In hierarchical models of galaxy formation, large galaxies grow by ingesting dwarf galaxies and the spherical minihalos of dark matter that envelop them. Some of those dwarfs would be tidally shredded when they venture too close to the center of the “host” galaxy’s enormous halo, leaving streams of stars as long-lasting relics of the carnage. Searching within the halo of our neighboring Andromeda galaxy for such relics and for dwarf galaxies surviving intact, the Pan-Andromeda Archaeological Survey has been systematically imaging the galaxy’s environs out to a radius of half a million light-years. The newly published PANDAS stellar population-density map of the halos of Andromeda and the adjacent Triangulum galaxy shows seven relic tidal streams (circled numbers in the figure) stretching over enormous distances. One of them, emanating from Triangulum, gives evidence of a cataclysmic encounter between the two galaxies two or three billion years ago, as shown in the linked animation. The survey results thus far already put an upper limit of about 90 on the total number of surviving dwarf galaxies brighter than the detection limit (Roman numerals in the figure) within Andromeda’s halo. That’s an order of magnitude less than the number of minihalos expected from dark-matter simulations. There has long been evidence of such shortfalls around large galaxies, but PANDAS is the most systematic census of satellite dwarf galaxies to date. Perhaps, theorists suggest, dark-matter minihalos are as abundant as the simulations predict but the shortfall appears because actual star formation in most minihalos is aborted by some threshold-mass effect. (A. McConnachie et al., Nature 461, 66, 2009.)--Bertram Schwarzschild
