A duo of astronomers, Dr. Youichi Ohyama (Institute
of Astronomy and Astrophysics, Academia Sinica or ASIAA, Taiwan) and Dr.
Ananda Hota (UM-DAE Centre for Excellence in the Basic Sciences or CBS,
India), has discovered a Blue Supergiant star located far beyond our
Milky Way Galaxy in the constellation Virgo (Figure).
Over fifty-five million years ago, it emerged in an extremely wild
environment, surrounded by intensely hot plasma (a million degrees
centigrade) and amidst raging cyclone winds blowing at four-million
kilometers per hour. Research using the Subaru Telescope, the
Canada-France-Hawaii-Telescope (CFHT) and the National Aeronautics and
Space Administration's (NASA) Galaxy Evolution Explorer (GALEX) revealed
unprecedented views of the star formation process in this intergalactic
context and showed the promise of future investigations of a possibly
new mode of star formation, unlike that within our Milky Way.
Figure: Pseudo-color
GALEX ultraviolet image of the galaxy IC 3418 falling into the Virgo
cluster. Notice the young star-forming clumps in its 55,000
light-years-long trail, as the galaxy moves towards the top-right area.
Zooming into one of the blobs, marked by the arrow, the colour optical
image from CFHT shows the bright Blue Supergiant star in the middle of
the inset image in the top-left area. The optical spectrum from the same
star (bottom-right area), which was obtained by Subaru Telescope's
Faint Object Camera and Spectrograph (FOCAS), shows only one bright red
emission line (H-alpha) due to the stellar wind and none of the other
usual signs of star-forming regions. (Credit: NAOJ, CFHT, GALEX, Y.
Ohyama & A. Hota)
About one thousand galaxies reside in a cluster
filled with million-degree hot plasma and dark matter. The Virgo
cluster, the nearest cluster of galaxies located about 55 million light
years from Earth in the constellation Virgo, is an ideal laboratory to
study the fate of gas stripped from the main body of galaxies falling
into the intra-cluster medium. Does star formation take place in the
clouds of stripped gas? If so, how? Dr. Ohyama and Dr. Hota focused on
the trail of IC 3418 to explore a potentially new mode of star
formation. Dr. Hota has been collecting data from multiple telescopes
since 2006 to understand this galaxy, which he first spotted in the
GALEX data during his Ph.D. research.
IC 3418 is a small galaxy falling into the Virgo
cluster of galaxies at such a high speed (a thousand kilometers per
second) that its blanket of cool gas strips off. As it passed through
the cluster, its stripped-off cool gas formed a 55,500 light-years-long
trail that looks very much like the water vapour condensation trail from
a supersonic jet's path. Hot plasma surrounds the trail of IC 3418, and
it has not been clear whether the clouds of cool gas would vaporize
like water sprinkled on a hot frying pan or condense further to form new
young massive stars. The GALEX ultraviolet image shows that new massive
stars do form in the trail. How did the stripped gas form new stars in
the hot plasma? This process does not conform to star formation in our
Milky Way Galaxy where massive stars develop in groups inside of stellar
nurseries sheltered within giant cold molecular gas clouds.
Dr. Ohyama suspected that a tiny dot of light
emission in the trail of IC 3418 might be different from other blobs of
ultraviolet light emissions in the trail. Spectroscopy of the little dot
from Subaru Telescope's Faint Object Camera and Spectrograph (FOCAS)
revealed something stunning. Dr. Ohyama recalls, "When I first saw the
spectrum, I was so puzzled, since it did not look like anything I had
known of in extra-galactic astronomy." Unlike typical star-forming
regions, the telltale signs of stellar nurseries were missing.
Intense UV-radiation usually ionizes/heats-up the
surrounding gas when a star is born. Instead of any sign of heated gas,
the observation showed fast winds blowing out of the stellar atmosphere
at a speed of about 160 kilometers per second. Comparison with emissions
from nearby stars made it clear that this massive, hot (O-type) star
had passed its youth and was now aging; it was at a stage known as Blue
Supergiant star and would soon face its explosive death as a supernova.
Dr. Ohyama commented on the significance of the
research: "If our interpretations are correct, this is probably the
farthest star ever discovered with spectroscopic observation. Since we
only observed for a fraction of the night with the 8.2 m Subaru
Telescope, there is huge potential for stellar spectroscopy with
extremely large telescopes, e.g., the Thirty Meter Telescope, being
planned for the future. We look forward to that exciting time."
Dr. Hota emphasized how important it is for
astronomers to pay attention to this exotic system: "Precisely because
the thermal and dynamic contrast of star formation that our research
shows cannot be observed within our Milky Way, the details revealed by
the Subaru Telescope's spectroscopy and the deep, sharp imaging of CFHT
are opening up a new avenue for investigating the baffling fundamentals
of star formation." Future in-depth investigations of this cocktail of
hot plasma and turbulent, cold gas may reveal very different
characteristics of stars, which may remain wild, exotic objects,
challenging current theories of star formation.
References:
- The research paper entitled "Discovery of a Possibly Single Blue Supergiant Star in the Intra-Cluster Region of Virgo Cluster of Galaxies" on which this article is based was published in the Astrophysical Journal Letters, Volume 767, No. 2.
- Three other galaxies similar to IC 3418 were discovered earlier by the Hubble Space Telescope, Subaru Telescope and GALEX. Refer to the following links for more information:
- Hubble Sees 'Comet Galaxy' Being Ripped Apart By Galaxy Cluster
(Hubble Space Telescope) - Galaxy Ramming Through Space Creates Fireballs (Subaru Telescope)
- Astronomers Discover Star-Studded Galaxy Tail (GALEX)
Acknowledgements:
This research was partially supported by the following:
This research was partially supported by the following:
- National Science Council of Taiwan (grant to Dr. Ohyama)
- National Centre for Radio Astrophysics of Tata Institute of Fundamental Research (NCRA-TIFR) and Inter-University Centre for Astronomy and Astrophysics (IUCAA), both in India (for Visiting Astronomer position to Dr. Hota).
