The Virgo Cluster is the nearest massive cluster of galaxies (~50 million light years away) and it contains well over a thousand member galaxies. Such a massive cluster of galaxies creates a huge gravitational potential well around it and galaxies from outside do occasionally fall in at very high speeds. One such galaxy is IC 3418 - a dwarf irregular galaxy that is falling into the Virgo Cluster at a very high speed of ~1000 km/s. IC 3418 features a long trail of material measuring over 50,000 light years in length. This trail is believed to have formed behind IC 3418 due to ram pressure stripping of material from IC 3418 as it plows through the intergalactic medium on its plunge into the Virgo Cluster.
Signs of star formation are evident within the long trail of
material behind IC 3418. This is a unique environment for star formation
because stars that form here are technically in intergalactic space where the
density of gas and dust is a lot sparser than in a galaxy. In contrasts,
traditional star formation occurs within the denser environment of a galaxy.
Spectroscopic observations of the star-forming trail of IC 3418 have revealed
the presence of a compact and luminous source of optical emission located near
the far end of the trail. This compact source is catalogued as SDSSJ1229+1122
and it is spectroscopically consistant with either a collection of 50 to 200 B-type
stars or a single blue supergiant star. In turns out that SDSSJ1229+1122 is
more likely to be a single blue supergiant star because a collection of so many
B-type stars requires a very unlikely star formation scenario. Additionally, spectroscopic
observations have revealed possible hints of a powerful stellar wind that is
consistant with a single blue supergiant star.
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Image: Artist’s impression of Rigel - a blue supergiant
star. Credit: Guillermo Krieger
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Image: Size comparison of Gamma Orionis (a typical blue
giant star) with Algol B (left) and the Sun (bottom). The single blue
supergiant star in SDSSJ1229+1122 would appear many times larger and more
luminous than Gamma Orionis.
As a single blue supergiant star, SDSSJ1229+1122 could only
have formed close to where it currently is, which is within the trail of material
behind IC 3418. This is because such a massive star has a lifespan of only
several million years and that is too short a time for it to have formed within
the galactic environment of IC 3418 before moving to its current location.
Instead, the blue supergiant star probably formed through a process of turbulence-driven
star formation. Turbulence in the trail of material behind IC 3418 creates
eddies that aggregate clouds of gas and dust into dense clumps which collapse
under their own gravity to form stars. As a result, turbulence-driven star
formation can explain the presence of a short-lived blue supergiant star that
is on its own in intergalactic space, thousands of light years from the nearest
galaxy.
Reference:
Youichi Ohyama and Ananda Hota (2013), “Discovery of a
possibly single blue supergiant star in the intra-cluster region of Virgo
cluster of galaxies”, arXiv:1304.2560 [astro-ph.CO]