Andromeda Galaxy
Image courtesy of John Lanoue, Wikimedia Commons

The current rage in galactic astronomy is “hierarchical formation” — the buildup of big galaxies by devouring little ones — and the Pan-Andromeda Archaeological Survey (PAndAS) is adding fodder to the flame.

Using the 3.6-meter Canada-France-Hawaii Telescope, PAndAS is digging up table scraps from our nearest spiral galaxy neighbor, Andromeda, in the largest single-galaxy survey of its kind. The observations reveal several previously-unobserved star streams spanning tens to hundreds of light-years, likely leftovers from Andromeda’s previous feedings. Hordes of red giant stars also trace a dim structure 100 times larger than the galactic disk we see with our eyes. Because the astronomers do not detect enough gas in the area to form the stars in situ, the stars must be crumbs captured in previous interactions, claims the team’s Nature paper, lead-authored by Alan McConnachie of the Herzberg Institute for Astrophysics in British Columbia.

What’s most intriguing, however, is what the survey doesn’t show, explains Edward Fitzpatrick, an astronomer at Villanova University. The team’s survey found fewer crumbs from mergers than the astronomers expected to see. “The galaxy formation people essentially suggest that the number of mergers which must have happened to build the big galaxies is some ten times larger than we see evidence for,” Fitzpatrick writes, referring to the dearth of “digested remnants” observed.

Thick star spreads near Andromeda’s disk may hide the missing crumbs, the team says. If not, Andromeda may have to blame its size on something other than its appetite.