178 research outputs found
Evolutionary synthesis models for the formation of S0 galaxies in clusters
No full textRich galaxy clusters in the local Universe show a large population of S0 galaxies
(~40% of all luminous galaxies). With increasing redshift the fraction of this S0
galaxy population is observed to strongly decrease (e.g. by a factor ~ to
) in favor of the spiral galaxy fraction while the number of bright ellipticals
does not seem to change. The infalling field galaxy population that successively builds
up the cluster also is spiral rich and S0 poor. It has hence been suspected that galaxy
transformation processes, either due to galaxy – galaxy or to galaxy – ICM
interactions, are responsible for this change. Complementing dynamical and morphological
studies, we use evolutionary synthesis models describing various possible effects of
those interactions on the star formation rates of the infalling spirals. We study the
effects of starbursts of various strengths as well as of the truncation of star formation
on the color and luminosity evolution of model galaxies of various spectral types.
Comparison with observed properties of the local S0 galaxy population is used to
constrain possible S0 formation mechanisms. We find that star formation truncation
in spiral galaxies earlier than Sd-type, if occurring not too long ago, as well as
starbursts more than 3 Gyr ago and followed by complete star formation extinction in
spirals – again earlier than Sd– may well account for the observed average S0
luminosities and colors. Late-type galaxies (Sd), even after a strong burst, remain
either too blue or too faint. Our results are in agreement with studies of spectral
features of cluster S0s but allow for stronger constraints
Decomposition and nanocrystallization in reactively sputtered amorphous Ta-Si-N thin films
No full textThe nanocrystallization process of reactively sputtered thin amorphous Ta-Si-N films is investigated by anomalous small angle x-ray scattering (ASAXS) and x-ray diffraction (XRD). Changes in the microstructure in Ta40Si14N46 films, density variations in the amorphous matrix, decomposition, formation, and growth of nanocrystals after vacuum anneals at different temperatures in the range between 800 and 1000 degreesC are observed and the results of the different techniques are compared. From a Fourier analysis of ASAXS intensities the nanostructure of the investigated ternary system is derived using a model of hard spheres according to Guinier and Fournet. ASAXS investigations indicate that the noncrystalline samples can be described by a monophase fit and the crystallized samples by a bimodal-phase fit, the latter results being consistent with XRD which identifies TaN and Ta5Si3 phases. Detailed analysis shows that TaN nanograins of approximately 2 nm size develop after a decomposition process. Larger grains of Ta5Si3 are observed in addition to the TaN grains if annealing is performed at temperatures higher than 950 degreesC. The aim of these investigations is to give a generally applicable explanation of the barrier failure mechanism for Ta-Si-N diffusion barriers, which is actually observed at temperatures below the crystallization temperature if the films are used in contact with Cu or Al. (C) 2001 American Institute of Physics
Nanocrystallization of amorphous-Ta40Si14N46 diffusion barrier thin films
No full textThe nanocrystallization process in thin amorphous-Ta40Si14N46 films, annealed in the range between 800 and 1000 degreesC for 1 h, is investigated by high-resolution transmission electron microscopy and high-angle annular dark-field and energy-dispersive x-ray analyses. At 800 degreesC clusters of about 2 nm in size indicate that compositional inhomogeneities have developed while the film has still remained structurally amorphous. The sample annealed at 900 degreesC contains a high density of nanograins of TaN measuring about 2 nm as well as amorphous structures measuring 75-100 nm having a high tantalum content. After annealing at 1000 degreesC, an almost entirely crystalline structure is observed with 4-nm-sized particles of cubic TaN and 15-nm-sized grains of Ta5Si3. Possible mechanisms driving these structural changes are discussed. (C) 2001 American Institute of Physics
The Impact of Starbursts and Post-Starbursts on the Photometric Evolution of High Redshift Galaxies
No full textWe present evolutionary synthesis models for galaxies of spectral types Sa
through Sd with starbursts of various strengths triggered at various redshifts
and study their photometric evolution before, during, and after their bursts in
a cosmological context. We find that bursts at high redshift, even very strong
ones, only cause a small blueing of their intrinsically blue young parent
galaxies. At lower redshift, even small bursts cause a significant blueing of
