101 research outputs found
Formation of Globular Clusters in Hierarchical Cosmology: ART and Science
We test the hypothesis that globular clusters form in supergiant molecular
clouds within high-redshift galaxies. Numerical simulations demonstrate that
such large, dense, and cold gas clouds assemble naturally in current
hierarchical models of galaxy formation. These clouds are enriched with heavy
elements from earlier stars and could produce star clusters in a similar way to
nearby molecular clouds. The masses and sizes of the model clusters are in
excellent agreement with the observations of young massive clusters. Do these
model clusters evolve into globular clusters that we see in our and external
galaxies? In order to study their dynamical evolution, we calculate the orbits
of model clusters using the outputs of the cosmological simulation of a Milky
Way-sized galaxy. We find that at present the orbits are isotropic in the inner
50 kpc of the Galaxy and preferentially radial at larger distances. All
clusters located outside 10 kpc from the center formed in the now-disrupted
satellite galaxies. The spatial distribution of model clusters is spheroidal,
with a power-law density profile consistent with observations. The combination
of two-body scattering, tidal shocks, and stellar evolution results in the
evolution of the cluster mass function from an initial power law to the
observed log-normal distribution. However, not all initial conditions and not
all evolution scenarios are consistent with the observed mass function.Comment: 8 pages, invited review for conference "Globular Clusters, Guide to
Galaxies", 6-10 March 2006, University of Concepcion, Chile, ed. T. Richtler,
et a
Spurious heating of stellar motions in simulated galactic discs by dark matter halo particles
Galaxie
On the distribution of initial masses of stellar clusters inferred from synthesis models
The fundamental properties of stellar clusters, such as the age or the total
initial mass in stars, are often inferred from population synthesis models. The
predicted properties are then used to constrain the physical mechanisms
involved in the formation of such clusters in a variety of environments.
Population synthesis models cannot, however, be applied blindy to such systems.
We show that synthesis models cannot be used in the usual straightforward way
to small-mass clusters (say, M < few times 10**4 Mo). The reason is that the
basic hypothesis underlying population synthesis (a fixed proportionality
between the number of stars in the different evolutionary phases) is not
fulfilled in these clusters due to their small number of stars. This incomplete
sampling of the stellar mass function results in a non-gaussian distribution of
the mass-luminosity ratio for clusters that share the same evolutionary
conditions (age, metallicity and initial stellar mass distribution function).
We review some tests that can be carried out a priori to check whether a given
cluster can be analysed with the fully-sampled standard population synthesis
models, or, on the contrary, a probabilistic framework must be used. This leads
to a re-assessment in the estimation of the low-mass tail in the distribution
function of initial masses of stellar clusters.Comment: 5 pages, 1 figure, to appear in ``Young Massive Star Clusters -
Initial Conditions and Environments'', 2008, Astrophysics & Space Science,
eds. E. Perez, R. de Grijs, R. M. Gonzalez Delgad
Super star clusters and Supernovae in interacting LIRGs unmasked by NIR adaptive optics
We report on an on-going near-IR adaptive optics survey targeting interacting
luminous IR galaxies. High-spatial resolution NIR data are crucial to enable
interpretation of kinematic, dynamical and star formation (SF) properties of
these very dusty objects. Whole progenitor nuclei in the interactions can be
missed if only optical HST imaging is used. Here we specifically present the
latest results regarding core-collapse supernovae found within the highly
extincted nuclear regions of these galaxies. Direct detection and study of such
highly obscured CCSNe is crucial for revising the optically-derived SN rates
used for providing an independent measurement of the SF history of the
Universe. We also present thus-far the first NIR luminosity functions of super
star cluster (SSC) candidates. The LFs can then be used to constrain the
formation and evolution of SSCs via constraints based on initial mass functions
and cluster disruption models.Comment: 6 pages. To appear in proceedings of 'Galaxies and their Masks'
(Namibia, April 2010), published by Springer, New York, eds. D.L. Block, K.C.
