Synthetic spectra of H Balmer and HeI absorption lines. II. Evolutionary synthesis models for starburst and poststarburst galaxies

DOI: 
Publication date: 
01/12/1999
Main author: 
Delgado, RMG
IAA authors: 
Delgado, RMG
Authors: 
Delgado, RMG; Leitherer, C; Heckman, TM
Journal: 
ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES
Publication type: 
Article
Volume: 
125
Pages: 
489-509
Number: 
Abstract: 
We present evolutionary stellar population synthesis models to predict the spectrum of a single-metallicity stellar population, with a spectral sampling of 0.3 Angstrom in five spectral regions between 3700 and 5000 Angstrom. The models, which are optimized for galaxies with active star formation, synthesize the profiles of the hydrogen Balmer series (H beta, H gamma, H delta, H8, H9, H10, H11, H12, and H13) and the neutral helium absorption lines (He I lambda 4922, He I lambda 4471, He I lambda 4388, He I lambda 4144, He I lambda 4121, He I lambda 4026, He I lambda 4009, and He I lambda 3819) for a burst with an age ranging from 10(6) to 10(9) yr, and different assumptions about the stellar initial mass function (IMF). Continuous star formation models lasting for 1 Gyr are also presented. The input stellar library includes non-LTE absorption profiles for stars hotter than 25,000 K and LTE profiles for lower temperatures. The temperature and gravity coverage is 4000 K less than or equal to T-eff less than or equal to 50,000 K and 0.0 less than or equal to log g less than or equal to 5.0, respectively. The metallicity is solar. It is found that the Balmer and He I Line profiles are sensitive to the age, except during the first 4 Myr of the evolution, when the equivalent widths of these lines are constant. At these early stages of the evolution, the profiles of the lines are also sensitive to the IMF. However, strong H Balmer and He I lines are predicted even when the low-mass cutoff of the IMF is as high as 10 M.. The equivalent widths of the Balmer lines range from 2 to 16 Angstrom and those of the He I lines from 0.2 to 1.2 Angstrom. During the nebular phase (cluster younger than about 10 Myr), H beta ranges from 2 to 5 Angstrom and He I lambda 4471 ranges between 0.5 and 1.2 A. The strength of the lines is maximum when the cluster is a few hundred (for the Balmer lines) and a few tens (for the He I lines) of Myr old. In the continuous star formation scenario, the strength of the Balmer and He I lines increases monotonically with time until 500 and 100 Myr, respectively. However, the lines are weaker than in the burst models owing to the dilution of the Balmer and He I lines by the contribution from very massive stars. The high spectral resolution of the profiles is useful to reproduce the absorption wings observed in regions of recent star formation and to estimate the effect of the underlying absorption on the nebular emission Lines. The strength of the nebular emission Balmer and He I lines compared with the stellar absorption components indicates that H delta and the higher order terms of the Balmer series and He I are dominated by the stellar absorption component if an instantaneous burst is older than similar or equal to 5 Myr. Some of the He I lines (e.g., He I lambda 3819 and He I lambda 4922) are more favorable than others (e.g., He I lambda 4471) for the detection of stellar features in the presence of nebular emission. We estimate that the correction to the He I lambda 4471 nebular emission line due to the stellar absorption is between 5% and 25%, if the nebular emission has equivalent width between 10 and 2.5 Angstrom (corresponding to a burst age between 1 and 3 Myr). The models can be used to date starburst and poststarburst galaxies until 1 Gyr. They have been tested on data for clusters in the LMC, the super-star cluster B in the starburst galaxy NGC 1569, the nucleus of the dwarf elliptical NGC 205 and a luminous 'E + A' galaxy. The full data set is available for retrieval at our websites or on request from the authors.
Database: 
WOK
Keywords: 
galaxies : evolution; galaxies : fundamental parameters; galaxies : starburst; galaxies : stellar content; line : profiles