| Issue |
A&A
Volume 477, Number 1, January I 2008
|
|
|---|---|---|
| Page(s) | 117 - 123 | |
| Section | Extragalactic astronomy | |
| DOI | https://doi.org/10.1051/0004-6361:20078359 | |
| Published online | 06 November 2007 | |
A peculiar object in M 51: fuzzy star cluster or a background galaxy?*
Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands e-mail: scheepmaker@astro.uu.nl
Received:
26
July
2007
Accepted:
19
October
2007
Aims. We study a peculiar object with a projected position close to the nucleus of M 51. It is unusually large for a star cluster in M 51 and we therefore investigate the three most likely options to explain this object: (a) a background galaxy, (b) a cluster in the disk of M 51 and (c) a cluster in M 51, but in front of the disk.
Methods. We use broad-band images of the Advanced Camera for Surveys and the Near Infrared Camera and Multi-Object Spectrometer, both on board the Hubble Space Telescope, 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 M 51, we
estimate an age and mass of 
Gyr and
, respectively (with the
extinction fixed to 
). 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 M 51, but in front of the disc, with an age of
Gyr, mass 
. 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.
Key words: galaxies: individual: M 51 / galaxies: star clusters
© ESO, 2007
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