248 research outputs found
Auto-consistent metallicity and star formation history of the nearest blue compact dwarf galaxy NGC 6789
We present a detailed auto-consistent study of the nearest blue compact dwarf
galaxy NGC 6789 by means of optical and UV archive photometry data and optical
long-slit ISIS-WHT spectroscopy observations of the five brightest star-forming
knots. The analysis of the spectra in all knots allowed the derivation of ionic
chemical abundances of oxygen, nitrogen, sulphur, argon and neon using measures
of both the high- and low-excitation electron temperatures, leading to the
conclusion that NGC 6789 is chemically homogeneous with low values of the
abundance of oxygen in the range 12+log(O/H) = 7.80-7.93, but presenting at the
same time higher values of the nitrogen-to-oxygen ratio than expected for its
metal regime.
We used archival HST/WFPC2 F555W and F814W observations of NGC 6789 to
perform a photometric study of the colour-magnitude diagram (CMD) of the
resolved stellar populations and derive its star formation history (SFH), which
is compatible with the presence of different young and old stellar populations
whose metallicities do not necessarily increase with age. We fit the observed
optical spectrum in all the five knots using the STARLIGHT code and a
combination of single stellar populations following the SFH obtained from the
CMD. We compare the resulting stellar masses and the relative fractions of the
ionising populations with a non-constrained SFH case. The properties of the
younger populations were obtained using CLOUDY photoionisation models, giving
similar ages in all the knots in the range 3-6 Myr and the estimation of the
dust absorption factor, which correlates with the observed GALEX FUV-NUV colour
indices. The total photometric extinction and dust-absorption corrected
H\alpha\ fluxes were finally used to derive the star formation rates.Comment: 18 pages, 6 figures, 10 tables; accepted by MNRA
The nature of the Wolf-Rayet galaxy Mrk 209 from photoionization models
This is an electronic version of an article published in Monthly Notices of the Royal Astronomical Society. PĂ©rez-Montero, E. and A.I. DĂaz. The nature of the Wolf-Rayet galaxy Mrk 209 from photoionization models. Monthly Notices of the Royal Astronomical Society 377 (2007): 1195-120
Chemical distribution of HII regions towards the Galactic anticentre
We study the physical and chemical properties of a sample of HII regions
located at RG >11 kpc and present the radial distribution of abundances towards
the Galaxy anticentre.
We carried out optical spectroscopic observations of nine HII regions with
the WHT. The sample was increased by searching the literature for optical
observations of regions towards the Galactic anticentre, re-analysing them to
obtain a single sample of 23 objects covering the Galactocentric radius from 11
kpc to 18 kpc to be processed in a homogeneous and consistent manner. Accurate
electron densities and temperatures of several ionic species were derived in 13
HII regions. These physical parameters were applied to the spectra to determine
direct total chemical abundances. For those regions without direct estimations
of temperature, chemical abundances were derived by performing tailor-made
photoionisation models and/or by using an empirical relation obtained from
radio recombination and optical temperatures.
We performed weighted least-squares fits to the distribution of the derived
abundances along the Galactocentric distances to study the radial gradients of
metallicity. The distributions O/H, N/H, S/H, and Ar/H towards the anticentre
can be represented by decreasing linear radial gradients, while in the case of
N/O abundances the radial distribution is better fitted with a two-zone model.
