56 research outputs found
Stellar population models in the Near-Infrared
The study of early-type elliptical and lenticular galaxies provides important information about the formation and evolution of galaxies in the early Universe. These distant systems cannot be studied by looking at their individual stars but information can still be obtained by studying their unresolved spectrum in detail.During my PhD I have constructed accurate unresolved stellar population models for populations of a single age and metallicity in the near-infrared range. The extension to the NIR is important for the study of early-type galaxies, since these galaxies are predominantly old and therefore emit most of their light in this wavelength range. The models are based on the NASA IRTF library of empirical stellar spectra. Integrating these spectra along theoretical isochrones, while assuming an initial mass function, we have produced model spectra of single age-metallicity stellar populations at an intermediate resolution.Comparison to literature results show that our models are well suited for studying stellar populations in unresolved galaxies. They are particularly useful for studying the old and intermediate-age stellar populations in galaxies, relatively free from contamination of young stars and extinction by dust. Subsequently, we use the models to fit the observed spectra of globular clusters and galaxies, to derive their age distribution, chemical abundances and IMF properties. We show that the contribution of AGB stars to the galaxy spectrum is clearly larger in the field than it is in the Fornax cluster. This implies that the environment plays an important role in driving the evolutionary histories of the galaxies
Single Stellar Populations in the Near-Infrared - I. Preparation of the IRTF spectral stellar library
We present a detailed study of the stars of the IRTF spectral library to
understand its full extent and reliability for use with Stellar Population (SP)
modeling. The library consist of 210 stars, with a total of 292 spectra,
covering the wavelength range of 0.94 to 2.41 micron at a resolution R = 2000.
For every star we infer the effective temperature (Teff), gravity (logg) and
metallicity ([Z/Zsun]) using a full-spectrum fitting approach in a section of
the K band (2.19 to 2.34 micron) and temperature-NIR colour relations. We test
the flux calibration of these stars by calculating their integrated colours and
comparing them with the Pickles library colour-temperature relations. We also
investigate the NIR colours as a function of the calculated effective
temperature and compared them in colour-colour diagrams with the Pickles
library. This latter test shows a good broad-band flux calibration, important
for the SP models. Finally, we measure the resolution R as a function of
wavelength. We find that the resolution increases as a function of lambda from
about 6 angstrom in J to 10 angstrom in the red part of the K-band. With these
tests we establish that the IRTF library, the largest currently available
general library of stars at intermediate resolution in the NIR, is an excellent
candidate to be used in stellar population models. We present these models in
the next paper of this series.Comment: 17 pages, 19 figures. Accepted for publication in Astronomy and
Astrophysic
Single stellar populations in the near-infrared. II. Synthesis models
We present unresolved single stellar population synthesis models in the near-infrared (NIR) range. The extension to the NIR is important for the study of early-type galaxies, since these galaxies are predominantly old and therefore emit most of their light in this wavelength range. The models are based on a library of empirical stellar spectra, the NASA Infrared Telescope Facility (IRTF) spectral library. Integrating these spectra along theoretical isochrones, while assuming an initial mass function (IMF), we have produced model spectra of single age-metallicity stellar populations at a resolution R ~ 2000. These models can be used to fit observed spectral of globular clusters and galaxies, to derive their age distribution, chemical abundances and IMF. The models have been tested by comparing them to observed colours of elliptical galaxies and clusters in the Magellanic Clouds. Predicted absorption line indices have been compared to published indices of other elliptical galaxies. The comparisons show that our models are well suited for studying stellar populations in unresolved galaxies. They are particularly useful for studying the old and intermediate-age stellar populations in galaxies, relatively free from contamination of young stars and extinction by dust. These models will be indispensable for the study of the upcoming data from JWST and extremely large telescopes, such as the E-ELT. Appendices are available in electronic form at http://www.aanda.orgModel spectra are available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/582/A97 and at smg.astro-research.ne
SDSS-IV MaNGA: local and global chemical abundance patterns in early-type galaxies
Chemical enrichment signatures strongly constrain galaxy formation and
evolution, and a detailed understanding of abundance patterns provides clues
regarding the nucleosynthetic production pathways of elements. Using the
SDSS-IV MaNGA IFU survey, we study radial gradients of chemical element
abundances in detail. We use stacked spectra out to 1 Re of 366 early-type
galaxies with masses 9.9 - 10.8 log to probe the abundances of
the elements C, N, Na, Mg, Ca, and Ti, relative to the abundance of Fe, by
fitting stellar population models to a combination of Lick absorption indices.
We find that C, Mg, and Ti trace each other both as a function of galaxy radius
and galaxy mass. These similar C and Mg abundances within and across galaxies
set a lower limit for star-formation timescales. Conversely, N and Ca are
generally offset to lower abundances. The under-abundance of Ca compared to Mg
implies delayed enrichment of Ca through Type Ia supernovae, whereas the
correlated behaviour of Ti and the lighter elements, C and Mg, suggest
contributions to Ti from Type II supernovae. We obtain shallow radial gradients
in [Mg/Fe], [C/Fe], and [Ti/Fe], meaning that these inferences are independent
of radius. However, we measure strong negative radial gradients for [N/Fe] and
[Na/Fe], of up to and dex/Re respectively. These
gradients become shallower with decreasing galaxy mass. We find that N and Na
abundances increase more steeply with velocity dispersion within galaxies than
globally, while the other elements show the same relation locally and globally.
