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Early Star-Forming Galaxies and Cosmic Reionisation

Periodic Reporting for period 4 - FIRST LIGHT (Early Star-Forming Galaxies and Cosmic Reionisation)

Reporting period: 2020-04-01 to 2021-09-30

`First Light’ refers to the period when the Universe emerged from darkness and the first stars and galaxies were born. At this early time the Universe became bathed in ultraviolet light for the first time and dark clouds composed primarily of hydrogen atoms were gradually ionised into constituent protons and electrons, a transition termed `cosmic reionisation’. Exploring and determining the physical processes that governed this final uncharted period of cosmic history is the basis of Professor Ellis’ ERC programme.

Although the Universe began in a `Big Bang’ 13.8 billion years ago, the birth of starlight explored via this programme is in many ways a more fundamental milestone in cosmic history. All life forms are based on chemical elements synthesised in stars, so the First Light programme directly explores our origins. Astronomy has enormous public appeal and the quest for the earliest galaxies receives widespread media and popular coverage. Numerous studies have shown that such astronomical discoveries regularly inspire young people to enter STEM-based careers.

Ellis’ programme is largely observational in nature, exploiting his access to the world’s most powerful astronomical telescopes, aided with a theoretical component based on state-of-the-art numerical simulations. To accelerate the research programme, he has established a number of international collaborations with colleagues in Japan and the USA. The programme addresses 3 fundamental questions:

(i) When did First Light occur and over what period did the Universe transition from a neutral (i.e. atomic) form to one that is fully ionised?

(ii) Were hot stars in early galaxies the principal source of ionising radiation or was there a contribution from non-thermal radiation emanating from massive black holes?

(iii) Planning for the upcoming James Webb Space Telescope, can we find examples of the very earliest `first generation’ sources verified via their pristine chemical composition?

The above questions are identical to those proposed in the 2014 submission and excellent progress has been made on all three items.
Professor Ellis arranged to be based at the European Southern Observatory (ESO) Headquarters in Germany for the first two years of the programme in order to ensure rapid familiarisation with European astronomical facilities after his 16 year sojourn in the USA. This proved very effective as his group has enjoyed a high success rate in securing observing time on ESO’s Very Large Telescope (VLT) in Chile. In September 2017, he returned to University College London (UCL) full time.

The overall programme combines challenging and innovative observations of high redshift galaxies and analogues at intermediate redshift with theoretical simulations which serve to interpret the results in the context of realistic models of galaxy formation and evolution. The observational aspects of the proposal remain ahead of schedule (c.f. the timeline proposed in 2014) with continued access not only to the ESO VLT but also to the Atacama Large Millimetre Array (ALMA) and the Keck telescopes in Hawaii. A major campaign was recently completed on the Hubble Space Telescope and ambitious proposals for the James Webb Space Telescope due for launch towards the end of the programme are at an advanced stage of preparation. The study of early galaxies and their role is a highly competitive one with as many as a dozen research groups seeking observing time worldwide. Professor Ellis’ group remains at the forefront in this area as evidenced not only by the success in securing observing time but also frequent invitations to give plenary talks on the work of his group at international conferences.

The UCL group is now up to its full complement with three postdoctoral associates and two graduate students. A modest amount of funding has been set aside for international visitors given the importance of Ellis’ collaborative links with Japan and the USA. Up to the end of the reporting period 24 refereed publications have been published and a further 6 are under review (see group web page).

Scientific highlights of the programme to date include:

(i) the discovery of dust emission from a source within the reionisation era which promises a new probe of early chemical enrichment from star-forming galaxies, and hence the ability to pinpoint when first light occurred. Further observations of this nature with ALMA are now approved,

(ii) the discovery of spectroscopic signatures indicative of non-thermal radiation from nuclear black holes in early galaxies providing the first evidence for a contribution from active galactic nuclei to the reionisation process.

(iii) detailed analysis of a large sample of intermediate redshift galaxies thought to be analogues of faint sources in the reionisation era. As well as characterising the nature of their stellar radiation, Hubble Space Telescope imaging reveals a sizeable fraction show leaking ionising radiation. This work provides the first quantitative confirmation that metal-poor galaxies can dominate the reionising process,

(iv) demonstration of a productive new route for determining the fraction of radiation that can escape into the intergalactic medium from high redshift galaxies by correlating their spatial distribution with the fluctuating transmission in the absorption line spectra of quasars. The innovative method removes a long-standing impasse in this area,

(v) a detailed analysis of the most distant known galaxy at a redshift of 9.1 with ALMA and the VLT, whose stellar population is estimated to be 300 million years old even at this remote epoch. The bulk of the stars in this galaxy thus formed at a redshift 15, well beyond the current horizon of observation. The discovery provides a major impetus for searches for first light with James Webb Space Telescope (JWST).
Highlights (iii) and (v) described in the previous section represent discoveries unforeseen in the programme proposed in 2014.

Regarding (iii), the Hubble Space Telescope study of ~60 redshift z=3.1 Lyman alpha – emitting galaxies has provided a rich set of opportunities. Not only is there strong evidence that ionising radiation can escape from many of these sources but the fact that the strength of this radiation can vary significantly provides additional information on the physical conditions necessary for it to escape. Additional diagnostic spectroscopy of these sources gathered with Keck and the VLT as part of the programme has placed Ellis’ group in the forefront of this competitive area.

Regarding (v), establishing the moment of `first light’ has gathered momentum with the recent announcement (by an independent group) of radio spectral line absorption at a redshift z~15-20. The detailed history of the z=9.1 galaxy provides a direct connection with this result and offers the very exciting possibility of a direct search for first light with JWST.

The plan for the remaining period of the project will build on the highlights described in the previous section, as well as to develop further the promise shown in the two topics above. The group has submitted over a dozen proposals to continue all aspects of the programme for 2019 and prepared draft proposals for JWST. At the time of writing, JWST proposals for the first year of operations are due in February 2019.
ALMA observations of A2744_YD4 reveal dust providing new information on when star formation began.
MACS1149-JD1 has a stellar population which first formed 250 million years after the Big Bang.