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Scientific Report 2007-2009
Astronomy & Astrophysics
A1. Structure and Evolution of Galaxies
This research is focused on the formation and evolution
of spheroidal and largely-populated star systems,
such as globular clusters and first-type elliptical galaxies,
which cannot usually be ”resolved” into individual
objects. For elliptical galaxies the issues concern the progressive
metal enrichment (owing to stellar nucleosynthesis
and supernovae explosions) and the spatial distribution
of the stellar populations, that were being formed in
time (with corresponding increasing metallicities). The
topic is relevant to the population synthesis, which consists
in computing the integrated brightness, spectrum
and colours, and allows the comparison of models with
observational data. Observations are obtained through
rectangular slits or concentric circular apertures. They
can produce projected (on the disk image of a galaxy)
radial profiles of photometric and spectral indices.
Information on the dynamical evolution of the stars in
the various populations is not available in the literature
at the levels needed to achieve an adequate comparison
with observations, because of limits in the theoretical
approach and difficulties in the numerical treatment. In
order to reach a satisfactory spectro-photometric synthesis
and compare the computed surface radial gradients
with the observed profiles it is essential to account for the
different galactic locations of the stars formed at different
ages and with different metallicities and, hence, with
differences in colours and spectra, Indeed, these data are
produced by the cumulative contributions of all the stars
of the galaxy located along the line of sight intersecting
the disk at any specified projected radial distance.
Angeletti and Giannone (2003, 2008, 2010a, 2010b)
bypassed the lack of information on the stellar dynamics
by modelling the spatial radial distribution of the
galactic mass, as deduced from the observed surface
brightness, and the stellar metallicity, as derived from
the central progressive concentration of the star forming
gas. The strategy consisted in relating the stellar metal
abundances to the stellar binding energies and angular
momenta, in the scheme of a dissipative contraction of
the proto-galactic gas accompanied by the simultaneous
formation of stars with the metal abundances equal to
that of the ambient gas at the time of the star formation.
The approach combined a set of different schemes,
in particular the ”Concentration model” (CM) and the
”Best Accretion Model” (BAM) by Lynden-Bell (1975),
and the ”Simple model” (SM) by Pagel and Patchett
(1975). Models with a large set of choices for the free
parameters (the exponent of the R 1/n law for the radial
surface brightness, the concentration index c, the
metal yield p , and the amount of the accreted mass
M) have been computed. The results have been then
compared with the observations of four spectroscopic indices
(Mg 1 , Mg 2 , < F e >, Hβ) by Davies et al. (1993)
and two photometric indices (B − R C ) and (U − R C ) by
Peletier et al. (1990) for a sample of eleven galaxies.
Figure 1: The distributions of the stellar metal abundances
for the elliptical galaxy NGC 4278 from the CM+BAM (with
n = 4, c = 0.70, p = 1.0Z sun, M = 3) along the lines of sight
through the projected radii Re B , 0.5Re B , and 0.1Re B , from left
to right (solid curves). The dashed curve gives the distribution
within the circular aperture with projected radius 0.5Re B ,
and the lowest solid curve the distribution integrated on the
whole galaxy. The numbers of the stars are normalized to
1. In the inset, the model radial profile of Mg 2 is compared
to the observations (dots; those on the right of the cross are
unaffected by the seeing). The radius R is in unit of the
effective radius Re
B = 32 ′′ .9 in the Johnson B band.
The inset in Figure 1 shows how the observational
data for index Mg 2 for the elliptical galaxy NGC 4278
are fitted by the model. In the figure the spatial (in the
galaxy) radial distribution of the metal abundances Z
is plotted for three surface radial distances, a circular
aperture concentric to the galaxy image, and the
whole galaxy. The best agreement of the models with
the observational data of the studied galaxy sample
indicates that the degrees of dissipation vary from
moderate to large, the mean stellar metallicities range
from the solar value to significantly oversolar values,
the masses of the accreted matter can be relevant, the
dispersions of velocities are isotropic in the majority
of the selected galaxies, and their ages are not in
disagreement with the age of 13 billion years. This is
the age of the oldest globular clusters in our galaxy (estimated
to be the lowest limit to the age of the Universe).
References
1. L. Angeletti, et al., A.I.P. Conf.Proc. 1059, 103 (2008).
Authors
L. Angeletti, P. Giannone
Sapienza Università di Roma 148 Dipartimento di Fisica