Australian researchers have discovered why some galaxies are
“clumpy” rather than spiral in shape, and it appears low spin is to
blame.The inding challenges an earlier theory that high levels of
gas cause clumpy galaxies, and sheds light on the conditions that
brought about the birth of most of the stars in the universe.
Ten billion years ago the universe was full of clumpy galaxies,
but these developed more regularity as they evolved. Dr Danail
Obreschkow of TheUniversity of WesternAustralia node of the
International Centre for Radio Astronomy Research (ICRAR) said
the majority of stars in the sky today, including ourive-billion-
year-old Sun, wereprobably born inside these clumpy formations.
“The clumpy galaxies produce stars at phenomenal rates,”
Obreschkow said. “A new star pops up about once a week, whereas
spiral galaxies like our Milky Way only form about one new star
a year.”
ICRAR and SwinburneUniversityastronomers focused on a
few rare galaxies, known as theDYNAMOgalaxies, that still look
clumpy even though they’reobserved“a mere”500 millionyears
in the past.Obreschkowsaid this was like looking at a passport
photo taken only last year whereas “the galaxies that are10 billion
lightyears away, that’scomparable to a picture from when you
were 3 or 4 years old.That’s very different.”The team used the Keck and Gemini observatories in Hawaii
to measure the spin of the galaxies, along with millimetre and
radio telescopes to measure the amount of gas they contained.
Obreschkow said the DYNAMO galaxies had a low spin and
this was the dominant cause of their clumpiness rather than their
high gas content, as previously thought. “While the Milky Way
appears to have a lot of spin, the galaxies we studied here have a low
spin, about three times lower,” he said.
It seems that the spin of the initial cloud of gas plays an impor-
tant part in galaxy formation, suggesting that spin explains why early
galaxies are gas-rich and lumpy while modern galaxies display
beautiful symmetric patterns.Black holes at the heart of galaxies could swell to 50 billion times the
mass of the Sun before losing the discs of gas they rely on to sustain
themselves.
A landmark study by Prof Andrew King of the University of Leicester
has explored supermassive black holes at the centre of galaxies, around
which are regions of space where gas settles into an orbiting disc. This
gas can lose energy and fall inwards, feeding the black hole, but these
discs are also unstable and prone to crumbling into stars. King calculated
how big a black hole would have to be for its outer edge to keep a disc
from forming, and arrived at the figure of50 billion solar masses!
The study suggests that the black hole would stop growing without a
disc, meaning 50 billionSuns would roughly be the upper limit. The only
way it could get larger is if a star happened to fall straight in or another
black hole merged with it. With the recent news confirming the detection
of gravitational waves, this scenario is very compelling.
“The significance of this discovery is that astronomers have found
black holes of almost the maximum mass, by observing the huge amount
of radiation given off by the gas disc as it falls in,” King said. “The mass
limit means that this procedure should not turn up any masses much
bigger than those we know, because there would not be a luminous disc.”
King suggests that bigger black hole masses are possible.For
example, a hole near the maximum mass could merge with another black
hole, and the result would be bigger still. But no light would be produced
in this merger, and the bigger merged black hole could not have a disc of
gas that would make light.
“One might nevertheless detect it in other ways, for example as it bent
light rays passing very close to it as in gravitational lensing, or perhaps in
future from the aforementioned gravitational waves that Einstein’s
general theory of relativity predicted would be emitted as it merged.”
Sagittarius A, a black hole measuring 4.3 million solar masses at the
centre of the Milky Way, is the only black hole whose mass has been
measured directly by analysing the full orbit of a circling star.APRIL 2016|| 45David Reneke is an astronomylecturer and teacher, a feature writer formajorAustralian newspapers andmagazines, and a science correspondent for ABC and commercialradio.
Subscribe toDavid’s free Astro-Space newsletter atwww.davidreneke.comOUT OF THIS WORLD David RenekeBlack Holes Could Grow as Large as 50 Billion Suns
A New Spin on Star-Forming
Galaxies
An artist’s impression of a
star being distorted by its
close passage to a
supermassive black hole
at the centre of a galaxy.
Credit: NASARegular spiral galaxies (left) form far fewer stars than the clumpy
galaxy on the right. Blue regions have the least star-forming gas
and red-yellow have the most. Dr Danail Obreschkow, ICRAR