Dark Matter Filament Detected Near the Milky Way

Dark Matter Filament Detected Near the Milky Way
Scientists at the Tel Aviv University discovered a string of galaxies only 15 million light years from the Milky Way. This group stands out because despite no apparent mutual interaction they display similar behavior. They all had recent star formation activity, and in some of the galaxies it is ongoing. This synchronized activity doesn’t seem to have stemmed from mutual effects but from accretion of intergalactic gas by the dark matter surrounding the galaxies.




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 The Wise Observatory in Mitzpe Ramon, in the Israeli Negev region (Credit: Wise Observatory)
The Wise Observatory in Mitzpe Ramon,
in the Israeli Negev region
(Credit: Wise Observatory)

Two components of our Universe are baryonic matter, visible matter that accounts for about 4% of the matter, and dark matter, a mysterious substance that can’t be directly observed, and makes up close to a quarter of the universe. Both types of matter seem to be structured, and not uniformly distributed. These structures include clusters connected by filaments and surrounding voids, where the matter density is very low. When baryonic matter streams into dark matter halos, galaxies can form. 

Adi Zitrin, a graduate student, and his instructor Dr. Noah Brosch, both from the Tel Aviv University, discovered a group of galaxies that seem to have formed along a dark matter filament. While studying galaxies as part of a sky survey, they stumbled upon a group of 14 galaxies located a mere 15 million light years from us, closer than most galaxies. This group stood out because the galaxies are lined up next to each other, in a region of lower than average galaxy density, forming a string at least 1.5 million light years long. All 14 galaxies are dwarf galaxies containing only several billion stars, not dozens or hundreds of billions stars, and are located at approximately the same distance from the Milky Way. From their color they were determined to be young, 10 billion years old at most. 

Their study revealed that after more than a billion years of inactivity, all 14 galaxies began forming stars again less than 30 million years ago and most of them still host ongoing star formations. Synchronized star formations in neighboring galaxies is a known phenomenon, usually credited to mutual interaction between galaxies caused by collisions or tidal forces, but in this case the galaxies in the group seem to keep to themselves. “This is a strange thing,” Brosch said, “A priori, one would not expect galaxies that have nothing to do with each other, a few million light years apart, to make stars at the same time.”  

The picture shows the 14 galaxies studied at the Wise Observatory. The image of each galaxy was assembled from three digital frames obtained through different filters. The colors are “blue”, “red”, and a special color produced by ionized hydrogen gas. The five faintest galaxies are synthetic reproductions for display. The sky region where the galaxies appear is six degrees wide, while each galaxy is only about one arc-minute, thus the galaxy images are highly enlarged to show individual internal details. The relative sizes of the galaxies are maintained, but their distances are reduced to fit the image and maintain relative locations. The colors have been modified to allow easy visibility of details. The background is artificial. Note the alignment of the galaxies along a line stretching from the lower-right corner of the image to the top-left corner. The star-forming regions in the galaxies are highlighted as shining reddish points (Credit: Adi Zitrin) 
The picture shows the 14 galaxies
studied at the Wise Observatory.
The image of each galaxy was
assembled from three digital frames
obtained through different filters.
The colors are “blue”, “red”, and a
special color produced by ionized
hydrogen gas. The five faintest galaxies
are synthetic reproductions for display.
The sky region where the galaxies
appear is six degrees wide, while each galaxy
is only about one arc-minute, thus the
galaxy images are highly enlarged to
show individual internal details. The relative
sizes of the galaxies are maintained,
but their distances are reduced to
fit the image and maintain relative
locations. The colors have been modified
to allow easy visibility of details.
The background is artificial. (Credit: Adi Zitrin)

Theoretical calculations done by astrophysicists at the Hebrew University in Jerusalem led by Professor Avishai Dekel, show that low mass galaxies can form when intergalactic gas streams into dark matter haloes and not by intergalactic gas heated by a virial shock to millions of degrees. This accretion mode yields efficient star formation in low mass galaxies. This could be used to explain the behavior of the 14 galaxies in this study. According to this theory, these galaxies are lined up along a dark matter filament, and their recent and present star formation is triggered by the accretion of cold gas from intergalactic space that is gravitationally attracted by the dark matter.

Some of the galaxies in this group were familiar to scientists before this study, but it was unknown that they exhibited similar properties. The initial detection of the group occurred when analyzing data from a radio sky survey, ALFALFA, executed at the Arecibo Observatory in Puerto Rico. Further optical observations took place at the Wise Observatory in Mitzpe Ramon in Israel. Brosch said the results of this study are still preliminary, and further observations with larger telescopes could reveal more about the star formation history of the galaxies. 

TFOT also reported on a new theory regarding the formation of massive stars. According to this theory, proposed by astrophysicists from Princeton University and the University of California at Berkeley, massive stars form inside an interstellar cloud of gas and dust, with the “help” of smaller stars that create the suitable conditions for massive stars’ creation. In another article TFOT covered NASA’s discovery of large amounts of simple organic gases and water vapor in a possible planet forming region around an infant star. 

Further information on the new study, scheduled to appear in the Monthly Notices of the Royal Astronomical Society, can be found in the Arxiv website and in the TAU press release.

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About the author

Shalhevet Bar-Asher

Shalhevet is finishing her B.A. degree in physics and mathematics at the Hebrew University in Jerusalem. She will begin her M.A. studies in physics next year and will focus on cosmology and astrophysics, her main topics of interest.

View all articles by Shalhevet Bar-Asher