Galaxies Die from the Inside Out

When the first stars and galaxies started to form, it was like a spark of a massive chain reaction where the vast amounts of gas and dust that had clumped together were quickly converted into dense, luminous star clusters.  This was the beginning of the formation of the heavier elements that would eventually make up all that we see on the planet Earth.  But when did this massive tirade of star formation end? When we look at galaxies in the present epoch, most don’t form stars very rapidly at all, and giant elliptical galaxies are all but devoid of gas, and consist of a vast ageing stellar population.  When did it all stop?

Galaxy star formation stops from the inside out, says new ESO observations. Credit: ESO

Using the Very Large Telescope (VLT) at the European Southern Observatory (ESO), along with NASA’s Hubble Space Telescope (HST), Astronomers have peered deep into the cosmos to look at galaxies as they were 3 Billion years after the Big Bang.  They noticed that the galaxies still made stars on their outskirts, but not in their centres.  Until now, how galaxies ended their initial torrents of star formation was unknown, even though we knew roughly when in history it happened.

Giant Elliptical Galaxies pack in stars at ten times the density of the central regions of our own Milky Way.  Known as ‘red and dead,’ these galaxies contain no young blue stars, and an ageing population of red stars that were estimated to have finished forming around ten Billion years ago.  These giant galaxies are thought to contain about half of all the stars the Universe has produced in its entire lifetime, and the understanding of how these galaxies evolved is essential to our overall picture of the universe.

There are many different theories as to why the star formation ended in an inside out fashion, but it seems that with the density of material in the centre, stars would form more rapidly in the central regions of the galaxy, using up all the gas and shutting down the star formation earlier.  There is more to the story however, as newly formed stars affect their environment, blowing away gas and triggering the formation of nearby stars.  Its possible that the rapid formation in the central regions blew the remaining gas into the outskirts.

It’s the first time we’ve seen evidence of how these galaxies became stagnant, and further study with the newest observational technology will reveal more details and give us a better understanding of how these giants formed and ended up red and dead.


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