how hot does it get be­tween gal­ax­ies in a clus­ter?

All About Space - - Ask Space - Dr Carolin Craw­ford is a pub­lic as­tronomer at the In­sti­tute of Astron­omy, Univer­sity of Cam­bridge.

“Dark mat­ter squeezes the gas [in­side clus­ters] to tem­per­a­tures of mil­lions of de­grees”

Many gal­ax­ies in the uni­verse live along­side oth­ers, teth­ered to each other by their mu­tual grav­ity to form gi­ant clus­ters; the largest grav­i­ta­tion­ally bound struc­tures in the uni­verse. The gal­ax­ies in a clus­ter are only a tiny frac­tion of the to­tal mat­ter present – there’s up to ten-times as much mass in the form of a hot ten­u­ous at­mos­phere fill­ing the space be­tween the gal­ax­ies, known as the in­tr­a­clus­ter medium.

The enor­mous grav­i­ta­tional field of the whole clus­ter – par­tic­u­larly that of the dom­i­nant, but un­ob­serv­able dark mat­ter – squeezes this gas to heat it to ex­traor­di­nar­ily high tem­per­a­tures of mil­lions of de­grees. There is so much en­ergy in this gas that the elec­trons are no longer bound to their atomic nu­cleus, but in­stead form a plasma com­posed com­pletely of elec­tri­cally charged par­ti­cles. Too hot to be seen in op­ti­cal im­ages, this plasma is only ob­serv­able in the X-ray wave­band where it ap­pears as a gi­ant, smooth pud­dle cen­tred on the most mas­sive galaxy in the clus­ter.

The tem­per­a­ture of the in­tr­a­clus­ter medium is a puz­zle – it ra­di­ates so much en­ergy through its X-ray emis­sion that it should have long since cooled down. The fact that it re­mains so hot re­quires a con­tin­ual heat­ing mech­a­nism within the clus­ter, most likely re­lated to the me­chan­i­cal en­ergy of sound waves trav­el­ling through the clus­ter gas. These waves in turn orig­i­nate from dis­tur­bances caused by the ac­tive su­per­mas­sive black hole found at the core of the cen­tral and most mas­sive galaxy.

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