Projects
The following list is intended to give an impression of the manifold of
projects that profit from our unique chemically consistent galaxy
evolutionary synthesis code.
We are always looking for new collaborators. So if you are interested in any
of the projects listed below and considering using GALEV for the analysis, or
have an idea on how GALEV can be used for a project not listed below, we would
be more than happy to hear from you. Just write us an email to contact at
galev.org.
Star Formation, Chemical Enrichment and Mass Assembly Histories of
Galaxies: The Observational Cosmology Approach
R. Kotulla, U. Fritze
In this project we aim at deriving - for the first time
simultaneously -
1. the Star Formation Histories
(SFHs)
2. the Mass Assembly Histories
(MAHs),
and
3. the Chemical Enrichment
Histories (CEHs)
of galaxies from
the highest redshifts to the present time using two complemen-
tary approaches (also see below). In the Observational Cosmology approach we analyse the evolution of
the integrated light of galaxies over a wide range in wavelengths and over
the full redshift range from the earliest epochs to the present.
Most
previous results dealing with the SFHs, MAHs and CEHs of galaxies have been
based on spectroscopic observations. Notwithstanding its merits, the
spectroscopic approach is biased towards the most luminous - certainly
not the typical - galaxies at any redshift, and limited to the actively
star-forming ones which offer emission lines for redshift
determinations. Instead, we use an approach based
on multi-wavelength photometry of
galaxies, analysed with dedicated tools and interpreted by comparing with
results from our chemically consistent evolutionary synthesis code
GALEV.
Star Formation, Chemical Enrichment and Mass Assembly
Histories of
Galaxies: The Astro-Archaeological Approach
R. Kotulla,
U. Fritze
in collaboration with P. Anders, R. de Grijs
In the
Astro-Archaeological approach we use star clusters, their ages
and chemical abundances in local galaxies of various types/masses as fossil
records of their parent galaxy SFH, CEH and
MAH.
The
Astro-Archaeological is
an entirely new one. Its power has only recently been realised, partly thanks
to our involvement. It has become clear that
1. star cluster formation is a major
mode of all active star formation and
the dominant mode in the strongest starbursts, i.e. those
accompanying mergers of gas-rich spirals where, together with short-lived,
low-mass clusters significant populations of long-lived, high-mass clusters
are formed from gas pre-enriched in their parent galaxies, and
that
2. since star clusters can
be analysed one-by-one, after careful background subtraction, the age
distribution of a population of clusters and their chemical abundances yield
direct information about the (violent) star formation and chemical enrichment
history, over cosmological time scales, of their parent galaxy.
Young Star Cluster Systems:
Star Formation vs. Star Cluster
Formation, violent vs normal SF
U. Fritze, P. Anders (Utrecht)
in
collaboration with R. de Grijs, Sheffield; S. Larsen, H. Lamers,
Utrecht;
Rich populations of massive, compact, long-lived star clusters
(
SCs) form in starbursts resulting
from mergers of massive gas-rich galaxies (cf. Schweizer 2002, 2003,
Fritze & Burkert 1995).
A good fraction of those SCs with radii
and masses in the range of those observed for globular cluster
(
GCs) are expected to survive for
many Gyrs and turn into objects similar to GCs (Goudfrooij et al. 2001,
2004, Fritze 2004). These "secondary GCs" will stand out from GCs
formed in the early universe, by their younger ages and enhanced
metallicities since they were formed out of gas which was pre-enriched in
spiral galaxies before a merger event. Provided their ages and
metallicities can be disentangled, they will give testimony of the merger,
of the epoch when it happened, and of the properties of the merging
galaxies.
Galaxy transformation in clusters
Ralf Kotulla, Uta Fritze,
A. Falkenberg (Göttingen);
in collaboration with: P. Woudt,
P. Kotze, Univ. Cape Town; T. Lisker, H. T. Meyer, Univ. Heidelberg;
E. Brinks, T. Scott (UH).
Galaxy populations in the field and in clusters are markedly
different, and yet clusters keep forming and growing by the continuous
infall of field galaxies. The field galaxy population is rich in spirals
like our Milky Way with active star formation (=SF), while the inner parts
of rich galaxy clusters are dominated by S0 and dwarf galaxies without any
gas and SF. A variety of processes have been proposed, and indeed observed
to work in individual cases, that can transform gas-rich SFing field
spirals into gas-poor passive cluster S0s or dwarfs. Some processes are
due to interactions of the infalling spirals with the hot and dense
intracluster gas observed in X-rays in the central regions of rich
clusters (
ram pressure
stripping/sweeping), others have to do with the frequent high-speed
encounters between galaxies (
harassment), or with the increased galaxy merger rate
within infalling groups. All processes occur preferentially at different
distances from the cluster center, probably have different timescales and
transition stages (blue/red E+A/Hdelta-strong galaxies), and, partly,
different end products (S0s, dEs, dSphs).
