XRISM clocked M82s hot wind at over 2 million miles per hour and found that supernovae, not cosmic rays, do the driving. Sixty percent of every blasts energy goes into the wind. Three solar masses per year still go missing.
image from grok
The Cigar Galaxy is blowing gas into space at over 2 million miles per hour, and for the first time astronomers have measured exactly how fast the hot wind is moving. M82, 12 million light-years away in Ursa Major, has been studied for decades as the archetypal starburst galaxy (one forming stars at 10 times the Milky Way rate), but nobody could directly clock the fast-moving gas at its core. That changed when the XRISM Resolve instrument measured the velocity of superheated iron atoms in the galactic center, producing a result that settles a 40-year argument about what drives these outflows.
The short answer: supernovae, doing what supernovae do. The thermal pressure from exploded stars is sufficient to launch and sustain the multiphase wind in M82, according to a 139-author paper published March 25 in Nature. Cosmic rays, which some models held as the primary driver, are not required, though they may contribute. The estimated supernova rate implies that approximately 60 percent of the supernova energy must be thermalized in hot gas to produce the observed outflow. That is a lot of energy conversion, but the numbers check out.
What makes the measurement possible is Resolve itself. The Japan Aerospace Exploration Agency XRISM (X-ray Imaging and Spectroscopy Mission) satellite carries an instrument with an energy resolution of 4.5 electronvolts, precise enough to detect the Doppler broadening of FeXXV, the emission line from iron atoms stripped of 24 electrons in M82 25-million-Celsius plasma. When gas moves at extreme velocities, its spectral lines spread wider. Resolve caught that spread. Prior to XRISM, astronomers did not have the ability to measure the velocities needed to test that hypothesis. The instrument was built specifically for this kind of observation.
The gas being measured is not the cool extended wind. That slower, diffuse stuff stretches 40,000 light-years from M82 core and has been visible in optical telescopes for years. The hot wind at the galaxy heart runs at 45 million degrees Fahrenheit (25 million Celsius), and its line-of-sight velocity dispersion is 595 kilometers per second with substantial uncertainty. Resolve caught it in the act.
The paper, led by Erin Boettcher of the University of Maryland and NASA Goddard with the XRISM Collaboration, puts the mass outflow rate at roughly 7 solar masses per year from the galactic center. But only about 4 solar masses per year actually go into the wind. The remaining 3 solar masses per year go somewhere, and nobody can currently account for them. Where do the three extra solar masses go?, co-author Edmund Hodges-Kluck of NASA Goddard asked. Good question.
This matters beyond M82. Outflows like these are one of the primary mechanisms galaxies use to regulate their own growth; blowing gas outward suppresses star formation, redistributes metals across the intergalactic medium, and shapes the relationship between a galaxy mass and its star-formation rate. Outflows like this are one of the most important ways that galaxies regulate their growth, said Skylar Grayson, a graduate student at Arizona State University and a co-author. Understanding the mechanics matters for any model of galaxy evolution.
Some of the early models of starburst galaxies were developed in the 1980s. Testing them required a telescope that did not exist until XRISM flew. The new data shows the supernova-driven thermal model working exactly as predicted, which is both a vindication of decades of theoretical work and a reminder that the universe often does exactly what the physics says it should.
The missing 3 solar masses per year remain an open problem. For now, the rest is settled.
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Research completed — 0 sources registered. XRISM Resolve instrument measured hot wind in starburst galaxy M82 at >2M mph for first time. Temperature ~2×10^7 K, mass outflow ~7 M_sun/yr, energy
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