Science Results

A giant tidal tail has been discovered emanating from a dwarf galaxy in the nearby M81 Group. The galaxy, named F8D1, is remarkable on account of its low luminosity and large size and is now recognized to be one of the closest examples of an "ultra-diffuse" galaxy (UDG). The origin of these enigmatic galaxies has puzzled astronomers for several decades – are they born this way or are their present-day properties the result of processes which have shaped them over their lifetimes? Using observations with Hyper Suprime-Cam (HSC) on the Subaru Telescope and the MegaCam imager on the Canada-France-Hawaii Telescope, a team of researchers has mapped the tidal stream of stars from F8D1 over 1 degree on the sky, corresponding to 200,000 light-years at the distance of the galaxy. This is the first time that such a stellar stream has been discovered in a UDG. Revealing F8D1 to be a galaxy in an advanced state of tidal disruption has implications for both the dynamical evolution of the M81 Group and for the origin of galaxies that exhibit UDG properties.

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Figure 1: (Left) M81 Group survey footprint (white and red circles) overlaid on a Sloan Digital Sky Survey image. (Right) The spatial distribution of red giant branch stars at the distance of F8D1 in the field delineated by the red circle in the left panel. The upper right image is a zoom in on the main body of the F8D1 galaxy taken with HSC. A high resolution version of the figure is available here (1.2 MB) (Credit: NAOJ)

Since 2014, a science team led by researchers of NAOJ and the University of Edinburgh has conducted a deep contiguous photometric survey of the M81 Group using HSC on the Subaru Telescope (Figure 1 left). Lying at 12 million light-years, the M81 Group is one of the nearest galaxy groups. Its proximity and resemblance to the Local Group have fueled much astronomical research over several decades. It contains more than 40 member galaxies, including the large spiral galaxy M81, the peculiar galaxies M82 and NGC3077, 9 late-type galaxies, at least 20 low-luminosity early-type dwarfs, and a variety of stellar debris features, some of which are tidal dwarf galaxy candidates. Strong tidal interactions between M81, M82 and NGC 3077 had been revealed through the neutral hydrogen gas studies. In 2015, the same science team showed, for the first time, that the signatures of these interactions are also present in the low surface brightness stellar distribution (Note 1).

The F8D1 stream was revealed through analyzing the spatial distribution of individual stars with properties which place them at the distance of the M81 Group. Since F8D1 lies at the edge of the survey footprint (Figure 1), only one tidal arm can be seen, extending approximately 200,000 light-years to the northeast. The team has recently been awarded further observing time to search for a counterpart stream to the southwest.

The discovery of a huge tidal tail from F8D1 is compelling evidence that the galaxy’s present day properties have been strongly shaped by events which have occurred in the past billion years. The team estimates that more than one-third of F8D1’s luminosity is contained in the tidal tail and they suggest that the source of the disruption has been a recent close passage to the massive spiral M81.

Rokas Žemaitis, a Ph.D. student at the University of Edinburgh who led the work, comments that, "The discovery that F8D1 is tidally disrupting is very exciting and it will be important to establish how many other UDGs also show faint tidal tails."


These results appeared as Žemaitis et al. "A tale of a tail: a tidally disrupting ultra-diffuse galaxy in the M81 group" in the Monthly Notices of the Royal Astronomical Society on November 2, 2022.


(Note 1) See Subaru Telescope August 4, 2015 press release for the M81 Galactic Archaeology study.