{"id":46590,"date":"2023-06-26T08:21:44","date_gmt":"2023-06-26T08:21:44","guid":{"rendered":"https:\/\/www.skyatnightmagazine.com\/?p=119725"},"modified":"2023-06-26T08:32:36","modified_gmt":"2023-06-26T08:32:36","slug":"studying-local-galaxies-from-an-outside-perspective-and-what-it-may-mean-for-dark-matter","status":"publish","type":"rss_feed","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/rss_feed\/studying-local-galaxies-from-an-outside-perspective-and-what-it-may-mean-for-dark-matter\/","title":{"rendered":"Studying local galaxies from an \u2018outside\u2019 perspective \u2013 and what it may mean for dark matter"},"content":{"rendered":"

Studying our cosmic neighbourhood from \u2018outside\u2019 is an eye-opener. <\/p>

By Chris Lintott\n \t\t<\/p>

Published: Monday, 26 June 2023 at 12:00 am<\/p>

\n\n

T<\/span>ravel is all very well, but sometimes you can learn more by staying close to home.<\/p>\n

As astronomers get excited by the views of distant galaxies<\/a> provided by JWST, it\u2019s good to be reminded that we can benefit from paying close attention to our surroundings.<\/p>\n

Some of the most exciting areas of research happening at the moment consider our local galaxies as if they were observed from a distance, something that can be done thanks to extensive surveys of stars in the Milky Way<\/a> and deep imaging of other members of our Local Group of galaxies<\/a>.<\/p>\n

\""Stretching\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
Stretching across 4 million lightyears, most of the galaxies in the Local Group (top) are dwarfs, but the two largest, M31 and our own Milky Way (bottom), are giant spirals. Credit: Paul Wootton. Original creation by David Rumsey\/National Geographic Society (U.S.): www.davidrumsey.com<\/span><\/figcaption><\/div>\n

Considering our own home in this way allows us to test the accuracy of techniques used to study the distant Universe \u2013 though in the case of the Milky Way, living in our object of study does make things slightly more complicated at times.<\/p>\n

In a new paper, astronomer Stacy McGaugh takes a close look at the relationships between how stuff moves in Local Group galaxies and the way that mass in those systems is distributed.<\/span><\/p>\n

This is crucial if you\u2019re interested in where to find the mysterious dark matter<\/a> that most cosmologists believe makes up most of the mass in the cosmos.<\/span><\/p>\n

These measurements are even more important if, like McGaugh, rather than invoking the presence of dark matter to explain some of the oddities we observe in the Universe, our observations are best explained by modifying Newton\u2019s equations of gravity<\/a>.<\/span><\/p>\n

\""We\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
We can only see dark matter from its effect on stars and galaxies. Here, astronomers used the images of galaxies distorted by gravitational lensing to map out the dark matter, shown in blue. Credit: NASA, ESA, M. Jee and H. Ford (Johns Hopkins University)<\/span><\/figcaption><\/div>\n

Differences between our local galaxies and more distant neighbours may, he reckons, suggest problems with the dark matter model that we should pay attention to.<\/span><\/p>\n

Things start off looking good for lovers of dark matter.<\/span><\/p>\n

One of the fundamental rules that seems to govern how galaxies are assembled is the Tully\u2013Fisher relation, which says that if you know the mass of a spiral galaxy you can work out how fast the material in it should be rotating around the centre.<\/span><\/p>\n

Make these measurements for the Milky Way, the Andromeda Galaxy<\/a> and a host of the small dwarfs that share the Local Group with us, and you find that they follow the same rule as distant galaxies.<\/span><\/p>\n

No signs of anything untoward here.<\/span><\/p>\n

When we consider what the masses of the big galaxies are, problems appear.<\/p>\n

\""This\"<\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/source><\/picture><\/div>
A Hubble Space Telescope image showing part of the Andromeda Galaxy. Credit: NASA, ESA, J. Dalcanton (University of Washington, USA), B. F. Williams (University of Washington, USA), L. C. Johnson (University of Washington, USA), the PHAT team, and R. Gendler.<\/span><\/figcaption><\/div>\n

There are discrepancies between the mass of the Milky Way and Andromeda predicted by using the same sort of rules we use on distant galaxies, and what we observe by looking directly at their stars, or at how the material in their discs moves.<\/p>\n

Essentially, McGaugh points out that both of the big denizens of the Local Group seem to have many more stars than we might otherwise expect for galaxies of their mass.<\/p>\n

Put another way, something the size of the Local Group would typically have one large galaxy, the mass of the Milky Way \u2013 Andromeda is superfluous to requirements, and yet here it is.<\/p>\n

McGaugh hints that this is another problem that dark matter can\u2019t solve.<\/p>\n

Others will point to the possibility that the particular history of Local Group\u2019s galaxies may have led them to have particularly efficient star formation, or perhaps that there is some subtlety to our studies of distant galaxies we have yet to understand.<\/p>\n

Either way, it\u2019s another reminder to look around us, and not always to seek answers in distant realms.<\/p>\n

Chris Lintott was reading <\/em>Local Group Galaxies from an External Perspective by Stacy McGaugh.<\/em><\/strong><\/p>\n

Read it online at: ui.adsabs.harvard.edu\/abs\/2023arXiv230500858M\/abstract<\/a>.<\/em><\/strong><\/p>\n

This article originally appeared in the July 2023 issue of BBC Sky at Night Magazine.<\/strong><\/em><\/p> <\/body><\/html>\n

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