Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have mapped the distribution of molecular gas in NGC 6240, a nearby merging galaxy system that hosts two supermassive black holes growing simultaneously.
NGC 6240 as seen with ALMA (top right) and the NASA/ESA Hubble Space Telescope (combined image on the left and zoomed in on the bottom right). In the ALMA image, the molecular gas is blue and the black holes are the red dots. The ALMA image provides the sharpest view of the molecular gas around the black holes in this merging galaxy. Image credit: ALMA / ESO / NAOJ / NRAO / E. Treister / AUI / NSF / S. Dagnello / NASA / ESA / Hubble.
NGC 6240, also known as IC 4625 or UGC 10592, is a pair of merging galaxies 400 million light-years away in the constellation Ophiuchus.
It spans 300,000 light-years and has an elongated shape with branching wisps, loops and tails.
This mess of gas, dust and stars bears more than a passing resemblance to a butterfly and, though perhaps less conventionally beautiful, a lobster.
To understand what is happening within NGC 6240, astronomers want to observe the dust and gas surrounding the two black holes in detail, but previous images have not been sharp enough to do that.
“The key to understanding this galaxy system is molecular gas,” said Dr. Ezequiel Treister, an astronomer at the Pontificia Universidad Católica.
“This gas is the fuel that is needed to form stars, but it also feeds the supermassive black holes, which allows them to grow.”
Most of the gas is located in a region between NGC 6240’s supermassive black holes.
Less detailed observations taken previously suggested that the gas mass might be a rotating disk.
“We don’t find any evidence for that. Instead, we see a chaotic stream of gas with filaments and bubbles between the black holes,” Dr. Treister said.
“Some of this gas is ejected outwards with speeds up to 500 km/sec (342 miles/sec). We don’t know yet what causes these outflows.”
This image shows NGC 6240. Image credit: NASA / ESA / Hubble Heritage / STScI / AURA / Hubble Collaboration / A. Evans, University of Virginia / NRAO / Stony Brook University.
Another reason to observe the molecular gas in NGC 6240 is that it helps to determine the mass of the supermassive black holes.
“Previous models, based on surrounding stars, indicated that the black holes were much more massive than we expected, around a billion times the mass of our Sun,” said Dr. Anne Medling, an astronomer at the University of Toledo.
“But these new ALMA images for the first time showed us how much gas is caught up inside the black holes’ sphere of influence.”
“This mass is significant, and therefore we now estimate the black hole masses to be lower: around a few hundred million times the mass of our Sun.”
“Based on this, we think that most previous black hole measurements in systems like this could be off by 5-89%.”
The gas also turned out to be even closer to the black holes than the astronomers had expected.
“It is located in a very extreme environment. We think that it will eventually fall into the black hole, or it will be ejected at high speeds,” Dr. Medling said.
Additionally, the researchers didn’t find evidence for a supermassive third black hole in NGC 6240, which another team recently claimed to have discovered.
“We don’t see molecular gas associated with this claimed third nucleus,” Dr. Treister said.
“It could be a local star cluster instead of a black hole, but we need to study it much more to say anything about it with certainty.”
The findings appear in two papers in the Astrophysical Journal and the Astrophysical Journal Letters.
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E. Treister et al. 2020. The Molecular Gas in the NGC 6240 Merging Galaxy System at the Highest Spatial Resolution. ApJ, in press; arXiv: 2001.00601
Anne M. Medling et al. 2019. How to Fuel an AGN: Mapping Circumnuclear Gas in NGC 6240 with ALMA. ApJL 885, L21; doi: 10.3847/2041-8213/ab4db7
