Friday, April 12, 2013

A Stray Blue Supergiant Star

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.

Image: Artist’s impression of Rigel - a blue supergiant star. Credit: Guillermo Krieger

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.

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]