452 research outputs found
Modelling galaxy spectra in presence of interstellar dust. I. The model of ISM and the library of dusty SSPs
In this paper, the first of a series of two devoted to modelling the spectra
of galaxies of different morphological type in presence of dust, we present our
description of the dust both in the diffuse ISM and the molecular clouds. Our
model for the dust takes into account three components, i.e. graphite,
silicates and polycyclic aromatic hydrocarbons. We consider and adapt to our
aims two prescriptions for the size distribution of the dust grains and two
models for the emission of the dusty ISM. We cross-check the emission and
extinction models of the ISM by calculating the extinction curves and the
emission for the typical environments of the Milky Way and the Large and Small
Magellanic Clouds and by comparing the results with the observational data. The
final model we have adopted is an hybrid one which stems from combining the
analysis of Guhathakurta et al (1989) for the emission of graphite and
silicates and Puget et al. (1985) for the PAH emission, and using the
distribution law of Weingartner et al. (2001a) and the ionization model for
PAHs of Weingartner et al. (2001b). We apply the model to calculate the SED of
SSPs of different age and chemical composition, which may be severely affected
by dust in young, massive stars while they are still embedded in their parental
MCs. We use the "Ray Tracing" method to solve the problem of radiative transfer
and to calculate libraries of SSP SEDs. Particular care is paid to model the
contribution from PAHs, introducing different abundances of C in the population
of very small carbonaceous grains (VSGs) and different ionization states in
PAHs. The SEDs of young SSPs are then compared with observational data of star
forming regions of four local galaxies successfully reproducing their SEDs from
the UV-optical regions to the mid and far infrared.Comment: 24 pages, 29 figure
New response functions for absorption-line indices from high-resolution spectra
Basing on the huge library of 1-A resolution spectra calculated by Munari et
al. over a large range of logT, log g, [Fe/H] and both for solar and a-enhanced
abundance ratios [a/Fe], we present theoretical absorption-line indices on the
Lick system. First we derive the so-called response functions (RFs) of Tripicco
& Bell for a wide range of logT, log g, [Fe/H] and [a/Fe]=+0.4 dex. The RFs are
commonly used to correct indices with solar [a/Fe] ratios to indices with
[a/Fe]>0. Not only the RFs vary with the type of star but also with the
metallicity. Secondly, with the aid of this and the fitting functions (FFs) of
Worthey et al., we derive the indices for single stellar populations and
compare them with those obtained by previous authors, e.g. Tantalo & Chiosi.
The new RFs not only supersede the old ones by Tripicco & Bell, but also show
that Hb increases with the degree of enhancement in agreement with the results
by Tantalo & Chiosi. The new indices for single stellar populations are used to
derive with aid of the recursive Minimum Distance method the age, metallicity
and degree of enhancement of a sample of Galactic Globular Clusters for which
these key parameters have been independently derived from the Colour-Magnitude
Diagram and/or spectroscopic studies. The agreement is remarkably good.Comment: 18 pages, to appear in Astronomy & Astrophysic
Model color-magnitude diagrams for Hubble Space Telescope observations of Local Group dwarf galaxies
In this paper, we discuss a method to conduct a quantitative study of the
star formation history (SFH) of Local Group (LG) galaxies using (HST) data.
This method has proven to be successful in the analysis of the SFH of the same
kind of galaxies using ground-based observations. It is based on the comparison
of observed CMDs with a set of model CMDs. The latter are computed assuming
different evolutionary scenarios, and include a detailed simulation of
observational effects. HST CMDs are ~3 mags deeper than typical ground-based
CMDs, allowing the observation, for all LG galaxies, of a part of the CMD that
up till now had remained accessible only for the very nearest galaxies. A very
important feature that will become accessible is the HB+red-clump. The
distribution of stars along this structure is quite sensitive to age and
metallicity and should provide a very important improvement in the time
resolution of the SFH for stars older than ~2-3 Gyr. We show and discuss four
model CMDs which would be comparable with CMDs from deep HST observations.
These model CMDs represent the following evolutionary scenarios corresponding
to a wide range of dwarf galaxy sub-types from dI to dE: A) a constant SFR from
15Gyr ago to the present time; B) as A), but with the SFR stopped 0.5 Gyr ago;
C) a constant SFR in the age range 10-9Gyr and D) as C) but in the age range
15-12 Gyr. In all four cases a range of metallicity from Z=0.0001 to Z=0.004
has been assumed. The present analysis is just a first qualitative approach to
what one may expect to find in the CMDs of LG galaxies. However a complete set
of model CMDs must be computed to analize the data for each galaxy, using the
crowding effects derived for that particular galaxy.Comment: 2 fi
A chemical evolution model for galaxy clusters
We develop a toy-model for the chemical evolution of the intracluster medium,
polluted by the galactic winds from elliptical galaxies. The model follows the
"galaxy formation history" of cluster galaxies, constrained by the observed
luminosity function.Comment: 6 pages, 10 figures, to appear in the Proceedings of the Workshop
"Chemical Enrichment of Intracluster and Intergalactic Medium", Vulcano,
Italy, 14-18 May 2001, ASP Conference Serie
Spectro-photometric Evolution of Elliptical Galaxies. II. Models with infall
In this paper we present new chemo-spectro-photometric models of elliptical
galaxies in which infall of primordial gas is allowed to occur. They aim to
simulate the collapse of a galaxy made of two components, i.e. luminous
material and dark matter. The mass of the dark component is assumed to be
constant in time, whereas that of the luminous material is supposed to accrete
at a suitable rate. They also include the effect of galactic winds powered by
supernova explosions and stellar winds from massive, early-type stars. The
models are constrained to match a number of properties of elliptical galaxies,
i.e. the slope and mean colours of the colour-magnitude relation (CMR), V
versus (V--K), the UV excess as measured by the colour (1550--V) together with
the overall shape of the integrated spectral energy distribution (ISED) in the
ultraviolet, the relation between the Mg2 index and (1550--V), the mass to blue
luminosity ratio M/Lb as a function of the B luminosity, and finally the
broad-band colours (U--B), (B--V), (V--I), (V--K), etc.Comment: pages 22, 20 postscript figures, 2 external table (tab2_infall using
supertabular.sty and tab5_infall using supertabular1.sty
Star Formation History of Early-Type Galaxies in Low Density Environments V. Blue line-strength indices for the nuclear region
We analyze the star formation properties of a sample of 21 shell galaxies and
30 early-type galaxies members of interacting pairs, located in low density
environments (Longhetti et al 1998a, 1998b).
The study is based on new models developed to interpret the information
coming from `blue' H/FeI, H+K(CaII) and \D4000 line-strength indices
proposed by Rose (1984; 1985) and Hamilton (1985).
We find that the last star forming event that occurred in the nuclear region
of shell galaxies is statistically old (from 0.1 up to several Gyr) with
respect to the corresponding one in the sub-sample of pair galaxies (<0.1 Gyr
or even ongoing star formation).
If the stellar activity is somehow related to the formation of shells, as
predicted by several dynamical models of galaxy interaction, shells have to be
considered long lasting structures.
Since pair members show evidence of very recent star formation, we suggest
that either large reservoirs of gas have to be present to maintain active star
formation, if these galaxies are on periodic orbits, or most of the pair
members in the present sample are experiencing unbound encounters.Comment: 12 pages, including 7 figures - Accepted for publication in A&
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