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Expansion maps of the three detected bubbles, which show the detected
expansion velocity in each pixel, all in the same velocity scale.
are contours of the region's Hydrogen alpha
emission; it can be seen that the bubbles are roughly concentric with
each other and the region.
Figure extracted from Camps Fariña et al. (2016). Large format:JPEG
A group of astronomers, led by researchers at the Instituto de
Astrofísica de Canarias (IAC), has found the first known case of three
supernova remnants one inside the other.
Using a method developed within the group for detecting huge expanding
bubbles of gas in interstellar space, they were observing the galaxy M33
in our Local Group of galaxies and found an example of a triple-bubble.
The results help to understand the feedback phenomenon, a fundamental
process controlling star formation and the dissemination of metals
produced in massive stars.
The group has been building up a database of these superbubbles with
observations of a number of galaxies and, using the very high resolution
2D spectrograph GHaFaS (Galaxy Halpha Fabry-Perot System) on the
William Herschel Telescope (WHT), has been able to detect and measure
some tens of them in different galaxies, which range in size from a few
light years to as big as a thousand light years across.
Superbubbles around large young star clusters are known to have a
complex structure due to the effects of powerful stellar winds and
supernova explosions of individual stars, whose separate bubbles may end
merging into a superbubble, but this is the first time that they, or any
other observers, have found three concentric expanding supernova
shells. They are concentric because the supernovae which produced them
exploded at intervals of only 10,000 years, close to simultaneously on
astronomical timescales, so they are still relatively spherical and
surround their parent star cluster.
"This phenomenon—says John Beckman, one of the co-authors on the
paper—allows us to explore the interstellar medium in a unique way, we
can measure how much matter there is in a shell, approximately a couple
of hundred times the mass of the sun in each of the shells". However, if
it is known that a supernova expels only around ten times the mass of
the sun, where do the second and third shells get their gas from if the
first supernova sweeps up all the gas?
The answer to that must come from the structure of the surrounding gas:
the inhomogeneous interstellar medium. "It must be—says Artemi Camps
Fariña, who is first author on the paper—that the interstellar medium is
not at all uniform, there must be dense clumps of gas, surrounded by
space with gas at a much lower density. A supernova does not just sweep
up gas, it evaporates the outsides of the clumps, leaving some dense gas
behind which can make the second and the third shells".
"The presence of the bubbles—adds Artemi— explains why star formation on
cosmological timescales has been much slower than simple models of
galaxy evolution predicted. These bubbles are part of a widespread
feedback process in galaxy discs and if it were not for feedback, spiral
galaxies would have very short lives, and our own existence would be
improbable", concludes. The idea of an inhomogeneous interstellar medium
is not new, but the triple bubble gives a much clearer and quantitative
view of the structure and the feedback process. The results will help
theorists working on feedback to a better understanding of how this
process works in all galaxy discs.
A. Camps Fariña, J. E. Beckman, J. Font, A. Borlaff, J. Zaragoza, P.
Amram, 2016, "Three supernova shells around a young star cluster in
M33", MNRAS, 461, L87. DOI: 10.1093/mnrasl/slw106. Paper on MNRAS|arXiv.