Outer Disk Ring?
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The spiral disk of the Milky Way
may be surrounded by a ghostly
ring of stars, formed of gas and
stars that may have been torn
from a smaller satellite galaxy
billions of years ago (more).
Ring of Stars around the Spiral Disk?
On January 6, 2003, two teams of astronomers announced the discovery of a vast ring of stars around the Milky Way galaxy -- at the American Astronomical Society's 201st meeting in Seattle, Washington, USA (see Heidi Jo Newberg, 2003)). The larger team (led by Brian Yanny and Heidi Jo Newberg) with the Sloan Digital Sky Survey (SDSS) noticed over four years of study that a crowded patch of space on the outskirts of the galactic disk in the direction of Constellation Monoceros had unusual properties. The stars were distributed over a wide arc of space outwide the edge of the Galaxy's thin disk and moving in a similar direction at similar speeds, which indicated that they were moving in a ring and not a cloud like an undisturbed, dwarf satellite galaxy (press releases from SDSS and the Rensselaer Polytechnic Institute; Yanny et al, 2003).
The other team (including Annette Ferguson, Rodrigo Ibata, Mike Irwin, Geraint Lewis, and Nial Tanvir) pointed the new Isaac Newton Group (ING) of telescopes in the Canary Islands at a patch of sky the same distance from the Galaxy's center in the direction of the Andromeda galaxy (M31). They soon found a similar patch of stars about 100 degrees around from the first patch in Monoceros, whose existence they also confirmed (press release; and Ibata et al, 2003). Combined, the two observed patches of stars span about one-sixth of the Milky Way's circumference, suggesting that further observations will find additional stars that form a complete outer ring around the entire galaxy.
Assuming that the newly discovered distribution of stars do completely encircle the Milky Way, the inner edge of the hypothesized structure may be around 120,000 light-years (ly) across, as estimated by the SDSS team. Located at at least 59,000 ly from the galactic center, the ring may contain somewhere between 360 million to nine billion stars with around 27 million to 500 million Solar-masses, according to the SDSS team (Yanny et al, 2003); or it may contain 20 million to one billion stars located between 49,000 and 65,000 ly from galactic center according to the ING team (Ibata et al, 2003). Previously, astronomers thought that the Milky Way's spiral thin disk petered out at its furthest hypothesized extent of as much as 50,000 ly from the galactic center.
Possibly 10 times thicker than the spiral disk of the Milky Way, the ring encircles the disk -- which has a diameter up to around 100,000 ly -- like a giant torus (or "doughnut"). In the Monoceros patch observed by the SDSS team, the ring appears to extend over 16,000 ly (5,000 parsecs or pc) above and below the galactic plane, with stars below the plane extending about 2,000 pc further from galactic center than those located above the plane; it also appear to be somewhat less than 13,000 ly (4,000 pc) wide. In the patch observed by the ING team, however, the ring appears to have a lower scale height of around 2,400 ly (750 pc) with a width around 6,500 ly (2,000 pc). Looking at the Milky Way from above, the ring is turning clockwise (prograde rotation) at about 68 +/- 16 miles (or 110 +/- 25 km) per second, assuming that its stars move on average in a circular orbit (Yanny et al, 2003). Our Sun, Sol, moves in the same direction at twice this speed but lies only about 26,000 ly from galactic center.
If a complete ring, the structure does not appear to be completely round. It also appears to be warped, probably from encounters with satellite galaxies orbiting the Milky Way such as the Large and Small Magellanic Clouds and the Sagittarius dwarf galaxy. Its stars are also very dispersed so that observers from a good distance outside the Milky Way would see only a "ghostly" ring because of its low surface brightness (Ibata et al, 2003). Initial observations suggest that stars in the ring are bluer than those found in the galaxy's thick disk (Yanny et al, 2003). Their initial metallicities are low (averaging around 2.5 percent of Sol's abundance of iron, and mostly ranging from 1.3 to 5.0 percent of Sol's) suggest that they are as scarce in elements heavier than hydrogen and helium as many thick disk and halo stars.
Some astronomers believe that the ring may be the remains of a satellite galaxy, spun apart by the stronger gravity of the Milky Way. As smaller galaxies are pulled apart, they dissolve into streams of stars and gas around their host galaxies (see the discussion or "star streams" or "tidal trails" from the Sagittarius Dwarf Elliptical Galaxy). Gravity, primarily from unseen dark matter, can hold the ring in a nearly circular orbit around the host galaxy. Previous studies of stellar rings around other galaxies suggest that a substantial portion of galaxies like the Milky Way are formed by a lot of smaller galaxies mixing together, according to SDSS team co-leader Heidi Jo Newberg. If the ring was created by a merger, then it may have occurred billions of years ago, according to SDSS team co-leader Brian Yanny.
