Artist’s impression of a fast radio burst traveling through space and reaching Earth
Infographic showing the path of FRB 18112 passing through the halo of an intervening galaxy
VLT image of the location of FRB 181112
Videos
ESOcast 207 Light: Enigmatic radio burst illuminates a galaxy’s tranquil halo (4K UHD)
Astronomers using ESO’s Very Large
Telescope have for the first time observed that a fast radio burst
passed through a galactic halo. Lasting less than a millisecond, this
enigmatic blast of cosmic radio waves came through almost undisturbed,
suggesting that the halo has surprisingly low density and weak magnetic
field. This new technique could be used to explore the elusive halos of
other galaxies.
Using one cosmic mystery to probe another, astronomers analysed the signal from a fast radio burst to shed light on the diffuse gas in the halo of a massive galaxy [1]. In November 2018 the Australian Square Kilometre Array Pathfinder
(ASKAP) radio telescope pinpointed a fast radio burst, named FRB
181112. Follow-up observations with ESO’s Very Large Telescope (VLT) and
other telescopes revealed that the radio pulses have passed through the
halo of a massive galaxy on their way toward Earth. This finding
allowed astronomers to analyse the radio signal for clues about the
nature of the halo gas.
“The signal from the fast radio burst exposed the
nature of the magnetic field around the galaxy and the structure of the
halo gas. The study proves a new and transformative technique for
exploring the nature of galaxy halos,” said J. Xavier Prochaska,
professor of astronomy and astrophysics at the University of California
Santa Cruz and lead author of a paper presenting the new findings
published today in the journal Science.
Astronomers still don’t know what causes fast radio bursts
and only recently have been able to trace some of these very short, very
bright radio signals back to the galaxies in which they originated.
“When we overlaid the radio and optical images, we could see straight
away that the fast radio burst pierced the halo of this coincident
foreground galaxy and, for the first time, we had a direct way of
investigating the otherwise invisible matter surrounding this galaxy,”
said coauthor Cherie Day, a PhD student at Swinburne University of
Technology, Australia.
A galactic halo
contains both dark and ordinary—or baryonic—matter that is primarily in
the form of a hot ionised gas. While the luminous part of a massive
galaxy might be around 30 000 light years across, its roughly spherical
halo is ten times larger in diameter. Halo gas fuels star formation
as it falls towards the centre of the galaxy, while other processes,
such as supernova explosions, can eject material out of the star-forming
regions and into the galactic halo. One reason astronomers want to
study the halo gas is to better understand these ejection processes
which can shut down star formation.
“This galaxy’s halo is surprisingly tranquil,” Prochaska said. “The
radio signal was largely unperturbed by the galaxy, which is in stark
contrast to what previous models predict would have happened to the
burst.”
The signal of FRB 181112 was comprised of a few pulses,
each lasting less than 40 microseconds (10 000 times shorter than the
blink of an eye). The short duration of the pulses puts an upper limit
on the density of the halo gas because passage through a denser medium
would broaden the duration of the radio signal. The researchers
calculated that the density of the halo gas must be less than 0.1 atoms
per cubic centimeter (equivalent to several hundred atoms in a volume
the size of a child’s balloon) [2].
“Like the shimmering air on a hot summer’s day, the
tenuous atmosphere in this massive galaxy should warp the signal of the
fast radio burst. Instead we received a pulse so pristine and sharp that
there is no signature of this gas at all,” said coauthor
Jean-Pierre Macquart, an astronomer at the International Center for
Radio Astronomy Research at Curtin University, Australia.
The study found no evidence of cold turbulent clouds or
small dense clumps of cool halo gas. The fast radio burst signal also
yielded information about the magnetic field in the halo, which is very
weak—a billion times weaker than that of a refrigerator magnet.
At this point, with results from only one galactic halo,
the researchers cannot say whether the low density and low magnetic
field strength they measured are unusual or if previous studies of
galactic halos have overestimated these properties. Prochaska said he
expects that ASKAP and other radio telescopes will use fast radio bursts
to study many more galactic halos and resolve their properties.
“This galaxy may be special,” he said. “We
will need to use fast radio bursts to study tens or hundreds of galaxies
over a range of masses and ages to assess the full population.”
Optical telescopes like ESO’s VLT play an important role by revealing
how far away the galaxy that played host to each burst is, as well as
whether the burst would have passed through the halo of any galaxy in
the foreground.
Notes
[1] A vast halo of low-density gas extends far beyond the
luminous part of a galaxy where the stars are concentrated. Although
this hot, diffuse gas makes up more of a galaxy’s mass than stars do, it
is very difficult to study.
[2] The density constraints
also limit the possibility of turbulence or clouds of cool gas within
the halo. Cool here is a relative term, referring to temperatures around
10 000°C, versus the hot halo gas at around 1 million degrees.
