Stunning spiral galaxies may look serene, but in their dim outskirts there are signs of past violence: the shredded remains of other galaxies. Now a team of astronomers hope to figure out how common these remnants are. If they succeed, the survey could tell us more about the origin of galaxies like our own.
Spiral galaxies like the Milky Way are thought to bulk up in collisions and mergers with other galaxies. Simulations suggest this process will involve major mergers between equal-sized galaxies as well as the consumption of hundreds of dwarfs just a fraction of the size of our own.
Some of the best evidence of these "minor mergers" comes from the shredded remains of dwarf galaxies in the Milky Way's own halo. The most prominent example is the Sagittarius stream, which wraps around the Milky Way and has been used to model the shape of the dark matter haloMovie Camera thought to surround our galaxy.
Now astronomers are turning to other galaxies to see how common such tidal streams are. "The models are predicting the streams are out there, so we are trying to find them," says David Martínez-Delgado of the Max Planck Institute for Astronomy in Heidelberg, Germany, who is leading a survey of nearby spiral galaxies to hunt for stellar tidal streams.
... See more: Deep images reveal the shady past of cannibal galaxies
Nearby M81 is the model of a spiral galaxy, with a bright central region and long sweeping arms and dust lanes. But "deep" images of the galaxy – images that see fainter detail – reveal a complicated halo full of wispy blue streams of gas and dust.
These features were churned up like tides by past encounters with M81's neighbours M82 and NGC 3077 about 200 million years ago. M81 sits about 12 million light years away and appears in the constellation Ursa Major.
(Image: M. K. Barker & A. M. N. Ferguson, University of Edinburgh/M. Irwin, University of Cambridge/N. Arimoto, NAOJ – Graduate University for Advanced Studies/P. Jablonka, Geneva Observatory)
These deep images of the haloes of nearby spiral galaxies turn up a wealth of features that are fossils of past interactions between massive spirals and wayward dwarf galaxies. They show – in negative – the diversity of features that can be found beyond a galaxy's bright disc, including long stellar streams, spikes and wedges, partially disrupted galaxies, and bright haloes of mostly absorbed debris.
Colour images of the disc of each galaxy are inset on the deep images.
(Images: David Martínez-Delgado, Max Planck Institute for Astronomy/IAC, and colleagues)
Arcing loops of debris surround the warped spiral galaxy NGC 5907, which lies some 40 million light years away and appears in the constellation Draco.
These streams, which are visible only in images with long exposures, are probably the remains of a dwarf galaxy that was torn apart some 4 billion years ago. The original dwarf may be responsible for the twisted shape of NGC 5907's disc.
The path of this "great circle" stream is similar to the Milky Way's own Sagittarius stream, which is also made of stars torn from an orbiting dwarf galaxy.
An umbrella-shaped cloud of stars extends from the spiral galaxy NGC 4651. The unusual feature is thought to have been formed by the disruption of a dwarf galaxy on an orbit that took it close to NGC 4651's centre.
This umbrella structure extends tens of thousands of light years beyond the galaxy's bright central disc.
A faint, looping structure (upper left) extends from the edge-on spiral galaxy NGC 4013, which sits some 50 million light years away and appears in the constellation Ursa Major. Estimated to be some 3 billion years old, this stream seems to have been pulled from a smaller galaxy as it orbited NGC 4013. It extends some 85,000 light years from the centre of the galaxy.
Spiral galaxies grow by swallowing smaller dwarf galaxies. As they are digested, these dwarf galaxies are severely distorted, forming structures such as surreal tendrils and stellar streams that surround their captors. Now, for the first time, a new survey has detected such tell-tale structures in galaxies more distant than our immediate galactic neighbourhood. This opens up the possibility of testing our current views of galaxy evolution in a new way.
Around the Milky Way galaxy and in the vicinity of our immediate cosmic neighborhood, known as the “Local Group” of galaxies, traces of spiral galaxies swallowing dwarf galaxies have been known since 1997. But the Local group with its three spiral galaxies and numerous dwarfs is much too small a sample to see whether theoretical predictions of the frequency of such digestive processes match observations. Now, for the first time, a new survey has managed to detect the tell-tale tendrils of galactic digestion beyond the Local Group. An international group of researchers led by David Martínez-Delgado (Max Planck Institute for Astronomy and Instituto de Astrofísica de Canarias) has completed a pilot survey of spiral galaxies at distances of up to 50 million light-years from Earth, discovering the tell-tale signs of spirals eating dwarfs.
When a spiral galaxy is approached by a much smaller companion, such as a dwarf galaxy, the larger galaxy's uneven gravitational pull severely distorts the smaller star system. Over the course of a few billions of years, tendril-like structures develop that can be detected by sensitive observation. In one typical outcome, the smaller galaxy is transformed into an elongated “tidal stream” consisting of stars that, over the course of additional billions of years, will join the galaxy's regular stellar inventory through a process of complete assimilation. The study shows that major tidal streams with masses between 1 and 5 percent of the galaxy's total mass are quite common in spiral galaxies.
Stellar Tidal Streams in Spiral Galaxies of the Local Volume:
A Pilot Survey with Modest Aperture Telescopes - D Martinez-Delgado et al
arXiv.org > astro-ph > arXiv:1003.4860 > 25 Mar 2010 (v1), 17 Aug 2010 (v2)
When a spiral galaxy is approached by a much smaller companion, such as a dwarf galaxy, the larger galaxy's uneven gravitational pull severely distorts the smaller star system. Over the course of a few billions of years, tendril-like structures develop that can be detected by sensitive observation. In one typical outcome, the smaller galaxy is transformed into an elongated “tidal stream” consisting of stars that, over the course of additional billions of years, will join the galaxy's regular stellar inventory through a process of complete assimilation. The study shows that major tidal streams with masses between 1 and 5 percent of the galaxy's total mass are quite common in spiral galaxies.
Stellar Tidal Streams in Spiral Galaxies of the Local Volume:
A Pilot Survey with Modest Aperture Telescopes - D Martinez-Delgado et al
arXiv.org > astro-ph > arXiv:1003.4860 > 25 Mar 2010 (v1), 17 Aug 2010 (v2)
Beautiful images. Note the similarity between Ray GaBany's image of NGC 4651 and this SDSS image of NGC3310. Both these galaxies are "bow and arrow" objects.
