Figure 1：Illustration of a galactic structure in the edge-on view. A stellar halo has a huge size with a diameter of over 500,000 light years and contains old halo stars and globular clusters.
Figure 2：Traditional view of the Andromeda galaxies, showing only its bright bulge and inner disk and extending out to a projected distance of only about 65,000 light years from the galaxy's center. Image credit: National Astronomical Observatory of Japan.
Figure 3：False-color map of the density of red giant stars in Andromeda, constructed from Subaru/Suprime-Cam images. The stellar density enhancements in Streams E, F, and SW are indicated. The map extends out to a projected distance of 300,000 light years from Andromeda's center. Image credit for star count map: Mikito Tanaka (Tohoku University).
Figure 4：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. Image credit: Puragra Guhathakurta (University of California, Santa Cruz).+
A team of astronomers from Tohoku University, University of Tokyo, NAOJ, University of California Santa Cruz, and other universities (Note 1) have discovered new stellar streams in a vast region surrounding the disk of Andromeda, in its so-called stellar halo. These stellar or tidal streams (Note 2), which are localized in space and move as a coherent group through the parent galaxy, intensify the density of stars and are remnants of past mergers of relatively small (i.e., dwarf) galaxies. The data from the team's observations using both Subaru's Suprime-Cam for photometry and Keck II's Deep Extragalactic Imaging Multi-Object Spectrograph (DEIMOS) for spectroscopy provided detailed spatial and velocity distributions of the stellar streams and led to this discovery.
Stars spread over the vast reaches of a halo in a big galaxy like the Milky Way or Andromeda Galaxy (Figure 1) are characterized by old age, few elements other than helium and hydrogen (i.e., low metallicities), and high velocities. The exceptional nature of these halo stars, when compared to stars in a galaxy's disk, reflects the early dynamics and chemical evolution of the galaxy when its appearance differed significantly from what we see today. Consequently, the halo provides important insights into the processes involved in the formation and evolution of a massive galaxy. According to the current theory of galaxy formation, we expect a halo to preserve evidence of past galaxy mergers and/or tidal dissolution in the course of halo formation.
Since the merging and dissolution of a dwarf galaxy typically last for a couple of billion years, these events are occasionally seen in a large galaxy. Given the assumption that past merging events are recorded as stellar streams, identification of these stellar substructures in a halo plays a key role in studying the past history of galaxies. The Andromeda Galaxy is an excellent test case for this purpose: it is the nearest, large spiral galaxy similar to our own Milky Way Galaxy (Figure 2) and is close enough for individual stars to be studied in great detail.
Motivated by the scientific significance of examining Andromeda's halo, an international research team led by Mikito Tanaka (Tohoku University) carried out photometric observations of Andromeda's halo fields with V and I bands of Suprime-Cam, a wide field imager mounted on the Subaru Telescope. Rather than spending an enormous number of observation nights mapping its entire halo, the team looked at specific portions of Andromeda's minor axis fields, including the hitherto uncharted north side as well as some fields at the major axis. This survey led to the discovery of two stellar streams to the northwest (Streams E and F: Figure 3) 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: Figure 3), which lies at a projected distance of 200,000 to 300,000 light years from Andromeda's center.
Another scientific team led by Puragra Guhathakurta (University of California, Santa Cruz) followed up the photometric observations with a spectroscopic survey of several hundred red giant stars in Streams E, F, and SW, using Keck II's 10-meter telescope fitted with DEIMOS. Red giant stars are large, bright stars with low or intermediate mass that are in a late phase of stellar evolution. Because the spectrograph spreads out the light from each star into a spectrum, it allows astronomers to measure the star's velocity and thus distinguish Andromeda red giant stars from foreground stars in the Milky Way. The spectral data confirmed the presence of coherent groups of Andromeda red giant stars moving with a common velocity (Figure 4).
The features of these newly discovered stellar streams in Andromeda are evidence of past galaxy mergers associated with the formation of a stellar halo. The next research step will be to measure in detail the chemical properties of Andromeda's giant red stars within their stellar streams. Mikito Tanaka anticipated the significance of future research by saying, "Further observational surveys of an entire halo region in Andromeda will provide very useful information on galaxy formation, including how many and how massive individual dwarf galaxies as building blocks are and how star formation and chemical evolution proceeded in each dwarf galaxy."
The photometric survey of Andromeda with Subaru's Suprime-Cam was published in a recent ApJ article (Note 3). Findings from the spectroscopic survey with Keck/DEIMOS were presented at the 215th meeting of the American Astronomical Society in Washington, D.C. (Note 4).
Note 1: Mikito Tanaka (Tohoku University, University of Tokyo), Masashi Chiba (Tohoku University), Yutaka Komiyama (NAOJ), Masanori Iye (NAOJ), Puragra Guhathakurta (University of California, Santa Cruz), Jason S. Kalirai (Space Telescope Science Institute), and other collaborators at the University of Virginia, UC Irvine, University of Massachusetts, Yale University, University of Washington, Columbia University, and California Institute of Technology.
Note 2: Stellar streams represent enhancements in the density of stars, localized in space and moving as a coherent group through the parent galaxy. Because tidal forces play a role in their creation, they are also referred to as tidal streams. These substructures in stellar distribution are remnants of past merging events of dwarf galaxies when they fall into a big galaxy like the Milky Way and Andromeda.
Note 3: "Structure and Population of the Andromeda Stellar Halo from a Subaru/Suprime-Cam Survey" by Mikito Tanaka, Masashi Chiba, Yutaka Komiyama, Puragra Guhathakurta, Jason S. Kalirai, Masanori Iye, 2010, ApJ, 708, 1168-1203
Note 4: "The SPLASH Survey: Spectroscopy of Newly Discovered Tidal Streams in the Outer Halo of the Andromeda Galaxy" by Puragra Guhathakurta, R. Beaton, J. Bullock, M. Chiba, M. Fardal, M. Geha, K. Gilbert, K. Howley, M. Iye, K. Johnston, J. Kalirai, E. Kirby, Y. Komiyama, S. Majewski, R. Patterson, M. Tanaka, E. Tollerud, SPLASH collaboration, 2010, AAS Meeting #215, #354.01
