If you wanted to build the brightest galaxy in the Universe, how would you assemble it? There are some good options to consider:
- Would you simply gather up as many stars as you possibly could, under the logic that more stars leads to a greater brightness?
- Would you try and gather up as many of the absolute brightest stars you could, and try to build a galaxy out of those?
- Would you put all your eggs in one basket, and construct a supermassive black hole that was feeding on a tremendous amount of matter, expelling radiation all across the electromagnetic spectrum?
- Or would you simply do everything you could to remove all the light-blocking dust present in your galaxy, allowing 100% of the light you generated to shine through the Universe?
While all of these options sound good, it turns out that only two of them really matter in the end.
The sheer number of stars in your galaxy, quite surprisingly, doesn't matter nearly as much as you might think. The largest and most massive galaxy in the known Universe, IC 1101 (above), contains an estimated 100 trillion (10^14) stars, or many hundreds of times the number of stars in our own Milky Way. Yet this galaxy is quite red in color: most of the stars in there are M-class stars, which are the lowest mass, faintest and longest-lived of all stars. Proxima Centauri, the closest star to us, is an example of one of these M-class red dwarfs; it would take six hundred of these stars just to equal the brightness of one Sun.
On the other hand, a very, very bright star -- like R136a1 in the Tarantula Nebula -- can be as bright as over eight million Suns, teaching us that a small number of the brightest stars can make all the difference to a galaxy.
Image credit: NASA, ESA, F. Paresce (INAF-IASF, Bologna, Italy), R. O'Connell (University of... [+]
These very bright stars don't live very long: only one or two million years at most, an extremely short length of time on the scales that galaxies shine. It's only the galaxies that are presently undergoing the greatest star formation activity that can house large numbers of the brightest, bluest stars. And the easiest way to get a large burst of star formation is to have two gas-rich galaxies of comparable size merge together. Suddenly, all that neutral matter that was working against you -- blocking light and obscuring large amounts of what would otherwise be visible stars -- contracts gravitationally to form new stars: the hottest, brightest and bluest ones of all.
But a major merger like this can not only give rise to a large number of these incredibly luminous stars, but can also cause the supermassive black hole at the center of one (or both) galaxies to become active, resulting in a tremendous amount of emitted light across the entire electromagnetic spectrum.
Image credit: NASA, ESA, CXC, STScI, and B. McNamara (University of Waterloo) / NRAO, and L. Birzan... [+]
This emission comes not from an actual "feeding frenzy" but from large amounts of matter near the black hole accelerating in a circular path, speeding up and heating up as it collides with all the other matter present. The gas and dust that you were worried might block the light from these galaxies actually turns out to be the fuel for the engines of these active galaxies.
It's only a very small percentage of supermassive black holes that are active at any given time, but a tiny fraction of those remain active not just for a few million years, but for as much as 100 million years! And it's the galaxies like these, the extreme ones, that can outshine everything else. Put all of these facts together, and you've got two options for the most luminous galaxy in the Universe:
- it would either be a very, very young, blue galaxy undergoing a very large starburst, or
- it would be a somewhat redder galaxy whose central black hole is undergoing tremendous, sustained activity.
Well, we've just broken the record! It's thanks to the activity of the supermassive black hole in the galaxy WISE J224607.57-052635.0, which we're seeing from when the Universe was just 10% of its current age. The WISE spacecraft has just discovered the most luminous galaxy in the Universe, with a brightness of 300 trillion Suns. What settles the issue is that this galaxy is red in color, not due to redshift but to the intrinsic color of its stars. Known as an ELIRG (extremely luminous infrared galaxy), it's a puzzle as to how this galaxy got to be so bright so fast. Researcher Andrew Blain, coauthor of the report, proposes the following explanation:
"The massive black holes in ELIRGs could be gorging themselves on more matter for a longer period of time. It's like winning a hot-dog-eating contest lasting hundreds of millions of years."
Every new discovery seems to bring a new mystery and a new puzzle to solve along with it, and this one is no exception. It's all part of the beauty and joy of science: the more we learn, the more we realize there is to learn. At least, though, we now know how to create the most luminous galaxy in the Universe: just give it a huge supermassive black hole, and feed it plenty of gas and dust, until what once blocked the light now emits it of its own accord. Who knew? To achieve a maximum brightness, it isn't about stars at all! While this galaxy won't be the brightest one for very long -- it should fade by time the Universe ages another 1% -- it sure is spectacular today.
