Wednesday, September 19, 2012

The Dawn of Galaxies

One of the most exciting areas of astrophysics today is understanding how the very first stars and galaxies lit up the Universe.  This happened during the Epoch of Reionization (EoR), which was highlighted in the latest Astronomy Decadal Survey report New Worlds, New Horizons as the area of astronomy with the greatest discover potential in the next decade.  It is a central goal of CANDELS to probe galaxies in the EoR.

The Universe began in a hot Big Bang.  Early on, it was too hot for the protons and electrons to combine into atoms, because the high temperature made particles smash into each other too often.  So the Universe after about three minutes consisted of a fully ionized plasma -- that is, an admixture of positively-charged hydrogen and helium nuclei, and negatively-charged electrons.  Finally, after about 380,000 years, the Universe became cool enough that protons and electrons could bind together into neutral hydrogen atoms.  Thus began the Cosmic Dark Ages, so called because no sources of light were present, and all of the cosmos was enshrouded in a fog of neutral hydrogen and helium gas.

The Dark Ages lasted until a few hundred million years after the Big Bang.  It was then that the very first sources of light appeared, providing energy that ate away at the neutral hydrogen fog.  And so the Universe became ionized again, slowly and inhomogeneously, with electrons and protons being separated by energetic photons emitted by the earliest stars and galaxies.  We call this process cosmic re-ionization.  The Epoch of Reionization lasted until about one billion years after the Big Bang, and left the bulk of the Universe fully ionized and transparent as we see it today.  The EoR is the last major phase transition that the Universe undergoes, and it is the frontier of galaxy evolution studies today.

CANDELS has already detect a few EoR galaxies, which is pretty exciting in of itself.  But that's only part of the job.  What we really want is to understand what these galaxies look like, how they got there, and what they imply for the process of reionization. This is a job for the CANDELS theory crew.

So what do we want to figure out?

The biggest questions here are among the most basic:
a) What are the sources responsible for reionization? and
b) What is the topology of reionization?

While we know what galaxies look like today, there are good reasons to think that the first galaxies responsible for reionization might have looked quite different.  For one thing, like in a new house, there hasn't been much time for dust to accumulate. This is critical, because it turns out that star formation as we know it today requires dust as a catalyst. So how can stars form in the first galaxies with little or no dust?

The answer is: very slowly. But the stars that do form can be incredibly massive -- perhaps hundreds of times heavier than the Sun! These first stars, known as Population III stars, are copious emitters of ionizing radiation that can eat away at the cold fog of neutral hydrogen. On the flip side, because they form slowly, they are rare, so it's unknown whether there will be enough of them to power reionization.  Current thinking says probably not, but don't bet your first-born on it.

  This animation of a simulation from John Wise shows heavy elements (yellow) surrounding Population III stars after they have exploded.  

Moreover, these massive Population III stars have a short life, exploding in spectacular hypernovae after just a few million years. The details of these explosions are crucial:  Heavy elements like carbon, oxygen, and silicon are catalyzed in copious amounts during their short lives, and the hypernova could disperse them widely to form dust that quickly transitions star formation to the more familiar Population II (dust-catalyzed) mode. On the other hand, if the star collapses directly to a massive black hole, it would suck most of these heavy elements into oblivion, and the Population III era would continue for longer. Since Population III stars are no longer around today, it is difficult to see how they work in detail, and insights from models are often all we have to go on.

Even after the Population III epoch ends, it remains unclear whether there are enough stars to power re-ionization. CANDELS, as impressive as it is, only allows us to view the brightest of reionization-epoch galaxies -- Hubble cannot directly detect the fainter galaxies (this is what JWST will do). If there are not enough galaxies to provide the reionizing photons needed, it may indicate that there are more exotic contributors such as early black holes or an unexpected preponderance of Population III stars. Yet many models indicate that these faint galaxies are so numerous, that they actually dominate the radiation output! Clearly, tallying the total photon budget from CANDELS galaxy counts remains a poorly constrained yet critical aspect for understanding the sources of re-ionization.

This movie of a simulation from Tiziana di Matteo shows an evolving cube of the cosmos as sources begin to ionize the surrounding gas, eventually leaving a transparent Universe after about 1 billion years. Note the complex topology of filaments and sheets that houses early galaxy formation; this is known as the Cosmic Web.

As if those uncertainties aren't enough, there is the issue of topology. Topology refers to the spatial distribution, in this case of the protons and electrons.  While radiation from early galaxies can ionize hydrogen, the Universe is still sufficiently dense that the dissociated protons and electrons can quickly re-join back into hydrogen. This is a process known as recombination.

While recombinations are (cosmically) rare today, nature has perversely arranged the timescales for reionization and recombination to be annoyingly comparable during the EoR. This means one has to understand the spatial clustering or topology of protons and electrons, in order to know how often after being so cruelly separated, they will bump into each other again and re-discover their lost electrochemical bonds of love. This can be quantified by the clumping factor of protons and electrons.  If the clumping factor is high, it requires many photons to ionize a single atom, since protons and electrons remain close enough to recombine again after being ionized. If clumping is low, a single photon might be enough to keep an atom ionized. Hence we not only have to count how many photons are being emitted, but we also have to understand matter clumping in order to know how effective each photon is at reionizing the cosmic fog.

With all this uncertain physics flying around, it's not surprising that the EoR represents one of the most difficult modeling problems in astronomy today. Our most sophisticated simulations include all the complex processes we use to model galaxy formation at later epochs, plus the dispersal of heavy elements via outflows along with radiative transfer -- the emission and propagation of photons from cosmic sources. This last aspect is particularly challenging, requiring massive supercomputers to move not only mass but light around the simulated cosmos.

CANDELS theorists have developed a remarkable simulation code, called MARCH, capable of handling all these physical effects with essentially no simplifying approximations. Using MARCH, we have been able to show that the clumping factor is around 3, in contrast to earlier estimates of 10-30, and that CANDELS is directly detecting the sources that provide about one-quarter of the photons needed for reionization. While these results are encouraging, there remain many uncertainties in such calculations, particularly the escape fraction, i.e. the number of ionizing photons that escape from within galaxies. There is a long way to go before we can confidently model all the processes going on during the EoR.

Nonetheless, the EoR remains one of the most vibrant and revolutionary areas of study in the CANDELS team. CANDELS data provides the boundary conditions for theorists' models, while the models inform the interpretation of the observations. The recent CANDELS team meeting in Santa Cruz enabled the High-Redshift Working Group to assess where we stand now in both observations and theory, and how to best proceed in concert. Together, we are shining a new light on the Cosmic Dark Ages by peering boldly into the dawn of galaxies.

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