Astronomers Observe Distant Galaxy Powered by Primordial Cosmic Fuel
Astronomers have detected cold streams of primordial hydrogen, vestigial matter left over from the Big Bang, fueling a distant star-forming galaxy in the early Universe. Profuse flows of gas onto galaxies are believed to be crucial for explaining an era 10 billion years ago, when galaxies were copiously forming stars. To make this discovery, the astronomers – a team led by Neil Crighton (Max Planck Institute for Astronomy and Swinburne University) – made use of a cosmic coincidence: a bright, distant quasar acting as a "cosmic lighthouse" illuminates the gas flow from behind. Their results have just been published in the Astrophysical Journal Letters.
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In the current narrative of how galaxies like our own Milky Way formed, cosmologists postulate that they were once fed from a vast reservoir of pristine hydrogen in the intergalactic medium, which permeates the vast expanses between galaxies. Approximately ten billion years ago when the Universe was just one fifth of its current age, early proto-galaxies were in a state of extreme activity, forming new stars at 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 (Birnboim & Dekel 2003, MNRAS, 345, 349; Dekel et al. 2009, Nature, 457, 451).
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, the team of MPIA 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, a team of astronomers led by Neil Crighton (Max Planck Institute for Astronomy; now at Swinburne University of Technology, Melbourne) has found the best evidence to date for a flow of pristine intergalactic gas onto a galaxy. The galaxy, denoted 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.
A crucial element of their discovery is the detection of the spectral signature of cosmic deuterium, a stable isotope of hydrogen (with an extra neutron in the nucleus). Cosmologists have demonstrated that hydrogen and helium and their stable isotopes like deuterium were all synthesized just minutes after the Big Bang, when the Universe was hot enough to power nuclear reactions. All heavier elements like carbon, nitrogen, and oxygen were created much later in the hot nuclear furnaces of stars. Because the hostile physical conditions in the centers of stars would destroy the fragile deuterium isotope, the discovery of deuterium in the gas confirms that the gas falling onto the galaxy is indeed pristine material left over from the Big Bang.
Crighton explains: "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."
This discovery of this system is part of a large survey for quasar sightlines which pass near galaxies, which is coordinated by Joseph Hennawi, the leader of the ENIGMA research group at the Max Planck Institute for Astronomy.
Hennawi adds: "Since this discovery is the result of a systematic search, we can now deduce that such cold flows are quite common: We only had to search 12 quasar-galaxy pairs to discover this example. This rate is in rough agreement with the predictions of supercomputer simulations, which provides a vote of confidence for our current theories of how galaxies formed."
The astronomers' long-term goal is to find about ten similar examples of these cold flows, which would allow for a much more detailed comparison of their observations with the predictions of numerical models. To this end, they are currently searching for more quasar-galaxy pairs using the Large Binocular Telescope in Arizona and the Very Large Telescope of the European Southern Observatory (ESO) in Chile. J. Xavier Prochaska (University of California at Santa Cruz), a collaborator on the survey, concludes: "Previous studies of these galaxies had shown evidence for gas flowing out of them, something we also see evidence for. However with Neil's much more precise analysis, we can also detect the raw material fueling galaxies, and thereby trace how much gas they take in, and when. That is a key piece in the puzzle of galaxy formation."
Avishai Dekel (Hebrew University, Jerusalem) was instrumental in theoretically and numerically establishing the current model of cold-flow accretion onto galaxies. While not involved in this research, he commented on the results: "This is a very interesting finding. It is consistent with the theoretical prediction, based both on physical analysis and on cosmological simulations, for the feeding of high-redshift galaxies by cold streams from the cosmic web. […] The low metallicity makes this case for inflow more convincing than earlier detections."
Background information
The work described here will be published as N. H. M. Crighton et al., "Metal-Poor, Cool Gas in the Circumgalactic Medium of a z = 2.4 Star-Forming Galaxy: Direct Evidence for Cold Accretion?" in Astrophysical Journal Letters.
The co-authors are Neil H. M. Crighton (MPIA and Swinburne University of Technology), Joseph F. Hennawi (MPIA), and J. Xavier Prochaska (University of California at Santa Cruz).
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The full quote by Avishai Dekel is: "This is a very interesting finding. It is consistent with the theoretical prediction, based both on physical analysis and on cosmological simulations, for the feeding of high-redshift galaxies by cold streams from the cosmic web. The detected gas properties, including column density, temperature and thickness, indeed match the predictions, and the velocity relative to the galaxy center is consistent with instreaming at a velocity comparable to the virial velocity. The low metallicity makes this case for inflow more convincing than earlier detections."
Questions and Answers
What is hot, cool and cold gas in this context, and why is this important?
