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 e latter two are also known as If one looks at the night sky,new moon, it appears completely. Using powerful telescopes, astronomers discovered that it isnot entirely dark, butfaint diffuse component is barelydetectable. The major contributors to this emission, spanning thewavelengths from very high energy (gamma, X-ray) to very lowfrequencies (far-IR, sub-mm, radio), are: particles within the Solarsystem; gas and dust in the Milky Way (our Galaxy); stellar plus dustradiation by galaxies beyond our own, and Active Galactic Nuclei (AGN) . Thlatter twoknown as Extragalactic Background Light (EBL)





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 ultraviolet light. Thus, there wa s 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 as there is thought to have been a brief period within the first 100,000 years after the ionized. This transition from neutral to ionized hydrogen is also known as the End of the Dark Ages. The Universe today is around 13.7 billion years old. During thislong 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 itwas young. Before the first stars and galaxies formed, the Universewas filled with electrically neutral hydrogen gas (HI), which absorbsultraviolet light. Thus, theres an epoch, when theUniverse had an age less than approximately 400-780 million years (orredshift grater than 7-12), when it was completely dark. As the ultravioletradiation from the first galaxies and AGN excited the gas, making itelectrically charged (ionized), it gradually became transparent toultraviolet light. This process is technically known as reionization as there is thought to have been a brief period within the first100,000 years after the Big Bang in which the hydrogen was alsoionized. This transition from neutral to ionized hydrogen is alsoas the End of the Dark Ages.





Cosmic Time and Reionization epoch. From cosmic dark ages to

light. Credit: isciencetimes formed and when this reionization process started to occur. An international team of astronomers used the 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. But scientists do not know exactly when the first sources of lightformed and when this reionization process started to occur. Aninternational team of astronomers used the VLT as a time machine, tolook back into the early Universe and observe several of the mostdistant galaxies ever detected. They have been able to measure theirdistances accurately and find that we are seeing them as they werebetween 780 million and a billion years after the Big Bang (or atredshift greater than 7). These new observations have allowed astronomersto establish a time-line for the epoch of reionization for the firsttime. During this phase the fog of hydrogen gas in the early Universewas clearing, allowing ultraviolet light to pass unhindered for thefirst time.





Universe is the real nature of the t he 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 gr e ater 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 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. Another pressing question for astronomers working on the high redshiftUniverse is the real nature of the first sources that endedheDark Ages. Plausible candidates for the reionization processes arestars in galaxies and AGN. At high redshifts, however, the numberdensity of luminous AGN starts to decrease, and it is rare to findsuper massive Black Holes actively accreting matter (AGN) at redshiftgrater than 6-7. Although other exotic explanations can be found, thesimplest explanation for Reionization is the ubiquitous presence ofgalaxies in the high redshift Universe. With HST and its newinstrument WFC3 working on the near-IR wavelengths, astronomers have started to findcandidate galaxies at redshift greater than 7, routinely observinggalaxies at redshift 8-9, and possibly a few candidates at redshiftgreater than 10. However, these sources have not yet been confirmed throughspectroscopic observations (as has been done at redshift 7 by the VLT): atthe moment they are only candidates for being the most distantsources observed in the Universe.









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 ‘ 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. Because the Universe is expanding, the wavelength of light fromgalaxies gets stretched as it passes through space. The further lighthas to travel, the more its wavelength is stretched. As red is thelongest wavelength visible to our eyes, the characteristic red colourthis gives to extremely distant objects has become known as redshift .’ Although it is technically a measure of how the colour ofan object’s light has been affected, it is also by extension a measureboth of the object’s distance, and of how long after the Big Bang wesee 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,



Credit: corresponding to 900 Angstrom rest frame).Credit: R. Ellis thousands of galaxies at redshift greater than 3, most of which have been successfully confirmed through spectroscopic observations. Astronomical spreading out the light from the galaxy into its component colours, much like a prism splits sunlight into a rainbow. The so called drop-out technique has allowed astronomers to findthousands of galaxies at redshift greater than 3, most of which havebeen successfully confirmed through spectroscopic observations.Astronomical spectroscopy is a technique which involves splitting andspreading 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 luminosit y which is 1.6 billion fainter than the faintest stars we can see with the naked eye o n a dark night.





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





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 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 th at have been derived through long and complex s imulations. In particular, one of the most important factors in these simulations is the If two galaxies have the same luminosity, but different sizes, it is easier to find the compact galaxy (with high surface brightness) than the large r one (characterized by low surface brightness). Using HST data,have measured the number density ofgalaxies at redshift around 7. This is a very delicate measurement, sinceit involves a number of corrections for incompleteness and systematics thhave been derived through long andcompleximulations.In particular, one of the most important factors in these simulationsis 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 largeone (characterized by low surface brightness).





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 At redshift 7 there is a relation between the size and luminosity ofgalaxies, with fainter galaxies being smaller, and hence withhigher surface brightness.thiswere able to derive with great accuracy the numberdensity 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.