The He/H radial gradient is presented here for the first time; we find a slope
that is not significantly different from zero. The derived gradient for oxygen
shows a clear decrease with distance with a slope of -0.053dex/kpc. Although a
shallower slope at large Galactocentric distances is suggested by our data, the
flattening of the distribution cannot be confirmed and more objects towards the
anticentre need to be studied in order to establish the true form of the
metallicity gradient.Comment: Accepted for publication in Section 5. Galactic structure, stellar
clusters and populations of Astronomy and Astrophysics. The official date of
acceptance is 13/09/2016. 19 pages, 15 figures and 10 table
An extreme [O III] emitter at z = 3.2: A low metallicity Lyman continuum source
Aims. Cosmic reionization is an important process occurring in the early epochs of the Universe. However, because of observational limitations due to the opacity of the intergalactic medium to Lyman continuum photons, the nature of ionizing sources is still not well constrained. While high-redshift star-forming galaxies are thought to be the main contributors to the ionizing background at z > 6, it is impossible to directly detect their ionizing emission. Therefore, looking at intermediate redshift analogues (z ⌠2?4) can provide useful hints about cosmic reionization. Methods. We investigate the physical properties of one of the best Lyman continuum emitter candidate at z = 3.212 found in the GOODS-S/CANDELS field with photometric coverage from the U to the MIPS 24 ?m band and VIMOS/VLT and MOSFIRE/Keck spectroscopy. These observations allow us to derive physical properties such as stellar mass, star formation rate, age of the stellar population, dust attenuation, metallicity, and ionization parameter, and to determine how these parameters are related to the Lyman continuum emission. Results. Investigation of the UV spectrum confirms a direct spectroscopic detection of the Lyman continuum emission with S/N > 5. Non-zero Lyaα flux at the systemic redshift and high Lyman-aα ESCape fraction (f(Lyaα) â„ 0.78) suggest a low H i column density. The weak C and Si low-ionization absorption lines are also consistent with a low covering fraction along the line of sight. The subsolar abundances are consistent with a young and extreme starburst. The [O iii] iiiλλ4959, 5007+HÎČ equivalent width (EW) is one of the largest reported for a galaxy at z > 3 (EW([O iii] λλ4959, 5007 + HÎČ) â1600 Ă
, rest-frame; 6700 Ă
observed-frame) and the near-infrared spectrum shows that this is mainly due to an extremely strong [O iii] emission. The large observed [O iii]/[O ii] ratio (>10) and high ionization parameter are consistent with prediction from photoionization models in the case of a density-bounded nebula scenario. Furthermore, the EW([O iiiλλ4959, 5007+HÎČ) is comparable to recent measurements reported at z ⌠7?9, in the reionization epoch. We also investigate the possibility of an AGN contribution to explain the ionizing emission but most of the AGN identification diagnostics suggest that stellar emission dominates instead. Conclusions. This source is currently the first high-z example of a Lyman continuum emitter exhibiting indirect and direct evidences of a Lyman continuum leakage and having physical properties consistent with theoretical expectation from Lyman continuum emission from a density-bounded nebula. A low H i column density, low covering fraction, compact star formation activity, and a possible interaction/merging of two systems may contribute to the Lyman continuum photon leakage. © 2015 ESO.We acknowledge the financial contribution from PRIN-INAF 2012.Peer Reviewe
Optical and near-IR spectroscopy of low excitation H II regions with the GTC
This is an electronic version of an article published in Revista Mexicana de AstronomĂa y AstrofĂsica. PĂ©rez-Montero, E., Castellanos, M. and Ă.I.DĂaz. Optical and near-IR spectroscopy of low excitation H II regions with the GTC. Revista Mexicana de AstronomĂa y AstrofĂsica 16 (2003): 304-30
Disentangling the metallicity and star formation history of HII galaxies through tailor-made models
We present a self-consistent study of the stellar populations and the ionized
gas in a sample of 10 HII galaxies with, at least, four measured electron
temperatures and a precise determination of ionic abundances following the
"direct method". We fitted the spectral energy distribution of the galaxies
using the program STARLIGHT in order to quantify the contribution of the
underlying stellar population to EW(Hbeta), which amounts to about 10% for most
of the objects. We then studied the Wolf-Rayet stellar populations detected in
seven of the galaxies. The presence of these populations and the corrected
EW(Hbeta) values indicate that the ionizing stellar populations were created
following a continuous star formation episode of 10 Myr duration, hence WR
stars may be present in all of objects even if they are not detected in some of
them. The derived stellar features, the number of ionizing photons and the
relative intensities of the strongest emission lines were used as input
parameters to compute tailored models with the photoionization code CLOUDY. Our
models are able to y reproduce their thermal and ionization structure as
deduced from their collisionally excited emission lines and, hence, no
abundance discrepancy factors are implied for this kind of objects. Only the
electron temperature of S+ is overestimated by the models, pointing to the
possible presence of outer shells of diffuse gas in these objects. This kind of
geometrical effects can affect the determination of the equivalent effective
temperature of the ionizing cluster using calibrators which depend on
low-excitation emission lines.Comment: 20 pages, 12 figures, MNRAS (in press
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