Discovery of a Blue Supergiant Star Born in the Wild

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:
1. Hubble Sees 'Comet Galaxy' Being Ripped Apart By Galaxy Cluster
   (Hubble Space Telescope)
2. Galaxy Ramming Through Space Creates Fireballs (Subaru Telescope)
3. Astronomers Discover Star-Studded Galaxy Tail (GALEX)

Acknowledgements:
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).

Astronomers Discover Star-Studded Galaxy Tail

NASA's Galaxy Evolution Explorer found a tail behind a galaxy called IC 3418. The star-studded tail can be seen in the image on the left, as detected by the space telescope in ultraviolet light. The tail has escaped detection in visible light, as shown by the image on the right. Credit: NASA/JPL-Caltech - Full image and caption

NASA's Galaxy Evolution Explorer has discovered a galaxy tail studded with bright knots of new stars. The tail, which was created as the galaxy IC 3418 plunged into the neighboring Virgo cluster of galaxies, offers new insight into how stars form.

"The gas in this galaxy is being blown back into a turbulent wake," said Janice Hester of the California Institute of Technology in Pasadena, lead author of a recent study published in the Astrophysical Journal Letters. "The gas is like sand caught up by a stiff wind. However, the particular type of gas that is needed to make stars is heavier, like pebbles, and can't be blown out of the galaxy. The new Galaxy Evolution Explorer observations are teaching us that this heavier, star-forming gas can form in the wake, possibly in swirling eddies of gas."

Collisions between galaxies are a fairly common occurrence in the universe. Our Milky Way galaxy will crash into the Andromeda galaxy in a few billion years. Galaxies tangle together, kicking gas and dust all around. Often the battered galaxies are left with tails of material stripped off during the violence.

Hester and her team studied the tail of IC 3418, which formed in a very different way. IC 3418 is mingling not with one galaxy, but with the entire Virgo cluster of galaxies 54 million light-years away from Earth. This massive cluster, which contains about 1,500 galaxies and is permeated by hot gas, is pulling in IC 3418, causing it to plunge through the cluster's gas at a rate of 1,000 kilometers per second, or more than 2 million miles per hour. At this incredible speed, the little galaxy's gas is being shoved back into a choppy tail.

The astronomers were able to find this tail with the help of the Galaxy Evolution Explorer. Clusters of massive, young stars speckle the tail, and these stars glow with ultraviolet light that the space telescope can see. The young stars tell scientists that a crucial ingredient for star formation - dense clouds of gas called molecular hydrogen - formed in the wake of this galaxy's plunge. This is the first time astronomers have found solid evidence that clouds of molecular hydrogen can form under the violent conditions present in a turbulent wake.

"IC 3418's tail of star-formation demonstrates that strong turbulence promotes cloud formation," said Mark Seibert, a co-author of the paper and a member of the Galaxy Evolution Explorer science team at the Carnegie Institute for Science in Pasadena.

Hester added that galaxy tails provide the perfect environment for isolating the factors controlling star formation.

"These tails are unique, exotic locations where we can probe the precise mechanisms behind star formation," said Hester. "Understanding star formation is pivotal to understanding the lifecycles of galaxies and the dramatic transformations that some galaxies undergo. We can also study how the process affects the development of planets like our own."

Other authors of the paper are James D. Neill, Ted K. Wyder and Christopher Martin of Caltech; Armando Gil de Paz of the Universidad de Computense de Madrid, Spain; Barry F. Madore of the Carnegie Institute of Washington; David Schiminovich of Columbia University, N.Y., N.Y; and Michael Rich of UCLA.

Caltech leads the Galaxy Evolution Explorer mission and is responsible for science operations and data analysis. NASA's Jet Propulsion Laboratory in Pasadena manages the mission and built the science instrument. The mission was developed under NASA's Explorers Program managed by the Goddard Space Flight Center, Greenbelt, Md. Researchers sponsored by Yonsei University in South Korea and the Centre National d'Etudes Spatiales (CNES) in France collaborated on this mission.

Graphics and additional information about the Galaxy Evolution Explorer is online at http://www.nasa.gov/galex/ and http://www.galex.caltech.edu

Whitney Clavin 818-354-4673

Jet Propulsion Laboratory, Pasadena, Calif.

Whitney.clavin@jpl.nasa.gov