ALBERT

Description: Gas flows in and out of galaxies are one of the key unknowns in today's galaxy evolution studies. Because gas flows carry mass, energy, and metals, they are believed to be closely connected to the star formation history of galaxies. Most of these processes take place in the circum-galactic medium (CGM) which remains challenging to observe in emission. A powerful tool to study the CGM gas is offered by combining observations of the gas traced by absorption lines in quasar spectra with detection of the stellar component of the same absorbing-galaxy. To this end, we have targeted the z abs = 1.825 sub-damped Lyα absorber (sub-DLA) towards the z em = 2.102 quasar 2dF J 223941.8-294955 (hereafter Q 2239-2949) with the ESO VLT/X-Shooter spectrograph. Our aim is to investigate the relation between its properties in emission and in absorption. The derived metallicity of the sub-DLA with log N (H  i ) = 19.84 ± 0.14 cm –2 is [M/H] 〉 –0.75. Using the Voigt profile optical depth method, we measure v 90 (Fe  ii ) = 64 km s –1 . The sub-DLA galaxy counterpart is located at an impact parameter of 2 ${^{\prime\prime}_{.}}$ 4 ± 0 ${^{\prime\prime}_{.}}$ 2 (20.8 ± 1.7 kpc at z = 1.825). We have detected Lyα and marginal [O  ii ] emissions. The mean measured flux of the Lyα line is F Ly α  ~ 5.7  x  10 –18 erg s –1  cm –2  Å –1 , corresponding to a dust uncorrected SFR of ~0.13 M  yr –1 .

Description: Aims. The distribution of neutral hydrogen in the intergalactic medium (IGM) is currently explored at low redshift by means of UV spectroscopy of quasars. We propose here an alternative approach based on UV colours of quasars as observed from GALEX surveys. We built a NUV-selected sample of 9033 quasars with (FUV−NUV) colours. The imprint of HI absorption in the observed colours is suggested qualitatively by their distribution as a function of quasar redshift. Methods. Because broad band fluxes lack spectral resolution and are sensitive to a large range of HI column densities a Monte Carlo simulation of IGM opacity is required for quantitative analysis. It was performed with absorbers randomly distributed along redshift and column density distributions. The column density distribution was assumed to be a broken power law with index β1 (1015 cm−2 〈  NHI 〈  1017.2 cm−2) and β2 (1017.2 cm−2 〈  NHI 〈  1019 cm−2). For convenience the redshift distribution is taken proportional to the redshift evolution law of the number density of Lyman limit systems (LLS) per unit redshift as determined by existing spectroscopic surveys. The simulation is run with different assumptions on the spectral index αν of the quasar ionising flux. Results. The fits between the simulated and observed distribution of colours require an LLS redshift density larger than that derived from spectroscopic counting. This result is robust in spite of difficulties in determining the colour dispersion other than that due to neutral hydrogen absorption. This difference decreases with decreasing αν (softer ionising quasar spectrum) and would vanish only with values of αν which are not supported by existing observations. Conclusions. We provide arguments to retain αν = −2, a value already extreme with respect to those measured with HST/COS. Further fitting of power law index β1 and β2 leads to a higher density by a factor of 1.7 (β1 = −1.7, β2 = −1.5), possibly 1.5 (β1 = −1.7, β2 = −1.7). Beyond the result in terms of density the analysis of UV colours of quasars reveals a tension between the current description of IGM opacity at low z and the published average ionising spectrum of quasars.

Description: We simulate the flux emitted from galaxy haloes in order to quantify the brightness of the circumgalactic medium (CGM). We use dedicated zoom-in cosmological simulations with the hydrodynamical adaptive mesh refinement code ramses, which are evolved down to z = 0 and reach a maximum spatial resolution of 380 h−1 pc and a gas mass resolution up to $1.8imes 10^{5} , h^{-1}, m {M}_{odot }$ in the densest regions. We compute the expected emission from the gas in the CGM using cloudy emissivity models for different lines (e.g. Lyα, C iv, O vi, C vi, O viii) considering UV background fluorescence, gravitational cooling and continuum emission. In the case of Lyα, we additionally consider the scattering of continuum photons. We compare our predictions to current observations and find them to be in good agreement at any redshift after adjusting the Lyα escape fraction. We combine our mock observations with instrument models for Faint Intergalactic Redshifted Emission Balloon-2 (FIREBall-2; UV balloon spectrograph) and HARMONI (visible and NIR IFU on the ELT) to predict CGM observations with either instrument and optimize target selections and observing strategies. Our results show that Lyα emission from the CGM at a redshift of 0.7 will be observable with FIREBall-2 for bright galaxies (NUV∼18 mag), while metal lines like O vi and C iv will remain challenging to detect. HARMONI is found to be well suited to study the CGM at different redshifts with various tracers.