Astronomers Detect Missing Ingredient in Cooking Up Stars
This image of Arp 220 was taken by NASA’s Hubble Space Telescope. Arp 220 is the aftermath of a collision between two spiral galaxies. It is the brightest of the three galactic mergers closest to Earth, about 250 million light-years away. Astronomers studied Arp 220 with the Submillimeter Array (SMA) to determine the role magnetic fields play in the formation of stars.
Credit: NASA/ESA/STScI/HST
Cambridge, MA - The missing ingredient for cooking up stars has been spotted for the first time by astronomers. Much like a pressure cooker has a weight on top of its lid to keep the pressure in, merging galaxies may need magnetic fields to create the ideal conditions for star formation.
Previously, the existence of such magnetic fields had only been theoretical. Now, an international team, including researchers from the Center for Astrophysics | Harvard & Smithsonian (CfA), has announced evidence of magnetic fields associated with a disk of gas and dust a few hundred light-years across deep inside a system of two merging galaxies known as Arp 220.
Arp 220 is one of the brightest objects beyond our Milky Way in infrared light. Astronomers think it is the result of a merger between two spiral galaxies full of gas, which has triggered great bursts of star formation.
Astronomers think disks of gas and dust could be the key to making the centers of interacting galaxies like Arp 220 just right for cooking lots of hydrogen gas into young stars. Magnetic fields may be able to stop intense bursts of star formation in the cores of merging galaxies from effectively ‘boiling over’ when the heat is turned up too high.
"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."
Researchers used the Submillimeter Array (SMA) on Maunakea in Hawaii to probe deep inside Arp 220. Located near the summit of Maunakea on the Big Island of Hawaii, the SMA is one of the flagship observatories of the Smithsonian Astrophysical Observatory, which is part of the CfA, and consists of eight radio dishes working together as one telescope.
To form a lot of stars in a short period of time, a large amount of gas needs to squeeze together. As the heat from young stars builds, the gas gets dispersed, thereby inhibiting more stars from forming.
"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 Clement.
Astronomers have long been looking for the magic ingredient that makes some galaxies form stars more efficiently than is normal. One of the issues about galaxy mergers is that they can form stars very quickly, in what is known as a starburst. This means they're behaving differently to other star forming galaxies in terms of the relationship between star formation rate and the mass of stars in the galaxy – they seem to be turning gas into stars more efficiently than non-starburst galaxies. Astronomers are baffled as to why this happens.
One possibility is that magnetic fields could act as an extra ‘binding force’ that holds the star forming gas together for longer, resisting the tendency for the gas to expand and dissipate as it is heated by young, hot stars, or by supernovae as massive stars die.
Theoretical models have previously suggested this, but the new observations are the first to show that magnetic fields are present in the case of at least one 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."
The next step for the research team will be to search for magnetic fields in galaxies similar to Arp 220. With their result, and further observations, the researchers hope the role of magnetic fields in some of the most luminous galaxies in the local universe will become much clearer.
A paper revealing the discovery appeared in a recent issue of the Monthly Notices of the Royal Astronomical Society. It is available online at https://arxiv.org/abs/2412.14770
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