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“Dark matter” mystery deepens

Oct. 17, 2011
Courtesy of the Harvard-Smithsonian Center for Astrophysics
and World Science staff

Like all ga­lax­ies, most as­tro­no­mers be­lieve ours is filled with a strange, in­vis­i­ble sub­stance that be­trays its pres­ence only through its gravita­t­ional pull. The gal­ax­y’s stars would fly apart with­out this so-called dark mat­ter hold­ing them to­geth­er.

But the na­ture of “dark mat­ter” is a riddle—and a new study has only deep­ened the mys­tery. Now, “we know less about dark mat­ter than we did be­fore,” la­ment­ed Matt Walk­er of the Har­vard-Smith­son­ian Cen­ter for As­t­ro­phys­ics in Cam­bridge, Mass., lead au­thor of a re­port on the work to ap­pear in the The As­t­ro­phys­i­cal Jour­nal.

Mod­els de­vel­oped by cos­mol­o­gists pre­dict dark mat­ter con­sists of some sort of ex­ot­ic par­t­i­cle that through gra­vity clumps to­geth­er with oth­ers of its kind. These ti­ny grains are be­lieved to be rath­er slug­gish, so dark mat­ter is typ­ic­ally dubbed “cold dark mat­ter,” slow par­t­i­cles be­ing nor­mally as­so­ci­at­ed with cold­ness.

Over cosmic his­to­ry, cos­mol­o­gists think clumps of dark mat­ter grew and at­tracted nor­mal mat­ter, form­ing the ga­lax­ies. Cos­mol­o­gists use pow­er­ful com­put­ers to sim­u­late this pro­cess. Their sim­ula­t­ions show that dark mat­ter should be densely packed in the cen­ters of ga­lax­ies. “If a dwarf gal­axy were a peach, the stand­ard cos­mo­lo­g­i­cal mod­el says we should find a dark mat­ter ‘pit’ at the cen­ter,” ex­plained Jor­ge Peñar­ru­bia of the Uni­vers­ity of Cam­bridge in the U.K., co-au­thor of the new stu­dy.

But mea­sure­ments of two dwarf ga­lax­ies show they con­tain a smooth dis­tri­bu­tion of dark mat­ter, Walk­er said. So “our mea­sure­ments con­tra­dict a bas­ic pre­diction about the struc­ture of cold dark mat­ter in dwarf ga­lax­ies.” In oth­er words, “the first two dwarf ga­lax­ies we stud­ied are like pit­less peach­es,” said Peñar­ru­bia.

Some as­tro­no­mers be­lieve dark mat­ter does­n’t even ex­ist, al­though the ma­jor­ity claim that many ob­served mo­tions of stars and ga­lax­ies are un­ex­plain­a­ble with­out it.

Dwarf ga­lax­ies are thought to con­sist of up to 99 per­cent dark mat­ter and only one per­cent nor­mal mat­ter, like stars. This makes dwarf ga­lax­ies ide­al sub­jects for dark mat­ter re­search­ers. Walk­er and Peñar­ru­bia an­a­lyzed the “dark mat­ter dis­tri­bu­tion” in dwarf ga­lax­ies neigh­bor­ing the Milky Way called For­nax and Sculp­tor. These each hold a mil­lion to 10 mil­lion stars, a mere hand­ful com­pared to the 400 bil­lion or so in the Milky Way. The team meas­ured the loca­t­ions, speeds and bas­ic chem­i­cal com­po­si­tions of 1500 to 2500 stars.

“S­tars in a dwarf gal­axy swarm like bees in a bee­hive in­stead of mov­ing in nice, cir­cu­lar or­bits like a spir­al gal­axy,” ex­plained Peñar­ru­bia. “That makes it much more chal­leng­ing to de­ter­mine the dis­tri­bu­tion of dark mat­ter.” The da­ta in­di­cat­ed that in both cases, the dark mat­ter is spread evenly over a huge re­gion sev­er­al hun­dred light-years across. A light-year is the dis­tance light trav­els in a year.

Some sci­en­tists have sug­gested in­ter­ac­tions be­tween nor­mal and dark mat­ter could spread out the dark stuff, but sim­ula­t­ions don’t show this hap­pens in dwarf ga­lax­ies, Walk­er and Peñar­ru­bia said. The new mea­sure­ments imply that ei­ther dark mat­ter is­n’t “cold,” or that it’s sur­pris­ingly strongly af­fect­ed by nor­mal mat­ter, added the re­search­ers, who are un­de­terred from their bas­ic as­sump­tion that dark mat­ter ex­ists. They hope to learn which of the two ex­plana­t­ions is bet­ter by stu­dying more dwarf ga­lax­ies, par­tic­u­larly those with an even high­er per­centage of dark mat­ter.


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Like all galaxies, most astronomers believe ours is filled with a strange, invisible substance that betrays its presence only through its gravitational pull. The galaxy’s stars would fly apart without this so-called dark matter holding them together. But dark matter’s nature is a mystery—and a new study has only deepened that quandary. Now, “we know less about dark matter than we did before,” lamented Matt Walker of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., lead author of a report on the work to appear in the The Astrophysical Journal. Models developed by cosmologists predict that dark matter consists of some sort of exotic particle that through gravity clumps together with others of its kind. These tiny grains are believed to be rather sluggish, so dark matter is typically dubbed “cold dark matter,” slow particles being normally associated with coldness. Over the history of the universe, cosmologists think clumps of dark matter clumps grew and attracted normal matter, forming the galaxies. Cosmologists use powerful computers to simulate this process. Their simulations show that dark matter should be densely packed in the centers of galaxies. “If a dwarf galaxy were a peach, the standard cosmological model said we should find a dark matter ‘pit’ at the center,” explained Jorge Peñarrubia of the University of Cambridge in the U.K., co-author of the new study. But measurements of two dwarf galaxies show that they contain a smooth distribution of dark matter, Walker said. So “our measurements contradict a basic prediction about the structure of cold dark matter in dwarf galaxies.” In other words, “the first two dwarf galaxies we studied are like pitless peaches,” said Peñarrubia. Some astronomers believe dark matter doesn’t even exist, although the majority claim that many observed motions of stars and galaxies are unexplainable without it. Dwarf galaxies are thought to consist of up to 99 percent dark matter and only one percent normal matter, like stars. This makes dwarf galaxies ideal subjects for dark matter researchers. Walker and Peñarrubia analyzed the “dark matter distribution” in dwarf galaxies neighboring the Milky Way called Fornax and Sculptor. These hold a million to 10 million stars, a mere handful compared to the 400 billion or so in the Milky Way. The team measured the locations, speeds and basic chemical compositions of 1500 to 2500 stars. “Stars in a dwarf galaxy swarm like bees in a beehive instead of moving in nice, circular orbits like a spiral galaxy,” explained Peñarrubia. “That makes it much more challenging to determine the distribution of dark matter.” The data indicated that in both cases, the dark matter is spread evenly over a huge region several hundred light-years across. A light-year is the distance light travels in a year. Some scientists have suggested interactions between normal and dark matter could spread out the dark stuff, but simulations don’t show this happens in dwarf galaxies, Walker and Peñarrubia said. The new measurements imply that either dark matter isn’t “cold,” or that it’s surprisingly strongly affected by normal matter, added the researchers, who are undeterred from their basic assumption that dark matter exists. They hope to learn which of those explanations is better by studying more dwarf galaxies, particularly those with an even higher percentage of dark matter.