Where Little Galaxies Come from
Cosmic collisions between dwarfs
Galaxies grow by attracting and ingesting smaller galaxies or by merging with galaxies of comparable size. Recently, a team of astronomers, including Glenn van de Ven from the Max Planck Institute of Astronomy, have identified the smallest example of a remnant of such a galactic merger: the dwarf spheroidal galaxy Andromeda II (AndII), a satellite of the well-known Andromeda galaxy. From the motion of stars within this galaxy, the researchers identified two distinct groups of stars – what appears to be stars of the original dwarf galaxy, and stars from another dwarf galaxy that merged with AndII.
The discovery started out as an anomaly: A group of astronomers in the USA, led by Marla Geha, had measured velocities of more than 700 stars in AndII, and found that the overall rotation was not only much faster than in other dwarf spheroidal galaxies but also rather unusual in orientation: Usually, galaxies spin like a wheel along its axis. In comparison, this galaxy tumbled head over heel.
With this anomaly in mind, the astronomers kindly provided their data to a group of researchers well known for their experience in modeling stellar motion in galaxies: Glenn van de Ven of the Max Planck Institute for Astronomy, Wyn Evans of Cambridge University, and Nicola Amorisco of the University of Copenhagen.
Amorisco, Evans, and van de Ven carefully re-analyzed the velocities of the stars within AndII, and found an explanation for the anomaly: AndII seems to be the remnant of a merger of two even smaller dwarf galaxies. The unusual rotation is then a consequence of the smaller dwarf galaxy orbiting the larger one before being absorbed.
As van de Ven explains: "Through a detailed analysis of the motions of more than 700 individual stars, we could show that there are two distinct groups of stars: Those of the original dwarf galaxy, and those in a stellar stream wrapped around the inner regions of AndII like a belt."
Interestingly, already in 2007 in a study of the distribution of stars in AndII, astronomers had speculated about a possible relic of an earlier merger: an unusual group of very old stars within the galaxy. The discovery of the stellar stream puts this speculation onto much firmer ground.
Such stellar streams, the characteristic remnant of a smaller galaxy captured by a larger one, have been found around our own Milky Way galaxy and other larger galaxies – but never for a galaxy with a mass of less than a billion Suns. And II has a much lower mass, corresponding to a mere 10 million Suns. Amorisco says: "Up until now, astronomers have only found traces of later stages of galaxy evolution – where at least one of the galaxies was already fairly massive, the result of many previous mergers and captures. This is the first example for a stellar stream in a very small galaxy." Van de Ven adds: "Current galaxy formation models assume that this is how galactic growth began – with small dwarf galaxies swallowing even smaller dwarfs. But before this, nobody had found an example at such low mass scales."
Amorisco, Evans and van de Ven hope that the next discoveries of this kind might be just around the corner. Van de Ven says: "We know there are astronomers out there who have similar data for other dwarf galaxies. With our method of analysis, they should be able to look for traces of stellar streams in other dwarf galaxies – and maybe identify even less massive examples." Even with this one example, it looks like the current model of galaxy evolution is right on the money when it comes to explaining where little galaxies come from.
Background information
The work described here has been published as N. C. Amorisco (University of Copenhagen), N. W. Evans (University of Cambridge) and G. van de Ven (Max Planck Institute for Astronomy), "Kinematic detection of a stellar stream in the galaxy Andromeda II" online in the journal Nature.
After the end of the embargo, the article will be online at http://dx.doi.org/10.1038/nature12995
Before the end of the embargo, the article is accessible in the restricted press section of Nature at http://www.nature.com
Additional information
The work reported here is a re-analysis of observational data taken with the DEIMOS spectrograph at the Keck telescopes by Ho, Geha, Muñoz et al. 2012. The re-analysis focuses on the radial velocities of more than 700 individual stars in the field of Andromeda II. Among these stars, there is a subgroup that exhibits a more ordered kind of motion ("kinematically colder") consistent with a stellar stream within that galaxy: The remnant of a smaller galaxy that has merged with the original Andromeda II. The overall properties of the stellar motion (in particular the small overall degree of disordered motion of the stars among themselves) are strong evidence that this was indeed a merger event, not, say, a fly-by in which Andromeda II was gravitationally disturbed by another galaxy.
For the merger time, only a lower limit can be estimated: In order to reach the state actually observed, the merger must have taken place more than 3 billion years ago. There is no indication, however, whether this happened 3 billion years after the Big Bang, or 3 billion years before the present.
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