Galactic Cannibalism

Many astronomers believe that the large galaxies seen today were formed from smaller "dwarf" galaxies, which formed first after the Big Bang. Many of these dwarfs either clumped together to form larger galaxies or were gradually swallowed up by larger galaxies that continued to grow by "cannibalizing" smaller ones. This hypothesis appears to be supported by direct and indirect observations of the destruction of dwarf galaxies in distant (and therefore ancient) reaches of the universe. [On the other hand, there is growing evidence that supports a complementary theory of galaxy formation and enlargement from gas being funneled into the dark-matter haloes surrounding developing spiral galaxies at the nodes of a cosmic web of dark and normal matter (Eugenie Samuel Reich, New Scientist, July 17, 2009.)]

Astronomers have long suspected that the Milky Way Galaxy was formed from smaller galaxies. Moreover, after becoming a relatively large galaxy, it may have continued to acquire a substantial part of its mass by "devouring" smaller galactic companions that moved too close. Apparently confirming that hypothesis was the discovery of a new object in 1994 now commonly referred to as the Sagittarius Dwarf Elliptical Galaxy (SagDEG), found very close to the Milky Way on the opposite side of the Galactic Center from the Solar System (Ibata et al, 1995 and 1994). Not to be confused with the Sagittarius Dwarf Irregular Galaxy (SagDIG), SagDEG is the Milky Way's nearest known neighbor and comprised of mostly old, yellowish stars. Astrophysicist Rosemary Wyse of Johns Hopkins University has estimated that as much as 10 percent of the stars in the Milky Way's halo came from dwarf galaxies like SagDEG, merging with the Milky Way over the past eight billion years or so. (In November 2003, astronomers announced that an even closer galaxy (located 25,000 ly from Sol and 42,000 ly from the galactic center) called the Canis Major dwarf may be losing stars to the Milky Way's disk as well.)

Patrick Cseresnjes, l'Observatory de Paris

(Used with permission)



Larger illustration.





From the Solar System, one of the brighest parts

of SagDEG is located on the far side of the center

of the Milky Way (more).



Also known as the Sagittarius Dwarf Spheroidal Galaxy, SagDEG can be found in (18:55.1-30:29, ICRS 2000) Constellation Sagittarius, the Archer. One of nine known nearby, dwarf spheroidal companion galaxies of the Milky Way, parts of SagDEG are so close that their stars are found within the outermost regions of the Milky Way's spiral disk, as close to the galactic center as the Solar System. Despite its proximity to the Milky Way, it was not discovered until 1994 because it was hidden from observers in the Solar System by the Milky Way's central region which presents a dense mask of foreground stars and dust that further obscures detection of SagDEG's extremely low surface brightness (Ibata et al, 1995 and 1994). Astronomers, however, found that some of the stars observed in the line-of-sight from the Solar System were not found to be moving as they should if they are located in the central regions of the Milky Way, and so they were gradually apportioned to a small, previously unseen galaxy.

Rodrigo Ibata, Rosemary Wyse;

Richard Sword, Laustsen et al,

1986; Ibata et al, 1997





Larger and jumbo contoured images.





Recent observations indicate that

in the next 100 million years, the

galaxy will move though the disk

of the Milky Way again possibly

for the 11th time (more at APOD

or JHU).



At first, many astronomers thought that the SagDEG had already reached an advanced state of destruction, so that a large part of its original matter was already mixed with that of the Milky Way. Subsequent observations indicated, however, that substantial parts of SagDEG still exist in a severely distorted fashion. Moving in a roughly polar orbit around the Milky Way as close as 50,000 ly from its galactic center, SagDEG comes far too close to the Milky Way for the dwarf to remain intact from the resulting gravitational tides. Although it may have begun as a ball of stars before falling towards the Milky Way, SagDEG is now being torn apart by immense tidal forces over hundreds of millions of years. Numerical simulations suggest that stars ripped out from the dwarf would be spread out in long streamers along its path, which were subsequently detected.

Unknown artist (source),

(Permission sought)





Larger alternate illustration.





Until stellar extensions of SagDEG

were found, the galaxy was thought to

lie near and mostly on the other side

of Milky Way's spiral disk (source).



Astronomers has been eager to view directly the destruction of the dwarf galaxy (as it is engulfed by the much larger Milky Way -- to study in detail and at first hand the mechanism governing the formation of large galaxies. They also hoped to find stars that were originally part of the dwarf galaxy and that would now be strewn along its entire orbit, thereby forming two streams that encircle the Milky Way. Unfortunately, these stellar streams would be extremely diffuse, so diffuse that they might be completely indistinguishable from Milky Way stars, even at short distances from the center of Sagittarius itself (Ibata et al, 1996; Christophe Alard, 1996; and Johnston et al, 1995).

© Kathryn V. Johnston, Chris Mihos,

Van Kleck Observatory/Wesleyan University

(Used with permission)



Larger and similar simulation images.





