A University of Cambridge-led team of astronomers has discovered a surprising connection between a supermassive black hole and the giant galaxy where it resides: radio jets from the black hole — which normally suppress star formation — are stimulating the production of star-forming molecular gas in the galaxy’s halo. The research paper reporting this discovery has been accepted for publication in the Astrophysical Journal (arXiv.org preprint).
Composite image showing how powerful radio jets from the supermassive black hole at the center of a galaxy in the Phoenix Cluster inflated huge ‘bubbles’ in the hot, ionized gas surrounding the galaxy (the cavities inside the blue region imaged by NASA’s Chandra X-ray observatory). Hugging the outside of these bubbles, ALMA discovered an unexpected trove of cold gas, the fuel for star formation (red). The background image is from the NASA/ESA Hubble Space Telescope. Image credit: ALMA / ESO / NAOJ / NRAO / NASA / ESA / Hubble / CXC / MIT / H. Russell et al / M. McDonald et al / B. Saxton.
The team analyzed observations of a huge cluster of galaxies gathered by the Atacama Large Millimeter Array (ALMA).
Named after the constellation it resides in, about 5.7 billion light-years from Earth, the Phoenix Cluster (SPT-CLJ2344-4243) is an enormous accumulation of about 1,000 galaxies.
At its center lies a massive galaxy, which appears to be spitting out stars at a rate of 500-800 solar masses per year.
This galaxy harbors a supermassive black hole that is in the process of devouring star-forming gas, which fuels a pair of powerful jets that erupt from the black hole in opposite directions into intergalactic space.
Astronomers refer to this type of black-hole powered system as an active galactic nucleus (AGN).
Earlier research with NASA’s Chandra X-ray observatory revealed that the jets from this AGN are carving out a pair of giant ‘radio bubbles,’ huge cavities in the hot, diffuse plasma that surrounds the galaxy.
These expanding bubbles should create conditions that are too inhospitable for the surrounding hot gas to cool and condense, which are essential steps for future star formation.
The new ALMA observations, however, reveal long filaments of cold molecular gas condensing around the outer edges of the radio bubbles.
These filaments extend up to 82,000 light-years from either side of the AGN, and collectively contain enough material to make about 10 billion Sun-mass stars.
“With ALMA we can see that there’s a direct link between these radio bubbles inflated by the supermassive black hole and the future fuel for galaxy growth,” said lead author Dr. Helen Russell, from the University of Cambridge.
“This gives us new insights into how a black hole can regulate future star birth and how a galaxy can acquire additional material to fuel an active black hole.”
“We have thought the role of black hole jets and bubbles was to regulate star formation and to keep cooling from happening,” added co-author Dr. Michael McDonald, from MIT’s Kavli Institute for Astrophysics and Space Research.
“We kind of thought they were one-trick ponies, but now we see they can actually help cooling, and it’s not such a cut-and-dried picture.”
The ALMA observations reveal previously unknown connections between an AGN and the abundance of cold molecular gas that fuels star birth.
“To produce powerful jets, black holes must feed on the same material that the galaxy uses to make new stars. This material powers the jets that disrupt the region and quenches star formation. This illustrates how black holes can slow the growth of their host galaxies,” Dr. McDonald said.
Without a significant source of heat, the most massive galaxies in the Universe would be forming stars at extreme rates that far exceed observations. Astronomers believe that the heat, in the form of radiation and jets from an actively feeding supermassive black hole, prevents overcooling of the cluster’s hot gas atmosphere, suppressing star formation.
This story, however, now appears more complex. In the Phoenix Cluster, the team found an additional process that ties the galaxy and its black hole together.
The radio jets that heat the core of Phoenix Cluster’s hot atmosphere also appear to stimulate the production of the cold gas required to sustain the AGN.
“That’s what makes this result so surprising. This supermassive black hole is regulating the growth of the galaxy by blowing bubbles and heating the gases around it. Remarkably, it also is cooling enough gas to feed itself,” said co-author Prof. Brian McNamara, from the University of Waterloo.
This result helps astronomers understand the workings of the cosmic ‘thermostat’ that controls the launching of radio jets from the supermassive black hole.
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H.R. Russell et al. 2017. ALMA observations of massive molecular gas filaments encasing radio bubbles in the Phoenix cluster. ApJ, accepted for publication; arXiv: 1611.00017
