Data from Shockingly Bursty: The History of Stars Formed in M82's Outflow

The data here comprises F475W and F814W observations of M82's Southern Arc.
Abstract: Starburst galaxies, like M82, launch kiloparsec-scale galactic outflows that interact with the circumgalactic medium (CGM) in complex ways. Apart from enriching the CGM with metals and energy, these outflows may drive shocks far into the CGM, triggering star formation. To investigate such processes, we analyze the star formation history (SFH) of the Southern Arcs—arc-like stellar features located ~5 kpc from M82’s star-forming disk along the minor axis—using Hubble Space Telescope Wide Field Camera 3 photometry. From resolved stellar populations, we derive SFHs over the last ~500 Myr, finding that ~85% of the stellar mass formed between ~150 and ~70 Myr ago, followed by a brief pause, with the remaining ~15% forming since ~30 Myr ago. The timing of these bursts aligns with star formation in the M82 disk and the age distribution of its star clusters, suggesting a causal link between the starburst-driven outflow and star formation in the CGM. We propose that these stars formed when shocks, generated by the interaction between hot outflowing gas and cooler halo gas, compressed a cold, dense CGM cloud, triggering gravitational collapse. The observed ~70–80 Myr separation between bursts provides the first direct observational constraint on the cadence of a starburst outflow. Our results support a scenario in which bursty star formation in the M82 disk launched episodic outflows, resulting in the multiple star formation epochs observed in the Southern Arcs.