New Hub­ble find chal­lenges our ideas about gal­ax­ies

Tehran Times - - SCIENCE - (Source: astron­

Ob­jects in the dis­tant uni­verse ap­pear small and dif­fi­cult to see – un­less they’re sit­ting be­hind a cos­mic mag­ni­fy­ing glass. That’s ex­actly the case for MACS 2129-1, a galaxy lensed by a mas­sive fore­ground galaxy clus­ter. Us­ing the Hub­ble Space Tele­scope, as­tronomers have man­aged to catch a glimpse of this un­usual ob­ject, which ap­pears to be an old, “dead” galaxy that’s al­ready stopped mak­ing new stars just a few bil­lion years after the Big Bang. Not only is this galaxy fin­ished with its star for­ma­tion ear­lier than ex­pected, it’s also shaped like a disk, rather than the fuzzy ball of stars that as­tronomers as­sumed they’d see.

The re­sults, which ap­pear in the June 22 is­sue of Na­ture, de­scribe a galaxy half the size of the Milky Way, but three times as mas­sive. Its com­pact disk of old, red stars is spin­ning rapidly, over two times the speed of the stars or­bit­ing the cen­ter of our own galaxy.

Grav­i­ta­tional lens­ing

As­tronomers were able to spot it via a phe­nom­e­non called grav­i­ta­tional lens­ing, which oc­curs when a mas­sive ob­ject, such as a galaxy clus­ter, bends the light from a dis­tant ob­ject as it trav­els to Earth, mag­ni­fy­ing the im­age we see on the sky. This al­lows re­searchers to probe very early epochs of the uni­verse that are oth­er­wise un­re­solv­able with to­day’s cur­rent in­stru­ments.

Based on archival data from the Clus­ter Lens­ing And Su­per­nova sur­vey with Hub­ble (CLASH), the team that dis­cov­ered the galaxy was able to mea­sure the ages of its stars, its to­tal stel­lar mass, and its rate of star for­ma­tion.

In our cur­rent pic­ture of galaxy for­ma­tion, diskshaped gal­ax­ies (like our own Milky Way) in the early uni­verse make stars through­out their youth, ap­pear­ing blue with bright, young stars be­fore evolv­ing into “red and dead” el­lip­ti­cal gal­ax­ies in our lo­cal uni­verse. This tran­si­tion is largely thought to oc­cur through merg­ers, which ran­dom­ize the or­bits of the stars in the re­sult­ing galaxy, trans­form­ing it from an ordered disk into an el­lip­ti­cal shape. Thus, older, more mas­sive gal­ax­ies should be el­lip­ti­cal balls of stars, not co­her­ent disks.

So as a disk galaxy in the early uni­verse that’s evolved past its star-form­ing phase into the dead phase with­out merg­ers, MACS 2129-1 chal­lenges that pic­ture. “This new in­sight may force us to re­think the whole cos­mo­log­i­cal con­text of how gal­ax­ies burn out early on and evolve into lo­cal el­lip­ti­cal-shaped gal­ax­ies,” said lead re­searcher Sune Toft of the Dark Cos­mol­ogy Cen­ter at the Niels Bohr In­sti­tute, Univer­sity of Copenhagen, in a press re­lease.

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