnuclear star-forming regions (CNSFRs) in several
early type spirals has been made in order to investigate their main properties:
stellar and gas kinematics, dynamical masses, ionising stellar masses, chemical
abundances and other properties of the ionised gas. Both high resolution (R20000) and moderate resolution (R ~ 5000) have been used.
In some cases these regions, about 100 to 150 pc in size, are seen to be
composed of several individual star clusters with sizes between 1.5 and 4.9 pc
estimated from Hubble Space Telescope (HST) images. Stellar and gas velocity
dispersions are found to differ by about 20 to 30 km/s with the H
emission lines being narrower than both the stellar lines and the [OIII]
5007 \AA lines. The twice ionized oxygen, on the other hand, shows
velocity dispersions comparable to those shown by stars. We have applied the
virial theorem to estimate dynamical masses of the clusters, assuming that
systems are gravitationally bounded and spherically symmetric, and using
previously measured sizes. The measured values of the stellar velocity
dispersions yield dynamical masses of the order of 10 to 10 solar
masses for the whole CNSFRs.
...Comment: 7 pages, 8 figures, proceeding of the meeting "Young massive star
clusters - Initial conditions and environments", Granada, Spain, 200
Analysis of Balmer Profiles of early type stars
The spectral energy distribution (SED) of recent star formation regions is
dominated by the more massive and early stars (O and B types). These stars show
large and very significant absorption features, the most prominent being the
recombination lines of H, HeI and HeII. In particular, the shape of their
profiles are very dependent on the luminosity of the star. We have explored the
potential use of high resolution profiles to discriminate between different
luminosity classes and spectral types, by using profiles of the He and Balmer
lines. We have calculated growth curves for each of the lines and their
dependence on gravity and effective temperature. We show some of these
theoretical growth curves and our preliminary conclusions are analyzed and
discussed.Comment: 1 pag, Contribution to the conference "Cosmic Evolution and Galaxy
Formation: Structure, Interactions and Feedback", Nov. 1999 (Puebla, Mexico),
to be published in ASP. Conf. Series, Eds. J. Franco, E. Terlevich. O.
Lopez-Cruz, I. Aretxag
Line temperatures and elemental abundances in HII galaxies
We present long-slit spectrophotometric observations in the red and near
infrared of 12 HII galaxies. The spectral range includes the sulphur lines
[SII] at wavelengths 6716, 6731 angstroms and [SIII] at 6312 angstroms and
9069, 9532 angstroms. For all of the observed galaxies, at least three
ion-weighted temperatures from forbidden auroral to nebular line ratios have
been obtained and the relations between the different line temperatures have
been discussed. It is found that, for some objects, the [OII] temperatures
derived from those of [OIII] through the use of photo-ionisation models,
without taking into account the effect of density, can lead to a significant
underestimate of the O+/H+ ionic abundance and hence of the total oxygen
abundance.Comment: 16 pages, 11 Postscript figures. Accepted for publication by MNRA
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