This implies that the high Na and N abundances found in massive early type
galaxies are generated by internal processes within galaxies. These are
strongly correlated with the total metallicity, suggesting
metallicity-dependent Na enrichment, and secondary N production in massive
early-type galaxies.Comment: 17 pages, 9 figures, 4 tables. MNRAS in pres
Carbon stars in the X-shooter Spectral Library
We provide a new collection of spectra of 35 carbon stars obtained with the
ESO/VLT X-shooter instrument as part of the X-shooter Spectral Library project.
The spectra extend from 0.3m to 2.4m with a resolving power above
8000. The sample contains stars with a broad range of (J-K) color and
pulsation properties located in the Milky Way and the Magellanic Clouds. We
show that the distribution of spectral properties of carbon stars at a given
(J-K) color becomes bimodal (in our sample) when (J-K) is larger than about
1.5. We describe the two families of spectra that emerge, characterized by the
presence or absence of the absorption feature at 1.53m, generally
associated with HCN and CH. This feature appears essentially only in
large-amplitude variables, though not in all observations. Associated spectral
signatures that we interpret as the result of veiling by circumstellar matter,
indicate that the 1.53m feature might point to episodes of dust production
in carbon-rich Miras.Comment: 29 pages, 21 figures, 9 tables, Accepted for publication in A&
Stellar population models based on the SDSS-IV MaStar library of stellar spectra. I. Intermediate-age/old models
We use the first release of the SDSS/MaStar stellar library comprising ~9000,
high S/N spectra, to calculate integrated spectra of stellar population models.
The models extend over the wavelength range 0.36-1.03 micron and share the same
spectral resolution (R~1800) and flux calibration as the SDSS-IV/MaNGA galaxy
data. The parameter space covered by the stellar spectra collected thus far
allows the calculation of models with ages and chemical composition in the
range t>200 Myr, -2 <=[Z/H]<= + 0.35, which will be extended as MaStar
proceeds. Notably, the models include spectra for dwarf Main Sequence stars
close to the core H-burning limit, as well as spectra for cold, metal-rich
giants. Both stellar types are crucial for modelling lambda>0.7 micron
absorption spectra. Moreover, a better parameter coverage at low metallicity
allows the calculation of models as young as 500 Myr and the full account of
the Blue Horizontal Branch phase of old populations. We present models adopting
two independent sets of stellar parameters (T_eff, logg, [Z/H]). In a novel
approach, their reliability is tested 'on the fly' using the stellar population
models themselves. We perform tests with Milky Way and Magellanic Clouds
globular clusters, finding that the new models recover their ages and
metallicities remarkably well, with systematics as low as a few per cent for
homogeneous calibration sets. We also fit a MaNGA galaxy spectrum, finding
residuals of the order of a few per cent comparable to the state-of-art models,
but now over a wider wavelength range.Comment: 37 pages, 31 figures, MNRAS in press, models available at
http://www.icg.port.ac.uk/masta
SDSS-IV MaNGA:the spatially resolved stellar initial mass function in ∼ 400 early-type galaxies
MaNGA provides the opportunity to make precise spatially resolved
measurements of the IMF slope in galaxies owing to its unique combination of
spatial resolution, wavelength coverage and sample size. We derive radial
gradients in age, element abundances and IMF slope analysing optical and
near-infrared absorption features from stacked spectra out to the half-light
radius of 366 early-type galaxies with masses .
We find flat gradients in age and [/Fe] ratio, as well as negative
gradients in metallicity, consistent with the literature. We further derive
significant negative gradients in the [Na/Fe] ratio with galaxy centres being
well enhanced in Na abundance by up to 0.5 dex. Finally, we find a gradient in
IMF slope with a bottom-heavy IMF in the centre (typical mass excess factor of
1.5) and a Milky Way-type IMF at the half-light radius. This pattern is
mass-dependent with the lowest mass galaxies in our sample featuring only a
shallow gradient around a Milky Way IMF. Our results imply the local
IMF- relation within galaxies to be even steeper than the global
relation and hint towards the local metallicity being the dominating factor
behind the IMF variations. We also employ different stellar population models
in our analysis and show that a radial IMF gradient is found independently of
the stellar population model used. A similar analysis of the Wing-Ford band
provides inconsistent results and further evidence of the difficulty in
measuring and modelling this particular feature.Comment: 28 pages, 24 figures, 9 tables. MNRAS in pres
SDSS-IV MaNGA: The Spatially Resolved Stellar Initial Mass Function in ~400 Early-Type Galaxies
Mapping Nearby Galaxies at Apache Point Observatory provides the opportunity to make precise spatially resolved measurements of the IMF slope in galaxies owing to its unique combination of spatial resolution, wavelength coverage, and sample size. We derive radial gradients in age, element abundances, and IMF slope analysing optical and near-infrared absorption features from stacked spectra out to the half-light radius of 366 early-type galaxies with masses 9.9–10.8 log M/M⊙. We find flat gradients in age and [α/Fe] ratio, as well as negative gradients in metallicity, consistent with the literature. We further derive significant negative gradients in the [Na/Fe] ratio with galaxy centres being well enhanced in Na abundance by up to 0.5 dex. Finally, we find a gradient in IMF slope with a bottom-heavy IMF in the centre (typical mass excess factor of 1.5) and a Milky Way-type IMF at the half-light radius. This pattern is mass dependent with the lowest mass galaxies in our sample featuring only a shallow gradient around a Milky Way IMF. Our results imply the local IMF–σ relation within galaxies to be even steeper than the global relation and hint towards the local metallicity being the dominating factor behind the IMF variations. We also employ different stellar population models in our analysis and show that a radial IMF gradient is found independently of the stellar population model used. A similar analysis of the Wing-Ford band provides inconsistent results and further evidence of the difficulty in measuring and modelling this particular feature
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