Astronomer Avon Huxor from the University of Heidelberg, with collaborators Steve Phillipps and James Price of the University of Bristol, have used the ACAM instrument on the William Herschel Telescope (WHT) in the discovery of a unique, isolated, compact elliptical galaxy.

Compact ellipticals (cEs) are a rare type of dwarf galaxy, possessing small effective radii and high central surface brightnesses. The best example is M32, a companion to the nearby Andromeda Galaxy. All of those found to date are close to massive galaxies, so many astronomers believe that their unusual features are the consequence of the tidal stripping of a much larger progenitor galaxy by the very massive host. Others, however, have argued that cEs are formed in the same manner as all elliptical galaxies, and are simply the rare low-mass examples.

The team identified cE candidates from the SDSS to test these scenarios, seeking isolated cEs that could not have been formed by stripping. One candidate was subsequently observed with the WHT/ACAM in good seeing to have an effective radius of 500 pc. Along with the measured surface brightness, this makes the galaxy a cE. It is at a projected distance of almost one megaparsec from the nearest galaxy, and so cannot have been formed by stripping. Although other cEs are the result of stripping (see, for example, Huxor et al. 2011), this cE shows that they can also form in a manner similar to the more massive elliptical galaxies.

A model fit, using GALFIT, to the isolated cE galaxy cE0. The ACAM image (left panel) shows an area of ~10kpc×10kpc. The best fit GALFIT model (centre panel) is subtracted from the data to show the residual (right panel) [ JPEG ].

More information

• Huxor, A. P., Phillipps, S., Price, J. and Harniman, R. 2011, "Tidal streams in newly discovered M32 analogues: evidence for the stripping scenario", MNRAS, 414, 3557. Paper.
• Huxor, A. P., Phillipps, S. and Price, J., 2013, "Discovery of an isolated compact elliptical galaxy in the field", MNRAS, 430, 1956. Paper.