Stars are produced when regions within a cloud of (mainly hydrogen) gas collapse under their own gravity, heating up in the process until temperatures and densities are sufficient for nuclear fusion to ignite. This, however, can happen only in very cold clouds with temperatures not far above absolute zero, with typical temperatures around 10-100 K (about -260 to 160 degrees Celsius); in warmer gas, collapse will be prevented by internal pressure of the gas.
That is why studies of star formation logistics are interested in cold and in cool gas, where cool gas in this context means gas with temperatures of up to tens of thousands of Kelvin. That might seem hot by everyday standards – tens of thousands degrees Celsius! – but is still much colder than the temperatures of hot gas with temperatures of millions of Kelvins and above, which is thought to be the typical temperature of the majority of the gas around proto-galaxies. This distinction is important: for hot gas to cool down sufficiently to serve as the raw material for star formation, inordinate amounts of time are needed – possibly even longer than the present age of the universe. Cool gas accreting from the intergalactic medium takes much less time to cool down to star-production levels, and supplies of cool gas thus play a fundamental role for star formation within the past billions of years.
What are cold accretion flows?
When gas from the intergalactic medium flows towards a galaxy, it will generally heat up considerably. As an analogy, imagine a public square full of people, and imagine that all those people suddenly rush towards a central location. As the crowd converges on their goal, it grows ever more densely packed, and people cannot help jostling each other. In a similar manner, infalling gas will, in general, grow ever more dense as it approaches a galaxy, its atoms jostling each other, raising the temperature of the gas in the process ("shock heating"). It takes such a long time for the resulting hot gas, heated to more than a million degrees Kelvin, to cool down that it is unlikely to play a significant role in fueling star-formation.
However, there is a different mode of gas flow. On scales of about ten million light-years and above, matter in the universe is not distributed homogeneously. Instead, the predominant matter component, dark matter, forms a complex network of long, thin filaments joining thickened nodes which is known as the cosmic web. The nodes of this web house the largest galaxies and their entourage of smaller companion galaxies. Gas flowing along the filaments can avoid the crowding effects that lead to heating. In our human crowd analogy, this corresponds to a column of people marching towards a common goal in an orderly queue. The result are dense and narrow streams called "cold-accretion flows" of intergalactic matter onto galaxies, supplying the galaxies with cool gas that, after a modicum of further cooling, can serve as the raw material for star formation.
What telescopes and instruments were used?
The systematic survey of absorption systems comprises observations with the Large Binocular Telescope on Mount Graham in Arizona and with the FORS2 instrument at the European Southern Observatory's Very Large Telescope. It also includes archival data. For many of the largest astronomical telescopes, observational data is archived and, after a certain time period that allows the principal observers exclusive access to their data, it is made publicly accessible. The data analysis described here used archival observations of the background QSO J1444535+291905 taken with the HIRES echelle spectrograph at the 10 m (segmented mirror) Keck I telescope on Hawai'i. The foreground galaxy was discovered by Charles Steidel, Gwen Rudie (California Institute of Technology) and collaborators using the LRIS spectrograph on the same telescope.
How can the astronomers tell that this is pristine gas, left over from right after the big bang?
The absorption lines from the gas show the cloud's chemical composition. In particular, they show the presence of heavier chemical elements. Astronomers collectively call these heavier elements – that is everything other than hydrogen and helium – "metals". What these metals have in common is that all of them are produced by nuclear fusion inside stars or by nuclear processes in supernovae explosions. Pristine gas will contain next to none of these heavier elements; gas that has formed stars in a galaxy will contain a significant amount.
The gas studied by Crighton et al. has several different components moving at different speeds towards or away from us, which shows that these must be different gas clouds along our line of sight. One of these clouds has very few heavier elements – between 0.7 and 1.5% the amount found in our Sun – and, crucially, it contains significant amounts of the hydrogen isotope deuterium (hydrogen with one extra neutron in the nucleus). Deuterium is quickly destroyed inside stars, so its presence ensures that this cloud is indeed pristine.
What is new and special about the results described here?
Over the last years, there have been a few similar observations. In 2011, Fumagalli, O'Meara and Prochaska reported the detection of pristine gas clouds (no discernible elements other than hydrogen) via absorption lines in the light of a distant quasar, but there was no nearby galaxy involved. In July this year, Nicolas Bouché and collaborators published an observation of an absorption system consisting of a gas cloud, an associated galaxy, and a background quasar illuminating the gas cloud (http://www.sciencemag.org/content/341/6141/50.
abstract), i.e. similar to the Crighton et al. observations. The kinematics of the cloud in Bouche et al. are consistent with accreting gas, but that gas contains a factor of ten higher proportion of heavy elements (metals) than the Crighton et al. system. Therefore, it may also be caused by gas stripped from the nearby galaxy, or gas inside a smaller satellite galaxy.
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