This artist's conception shows a dwarf galaxy seen from the surface of a hypothetical exoplanet. A new study finds that the dark matter in dwarf galaxies is distributed smoothly rather than being clumped at their centers. This contradicts simulations using the standard cosmological model known as lambda-CDM.  Credit: David A. Aguilar (CfA)

Digging Deeper For Dark Matter

Article Updated: 24 Dec , 2015

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Dark matter… If it can’t be seen, then how do we know it’s there? If it wasn’t for the effects of gravity, we wouldn’t. We’d have a galaxy filled with runaway stars and no galaxy would exist for long. But how it behaves and how it is distributed in one of the biggest cosmic cryptograms of all. Even with new research, there seems to be more questions than answers!

“After completing this study, we know less about dark matter than we did before,” said lead author Matt Walker, a Hubble Fellow at the Harvard-Smithsonian Center for Astrophysics.

It is generally accepted that our Universe is predominately composed of dark matter and dark energy. Of the former, it is considered to be “cold”, stately exotic particles which coalesce through gravitation. As they evolve, these dark matter “clumps” then attract “normal” matter which forms present day galaxy structures. Through computer modeling, astronomers have simulated this growth process which concludes that galactic centers should be dense with dark matter. However, these models aren’t consistent with findings. By measuring two dwarf galaxies, scientists have found a even distribution instead.

“Our measurements contradict a basic prediction about the structure of cold dark matter in dwarf galaxies. Unless or until theorists can modify that prediction, cold dark matter is inconsistent with our observational data,” Walker stated.

Why study a dwarf instead of a spiral? In this case, the dwarf galaxy is a perfect candidate because of its composition – 99% dark matter and 1% stars. Walker and his co-author Jorge Penarrubia (University of Cambridge, UK) chose two nearby representatives – the Fornax and Sculptor dwarfs – for their study. In comparison to the Milky Way’s estimated 400 billion stars, this pair averages around 10 million instead. This allowed the team to take a comprehensive sample of around 1500 to 2500 stars for location, speed and basic chemical composition. But even at a reduced amount, this type of stellar accounting isn’t exactly easy picking.

“Stars in a dwarf galaxy swarm like bees in a beehive instead of moving in nice, circular orbits like a spiral galaxy,” explained Penarrubia. “That makes it much more challenging to determine the distribution of dark matter.”

What the team found was somewhat surprising. According to the modeling techniques, dark matter should have clumped at the core. Instead they found it evenly distributed over a distance measuring several hundred light years across.

“If a dwarf galaxy were a peach, the standard cosmological model says we should find a dark matter ‘pit’ at the center. Instead, the first two dwarf galaxies we studied are like pitless peaches,” said Penarrubia.

It is hypothesized that interactions between normal and dark matter might be responsible for the distribution, but the computer simulations say it shouldn’t happen to a dwarf. New queries to new findings? Yes. This revelation may suggest that dark matter isn’t always “cold” and that it could be impacted by normal matter in unexpected ways.

Original Story Source: Harvard Smithsonian Center for Astrophysics News Release. For Further Reading: A Method Of Measuring (Slopes Of) the Mass Profiles of Dwarf Spheroidal Galaxies.

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Lawrence B. Crowell
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Lawrence B. Crowell
October 18, 2011 1:35 AM
The contribution by matter, luminous plus dark matter, to the Omega term increases to ?_m ~ .28 at the scale of a large galaxy and remains on all larger structures. For structure larger than this the contribution levels off. Dwarf galaxies have a suppressed contribution. The mass to luminosity ratio scales more or less linearly with length scale until you reach the dimensions of a galaxy, where it saturates out at the ?_m ~ .28 above. So dwarf galaxies are in a sort of “hinderland” or boundary of being large scale structures. There is still a possible contribution by neutrinos to dark matter. This will form a warm dark matter component, as opposed to dark. The contribution, stated… Read more »
metamaterials
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metamaterials
October 18, 2011 3:06 AM
This will seem like and be labeled as sheer science fiction and fantasy here, but I must show my findings, and wonder about new fields of science, called biocosmology, exobiology, astrobiochemistry, etc. What are Dwarf Galaxies MADE OF AND LOOK LIKE ? Filamentary emissions of mitochondria muscle cells of rats !! See photo : http://microscopyu.com/articles/fluorescence/filtercubes/violet/v2a/v2aa10desminlarge.html Note how the dense beady swarming stars of a dwarf galaxy, especially our companion dwarf Seque 1, and the whole entire galaxy, looks like my “Rat Cell Galaxy.” This is a filamentary emission photograph of the light of a rat cell mitochondria, the energy source and powerhouse of the cell, where ATP is. Only dwarf galaxies have 99% dark matter composition! So a… Read more »
Anonymous
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Anonymous
October 19, 2011 7:07 PM

Very interesting thoughts. It makes me think of those photos comparing what the Universe might look like with a neuron.

