Combined image of NGC 1332 shows the central disk of gas surrounding the supermassive black hole at the center of the galaxy. New ALMA observations traced the motion of the disk, providing remarkably precise measurements of the black hole's mass: 660 million times the mass of our Sun. The red region in the ALMA image represents emission that has been redshifted by gas rotating away from us; the blue represents emission blue-shifted by gas rotating toward us. The range of colors represents rotational speeds up to 500 kilometers per second. Credit: A. Barth (UCI), ALMA (NRAO/ESO/NAOJ); NASA/ESA Hubble; Carnegie-Irvine Galaxy Survey.
Supermassive black holes, some weighing millions to billions of times
the mass of the Sun, dominate the centers of their host galaxies. To
determine the actual mass of a supermassive black hole, astronomers must
measure the strength of its gravitational pull on the stars and clouds
of gas that swarm around it.
Using the Atacama Large Millimeter/submillimeter Array (ALMA),
a team of astronomers has delved remarkably deep into the heart of a
nearby elliptical galaxy to study the motion of a disk of cold
interstellar gas encircling the supermassive black hole at its center.
These observations provide one of the most accurate mass measurements to
date for a black hole outside of our Galaxy, helping set the scale for
these cosmic behemoths.
To obtain this result, Aaron Barth, an astronomer at the University of California, Irvine, and lead author on a paper published in the Astrophysical Journal Letters,
and his team used ALMA to measure the speed of carbon monoxide gas in
orbit around the black hole at the center of NGC 1332, a massive
elliptical galaxy approximately 73 million light-years from Earth in the
direction of the southern constellation Eridanus.
"Measuring
the mass of a black hole accurately is very challenging, even with the
most powerful telescopes on Earth or in space," Barth said. "ALMA has
the revolutionary ability to observe disks of cold gas around
supermassive black holes at small enough scales that we can clearly
distinguish the black hole's influence on the disk's rotational speed."
The
ALMA observations reveal details of the disk's structure on the order
of 16 light-years across. They also measure the disk's rotation well
within the estimated 80 light-year radius of the black hole's "sphere of
influence" – the region where the black hole's gravity is dominant.
Near
the disk's center, ALMA observed the gas traveling at more than 500
kilometers per second. By comparing these data with simulations, the
astronomers calculated that the black hole at the center of NGC 1332 has
a mass 660 million times greater than our Sun, plus or minus ten
percent. This is about 150 times the mass of the black hole at the
center of the Milky Way, yet still comparatively modest relative to the
largest black holes known to exist, which can be many billions of solar
masses.
ALMA's close-in observations were essential, the
researchers note, to avoid confounding the black hole measurement with
the gravitational influence of other material – stars, clouds of
interstellar gas, and dark matter – that comprises most of the galaxy's
overall mass.
"This black hole, though individually massive,
accounts for less than one percent of the mass of all the stars in the
galaxy," noted Barth. "Most of a galaxy's mass is in the form of dark
matter and stars, and on the scale of an entire galaxy, even a giant
black hole is just a tiny speck in the center. The key to detecting the
influence of the black hole is to observe orbital motion on such small
scales that the black hole's gravitational pull is the dominant force."
This observation is the first demonstration of this capability for ALMA.
Astronomers
use various techniques to measure the mass of black holes. All of them,
however, rely on tracing the motion of objects as close to the black
hole as possible. In the Milky Way, powerful ground-based telescopes
using adaptive optics can image individual stars near the galactic
center and precisely track their trajectories over time. Though
remarkably accurate, this technique is feasible only within our own
Galaxy; other galaxies are too distant to distinguish the motion of
individual stars.
To make similar measurements in other
galaxies, astronomers either examine the aggregate motion of stars in a
galaxy's central region, or trace the motion of gas disks and
mega-masers -- natural cosmic radio sources.
Previous studies of
NGC 1332 with ground- and space-based telescopes gave wildly different
estimates for the mass of this black hole, ranging from 500 million to
1.5 billion times the mass of the Sun.
The new ALMA data confirm that the lower estimates are more accurate.
Crucially,
the new ALMA observations have higher resolution than any of the past
observations. ALMA also detects the emission from the densest, coldest
component of the disk, which is in a remarkably orderly circular motion
around the black hole.
Many past black hole measurements made
with optical telescopes, including the Hubble Space Telescope, focused
on emission from the hot, ionized gas orbiting in the central regions of
galaxies. Ionized-gas disks tend to be much more turbulent than cold
disks, which leads to lower precision when measuring a black hole's
mass.
"ALMA can map out the rotation of gas disks in galaxy
centers with even sharper resolution than the Hubble Space Telescope,"
noted UCI graduate student Benjamin Boizelle, a co-author on the study.
"This observation demonstrates a technique that can be applied to many
other galaxies to measure the masses of supermassive black holes to
remarkable precision."
The National Radio Astronomy Observatory
is a facility of the National Science Foundation, operated under
cooperative agreement by Associated Universities, Inc.
Contacts:
Charles Bkue,
NRAO Public Information Officer
(434) 296-0314;
Email: cblue@nrao.edu
Brian Bell,
UCI Communications Officer
(949) 824-8249;
Email: bpbell@uci.edu
Reference:
"Measurement of the black hole mass in NGC 1332 from
ALMA observations at 0.044 arcsecond resolution," Aaron Barth et al.,
2016, Astrophysical Journal Letters [http://apjl.aas.org]. Preprint [http://arxiv.org/abs/1605.01346]
Notes:
The team is composed of Aaron Barth (University of California,
Irvine), Benjamin D. Boizelle (University of California, Irvine),
Jeremy Darling (University of Colorado, Boulder), Andrew J. Baker
(Rutgers, the State University of New Jersey, Piscataway), David A.
Buote (University of California, Irvine), Luis Ho (Kavli Institute of
Astronomy and Astrophysics, Peking University, China), and Jonelle L.
Walsh (Texas A&M University, College Station).
The Atacama
Large Millimeter/submillimeter Array (ALMA), an international astronomy
facility, is a partnership of the European Organisation for Astronomical
Research in the Southern Hemisphere (ESO), the U.S. National Science
Foundation (NSF) and the National Institutes of Natural Sciences (NINS)
of Japan in cooperation with the Republic of Chile. ALMA is funded by
ESO on behalf of its Member States, by NSF in cooperation with the
National Research Council of Canada (NRC) and the National Science
Council of Taiwan (NSC) and by NINS in cooperation with the Academia
Sinica (AS) in Taiwan and the Korea Astronomy and Space Science
Institute (KASI).
ALMA construction and operations are led by ESO
on behalf of its Member States; by the National Radio Astronomy
Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on
behalf of North America; and by the National Astronomical Observatory of
Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO)
provides the unified leadership and management of the construction,
commissioning and operation of ALMA.
