A peculiar object in M51: fuzzy star cluster or a background galaxy?

TL;DR

This study investigates a peculiar, large object near M51's nucleus, analyzing whether it is a background galaxy or a star cluster in or in front of M51, using photometry and structural modeling.

Contribution

It provides a detailed analysis combining photometry and structural modeling to determine the nature of a peculiar object near M51, proposing it as a fuzzy star cluster outside the galaxy disc.

Findings

The object is unlikely a background galaxy due to its colors and brightness.

If a star cluster in M51, it is either near dissolution or located in front of the disc.

The effective radius suggests it is a fuzzy star cluster of 12-25 pc.

Abstract

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.