Empirical Calibration of the Near-IR Ca II Triplet


Paper IV

Empirical calibration of the near-IR Ca II triplet - IV. The stellar populations synthesis models

Vazdekis A., Cenarro A.J., Gorgas J., Cardiel N. and Peletier R.F.

Abstract

We present a new evolutionary stellar population synthesis model, which predicts SEDs for single-age single-metallicity stellar populations, SSPs, at resolution 1.5 angstrom (FWHM) in the spectral region of the near-IR Ca II feature. The main ingredient of the model is a new extensive empirical stellar spectral library that has been recently presented in Cenarro et al. (2001a,b; 2002a), which is composed of more than 600 stars with an unprecedented coverage of the stellar atmospheric parameters.

Two main products of interest for stellar population analysis are presented. The first is a spectral library for SSPs with metallicities -1.7 < [Fe/H] < +0.2, a large range of ages (0.1-18 Gyr) and IMF types. They are well suited to model galaxy data, since the SSP spectra, with flux-calibrated response curves, can be smoothed to the resolution of the observational 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 (namely CaT*, CaT and PaT) for the same SSPs in the form of equivalent widths.

We find the following behaviour for the Ca II feature in old-aged SSPs: i) the strength of the CaT* index 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 of Armandroff & Zinn (1988), which are well reproduced, probing the validity of using the integrated Ca II feature for determining the metallicities of these systems. Fitting the models to two early-type galaxies of different luminosities (NGC 4478 and NGC 4365), we find that the Ca II measurements cannot be fitted unless a very dwarf-dominated IMF is imposed, or if the Ca abundance is even lower than the Fe abundance. More details can be found in Cenarro et al. (2003b).




Paper IV has been accepted for publication in MNRAS. It will soon be available at this page.

The SSPs model predictions:

(See also this web site for full details on the following model predictions as well as for other ones in different spectral ranges)

SSPs parameter coverage:

IMF Metallicity Age (Gyr)
Unimodal (UN)
slope
0.3
0.8
1.3 (=Salpeter IMF)
1.8
2.3
2.8
3.3
Bimodal (BI)
slope
0.3
0.8
1.3
1.8
2.3
2.8
3.3
Kroupa Universal (KU)
Kroupa Revised (KB)
[M/H] Z
-1.68
-1.28
-0.68
-0.38
  0.00
 0.20
0.0004
0.001
0.004
0.008
0.019
0.030
0.10
0.11
0.13
0.14
0.16
0.18
0.20
0.22
0.25
0.28
0.32
0.35
0.40
0.45
0.50
0.56
0.63
0.71
0.79
0.89
1.00
1.12
1.26
1.41
1.58
1.78
2.00
2.24
2.51
2.82
3.16
3.55
3.98
4.47
5.01
5.62
6.31
7.08
7.94
8.91
10.00
11.22
12.59
14.13
15.85
17.78


Age ranges (in gigayears) where SEDs predictions are safe:
  [M/H] = -1.68 [M/H] = -1.28 [M/H] = -0.68 [M/H] = -0.38 [M/H] = 0.0 [M/H] = +0.2
SAFE     [5.5,18] [2.5,18] [0.1,18] [1.0,18]
RELATIVELY SAFE [10,18] [10,18]        

Integrated indices predicted by using the fitting functions:

IMF type Data tables
Unimodal nIR-out_li_UN
Bimodal nIR-out_li_BI
Kroupa Universal nIR-out_li_KU
Kroupa Revised nIR-out_li_KB

SEDs at 1.5Å (FWHM) and dispersion 0.85Å/pix in the spectral range [8348.85 - 8951.50]:

UNITS: the total mass of the SSP is 1Mo. Multiplying by 3.826*1033 the spectra will be given in units of erg s-1cm-2-1.

The spectra were grouped within tar compressed files according to their spectral ranges. These files can be retrieved from the following table:

Age range IMF slope Unimodal Bimodal Kroupa Universal Kroupa Revised
SAFE 1.3 ASCII.tar.gz ASCII.tar.gz ASCII.tar.gz ASCII.tar.gz
SAFE 1.3 FITS.tar.gz FITS.tar.gz FITS.tar.gz FITS.tar.gz
ALL [0.3,3.3] ASCII.tar.gz ASCII.tar.gz ASCII.tar.gz ASCII.tar.gz
ALL [0.3,3.3] FITS.tar.gz FITS.tar.gz FITS.tar.gz FITS.tar.gz