Research
Activity
My research activity is organized around the relationship between
star
formation and activity in the nuclei of galaxies. This involves the use
of quite a few observational techniques, both photometric and
spectroscopic,
in various spectral ranges, from the far ultraviolet to the near
infrared.
Also, for the study of these data, I am proficient in the use of
several
techniques of spectroscopic analysis, such as the computation of the
physical
conditions (temperature, density, etc.), and chemical abundances,
dating
the age of stellar populations, use of complex numerical codes of
photoionization
(CLOUDY), stellar
population synthesis (SED@,
Starburst99),
stellar atmospheres (TLUSTY),
etc. The main research topics on which I am involved include the
following:
We present the single stellar
population (SSP) synthesis results
of our
new synthetic stellar atmosphere models library with a spectral
sampling of 0.3 Å, covering the wavelength range from 3000
Å to
7000Å for a wide range of metallicities (twice solar, solar, half
and
1/10 solar). The stellar library is composed of 1650 spectra
computed with the latest improvements in stellar atmospheres. In
particular it incorporates non-LTE line-blanketed
models for hot (Teff >= 27500 K) and LTE line-blanketed
models (Phoenix)
for cool (3000 K <= Teff <= 4500 K) stars. Because of
the high
spectral resolution of this library, evolutionary synthesis models can
be used to predict the strength of
numerous weak absorption lines, and the evolution of the profiles of
the
strongest lines over a wide range of ages. The SSP results have been
calculated for ages 1 Myr to 17 Gyr using the stellar evolutionary
provided by the Geneva and Padova-tracks groups. For young stellar
populations,
our results have a very detailed coverage of high-temperature stars
with similar
results for Padova and Geneva isochrones. For intermediate and old
stellar populations, our results,
once degraded to a lower resolution, are similar to the ones obtained
by
other groups apart from limitations imposed by the stellar evolutionary
physics. The limitations and advantages of our library for the analysis
of
integrated populations are described. The full set of the stellar
library and the
evolutionary models are available here for retrieval or on request from
authors.
- Building of stellar
absorption line profile libraries in
the far UV and
the
optical.
Stellar libraries are one of the basic ingredients of evolutionary
population synthesis models. I have built a stellar library at the
far-Ultraviolet,
using observations of massive stars (O and B) taken by the satellites
Copernicus
and HUT (Hopkins Ultraviolet Telesope) that trace the profile of the Lyb
and the resonance line of OVI 1032 in stellar winds. I have also built
a stellar library at optical wavelengths (3700--5000 Å) using
stellar
atmosphere models (TLUSTY)
that
trace mainly the profile of H Balmer and HeI photospheric lines.
- Stellar
population synthesis models: Balmer absorption lines.
The evolutionary synthesis is a powerful technique that allows to
interpret
the integrated light from a galaxy. It predicts the synthetic spectrum
of a stellar population as a function of age, SFR, and IMF. The basic
ingredients
are the evolutionary stellar tracks and the stellar libraries. I have
built
models to predict the far-ultraviolet spectrum and the H Balmer and HeI
absorption lines in star forming galaxies in the starburst and
post-starburst
phase.
- Extragalactic HII
regions.
(Catalogues
are available)
Giant extragalactic HII regions (RHIIs)
are amongst the brightest objects in galaxies. RHIIs
have been studied extensively because they are the best indicators of
the
conditions that lead to massive star formation, and they show the cloud
properties immediately after the moment when stars form. Equally
important,
they are favourable places where to measure the chemical abundances and
star formation rates in galaxies. RHIIs are
characterized
to have a size larger than 100 pc and Ha
luminosity
brighter than 1039 erg s-1. Therefore, the nebula
requires an ionizing photon luminosity larger than 1051 s-1;
this is provided by a stellar cluster that contains more than 100 young
massive stars. These characteristics are very similar to those of
starburst
galaxies. However, RHIIs are less luminous than
prototypical
starbursts, and thus they are referred to as mini-starbursts. I work in
the determination of the HII
region luminosity function, which provide information on the
luminosity
and mass function of the embedded stellar associations, as well as the
mass distribution of the HII regions. (The
catalogues of the HII regions from this work can
be retrieved
here.)
