Not long ago we talked about what the Milky Way would look like when seen from afar. I had mentioned Poul Anderson’s World Without Stars, which appeared in Analog in 1966 under the title The Ancient Gods. In the Anderson tale, a starship crew is sent to make contact with a recently discovered technological civilization that lives on a world hundreds of thousands of light years from the galactic core. Now a recent paper deepens our understanding of this environment deep in the galaxy’s outer halo.

Recall that the Milky Way is about 100,000 light years in diameter, and that the distance to the nearest large galaxy is roughly 2,500,000 light years. Anderson’s crew is over 200,000 light years from the core, which puts them in the outer halo, a sparse spherical volume of space that stretches out 500,000 light years, well beyond the familiar, highly visible disk. While the stars in the galactic disk are on nearly circular orbits in the plane of the galaxy, the halo stars are on more elliptical orbits that are randomly oriented, so that while inner halo stars can pass through the disk, most of their lives are spent well above or below the plane of the galaxy. The inner, visible part of the halo is where we find the ancient, metal-poor globular clusters.


Image: Structure of the Milky Way, showing the inner and outer halo. Credit: NASA, ESA, and A. Feild (STScI).

Astronomer John Bochanski (Haverford College, PA) and team, however, are looking out well beyond the globular clusters. The researchers note in their paper in The Astrophysical Journal Letters that there are few known outer halo stars at distances over 120 kiloparsecs, which works out to about 390,000 light years — in fact, the list of known halo stars at this distance yields a grand total of seven, with the paper adding an additional two. The galaxy’s outer halo, we learn, is largely unexplored, but as we’ll see, it holds implications for galaxy formation theories.

The team’s recent paper outlines the discovery of two cool red giants — ULAS J0744+25 and ULAS J0015+01 — that appear to be the most distant Milky Way stars yet detected, at distances of 775,000 and 900,000 light years respectively. The work draws on observations from the UKIRT Infrared Deep Sky Survey and Sloan Digital Sky Survey, with spectroscopic studies using the 6.5m telescope at the MMT Observatory in Arizona. The newly discovered stars are five times more distant than the Large Magellanic Cloud and almost a third of the way to the Andromeda galaxy. At these distances, both Andromeda and the Milky Way should appear quite faint in the visible spectrum. If Anderson’s crew were here, the night sky would be dark indeed.

The image below brightens the Milky Way to give some sense of its distance from these stars.


Image: This simulated image demonstrates how small the Milky Way would look from the location of ULAS J0744+25, nearly 775,000 light years away. This star, along with ULAS J0015+01, are the most distant stars ever associated with our Galaxy, and are about five times further away than the Large Magellanic Cloud, one of the Milky Way’s closest galactic neighbors. Credits: Visualization Software: Uniview by SCISS Data: SOHO (ESA & NASA), John Bochanski (Haverford College) and Jackie Faherty (American Museum of Natural History and Carnegie Institute’s Department of Terrestrial Magnetism).

Bochanski’s team has been looking at formation models for the Milky Way, an interesting issue given that, as he explains, “Most models don’t predict many stars at these distances. If more distant red giants are discovered, the models may need to be revised.” The halo itself may be the result of mergers over the galaxy’s lifetime with numerous smaller galaxies, with outer stars the remnant population of what had once been intact dwarf galaxies. If this is correct, we can study these outer halo stars as a way of probing the formation history of the entire spiral. The team hopes to identify up to 70 red giants in the halo, refining its selection criteria and aiding next generation surveys like Gaia that will help us deepen our catalog in this distant region.

The paper is Bochanski et al., “The Most Distant Stars in the Milky Way,” The Astrophysical Journal Letters, Volume 790, Issue 1, article id. L5 (abstract / preprint).