297 research outputs found
Dissipative N - body code for galaxy evolution
The evolving galaxy is considered as a system of baryonic fragments embedded
into the static dark nonbaryonic (DH) and baryonic (BH) halo and subjected to
gravitational and viscous interactions. Although the chemical evolution of each
separate fragment is treated in the frame of one -- zone close box model with
instantaneous recycling, its star formation (SF) activity is a function of mean
local gas density and, therefore, is strongly influenced by other interacting
fragments. In spite of its simplicity this model provides a realistic
description of the process of galaxy formation and evolution over the Hubble
timescale.Comment: 11 pages, LaTeX, 7 figures, using the article.sty, expected in
A&ApTr, 18, 83
Initial Mass Function Effects on the Colour Evolution of Disk Galaxies
Aims. In this work, we want to find out if the IMF can be determined from colour images, integrated colours, or mass-to-light ratios, especially at high redshift, where galaxies cannot be resolved into individual stars, which would enable us to investigate dependencies of the IMF on cosmological epoch.
Methods. We use chemo-dynamical models to investigate the influence of the Initial Mass Function (IMF) on the evolution of a Milky Way-type disk galaxy, in particular of its colours.
Results. We find that the effect of the IMF on the internal gas absorption is larger than its effect on the light from the stellar content. However, the two effects work in the opposite sense: An IMF with more high mass stars leads to brighter and bluer star-light, but also to more interstellar dust and thus to more absorption, causing a kind of “IMF degeneracy”. The most likely wavelength region in which to detect IMF effects is the infrared (i.e., JHK). We also provide photometric absorption and inclination corrections in the SDSS ugriz and the HST WFPC2 and NICMOS systems
UV (IUE) spectra of the central stars of high latitude planetary nebulae Hb7 and Sp3
We present an analysis of the UV (IUE) spectra of the central stars of Hb7
and Sp3. Comparison with the IUE spectrum of the standard star HD 93205 leads
to a spectral classification of O3V for these stars, with an effective
temperature of 50,000 K. From the P-Cygni profiles of CIV (1550 A), we derive
stellar wind velocities and mass loss rates of -1317 km/s +/- 300 km/s and
2.9X10^{-8} solar mass yr^{-1} and -1603 km/s +/- 400 km/s and 7X10^{-9} solar
mass yr^{-1} for Hb7 and Sp3 respectively. From all the available data, we
reconstruct the spectral energy distribution of Hb7 and Sp3.Comment: 4 pages, 3 figures, latex, accepted for publication in Astronomy &
Astrophysic
Gas Physics, Disk Fragmentation, and Bulge Formation in Young Galaxies
We investigate the evolution of star-forming gas-rich disks, using a 3D
chemodynamical model including a dark halo, stars, and a two-phase interstellar
medium with feedback processes from the stars. We show that galaxy evolution
proceeds along very different routes depending on whether it is the gas disk or
the stellar disk which first becomes unstable, as measured by the respective
Q-parameters. This in turn depends on the uncertain efficiency of energy
dissipation of the cold cloud component from which stars form. When the cold
gas cools efficiently and drives the instability, the galactic disk fragments
and forms a number of massive clumps of stars and gas. The clumps spiral to the
center of the galaxy in a few dynamical times and merge there to form a central
bulge component in a strong starburst. When the kinetic energy of the cold
clouds is dissipated at a lower rate, stars form from the gas in a more
quiescent mode, and an instability only sets in at later times, when the
surface density of the stellar disk has grown sufficiently high. The system
then forms a stellar bar, which channels gas into the center, evolves, and
forms a bulge whose stars are the result of a more extended star formation
history. We investigate the stability of the gas-stellar disks in both regimes,
as well as the star formation rates and element enrichment. We study the
morphology of the evolving disks, calculating spatially resolved colours from
the distribution of stars in age and metallicity, including dust absorption. We
then discuss morphological observations such as clumpy structures and chain
galaxies at high redshift as possible signatures of fragmenting, gas-rich
disks. Finally, we investigate abundance ratio distributions as a means to
distinguish the different scenarios for bulge formation.Comment: 16 pages, Latex, 14 figures, to appear in Astronomy and Astrophysics,
Version with high quality images available at
http://www.astro.unibas.ch/leute/ai.shtm
Physical Processes in Star-Gas Systems
First we present a recently developed 3D chemodynamical code for galaxy
evolution from the K**2 collaboration. It follows the evolution of all
components of a galaxy such as dark matter, stars, molecular clouds and diffuse
interstellar matter (ISM). Dark matter and stars are treated as collisionless
N-body systems. The ISM is numerically described by a smoothed particle
hydrodynamics (SPH) approach for the diffuse (hot) gas and a sticky particle
scheme for the (cool) molecular clouds. Physical processs such as star
formation, stellar death or condensation and evaporation processes of clouds
interacting with the ISM are described locally. An example application of the
model to a star forming dwarf galaxy will be shown for comparison with other
codes. Secondly we will discuss new kinds of exotic chemodynamical processes,
as they occur in dense gas-star systems in galactic nuclei, such as
non-standard ``drag''-force interactions, destructive and gas producing stellar
collisions. Their implementation in 1D dynamical models of galactic nuclei is
presented. Future prospects to generalize these to 3D are work in progress and
will be discussed.Comment: 4 pages, 4 figures, "The 5th Workshop on Galactic Chemodynamics" -
Swinburne University (9-11 July 2003). To be published in the Publications of
the Astronomical Society of Australia in 2004 (B.K. Gibson and D. Kawata,
eds.). Accepted version, minor changes relative to origina
The Formation of a Disk Galaxy within a Growing Dark Halo
We present a dynamical model for the formation and evolution of a massive
disk galaxy, within a growing dark halo whose mass evolves according to
cosmological simulations of structure formation. The galactic evolution is
simulated with a new 3D chemo-dynamical code, including dark matter, stars and
a multi-phase ISM. The simulations start at redshift z=4.85 with a small dark
halo in a LCDM universe and we follow the evolution until the present epoch.
