343 research outputs found
Reliable random error estimation in the measurement of line-strength indices
We present a new set of accurate formulae for the computation of random
errors in the measurement of atomic and molecular indices. The new expressions
are in excellent agreement with numerical simulations. We have found that, in
some cases, the use of approximated equations can give misleading line-strength
index errors. It is important to note that accurate errors can only be achieved
after a full control of the error propagation throughout the data reduction
with a parallel processing of data and error frames. Finally, simple recipes
for the estimation of the required signal-to-noise ratio to achieve a fixed
index error are presented.Comment: 9 pages, LaTeX file + 5 PostScript figures, psfig.sty and laa-s.sty
required, to be published in Astronomy & Astrophysics Supplement Serie
Near-IR line-strengths in elliptical galaxies: evidence for IMF variations?
We present new relations between recently defined line-strength indices in
the near-IR (CaT*, CaT, PaT, MgI, and sTiO) and central velocity dispersion
(sigma_0) for a sample of 35 early-type galaxies, showing evidence for
significant anti-correlations between CaII triplet indices (CaT* and CaT) and
log(sigma_0). These relations are interpreted in the light of our recent
evolutionary synthesis model predictions, suggesting the existence of important
Ca underabundances with respect to Fe and/or an increase of the dwarf to giant
stars ratio along the mass sequence of elliptical galaxies.Comment: 5 pages, 3 figures, LaTeX. Accepted for publication in MNRAS Pink
Pages. See related information in
http://www.ucm.es/info/Astrof/ellipt/CATRIPLET.htm
Empirical calibration of the near-IR Ca II triplet - I. The stellar library and index definition
A new stellar library at the near-IR spectral region developed for the
empirical calibration of the Ca II triplet and stellar population synthesis
modeling is presented. The library covers the range 8348-9020 Angstrom at 1.5
Angstrom (FWHM) spectral resolution, and consists of 706 stars spanning a wide
range in atmospheric parameters. We have defined a new set of near-IR indices,
CaT*, CaT and PaT, which mostly overcome the limitations of previous
definitions, the former being specially suited for the measurement of the Ca II
triplet strength corrected for the contamination from Paschen lines. We also
present a comparative study of the new and the previous calcium indices, as
well as the corresponding transformations between the different systems. A
thorough analysis of the sources of index errors and the procedure to calculate
them is given. Finally, index and error measurements for the whole stellar
library are provided together with the final spectra.Comment: 24 pages, 10 figures, LaTeX. For associated spectra, tables and
software, see http://www.ucm.es/info/Astrof/ellipt/CATRIPLET.html or
http://www.nottingham.ac.uk/~ppzrfp/CATRIPLET.htm
MIUSCAT: extended MILES spectral coverage. I. Stellar populations synthesis models
We extend the spectral range of our stellar population synthesis models based
on the MILES and CaT empirical stellar spectral libraries. For this purpose we
combine these two libraries with the Indo-U.S. to construct composite stellar
spectra to feed our models. The spectral energy distributions (SEDs) computed
with these models and the originally published models are combined to construct
composite SEDs for single-age, single-metallicity stellar populations (SSPs)
covering the range 3465 - 9469\AA at moderately high, and uniform, resolution
(FWHM=2.51\AA). The colours derived from these SSP SEDs provide good fits to
Galactic globular cluster data. We find that the colours involving redder
filters are very sensitive to the IMF, as well as a number of features and
molecular bands throughout the spectra. To illustrate the potential use of
these models we focus on the NaI doublet at 8200 \AA and with the aid of the
newly synthesized SSP model SEDs we define a new IMF-sensitive index that is
based on this feature, which overcomes various limitations from previous index
definitions for low velocity dispersion stellar systems. We propose an
index-index diagram based on this feature and the neighboring CaII triplet at
8600 \AA, to constrain the IMF if the age and [Na/Fe] abundance are known.
Finally we also show a survey-oriented spectrophotometric application which
evidences the accurate flux calibration of these models for carrying out
reliable spectral fitting techniques. These models are available through our
user-friendly website.Comment: 16 pages, 17 figures, 2 tables; MNRAS in press. Model predictions
available at our website: http://miles.iac.e
Evidence for Blue Straggler Stars Rejuvenating the Integrated Spectra of Globular Clusters
Integrated spectroscopy is the method of choice for deriving the ages of
unresolved stellar systems. However, hot stellar evolutionary stages, such as
hot horizontal branch stars and blue straggler stars (BSSs), can affect the
integrated ages measured using Balmer lines. Such hot, "non-canonical" stars
may lead to overestimations of the temperature of the main sequence turn-off,
and therefore underestimations of the integrated age of a stellar population.
Using an optimized Hbeta index in conjunction with HST/WFPC2 color-magnitude
diagrams (CMDs), we show that Galactic globular clusters exhibit a large
scatter in their apparent "spectroscopic" ages, which does not correspond to
that in their CMD-derived ages. We find for the first time that the specific
frequency of BSSs, defined within the same aperture as the integrated spectra,
shows a clear correspondence with Hbeta in the sense that, at fixed
metallicity, higher BSS ratios lead to younger "apparent" spectroscopic ages.
Thus, the specific frequency of BSSs in globular clusters sets a fundamental
limit on the accuracy for which spectroscopic ages can be determined for
globular clusters, and maybe for other stellar systems like galaxies. The
observational implications of this result are discussed.Comment: 5 pages, 2 figures, 1 table. Accepted for publication in ApJ
MIUSCAT: extended MILES spectral coverage. II. Constraints from optical photometry
In the present work we show a comprehensive comparison of our new stellar
population synthesis MIUSCAT models with photometric data of globular clusters
and early-type galaxies. The models compare remarkably well with the colours of
Milky Way globular clusters in the optical range. Likewise, the colours of M31
globular clusters can also be explained by the models by assuming younger ages
then their Galactic counterparts. When compared with quiescent galaxies we
reproduce the colour evolution at intermediate redshift. On the other hand we
find that the colour relations of nearby early-type galaxies are still a
challenge for present-day stellar population synthesis models. We investigate a
number of possible explanations and establish the importance of alpha-enhanced
models to bring down the discrepancy with observations.Comment: 20 pages, 17 figures; MNRAS in press. Model predictions available at
our website: http://miles.iac.e
Early-type galaxies in the Coma cluster: a new piece in the calcium puzzle
We present measurements of the CaII triplet and the Ca4227 Lick-index for a
sample of early-type galaxies in the Coma cluster, deriving, for the first
time, their corresponding relationships with velocity dispersion. Compared with
a similar subsample of ellipticals in the field, Coma galaxies with velocity
dispersions in the range ~ 180-270 km/s exhibit significant differences in the
strengths of the Ca features, suggesting an influence of the environment on the
star formation histories of these galaxies. We argue that the main scenarios
previously proposed to explain the relatively low CaII triplet of galaxies are
not able by themselves to simultaneously reconcile the strengths of the two Ca
indices in both environments.Comment: 6 pages including 2 figures. Accepted for publication in ApJL. For
associated data tables see
http://www.ucm.es/info/Astrof/ellipt/CATRIPLET.htm
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