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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.
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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.
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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.
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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  highlight
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 ( SDSS), 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 (astro-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 (astro-ph/0605300)
Further information:
Stellar population synthesis at the resolution of 2003,
2003, MNRAS, 344, 1000 (astro-ph/0309134)
MPA/JHU value added catalog
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