Pixel-by-pixel analysis of starburst galaxies
R. Kotulla,
U. Fritze
in collaboration with: J. Gallagher, Madison; R. de
Grijs, Sheffield
Pixel-by-pixel analyses of the ACS Early Release data for
the Tadpole and Mice interacting galaxy systems with GALEV models have
revealed the spatial distribution of stars of various ages
and, surprisingly, found star formation (SF) and even star cluster
formation (SCF) all along the ~200 kpc long tidal tails. Other instances
of extragalactic SF are found in deep H-alpha imaging (Ferguson+98), in
the UV with Galex (Thilker+05). In collaboration with J. Gallagher
(Madison) and E. Wehner (Canada) we investigate SF in low surface
brightness structures around galaxies, the extremely low SFR density
regime (Wehner+06).
Tidal Dwarf Galaxies
in collaboration with: P.-A. Duc (Paris);
P. Weilbacher (Potsdam); Elias Brinks (Hertfordshire)
An intriguing feature of strong galaxy
interactions is the formation of so-called Tidal Dwarf Galaxies (TDGs) in
the extended tidal tails thrown out of spiral disks. P. Weilbacher
analysed the first sample of TDGs in his Diploma and PhD theses in
Göttingen. Combining optical and NIR data, he investigated the ratio
of stars formed in situ and stars inherited from the spiral and could, for
the first time, show by means of VLT tilted slit commissioning spectra
that some of these systems already show rotation, i.e. dynamical
decoupling from their surrounding tail, in combination with infall (still
in formation) (Weilbacher+00, 01, 02, 03). The cosmological implications
of this recycling mode of dwarf galaxy formation still going on in the
local universe are being explored in collaboration with P. Weilbacher (now
Potsdam).
QSO Absorption Lines, Attenuation and the Cosmic Baryon
Content
in collaboration with: T. Tepper Garcia, P. Richter,
Potsdam
During his PhD in
Göttingen, T. Tepper Garcia has investigated the stochastic effects
in the attenuation by intergalactic HI, mostly in the form of Lyman-alpha
clouds, on the luminosities and colours of distant galaxies. Using recent
observed distributions for the number densities of clouds as a function of
redshift, their column densities and Doppler parameters, he performed Monte
Carlo realisations of thousands of lines of sight with hundreds to
thousands of Lyman-alpha clouds each. His results for the 1-, 2-, 3-sigma
scatter around the mean attenuation are included in the GALEV models and used
in our interpretation of high-z galaxies.
Cosmo-chemical Evolution of Galaxies
U. Fritze
Several years ago, we have used our chemically consistent GALEV models for
the chemical evolution of galaxies in terms of a large number of individual
elements in comparison to observed abundances of Damped Ly-alpha Absorbers
(DLAs) in the redshift range from z>4 to z~1.5. Our results indicated
that the DLA abundances and their redshift evolution could well be explained
by our spiral galaxy models, i.e. these DLAs could be the high-z progenitors
of present-day spirals (Lindner & Fritze 99). We predicted that
these galaxies should already be very massive at these high redshifts with
at least half of their present mass, albeit largely in the form of gas, and
we also predicted very faint luminosities.
Resolved stellar populations vs. integrated light:
Accuracies and
limitations in the recovery of Star Formation Histories
T. Lilly,
U. Fritze
in collaboration with D. Alloin, ESO Santiago; P. Demarque,
Yale; C. Gallart, IAC; S. Yi, Oxford/Korea;
An important methodological study - that is only
possible with GALEV as the only evolutionary synthesis code that can
describe the evolution of a resolved stellar population in terms of
CMDs
and, at the same time,
the evolution of its integrated light - is the comparison to what
accuracy and how far back in time the SFH of a composite stellar population
can be reconstructed using these different approaches. While CMD analyses
are limited to low stellar density systems in the local group, integrated
light studies are the only approach to SFHs of distant
galaxies.
