Observable signatures of the low-z circumgalactic and intergalactic media: ultraviolet line emission in simulations

Abstract

We present for the first time predictions for ultraviolet (UV) line emission of intergalactic and circumgalactic gas from Adaptive Mesh Resolution (AMR) large-scale structure simulations at redshifts 0.3 < z < 1.2, with a specific emphasis on its observability with current and near-future UV instrumentation. In the three UV transitions of interest (Lyα, O vi and C iv), there is a clear bimodality in the type of emitting objects: the overwhelming majority of the flux stems from discrete, compact sources, while a much larger fraction of the volume is filled by more tenuous gas. We characterize both object types with regard to their number densities, physical sizes and shapes, brightnesses and luminosities, velocity structures, masses, temperatures, ionization states, and metal content. Degrading our AMR grids to characteristic resolutions offered by available (such as FIREBall) or foreseeable instrumentation allows us to assess which inferences can be drawn from currently possible observations, and to set foundations to prepare observing strategies for future missions. In general, the faint emission of the intergalactic medium (IGM) and filamentary structure remains beyond the capabilities of instruments with only short-duration exposure potential (i.e. stratospheric balloons), even for the most optimistic assumption for Lyα, while the yet fainter metal line transitions (O vi and C iv) for these structures will actually remain challenging for long-duration exposures (i.e. space-based telescopes), mostly due to their low metallicities pushing them more than three orders of magnitudes in brightness below the Lyα radiation. For the bright, circumgalactic medium, the situation is much more promising, and it is foreseeable that in the near future we will not only just detect such sources, but also the combination of all three lines in addition to velocity information will yield valuable insight into the physical processes at hand, illuminating (and discriminating between) important mechanisms during the formation of galaxies and their backreaction on to the IGM from whence they formed.