"Andromeda Galaxy Harbors Twice as Much Dark Matter as the Milky Way" --Royal Astronomical Society
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July 30, 2014

"Andromeda Galaxy Harbors Twice as Much Dark Matter as the Milky Way" --Royal Astronomical Society


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The Milky Way is smaller than astronomers previously thought, according to new research. For the first time, scientists have been able to precisely measure the mass of the galaxy that contains our solar system. Researchers have found that the Milky Way is approximately half the weight of ou neighboring galaxy – Andromeda – which has a similar structure to our own. The Milky Way and Andromeda are the two largest in a region of galaxies which astronomers call the Local Group.

In this new study, researchers were also able to work out the mass of invisible matter found in the outer regions of both galaxies, and reveal their total weights. They say 90 per cent of both galaxies' matter is invisible. Scientists say that Andromeda's extra weight must be present in the form of dark matter, the little-understood invisible substance which makes up most of the outer regions of galaxies. They estimate that Andromeda contains twice as much dark matter as the Milky Way, causing it to be twice as heavy.

In previous studies, researchers were only able to estimate the mass of the Milky Way and Andromeda based on observations made using their smaller satellite dwarf galaxies. In the new study, researchers culled previously published data that contained information about the distances between the Milky Way, Andromeda and other close-by galaxies — including those that weren't satellites — that reside in and right outside an area referred to as the Local Group.

Galaxies in the Local Group are bound together by their collective gravity. As a result, while most galaxies, including those on the outskirts of the Local Group, are moving farther apart due to expansion, the galaxies in the Local Group are moving closer together because of gravity. For the first time, researchers were able to combine the available information about gravity and expansion to complete precise calculations of the masses of both the Milky Way and Andromeda.

"Historically, estimations of the Milky Way's mass have been all over the map," said Walker, an assistant professor of physics at Carnegie Mellon. "By studying two massive galaxies that are close to each other and the galaxies that surround them, we can take what we know about gravity and pair that with what we know about expansion to get an accurate account of the mass contained in each galaxy. This is the first time we've been able to measure these two things simultaneously."

By studying both the galaxies in and immediately outside the Local Group, Walker was able to pinpoint the group's center. The researchers then calculated the mass of both the ordinary, visible matter and the invisible dark matter throughout both galaxies based on each galaxy's present location within the Local Group. Andromeda had twice as much mass as the Milky Way, and in both galaxies 90 percent of the mass was made up of dark matter.

Researchers say their work should help them learn more about how the outer regions of galaxies are structured. Their findings also provide further evidence in support of a theory which suggests that the universe is expanding.

A team of scientists led by the University of Edinburgh used recently published data on the known distances between galaxies – as well as their velocities – to calculate the total masses of Andromeda and the Milky Way.

"We always suspected that Andromeda is more massive than the Milky Way, but weighting both galaxies simultaneously proved to be extremely challenging," said
Dr Jorge Peñarrubia, of the University of Edinburgh's School of Physics and Astronomy, who led the study. "Our study combined recent measurements of the relative motion between our galaxy and Andromeda with the largest catalogue of nearby galaxies ever compiled to make this possible."

The study, published in the journal Monthly Notices of the Royal Astronomical Society, was carried out in collaboration with the University of British Colombia, Carnegie Mellon University and NRC Herzberg Institute of Astrophysics. Findings from the study are supported by research carried out by the University of Cambridge, which used different data and methods and produced very similar results.

The Daily Galaxy via RAS, Carnegie Mellon, and University of Edinburgh's School of Physics and Astronomy

Image Credit Andomeda Galaxy


If Andromeda is twice as large as the Milky Way, it should have twice the mass. Duh! It should have twice as much dark matter, twice the number of stars, etc. These scientists are twice as brilliant as I am.

I've ran some nifty experiments awhile back. They seem off by 90%. I used to call that the fudge factor but my grant money dried up. I now call it Dark BS and the money came back. All's well in Astrophysics.

@Expat: Not in AP, but I would certainly hope so!

A specified fudge factor means you have a predictive model with a large systematic error which is interesting, but not generally fruitful. After DM was accepted it means we know there is a real feature, and have some ideas how to go about exploring it.

You don't tell the timing, but if you also didn't accept DM when the rest of the AP community did, that should have been a problem too.

In this entire article about the newly measured masses of the galaxies, the actual masses of the galaxies are never mentioned.

regardless, Andromeda is being depicted mirror wise.

@Peter: Sometimes it is not what you think you know it's what you can prove.

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