Astronomy

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Predicting supermassive black hole collisions

SMOKE SIGNALS. Rochester Institute of

Technology researchers simulated the inward spiral

of two supermassive black holes to identify visible

signals astronomers might see before such a pair

collides. Although the collisions of several stellar-

mass black holes have been observed, a pair of

colliding supermassive black holes has not. These

frames from a time-lapse simulation show two

in-spiraling black holes (the black dot at the center of

each frame is not part of the simulation) surrounded

by individual disks of gas that grow periodically

denser and brighter, and then thinner and less

bright. Time is measured in units of the binary’s

orbital period (tbin); areas that are yellow and red are

most dense, and those that are blue and black are

least dense. Knowing the signature hallmarks of the

last dance between supermassive black holes fated

to collide can prepare observers to follow up such an

event as soon as it is spotted. — A.K.

Supermassive black holes sit in the centers

of most massive galaxies. Actively accreting

black holes are called active galactic nuclei,

or AGN. Astronomers use the unified model

of AGN to describe these objects as a black

hole surrounded by a bright accretion disk of

infalling material, all inside a larger doughnut-

shaped torus of dusty material.

Now, the resolution afforded by the

Atacama Large Millimeter/submillimeter Array

(ALMA) has allowed astronomers to clearly

image the rotation of a dusty torus around a

supermassive black hole for the first time. The

target was the spiral galaxy M77, 47 million

light-years away. Using ALMA, researchers

identified emission from hydrogen cyanide

molecules and formyl ions associated with

the gas and dust in the center of the galaxy

around the black hole, zeroing in on a dense

doughnut of material immediately around it.

The work was published February 1 in The

Astrophysical Journal Letters.

The torus spans only about 20 light-years,

an extremely small region compared with

M77’s diameter of about 100,000 light-years.

Data showed Doppler shifting of the material

in the doughnut, with some material moving

away from Earth and some moving toward it

— a clear sign of rotation.

While the torus is rotating as expected, it is

asymmetric and shows other possible signs of

disruption, such as a past merger with another

galaxy. These hints support separate observa-

tions with the Subaru Telescope indicating M77

merged with a smaller galaxy several billion

years ago and explain M77’s extremely active

AGN, which is at odds with the galaxy’s ordered

shape. (Galaxies that have undergone recent

mergers show obvious signs of disruption in

their shapes, but M77 does not.) More work is

needed to determine the history of M77 and its

AGN, but this first image of a rotating torus is a

significant step forward in the study of galaxies

and their supermassive black holes. — A.K.

A black hole’s dusty doughnut

RISING WATERS. Twenty-five years of satellite data confirm

that Earth’s sea levels are rising at an accelerating rate.

RIT CENTER FOR COMPUTATIONAL RELATIVITY AND GRAVITATION

ZOOMING IN. ALMA allowed astronomers to image the central region of the galaxy M77, showing

a 700-light-year-wide horseshoe-shaped filament of hydrogen cyanide (green) around a compact region

of formyl ions (red). Zooming in, astronomers imaged Doppler rotation in the 20-light-year-wide torus

directly around the galaxy’s supermassive black hole, with red showing gas moving away from Earth

and blue showing gas moving toward Earth. ALMA (ESO/NAOJ/NRAO), IMANISHI ET AL., NASA, ESA AND A. VAN DER HOEVEN