New tidal streams found in Andromeda reveal history of galactic mergers
WASHINGTON D.C.—The Andromeda galaxy has two previously unknown tidal streams, according to data recently taken at the W. M. Keck Observatory and Subaru Telescope. The coherent flows of stars are remnants of dwarf galaxies that Andromeda has been consuming over the last one to two billion years.
The Andromeda galaxy is a unique test bed for studying the formation and evolution of a large galaxy, said Puragra Guhathakurta, of the University of California, Santa Cruz. He leads the Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo (SPLASH), an international collaboration conducting a large survey of red giant stars in Andromeda.
Tidal streams are important because they represent a conceptual “link” or “bridge” between the victims and survivors of galactic cannibalism, an intermediate stage between the population of intact dwarf galaxies and the merged or dissolved dwarf galaxies whose stars are now well mixed in the parent galaxy’s halo, he explained.
Guhathakurta announced the discovery of the two new streams at the 215th meeting of the American Astronomical Society held Jan. 4-7, 2010 in Washington D.C.
In the currently favored Lambda Cold Dark Matter paradigm of structure formation in the Universe, the outer halos of large galaxies like the Milky Way galaxy and the neighboring Andromeda galaxy are built up through the merger and dissolution of smaller “dwarf” satellite galaxies. “This process of galactic cannibalism is an integral part of the growth of galaxies,” he said.
Discovery of the two tidal streams supports this idea of galactic cannibalism. Japanese astronomers first observed them when using the Subaru 8-meter telescope and Suprime-Cam camera to map the density of red giant stars in large portions of the Andromeda galaxy, including the previously uncharted north side. This revealed the streams on the northwest (streams E and F) at projected distances of 200,000 and 300,000 light years from Andromeda’s center. The study also confirmed a few previously known streams, including the little-studied diffuse stream to the southwest (stream SW), which lies at a projected distance of 200,000-300,000 light years from Andromeda’s center.
The SPLASH researchers followed up with a spectroscopic survey of several hundred red giant stars in Streams E, F, and SW, using the Keck II 10-meter telescope and DEIMOS spectrograph at the W. M. Keck Observatory in Hawai’i. The spectrograph spreads out the light from each star in to a spectrum, which allows astronomers to measure the velocity of the star and distinguish Andromeda red giant stars from foreground stars in the Milky Way. The spectral data confirmed the presence of the groups of Andromeda red giant stars moving with a common velocity.
One of the next steps using the Keck data will be to measure the chemical properties of red giant stars in these newly discovered tidal streams in Andromeda, Guhathakurta said.
Comparing the chemical properties of tidal streams, intact dwarf satellites and the smooth halo will provide details about galaxy cannibalism.
Dwarf galaxies are less effective at recycling chemical elements than massive galaxies. This is partly because the weaker gravity of a dwarf galaxy makes it harder for it to retain the chemically enriched gas that is blown out of massive stars during supernova explosions. As a result, stars in dwarf galaxies are more anemic (have a smaller fraction of complex elements) than those in the interior of massive galaxies. Moreover, the action of merging with a larger galaxy causes a dwarf galaxy to lose its gas, breaking the chemical cycle altogether.
“The cannibalized victims have had less time to recycle their chemicals than dwarf galaxy survivors, and this should be reflected as a difference between their chemical properties,” Guhathakurta said. “Tidal streams should be somewhere between the victims and the survivors in terms of their chemical properties.”
The W. M. Keck Observatory operates two 10-meter optical/infrared telescopes on the summit of Mauna Kea on the island of Hawai’i and is a scientific partnership of the California Institute of Technology, the University of California and NASA. For more information please call 808.881.3827 or visit http://www.keckobservatory.org.
Subaru is an 8.2-meter optical-infrared telescope at the summit of Mauna Kea, Hawaii, operated by the National Astronomical Observatory of Japan (NAOJ), National Institutes of Natural Sciences.
Caption: Distribution of line-of-sight velocities of stars in the Stream SW field. The filled portion of the histogram corresponds to Andromeda red giant stars while the open portion corresponds to foreground Milky Way stars. The concentration of red giant stars (at a velocity of -370 kilometers per second) is characteristic of tidal streams. The mean velocity of the Andromeda galaxy is -300 kilometers per second, so this shows that Stream SW is moving towards us at 70 kilometers per second relative to the parent galaxy. In addition to the red giants in Stream SW, there is a population of red giants with a broad distribution of velocities that represents the smooth halo of Andromeda built from the dissolved dwarf galaxy victims of the cannibalism process. Credit: Raja Guhathakurta, UCSC.