290 research outputs found
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
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
Empirical calibration of the near-IR CaII triplet - IV. The stellar population synthesis models
We present a new stellar population synthesis model, which predicts SEDs for
single-age single-metallicity stellar populations, SSPs, at resolution 1.5A
(FWHM) in the spectral region of the near-IR CaII triplet feature. The main
ingredient of the model is a new extensive empirical stellar spectral library
presented in Cenarro et al., which is composed of more than 600 stars. Two main
products of interest for stellar population analysis are presented. The first
is a library of SEDs for SSPs covering a large range of ages, metallicities and
different IMF types. They are well suited to model galaxy data, since the SSP
spectra, with flux-calibrated response curves, can be smoothed to match the
resolution of galaxy data, taking into account the internal velocity dispersion
of the galaxy, allowing the user to analyze the observed spectrum in its own
system. We also produce integrated absorption line indices for the same SSPs in
the form of equivalent widths. We find the following behaviour for the CaII
feature in old-aged SSPs: i) the CaII triplet feature does not change much with
time for all metallicities for ages larger than ~3 Gyr, ii) this index shows a
strong dependence with metallicity for values below [M/H] ~-0.5 and iii) for
larger metallicities this feature does not show a significant dependence either
on age or on the metallicity, being more sensitive to changes in the slope of
power-like IMF shapes. The SSP spectra have been calibrated with measurements
for globular clusters. Fitting the models to two early-type galaxies of
different luminosities, we find that the CaII measurements cannot be fitted
unless a very dwarf-dominated IMF is imposed, or if the Ca abundance is even
lower than the Fe abundance.Comment: 32 pages, 20 postscript figures, LaTeX. MNRAS, in press. Model
predictions can be found at http://www.iac.es/galeria/vazdekis/ or
http://www.ucm.es/info/Astrof/ellipt/CATRIPLET.htm
Extracting H flux from photometric data in the J-PLUS survey
We present the main steps that will be taken to extract H emission
flux from Javalambre Photometric Local Universe Survey (J-PLUS) photometric
data. For galaxies with , the H+[NII] emission is
covered by the J-PLUS narrow-band filter . We explore three different
methods to extract the H + [NII] flux from J-PLUS photometric data: a
combination of a broad-band and a narrow-band filter ( and ), two
broad-band and a narrow-band one (, and ), and a SED-fitting
based method using 8 photometric points. To test these methodologies, we
simulated J-PLUS data from a sample of 7511 SDSS spectra with measured
H flux. Based on the same sample, we derive two empirical relations to
correct the derived H+[NII] flux from dust extinction and [NII]
contamination. We find that the only unbiased method is the SED fitting based
one. The combination of two filters underestimates the measurements of the
H + [NII] flux by a 28%, while the three filters method by a 9%. We
study the error budget of the SED-fitting based method and find that, in
addition to the photometric error, our measurements have a systematic
uncertainty of a 4.3%. Several sources contribute to this uncertainty:
differences between our measurement procedure and the one used to derive the
spectroscopic values, the use of simple stellar populations as templates, and
the intrinsic errors of the spectra, which were not taken into account. Apart
from that, the empirical corrections for dust extinction and [NII]
contamination add an extra uncertainty of 14%. Given the J-PLUS photometric
system, the best methodology to extract H + [NII] flux is the
SED-fitting based one. Using this method, we are able to recover reliable
H fluxes for thousands of nearby galaxies in a robust and homogeneous
way.Comment: 11 pages, 14 figures. Minor changes to match the published versio
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