Jamie Horsburgh
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Jamie Horsburgh
October 18, 2011 7:01 AM
Hello Tammy, Another great article. A few grammatical typos which you know I can’t help but point out. At the bottom of the first paragraph you should use “seem” instead of “seems” since it is referring to the plural “questions”. In the third sentence of the third paragraph, since dark matter is singular you should use “this” and then “attracts” with an s. Also, I know it is idiomatic, but it might sound better if you used “easy pickings” with an s at the end of the 5th paragraph. I wish I were knowledgeable enough to make comments about the actual astronomy like lcrowell, but alas we all have our bailiwicks. Thanks for writing fascinating articles that even… Read more »
Michael Lowe
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Michael Lowe
October 18, 2011 10:02 PM

your an idiot

Anonymous
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Anonymous
October 19, 2011 7:03 PM

That’s funny. I bet you Jamie laughed at that one. smile

Torbjorn Larsson OM
Member
Torbjorn Larsson OM
October 18, 2011 8:36 AM
“If a dwarf galaxy were a peach, the standard cosmological model says we should find a dark matter ‘pit’ at the center. Instead, the first two dwarf galaxies we studied are like pitless peaches,” said Penarrubia. It is hypothesized that interactions between normal and dark matter might be responsible for the distribution, but the computer simulations say it shouldn’t happen to a dwarf. These results follow very close to the almost year long Eris simulation that made CDM successfully predict a galaxy for the first time. And it didn’t give a “pit” central density peak but follows a power law distribution fit (fig 5)*, which makes me think Penarrubia et al is referring to older results. This revelation… Read more »
Lawrence B. Crowell
Member
Lawrence B. Crowell
October 18, 2011 12:07 PM
A dwarf galaxy is likely to have ?_m less than that of a larger galaxy, which is about ?_m ~ .28. This might then attenuate the formation of mass concentrations at the center. A small collection of masses in a mutual gravitational system that is at the Virial limit will maintain that configuration for a considerable time with little loss of phase space volume. This is a fancy way of saying that the stars will orbit with few being either ejected or colliding and imploding into black holes. For matter in a distribution with a density ? the force per unit volume on a test mass m is f = G?m/r^2. I assume the density is constant in… Read more »
metamaterials
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metamaterials
October 20, 2011 5:30 PM

Hooke’s Law of motion is like a spring, and works very well for dark matter, to explain the constant velocity that all stars have orbiting the galaxy, no difference in distance from the center of the galaxy.

devillotus
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devillotus
October 18, 2011 6:06 PM

Dark matter is composed of dark energy stars. There are no black holes.

Anonymous
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Anonymous
October 19, 2011 7:02 PM

that’s a cool thought

ITSRUF
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ITSRUF
October 20, 2011 2:02 AM

Love the line: “…cold dark matter is inconsistent with our observational data,”

Then why should we believe it?

uniontera6
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uniontera6
October 28, 2011 2:44 PM
Uniontera number 11-1, hope it helps. Existence is the time expressed by light itself. http://www.youtube.com/watch?v=AN1jTwHUC7E 1. Arrow A is the laser work. (Same time work) 2. Arrow B is the reason of gravity. (also, Higgs boson individually / Different time work) 3. The rose line is the beginning light. (result from Bible) 4. The existence of different time. (result from relativity of light) 5. 2 dimension is not a space. (result from 2-axis, math) 6. Universe came from nothing. (result from No.5 / cf. Playing with bubbles) 6-1. A barrier[posititon] exist between the inner and outer bubbles. But universe do not have that barrier. (Cause of Inertia / Reason of Sun / Position of Antimatter / cf. Expansion)… Read more »
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