Title: Stars Born in the Wind: M82’s Outflow and Halo Star Formation
Authors: Vaishnav V. Rao, Adam Smercina, Eric F. Bell, Benjamin Williams, Julianne J. Dalcanton, Andrew Dolphin, Adam Leroy, Antonela Monachesi, Jeremy Bailin, Roelof S. de Jong, Fabian Walter
First Author’s Institution: Department of Astronomy, University of Michigan, Ann Arbor, Michigan, USA
Status: Accepted to the Astrophysical Journal (open access), available on arXiv
Outflows and Star Formation
Some galaxies form stars at exceptionally high rates, known as starburst galaxies. 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 (SNe). While these stellar explosions occur on relatively small scales, they can collectively drive galactic-scale expulsions of gas and dust known as outflows.
Outflows inject 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. M82, a quintessential local starburst galaxy, is the focus of today’s paper (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.
The Southern Arcs of M82
The main focus of today’s paper are arc-like groups of stars located near M82’s southern outflow, called the Southern Arcs. 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 Hertsprung-Russel (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 (CMD), 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 CMDs of the Southern Arc to derive star formation histories (SFHs) for the region. SFHs 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 SFHs, the authors use the MATCH CMD fitting code, which determines the combination of stellar populations which reproduce the observed CMD, accounting for observational biases along the way. Figure 2 shows the SFHs 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 70-150 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.
So what’s the deal with the Southern Arcs?
The authors explore two mechanisms which could explain the SFHs. 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.
The authors emphasize that distinguishing between the two scenarios requires further observations, specifically to determine metallicities of the stars in the Southern Arcs. If the two distinct stellar populations have different metallicities, it is more likely that the stars formed within the outflow in multiple gas clouds. If the populations have similar metallicities, it hints that the outflow shock triggered star formation, leading to similar stellar populations. So, as the age old saying goes, ‘further data is needed’ to better understand the origin of the Southern Arcs!
Astrobite edited by Jesse Thwaites
Featured image credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA). Acknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI) and P. Puxley (NSF).
