KISSIMMEE, FLORIDA—A decade ago, astronomers discovered that the gas in our Milky Way galaxy is not spread out into a completely flat disk but has ripples, launching a search for the disturbances that caused them. Now, researchers in the nascent field of galactic seismology have found a possible cause of at least some of those ripples: a dwarf galaxy that shot like a bullet through the galactic disk some half a billion years ago. The team proposes that its technique could eventually be used to identify other dwarf galaxies—which are notoriously dark and hard to see—through the disturbances they cause in our galactic disk.
To try to explain what caused the ripples in our galaxy’s gas disk, astronomer Sukanya Chakrabarti of the Rochester Institute of Technology in New York built a computer simulation of it and examined various scenarios. She found that the observed ripples could have been caused by a dwarf galaxy shooting through. She predicted that the dwarf would now be traveling away from us at 200 kilometers/second and be roughly 300,000 light-years away.
Chakrabarti and her colleagues then set about looking for a likely candidate. One challenge was the dimness of dwarf galaxies, which typically have fewer than 1% of the number of stars in a galaxy like the Milky Way and are often dominated by dark matter, the mysterious stuff that holds galaxies together. In addition, the team predicted that this particular dwarf was moving almost parallel to the plane of the Milky Way, and so would be hidden by the gas and dust in our galaxy.
The researchers scoured surveys by infrared observatories because such light is better at penetrating obscuring clouds, and found three particular stars that were close together and could be part of the same dwarf galaxy. The stars are all Cepheid variables, a type of star that has a known brightness, making it possible to calculate a Cepheid’s distance from its apparent brightness. The stars suggested that the dwarf, if they are indeed part of it, was at the same distance predicted by the computer simulation.
As Chakrabarti told a meeting of the American Astronomical Society here today, further studies of the three stars showed they are also traveling at the speed predicted for the ripple-spawning dwarf galaxy. She conceded that it was not possible to get a precise trajectory for the dwarf galaxy and thus confirm it is the smoking bullet, but future observations with better telescopes may be able to nail it down.
Down the line, she hopes that that through more detailed study of the Milky Way’s ripples her team can tease out the influence of other dwarf galaxies, along with that of clumps of dark matter that are entirely hidden from conventional telescopes. “We can use ripples [in the galaxy] to map out dark matter content” in the local universe, she says.
“The discovery reported is potentially very interesting,” says astronomer Lawrence Widrow of Queen’s University in Kingston, Canada, who is not involved in the research. “It ties together the dynamics of the disk of the galaxy and the system of dwarfs that inhabit the galaxy's halo.”