Abstract
We study the soft X-ray emission (0.1-1 keV) from the warm-hot intergalactic medium (WHIM) in a hydrodynamic simulation of a cold dark matter universe. Our main goal is to investigate how such emission can be explored with a combination of imaging and spectroscopy and to motivate future X-ray missions. We first present high-resolution images of the X-ray emission in several energy bands in which emission from different ion species dominates. We pick three different areas to study the high-resolution spectra of X-rays from the WHIM: (1) a galaxy group, (2) a filament, and (3) an underluminous region. By taking into account the background X-ray emission from AGNs and foreground emission from the Galaxy, we compute composite X-ray spectra of the selected regions. We briefly investigate angular clustering of the soft X-ray emission, finding a strong signal. Most interestingly, the combination of high spectral resolution and angular information allows us to map the emission from the WHIM in three dimensions. We cross-correlate the positions of galaxies in the simulation with this redshift map of emission and detect the presence of six different ion species (Ne IX, Fe XVII, O VII, O VIII, N VII, and C VI) in the large-scale structure traced by the galaxies. Finally, we show how such emission can be detected and studied with future X-ray satellites, with particular attention to a proposed mission, the Missing Baryon Explorer (MBE). We present simulated observations of the WHIM gas with MBE.
Highlights
Cosmological hydrodynamic simulations provide us with quantitative predictions for the state of baryonic matter in the universe
While a substantial fraction (∼ 30%) of baryons are trapped within the gravitational potential wells of these large scale structures in the form of hot intracluster or intragroup gas, the remaining baryons are left in intergalactic space and are predicted to form filamentary structures seen in cosmological hydrodynamic simulations
Green lines are for the intergalactic medium (IGM) emission, red lines are for the Galactic foreground plus AGNs, and black lines stand for total X-ray emission. (a) Hot gas in a galaxy group, it is clear that the IGM emission dominates the whole spectrum, especially at high energy; (b) diffuse X-ray emission in a filament; The Galactic foreground plus AGNs’ emission dominate, but some strong emission lines from a filament at z ≈ 0.21 show up in the total spectrum; and (c) a void, in which the entire spectrum is dominated by the Galactic foreground plus AGNs
Summary
Cosmological hydrodynamic simulations provide us with quantitative predictions for the state of baryonic matter in the universe (see, e.g, Cen & Ostriker 1999; Dave et al 2001). Simulations predict that the mean intensity of emission from the WHIM gas is between (2 – 4) ×10−13 ergs s−1cm−2deg−2, only about 5 – 15% of the total extragalactic X-ray emission between 0.5 – 2 keV (Phillips, Ostriker, & Cen 2001; Croft et al 2001). We expect that future X-ray missions will enable high resolution imaging/spectroscopic observations of the emission from the WHIM gas We explore such a scenario with a high resolution hydrodynamic simulation in this paper. Yoshikawa et al (2003) examined the detectability of the X-ray emission from the WHIM gas independently with a similar numerical approach They presented a detailed study of O VII and O VIII emission lines from WHIM. We investigate the detectability of emission with the proposed MBE X-ray telescope in §5, and §6 is our discussion and summary
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