Wednesday, November 14, 2012

The Clearing of the Cosmic Fog and the End of the Dark Ages

If one looks at the night sky, during new moon, it appears completely dark. Using powerful telescopes, astronomers discovered that it is not entirely dark, but the faint diffuse component is barely detectable. The major contributors to this emission, spanning the wavelengths from very high energy (gamma, X-ray) to very low frequencies (far-IR, sub-mm, radio), are: particles within the Solar system; gas and dust in the Milky Way (our Galaxy); stellar plus dust radiation by galaxies beyond our own, and Active Galactic Nuclei (AGN). The latter two are also known as Extragalactic Background Light (EBL).

The Universe today is around 13.7 billion years old. During this long period, stars and AGNs had time to produce the EBL we see today, but at present we know that the Universe was very different when it was young. Before the first stars and galaxies formed, the Universe was filled with electrically neutral hydrogen gas (HI), which absorbs ultraviolet light. Thus, there was an epoch, when the Universe had an age less than approximately 400-780 million years (or redshift grater than 7-12), when it was completely dark. As the ultraviolet radiation from the first galaxies and AGN excited the gas, making it electrically charged (ionized), it gradually became transparent to ultraviolet light. This process is technically known as reionization, as there is thought to have been a brief period within the first 100,000 years after the Big Bang in which the hydrogen was also ionized. This transition from neutral to ionized hydrogen is also known as the End of the Dark Ages.

Cosmic Time and Reionization epoch. From cosmic dark ages to
light. Credit: isciencetimes
But scientists do not know exactly when the first sources of light formed and when this reionization process started to occur. An international team of astronomers used the VLT as a time machine, to look back into the early Universe and observe several of the most distant galaxies ever detected. They have been able to measure their distances accurately and find that we are seeing them as they were between 780 million and a billion years after the Big Bang (or at redshift greater than 7). These new observations have allowed astronomers to establish a time-line for the epoch of reionization for the first time. During this phase the fog of hydrogen gas in the early Universe was clearing, allowing ultraviolet light to pass unhindered for the first time.

Another pressing question for astronomers working on the high redshift Universe is the real nature of the first sources that ended the Dark Ages. Plausible candidates for the reionization processes are stars in galaxies and AGN. At high redshifts, however, the number density of luminous AGN starts to decrease, and it is rare to find super massive Black Holes actively accreting matter (AGN) at redshift greater than 6-7. Although other exotic explanations can be found, the simplest explanation for Reionization is the ubiquitous presence of galaxies in the high redshift Universe. With HST and its new instrument WFC3 working on the near-IR wavelengths, astronomers have started to find candidate galaxies at redshift greater than 7, routinely observing galaxies at redshift 8-9, and possibly a few candidates at redshift greater than 10. However, these sources have not yet been confirmed through spectroscopic observations (as has been done at redshift 7 by the VLT): at the moment they are only candidates for being the most distant sources observed in the Universe.

The Hubble Ultra Deep Field (HUDF). Credit: wikipedia
The search for candidate galaxies at high redshift is routinely carried out with different techniques (for an example see this post by K. Caputi), but a simple and widely used technique is the so-called drop-out selection. The neutral hydrogen in each galaxy, part of the Interstellar Medium (ISM), imprints a characteristic shape on the spectral energy distribution of galaxies at high redshift, absorbing almost totally the light at rest frame wavelengths less than 912 Angstroms, and also partially the light at between 912 and 1216 Angstroms at redshifts greater than 3.

Because the Universe is expanding, the wavelength of light from galaxies gets stretched as it passes through space. The further light has to travel, the more its wavelength is stretched. As red is the longest wavelength visible to our eyes, the characteristic red colour this gives to extremely distant objects has become known as redshift.’ Although it is technically a measure of how the colour of an object’s light has been affected, it is also by extension a measure both of the object’s distance, and of how long after the Big Bang we see it.

Thus, the combination of the Universe's expansion and ISM absorption turns into a typical colour combination for galaxies at high redshifts, which are typically "red at short wavelengths and blue at long wavelengths", or more simply "drop-out" galaxies, since they tend to disappear in the blue bands.

A galaxy candidate at redshift 3.
Galaxies at redshift 3 are "drop-out" in the
U band (around 3600 Angstrom observed,

corresponding to 900 Angstrom rest frame).
Credit: R. Ellis
The so called drop-out technique has allowed astronomers to find thousands of galaxies at redshift greater than 3, most of which have been successfully confirmed through spectroscopic observations. Astronomical spectroscopy is a technique which involves splitting and spreading out the light from the galaxy into its component colours, much like a prism splits sunlight into a rainbow.

Thanks to the drop-out technique and to the availability of powerful near-IR instruments like WFC3 on-board HST, we were able to find more than 150 candidate galaxies around z=7, reaching very faint luminosities. The faintest galaxies we found have a luminosity which is 1.6 billion fainter than the faintest stars we can see with the naked eye on a dark night.

To derive the contributions of these galaxies to the Reionization processes we need to know three quantities: their number density, their efficiency at emit ionizing radiation (called escape fraction, see this post by H. Teplitz), and a measurement of the non homogeneity of the Universe (called clumpiness). If galaxies at redshift around 7 are ubiquitous and numerous in number, if they emit a lot of UV ionizing radiation and if the material between one galaxy and another (called Intergalactic Medium or IGM) is distributed in a homogeneous way, than it is easy for galaxies at redshift 7 to reionize the Universe.

High surface brightness galaxy (upper left) compared with
three low surface brightness galaxies. Low surface brightness
galaxies, which have low contrast compared to the brightness
of the sky, are hard to find, even at low redshift!

Credit: University of Arizona
Using HST data, we have measured the number density of galaxies at redshift around 7. This is a very delicate measurement, since it involves a number of corrections for incompleteness and systematics that have been derived through long and complex simulations. In particular, one of the most important factors in these simulations is the surface brightness bias effect. If two galaxies have the same luminosity, but different sizes, it is easier to find the compact galaxy (with high surface brightness) than the larger one (characterized by low surface brightness).

At redshift 7 there is a relation between the size and luminosity of galaxies, with fainter galaxies being smaller, and hence with higher surface brightness. Using this relation, we were able to derive with great accuracy the number density of galaxies at redshift 7, also known as the Luminosity Function.

The Luminosity Function of galaxies at redshift 7. Blue and magenta
points and lines have been derived using CANDELS+HUDF data.
An interesting result of this work is that the number of ionizing photons emitted from galaxies at redshift around 7 cannot keep the Universe reionized if the IGM is clumpy and the ionizing escape fraction of high-z galaxies is relatively low (less than 30%). We are currently waiting for deeper and wider data from HST to confirm this result and put strict limits on the role of galaxies in the reionization of the Universe.

This research was presented in a paper called “The size-luminosity relation at z=7 in CANDELS and its implication on reionization”, to appear in the Astronomy & Astrophysics journal (A&A).

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