African Physics Newsletter

MeerKAT telescope discovers radio bubbles at center of our galaxy

MeerKAT at sunset Source: SARAO

Images show entirely new features, formerly unknown, of which the most striking is a pair of bubble-shaped structures across the galactic plane.

The South African radio telescope MeerKAT has discovered giant bubbles at the center of our galaxy.

The discovery was serendipitous. The telescope pointing was set up near the time of inauguration in July 2018 to obtain images showing the capability of this new array of 64 radio antennas. When the data were analyzed over the following year, these previously unknown enormous radio bubbles emerged. This first major result reported with MeerKAT was published in Nature.

The bubbles extend out on either side of the galactic plane for a distance of about 700 light years and are expanding outwards. Their symmetry around the galactic plane is a factor that is consistent with an unusually energetic explosive event associated with the Galactic Centre.


The galactic plane, marked by a series of bright features (left, dashed line on the right drawing), runs horizontally through the image. The newly discovered radio bubbles extend vertically above and below (right: pale blue outline). The bright patch in the lower-left corner (right: red outline) is an HII region in the foreground of the image. The HII line is emitted when ionized hydrogen recombines.
Source: SARAO, Oxford University, and the National Radio Astronomy Observatory (NRAO) and adapted by I. Gledhill from Heywood et al., 2019.

A supermassive explosion

The speed of the gas near the base of the bubbles and their clearly delineated outline indicates a massive short-duration explosion in the Galactic Centre approximately 7 million years ago. Could this have been an object falling into the supermassive black hole at the center of our galaxy? The event certainly took place in the close vicinity, but it is not yet clear whether it was due to a black hole-related event, or due to a starburst (a period of much heightened star formation and supernova explosions), or a combination of the two.

In comparison with the central regions of many other galaxies, the black hole that has been detected within the radio source Sgr A* appears to be relatively quiescent, although record-breaking X-ray flares and sudden optical brightening events have been observed from this source and are consistent with bodies falling in via an accretion disk.

A tough neighborhood

With a mass of about 4.4 million times the mass of our own sun, the black hole at the center of our galaxy has an event horizon radius – the Schwarzschild radius – of about 0.08 Astronomical Units. A handy way to visualize this is to imagine that our sun is replaced by the Sagittarius Sgr A* black hole: it would subtend an angle 20 times that of our sun.

In the galactic nucleus, extending about 4 light years from the center, thousands of stars move in complex orbits. Star formation has been taking place at a relatively constant rate over the last 100 million years, through occasional short-duration starburst episodes.

At present, we observe large volumes which are close to the critical density for star formation. Close to Sgr A*, in this extreme environment, is a magnetar, a neutron star with an extraordinarily high magnetic field, in this case between 109 and 1011 Tesla (the strongest permanent magnets on earth are less than 5 Tesla).

An unknown mechanism

The 800 light year-wide inner part of the galactic core contains “large amounts of warm molecular gas, a high cosmic ray ionization rate, unusual gas chemistry, enhanced synchrotron emission, and a multitude of radio-emitting magnetized filaments, the origin of which has not been established”, according to the authors of the Nature's paper. Extending on either side of the Galactic Centre are two lobes of plasma radiating gamma rays and X-rays known as the Fermi bubbles, which extend out to 25 000 light years from the core.

The authors of the MeerKAT paper remark that the event that formed the radio bubbles could have been the mechanism that generated a very high density of cosmic ray particles, and accelerated the relativistic particles that light up the bright magnetized filaments in the images of the galactic center.

The authors

In an international effort of this scale, collaborative authorships acknowledge a multitude of roles. Explaining the shared contributions is useful because the practice can be puzzling to authors in other fields: how can so many people make real contributions to one paper? In this case, Ian Heywood and Fernando Camilo planned these MeerKAT observations; both are part of the South African Radio Astronomy Observatory, and Ian Heywood is also affiliated with both Rhodes and Oxford Universities. Ian Heywood performed the calibration and imaging of the observations. These two authors wrote the paper with Farhad Yusef-Zadeh and William Cotton, both from the USA. The other 94 authors, most of whom are South Africans, have contributed to one or more of the planning, design, construction, commissioning or operation of the MeerKAT radio telescope, and all are collectively part of what is called the Builders’ List of the MeerKAT telescope. Many of them are students or former students within the SKA-South Africa program of postgraduate bursaries, grants, and fellowships.

Igle Gledhill, South Africa

This article has first been published in the African Physics Newsletter © 2020 American Physical Society


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