Sky and Telescope - February 2017 - 18

BLACK HOLES, PART II

by Camille M. Carlisle

Of Black Holes
andGalaxies
How much influence do supermassive
black holes have on their galaxies,
or vice versa?

X MONSTER WITHIN The galaxy M106 has a pair of "anomalous arms" that
intersect with its spiral disk. Astronomers suspect that the jets of the active
black hole at the galaxy's heart are creating shock waves in the interstellar
gas, thereby heating the gas and shoving it out of the galaxy. This outflow
appears as the arms, which glow in radio, optical, and X-ray wavelengths.

18

F E B R U A R Y 2 0 1 7 * SK Y & TELESCOPE

CREDIT.LEF T X X X X X X X X X X X X X X X

A gaping maw in the fabric of spacetime, devouring anything fool
enough to come close, inexorable in its dominance - that's basically
the dictionary entry for a tyrant.
But in fact, these objects have surprisingly tiny spheres of influence. Their gravitational reach is small, only a few light-years. Stars
can even come within a few thousandths of a light-year and survive
unscathed. As far as a galaxy should be concerned, its black hole
could just as well not be there.
Yet that's not the case. In the 1990s and 2000s, astronomers
noticed some unexpected correlations between galaxies and the supermassive beasts enthroned in their hearts. The black holes' masses
increased or decreased in tandem with other galaxy properties: the
more massive the black hole, the bigger and brighter the galaxy's central bulge of mature stars, and the faster those stars zipped around.
"[These connections] tell us that either the black hole cares about
the galaxy that it lives in, or the galaxy cares about the black hole
that's in it," says Kayhan Gültekin (University of Michigan), who
has been one of many to investigate the link with star speeds, called
the M-sigma relation (M for the black hole's mass, sigma for the range
of stellar velocities).
But bulges can easily span 10,000 light-years, far beyond the black
hole's presumed reach. In other words, these correlations shouldn't
exist. Yet preliminary data showed relations that looked so good,
they appeared to have "no scatter": all the systems tightly hugged a
straight line on a graph.
"That seemed like it had to be magic," says Jenny Greene (Prince-

N ASA / CXC / JPL- CA LTEC H / STSCI / N S F / N R AO / V L A

Black holes might seem like control freaks



Sky and Telescope - February 2017

Table of Contents for the Digital Edition of Sky and Telescope - February 2017

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Sky and Telescope - February 2017 - Cover1
Sky and Telescope - February 2017 - Cover2
Sky and Telescope - February 2017 - 1
Sky and Telescope - February 2017 - Contents
Sky and Telescope - February 2017 - 3
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Sky and Telescope - February 2017 - Cover3
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