The Uni­verse ap­pears to be in­ex­pli­ca­bly lop­sided

Dwarf gal­ax­ies clus­ter be­tween larger ones in the Lo­cal Group and else­where, but we don’t know why

Sky at Night Magazine - - BULLETIN - CHRIS LIN­TOTT was read­ing… The Lop­sid­ed­ness of Satel­lite Galaxy Sys­tems in RCDM sim­u­la­tions by Mar­cel S Pawlowski, Ro­drigo A Ibata and James S Bul­lock Read it on­line at

Our Lo­cal Group of gal­ax­ies is lop­sided, and this has been wor­ry­ing cos­mol­o­gists. The group is made up of two large sys­tems (An­dromeda and our own Milky Way) and a host of smaller satel­lites, and it’s the lat­ter that are the prob­lem. Rather than be­ing dis­trib­uted ran­domly, it turns out they are much more likely to be in the re­gion be­tween the two big­gest gal­ax­ies than any­where else, and it isn’t im­me­di­ately ob­vi­ous why that should be the case.

The ef­fect isn’t small. About 80 per cent of the satel­lite gal­ax­ies that or­bit around An­dromeda are on the side that faces us, and sim­i­lar ef­fects have been seen around other pairs of large gal­ax­ies ob­served by the Sloan Dig­i­tal Sky Sur­vey. This re­sult in­spired a team led by Mar­cel Pawlowski in Irvine, Cal­i­for­nia, to in­ves­ti­gate whether the same thing hap­pens in large-scale sim­u­la­tions of the Uni­verse. If dif­fer­ences were found, it would sug­gest that some­thing is wrong with our pic­ture of how cos­mol­ogy has steered the evo­lu­tion of the Uni­verse over the last 13.8 bil­lion years. The cur­rently favoured ‘Lambda-CDM’ cos­mol­ogy al­ready has prob­lems ac­count­ing for lots of small-scale fea­tures of the Uni­verse, such as the num­ber of satel­lite gal­ax­ies seen, so a new test would be very wel­come.

The ex­per­i­ment is an easy one to imag­ine. All you have to do is take one sim­u­la­tion of the Uni­verse, and count the small gal­ax­ies that ap­pear around pairs of mas­sive sys­tems within it. The au­thors used a suite of dif­fer­ent sim­u­la­tions, torn be­tween pick­ing one that in­cludes the small­est gal­ax­ies and choos­ing one that cov­ers a large enough vol­ume of the Uni­verse to make for a good test.

We know much more about these sim­u­lated Uni­verse than the real one. It is, for ex­am­ple, pos­si­ble in the sim­u­la­tion to iden­tify with cast-iron con­fi­dence which gal­ax­ies re­ally are satel­lites of any given pair, while ob­servers look­ing at the real thing might be con­fused by back­ground gal­ax­ies that just

“About 80 per cent of the satel­lite gal­ax­ies that or­bit around An­dromeda are on the side that faces us”

hap­pen to ap­pear nearby. This prob­lem is made worse be­cause we only have es­ti­mated red­shifts and hence dis­tances for most of the smaller gal­ax­ies, but with some care­ful sta­tis­tics the au­thors can make a sen­si­ble com­par­i­son.

When they do, it seems that in the sim­u­la­tions, just as in re­al­ity, the pres­ence of a pair of big gal­ax­ies is associated with a lop­sided dis­tri­bu­tion of smaller sys­tems. The ef­fect is seen in all but one sim­u­la­tion, and though that one is the most com­plex, in­clud­ing much more physics, it is also the small­est and so the ab­sence of an ef­fect is prob­a­bly just co­in­ci­dence. Cos­mol­ogy passes the test, but we’re still left with a mys­tery.

Satel­lites are most likely to be found be­tween two large gal­ax­ies, but the next most likely sim­u­lated po­si­tion is on the far side of each of the pair of gal­ax­ies. It’s not at all clear why this is the case, or whether that ef­fect holds in the real world. Plenty for both ob­servers and the­o­rists to get to grips with, there­fore – even if cos­mol­ogy passes the test this time.

Milky Way The An­dromeda Galaxy (M31) The Milky Way and M31 are dom­i­nant gal­ax­ies in the Lo­cal Group; M33, the third largest, may be a satel­lite of M31 The Tri­an­gu­lum Galaxy (M33)

CHRIS LIN­TOTT is an as­tro­physi­cist and co-pre­sen­ter of The Skyat Night on BBC TV. He is also the di­rec­tor of the Zooni­verse project.

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