"Long before it's in the papers"
January 27, 2015


Filament of “cosmic web” reported seen for first time

Jan. 20, 2014
Courtesy of Nature
and World Science staff

As­tro­no­mers say they have found a dis­tant gal­axy light­ing up a vast cloud of thin gas, re­veal­ing for the first time part of a net­work of fil­a­ments thought to link ga­lax­ies through­out the uni­verse.

Re­search­ers at the Uni­vers­ity of Cal­i­for­nia, San­ta Cruz, led the stu­dy, pub­lished Jan­u­ary 19 in the re­search jour­nal Na­ture.

A neb­u­la (blue) around the qua­sar, or bright gal­axy co­re, UM28 at the cen­ter. The qua­sar's ra­di­a­tion makes the sur­round­ing gas glow, re­veal­ing a "cosmic web" fil­a­ment, ac­cord­ing to astro­nom­ers. The im­age was ob­tained at the W.M.Keck Ob­serv­a­to­ry. (Cred­it: S. Can­talupo )

Us­ing the 10-meter Keck I Tel­e­scope at the W. M. Keck Ob­serv­a­to­ry in Ha­waii, the re­search­ers said they de­tected a very large, lu­mi­nous “neb­u­la” or cloud of gas ex­tend­ing about 2 mil­lion light-years across space. A light-year is the dis­tance light trav­els in a year.

“This is a very ex­cep­tion­al ob­ject: it’s huge, at least twice as large as any neb­u­la de­tected be­fore, and it ex­tends well be­yond the ga­lac­tic en­vi­ron­ment of the qua­sar,” said study co-author Se­bas­tia­no Can­talupo, a post­doc­tor­al fel­low at UC San­ta Cruz. 

Main­stream think­ing pre­dicts that ga­lax­ies are em­bed­ded in a cos­mic web of mat­ter, most of which (a­bout 84 per­cent) is in­vis­i­ble dark mat­ter. This web is seen in the re­sults from com­put­er sim­ula­t­ions of the ev­o­lu­tion of struc­ture in the uni­verse, which show the dis­tri­bu­tion of dark mat­ter on large scales, in­clud­ing dark mat­ter “ha­los” in which ga­lax­ies form and the cos­mic web of “fil­a­ments” that con­nect them. 

Gra­vity causes or­di­nary mat­ter to fol­low dark mat­ter, so fil­a­ments of thin gas are ex­pected to trace a pat­tern si­m­i­lar to that seen in dark mat­ter sim­ula­t­ions.

Un­til now, though, these fil­a­ments haven’t been seen. Interga­lac­tic gas has been de­tected by its ab­sorp­tion of light from bright back­ground sources, but those re­sults don’t re­veal how the gas is dis­trib­ut­ed. In the new stu­dy, the re­search­ers de­tected the flu­o­res­cent glow of hy­dro­gen gas re­sult­ing from its il­lu­mina­t­ion by in­tense radia­t­ion from the qua­sar.

“This qua­sar is il­lu­minating dif­fuse gas on scales well be­yond any we’ve seen be­fore, giv­ing us the first pic­ture of ex­tend­ed gas be­tween ga­lax­ies. It pro­vides a ter­rif­ic in­sight in­to the over­all struc­ture of our uni­verse,” said co­au­thor J. Xa­vi­er Pro­chaska, pro­fes­sor of as­tron­o­my and as­t­ro­phys­ics at UC San­ta Cruz.

“We have stud­ied oth­er qua­sars this way with­out de­tect­ing such ex­tend­ed gas,” Can­talupo said. “The light from the qua­sar is like a flash­light beam, and in this case we were lucky that the flash­light is point­ing to­ward the neb­u­la and mak­ing the gas glow. We think this is part of a fil­a­ment that may be even more ex­tend­ed than this, but we only see the part of the fil­a­ment that is il­lu­minated by the beamed emis­sion from the qua­sar.”

