Star formation in the HI bridge between M81 and M82

We present multiwavelength 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 ( UV) observations. Four of these objects are also detected at Ha. 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 similar to 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 (similar to 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 and 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.