An international team, including researchers from the Center for Astrophysics | Harvard and Smithsonian (CfA), has detected magnetic fields within a gas and dust disk several hundred light-years across, deep inside Arp 220-a system formed by the merger of two galaxies.

Arp 220, one of the most infrared-luminous objects beyond the Milky Way, is thought to be the product of two colliding spiral galaxies rich in gas. This intense collision has triggered a surge in star formation, making it an ideal laboratory for studying the conditions necessary for stars to emerge.

Astronomers believe that gas and dust disks in merging galaxies play a key role in star formation, but they have long sought to understand why some galaxies convert gas into stars more efficiently than others. The presence of magnetic fields may help explain this phenomenon by preventing excessive bursts of star formation, akin to a pressure cooker's ability to regulate heat and pressure.

"This is the first time we've found evidence of magnetic fields in the core of a merger," said David Clements of Imperial College, United Kingdom, who led the study. "But this discovery is just a starting point. We now need better models, and to see what's happening in other galaxy mergers."

The team used the Submillimeter Array (SMA), a collection of eight radio dishes located near the summit of Maunakea in Hawaii, to peer deep inside Arp 220. The SMA, operated by the Smithsonian Astrophysical Observatory and part of the CfA, is renowned for its high-resolution capabilities in studying celestial phenomena.

For a galaxy to form a significant number of stars in a short period, large quantities of gas must condense and collapse. However, as young stars heat their surroundings, this gas tends to disperse, reducing further star formation. "To stop this happening, you need to add something to hold it all together - a magnetic field in a galaxy, or the lid and weight of a pressure cooker," added Clements.

Galaxy mergers often experience rapid bursts of star formation, termed starbursts, which make them behave differently from typical star-forming galaxies. In these starbursts, gas appears to convert into stars at a significantly higher rate. Scientists have long sought to understand the mechanisms behind this efficiency.

One possibility is that magnetic fields provide additional stabilization, helping to keep the star-forming gas from expanding and dissipating too quickly. By acting as a binding force, these fields may sustain dense star-forming regions longer than expected, counteracting the disruptive effects of stellar heat and supernovae.

Theoretical models have long predicted such magnetic influences, but the new observations provide the first concrete evidence of their existence in a merging galaxy.

"Another effect of the magnetic field is that it slows down the rotation of gas in the disks of merging galaxies. This allows the force of gravity to take over, pulling the sluggish gas inward to fuel starbursts," said Qizhou Zhang of the CfA, a co-author of the study. "The SMA has been one of the leading telescopes for high angular resolution observations of magnetic fields in molecular clouds in the Milky Way. It's great to see that this study breaks new ground by measuring magnetic fields in merging galaxies."

Next, the research team plans to search for magnetic fields in other galaxies similar to Arp 220. By expanding their observations, they aim to gain a clearer understanding of the role magnetic fields play in some of the most luminous galaxies in the local universe.

Research Report:Polarized Dust Emission in Arp220: Magnetic Fields in the Core of an Ultraluminous Infrared Galaxy

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Center for Astrophysics | Harvard and Smithsonian
Stellar Chemistry, The Universe And All Within It