The first billion years of the Universe define the current frontier of modern cosmology, both observationally and theoretically. During this time the first stars and galaxies assembled from the primordial gas, and the atomic hydrogen permeating the early Universe became ionised. The epoch of reionisation, which started at redshift z~10 and lasted to redshift z~6, represents the last major phase transition in cosmic history, which impacted almost every baryon in the Universe.
The main scientific goal of this survey is to draw a comprehensive picture of the Universe in the second half of the reionization process.
This will be achieved by observing for 250 hours with the spectrograph XSHOOTER at the ESO VLT telescope to collect an unparalleled, homogeneous sample of intermediate resolution, high signal-to-noise ratio spectra of 30 quasars at 5.8 ∼ < z ∼< 6.6.
The main science cases that will be addressed by this large initiative are grouped in three work packages (WP).
Characterizing the second half of the re-ionization process.
Quasars at z ~ 6.0 (~1 Gyr after the Big Bang), due to their high intrinsic luminosities and prominent emission line due to the Lyman-alpha transition in neutral hydrogen, have played a relevant role in the early characterization of the reionization process.
The diffuse intergalactic gas, which contains the majority of baryons at these redshift, imprints a characteristic signature in the spectra of the high-redshift quasars which allows us to estimate its ionisation status.
Thanks to the XQR-30 survey it will be possible to derive more sensitive constraints on the neutral hydrogen fraction at the studied redshifts to be compared with the predictions of numerical models for a more accurate understanding of the final phases of the reionization process.
Metal enrichment and the nature of the first ionizing sources.
The detection and analysis of metal ion absorptions at redshifts beyond z ~ 5 is ideally suited to investigate the mechanisms by which heavy elements are expelled from galaxies before the cumulative effect of several generations of stars pollutes the circum-galactic and inter-galactic medium. On the other hand, low-ionisation metal lines trace the dense gas in and around galaxies, offering insights into their interstellar media and allowing to look for the chemical signatures of the first generation of stars, the so called Population III.
XQR-30 will allow us to significantly increase the sample of metal absorption lines to study their properties in a statistical sense and understand in more detail the processes that expel gas from galaxies.
Quasars in the early Universe and their environment.
The mere presence of luminous quasars, powered by giant black holes less than 1 Gyr after the Big Bang represents a challenge for models of massive black hole formation and early galaxy growth. As we push the redshift frontier, it becomes increasingly difficult to explain the formation of these black holes in terms of remnants of extremely massive, metal-poor stars (PopIII). Alternative, more exotic scenarios involving the direct collapse of a single massive cloud of pristine gas have been explored. All these models yield distinctive predictions on the properties of the black hole and of the galaxy that hosts it.
XQR-30 can provide the accurate BH mass and accretion rate measurements necessary to test these models and shed light on the first stages of star and galaxy formation.