how the cosmic web’s fingers can shape galaxies
We all know that our vast universe is lousy with galaxies. Trillions of trillions of the things sprawl across the known cosmos and more than likely, the unknown one as well. We know a lot about them, including how many of them form. Enormous halos of dark matter and gas collapse into a massive black hole that consumes the matter spiraling around it, producing a bright quasar that takes eons to cool off and settle into a quiet, normal, mature galaxy that prefers a nice nap to a billion year rave under the blazing light of superheated plasma, and has a diversified portfolio of stars that will come and go until the last one flickers out of existence many epochs from now, as the second universal dark age begins. But often times, when astronomers look back toward the earliest galaxies, they find plenty of seemingly fully grown, mature galaxies among the quasars, something that technically shouldn’t happen if all galaxies come from halo collapse events.
But if galaxies don’t start with a bang, how would they accumulate their heft? Well, they’d simply accreate it from relatively cool gas flowing into them from the structure of the universe itself. As these jets of gas flow away from existing galaxies and are pushed by massive energetic events, they can eventually funnel down into a proto-galaxy with a lot of angular momentum, sending it spinning and quickly accumulating more and more matter. While this sounds like a very hot and tumultuous process, it’s actually anything but. The filament feeds dark matter and gas at steady rates and the spin it imparts is barely faster than what’s typical for a mature galaxy. Known as a cold flow model, it explains why some galaxies appear to have aged before their time, a factoid used in many speculative cosmology papers and blog posts to question the age of the universe to imply it was much older than we think. These early galaxies did not actually age quickly, they were still quite young. They simply didn’t have a violent birth and didn’t need to stabilize.
Even though this model has been around for a long time but we’ve never been quite sure if it’s happening in the wild. Until now, when a team of astronomers at Caltech saw a disk of gas and dust some 400,000 light years wide nearly 10 billion light years away steadily fed by a cool gas filament just the model predicted. Not only do we have a good explanation for the discrepancy between early galaxies, we also have direct observational evidence that the theory is correct. A few hundred million years of steady feeding and this galaxy will grow into a huge, stable wheel that looks as if it’s been around for at least an order of magnitude longer than it has, evidence that the universe’s biggest and most important structures don’t always have to be born from an immense cataclysm but gentle nudges from gravity and enough time can do the trick. But we’re not yet done with the cold flow model. Now that we know that the basics are right, we can flesh out exactly what happens when cool gas filaments fuel star birth how they affect the distribution of those stars infants. But that’s space for you. There’s always something more to discover…