T.P. Idiart and J.A. de Freitas Pacheco

1
T.P. Idiart? and J.A. de Freitas Pacheco?
?Universidade de São Paulo (Brasil)
?Observatoire de la Côte dAzur (France)
Introduction
RESULTS
Elliptical galaxies are usually think as old,
complex stellar systems, whose the stellar
population bulk was formed from an interstellar
medium enriched in heavier elements produced
mainly by massive stars (? 9M?). The chemical
clocks used to date the stellar population are
basically the yields of type II and Ia
supernovae. Type II supernovae (SNII) ejecta are
rich in ? elements like Mg, Ca, Ti, O and Si,
whereas type Ia supernovae (SNIa) contribute to
enrichment of the interstellar medium mainly in
the elements of the iron peak. The high iron
content and the mean non-solar ?/Fe ratio in E
galaxies indicate that most of the stars were
formed very rapidly. We developed a multi-popul
ation model taking into account stellar
evolutionary tracks with non-solar ?/Fe ratios.
These produce a more adequate scaling between
?-elements and iron abundances, mainly at high
metallicities. In our scenario is included the
possibility of a two phase interstellar medium
produced by SN explosions. This model is applied
to study the chemical and photometrical evolution
of E galaxies. Our picture provides a natural way
to stop the star formation after the wind onset,
when the remaining gas is only in the hot and
ionized phase.
Calibration of the free parameters star
formation efficiency and initial mass
function using the MV (U-V) diagram for
Virgo-Coma ellipticals
Fiducial models at zero redshift
The model
K star formation efficiency ? coefficient of
IMF
A two-phase one-zone model with wind outflow
Main hypotheses the interstellar medi
um contains hot (T ? 50000 K) and cold gas
hot gas mass ionized gas mass
the gas is heated by type II and Ia explosions
winds are originated from the SN events
stars are not formed in hot gas conditions
These models show that the most massive galaxies
have higher star formation efficiency, flatter
IMF, higher mean stellar population ages, more
metallic stars and higher non-solar ?/Fe ratio.
The predicted observed stellar absorption indices
Mg2 and H? at z0 are also shown.
Evolution of the Elliptical Interstellar Gas
The stellar population at zero redshift
Basic equations
This figure shows the evolution of the hot and
cold gas for galaxies of distinct initial masses
MGAL. The ordinate shows the mass fraction of hot
and cold gas in units of MGAL, and the abscissa
the time evolution in units of age of the galaxy
?UNIV. Note that only the cold gas component is
available to form stars, implying that the vast
majority of stars is already formed in early
times of galaxy evolution.
As show in histograms, more than 90 of the stars
are formed in the first 4 Gyrs of E galaxy
evolution. The number of younger stars increases
as the initial galaxy mass MGAL decreases. This
happens because a lower star formation efficiency
implies in a lower quantity of hot gas and hence
an extented star formation period.
fG(t) total gas mass fraction
fHOT(t) hot gas mass fraction
?(m) initial mass function MR mass of remanen
t star k star formation efficiency tW time p
arameter of the wind
C1 function of ionizing energy
C2 function of physical properties of the gas
(hot gas)
?SN supernova frequency
Database for stellar population synthesis of E
galaxies
The evolutionary tracks for ?-enhanced stars
The high efficiency of star formation and the
existence of a hot gas component limits
drastically the number of stars formed more
recently. This is crucial to achieve the MV U-V
relation for E galaxies, because the U-V color is
very sensible to the existence of younger
population of stars.
The conversion from Z to Fe/H for an ?-enhanced
mixture of 0.4 can be estimated directly from
the expression by Salaris et al. (1993), using
the correction by Yong-Cheol et al. (2002) for
higher values of metallicity. This relation is
scaled to the solar abundances by Anders
Grevesse (1989), which are compatible with the
abundance catalog by Thévenin (1998).
Lick spectroscopic indices for ?-enhanced stars
As a stellar atmospheric database, we used
Thévenins (1998) catalog, which has a set of
homogeneous parameters effective temperature,
surface gravity and chemical abundances of
various nuclear species determined by a detailed
spectral analysis. The ?-elements considered for
non-solar ?/Fe classifications were O, Mg,
Si, Ca and Ti. At least three different elements
were used to classify stars with an ?-enhanced
abundance pattern (element/Fe ? 0.2). In these
conditions a total of 62 stars with observed
spectroscopic indices were selected. The figure
shows the relationship between atmospheric
parameters and spectroscopic indices for the
selected ?-enhanced stars. For comparison, a
sample of solar ?/Fe (element/Fe are overplotted.
Model predictions for different redshifts
The graphs show the behavior of Lick indices as a
function of the central velocity dispersion (a
mass indicator) for different redshifts.
According to the parameters of the fiducial
models, the indices show significant variations
in this redshifts range.
Project supported by IAG/USP, FAPESP and CNPq