Into the (Double) Void

Title: First direct dynamical detection of a dual supermassive black hole system at sub-kiloparsec separation

Authors: Karina. T. Voggel, Anil C. Seth, Holger Baumgardt, Bernd Husemann, Nadine Neumayer, Michael Hilker, Renuka Pechetti, Steffen Mieske, Antoine Dumont, Iskren Georgiev

First Author’s Institution: Universite de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France

Status: Accepted for publication in Astronomy & Astrophysics, pre-print available on the arxiv

Today’s authors have discovered not one, but two nearby black holes millions of times the mass of the Sun!

What is a supermassive black hole? Hint: it’s not the song by Muse.

A supermassive black hole (SMBH) is the largest type of black hole; their masses are millions or billions times that of the Sun. A stellar mass black hole, which is roughly 5 to 20 times more massive than the Sun, forms when a massive star collapses under its own gravity. This collapse generates a supernova, and forms a region of spacetime that is so dense that not even light can escape: a black hole. The formation mechanism for SMBHs is still up for debate, but most astrophysicists agree that they form by accreting matter or merging with other black holes (check out this recent Astrobite to learn more about SMBHs).

MUSE is an Instrument, Not a Band

Today’s authors observed a pair of SMBHs with the Multi-Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope (VLT) in Chile. The VLT is the world’s most advanced visible-light astronomical observatory, and MUSE uses the photons gathered by the VLT to obtain imaging and spectroscopic data of the sky. The very high spatial resolution of the instrument allowed today’s authors to separate and study the two SMBHs in the peculiar galaxy NGC 7727 in the constellation Aquarius.

Into the (Double) Void

Inside the galaxy, the authors found two SMBHs: one that is 6 million times the mass of the Sun, and one that is 154 million times the mass of the Sun. The authors determined the masses of these SMBHs by measuring the speeds of the stars close to them. The stars around the more massive SMBH move much more quickly as they orbit their host (check out this video for a visualization of these speedy stars).

The larger of the pair is at the center of NGC 7727. The smaller SMBH is likely embedded in the ravaged center of a galaxy that merged with NGC 7727 billions of years ago. Astrophysicists believe that almost every large galaxy has a SMBH at the galaxy’s center. Because NGC 7727 merged with another galaxy, the smaller SMBH was stripped of its stars, and thus of its mass. Figure 1 shows how this affected the scaling relation for the smaller SMBH, making it an intriguing SMBH outlier.

Plot with mass of the black hole on the y-axis and mass of the bulge on the x-axis. A black line intersects the x-axis at roughly 10^9 and trends approximately linearly upwards. Black dots and the green star follow this trend. The blue dots and red star follow a dashed black line that crosses the x-axis at roughly 10^7 and trends upward linearly.
Figure 1. Black hole galaxy scaling relations. Black hole mass is on the y-axis and the mass of the bulge (the stars that orbit close to the black hole at the center of a galaxy) is on the x-axis. Data points from Saglia et al. (2016) are shown in black. Confirmed stripped nuclear star clusters with SMBHs are shown in blue. The larger SMBH is marked with a green star, and the smaller SMBH is marked with a red star. The relation of Pacucci et al. (2018) is shown by a solid black line, and a constant SMBH mass fraction of 5% is shown as a dashed black line. This plot shows how Nucleus 2 (the smaller SMBH) was stripped of stars, lowering its stellar mass while the black hole mass remained the same (Figure 15 in the paper).

Because of this merger, the two SMBHs discovered by today’s authors are (astronomically speaking) rather close to one another. They are roughly 1600 light-years apart. This makes these black holes the most closely-separated pair of SMBHs ever discovered, and the only known SMBH pair with a separation less than 1000 parsecs (3200 light-years). And these goliaths are only getting closer; the authors estimate that they will merge to form an even bigger black hole in 250 million years. This collision will send gravitational waves out across the Universe. But don’t worry; although these black holes are the closest known SMBH pair, they are much too far (at roughly 89 million light-years) to cause any Earthly damage during their collision.

Searching for the Supermassive

In addition to being really cool, this SMBH pair also gives astrophysicists insight into why these systems are hard to detect and how we can find more anyway.

When NGC 7727 merged with another galaxy long ago, it stripped the stars away from the smaller SMBH. This left the SMBH with very little material around it, meaning there is little matter for it to accrete. Typically, accretion is how we “see” black holes. Black holes emit no light, but the material that surrounds a black hole (that the black hole is accreting) is bright and glowing, allowing astrophysicists to infer the presence of a black hole in the center. Since the smaller SMBH has no material to accrete and heat and light up, it is hard to detect via emission lines (Figure 2).

Three panels show a mass of gray dots overlaid with a red line that slopes downward from the y-axis at roughly one to the x-axis. A green line is also overlaid that extends upwards from the red line and divides the gray points. A dashed blue line is overlaid on the first panel and has a similar shape to the red line, but slopes downward more steeply. The black dot is located outside the gray data points in the upper right corner. The red triangle is located above and to the left of the black dot.
Figure 2. Three diagnostic BPT diagrams for the larger SMBH (black point) and the smaller SMBH (red triangle). The gray data points represent galaxy properties from Brinchmann et al. (2004). The solid red line denotes the maximum location of starburst regions suggested by Kewley et al. (2001). The blue line is the star-forming boundary suggested by Kauffmann et al. (2003). The green line is the division between Seyfert galaxies and LINERS suggested by Kewley et al. 2006. These figures show that only an upper limit on the flux of Nucleus 2 (the smaller SMBH in the pair) can be determined due to its lack of accretion and therefore emission lines (Figure 14 in the paper).

However, the results from today’s paper do provide evidence that SMBHs exist not only at the center of galaxies, but also as stripped relics of former galaxies. So how do we find more of these monstrous pairs?

Because this system is so close-by, astronomers can use it as a blueprint to find hidden SMBHs in more distant galaxies. Furthermore, once the HARMONI instrument is commissioned at the Extremely Large Telescope, astrophysicists will be able to find smaller SMBHs that escape detection by MUSE. These results will clue us in to how galaxies tangle and tussle with one another, spewing stars and leaving black holes in their wake.

Astrobite edited by Katya Gozman

Featured image credit: ESO/Voggel et al.

About Catherine Clark

Catherine Clark is a PhD candidate at Northern Arizona University and Lowell Observatory. Her research focuses on the smallest, coldest, faintest stars, and she uses high-resolution imaging techniques to look for them in multi-star systems. She is also working on a Graduate Certificate in Science Communication. Previously she attended the University of Michigan, where she studied Astronomy & Astrophysics, as well as Spanish. Outside of research, she enjoys spending time outdoors hiking and photographing, and spending time indoors playing games and playing with her cats.

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