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Exploring the Inception of Galaxies and the Epoch of Reionization

Periodic Report Summary 2 - EIGER (Exploring the Inception of Galaxies and the Epoch of Reionization)

The ERC consolidator grant Exploring the Inception of Galaxies and the Epoch of Reionization (EIGER) is designed to reveal the physical properties of the galaxies which formed in the early history of the Universe. Due to the finite speed of light, on Earth we view very distant galaxies as they were in the distant past. For example, the current age of the Universe is 13.8 billion years,
but the goal of EIGER is understand the properties of galaxies as they were when the Universe was only about 5% of its current age. Moreover, due to the continual expansion of the Universe, the light from these distant galaxies is stretched, or redshifted, as it travels towards us. The larger the distance, the greater the amount of redshifting of the light. Consequently, to study the UV and optical properties of very distant galaxies(i.e redshift z>6), the EIGER project primarily relies on imaging and spectroscopy carried out at red-optical and near-IR wavelengths.

EIGER is a four-year project, and many of the key scientific results will come late in the grant period as all of the inter-related observations and analysis work are drawn together. However, at the half-way stage, there have already been two major achievements. Firstly, early in the EIGER grant period McLure was Co-PI of the HUDF12 programme, which used the Hubble Space Telescope to obtain the most sensitive ever near-IR imaging of a small area of sky known as the Hubble Ultra Deep Field (HUDF). Due to the unprecedented sensitivity of these observations, the HUDF12 dataset allowed us to robustly identify and study the properties of galaxies at redshift z>7, when the Universe was only 0.7 billion years old (about 5% of its current age). One of the goals of the HUDF12 programme was to establish whether or not we could observe enough faint galaxies at high redshift to explain cosmic reionization. Cosmic reionization is the epoch early in the history of the Universe when the diffuse hydrogen gas which pervades space changed from being neutral to ionised. This process is thought to have been driven by high-energy UV photons emitted from the first generation of small galaxies, but until recently it has not been clear whether or not enough galaxies existed at high-redshift to explain this event. The new observations from the HUDF12 project successfully demonstrated that, under a plausible set of assumptions, we do observe sufficient numbers of faint high-redshift galaxies to explain cosmic reionization. Moreover, the HUDF12 data also allowed us to measure accurate colours for faint high-redshift galaxies, which suggested that their stellar populations are not dramatically different from those of similarly star-forming galaxies observed at much lower redshifts. Finally, another notable success of the HUDF12 project was to use the sensitivity of the Hubble Space Telescope to identify the first meaningful sample of extremely distant galaxies, at redshift z>9. Although the data obtained for the HUDF12 project was good enough to identify these extremely distant galaxies, it will require observations with the much more sensitive James Webb Space Telescope (JWST) to study their physical properties in detail (launch in 2018).

Another achievement of the EIGER project has been the award of the ESO public spectroscopy survey VANDELS (vandels.inaf.it). Based on the high-redshift research being carried out within EIGER, McLure proposed the VANDELS survey (along with Co-PI L. Pentericci) to use the VIMOS multi-object spectrograph on the VLT telescopes in Chile. The science driver behind the VANDELS survey was to obtain very high sensitivity, red-optical, spectra of about 2500 high-redshift galaxies (mostly within the 3