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  Current Research Highlight :: June 2006 all highlights

The stellar populations of nearby galaxies as fossil records of galaxy evolution

The light emitted from stars in galaxies encodes important information about their age and chemical composition. These are crucial constraints for galaxy formation and evolution models. Scientists at the Max-Planck-Institute for Astrophysics have developed a new method to derive accurate estimates of such parameters for samples of hundreds thousand galaxies, thus providing a complete census of the ages and metallicities of galaxies in the local Universe.

Fig. 1: Optical image and spectrum of two example galaxies, the elliptical NGC 5846 (top panels) and the spiral galaxy NGC 450 (bottom panels). The images on the left are obtained as mosaics of the images in three optical bands (g,r,i) of the Sloan Digital Sky Survey (courtesy of David W. Hogg, Michael R. Blanton, and the Sloan Digital Sky Survey Collaboration). The corresponding optical spectrum is shown on the right (black line). Emission lines, produced by the ionized gas surrounding massive young stars, have to be removed in order to accurately measure the underlying stellar absorptions (spectrum in blue). Some of the most important spectral features sensitive to the age and/or metallicity of the stellar populations are highlighted: the characteristic break at 4000Å and the hydrogen Balmer lines, Hγ and Hβ, sensitive to the age of the stellar populations in the galaxy; the metallic lines Mgb and Fe5335, sensitive to the total metallicity and the relative element abundances in stars. The elliptical galaxy displays red colours and prominent absorption features, indicative of old stellar populations. The spiral galaxy displays instead a much bluer colour and weak absorption lines, indicative of younger stellar populations.

Fig. 2: Distribution in stellar metallicity (left) and age (right) as a function of stellar mass for galaxies of all morphological types. The solid line shows the median relation, and the dashed lines enclose the 68 percent interval of the distribution. Both stellar metallicity and age increase with stellar mass, with a rapid transition from young, metal-poor to old, metal-rich galaxies over the stellar mass range 3·109—3·1010 solar masses.

Fig. 3: The relation between the colour and the luminosity of local early-type galaxies. The colour-code in each panel reflects, as a function of the position in the diagram, the average stellar metallicity (panel a), age (panel b), stellar mass (panel c) and α-elements over iron abundance ratio (panel d - high α/Fe ratios reflect short timescales of star formation). The colour-magnitude relation is a sequence in stellar mass, along which total metallicity, element abundance ratios and age all increase.

The properties of the stellar populations in galaxies are the result of their past history of star formation and chemical evolution. Determining the age and the chemical composition of the stars in galaxies should thus shed light on the processes that led galaxies to look like they are today. The integrated spectra of galaxies contain important clues about the ages of their stellar populations and the amount of heavy elements (`metals') produced during nuclear reactions in stars and supernova explosions and then locked up in subsequent generations of stars. Such information is imprinted in the strength of spectral absorption features, produced by hydrogen and several heavier elements (see Fig. 1), which respond to changes in the age and chemical composition of the stars in galaxies. These, in turn, are determined by the rate at which gas is transformed into stars and by the cycle of production, ejection and re-processing of metals into new generations of stars. The strengths of stellar absorption features can be interpreted in terms of the physical properties of the underlying stellar populations by means of models that describe the emission by populations of stars born at different epochs and with different chemical compositions. Models with high spectral resolution, matching the resolution achieved by modern spectroscopic surveys, are needed to decrypt all the information contained in the absorption features.

Scientists at Max-Planck-Institute for Astrophysics (MPA), by adopting a new, high-resolution population synthesis code (see linkPfeil.gifhighlight of November 2002), have interpreted, for the first time, the optical spectra for large samples of nearby galaxies, from quiescent ellipticals to actively star forming galaxies. The sample is provided by the Sloan Digital Sky Survey (linkPfeilExtern.gifSDSS), one of the most ambitious redshift surveys in the local Universe, and it includes almost 200,000 galaxies. A novel statistical approach has allowed scientists at MPA to derive simultaneous estimates of physical parameters, such as the age, the amount of metals and the total mass in stars, for very large samples of galaxies, from a limited set of observational constraints (an optimally selected set of absorption features).

The new estimates of the ages and metallicities of nearby galaxies and the unprecedented statistics allowed scientists at MPA to provide a complete census of the physical parameters of galaxies today and their dependence on galaxy mass. Present-day galaxies appear to define two broad distinct classes in observational and physical properties. The work conducted at MPA has provided a quantitative description of the separation between massive, old, metal-rich galaxies and low-mass, young, metal-poor systems (see Fig. 2). The transition between the two regimes occurs over a very characteristic stellar mass of 3·1010 solar masses.

Elliptical galaxies dominate above the critical mass. These galaxies have mostly completed their star formation and obey several tight scaling relations. Among these, the colour-magnitude relation (see Fig. 3) links the mass of early-type galaxies to the properties of their stellar populations. Scientists at MPA provided a detailed description of how the age, chemical composition and mass of the stars in nearby ellipticals vary along the relation. This demonstrates that the observed relation witnesses a higher degree of chemical enrichment reached in more massive systems, whose stellar populations formed earlier and faster than in low-mass ellipticals.

A complete census of the ages and chemical compositions of the stellar populations in galaxies today is a fundamental constraint at redshift zero for the global histories of star formation and chemical enrichment in the Universe. The study conducted at MPA represents an important gauge for similar studies on large samples of high redshift galaxies, which will allow us to directly constrain the star formation and chemical enrichment histories of galaxies.


Anna Gallazzi and Stéphane Charlot


Publications:

The ages and metallicities of galaxies in the local universe,
2005, MNRAS, 362, 41 (linkPfeilExtern.gifastro-ph/0506539)

Ages and metallicities of early-type galaxies in the Sloan Digital Sky Survey: new insight into the physical origin of the colour-magnitude and Mg2--σV relations,
MNRAS accepted (linkPfeilExtern.gifastro-ph/0605300)


Further information:

Stellar population synthesis at the resolution of 2003,
2003, MNRAS, 344, 1000 (linkPfeilExtern.gifastro-ph/0309134)

linkPfeil.gifMPA/JHU value added catalog


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