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Figure 12:
The effective radii ![]() |
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Figure 13:
Histogram of the effective radii of objects in NGC 5236 with
V < 19.5 and B-V > 0.6, assumed to be mostly foreground stars. Only
objects in the region 300 < x < 1700 pixels and 300 < y < 1700 pixels
were included. For easier comparison with Fig. 14 the
effective radii have been converted into parsec (1 pc ![]() ![]() |
The effective radii were derived both on B and V band images for the 6 most cluster-rich galaxies in order to check the results. In Fig. 12 the effective radii (in pixel units) derived by ishape from B and V band frames are plotted against each other for objects with S/N > 50, leaving out about 20% of the objects. If objects with lower S/N were included, the scatter was found to increase somewhat, although the correlations in Fig. 12 would remain evident.
As another test, radii were derived for sources in the NGC 5236
frame with B-V > 0.6 and V < 19.5. Most of these objects are
expected to be foreground stars, and they are indeed distributed quite
uniformly across the CCD frame. Figure 13 shows the
distribution for all such objects located within the central
pixels of the image, roughly corresponding to the area
covered by the galaxy. For convenience, the effective radii have been
converted to parsec. It can be seen from Fig. 13 that
ishape finds
pc (0.05
) for the vast
majority of the objects with B-V>0.6, in agreement with the assumption
that they are in fact foreground stars. The fact that foreground stars appear
virtually unresolved by ishape adds confidence to the algorithm's
ability to recognise extended objects.
The number of clusters in most of the galaxies are really too sparse that
it makes sense to discuss the distributions for individual galaxies.
However, Fig. 14 shows
histograms for two of the most
cluster-rich galaxies, NGC 5236 and NGC 1313. These are
also the two most nearby galaxies, at m-M=27.84 and 28.2 respectively
(see Paper I). Separate histograms are given for the "red'' (U-B > -0.4)
and "blue'' (
) cluster samples. Several systematic effects
might influence the observed
distributions, for example the fact that
the most extended clusters are detected with a much lower efficiency, and
one should be careful not to overinterpret the data in Fig. 14.
Nevertheless, the objects in the "blue'' sample generally seem to be
less concentrated, which might imply that many of the blue objects are
loosely bound associations that do not survive for long, rather than bound
clusters. It is emphasised, however, that this statement could only be
justified by a more detailed study of the structure of the objects, something
that is not
feasible from ground-based observations.
Figure 14 also seems to indicate that many of the objects
in NGC 5236 are more concentrated than those in NGC 1313,
although the statistics are poor for the latter galaxy.
From the discussion in Sect. 4.3 and Fig. 12
the uncertainty on the cluster radii is estimated to be about 0.5 pixels.
1 DFOSC pixel () corresponds to about 7 pc at the distance of
NGC 5236 and NGC 1313, so we expect an uncertainty of
about 4 pc on the individual cluster radii in Fig. 14, as
indicated by the horizontal error bars. The fact that we do not
observe any lower limit in the
distribution may be an effect of the
limited resolution, causing clusters with larger radii to scatter into the
low
region, but the lower
limit in our sample cannot be higher
than a few pc.
How do the sizes of the clusters discussed here compare with other
results? Accurate measurements of cluster sizes are available only in the
Milky Way and in the LMC. Most globular clusters in the Milky Way have
pc with a peak in the
distribution at about 3 pc, although
clusters with
up to
pc exist
(Harris 1996).
Elson et al. (1987)
found typical half-mass radii for young
LMC clusters in the range 5-15 pc, corresponding to
values between
4 and 12 pc
(Spitzer 1987).
Effective radii have been estimated also for young clusters in the
Antennae
by
Whitmore & Schweizer (1995, WS95)
using HST/WFPC data, and in
the merger remnant NGC 7252
(Whitmore et al. 1993),
in
both cases by assuming Gaussian cluster profiles. WS95 found an average
of 12 pc for the Antennae clusters, but new WFPC/2 data lead to
a revised mean value of
pc
(Whitmore et al. 1999).
In the case of NGC 7252,
Whitmore et al. (1993)
found a mean effective radius of 9.9 pc for clusters located at distances
larger than 3.5
from the centre of NGC 7252.
Östlin et al. (1998)
used the model profile of
Lugger et al. (1995)
to estimate core radii (one half times the FWHM) for YMCs in the blue
compact galaxy ESO 338-IG04,
and found a distribution that peaked at 2.5 pc with all clusters having core
radii less than 10 pc. The
Lugger et al. (1995)
profile does not
have a well-defined effective radius, but
Östlin et al. (1998)
also found that if the clusters were instead fitted with Gaussian profiles
(for which the core radius equals
) the core radii were about 2 times
larger than for the
Lugger et al. (1995)
profiles, so their
distribution should peak at about 5 pc and have an upper limit at 20 pc.
Our observed distributions thus seem to resemble those observed for
YMCs in other galaxies quite well. We note, however, that the effective radii
have been estimated by a variety of methods in the different studies, and
this could account for some of the differences. The effective radii for the
YMCs in our sample are also comparable with those of Galactic globular
clusters and young LMC clusters.
The velocity dispersion in a star cluster or OB association is typically of the order of 1-10 km s-1, so an unbound object will expand by 1-10 pc/Myr. A U-B colour of -0.4 corresponds to an age of about 50 Myr (Girardi et al. 1995), so most of the objects in the "red'' group would have expanded to sizes much larger than what is observed and would have effectively disappeared if they were not gravitationally bound. Like all star clusters they will eventually be subject to dynamical erosion, but this acts on much longer timescales and the lifetimes are difficult to estimate without a detailed knowledge of the morphology of individual clusters and their orbits within the host galaxies.
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