Other important area of my research is the study of the ionized
structure and physical conditions of RHIIs,
and
the determination of the massive
stellar content and its evolutionary state.
- Super Star Clusters.
Super Star Clusters (SSCs) represent a particular mode of star
formation,
with stellar surface density exceeding that of giant HII
regions. They are considered the analog of young globular clusters.
This
statement is based on their luminosity, mass, size and age
determination.
The ages span from a few to several hundred Myrs; therefore, their
spectral
energy distribution show different characteristic features according to
their evolutionary state. They are also considered the building blocks
of Starburst galaxies, and therefore, they are benckmark objects to
study
how the star formation proceeds in Starburst galaxies. I work in
the study of the star
formation history of SSCs.
- Stellar populations in Starburst galaxies.
Starburst are objects in which the total energetics are dominated by
star formation and associated phenomena. They are ideal laboratories in
which to explore questions about the formation and evolution of
galaxies.
The spectra of Starbutrst galaxies are characterized by showing a
spectral
dichotomy: an absorption line spectrum observed at UV wavelengths and a
nebular emission line spectrum at optical wavelengths. This dichotomy
reflects
that Starburst galaxies are powered by hot stars. The strongest UV
absorption
lines are formed in the interstellar medium of the galaxy and the wind
of massive stars. The nebular lines are formed in the interstellar
medium
surrounding the stellar cluster. Thus, multiwavelength observations
from
UV to NIR combined with evolutionary synthesis constitute a powerful
tool
to study the evolution of Starburst galaxies, to derive the stellar
content
and to constrain the star formation history in these galaxies.
Starbursts
are major components of the local universe, and the sites of 25% of the
total high-mass star formation in the local Universe. I work on the
determination
of the main properties of nearby starbursts and the role of the
Starburst
phenomenon in the evolution of galaxies. I have studied in detail the
prototypical
nuclear starburst NGC 7714, at the UV,
optical,
and NIR.
-
Interaction of the massive stars with the
interstellar
medium in Starburst galaxies.
Massive stars are able to modify their surrounding interstellar medium.
They emit photons that are able to ionize the gas, and they have
strong stellar winds. Thus massive stars are able to heat the ISM not
only
radiatively but also mechanically through the deposition of
momentum.
RHIIs and Starburst, due to their intense star
formation
activity, show outflows of gas that are driven by the collective effect
of supernova and stellar winds. These outflows are detected at UV
and optical
wavelengths.
-
The
Starburst-AGN
connection in Seyfert galaxies.
Perhaps the most important unanswered question concerning the Seyfert
galaxy phenomenon is the fundamental nature of the energy source(s).
There
are sound theoretical arguments in favor of accretion onto supermassive
black holes, and the observational evidence that such 'monsters' exist
is growing. On the other hand, circumnuclear starbursts can have
bolometric
luminosities that rival even powerful QSO's, and there have been
recurring
suggestions that such starbursts may play an important role in the
Seyfert
galaxy phenomenon. I have investigated the possibility of a connection
between the Starburst and Seyfert phenomena throught the analysis
of the UV light emitted by the nucleus of Seyfert 2 galaxies. HST
observations
of four Seyfert 2 nuclei (Mrk 477 (link), NGC 7130, NGC 5135 and IC3639
(link)) that show high ultraviolet surface brightness indicate
the
UV light of these nuclei are produced by dusty, compact, and powerful
starburst.
- The Extended Narrow Line Region
of Seyfert and LINERs
The name of Extended Narrow Line Region (ENLR) refer to the
spatially
resolved extranuclear high excitation gas observed extending out from
the
active nucleus of some galaxies. The source of ionization is usually
thought
to be radiation escaping from the active nucleus, in a more or less
collimated
manner, and hitting the gas in the galactic disk. Studies of the ENLR
play
an important rôle in the understanding of the source of ionizing
radiation in the active nucleus. The spatially resolved ENLR gas
participates
from the normal insterstellar medium conditions, it is easier to
calculate
the physical properties of this gas, such as electron density and
ionization
parameter, and this provides a cleaner look into the properties of the
ionizing source of radiation.