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Galaxies detected by their “shadows”

Nov. 2, 2010
Courtesy of Rutgers
and World Science staff

As­tro­no­mers have re­ported find­ing 10 huge new clus­ters of ga­lax­ies us­ing a new meth­od that de­tects their “shad­ows” on the back­ground light that per­me­ates the uni­verse.

This so-called mi­cro­wave back­ground radia­t­ion, too low-energy to be vis­i­ble to the un­aided eye, is thought to be a left­o­ver glow from the “Big Bang,” a sort of ex­plo­sion that gave birth to the cos­mos.

Top row: four At­a­cama Cos­mol­o­gy Tel­e­s­cope im­ages of cos­mic back­ground ra­di­a­tion, with dark blue in­di­cat­ing "shad­ows" cast by gal­axy clus­ters. Be­low, four vis­i­ble-light pic­tures of the gal­axy clus­ters. Faint white con­tour lines (clear­er in the full-page image here) cor­re­spond to the cos­mic back­ground ra­di­a­tion in­ten­si­ty lev­els in the top-row im­ages. (Cred­its: top row: To­bi­as Mar­riage, Johns Hop­kins / Prince­ton; bot­tom grid: Fe­li­pe Me­n­an­teau, Rut­gers )


The in­ves­ti­ga­t­ion of the ga­lax­ies be­gan in 2008 with a new ra­di­o tel­e­scope in north­ern Chile’s At­a­cama Des­ert – one of the dri­est places on Earth. The in­stru­ment, known as the At­a­cama Cos­mol­o­gy Tel­e­scope, col­lects millimeter-length ra­di­o waves that re­veal im­ages of the back­ground radia­t­ion. These ra­di­o waves are easily blocked by wa­ter va­por, hence the tel­e­scope’s home high in the An­des Moun­tains.

“The ground­break­ing ob­serva­t­ions at At­a­cama, led by Ly­man Page of Prince­ton Uni­vers­ity, sur­veyed large ar­eas of the [South­ern] sky to re­veal shad­ows that point­ed as­tro­no­mers to these pre­vi­ously un­seen mas­sive gal­axy clus­ters,” said Fe­li­pe Menan­teau, a re­search sci­ent­ist at Rut­gers Uni­vers­ity in New Jer­sey.

Yale Uni­vers­ity cos­mol­o­gist Priyam­vada Natara­jan, who was­n’t in­volved with the stu­dy, said “it will build our in­ven­to­ry of the most mas­sive and dis­tant clus­ters in the uni­verse,” which will help sci­ent­ists bet­ter un­der­stand how the uni­verse was born and changes.

In a pa­per pub­lished in the Nov. 10 is­sue of As­t­ro­phys­i­cal Jour­nal, the in­ves­ti­ga­tors de­scribed vis­u­al tel­e­scope ob­serva­t­ions of these gal­axy clus­ters, con­sid­ered es­sen­tial to ver­i­fy the “shad­ow” sight­ings.

Phys­i­cists Ra­shid Sun­yaev and Yakov Zel’­dovich pre­dicted the shad­ow phe­nom­e­non 40 years ago, now known as the Sun­yaev-Zel’­dovich ef­fect. As­tro­no­mers lat­er ver­i­fied it by find­ing shad­ows cast by pre­vi­ously known gal­axy clus­ters. The new ra­di­o tel­e­scope now makes it prac­ti­cal for as­tro­no­mers to es­sen­tially re­verse the pro­ce­dure – to search the cos­mic back­ground radia­t­ion for shad­ows that in­di­cate the pres­ence of un­seen clus­ters.

The “shad­ows” that the in­stru­ment re­vealed “are not shad­ows in the tra­di­tion­al sense, as they are not caused by the gal­axy clus­ters block­ing light from an­oth­er source,” said Rut­gers as­tron­o­mer Jack Hughes, who part­i­ci­pated in the new work. “Rather, the hot gas­es with­in the gal­axy clus­ters cause a ti­ny frac­tion of the cos­mic back­ground radia­t­ion to shift to high­er en­er­gies.”

Cos­mic back­ground radia­t­ion was first ob­served by two Bell Labs as­tro­no­mers in New Jer­sey back in the 1960s, a dis­cov­ery that earned them the No­bel Prize in Phys­ics in 1978.

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Astronomers have reported finding 10 huge new clusters of galaxies using a new method that detects their “shadows” on the background light that permeates the universe. This so-called microwave background radiation, too low-energy to be visible to the unaided eye, is thought to be a leftover glow from the “Big Bang,” a sort of explosion that gave birth to the universe. The investigation of the galaxies began in 2008 with a new radio telescope in northern Chile’s Atacama Desert – one of the driest places on Earth. The instrument, known as the Atacama Cosmology Telescope, collects millimeter-length radio waves that reveal images of the otherwise invisible background radiation. These radio waves are easily blocked by water vapor, hence the telescope’s home high in the Andes Mountains. “The groundbreaking observations at Atacama, led by Lyman Page of Princeton University, surveyed large areas of the [Southern] sky to reveal shadows that pointed astronomers to these previously unseen massive galaxy clusters,” said Felipe Menanteau, a research scientist at Rutgers University in New Jersey. In a paper published in the Nov. 10 issue of Astrophysical Journal, the investigators described visual telescope observations of these galaxy clusters, considered essential to verify the “shadow” sightings. Both observations will help scientists better understand how the universe was born and continues to evolve, the researchers said. Physicists Rashid Sunyaev and Yakov Zel’dovich predicted the shadow phenomenon 40 years ago, now known as the Sunyaev-Zel’dovich effect. Astronomers later verified it by finding shadows cast by previously known galaxy clusters. The new radio telescope now makes it practical for astronomers to essentially reverse the procedure – to search the cosmic background radiation for shadows that indicate the presence of unseen clusters. The ‘shadows’ that the instrument revealed “are not shadows in the traditional sense, as they are not caused by the galaxy clusters blocking light from another source,” said Rutgers astronomer Jack Hughes. “Rather, the hot gases within the galaxy clusters cause a tiny fraction of the cosmic background radiation to shift to higher energies.” Cosmic background radiation was first observed by two Bell Labs astronomers in New Jersey back in the 1960s, a discovery that earned them the Nobel Prize in Physics in 1978. Hughes and Menanteau worked with Chilean astronomers to collect optical images of dozens of candidate galaxy clusters, which led to the discovery of 10 massive ones. Yale University cosmologist Priyamvada Natarajan, who wasn’t involved with the study, said “it will build our inventory of the most massive and distant clusters in the universe, which will provide important constraints on the currently accepted cosmological model.”