The energy release by massive stars and SNe prevents a rapid collapse of the
baryonic matter and delays the maximum star formation until z=1. The galaxy
forms radially from inside-out and vertically from halo to disk. The first
galactic component that forms is the halo, followed by the bulge, the disk-halo
transition region, and the disk. At z=1, a bar begins to form which later turns
into a triaxial bulge. There is a pronounced deficiency of low-metallicity disk
stars due to pre-enrichment of the disk ISM with metal-rich gas from the bulge
and inner disk (G-dwarf problem). The mean rotation and the distribution of
orbital eccentricities for all stars as a function of metallicity are not very
different from those observed in the solar neighbourhood, showing that
homogeneous collapse models are oversimplified. The approach presented here
provides a detailed description of the formation and evolution of an isolated
disk galaxy in a LCDM universe, yielding new information about the kinematical
and chemical history of the stars and the ISM, but also about the evolution of
the luminosity, the colours and the morphology of disk galaxies.Comment: 23 pages, LaTeX, 18 figures, A&A accepted, a high resolution version
of the paper can be found at http://www.astro.unibas.ch/leute/ms.shtm
Abundances for metal-poor stars with accurate parallaxes II. alpha-elements in the halo
Abundances for alpha-elements and Fe in about 150 field subdwarfs and early
subgiants with accurate parallaxes and kinematic data are used to discuss the
run of abundance ratios in metal-poor stars in the solar neighborhood. Based on
kinematics, we separated stars into two populations: the first one has a
positive velocity of rotation around the galactic center, and it is likely to
be related to the dissipational collapse of the galaxy; the second one has
either negligible or negative rotational velocity, and it is likely related to
an accretion component. The two populations show a large overlap in
metallicity. However, they show distinct chemical properties. For the first
population we found that there are close correlations (with small scatters
around) of the rotational velocity with metallicity and with the Fe/alpha
abundance ratio: this might be a signature of a not very fast collapse of the
progenitor clouds, with enough time for a significant contribution by SNe Ia,
although this result needs to be confirmed by a 3-D/non-LTE study. On the other
side, the second population exhibits a larger scatter in both the above
mentioned relations, and on average, a larger Fe/alpha ratio at a given
metallicity, suggesting a larger scatter in ages. We argue that the lack of
stars with moderate rotational velocities and high Fe/alpha abundance ratios is
due to the short merging time for protogalactic clouds with prograde motion,
while the presence of a group of counter-rotating stars with this
characteristics indicates a much longer typical lifetimes for protogalactic
fragments having such a motion. Finally, we found that perigalactic distances
correlate with the Fe/alpha abundance ratios better than the apogalactic
distances.Comment: 10 pages, 6 encapsulated figures, accepted for publication in A&
NLTE determination of the sodium abundance in a homogeneous sample of extremely metal-poor stars
Abundance ratios in extremely metal-poor (EMP) stars are a good indication of
the chemical composition of the gas in the earliest phases of the Galaxy
evolution. It had been found from an LTE analysis that at low metallicity, and
in contrast with most of the other elements, the scatter of [Na/Fe] versus
[Fe/H] was surprisingly large and that, in giants, [Na/Fe] decreased with
metallicity.
Since it is well known that the formation of sodium lines is very sensitive
to non-LTE effects, to firmly establish the behaviour of the sodium abundance
in the early Galaxy, we have used high quality observations of a sample of EMP
stars obtained with UVES at the VLT, and we have taken into account the non-LTE
line formation of sodium.
The profiles of the two resonant sodium D lines (only these sodium lines are
detectable in the spectra of EMP stars) have been computed in a sample of 54
EMP giants and turn-off stars (33 of them with [Fe/H]< -3.0) with a modified
version of the code MULTI, and compared to the observed spectra.
With these new determinations in the range -4 <[Fe/H]< -2.5, both [Na/Fe] and
[Na/Mg] are almost constant with a low scatter. In the turn-off stars and
"unmixed" giants (located in the low RGB): [Na/Fe] = -0.21 +/- 0.13 or [Na/Mg]
= -0.45 +/- 0.16. These values are in good agreement with the recent
determinations of [Na/Fe] and [Na/Mg] in nearby metal-poor stars. Moreover we
confirm that all the sodium-rich stars are "mixed" stars (i.e., giants located
after the bump, which have undergone an extra mixing). None of the turn-off
stars is sodium-rich. As a consequence it is probable that the sodium
enhancement observed in some mixed giants is the result of a deep mixing.Comment: 8 pages, 9 figures; accepted for publication in A&
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