Youngest Known Dwarf Galaxy Has Interacting Companion

CONTACT:
Lia Unrau (713) 831-4793
Reginald Dufour (713) 8101, ext. 3348

Youngest Known Dwarf Galaxy Has Interacting Companion

In a pairing of young and old, two
irregular dwarf galaxies, one famous for having the most unpolluted
primitive gas of any star-forming galaxy, are gravitationally
connected to one another, a Rice University astronomer announced
today.

The two dwarf galaxies share a nearly identical speed of
recession from the Earth and show evidence for a tidal pull between
them, Reginald Dufour, Rice professor of space physics and
astronomy, reported today at the American Astronomical Society
meeting in Madison, Wis. This may have implications for the
evolution of the gas in the younger of the two galaxies.

I Zw 18, the younger, brighter galaxy, named by astronomer Fritz
Zwicky, is a bluish, compact irregular dwarf galaxy believed to be
only a few tens of millions years old-a true galactic “baby”
compared to the age of most other galaxies (10 or more billion years
old). Located about 35 million light years away in Ursa Major (the
Big Dipper), I Zw 18 has been considered the most extreme case of a
system of stars forming out of unpolluted primitive gas-that is, the
primordial soup from the Big Bang containing only hydrogen and
helium gas, unpolluted by metal-rich ejections from dying stars.
Thus, I Zw 18 has been extensively studied and used as a benchmark
for determining the relative levels of hydrogen and helium in the
earliest days of the universe.

Last year Dufour and colleagues used the Hubble Space Telescope
to study I Zw 18 and nearby “blurry” objects previously seen from
ground-based telescopes. The images not only resolved I Zw 18 into
hundreds of stars, most younger than 10 million years old, but also
resolved stars in a nearby companion dwarf galaxy, similar to the
Small Magellanic Cloud (a companion galaxy to our Milky Way), which
could be only about 3-6,000 light years from I Zw 18.

Dufour
announced today that the association has been confirmed using
spectrographic observations from the 4.2-meter diameter William
Herschel Telescope (WHT) in the Canary Islands.
In a paper titled, “Discovery of a Dynamically Associated
Companion Galaxy to I Zw 18,” Dufour and his colleagues Cesar
Esteban of the Instituto de Astrofisica de Canarias (IAC), and
Hector O. Castaneda of the ISO Science Operations Centre and IAC,
report that measurements of the radial velocities-the relative
motion directly toward or away from Earth-from both the main star-forming body of I Zw 18 and the central bright emission knot in the
unnamed companion galaxy are practically identical, suggesting they
are moving at the same speed around one another and their gas is
responding to a tidal pull. The astronomers also see a link in the
hydrogen emission from I Zw 18 to the companion galaxy, providing
additional evidence for tidal interaction between the two galaxies.

“The nearly identical radial velocities of the two galaxies
indicate that they are at a similar distance and are indeed
gravitationally linked to each other,” Dufour says. “This is clearly
now a more complicated dual system than previously thought.”

The I Zw 18 galaxy has been the focus of many studies, due to
its nearly “pure” composition of neutral hydrogen and helium, and
extremely low abundance of heavier gases such as oxygen, carbon and
nitrogen, which are not associated with the Big Bang, but formed
later through fusion in massive stars, and ejected from these stars
upon dying. The relative levels of carbon and nitrogen in I Zw 18
hint that older stars produced some of these elements. However, the
recent HST studies show no evidence of such an older population of
stars in I Zw 18.

The companion galaxy may be the missing contributor of carbon
and nitrogen to I Zw 18, Dufour theorizes; if the two galaxies were
closer at one time, and the enriched interstellar gas of the
companion has been selectively pulled into the hydrogen clouds from
which stars are just forming in I Zw 18. The HST observations of the
companion galaxy indicate that it is older than I Zw 18 and contains
stars at least several hundreds of millions of years old and capable
of ejecting enriched material. In addition, the close passage of the
companion galaxy to I Zw 18 could have “triggered” the current star
formation, Dufour says.

Another scenario is that ejections from the companion system
occurred billions of years ago and dispersed into intergalactic
space before the two systems came close to each other, meaning the
gas now forming stars in I Zw 18 has not been significantly
polluted, Dufour and his colleagues write in a paper submitted for
publication in the Astrophysical Journal Letters.
If ejections from stars in the older companion galaxy have
indeed “polluted” the hydrogen clouds in I Zw 18, it may no longer
be considered an isolated system in terms of its chemical evolution,
Dufour notes.

To answer some of the new questions about the nature and
chemical evolution of I Zw 18 raised by this dual-galaxy scenario,
Dufour is collaborating with American and Spanish astronomers in
additional observations with HST planned for later this year, and
with WHT in early 1997.

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Color and black & white GIF and JPEG HST images of I Zw 18 and the
companion galaxy are available by anonymous ftp to spacsun.rice.edu
(pub/rjd/ZWCOMP). The press release materials and images are also
available on the World Wide Web at
http://riceinfo.rice.edu/projects/reno/Rice.news/pr/DwarfGalaxy.html..