9 research outputs found
A comparative study of star formation processes in different environments
In this thesis we have identified two star formation sites for star formation
in galaxies which differ widely in metal content. We conclude that `massive'
star formation that occurs in a high density, high metallicity environment,
like that encountered in circumnuclear star-forming regions, takes place in
systems that comply to the definition of super stellar clusters and that are
arranged in star forming complexes. These complexes have Halpha luminosities,
and therefore masses of ionizing stars, that overlap at the lower end with
those found in HII galaxies, which have a much lower metallicity. The fact that
their ionization structure and the temperature of their ionizing radiation
field are very similar, point to stellar clusters in these two environments
which have the same equivalent effective temperature. This is contrary to what
is expected from stellar evolution models which predict lower stellar effective
temperatures in high metallicity regions.Comment: PhD Thesis at Universidad Autonoma de Madrid, Sept. 200
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
Implications of the kinematical structure of circumnuclear star-forming regions on their derived properties
[Abbreviated] We review the results of high dispersion spectroscopy of 17
circumnuclear starforming regions (CNSFRs) in 3 nearby early spiral galaxies,
NGC2903, NGC3310 and NGC3351. We find that single Gaussian fitting to the
H and [OIII]5007A line profiles results in velocity dispersions around
32km/s and 52km/s, respectively, while the IR CaII triplet cross-correlation
technique provides stellar velocity dispersion values close to 50km/s. Even
though multiple kinematical components are present, the relation between gas
velocity dispersion and Balmer emission line luminosity (L-sigma relation)
reproduces the correlation for disk giant HII regions albeit with a larger
scatter. The scatter in the L-sigma relation is considerably reduced when
theoretical evolutionary corrections are applied suggesting that an age range
is present in the sample of CNSFRs. To analyse the observed complex profiles,
we performed multiple Gaussian component fits to the Hbeta and [OIII]?5007A
lines obtaining optimal fits with two Gaussians of different width. These best
fits indicate that the narrower component has average velocity dispersion close
to 23km/s while the broader component shows average values in the range
50-60km/s for both lines, close to the observed stellar velocity dispersions.
The fluxes of the broad and narrow Hbeta components are similar. This is not
the case for [OIII]5007A for which the broad components have higher fluxes than
the narrow ones, thus producing a clear segregation in their [OIII]/Hbeta
ratios. We suggest a possible scenario for understanding the behaviour of
CNSFRs in the L-sigma and sigma_gas-sigma_star diagrams involving an inner
gaseous disk responsible for the narrow component of the emission lines.Comment: 14 pages, 10 figures, accepted by MNRA
Metallicity evolution of AGNs from UV emission-lines based on a new index
We analyzed the evolution of the metallicity of the gas with the redshift for
a sample of AGNs in a very wide redshift range (0<z<4) using ultraviolet
emission-lines from the narrow-line regions (NLRs) and photoionization models.
The new index C43=log(CIV+CIII])/HeII is suggested as a metallicity indicator
for AGNs. Based on this indicator, we confirmed the no metallicity evolution of
NLRs with the redshift pointed out by previous works. We found that metallicity
of AGNs shows similar evolution than the one predicted by cosmic semi-analytic
models of galaxy formation set within the Cold Dark Matter merging hierarchy
(for z < 3). Our results predict a mean metallicity for local objects in
agreement with the solar value (12+log(O/H)=8.69). This value is about the same
that the maximum oxygen abundance value derived for the central parts of local
spiral galaxies. Very low metallicity log(Z/Z_{\odot})~ -0.8 for some objects
in the range 1.5 < z <3 is derived.Comment: 25 pages, 10 figures, accepted MNRA
Circumnuclear Regions of Star Formation in Early Type Galaxies
Circumnuclear star forming regions, also called hotspots, are often found in
the inner regions of some spiral galaxies where intense processes of star
formation are taking place. In the UV, massive stars dominate the observed
circumnuclear emission even in the presence of an active nucleus, contributing
between 30 and 50% to the H total emission of the nuclear zone.
Spectrophotometric data of moderate resolution (3000 < R < 11000) are
presented from which the physical properties of the ionized gas: electron
density, oxygen abundances, ionization structure etc. have been derived.Comment: 4 pages, 2 figures, proceeding of the meeting "Pathways Through an
Eclectic Universe", Tenerife, 200
Optical and mid-infrared neon abundance determinations in star-forming regions
We employed observational spectroscopic data of star-forming regions compiled
from the literature and photoionization models to analyse the neon ionic
abundances obtained using both optical and mid-infrared emission-lines.