A qua­sar is a type of ac­tive ga­lac­tic nu­cle­us, or co­re, that emits in­tense radia­t­ion pow­ered by a gi­ant black hole at the cen­ter of the gal­axy. In an ear­li­er sur­vey of dis­tant qua­sars us­ing the same tech­nique to look for glow­ing gas, Can­talupo and oth­ers de­tected so-called “dark ga­lax­ies,” the dens­est knots of gas in the cos­mic web. These dark ga­lax­ies are thought to be ei­ther too small or too young to have formed stars.

“The dark ga­lax­ies are much dens­er and smaller parts of the cos­mic web. In this new im­age, we al­so see dark ga­lax­ies, in ad­di­tion to the much more dif­fuse and ex­tend­ed neb­u­la,” Can­talupo said. “Some of this gas will fall in­to ga­lax­ies, but most of it will re­main dif­fuse and nev­er form stars.”

The re­search­ers es­ti­mat­ed the amount of gas in the neb­u­la to be at least ten times more than ex­pected from the re­sults of com­put­er sim­ula­t­ions. “We think there may be more gas con­tained in small dense clumps with­in the cos­mic web than is seen in our mod­els,” so those may have to be fixed, Can­talupo said.

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Astronomers say they have found a distant galaxy lighting up a vast cloud of thin gas, revealing for the first time part of the network of filaments thought to connect galaxies in a cosmic web. Researchers at the University of California, Santa Cruz, led the study, published January 19 in Nature. Using the 10-meter Keck I Telescope at the W. M. Keck Observatory in Hawaii, the researchers said they detected a very large, luminous “nebula” or cloud of gas extending about 2 million light-years across intergalactic space. A light-year is the distance light travels in a year. “This is a very exceptional object: it’s huge, at least twice as large as any nebula detected before, and it extends well beyond the galactic environment of the quasar,” said study co-author Sebastiano Cantalupo, a postdoctoral fellow at UC Santa Cruz. Mainstream thinking predicts that galaxies are embedded in a cosmic web of matter, most of which (about 84 percent) is invisible dark matter. This web is seen in the results from computer simulations of the evolution of structure in the universe, which show the distribution of dark matter on large scales, including the dark matter halos in which galaxies form and the cosmic web of “filaments” that connect them. Gravity causes ordinary matter to follow the distribution of dark matter, so filaments of diffuse, ionized gas are expected to trace a pattern similar to that seen in dark matter simulations. Until now, though, these filaments haven’t been seen. Intergalactic gas has been detected by its absorption of light from bright background sources, but those results don’t reveal how the gas is distributed. In the new study, the researchers detected the fluorescent glow of hydrogen gas resulting from its illumination by intense radiation from the quasar. “This quasar is illuminating diffuse gas on scales well beyond any we’ve seen before, giving us the first picture of extended gas between galaxies. It provides a terrific insight into the overall structure of our universe,” said coauthor J. Xavier Prochaska, professor of astronomy and astrophysics at UC Santa Cruz. “We have studied other quasars this way without detecting such extended gas,” Cantalupo said. “The light from the quasar is like a flashlight beam, and in this case we were lucky that the flashlight is pointing toward the nebula and making the gas glow. We think this is part of a filament that may be even more extended than this, but we only see the part of the filament that is illuminated by the beamed emission from the quasar.” A quasar is a type of active galactic nucleus, or core, that emits intense radiation powered by a giant black hole at the center of the galaxy. In an earlier survey of distant quasars using the same technique to look for glowing gas, Cantalupo and others detected so-called “dark galaxies,” the densest knots of gas in the cosmic web. These dark galaxies are thought to be either too small or too young to have formed stars. “The dark galaxies are much denser and smaller parts of the cosmic web. In this new image, we also see dark galaxies, in addition to the much more diffuse and extended nebula,” Cantalupo said. “Some of this gas will fall into galaxies, but most of it will remain diffuse and never form stars.” The researchers estimated the amount of gas in the nebula to be at least ten times more than expected from the results of computer simulations. “We think there may be more gas contained in small dense clumps within the cosmic web than is seen in our models. These observations are challenging our understanding of intergalactic gas and giving us a new laboratory to test and refine our models,” Cantalupo said.