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Primordial Deuterium abundance
The primordial deuterium (D/H) abundance is currently the most reliable probe of Big Bang Nucleosynthesis. It can be deduced to ~1% precision using near-pristine gas clouds that imprint DI and HI absorption lines on the spectrum of a background quasar. Cooke et al. (2018) selected a golden sample of seven systems with precise measurement of D/H, most of which are at redshift z~2.8-3.1, to derive the most precise determination of D abundance to date.
An efficient high-resolution UV spectrograph has several attractive properties: (1) At low redshifts, there is significantly less contamination, so there are more absorption line systems where D/H can be measured; (2) Less contamination translates to reliable measurements of D/H; (3) We already know suitable quasar sightlines that intersect near-pristine gas clouds at redshifts 2.3 < z < 2.8 with the desirable properties (e.g. quiescent, near-pristine, mostly neutral) to accurately infer the primordial D/H ratio. CUBES will therefore provide a unique opportunity to deliver a large, reliable sample of D/H estimates.
Fig.1 Golden sample of 7 absorption systems at z~2.8-3.1 (Cooke et al. 2018). D/H abundance determined to 1% precision.
Missing baryonic mass in the high-z circumgalactic medium (CGM)
The baryonic fraction in galaxies is smaller than the universal fraction (~17 %) by more than 60%. The baryonic mass of galaxies is generally measured as the mass in stars and cold interstellar medium (ISM). However, at low redshift (z~0.2), it has been shown that more than half of the missing baryons are residing in the circum-galactic medium (Werk et al. 2014). This diffuse gas is traced by the absorption lines it imprints in the spectra of bright background sources.
CUBES can extend these studies to higher redshift galaxies in the ranges z = 1.5 to 2 (and 2 < z < 2.8 considering OVI lines), immediately after the era of peak star-formation in the Universe (e.g. Madau & Dickinson, 2014).
Fig.2 Lines available in the ground-UV for studies of the circumgalactic medium of galaxies at 1.5 < z < 2 (or 2 < z < 2.8 for OVI absorbers). CUBES observations will neatly complement observations by UVES and ESPRESSO at the VLT, but also future observations in the blue-visible with ELT-MOSAIC, which is targeting the CGM in galaxy halos at z~3 (Puech et al. 2018; Morris et al. 2018).
The cosmic UV background
Cosmic reionisation is a major focus of cosmology but after almost 40 years (Sargent et al, 1980) the sources driving it and keeping the Universe ionised are still not well understood. Quasars are efficient at producing UV photons with an escape fraction of ~100 % (Cristiani et al. 2016; Grazian et al. 2018) but it is not clear if their volume density at intermediate and low luminosities is sufficient to provide the required UV rates. Galaxies are far more numerous, but we only have a few reliable detections of ‘leaking’ Lyman-continuum flux so far (e.g. Vanzella et al. 2015, 2018; Shapley et al. 2016; Bian et al. 2017).
CUBES with its unprecedented near-UV sensitivity could allow to probe the escape fraction of ionising photons from galaxies, f_esc, at peak star-formation epoch, in the redshift range 2.3 < z < 3.5.
Transient
Gamma-ray bursts (GRBs) are extremely powerful explosions of cosmological origin lasting just a few seconds (Piran et al. 2013). The UVES VLT instrument, with the Rapid Response Mode (RRM) system, can allow to obtain high-resolution spectroscopy of a GRB afterglow only a few minutes after the prompt event.
The primary diagnostics to determine the GRB-absorber distance are from Fe II. Its bluemost lines are at ~2400Å (rest frame), so this kind of analysis is difficult for GRBs at z<0.8 because the throughput of UVES dramatically reduces below 4000 Å. CUBES will give a S/N that is at least five times better than UVES for the same exposure time over 3000-4000 Å, so it will strongly increase the redshift range for which high-resolution spectroscopy of GRB afterglows can give information on the distance between the GRB progenitor and the absorbing gas. In particular, UVES has allowed this analysis for z≥1 GRBs; with CUBES we will expand this to events with z>0.25.
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