More Information
This research was presented in a paper published on 26 September 2019 in the journal Science.
The team is composed of J. Xavier Prochaska (University of
California Observatories-Lick Observatory, University of California, USA
and Kavli Institute for the Physics and Mathematics of the Universe,
Japan), Jean-Pierre Macquart (International Centre for Radio Astronomy
Research, Curtin University, Australia), Matthew McQuinn (Astronomy
Department, University of Washington, USA), Sunil Simha (University of
California Observatories-Lick Observatory, University of California,
USA), Ryan M. Shannon (Centre for Astrophysics and Supercomputing,
Swinburne University of Technology, Australia), Cherie K. Day (Centre
for Astrophysics and Supercomputing, Swinburne University of Technology,
Australia and Commonwealth Science and Industrial Research
Organisation, Australia Telescope National Facility, Australia), Lachlan
Marnoch (Commonwealth Science and Industrial Research Organisation,
Australia Telescope National Facility, Australia and Department of
Physics and Astronomy, Macquarie University, Australia), Stuart Ryder
(Department of Physics and Astronomy, Macquarie University, Australia),
Adam Deller (Centre for Astrophysics and Supercomputing, Swinburne
University of Technology, Australia), Keith W. Bannister (Commonwealth
Science and Industrial Research Organisation, Australia Telescope
National Facility, Australia), Shivani Bhandari (Commonwealth Science
and Industrial Research Organisation, Australia Telescope National
Facility, Australia), Rongmon Bordoloi (North Carolina State University,
Department of Physics, USA), John Bunton (Commonwealth Science and
Industrial Research Organisation, Australia Telescope National Facility,
Australia), Hyerin Cho (School of Physics and Chemistry, Gwangju
Institute of Science and Technology, Korea), Chris Flynn (Centre for
Astrophysics and Supercomputing, Swinburne University of Technology,
Australia), Elizabeth Mahony (Commonwealth Science and Industrial
Research Organisation, Australia Telescope National Facility,
Australia), Chris Phillips (Commonwealth Science and Industrial Research
Organisation, Australia Telescope National Facility, Australia), Hao
Qiu (Sydney Institute for Astronomy, School of Physics, University of
Sydney, Australia), Nicolas Tejos (Instituto de Fisica, Pontificia
Universidad Catolica de Valparaiso, Chile).
ESO is the foremost intergovernmental astronomy
organisation in Europe and the world’s most productive ground-based
astronomical observatory by far. It has 16 Member States: Austria,
Belgium, the Czech Republic, Denmark, France, Finland, Germany, Ireland,
Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland
and the United Kingdom, along with the host state of Chile and with
Australia as a Strategic Partner. ESO carries out an ambitious programme
focused on the design, construction and operation of powerful
ground-based observing facilities enabling astronomers to make important
scientific discoveries. ESO also plays a leading role in promoting and
organising cooperation in astronomical research. ESO operates three
unique world-class observing sites in Chile: La Silla, Paranal and
Chajnantor. At Paranal, ESO operates the Very Large Telescope and its
world-leading Very Large Telescope Interferometer as well as two survey
telescopes, VISTA working in the infrared and the visible-light VLT
Survey Telescope. Also at Paranal ESO will host and operate the
Cherenkov Telescope Array South, the world’s largest and most sensitive
gamma-ray observatory. ESO is also a major partner in two facilities on
Chajnantor, APEX and ALMA, the largest astronomical project in
existence. And on Cerro Armazones, close to Paranal, ESO is building the
39-metre Extremely Large Telescope, the ELT, which will become “the
world’s biggest eye on the sky”.
Link
Contact
J. Xavier Prochaska
UCO/Lick Observatory — UC Santa Cruz
USA
Tel: +1 (831) 295-0111
Email: xavier@ucolick.org
Cherie Day
Centre for Astrophysics and Supercomputing — Swinburne University of Technology
Australia
Tel: +61 4 5946 3110
Email: cday@swin.edu.au
Mariya Lyubenova
ESO Head of Media Relations
Garching bei München, Germany
Tel: +49 89 3200 6188
Email: pio@eso.org
Contact
J. Xavier Prochaska
UCO/Lick Observatory — UC Santa Cruz
USA
Tel: +1 (831) 295-0111
Email: xavier@ucolick.org
Cherie Day
Centre for Astrophysics and Supercomputing — Swinburne University of Technology
Australia
Tel: +61 4 5946 3110
Email: cday@swin.edu.au
Mariya Lyubenova
ESO Head of Media Relations
Garching bei München, Germany
Tel: +49 89 3200 6188
Email: pio@eso.org
Source: ESO/News
