Life's stationsPAVEL KROUPA
Life's stations
Research Interests:Dynamical properties of stellar systems (IMF, multiplicity), evolution of young multiple stellar systems in birth aggregates, star formation, dynamical evolution of open and globular clusters, spatial and kinematical distribution of stars, origin of field stars, structure and mass of the Galaxy, galactic dynamics, formation and evolution of dwarf satellite galaxies, dark matter content of galaxies.
Pavel Kroupa has been awarded in total grants worth of about 9
million Euro and is currently leading the stellar populations and
dynamics research (SPODYR) group at the University of Bonn in Germany
and the Charles University in Prague. He was director of the
Argelander-Institute of Astronomy in Bonn for one year. Two of his
students were awarded a Hubble Fellowship.
He has been actively contributing to the research field of the initial
mass function (IMF) of stars with the Kroupa et al. (1993) and Kroupa
(2001) publications receiving together more than 7843 citations. His
work introduced the correct treatment of the stellar mass-luminosity
relation, corrections for unresolved multiple systems and a Galactic
field model to analyse the observed star counts, and uniquely unified
the local trigonometric-parallax based star counts with the deeper
photometric-parallax based star counts thereby achieving a robust
measure of the IMF of stars representative of the local Galactic field
stellar population. This is referred to as the canonical IMF, a
two-part-power-law form. This research uncovered the true association
of brown dwarfs to stars (Kroupa & Bouvier 2003; Thies et al. 2015)
and, together with his to-be PhD student Carsten Weidner, he
introduced the IGIMF Theory in 2003 (Kroupa & Weidner 2003). The IGIMF
theory allows the calculation of the galaxy-wide stellar IMF, the gIMF
(also sometimes written gwIMF), showing this integrated galactic
initial mass function (IGIMF) to differ from the stellar IMF. The
IGIMF Theory immediately solved a number of outstanding problems: the
IMF for massive stars in the Solar neighbourhood is much steeper
(lacks massive stars) than the canonical stellar IMF (Kroupa & Weidner
2003), star-forming dwarf galaxies lack massive stars thus being
Halpha dimm for their UV luminosity (Pflamm-Altenburg et al. 2009),
star-forming galaxies have extended UV disks with shorter Halpha
radial cutoffs (Pflamm-Altenburg et al. 2008), star forming low mass
dwarf galaxies have very similar star formation efficiencies as
massive disk galaxies (Pflamm-Altenburg & Kroupa 2010), and low-mass
galaxies lack massive stars thus explaining the mass-metallicity
relation of galaxies (Koeppen et al. 2007; Recchi et al. 2009; Yan et
al. 2021). His research led to the suggestion that the stellar IMF
(sIMF, i.e. the IMF of all stars formed together in one molecular
cloud clump, i.e. as an embedded cluster) is an optimally sampled
distribution function rather than a probability density distribution
function (Kroupa et al. 2013).
Starting in 2009, Pavel Kroupa began, with his PhD students, a
systematic research project at the University of Bonn in order to
constrain the possible systematic variation of the stellar IMF (sIMF)
with the physical conditions of the star-forming gas. By studying in
detail the dynamical properties of ultra-compact dwarf galaxies
(Dabringhausen et al. 2009) and their X-ray emitting stellar
population (Dabringhausen et al. 2012) in combination with a detailed
stellar-dynamical analysis of globular star clusters (Marks et
al. 2012) a very clear systematic dependency of the stellar IMF on the
metallicity and density of the molecular cloud clump in which the
embedded cluster formed crystallises out. This dependency,
sIMF=sIMF(rho, Z), is robust because the three analysis methods and
data samples differ entirely but yield a consistent result.
Incorporating this into the IGIMF Theory now provides the only
existing realistic computational method linking stellar populations
born in molecular cloud clumps on the pc scale to those in a whole
galaxy and those at very high redshift relevant for JWST observations
(Jerabkova et al. 2018; Haslbauer et al. 2024). The rapid formation of
elliptical galaxies and classical bulges and their immensely rapid
chemical enrichment can only be understood within the framework of the
IGIMF Theory (Yan, Jerabkova, Kroupa 2021). Elliptical galaxies were
up to 5 orders of magnitude brighter than today when they were forming
(Zonoozi et al. 2025), implying that massive elliptical galaxies
mostly consist of neutron stars and stellar-mass black holes, with
important implications for fundamental cosmological problems (Gjergo &
Kroupa, in prep). Combining the IGIMF Theory together with basic
physics, the theory of general relativity and the rapid formation
times of elliptical galaxies and bulges beautifully explains how
super-massive black holes form rapidly and why quasars appear very
early in the Universe (Kroupa et al. 2020).
A chapter for the Encyclopedia of
Astrophysics on this matter is available in Kroupa, Gjergo et al. (2024).
