Multiple Galaxy Collisions Surprise Hubble Astronomers

CONTACT: Ray Villard
Space Telescope Science Institute, Baltimore, MD
(Phone: 410-338-4514)
(E-mail: villard@stsci.edu)

Kirk Borne
Goddard Space Flight Center, Greenbelt, MD
(Phone: 301-286-0696)
(E-mail: borne@xfiles.gsfc.nasa.gov)

Hubble astronomers conducting research on a class of galaxies calledultra-luminous infrared galaxies (ULIRG) have discovered that over twodozen of these are found within "nests" of galaxies, apparently engagedin multiple collisions that lead to fiery pile-ups of three, four oreven five galaxies smashing together.

Astronomers previously thought only pairs of galaxies were interactingin ULIRGs. But Hubble pictures show a surprising amount of complexityand structure that astronomer Kirk Borne of NASA's Goddard Space FlightCenter in Greenbelt, Md., and collaborators, photo-interpret asmultiple galaxy collisions.

Tangled-up looking images from complex computer simulations of multiplegalaxy collisions resemble what Borne sees through Hubble's eye. Forconclusive proof that more than two galaxies are interacting, he plansto do follow-up spectroscopic observations to measure the collisionspeeds of the wayward galaxies.

Using Hubble to conduct a three-year survey of 123 ULIRGs within 3billion light-years of Earth, Borne found that 30% of them show strongvisual evidence for multiple mergers, where astronomers previouslythought only two galaxies were interacting.

"We are seeing the final stage of the hierarchical evolution of theuniverse, where small fragments coalesce to build ever bigger objects,says Borne. "We see matter ripped out of galaxies in the form of longtails of stars, and matter contracting in the form of multiple nucleicrowded together. In some we see a 'nest' of galaxies where they allcoalesce."

These results offer a snapshot of what conditions were like in theearly universe, when galaxy collisions were commonplace.

First detected by the IRAS satellite in the early 1980s, ULIRGs glowfiercely in infrared light, one hundred times brighter than our MilkyWay galaxy. The brilliant infrared (IR) glow of these galaxies iscaused by a firestorm of star birth triggered by the collisions. The IRcomes from a large amount of dust absorbing and re-radiating the lightof the hot newborn stars that precipitated from the collisions.

It was realized early on that ULIRGs are oddly shaped enough to beconsidered pairs of colliding galaxies. It wasn't until 1998 that ateam of Japanese scientists, Y. Taniguchi and Y. Shioya, theorizedthat ULIRGs might be multiple-mergers. "The Hubble results supportthis hypothesis," says Borne.

In research that has been submitted for publication in theAstrophysical Journal Letters, Borne reports that the progenitors tothese bang-ups are probably similar to the so-called Hickson compactgroups -- clusters of at least four galaxies in a tight configurationthat is isolated from other galaxies. Under the relentless pull ofgravity, tidal forces exist which dissipate momentum from the galaxiesand make them fall together.

Borne's co-investigators are Howard Bushouse (Space Telescope ScienceInstitute), Luis Colina (Instituto de Fisica de Cantabria, Spain) andRay Lucas (Space Telescope Science Institute).

The Space Telescope Science Institute is operated by the Associationof Universities for Research in Astronomy, Inc. for NASA, undercontract with NASA's Goddard Space Flight Center, Greenbelt, MD. TheHubble Space Telescope is a project of international cooperationbetween NASA and the European Space Agency.

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PHOTO CAPTION

EMBARGOED UNTIL: 1 a.m. (EST) November 22, 1999

PHOTO NO.: STScI-PRC99-45

MULTIPLE GALAXY COLLISIONS SURPRISE HUBBLE ASTRONOMERS

Astronomers have interpreted the oddly shaped objects in these NASAHubble Space Telescope snapshots as strong visual evidence formultiple galaxies crashing into each other. These smashups create atangled clump of matter and trigger a burst of new stars.

The photo at upper right, for example, appears to possess the nucleiof several galaxies. In another picture [bottom row, center], athree-galaxy collision has ripped several streamers of stars fromtheir homes. The galaxies are converging into one central spot.

The Wide Field and Planetary Camera 2 pictures reveal a surprisingamount of complexity and structure in these galaxies, calledultra-luminous infrared galaxies because they glow fiercely ininfrared light. The bright, infrared glow is caused by a firestorm ofstar birth triggered by the multiple-galaxy pileups.

These images are part of a three-year study of 123 galaxies within 3billion light-years of Earth. The study was conducted in 1996, 1997,and 1999. False colors were assigned to these photos to enhance finedetails within these merging galaxies.

Credits: NASA, Kirk Borne (Raytheon and NASA Goddard Space FlightCenter, Greenbelt, Md.), Luis Colina (Instituto de Fisica de Cantabria,Spain), and Howard Bushouse and Ray Lucas (Space Telescope ScienceInstitute, Baltimore, Md.)