Comparing Ne++/H+ ionic abundances from distinct methods, we found that, in
average, the abundances obtained via IR emission-lines are higher than those
obtained via optical lines by a factor of 4. Photoionization models with
abundance variations along the radius of the hypothetical nebula provide a
possible explanation for a large part of the difference between ionic
abundances via optical and infrared emission-lines. Ionization Correction
Factor (ICF) for the neon is obtained from direct determinations of ionic
fractions using infrared emission-lines. A constant Ne/O ratio (logNe/O \approx
-0.70) for a large range of metallicity, independently of the ICF used to
compute the neon total abundance is derived.Comment: 17 pages, 14 figures, accepted by MNRA
X-rays as dominant excitation mechanism of [Fe ii] and H2 emission lines in active galaxies
We investigate the excitation mechanisms of near-infrared [Fe ii] and H2
emission lines observed in Active Galactic Nuclei (AGNs). We built a
photoionization model grid considering a two-component continuum, one accounts
for the Big Bump component peaking at 1Ryd and another represents the X-ray
source that dominates the continuum emission at high energies. Photoionization
models considering as ionizing source a spectral energy distribution obtained
from photometric data of the Sy 2 Mrk 1066 taken from the literature were
considered. Results of these models were compared with a large sample of
observational long-slit and Integral field Unit (IFU) spectroscopy data of the
nuclear region for a sample of active objects. We found that the correlation
between the observational [Fe ii]{\lambda}1.2570 {\mu}m/Pa{\beta} vs.
H2{\lambda}2.1218 {\mu}m/Br{\gamma} is well reproduced by our models as well as
the relationships that involve the H2 emission line ratios observed in the
spectroscopic data.We conclude that the heating by X-rays produced by active
nuclei can be considered a common and very important mechanism of excitation of
[Fe ii] and H2.Comment: 10 pages, 9 figures, accepted by MNRA
Discrepancies between the [O iii] and [S iii] temperatures in H ii regions
Context. Analysis of published [O iii] and [S iii] temperatures measurements of emission line objects consisting of Hii galaxies, giant extragalactic Hii regions, Galactic Hii regions, and Hii regions from the Magellanic Clouds reveal that the [O iii] temperatures are higher than the corresponding values from [S iii] in most objects with gas metallicities in excess of 0.2 solar. For the coolest nebulae (the highest metallicities), the [O iii] temperature excess can reach ∼3000 K. Aims. We look for an explanation for these temperature differences and explore the parameter space of models with the aim of reproducing the observed trend of T O iii > T S iii in Hii regions with temperatures below 14 000 K. Methods. Using standard photoionization models, we varied the ionization parameter, the hardness of the ionizing continuum, and the gas metallicities in order to characterize how models behave with respect to the observations. We introduced temperature inhomogeneities and varied their mean squared amplitude t 2. We explored the possibility of inhomogeneities in abundances by combining two models of widely different metallicity. We calculated models that consider the possibility of a non-Maxwell-Boltzmann energy distribution (a κ-distribution) for the electron energies. We also considered shock heating within the photoionized nebula. Results. Simple photoionization calculations yield nearly equal [O iii] temperature excess can reach ∼3000 K. Aims. We look for an explanation for these temperature differences and explore the parameter space of models with the aim of reproducing the observed trend of T O iii > T S iii in Hii regions with temperatures below 14 000 K. Methods. Using standard photoionization models, we varied the ionization parameter, the hardness of the ionizing continuum, and the gas metallicities in order to characterize how models behave with respect to the observations. We introduced temperature inhomogeneities and varied their mean squared amplitude t 2. We explored the possibility of inhomogeneities in abundances by combining two models of widely different metallicity. We calculated models that consider the possibility of a non-Maxwell-Boltzmann energy distribution (a κ-distribution) for the electron energies. We also considered shock heating within the photoionized nebula. Results. Simple photoionization calculations yield nearly equal [O iii and [S iii] temperatures in the domain of interest. Hence these models fail to reproduce the [O iii] temperature excess. Models that consider temperature inhomogeneities, as measured by the mean squared amplitude t 2, also fail in the regime where T O iii < 14 000 K. Three options remain that can reproduce the observed excess in T O iii temperatures: (1) large metallicity inhomogeneities in the nebula; a (2) κ-distribution for the electron energies; and (3) shock waves that propagate in the photoionized plasma at velocities ∼60 km s -1. Conclusions. The observed nebular temperatures are not reproduced by varying the input parameters in the pure photoionization case nor by assuming local temperature inhomogeneities. We find that (1) metallicity inhomogeneities of the nebular gas; (2) shock waves of velocities 60 km s -1 propagating in a photoionized plasma; and (3) an electron energy distribution given by a κ-distribution are successful in reproducing the observed excess in the [O iii] temperatures. However, shock models require proper 3D hydrodynamical simulations to become a fully developed alternative while models with metallicity inhomogeneities appear to fail in metal-poor nebulae, since they result in T recO++ T O iii T recO++ ≳ TO iii