ESA’s Herschel space observatory has made detailed observations of
surprisingly hot molecular gas that may be orbiting or falling towards
the supermassive black hole lurking at the centre of our Milky Way
galaxy.
Our local black hole is located in a region known as Sagittarius A* –
Sgr A* – after a nearby radio source. It has a mass about four million
times that of our Sun and lies around 26 000 light-years away from the
Solar System.
Even at that distance, it is a few hundred times closer to us than any
other galaxy with an active black hole at its centre, making it the
ideal natural laboratory to study the environment around these enigmatic
objects.
Vast amounts of dust lie in the plane of the Milky Way between here and
its centre, obscuring our view at visible wavelengths. But at
far-infrared wavelengths, it is possible to peer through the dust,
affording Herschel’s scientists the chance to study the turbulent
innermost region of our Galaxy in great detail.
Copyright: Radio-wavelength
image: National Radio Astronomy Observatory/Very Large Array (courtesy
of C. Lang); spectrum: ESA/Herschel/PACS & SPIRE/J.R. Goicoechea et
al. (2013).
Herschel has detected a great variety of simple molecules at the Milky
Way’s heart, including carbon monoxide, water vapour and hydrogen
cyanide. By analysing the signature from these molecules, astronomers
have been able to probe some of the fundamental properties of the
interstellar gas surrounding the black hole.
“Herschel has resolved the far-infrared emission within just 1
light-year of the black hole, making it possible for the first time at
these wavelengths to separate emission due to the central cavity from
that of the surrounding dense molecular disc,” says Javier Goicoechea of
the Centro de Astrobiología, Spain, and lead author of the paper
reporting the results.
The biggest surprise was quite how hot the molecular gas in the
innermost central region of the Galaxy gets. At least some of it is
around 1000ºC, much hotter than typical interstellar clouds, which are
usually only a few tens of degrees above the –273ºC of absolute zero.
While some of the heating is down to the fierce ultraviolet radiation
pouring from a cluster of massive stars that live very close to the
Galactic Centre, they are not enough to explain the high temperatures
alone.
In addition to the stellar radiation, Dr Goicoechea’s team hypothesise
that emission from strong shocks in highly-magnetised gas in the region
may be a significant contributor to the high temperatures. Such shocks
can be generated in collisions between gas clouds, or in material
flowing at high speed from stars and protostars.
“The observations are also consistent with streamers of hot gas speeding
towards Sgr A*, falling towards the very centre of the Galaxy,” says Dr
Goicoechea. “Our Galaxy’s black hole may be cooking its dinner right in
front of Herschel’s eyes.”
Just before material falls into a black hole, it is heated up enormously
and can cause high-energy X-ray and gamma-ray flares. While Sgr A*
currently shows little sign of such activity, this could change soon.
Using near-infrared observations, other astronomers have spotted a
separate, compact cloud of gas amounting to just a few Earth masses
spiralling towards the black hole. Located much closer to the black hole
than the reservoir of material studied by Herschel in this work, it may
finally be gobbled up later this year.
Spacecraft including ESA’s XMM-Newton and Integral will be waiting to
spot any high-energy burps as the black hole enjoys its feast.
“The centre of the Milky Way is a complex region, but with these
Herschel observations, we have taken an important step forward in our
understanding of the vicinity of a supermassive black hole, which will
ultimately help improve our picture of galaxy evolution,” says Göran
Pilbratt, ESA’s Herschel project scientist.
Notes for Editors
“Herschel Far-Infrared Spectroscopy of the Galactic Center. Hot
Molecular Gas: Shocks versus Radiation near Sgr A∗” by J.R. Goicoechea
et al., is published in Astrophysical Journal Letters, 7 May 2013. Access paper.
The study focuses on a spectral scan towards Sgr A∗ at wavelengths of
approximately 52–671 microns taken with the PACS and SPIRE
spectrometers, and is part of the PRISMAS (PRobing InterStellar
Molecules with Absorption line Studies) and SPECHIS (SPIRE Spectral Line
Surveys of HIFI-GT-KP Sources) Herschel Guaranteed-Time Programmes.
PACS spectra between 52 microns and 190 microns were obtained during
March 2011 and March 2012. SPIRE observations between 194 microns and
671 microns were obtained during February 2011.
Herschel launched on 14 May 2009 and completed science observations on 29 April 2013.
Herschel is an ESA space observatory with science instruments provided
by European-led Principal Investigator consortia and with important
participation from NASA.
For further information, please contact:
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int
Javier R. Goicoechea
Centro de Astrobiologia CSIC-INTA, Spain
Tel: +34 91 520 6422
Email: jr.goicoechea@cab.inta-csic.es
Göran Pilbratt
ESA Herschel Project Scientist
Tel: +31 71 565 3621
Email: gpilbratt@rssd.esa.int
