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Absorption lines from molecular hydrogen (H2) in the spectra of background sources are a powerful probe of the physical conditions in intervening cold neutral medium. At high redshift, z>2, H2 lines are conveniently shifted in the optical domain, allowing the use of ground-based telescopes to perform high-resolution spectroscopy, which is essential for a proper analysis of the cold gas. We describe recent observational progress, based on the development of efficient pre-selection techniques in low-resolution spectroscopic surveys such as the Sloan Digital Sky Survey (SDSS). The next generation of spectrographs with high blue-throughput, such as CUBES, will certainly significantly boost the efficiency and outcome of follow-up observations. In this paper, we discuss high priority science cases for CUBES, building on recent H2 observations at high-z: probing the physical conditions in the cold phase of regular galaxies and outflowing gas from active galactic nucleus.

We have provided an independent analysis of HD and H 2 absorption lines in several systems in our Galaxy using FUSE space telescope archival data and neutral carbon and metal lines in these systems using HST archival data. The obtained HD column densities lie in the range from ∼10 14 to ∼10 16 cm –2 . The obtained column densities were used to estimate physical conditions in the analyzed systems, including cosmic ray ionization rate, which were obtained in the range from ζ ∼ 10 –17 to ∼10 −15 . 5 s –1 .

We investigate evolution of physical parameters of the intergalactic medium using an analysis of Lya forest lines detected towards distant quasars. We used the enlarged sample of 98 quasars obtained with Keck/HIRES and VLT/UVES. We show that taking into account a finite spatial size of absorbers, regulated by pressure smoothing, significantly affects the inferred thermal parameters of the intergalactic gas, such as the hydrogen photoionization rate and parameters of the temperature-density relation. Using Bayesian framework we constrained for the first time the scale parameter between the Jeans length and characteristic size of the absorbers. We also discuss limitations of the method based on the analysis of the minimal broadending of Lya lines, which stem from the patchy nature of He II reionization.

State-of-the-art 19th century spectroscopy led to the discovery of quantum mechanics, and 20th century spectroscopy led to the confirmation of quantum electrodynamics. State-of-the-art 21st century astrophysical spectrographs, especially ANDES at ESO’s ELT, have another opportunity to play a key role in the search for, and characterization of, the new physics which is known to be out there, waiting to be discovered. We rely on detailed simulations and forecast techniques to discuss four important examples of this point: big bang nucleosynthesis, the evolution of the cosmic microwave background temperature, tests of the universality of physical laws, and a real-time model-independent mapping of the expansion history of the universe (also known as the redshift drift). The last two are among the flagship science drivers for the ELT. We also highlight what is required for the ESO community to be able to play a meaningful role in 2030s fundamental cosmology and show that, even if ANDES only provides null results, such ‘minimum guaranteed science’ will be in the form of constraints on key cosmological paradigms: these are independent from, and can be competitive with, those obtained from traditional cosmological probes.

We present a simplified semi-analytical description of the relative HD/H2 abundance in the cold neutral interstellar medium. With this description we was able to obtain three asymptotics of the relative HD/H2 abundance and its dependence on physical parameters in the medium, namely, the number density of the gas, the intensity of the ultraviolet field, the cosmic ray ionization rate and metallicity. Our calculations in presented simple formalism are in the reasonable agreement with the calculations using the Meudon PDR code. We found that in the case of low metallicity and a higher cosmic ray ionization rate, the relative abundance of HD/H2 is significantly enhanced, which can explain the observed difference between the local and high-z measurements of relative HD/H2 abundance.

We present a simplified semi-analytical description of the relative HD/H 2 abundance in the cold neutral interstellar medium. With this description we was able to obtain three asymptotics of the relative HD/H 2 abundance and its dependence on physical parameters in the medium, namely, the number density of the gas, the intensity of the ultraviolet field, the cosmic ray ionization rate and metallicity. Our calculations in presented simple formalism are in the reasonable agreement with the calculations using the Meudon PDR code. We found that in the case of low metallicity and a higher cosmic ray ionization rate, the relative abundance of HD/H 2 is significantly enhanced, which can explain the observed difference between the local and high-z measurements of relative HD/H 2 abundance.

We investigate a contribution of the Jeans smoothing to the minimal width of Ly forest lines and discuss how the accounting for this additional broadening affects the inferred parameters of the intergalactic matter equation of state. We estimate a power-law index γ of the equation of state, a temperature at the mean density T0 and a hydrogen photoionization rate Γ within 4 redshift bins. Furthermore, in each bin we obtain an upper limit on the scale-parameter fJ, which sets the relation between the Jeans length and the characteristic physical size of the absorber clouds.

Quasar spectra provide a unique opportunity to investigate the intergalactic medium at high redshifts. The spectral analysis of the Lya forest allows us to constrain the effective equation of state of the intergalactic medium and its changes during the evolution of the Universe. Based on the Voigt profile fitting of Lya forest lines in 50 high-resolution QSO spectra obtained at Keck telescope we present new measurements of the power-law index γ of temperature-density relation in IGM for six redshift bins in the range z = 1.6 - 3.7. We find that the IGM state is close to isothermal (γ ≈ 1) at z ∼ 3 which may indicate that HeII reionization occurred at this redshift.

We present a spectroscopic analysis of seven Extremely Strong Damped Lyα systems at redshifts z = 2-3, obtained with the intermediate-resolution spectrograph X-shooter on the Very Large Telescope. For all systems we estimated column densities of the neutral atomic hydrogen HI, metal abundances and dust depletion. We firmly detected molecular hydrogen H2 in two systems in our sample; for the remaining systems we set a conservative upper limits on the H2 column densities. The properties of the obtained systems are in consistency with the sample of the Extremely Strong Damped Lyα systems available in the literature.