G objects
“Part of the problem is angular momentum.
Orbiting gas and orbiting stars can’t usually get too
close because of their inherent angular momentum,
which presents what is called a centrifugal barrier
and makes it hard to feed the black hole. But gas
can apparently be pulled off the G objects as they
orbit close to the black hole, and that gas can
ultimately be drawn into the black hole as it joins
the accretion f low of matter onto the black hole.”
If correct, it would mean that G objects are
contributing to the growth of black holes. They’re
probably not the dominant contributor, but it
might help to explain why, in September last
year, the UCLA team was also able to observe the
brightest light in 24 years of supermassive black
hole observation, pointing to the galaxy having
a scrumptiously large feast of interstellar gas and
dust. The team is now looking for more G objects,
and it says it has identified some candidates. “We
will continue to identify more G objects and to
determine their orbits more precisely with more
measurements in coming years,” Morris says.
“The next generation of big telescopes will be
very useful for seeing in much improved detail
what happens as G objects swing closely around the
black hole in their orbits, and they will help to tell
Binary stars
have merged
As one of two most likely
scenarios, this theory
suggests the G objects
are merged binary stars
that have an outer layer of
gas and dust. It suggests
that close proximity to
black holes causes great
disturbance and increases
the chance of collision. As
material is expelled during
such events, the stellar mass
becomes hidden.Compact gas clouds
The other major theory
suggests the G objects are
dusty ionised gas clouds
that follow highly eccentric
orbits around the black hole.
The suggestion is that it is
ultra-rare for stars to exist
inside gas clouds that are so
thicktheypreventlightfrom
shiningthrough.G2wasalso
seentoslowdownfollowing
itsblackholeencounter- somethinglesslikelyfor
densestars.
TheoriginisstarS2
IftheGobjectsaresimply
puregasclouds,thena
theorypointstothestarS2
sheddingmaterialasitorbits
theblackhole.G1andG2
followsimilarorbits,so
couldbepartofthesame
streamofgas,butthat
doesn’texplainthewildly
differentorbitsthatare
takenbyG3throughtoG6.
Perhapstheyarenotallof
thesamecategory.THE THEORIES
Is a star really at
the centre of the
mysterious clouds of
gas and dust?“Thestars we’vebeenwatching
andnot understandingmay be
theendproductof mergersthat
arecalmnow” AndreaGhez
us how much mass they lose in each passage.” Key
quests will involve trying to find binary systems
which haven’t yet merged but which may later do
so and become G objects. They will also look at
whether the black hole becomes more active after a
G object passes nearby.
“The answer to that is something we are
pursuing now,” Morris says. “We recently reported
an infrared observation last year that the black hole
underwent an enormous f lare. Can we associate
that with a G object passage a few years earlier? Or
is that just some random lump of matter falling into
the black hole?”
One thing’s for sure, the issue of G objects is not
about to be forgotten, and they represent another
mysterious puzzle to solve.© ESO© NASA