86 research outputs found
The spatial distribution of star and cluster formation in M51
Aims. We study the connection between spatially resolved star formation and
young star clusters across the disc of M51. Methods. We combine star cluster
data based on B, V, and I-band Hubble Space Telescope ACS imaging, together
with new WFPC2 U-band photometry to derive ages, masses, and extinctions of
1580 resolved star clusters using SSP models. This data is combined with data
on the spatially resolved star formation rates and gas surface densities, as
well as Halpha and 20cm radio-continuum (RC) emission, which allows us to study
the spatial correlations between star formation and star clusters. Two-point
autocorrelation functions are used to study the clustering of star clusters as
a function of spatial scale and age. Results. We find that the clustering of
star clusters among themselves decreases both with spatial scale and age,
consistent with hierarchical star formation. The slope of the autocorrelation
functions are consistent with projected fractal dimensions in the range of
1.2-1.6, which is similar to other galaxies, therefore suggesting that the
fractal dimension of hierarchical star formation is universal. Both star and
cluster formation peak at a galactocentric radius of 2.5 and 5 kpc, which we
tentatively attribute to the presence of the 4:1 resonance and the co-rotation
radius. The positions of the youngest (<10 Myr) star clusters show the
strongest correlation with the spiral arms, Halpha, and the RC emission, and
these correlations decrease with age. The azimuthal distribution of clusters in
terms of kinematic age away from the spiral arms indicates that the majority of
the clusters formed 5-20 Myr before their parental gas cloud reached the centre
of the spiral arm.Comment: 14 pages, 21 figures, accepted for publication in A&
A peculiar object in M51: fuzzy star cluster or a background galaxy?
Aims: We study a peculiar object with a projected position close to the
nucleus of M51. It is unusually large for a star cluster in M51 and we
therefore investigate the three most likely options to explain this object: (a)
a background galaxy, (b) a cluster in the disk of M51 and (c) a cluster in M51,
but in front of the disk. Methods: We use HST/ACS and HST/NICMOS broad-band
photometry to study the properties of this object. Assuming the object is a
star cluster, we fit the metallicity, age, mass and extinction using simple
stellar population models. Assuming the object is a background galaxy, we
estimate the extinction from the colour of the background around the object. We
study the structural parameters of the object by fitting the spatial profile
with analytical models. Results: We find de-reddened colours of the object
which are bluer than expected for a typical elliptical galaxy, and the central
surface brightness is brighter than the typical surface brightness of a disc
galaxy. It is therefore not likely that the object is a background galaxy.
Assuming the object is a star cluster in the disc of M51, we estimate an age
and mass of 0.7 Gyr and 2.2 x 10^5 \msun, respectively (with the extinction
fixed to E(B-V) = 0.2). Considering the large size of the object, we argue that
in this scenario we observe the cluster just prior to final dissolution. If we
fit for the extinction as a free parameter, a younger age is allowed and the
object is not close to final dissolution. Alternatively, the object could be a
star cluster in M51, but in front of the disc, with an age of 1.4 Gyr and mass
M = 1.7 x 10^5 \msun. Its effective radius is between ~12-25 pc. This makes the
object a "fuzzy star cluster", raising the issue of how an object of this age
would end up outside the disc.Comment: 7 pages, 5 figures. Journal-ref and DOI added. 2 typos corrected.
Added corrections to proof including 1 referenc
The type IIb SN 2008ax: the nature of the progenitor
A source coincident with the position of the type IIb supernova (SN) 2008ax
is identified in pre-explosion Hubble Space Telescope (HST) Wide Field
Planetary Camera 2 observations in three optical filters. We identify and
constrain two possible progenitor systems: (i) a single massive star that lost
most of its hydrogen envelope through radiatively driven mass loss processes,
prior to exploding as a helium-rich Wolf-Rayet star with a residual hydrogen
envelope, and (ii) an interacting binary in a low mass cluster producing a
stripped progenitor. Late time, high resolution observations along with
detailed modelling of the SN will be required to reveal the true nature of this
progenitor star.Comment: 5 pages, 2 figures, resolution of figure 1 reduced, figure 2 revised,
some revision following referee's comments, accepted for publication in MNRAS
letter
Revealing a Ring-like Cluster Complex in a Tidal Tail of the Starburst Galaxy NGC 2146
We report the discovery of a ring-like cluster complex in the starburst
galaxy NGC 2146. The Ruby Ring, so named due to its appearance, shows a clear
ring-like distribution of star clusters around a central object. It is located
in one of the tidal streams which surround the galaxy. NGC 2146 is part of the
Snapshot Hubble U-band Cluster Survey (SHUCS). The WFC3/F336W data has added
critical information to the available archival Hubble Space Telescope imaging
set of NGC 2146, allowing us to determine ages, masses, and extinctions of the
clusters in the Ruby Ring. These properties have then been used to investigate
the formation of this extraordinary system. We find evidence of a spatial and
temporal correlation between the central cluster and the clusters in the ring.