SagDEG may have begun as a ball mass of stars,

at top center, before falling towards the Milky Way

along the dashed line and being ripped apart into

long streamers along the path (more on SagDEG

and similar simulations).



In 1996, a team of astronomers found a stream of stars that were apparently stripped from SagDEG by the Milky Way as a "tidal trail" (Mateo et al, 1996). Extending to the southwest, it can be traced out to 34 degrees from the center of Sagittarius. Although theoretical models predicted the symmetric presence of another stream, extending to the northwest, that could be so long as to completely encircle the Milky Way, this stream was more difficult to find because it would cross the disk of the Milky Way and so be obscured by the dense stars, gas, and dust of the Galactic Center.

Four years later, another team of astronomers using the 2.5-meter Isaac Newton Telescope at Roque de los Muchachos Observatory on La Palma (and relying on evidence supplied by their own dynamical models of Sagittarius and on preliminary results from the international Sloan Digital Sky Survey team) announced that they had found an excess of young stars belonging to a stellar system located at 183,000 ly (56,000 pc) from the center of the Milky Way. Its position in the sky indicates that it probably contains stars belonging to the northwest stream that was stripped out to 60 degrees from Sagittarius (equivalent to 212,000 ly or 65,000 pc when measured along the orbit of Sagittarius) from the center of SagDEG. These remnants are the furthest from the centre of a progenitor detected and confirm that SagDEG has formed an arc that completely surrounds the Milky Way, just as predicted by theoretical models (more from: IAC press release; Martínez-Delgado et al, 2003, 2002, and 2001; and Dolm-Palmer et al, 2001). Studies of highly evolved, Carbon stars in the Milky Way's halo and of SagDEG's halo streams suggest they are of comparable age and so imply that the Milky Way and Sag DEG have been a strongly interacting system for most of their existence (Ibata et al, 2001).

David Law,

University of Virginia









Larger animation still.







SagDEG's "tidal stream" was also

detected using red (M) giant stars

from the Two-Micron All Sky Survey

(2MASS), where Sol's location is

depicted by the yellow dot (more).



Another team of astronomers (including Rosemary Wyse, Nicholas Suntzeff, Rodrigo Ibata, Gerard Gilmore, and Mike Irwin) estimated in 1998 that SagDEG orbits the Milky Way within less than a billion years. Because SagDEG must have already passed the dense central region of the Milky Way at least about ten times, the dwarf should have been more disrupted that is observed. However, astronomers now suspect that SagDEG has more dark matter than it was originally suspected to have to hold onto so many of its stars for so long inspite of the Milky Way's strong gravitional pull (press release). Much of its current state of tidal disruption may actually have come from its last pass through the Milky Way, rather than from the current one (Velazquez and White, 1998). In 2003, some astronomers modeling SagDEG's movements with a full-sky map of red (M) giant stars attributed to the galaxy that were detected through the Two-Micron All Sky Survey (2MASS) speculated that it was once pulled through the Milky Way's disk very close to Sol's current location (Majewski et al, 2003; and Law et al, 2003).

© Kathryn V. Johnston, Chris Mihos,

Van Kleck Observatory/Wesleyan University

(Used with permission)



Larger illustration.



"Fossil" remnants of satellite galaxies that have

collided with the Milky Way may be observed as

star streams (more discussion and simulations).



In return, the close proximity of SagDEG has also affected the Milky Way's spiral disk, warping it like an old-fashioned phonograph record left out in the hot sun. Like a sun-baked record spinning on a turntable, the Milky Way spins around with similarly floppy and wobbly gyrations every 250 million or so years (Hallum and Byrd, 1994; and Frank J. Kerr, 1957). According to Jeremy Bailin, an astronomy graduate student at the University of Arizona in Tucson, SagDEG has the right orbital characteristics to have caused the warp. SagDEG is located only a third as far from the center of the Milky Way as the Magellanic Clouds and its orbital motion is linked to the rotation of the Milky Way's disk.



Edward L. Wright, COBE, DIRBE, NASA -- larger infrared image

(SagDEG may be warping the Milky Way's spiral disk -- more on this image at APOD.)

Bailin's analysis of the galactic warp is based on angular momentum -- a measure of how much a system is spinning or rotating. When two spinning objects spinning collide, their angular momentum is combined. When Bailin deduced the angular momentum of the warped portion of the Milky Way's disk and compared that measure with SagDEG's angular momentum, he found that, within the margins of measurement error, the two angular momenta are very similar in strength and direction. As such a great similarity in the angular momentum of the two bodies are not likely to be a coincidence, Bailin's findings appear to provide strong circumstantial evidence that the interaction of SagDEG with the Milky Way disk created its warp. With continued spinning, the warp should eventually disappear after a few hundred million years to a few billion years unless another dwarf galaxy like Sagittarius collides with the right orbital motion to warp the Milky Way's disk again (Jeremy Bailin, 2003; and Charles Liu, 2003). (An image of a warp in spiral galaxy ESO 510-13, that also may have developed from a galactic collision, is available at Astronomy Picture of the Day).