Star-formation in the HI bridge between M81 and M82

TL;DR

This study investigates star formation in the HI tidal bridge between M81 and M82, revealing both young and old stellar populations and suggesting different origins for these stars within the tidal debris.

Contribution

It provides the first detailed analysis of stellar populations in the M81-M82 tidal bridge using multi-wavelength observations and Hubble Space Telescope imaging.

Findings

Detected eight young star-forming regions in the bridge.

Found both young (<10 Myr) and old (>1 Gyr) stellar populations.

Old stars likely originated from M81/M82 disks and were ejected during a tidal interaction.

Abstract

We present multi-wavelength observations of stellar features in the HI tidal bridge connecting M81 and M82 in the region called Arp's Loop. We identify eight young star-forming regions from Galaxy Evolution Explorer ultraviolet observations. Four of these objects are also detected at H\alpha. We determine the basic star formation history of Arp's Loop using F475W and F814W images obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope. We find both a young (< 10 Myr) and an old (>1 Gyr) stellar population with a similar spatial distribution and a metallicity Z~0.004. We suggest that the old stellar population was formed in the stellar disk of M82 and/or M81 and ejected into the intergalactic medium during a tidal passage (~ 200-300 Myr ago), whereas the young UV-bright stars have formed in the tidal debris. The UV luminosities of the eight objects are modest and typical of small clusters or OB associations. The tidal bridge between M81-M82 therefore appears to be intermediate between the very low levels of star formation seen in the Magellanic bridge and actively star-forming tidal tails associated with major galaxy mergers.