The latter are about 4 Myr younger than the central cluster, which has an age
of 7 Myr. This result is supported by the H alpha emission which is strongly
coincident with the ring, and weaker at the position of the central cluster.
From the derived total H alpha luminosity of the system we constrain the star
formation rate density to be quite high, e.g. ~ 0.47 Msun/yr/kpc^2. The Ruby
Ring is the product of an intense and localised burst of star formation,
similar to the extended cluster complexes observed in M51 and the Antennae, but
more impressive because is quite isolated. The central cluster contains only 5
% of the total stellar mass in the clusters that are determined within the
complex. The ring-like morphology, the age spread, and the mass ratio support a
triggering formation scenario for this complex. We discuss the formation of the
Ruby Ring in a "collect & collapse" framework. The predictions made by this
model agree quite well with the estimated bubble radius and expansion velocity
produced by the feedback from the central cluster, making the Ruby Ring an
interesting case of triggered star formation.Comment: 11 pages, 7 figures, 1 table; Accepted for publication in MNRA
On the mass-radius relation of hot stellar systems
Most globular clusters have half-mass radii of a few pc with no apparent
correlation with their masses. This is different from elliptical galaxies, for
which the Faber-Jackson relation suggests a strong positive correlation between
mass and radius. Objects that are somewhat in between globular clusters and
low-mass galaxies, such as ultra-compact dwarf galaxies, have a mass-radius
relation consistent with the extension of the relation for bright ellipticals.
Here we show that at an age of 10 Gyr a break in the mass-radius relation at
~10^6 Msun is established because objects below this mass, i.e. globular
clusters, have undergone expansion driven by stellar evolution and hard
binaries. From numerical simulations we find that the combined energy
production of these two effects in the core comes into balance with the flux of
energy that is conducted across the half-mass radius by relaxation. An
important property of this `balanced' evolution is that the cluster half-mass
radius is independent of its initial value and is a function of the number of
bound stars and the age only. It is therefore not possible to infer the initial
mass-radius relation of globular clusters and we can only conclude that the
present day properties are consistent with the hypothesis that all hot stellar
systems formed with the same mass-radius relation and that globular clusters
have moved away from this relation because of a Hubble time of stellar and
dynamical evolution.Comment: 5 pages, 3 figures, MNRAS Letters (accepted
Mass loss rates and the mass evolution of star clusters
We describe the interplay between stellar evolution and dynamical mass loss
of evolving star clusters, based on the principles of stellar evolution and
cluster dynamics and on a grid of N-body simulations of cluster models. The
cluster models have different initial masses, different orbits, including
elliptical ones, and different initial density profiles. We use two sets of
cluster models: initially Roche-lobe filling and Roche-lobe underfilling. We
identify four distinct mass loss effects: (1) mass loss by stellar evolution,
(2) loss of stars induced by stellar evolution and (3) relaxation-driven mass
loss before and (4) after core collapse. Both the evolution-induced loss of
stars and the relaxation-driven mass loss need time to build up. This is
described by a delay-function of a few crossing times for Roche-lobe filling
clusters and a few half mass relaxation times for Roche-lobe underfilling
clusters. The relaxation-driven mass loss can be described by a simple power
law dependence of the mass dM/dt =-M^{1-gamma}/t0, (with M in Msun) where t0
depends on the orbit and environment of the cluster. Gamma is 0.65 for clusters
with a King-parameter W0=5 and 0.80 for more concentrated clusters with W0=7.
For initially Roche-lobe underfilling clusters the dissolution is described by
the same gamma=0.80. The values of the constant t0 are described by simple
formulae that depend on the orbit of the cluster. The mass loss rate increases
by about a factor two at core collapse and the mass dependence of the
relaxation-driven mass loss changes to gamma=0.70 after core collapse. We also
present a simple recipe for predicting the mass evolution of individual star
clusters with various metallicities and in different environments, with an
accuracy of a few percent in most cases. This can be used to predict the mass
evolution of cluster systems.Comment: 25 pages, 17 figures, 4 tables, 2 appendices; accepted for
publication in MNRA
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