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ING Science News
28 December, 2021

Tracing The Giant Stellar Stream Around the Sombrero Galaxy

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".

WHT image of Sombrero galaxy's tidal stream (the faint V-shaped feature just above the centre of the image). North is up and east is left; the field of view is 15 arcmin×15 arcmin. Figure extracted from Martínez-Delagado et al., 2021, MNRAS, 506, 5030. Large format: JPG

"Observational techniques in present day Astrophysics need advanced image processing. Our modelling of the bright stars around the Sombrero galaxy, and at the same time of the halo light of the galaxy itself has enabled us to unveil the nature of this tidal stream”, explains Javier Román (Instituto de Astrofísica de Canarias).

The work has been possible thanks to the collaboration between professional and amateur astronomers. "We have collaborated with the Spanish astrophotographer Manuel Jiménez, who took the images with a robotic telescope of 18 centimetre diameter, and the well-known astrophotographer David Malin, who discovered this tidal stream on photographic plates taken in the 1990s. This collaboration shows the potential of amateur telescopes to take deep images of nearby galaxies which give important clues about the process of their assembly which is continuing until the present epoch”, concludes Martínez-Delgado.

Journal article
Martínez-Delgado, David; Román, Javier; Erkal, Denis; Schirmer, Mischa; Roca-Fàbrega, Santi; Laine, Seppo; Donatiello, Giuseppe; Jiménez, Manuel; Malin, David; Carballo-Bello, Julio A., 2021, "A feather on the hat: tracing the giant stellar stream around the Sombrero galaxy", MNRAS, 506, 5030 [ Paper ]

"A large tidal stream observed in the Sombrero galaxy", IAC press release, 21st July 2021.

Research contact
David Martínez-Delgado (Instituto de Astrofísica de Andalucía, Spain)

Press contact
Javier Méndez (ING PR Officer)

About the William Herschel Telescope

Based on observations made with the William Herschel Telescope (WHT) operated on the island of La Palma by the Isaac Newton Group of Telescopes (ING) in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias (IAC). The ING is funded by the Science and Technology Facilities Council (STFC-UKRI) of the United Kingdom, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) of the Netherlands, and the IAC in Spain. IAC's contribution to ING is funded by the Spanish Ministry of Science, Innovation and Universities.

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Last modified: 28 December 2021