A Cosmic Phoenix
NGC 4889 and its attendants, members of the vast Coma Cluster. Note their smooth, milky-white appearances.
NASA Hubble (CC BY 2.0)
For the citizens of the universe's suburbs, like our Milky Way lives, life is relatively peaceful. Most of our neighbours are at comfortable distances of a few million light years and close visitations are one in a billion years. Out here, most galaxies are content to feed on the filaments of the intergalactic medium, slowly sipping hydrogen gas to fuel slow and steady star formation that colours their spiral arms with the elegant blue of young stars and the pretty pink of glowing nebulae. But for a significant minority of galaxies in the universe, life is not so tranquil.
About 10% of galaxies in the universe do not live alone or in sparse groups, but rather in dense clusters. These are some of the most extreme environments in the universe, with thousands of galaxies crammed together like college students, all buzzing and whirling around a common centre of mass at thousands of kilometres per second. Close encounters happen frequently and the resulting gravitational tugs violently warp and shake up galaxies in a process known fittingly as harassment. Giant supermassive black holes at the centres of cluster galaxies belch out massive amounts of radiation, driving strong winds which ram through the cluster's intergalactic medium. Some truly unfortunate galaxies might even end up colliding head-on with other cluster members, scattering stars and dust everywhere. With so much violence going on all the time, cluster galaxies cannot sustain star formation; collisions and harassment trigger furious bursts of star formation that consume their supplies of gas, while black hole winds blow away any gas trying to settle into their disks. Without star formation, cluster galaxies lose their beautiful arms, becoming inert, red and dead blobs of dim, old stars doomed to slowly fade forever.
But on special occasions, a select few galaxies can defy this fate and return to the land of the living.
Giant elliptical galaxy NGC 1275, which glows with a firestorm of young stars.
NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration (Public Domain)
Over time, the largest galaxies of a cluster will sink to its core as they lose gravitational energy to their smaller neighbours through the process of dynamical friction. Because of this, a massive pile-up collision eventually happens there and forms a giant elliptical galaxy made from the old, red, mixed-up stars of all those which came before. Now, because this giant elliptical galaxy is formed right on top of the galaxy cluster's center of mass, it experiences no unbalanced gravitational forces and so just ends up sitting there not moving. And it is this lethargy that will be key to its revival.
Eventually, the black holes at the hearts of cluster galaxies will run out of gas to eat as collisions, harassment and black hole winds rip it all away. Without their radiation, the cluster's intergalactic medium is allowed to cool off. And, as you might remember from grade-school science, warm gas rises and cool gas sinks. Cooling flows of intergalactic medium sink towards the cluster centre, becoming denser and denser until they collapse and fragment into dense pockets where new stars flare to life. Those new stars follow the inexorable pull of gravity along with the gas, raining onto the giant elliptical galaxy waiting at the cluster centre.
The galaxy you see above, NGC 1275, is an exemplar of this process in full swing. This giant elliptical galaxy sits at the heart of the Perseus Cluster, a crowded galactic megalopolis with over 1000 residents. Though most of its stars are old and red, cooling flows have showered it with billions of new stars that have given its core a noticeable blue tinge. The UV light emitted by these young blue stars have in turn ionized the cooling flows, forming vast red threads of glowing nebulae longer than the Milky Way is wide.
This glut of star formation cannot last forever. Eventually, enough gas will sink to the heart of NGC 1275 to reawaken the supermassive black hole at its heart, which will heat the gas to a boil and shut off the shower of young stars. But sometime in the far future, the Perseus Cluster will cool once more and NGC 1275 will once again give birth to a new generation. There is enough gas in the Perseus Cluster's intergalactic medium to repeat this cycle for trillions of years, far longer than the Milky Way will have with its comparatively meagre supplies of gas. So perhaps in the end it is the galaxy clusters which will have the last laugh before the universe grows cold and silent forevermore.