Alternatively, the stars of the ring could have formed within the Milky Way, according Mike Irwin of the ING team. Its stars could have originally come from the galaxy's spiral disk through warping of the spiral disk from ancient as well as more recent gravitational interactions with satellite galaxies or even larger galactic neighbors. Over time, however, their orbits have been warped or spread over time so that the stars in the ring now wander far from the plane of the spiral disk (Ibata et al, 2003).
© unknown artist for New
McClure-Griffiths et al, in pdf, submitted
(Permission being sought)
The large arc or arm of neutral
hydrogen gas has been detected
near part of the outer ring of
In May 2004, a team of astronomers (led by Naomi McClure-Griffiths) announced that they had found an arc or galactic arm of neutral hydrogen gas around 77,000 ly long and a few thousand ly thick along the Milky Way's outermost edge in the so-called "fourth quadrant" (McClure-Griffiths et al, in pdf, submitted to Astrophysical Journal Letters). Located between 59,000 to 78,000 ly (18,000 to 24,000 parsecs), it is very close to the streak or ring of stars along the rim of the Milky Way located some 60,000 ly from the galactic core (that was discovered in 2003 by the team of astronomers including Brian Yanny that was mentioned above) whose stellar composition suggests that they were torn from a small invading galaxy that was torn apart as it brushed past the Milky Way.
According to McClure-Griffiths, the arc or arm could be a tendril that was once part of another spiral arm. Indeed, it is not be unusual for a medium-sized galaxy like the Milky Way to have arms that extend so far, as nearby Andromeda, a similar spiral, has long gaseous arms. On the other hand, another possibility is that the gas was drawn out of the Milky Way in a collision with a dwarf galaxy early in its evolution. In the near future, astronomers hope to characterise the make-up of the new arm in more detail, and to run computer simulations to determine whether it could have been created by the near collision of a small galaxy with the Milky Way.
Examples of galactic rings abound. The Cartwheel galaxy is believed have been a spiral that collided with a smaller neighbor. Seyfert galaxy M77 (NGC 1068) has a spiral disk with a faint outer ring, as well as a small and very bright, active nucleus. The polar ring of NGC 4650A is oriented perpendicular to its spiral disk due to the nature of its collision with another galaxy. Some galaxies even have inner rings that encircle their nucleus, like NGC 6782.
Hubble Heritage Team, NASA
After an "accretion event" about
two to three billion years ago,
Hoag's Object developed a very
bright ring with lots of young blue
stars that spans about 120,000
light-years (ly). The galaxy is
located around 600 million ly
away towards Constellation
Serpens and has another ring
galaxy behind it (more).
As summarized by astronomers Ronald J. Buta and Françoise Combes in their paper on "Galactic Rings," about one-fifth of all spiral disk galaxies display a ring-shaped pattern, and an additional third appear to have broken or partial rings made up of spiral arms ("pseudo-rings"). In general, galactic rings are most often associated with galaxies that have central "bars" of stars (such as the one found in the Milky Way (now classified as a barred spiral galaxy) or other common non-axisymmetric perturbations such as ovals. In addition, most galactic rings are the sites of active star formation, and in some apparently old galaxies their rings are the only places where recent star formation can be found. A few rings are sites of the most spectacular "starbursts" of stellar formation known in "non-violently" interacting galaxies.
Linda Sparke, and Anne Kinney,
Hubble Heritage Team (AURA /
STScI / NASA)
NGC 4650A has a "polar ring"
that is believed to be the
result of a collision with
another galaxy perpendicular
to its disk (more).
Although a small fraction of galactic rings observed do appear to be the result of collisions or mergers of galaxies, or of accretion of intergalactic gas, the vast majority of rings are probably created by simple resonance phenomena that are caused by the actions of a rotating bar or some other non-axisymmetric disturbance on the motions of gas clouds within a galactic disk. Evidence for the resonance hypothesis has been accumulating for two decades, so that many astronomers now believe that rings are a natural consequence of barred galaxy dynamics, perhaps more easily understood than the bars and ovals that created them. However, some barred galaxies lack rings, while others (such as Hoag's Object) display rings despite the apparent lack of a bar. Questions remain regarding the role of mild tidal interactions, the origin of the gas that fuels star formation in rings, the existence of intrinsic bar/ring misalignment, and the simultaneous existence of different ring types that may have been created during very different eras in the life of the host galaxy.
Astronomer James L. Higdon also has an illustrated summary of research on notable starburst galaxies in "An HI/H-alpha Atlas of Ring Galaxies," based on research being undertaken with at least six other astronomers.
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