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Title: Stars Born in the Wind: M82’s Outflow and Halo Star Formation
Authors: Vaishnav V. Rao et al.
First Author’s Institution: University of Michigan
Status: Published in ApJ
Outflows and Star Formation
Some galaxies, known as starburst galaxies, form stars at exceptionally high rates. High star formation rates mean, you guessed it, lots of new stars! The most massive stars live comparatively short lives and can die in a brilliant cosmic explosion known as a supernova. So, when you have a starburst galaxy, you get lots of young stars, a fraction of which produce supernovae. While these stellar explosions occur on relatively small scales, they can collectively drive galactic-scale expulsions of gas and dust known as outflows.
Outflows expel gas laden with metals out of the galaxy, playing a pivotal role in the evolution of galaxies. They can also drive additional star formation in the areas surrounding the galaxy, which is exactly what today’s authors are interested in. Messier 82 (M82), a quintessential local starburst galaxy, is the focus of today’s article (see Figure 1). M82’s proximity makes its spectacular outflows a prime testing ground for studying the impact of outflows on a galaxy and its surroundings.
Figure 1: An image of the local starburst galaxy M82, taken by the Subaru Telescope. The Hubble Space Telescope field encompassing the Southern Arcs is highlighted as a green box. [Rao et al. 2025]
The Southern Arcs of M82
The main focus of today’s article are arc-like groups of stars called the Southern Arcs that are located near M82’s southern outflow. Using photometry from the Hubble Space Telescope, today’s authors derive star formation histories for the Southern Arcs, with the goal of understanding the impact that M82’s outflows have had on star formation in its halo. Figure 1 shows the Southern Arcs region of M82 highlighted in green alongside an image of M82.
Star Formation Histories
If you’ve ever taken an astronomy class, you’re probably familiar with the Hertzsprung–Russell (HR) diagram. HR diagrams are one of the most powerful tools available to astronomers, as they encode a ton of information regarding populations of stars and their formation. In practice, astronomers can construct HR diagrams using resolved stellar populations. That is, if you can resolve individual stars in a galaxy, you can construct a color–magnitude diagram, which is essentially the HR diagram you may be familiar with, but uses observed properties as a proxy for temperature (color) and luminosity (magnitude).
Today’s authors use the color–magnitude diagrams of the Southern Arc to derive star formation histories for the region. Star formation histories describe the star formation rate as a function of time, providing an insight into when and how stellar populations formed. To derive the Southern Arc star formation histories, the authors use the MATCH color–magnitude diagram fitting code, which determines the combination of stellar populations that reproduce the observed color–magnitude diagram, accounting for observational biases along the way. Figure 2 shows the star formation histories obtained using three different stellar evolution models. The authors find that about 85% of the stellar mass in the Southern Arc field formed sometime between 70 and 150 million years ago before star formation slowed down. About 30 million years ago, star formation picked up again, producing the rest of the stellar mass in the Southern Arc field.
Figure 2: The star formation history (left) and cumulative star formation history (right) of the Southern Arc region. Different stellar evolutionary models are marked in different colors. The star formation history shows the star formation rate as a function of time, while the cumulative star formation history shows the buildup of the stellar mass as a fraction of the total mass observed now. [Rao et al. 2025]
So, What’s the Deal with the Southern Arcs?
The authors explore two mechanisms that could explain the star formation histories. In the first scenario, M82’s outflows trigger star formation when impacting the cooler circumgalactic gas. When the outflow shocks collide with the cooler gas, it causes it to collapse and form stars. In the second scenario, star formation is occurring within the outflows themselves. Figure 3 is a schematic of the two proposed mechanisms.
Figure 3: A schematic outlining the two possible mechanisms for the formation of the Southern Arcs. The left panel shows the scenario in which shocks produced by the outflow collide with gas in the circumgalactic medium, triggering star formation. The right panel shows the scenario in which star formation is triggered within the outflow itself. [Rao et al. 2025]
Original astrobite edited by Jessie Thwaites.
About the author, Drew Lapeer:
Drew is a first-year PhD student at the University of Massachusetts Amherst. They are broadly interested in the evolution of galaxies, with a focus on the impact of cosmic feedback on the galactic ecosystem. In their free time, they enjoy reading, rock climbing, hiking, and baking!