Freeman, I. Puerar
Star and cluster formation in extreme environments
Current empirical evidence on the star-formation processes in the extreme,
high-pressure environments induced by galaxy encounters (mostly based on
high-resolution Hubble Space Telescope observations) strongly suggests that
star CLUSTER formation is an important and perhaps even the dominant mode of
star formation in such starburst events. The sizes, luminosities, and mass
estimates of the young massive star clusters (YMCs) are entirely consistent
with what is expected for young Milky Way-type globular clusters (GCs). Recent
evidence lends support to the scenario that GCs, which were once thought to be
the oldest building blocks of galaxies, are still forming today. Here, I
present a novel empirical approach to assess the shape of the
initial-to-current YMC mass functions, and hence their possible survival
chances for a Hubble time.Comment: 6 pages, LaTeX with Kluwer style files included; to appear in:
"Starbursts - from 30 Doradus to Lyman break galaxies" (Cambridge UK,
September 2004; talk summary), Astrophysics & Space Science Library, eds. de
Grijs R., Gonzalez Delgado R.M., Kluwer: Dordrech
Alignment of galaxy spins in the vicinity of voids
We provide limits on the alignment of galaxy orientations with the direction
to the void center for galaxies lying near the edges of voids. We locate
spherical voids in volume limited samples of galaxies from the Sloan Digital
Sky Survey using the HB inspired void finder and investigate the orientation of
(color selected) spiral galaxies that are nearly edge-on or face-on. In
contrast with previous literature, we find no statistical evidence for
departure from random orientations. Expressed in terms of the parameter c,
introduced by Lee & Pen to describe the strength of such an alignment, we find
that c<0.11(0.13) at 95% (99.7%) confidence limit within a context of a toy
model that assumes a perfectly spherical voids with sharp boundaries.Comment: 8 pages, 4 figures; v2 discussion expanded, references fixed, matches
version accepted by JCA
The Impact of Stellar Migration on Disk Outskirts
Stellar migration, whether due to trapping by transient spirals (churning),
or to scattering by non-axisymmetric perturbations, has been proposed to
explain the presence of stars in outer disks. After a review of the basic
theory, we present compelling, but not yet conclusive, evidence that churning
has been important in the outer disks of galaxies with type II (down-bending)
profiles, while scattering has produced the outer disks of type III
(up-bending) galaxies. In contrast, field galaxies with type I (pure
exponential) profiles appear to not have experienced substantial migration. We
conclude by suggesting work that would improve our understanding of the origin
of outer disks.Comment: Invited review, Book chapter in "Outskirts of Galaxies", Eds. J. H.
Knapen, J. C. Lee and A. Gil de Paz, Astrophysics and Space Science Library,
Springer, in press 39 pages, 15 figure
Emergence of the OXA-23 carbapenemase-encoding gene in multidrug-resistant Acinetobacter baumannii clinical isolates from the Principal Hospital of Dakar, Senegal
International audienc
The Baltimore and Utrecht models for cluster dissolution
The analysis of the age distributions of star cluster samples of different
galaxies has resulted in two very different empirical models for the
dissolution of star clusters: the Baltimore model and the Utrecht model. I
describe these two models and their differences. The Baltimore model implies
that the dissolution of star clusters is mass independent and that about 90% of
the clusters are destroyed each age dex, up to an age of about a Gyr, after
which point mass-dependent dissolution from two-body relaxation becomes the
dominant mechanism. In the Utrecht model, cluster dissolution occurs in three
stages: (i) mass-independent infant mortality due to the expulsion of gas up to
about 10 Myr; (ii) a phase of slow dynamical evolution with strong evolutionary
fading of the clusters lasting up to about a Gyr; and (iii) a phase dominated
by mass dependent-dissolution, as predicted by dynamical models. I describe the
cluster age distributions for mass-limited and magnitude-limited cluster
samples for both models. I refrain from judging the correctness of these
models.Comment: 3 pages, 1 figure, to appear in "Young Massive Star Clusters -
Initial Conditions and Environment", 2008, Astrophysics and Space Science,
Eds. E. Perez, R. de Grijs and R.M. Gonzalez Delgad
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