R E S E A R C H   H I G H L I G H T S N O V E M B E R   2 0 0 2   D E U T S C H E   V E R S I O N 

Decrypting high-resolution Galaxy Spectra


Scientists at the Max-Planck Institute for Astrophysics have developed a new model to decrypt high-resolution galaxy spectra. With this model, it will possible to interpret the high-quality spectra gathered by modern spectroscopic galaxy surveys and to set unprecedented constraints on the star formation and chemical enrichment histories of the Universe.


Reproduction of SDSS Galaxy Spectrum

Fig. 1: Reproduction of the observed spectrum (power per unit wavelength) of a galaxy from the Sloan Digital Sky Survey (SDSS, in blue) by a standard population synthesis model (in red). The continuum shape of the observed spectrum is well fitted by the model, but the model resolution ( $\lambda/\Delta \lambda\approx 250$) is far lower than the data resolution ( $\lambda/\Delta \lambda \approx1800$). Thus, the model cannot be used to interpret the plethora of absorption lines that contain crucial information about the ages and the metallicities of the stars making up the galaxy's light.


The same spectrum reproduced with high-resolution synthesis model

Fig. 2: Reproduction of the same SDSS spectrum as in Figure 1 (in blue) by the new, high-resolution ( $\lambda/\Delta \lambda \approx2000$) population synthesis model developed at the MPA (in red). The main stellar absorption features in the observed spectrum are now well reproduced by the model. The requirement to fit these features leads to valuable constraints on the stellar content of the observed galaxy (see text).

The star formation and chemical enrichment histories of the Universe are imprinted in the light emitted from galaxies. The integrated light of a galaxy is the sum of the emission from stars of various ages that were born at different cosmic epochs. Stars of a given age have the chemical composition of the gas out of which they formed, and the light they emit contains information about both their age and their chemical composition. If we can interpret the spectra of nearby galaxies in terms of the different stellar generations that compose them, we will be able to reconstruct the star formation and chemical enrichment histories of the Universe.

Stellar population synthesis, the modeling of the spectra emitted by specific populations of stars, is a natural approach to interpreting observed galaxy spectra. This technique combines stellar evolution theory with a library of individual stellar spectra that describe the emission from stars of any mass, age and metallicity. Current population synthesis models, however, suffer from a serious limitation: their spectral resolution is typically much lower than achieved in modern spectroscopic galaxy surveys. Figure 1 illustrates how current models compare to the high-quality spectra obtained by the Sloan Digital Sky Survey (external page SDSS), which the MPA joint as a Participating Institution in January 2001 (see highlight of April 2002). It is clear from this figure that the model resolution is far too low to help us decrypt the plethora of absorption lines in the observed spectrum. These lines are produced by atomic and molecular transitions in the stellar atmospheres, and they contain valuable information about the ages and the metallicities of the stars making up the galaxy's light.

G. Bruzual (Center for Astronomical Research, Mérida, Venezuela and MPA) and S. Charlot (MPA) have developed a new population synthesis model enabling the interpretation of high-resolution galaxy spectra. This model incorporates the most recent developments in stellar evolution theory and a external page new library of high-resolution, individual stellar spectra. Figure 2 shows how this model compares to the same observed SDSS spectrum as in Figure 1. The main stellar absorption features in the data are now well reproduced by the model. The requirement to fit these features leads to valuable constraints on the stellar content of the observed galaxy. In the example shown, 80% of the total stellar mass of $2\times
10^{10}$ solar masses can be attributed to stars of slightly sub-solar metallicity formed 5-13 Gyr ago, 16% to stars of slightly supra-solar metallicity formed 2.5-5 Gyr ago, and the remaining 4% to stars of supra-solar metallicity formed 1.0-2.5 Gyr ago.

The SDSS will obtain spectra of the type shown in Figure 2 for nearly a million nearby galaxies. Using the above model to constrain the stellar content of the galaxies, it will be possible address key cosmological questions, such as when did stars in the Universe form, whether star formation occurs continuously or in bursts, whether galaxies hold onto the heavy elements they produce or eject a substantial fraction, how the masses of galaxies evolve with time, and how their ages and star formation histories depend on luminosity, morphological type and environment.

Stephane Charlot



    R E S E A R C H   H I G H L I G H T S   M P A   H O M E P A G E   D E U T S C H E   V E R S I O N 
  Last modified: Wed Oct 30 17:43:52 CET 2002     •     Comments to: info@mpa-garching.mpg.de