A team of astronomers has taken a close look at a nearby galaxy — and discovered an unusual structure that sheds light on supermassive black holes’ relationships with their host galaxies.
This ultraviolet widefield image shows NGC 7582 (right bottom) with its galactic neighbors, NGC 7599 (left) and NGC 7590 (top middle).
NASA / JPL-Caltech
A spiral galaxy 70 million light-years from Earth hosts a supermassive black hole some 50 million times the mass of our Sun. Those numbers alone sound almost like science fiction, but a recent closer look at the center of this ordinary galaxy revealed something even more bizarre: a ring of dust, gas, and stars spanning 2,000 light-years that dwarfs the black hole itself. The result was presented at the 230th meeting of the American Astronomical Society in Austin, Texas.
From previous visible-light, near-infrared, and X-ray observations, astronomers already knew this galaxy, dubbed NGC 7582, was of a rare breed: its central black hole gorges on a gas buffet, but we hardly see the shining gas as it’s pulled into the maw, thanks to large swaths of dust and gas obscuring our view of the galaxy’s core.
This type of “hidden” active galaxy is rare in the sense that these galaxies are hard to find, but astronomers are pretty sure they actually outnumber “normal” active galaxies by at least 2 to 1. What remains unclear is what is doing the hiding: Do broad dust lanes hide galactic centers from view? Or does a wall of dust somehow form closer to the black hole?
Ring Around a Galaxy, Pocket Full of Outflows
To help answer those questions, Stéphanie Juneau (National Optical Astronomy Observatory and CEA-Saclay, France) and colleagues used the MUSE instrument on the Very Large Telescope in Chile to image the center of spiral galaxy NGC 7582.
Image of the barred spiral galaxy NGC 7582, which hosts a hidden black hole. Observations from the MUSE instrument at the Very Large Telescope in Chile (inset) reveal emission from a hot gaseous wind (in green) launched by the black hole.
Background image: S. Binnewies and J. Pöpsel (Capella Observatory); Inset: S. Juneau (NOAO, CEA-Saclay)
The result was more than a pretty picture — each of the image’s 90,000 pixels came with a spectrum attached, which reveals how that part of the galaxy is moving. Using these images cum spectra, Juneau’s team separated the overall rotation of stars around the galactic center from the rotation of an inner ring, made of dust, gas, and stars, that’s spinning even faster than the galaxy itself.
This spectral image of NGC 7582 shows which way the gas at each point in the galaxy is moving. Red indicates that the stars are moving away from our point of view, while blue indicates stars that are moving toward us. The barred spiral galaxy rotates, of course, which is why one side is red and the other is blue. But at its center, there's a ring that's spinning even faster. Since we see the ring edge-on, we see it as two circles (blue and orange/yellow) that stand out from the rest of the galaxy's core. The ring spans 2,000 light-years.
Juneau et al. 2017
Previous observations had shown that the black hole is feeding from a disk-shaped buffet, and we’re seeing that gas spiraling around the maw face-on, or nearly so. This accretion disk is less than a couple light-years wide. The dusty ring around it, on the other hand, is 2,000 light-years across — and we’re seeing it edge-on.
The images also revealed the outlines of a cone-shaped outflow: hot gas that’s spewing from the black hole. The direction of the black hole’s wind, though, appears to be shaped by the dusty ring, which deflects the wind and protects the galaxy from its power.
Evidence for a Minor Merger
Where did this galaxy-protecting ring come from? Previous radio observations have shown that NGC 7582 has a small tail of neutral hydrogen gas, probably the ghostly remnant of interaction with another galaxy. NGC 7582 doesn’t share the feature with any of its galactic neighbors. So, the authors speculate, the galaxy may have recently experienced a minor merger, perhaps with a dwarf galaxy that was once in orbit around it.
If the dwarf ventured too close, the spiral would have torn it apart, sending much of its gas toward the larger galaxy’s center. The interaction could have fed the black hole (which would in turn create an powerful wind), while at the same time creating the dusty ring that shields the galaxy from the black hole’s power.
It’s a neat explanation but, as Juneau notes, it’s not the only one. There are more ways than mergers to make a dusty nuclear ring, and the ring probably isn’t the only thing hiding NGC 7582 from view. Ironically, the way to better understand these galaxies that harbor hidden black holes is simply to find more of them.