In a paper published in the Astrophysical Journal Letters (arXiv.org), astronomers from the Max Planck Institute for Astronomy and the University of California have reported the discovery of a distant star-forming galaxy powered by pristine hydrogen, matter left over from the Big Bang.
To detect hydrogen stream near this galaxy, the astronomers made use of a cosmic coincidence – a bright quasar acting as a ‘cosmic lighthouse.’
In the current narrative of how galaxies like our own Milky Way formed, scientists postulate they were once fed from a vast reservoir of primordial hydrogen in the intergalactic medium, which permeates the vast expanses between galaxies. About 10 billion years ago when the Universe was one-fifth its current age, early proto-galaxies were in a state of extreme activity, forming new stars nearly one hundred times their current rate. Because stars form from gas, this fecundity demands a steady source of cosmic fuel.
In the past decade, supercomputer simulations of galaxy formation have become so sophisticated that they can actually predict how galaxies form and are fed: gas funnels onto galaxies along thin cold streams which, like streams of snow melt feeding a mountain lake, channel cool gas from the surrounding intergalactic medium onto galaxies, continuously topping up their supplies of raw material for star formation.
However, testing these predictions has proven to be extremely challenging, because such gas at the edges of galaxies is so rarefied that it emits very little light. Instead, astronomers systematically searched for examples of a very specific type of cosmic coincidence. Quasars constitute a brief phase in the galactic life-cycle, during which they shine as the most luminous objects in the Universe, powered by the infall of matter onto a supermassive black hole.
From our perspective on Earth, there will be rare cases where a distant background quasar and a stream of primordial gas near a foreground galaxy are exactly aligned on the night sky. As light from the quasar travels toward Earth, it passes by the galaxy and through the primordial gas, before reaching our telescopes. The cosmic gas selectively absorbs light at very specific frequencies which astronomers refer to as absorption lines. The pattern and shape of these lines provide a cosmic barcode, which astronomers can decode to determine the chemical composition, density, and temperature of the gas.
Using this technique, Dr Neil Crighton of the Max Planck Institute for Astronomy with colleagues has found the best evidence to date for a flow of pristine intergalactic gas onto a galaxy.
The galaxy, labeled Q1442-MD50, is so distant that it took 11 billion years for its light to reach us.
The primordial infalling gas resides a mere 190,000 light-years from the galaxy – relatively nearby on galactic length-scales – and is revealed in silhouette in the absorption spectrum of the more distant background quasar QSO J1444535+291905.
“This is not the first time astronomers have found a galaxy with nearby gas, revealed by a quasar. But it is the first time that everything fits together. The galaxy is vigorously forming stars, and the gas properties clearly show that this is pristine material, left over from the early Universe shortly after the Big Bang,” Dr Crighton said.
Bibliographic information: Neil H. M. Crighton et al. 2013. Metal-poor, Cool Gas in the Circumgalactic Medium of a z = 2.4 Star-forming Galaxy: Direct Evidence for Cold Accretion? ApJ 776, L18; doi: 10.1088/2041-8205/776/2/L18