EDP Sciences
Free access
Volume 477, Number 1, January I 2008
Page(s) 117 - 123
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20078359

A&A 477, 117-123 (2008)
DOI: 10.1051/0004-6361:20078359

A peculiar object in M 51: fuzzy star cluster or a background galaxy?

R. A. Scheepmaker, H. J. G. L. M. Lamers, S. S. Larsen, and P. Anders

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 0.7+0.1-0.1 Gyr and $2.2^{+0.3}_{-0.3}\times 10^{5}~\mbox{$M_{\odot}$ }$, respectively (with the extinction fixed to $\mbox{$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 M 51, but in front of the disc, with an age of 1.4+0.5-0.2 Gyr, mass $M = 1.7^{+0.8}_{-0.3}\times
10^{5}~\mbox{$M_{\odot}$ }$. 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