Weighing a Supermassive Black Hole

Combining a novel technique and a world-class telescope, astronomers have measured the mass of the supermassive black hole at the center of barred spiral NGC 1097.

ALMA sees NGC 1097

A composite image of the barred spiral galaxy NGC 1097 shows ALMA data in red (HCO+) and green/orange (HCN), superimposed on an optical image taken by the Hubble Space Telescope.
NRAO / ESO / NAOJ / K. Onishi / NASA / ESA Hubble Space Telescope / E. Sturdivant / NRAO / AUI / NSF

Astronomers speak casually about measuring the mass of supermassive black holes a million or a billion times heavier than the Sun. But weighing behemoths isn't as easy as it sounds. Take the Milky Way's resident beast, Sgr A*, whose gravity drives circling stars to incredible speeds; despite that, astronomers still had to wait for 17 years to wait for a single star to complete an orbit. The observationally complex endeavor needed no more calculations than what Johannes Kepler deduced in the early 17th century to give a precise mass, 4.1 million Suns' worth, for Milky Way's central black hole.

Outside the Milky Way, such direct mass measurements aren't possible, and astronomers rely instead on following ionized gas, megamasers, or the bulk motions of stars. Now ALMA is adding another measuring stick to astronomers' toolkit: cold molecular gas.

In a proof-of-concept observation, Kyoko Onishi (Graduate University of Advanced Studies, Japan, and National Astronomical Observatory of Japan) used the Atacama Large Millimeter/submillimeter Array (ALMA) to monitor the motions of cold gas within 440 light-years of the central black hole in spiral galaxy NGC 1097.

The observations, conducted in 2012 when ALMA was just coming online, used only 16 antennas, so the images only resolve down to 1.3 arcseconds near the galaxy's core. (That's an astounding feat for millimeter-wavelength astronomy, but now that all 66 antennas are online, ALMA can do even better.)

But the sharpness of a single image isn't everything — ALMA sees more than one image at a time. Tuning ALMA to detect the molecules cyanide (HCN) and formylium (HCO+), Onishi and colleagues stepped through velocity space, imaging first the slow-moving gas, then the fast-moving gas. Comparing these slices to theoretical models, the team calculated the mass lurking in NGC 1097's core: 140 million times the mass of the Sun.

NGC 1097

NGC 1097 as seen in visible light.

As massive as it is, this black hole only affects the molecular gas near the very center of the spiral galaxy — in fact, the beautiful ring of gas featured in the image above lies between 2,600 and 3,300 light-years from the black hole and doesn't feel its pull directly.

Yet somehow, the black hole's reach seems to extend beyond its grasp. NGC 1097 is yet another galaxy that follows the so-called M-sigma relation, which suggests that the galaxy's central black hole and its far larger bulge of stars grew and evolved together.

In recent years, astronomers have found that this evolutionary relation might work differently for fuzzy football-shaped elliptical galaxies than for spirals. The observations of NGC 1097 fit perfectly with what's expected for spiral galaxies, and don't fit with predictions for ellipticals. "It's exciting to think that we can now apply this same technique to other similar galaxies and better understand how these unbelievably massive objects affect their host galaxies," said Kartik Sheth (NRAO) in a press release.

Kyoko Onishi et al. "A Measurement of the Black-Hole Mass in GNC 1097 Using ALMA." Astrophysical Journal, 2015 June 10.

5 thoughts on “Weighing a Supermassive Black Hole

  1. Peter WilsonPeter Wilson

    “In a proof-of-concept observation, Kyoko Onishi used ALMA to monitor the motions of cold gas within 440 light-years of the central black hole in spiral galaxy NGC 1097.”

    Wait. For a proof-of-concept observation, wouldn’t you monitor the motions of cold gas within 440 light-years of the Milky Way’s central black hole, whose mass is already known?

  2. Anthony BarreiroAnthony Barreiro

    Fascinating work. I love ALMA.

    I looked up NGC 1097 on wikipedia. It’s tenth magnitude and 9 by 6 arcminutes, the brightest galaxy in Fornax. With a declination of -30 degrees, 1097 never gets very high as seen from mid-northern latitudes, but William Herschel discovered the galaxy from England in 1790, so it’s doable. At a distance of 45 million light years, 1097 is closer (and farther north) than the Fornax galaxy cluster.

  3. NightGazer

    Hey guys, nice report. I’m 16 why we are saying: weight. Isn’t weight the measure of gravity applying its forces on us? Why not mass? cheers.
    BTW this duck pic i got was made by me on a 3D animation program. Just saying before you luagh.

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