How far does dark matter extend from the Andromeda galaxy ?

[scains25]

Hi, thanks for the article in this week's email about mapping the Andromeda galaxy, extending halfway towards our galaxy. The article says the researchers were also able to measure the motion of the halo, but it didn't give details. Please tell me the rotation velocity is finally starting to slow down, this far out ? All the measurements of different galaxies so far always seem to show rotation velocity just staying constant as far out as can be measured, so the dark matter must also be high density as far out as can be measured, but it can't go on forever.

Thanks for the weekly email, it has become my favourite source for astronomy news, so many amazing stories explained very simply with beautiful photos. ( Is that Cygnus loop photo REALLY real ?? Wow.) I just recommended your weekly emails to my linkedin followers. Thanks.

[antoniseb]

Welcome scains25, your question presumes that somehow we have precise measurements of dark matter, and we don't. You will often see cutting-edge research in Astronomy say that there is an indication of something, and that implies that further study will be needed to get details. So the answer, rather vaguely expressed is that the dark matter that is dense enough to support the roughly constant rotation velocity doesn't extend much beyond the dense stars in M31. It's shape may be distorted by recent mergers and a pass-through of another galaxy or two (M32 & NGC205), but we don't have enough precision to say for sure. The dark matter halo around M31 DOES extend perhaps a third of the way to the Milky Way, but it gets thinner as it extends out, and again the specifics are only loosely known.

[SagittariusAStar]

The total diameter of the Milky Way has recently been estimated to be 1.9 million light-years. If Andromeda is similar, then that might be the approximate answer.