their intrinsically redder galaxies. While the burst phase is generally short,
typically a few hundred Myr in normal-mass galaxies, the postburst stage with
its red colors and, in particular the very red ones for early bursts at high
redshift, is much longer, of the order of several Gyr. Even without any dust,
which in the postburst stage is not expected to play an important role anyway,
models easily reach the colors of EROs in the redshift range z=2 through z=0.5
after starbursts at redshifts between 2 and 4. We therefore propose a third
alternative for the ERO galaxies beyond the two established ones of passive
galaxies vs. dusty starbursts: the dust-free post-(strong-)starbursts. A very
first comparison of our models to HDF data with photometric redshifts shows
that almost all of the outliers that could not be described with our chemically
consistent models for undisturbed normal galaxy types E through Sd can now be
explained very well. Galaxies in the redshift range from z=2.5 to z=0.5 that
are redder, and in some cases much redder, than our reddest undisturbed model
for a high-metallicity classical elliptical are well described by
post-starburst models after starbursts at redshifts between 2 and 4. Galaxies
bluer than our bluest low metallicity Sd model, most of which have redshifts
lower than 1, are well explained by ongoing starbursts.Comment: 12 pages, accepted by Astron. & Astrophysics Nov. 17, 200
Metallicity dependent calibrations of flux based SFR tracers
No full textWe present new calibrations of the widely used , , and UV luminosity vs. star
formation rate (SFR) relations. Using our evolutionary synthesis code GALEV we
compute the different calibrations for 5 metallicities, from 1/50 solar up to 2.5 solar. We find significant changes in the calibrations for lower
metallicities compared to the standard calibrations using solar input physics.
Chemically Consistent Evolutionary Synthesis
No full textTo account for the range of stellar metallicities in local galaxies and for the increasing importance of low metallicities at higher redshift we present chemically consistent models for the spectral and chemical evolution of galaxies over cosmological timescales. We discuss advantages, limitations and future prospects of our approach
Chemically consistent evolution of galaxies II. Spectrophotometric evolution from zero to high redshift
No full textThe composite stellar populations of galaxies comprise stars of a
wide range of metallicities. Subsolar metallicities become increasingly
important, both in the local universe when going from early towards later galaxy
types as well as for dwarf galaxies and for all types of galaxies towards higher
redshifts.
We present a new generation of chemically consistent evolutionary synthesis models
for galaxies of various spectral types from E through Sd. The models follow the
chemical enrichment of the ISM and take into account the increasing initial
metallicity of successive stellar generations using recently published metallicity
dependent stellar evolutionary isochrones, spectra and yields.
Our first set of closed-box 1-zone models does not include any spatial resolution or
dynamics. For a Salpeter initial mass function (IMF) the star formation rate
(SFR) and its time evolution are shown to successfully parameterise spectral
galaxy types E, ..., Sd. We show how the stellar metallicity distribution in various
galaxy types build up with time to yield after ~12 Gyr agreement with stellar
metallicity distributions observed in our and other local galaxies.
The models give integrated galaxy spectra over a wide wavelength
range (90.9 Å–160 μm), which for ages of ~12 Gyr are in good
agreement not only with observed broad band colours but also with template
spectra for the respective galaxy types.
Using filter functions for Johnson-Cousins , IC, as well as
for HST broad band filters in the optical and Bessel & Brett's NIR J, H, K filter
system, we calculate the luminosity and colour evolution of model galaxies over a
Hubble time.
Including a standard cosmological model () and the
attenuation by intergalactic hydrogen we present evolutionary and cosmological
corrections as well as apparent luminosities in various filters over the redshift
range from to the present for our galaxy types and compare to earlier
models using single (=solar) metallicity input physics only. We also resent a first
comparison of our cc models to HDF data. A more detailed comparison with Hubble
Deep Field (HDF) and other deep field data and an analysis and interpretation of high
redshift galaxies in terms of ages, metallicities, star formation histories and,
galaxy types will be the subject of a forthcoming paper
Application of β-D-glucuronides and glucose together suggests a new direction for cancer chemotherapy
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