How do galaxies form?
Seems like a simple question, right? We live in a galaxy – a sprawling city of gas, dust, and over a hundred billion stars – and we see hundreds of billions of them in the sky, so you’d think we have a decent handle on how these objects came to be.
That turns out not to be the case. Their formation is more complicated than you might expect, but a new set of observations reveals an important clue in the birth process of the largest discrete objects in the Universe.
First, the cool image:
OK, so it’s not the most photogenic thing in the world. But what is it?
Deep optical images of young galaxies reveal that they are surrounded a vast cloud of hydrogen gas, many times larger than the galaxy itself. In a fit of nomenclaturial acumen, astronomers have dubbed these “blobs”. The thing is, to be seen at all from this great distance, the blobs must be tremendously luminous, and there’s no clear source of energy for them. Something must be powering them, but what?
There were two competing ideas: one was that the gas is simply cooling as it falls in to the galaxy, and that gas radiates away its heat in the form of light (similar to the way a hot iron bar will radiate its heat away as infrared light). That will then power the gas on the outside, causing it to glow. The other idea was that there is some central source of power deep inside the galaxy itself, lighting up the blobs like a light bulb in a smoke-filled room.
But which is it? Ah, enter these new observations.
The image on the left is of such a young galaxy with a blob around it. This image is optical (from Japan’s Subaru telescope and Hubble) plus deep infrared (taken using the Spitzer Space Telescope). You can see the blob, falsely colored yellow, and the denser galaxy embedded in it. Look at the upper left part of the blob: see that reddish glow? What could that be?
The image on the right reveals it. That is the same picture, but added in (in blue) are observations using the Chandra X-ray Observatory. X-rays are only produced through very energetic and violent sources, such as matter swirling around a black hole, or exploding stars. That would certainly explain what’s powering the blobs’ light! But usually these events also produce a lot of optical light. Why don’t we see that?
The reddish glow in the left hand picture is the key. That indicates the galaxy is loaded with dust, made when stars are born and when the explode, too. Dust absorbs optical light, and what does get through can be highly reddened.
So now it looks like we have a complete picture of what’s going on here: as a galaxy forms from an infalling blob of gas a million light years across, a supermassive black hole coalesces in the center. Matter falls in, swirling madly around it, pouring out X-rays. Just outside this central region of the galaxy, stars are born at tremendous rates, creating lots of dust. The most massive stars explode in just a few million years, also blasting out X-rays, but also making even more dust. The dust blocks our optical view of the bright sources, but the X-rays still can leak out in quantities sufficient to heat up and light up the surrounding blobs of gas. What this means for the galaxy at large is that this huge amount of energy dumped into the blobs may slow and eventually reverse the infall, shutting off the process which forms the galaxy itself.
What we’re seeing here may be the last birth throes of a galaxy.
What I love about all this – besides the fact that we can know anything at all about what’s going on in an object a million light years across, billions of light years away, and billions of years in the past – is that we need all these observations together to figure this stuff out. The optical light alone presents us with a mystery, and the IR observations help but don’t solve the problem. But when you add the X-ray observations, they reveal the solution.
And think on this… the blobs we’re talking about here are huge, dwarfing the galaxies that are forming from them. They contain billions of times the Sun’s mass in raw gas, the building material of stars in a nascent galaxy. And these immense clouds are being lit up by not just supernovae — which are terrifying all by their lonesome, dumping out energy at rates that would turn the Earth into a crispy ember – but also by gigantic black holes smack dab in their galaxy’s hearts, which are blowing out energy in quantities to rival or exceed the supernovae themselves.
Yet all that power, the true source of energy illuminating the clouds so much we can see them from across half the Universe, is hidden from our telescopes. Or at least it was, until we learned to slip the surly bonds of Earth and loft our eyes into space, where X-rays can travel freely, unimpeded by our pesky atmosphere.
The universe is complex, and if we truly want to understand it, we will need to continue to explore it, and use the combined might of our scientific equipment to investigate it. There are hidden treasures out there, and the more we probe, the more we’ll find.