Emission from the circumgalactic medium: from cosmological zoom-in simulations to multiwavelength observables

Abstract

We simulate the flux emitted from galaxy halos 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 1.8×10 5 h −1 M 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, CIV, OVI, CVI, OVIII) 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 FIREBall-2 (UV balloon spectrograph) and HARMONI (visible and NIR IFU on the ELT) to predict CGM observations with either instrument and optimise 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 OVI and CIV will remain challenging to detect. HARMONI is found to be well suited to study the CGM at different redshifts with various tracers.

End-to-end ground calibration and in-flight performance of the FIREBall-2 instrument

“…For total flux (unresolved source), according to GALEX observations and simulations (Fig. 7 of Augustin et al 24 ), the ∼41.7 logðL Lyα Þ limit we find for both scenarios in Sec. 5.2 would be constraining by an order of magnitude for the Lyα integrated flux at z ∼ 0.7.…”

“…As can be seen in Fig. 7 of Augustin et al, 24 FIREBall-2 is presently lacking a factor of 3 in sensitivity to constrain the models at z ∼ 0.7.…”

“…For a spectrally unresolved object, the favorable ∼ × 30 cosmological emission line surface brightness dimming at z ∼ 3 with respect to 0.7 makes the FIREBall-2 single-object limit translate to ∼14.7 × 10 −19 ergs −1 cm 2 arcsec −2 at z ∼ 3, a value five times brighter than MUSE's. 4 Stacking 25 or more galaxies 24 are thus expected to provide a detection surface brightness limit at z ∼ 0.7 comparable to that of MUSE at z ∼ 3 for individual galaxies. FIREBall detection of extended emission would thus only be possible in presence of large spectral offsets due to strong winds.…”

The Cosmic Baryon and Metal Cycles

“…Only a minority of the baryons are scrutinized by observations of starlight from galaxies. The remaining baryons reside in low-density gas which is challenging to detect in emission (Wijers et al 2019;Augustin et al 2019). The baryons and metals associated with the majority of the gas that lies beyond the regions excited by hot stars is studied using resonant absorption lines in the spectra of background sources.…”

Predictions for the angular dependence of gas mass flow rate and metallicity in the circumgalactic medium

“…Due to its diffuse nature (densities n H <0.1 cm −3 ), direct detection of the CGM in emission is observationally challenging (Bertone et al 2010a, b;Frank et al 2012;van de Voort & Schaye 2013), although recent simulations provide ever more robust predictions (Augustin et al 2019;Wijers et al 2019;Corlies et al 2020). In contrast, the CGM is commonly detected in absorption against bright background sources (Bouché et al 2007a).…”