Exploring the Galaxy’s Outer Halo

by Paul Gilster on July 11, 2014

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.

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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.

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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).

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{ 11 comments… read them below or add one }

Alex Tolley July 11, 2014 at 10:08

I don’t understand how the simulated image could be correct. At 800 kly the galaxy would cover over 7 degrees of the sky (15x the angular diameter of the moon), The image seems to make the galaxy quite small to me.

Paul Gilster July 11, 2014 at 16:56

Alex, I’m checking with John Bochanski about this and hope to have a comment in the next day or so. Will report back with his response.

Hans Bausewein July 11, 2014 at 17:37

I cannot find the distance of the virtual observer from the star in this description. If that were 1 AU, the red giant would fill a huge part of the sky, I guess.

Paul Gilster July 11, 2014 at 19:18

Alex, Dr. Bochanski responds as follows:

“Thanks for writing this story. That image has been cropped and modified many times, so the scale has changed many times. But your reader is correct, 7 degrees is about the size of the MW’s disk from this distance (however the entire disk would probably not be visible). Perhaps a comparison with the moon may have been a better idea, but its too late for that.

EIther way, our Galaxy looks much different from these stars.”

Kamal Ali July 12, 2014 at 3:08

I couldnt find where it says how they determined the distance: I’m very curious about that. Anyone have access to the original article (behind paywall)?

Also, what is the apparent magnitude of these stars? I wonder how faint they are.

ljk July 12, 2014 at 8:32

How big would the Andromeda galaxy My look in their skies?

ljk July 12, 2014 at 8:34

How does a single piece of art get cropped and modified many times? Every news item I have seen on this subject shows the exact same artwork. Perhaps someone just did not bother to do their homework.

Ron S July 12, 2014 at 15:30

Kamal,

Paul provided the link to the pre-print right at the bottom of the article. I’ve skimmed through the pre-print, and the answers you seek are in there.

Andrew_W July 13, 2014 at 4:51

900,000 LY is over a third of the way to Andromeda, I wonder what the typical distances between stars in the outer halo is, maybe a story about the stepping stones between the galaxies?

kzb July 14, 2014 at 7:41

The funny thing is, these are large stars and are therefore relatively young (even though they are red giants). Does this imply there is (or was) a small amount of active star formation in the halo?

I remember there is an older paper surveying A-type stars out to large galactic radius also (although not as far as this by any stretch, from memory it was about 30kpc)

Ron S July 14, 2014 at 9:35

kzb, I wondered the same but in the paper they say there is no evidence (yet) of gas clouds in the outer halo from which stars could form. Also, red giants are generally end-of-life stars that could be many billions of years old, and therefore could have been either ejected from the MW or remnants of a colliding dwarf galaxy. This is all discussed in the paper.

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