His research on star cluster dynamics pioneered the treatment of an
initial 100% fraction in them. Frequent discussions with Sverre
Aarseth led to the improvement of the Nbody4,5,6 codes. This work
unified the local Galactic-field population that has a binary fraction of
about 50 per cent with the about 1 Myr old stellar populations seen in
nearby star forming regions that have a binary fraction near 100 per
cent, and it led to the deduction (Kroupa 1995a,b) that the vast
majority, if not all, of stars form in binary systems (with a few triples
triples and quadruples) and in embedded clusters. This allowed the
conclusion that embedded star clusters (i.e. star formation in
molecular cloud clumps) are the fundamental building blocks of
galaxies. Therewith is is clear that the encounters between forming
stars in the dense embedded clusters sometimes/often lead to
misaligned and even counter-rotating hot planets and jumbled-up
planetary systems (Thies et al. 2005, 2010, 2011). Galactic thick
disks can arise from the gas-expulsion process from embedded clusters
(Kroupa 2002) leading to the realisation that the Milky Way will have
looked like a chain galaxy some 10 Gyr ago (Zonoozi et al. 2019).
Pavel Kroupa's research was also focussed on the initial properties of
binary stars, deriving their birth distribution functions (of initial
periods, mass ratios and eccentricities, Kroupa 1995a,b) that now form
the basis for binary-star population synthesis studies. Knowledge of
these distribution function allowed the realistic quantification of OB
stellar ejections from very young clusters (Oh & Kroupsa 2016), and to
a new assessment of Cepheid populations (Dinnbier et al. 2024) and of the
frequency of stellar mergers (Dvorakova et al. 2024). Applying the
same concepts underlying the IGIMF theory, it has become possible to
predict the full binary star populations in dwarf star forming and
massive quenched galaxies (Marks & Kroupa 2011). This work also lead
to major discoveries on the properties of tidal tails of star clusters
(Kroupa et al. 2022, 2024), therewith pioneering tests of
gravitational theory on open star-cluster scales. The asymmetry of
tidal tails of open star clusters discovered with this research
demonstrates Milgromian dynamics (i.e. MOND) to be the relevant
theoretical framework for understanding the equations of motion of
stars in gravitational fields.
Pavel Kroupa published the very first proper motion measurements of
the Magellanic Clouds (Kroupa et al. 1994; Kroupa & Bastian 1997), and
he pioneered the satellite-galaxy-plane problem by pointing out that
the 11 classical (and the later-discovered) satellite galaxies of the
Milky Way form a disk which is incompatible with the predictions of
the standard LCDM cosmological model (Kroupa et al. 2005; Pawlowski et
al. 2012; Pawlowski & Kroupa 2020). His work led to the discovery that
the entire Local Group forms a highly symmetrical structure that is
incompatible with predictions of the standard LCDM cosmological model
(Pawlowski et al. 2013).
Therewith, Pavel Kroupa's research became involved with critically
testing cosmological models. He introduced the Chandrasekhar
dynamical friction test for the existence of dark matter (Kroupa 2015)
and applied it to the relative motion of the Small and Large
Magellanic Clouds as they fall past the Milky Way (Oehm & Kroupa
2024). This work proves that dark matter is non existent, verifying
the conclusions already reached by the galaxy-bar test (Roshan et
al. 2021) and the dwarf-galaxies-of-the-Fornax-galaxy-cluster test
(Asencio et al. 2022). Further tests of cosmological models involve
the El Gordo Galaxy cluster test (Asencio et al. 2021, 2023) and the
KBC void test (Haslbauer et al. 2020). Interestingly, MOND-based
cosmological modelling solves these LCDM-failures readily.
The research of Pavel Kroupa demonstrated that once the correct
(i.e. Milgromian) equations of motion are used, correct answers pop up
immediately: the fact that galaxy disks have exponential profiles is a
consequence of galaxy formation in MOND (Wittenburg et al. 2020), the
downsizing timescales of elliptical galaxies follow naturally from the
monolithic collapse of pre-galactic gas clouds in the early Universe
(Eappen et al. 2022), the planes of satellites arise naturally because
galaxies interact with each other (and merge rarely as there are no
extended dark matter halos such that Chandrasekhar dynamical friction
does not act in them) forming tidal tails in which condense dwarf
galaxies that later make the disks of satellites (Bilek et
al. 2021). Thus, the Milky Way and Andromeda had an encounter some 10
Gyr ago forming the satellite planes around both, the warped disk of
the Milky Way and its thick disk (Bilek et al. 2018; Banik et al. 2022). The
rapid formation of super massive black holes (SMBHs) and their
correlations with their host galaxy properties arise naturally as well
(Kroupa et al. 2020).
Pavel Kroupa organised and funded the creation of the Phantom of Ramses (PoR)
code (Lueghausen et al. 2015; Nagesh et al. 2021; Wittenburg et
al. 2023) used for galaxy simulations and for cosmological structure
formation simulations in MOND-based cosmologies. Innovative
cosmological models are being studied now in his research group, and
these lead to natural solutions to the KBC void and the Hubble tension
as well as the observed non-LCDM galaxy bulk flows (Haslbauer et
al. 2020; Mazurenko et al. 2024). The Bohemian Model of Cosmology is a
particularly interesting possible solution to the observed
cosmological and galaxy phenomena. A summary can be found in
Kroupa, Gjergo et al. (2023).
Career Synopsis:Background:
University Education and Employment: