The colossal superwind of the Cigar galaxy

The astronomer Rafael Bachiller discovers in this series the most spectacular phenomena of the Cosmos. Themes of palpitating research, astronomical adventures and scientific novelties about the Universe analyzed in depth.

Through observations with the SOFIA telescope, astronomers have studied the properties of a very violent superwind that emanates from the center of the Cigar galaxy, very active due to its exuberant star-forming buds.

Situated 12 million light years away in the constellation of the Big Dipper, the Cigar galaxy (Messier 82) is an irregular galaxy that has experienced spectacular outbreaks of star formation in its central region. This unbridled star-forming activity is thought to have been triggered by gravitational interaction with another smaller galaxy (Messier 81) close to it. The Cigar galaxy is of very special interest to astronomers, as it is the closest galaxy to the Earth to have this type of great activity in star formation.

The galaxy M82 is seen by singing and it is this orientation that gives it the peculiar aspect of a cigar. In this cosmic cigar, stars form ten times faster than in the Milky Way. The most recent and violent episodes of star formation happened about 10 million years ago, in the same nucleus of the galaxy, and 5 million years ago, in a ring around the central region.

M81, left, and M82 (Cigar galaxy, right)
After these innumerable stars have formed, the combination of individual stellar winds forms what astronomers call a ‘superwind’: a gigantic ejection of radiation, gas and dust, which emerges from the star-forming regions escaping through the galaxy, perpendicular to its plane, to reach extragalactic space.

The Stratospheric Observatory for Infrared Astronomy (SOFIA), the result of collaboration between the North American (NASA) and German (DLR) space agencies, is a very special instrument: a 2.7-metre diameter mirror telescope mounted on the stern of the fuselage of a Boeing 747 jumbo.

Its objective is to capture far-infrared radiation that, when shielded by the atmosphere, does not reach the telescopes installed on the Earth’s surface. The jumbo transports the SOFIA telescope at an altitude of 12,000 metres, above the most opaque layers of the atmosphere, and from this privileged position it is possible to observe the extreme infrared radiation of the stars.

The SOFIA telescope on board the 747 jumbo NASA/DLR/SOFIA
A group of astronomers led by Terry Jay Jones, of the University of Minnesota, has just used the SOFIA telescope to observe the superwind of the Cigar galaxy in the far infrared, specifically at wavelengths of 53 and 154 microns. The small dust grains of the superwind emit this type of infrared and, therefore, by studying this radiation, the characteristics of the dust and the superwind in which it is transported can be studied.

The grains of dust, like those found in the superwind of M82, are not spherical but tend to be oblong, i.e. a littlepepinned. Due to the electrical charge they contain, these grains tend to align with their major axis along the magnetic field of the place where they are located.

This makes the emission of such grains of dust always polarized (the electric field of the waves oscillates in a peculiar way), and the direction of the polarization reveals the direction of the magnetic field in the zone. Thus, by observing the infrared polarized light of M82’s superwind, astronomers have been able to reconstruct the magnetic structure of the wind.

It turns out that the superwind seems to carry the galaxy’s magnetic field with it up to a record distance of about 2000 light years from the galactic centre. The image at the top of this article shows (in yellow) that the magnetic field is perpendicular to the plane of the galaxy throughout that region and very well aligned with the wind. However, outside this zone, the magnetic field remains more or less horizontal, permeating the plane of the galaxy. These observations have also made it possible to determine that the superwind carries a mass of gas and huge dust, equivalent to about 55 million suns.

It is not known which are the detailed physical mechanisms that get to form superwinds so massive in active galaxies as the one of the Cigarette. These winds emerge spectacularly bipolar, from the centre of the galaxies and in the two opposite directions along the direction perpendicular to the galactic plane.

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