Measuring the Central Gas in a Cosmic Cooling Flow
The NGC 5044 galaxy group, the brightest X-ray group in the sky, viewed in a variety of wavebands from low-frequency radio to X-ray. Astronomers studying the "cooling flows" that develop in galaxy clusters as the very hot X-ray-emitting gas cools have used a variety of telescopes and spatial resolutions to trace accurately cooling flows through the distribution and motion of carbon monoxide.
Our Milky Way is a member of a cluster of galaxies, the co-called "Local Group," about fifty galaxies whose other large member is Andromeda about 2.3 million light-years away. Most galaxies lie in clusters, and the closest large cluster of galaxies to us is the Virgo Cluster with about 2000 members. The space between galaxies is not empty, but is filled with hot ionized gas, X-ray emitting material whose temperature is of order ten million kelvin, or even higher.
The development of clusters is a key feature of galaxy evolution but our understanding remains remarkably incomplete. One problem relates to the fate of its hot, X-ray emitting gas. In the cores of clusters the gas is heated to high temperatures as it falls toward the cluster center and by some other processes. The mystery is why this hot gas does not cool more efficiently and sink towards the center of the cluster where it might trigger more star formation. The common surmise has been that outflowing jets from supermassive black holes, or perhaps other kinds of feedback, inhibit the formation of such "cooling flows," but establishing the details of this mechanism has been elusive.
The galaxy cluster NGC 5044 is one of the brightest X-ray groups in the sky, and has been extensively studied. Its central region is known to contain about one hundred million solar-masses of cool molecular gas as determined from observations of carbon monoxide (CO) gas. These CO results, however, were obtained either with very low spatial resolution (“single dish”) telescopes that measured the total gas content or with the very high spatial resolution ALMA array that was primarily effective for seeing small substructures (dimensions on the order of fifty light-years at the distance of NGC 5044). These earlier results not only missed the gas in intermediate scale structures, essential to understanding how cooling flows behave, but also flagged a problem: the cold gas measured overall was more than double that seen in small substructures, implying that a large fraction was unaccounted for.
CfA astronomers Gerrit Schellenberger, Laurence David, Jan Vrtilek, Ewan O’Sullivan, Bill Forman, and Christine Jones led a team that used the Atacama Compact Array grouping of twelve telescopes from the ALMA facility to fill in the missing dataset, and combined its results with previous observations to construct a consistent picture of the cooling flow in this cluster. They confirm the total molecular mass value and, in a second step, make use of the superior spatial resolution of interferometers to detect ten individual clouds of cold molecular gas in the new data, which account for about 70% of the total cluster cold gas emission. Two of these clouds are seen in absorption against the supermassive black hole at the nucleus, apparently infalling towards it at a velocity of about 250 km/sec and currently an estimated fifty light-years away. The results resolve the previous ambiguities in this cluster’s cooling flow, and help address the outstanding puzzle about the fate of the gas in cooling flows at the centers of galaxy clusters.
"Atacama Compact Array Measurements of the Molecular Mass in the NGC 5044 Cooling Flow Group," Gerrit Schellenberger, Laurence P. David, Jan Vrtilek, Ewan O'Sullivan, Jeremy Lim, William Forman, Ming Sun, Francoise Combes, Philippe Salome, Christine Jones, Simona Giacintucci, Alastair Edge, Fabio Gastaldello, Pasquale Temi, Fabrizio Brighenti, and Sandro Bardelli, ApJ, 894, 72, 2020.