Using the PFIP imager on the William Herschel Telescope (WHT), astronomers have been able to trace for the first time the complete tidal stream surrounding the Sombrero galaxy, a galaxy which exhibits a rare morphology not yet fully explained.

The Sombrero galaxy, or Messier 104, is a galaxy some thirty million light years away, which is part of the Local Supercluster (a group of galaxies which includes the Virgo cluster and the Local Group containing the Milky Way). It has roughly one third of the diameter of the Milky Way, and shows characteristics of both of the dominant types of galaxies in the Universe, the spirals and the ellipticals. It has spiral arms, and a very large bright central bulge, which makes it look like a hybrid of the two types.

According to the latest cosmological models, large spiral galaxies such as the Milky Way grew by absorbing smaller galaxies, by a sort of galactic cannibalism. Evidence for this is given by very large structures, the tidal stellar streams, which are observed around them and are the remains of these satellite galaxies. But the full histories of the majority of these cases are hard to study, because these flows of stars are very faint, and only the remains of the most recent mergers have been detected.

Previous observations using the Hubble Space Telescope had shown that the halo, an extensive and faint region surrounding the Sombrero galaxy, shows many stars rich in metals, elements heavier than hydrogen and helium. This is a typical feature of new generations of stars, which are normally found in the discs of galaxies, and are quite rare in galactic halos, which are populated by old stars. To explain their presence astronomers suggested what is known as "a wet merger", a scenario in which a large elliptical galaxy is rejuvenated by large quantities of gas and dust from another massive galaxy, which went into the formation of the disc which we now observe.

David Martínez-Delgado (Instituto de Astrofísica de Andalucía) explains: "Our motive for obtaining very deep images of the Sombrero galaxy was to look for the remains of its merger with a very massive galaxy. In our images we have not found any evidence to support this hypothesis, although we cannot rule out that it could have happened several thousand million years ago, and the debris is completely dissipated by now. In our search we have in fact been able to trace for the first time the complete tidal stream which surrounds the disc of this galaxy, and our theoretical simulations have let us reconstruct its formation in the last three thousand million years, by cannibalism of a satellite dwarf galaxy".