Soft X-ray excess of clusters: a thermal filament model, and the strong lensing of background galaxy groups
📝 Abstract
The observational and theoretical status of the search for missing cosmological baryons is summarized, with a discussion of some indirect methods of detection. The thermal interpretation of the cluster soft X-ray and EUV excess phenomenon is examined in the context of emission filaments, which are the higher density part of the warm hot intergalactic medium (WHIM) residing at the outskirt of clusters. We derived an analytic radial profile of the soft excess surface brightness using a simple filament model, which provided us a means of observationally constraining the WHIM parameters, especially the total mass budget of warm gas associated with a cluster. We then pointed out a new scenario for soft excess emission, viz. a cluster that can strongly lens the soft X-rays from background WHIM knots. If, as seems quite likely, the missing baryons are mostly in the WHIM halos of galaxy groups, the lensing probability will be quite high ( $\sim$ 10 %). This way of accounting for at least part of a cluster’s soft excess may also explain the absence of O VII absorption at the redshift of the cluster.
💡 Analysis
The observational and theoretical status of the search for missing cosmological baryons is summarized, with a discussion of some indirect methods of detection. The thermal interpretation of the cluster soft X-ray and EUV excess phenomenon is examined in the context of emission filaments, which are the higher density part of the warm hot intergalactic medium (WHIM) residing at the outskirt of clusters. We derived an analytic radial profile of the soft excess surface brightness using a simple filament model, which provided us a means of observationally constraining the WHIM parameters, especially the total mass budget of warm gas associated with a cluster. We then pointed out a new scenario for soft excess emission, viz. a cluster that can strongly lens the soft X-rays from background WHIM knots. If, as seems quite likely, the missing baryons are mostly in the WHIM halos of galaxy groups, the lensing probability will be quite high ( $\sim$ 10 %). This way of accounting for at least part of a cluster’s soft excess may also explain the absence of O VII absorption at the redshift of the cluster.
📄 Content
The location of the ‘missing baryons’ in the Universe is an open and important question of contemporary cosmology, as interesting as it is ironic, because the problem manifests itself as a deficit in the mass budget which arises only at low redshifts, i.e. in the space near us. Observationally the total baryonic content in stars, galaxies, and clusters of galaxies (Ω b = (2.1 +2.0 -1.4 )h -2 0.7 %, Fukugita et al 1998) is only about half of the amount required by Big Bang Nucleosynthesis models (Ω b = (3.9 ± 0.5)h -2 0.7 %, Burles & Tytler 1998) or from measurements of the cosmic microwave background (Ω b = (4.6 ± 0.2)h -2 0.7 , Komatsu et al. 2008WMAP5, consistent with the Bennett et al 2003WMAP1 and Spergel et al 2006 WMAP3 results). Cosmological hydrodynamic simulations have shown that this missing 50 % of baryons is concealed in a tenuous filamentary gas of temperature 10 5 -10 7 K, currently referred by many to as the WHIM (the Warm Hot Intergalactic Medium, Cen & Ostriker 1999, Davé et al 2001). It is also possible to derive this result heuristically as follows. Let λ be a wavelength of bulk motion of the intergalactic medium in the near Universe; when these waves collide and break, the thermal velocity of the shocked gas will typically be v ∼ H 0 λ, or v ≈ 100h 0.7 (λ/1.5 Mpc) km s -1 for λ ∼ the cluster size. Thus if gas heating takes place mainly at the ’nodes’ of cluster-scale mass clumping, the thermal motion would involve a value of v that places the gas in the 10 5-7 K range of temperatures.
Since the arrival of its theoretical prediction, search for the WHIM has been an ongoing effort, with some success but no clinching evidence as yet. At the low end of the WHIM temperature scale, far UV absorption lines have been reported (e.g., Richter et al. 2008, Stocke et al. 2006, Tripp et al. 2006, Savage et al. 2005, Danforth and Shull 2005). At temperatures where the bulk of the WHIM is expected to be, the detection of O VII and O VIII absorption lines in the spectrum of distant quasars (Nicastro 2005) is a more debatable result (Kaastra et al 2006). Although soft X-ray and EUV emission in regions of galaxy concentration (Werner et al 2008, Mannucci et al 2007, Zappacosta et al 2002, Mittaz et al 1998) may also be the signature of warm filaments, the definitive proof of this interpretation, viz. an identifiable line at the appropriate redshift, is still not available. More precisely, the clinching line signature of O VII was claimed (Kaastra et al 2003, Finoguenov et al 2003) and refuted (Lieu & Mittaz 2005, Takei 2008).
We examine if limits can be placed on the WHIM from less direct measurements. The column density of free electrons in the intergalactic medium is proportional to the average WHIM density ρ WHIM in a given volume, and it is independent of its state of clumping. To see this, allow the WHIM matter to reside in clumps of number density n, each having mass density ρ and radius r. By mass conservation inside a large spherical volume of radius R, we have ρ WHIM ∼ nρr 3 . The average electron column along a random direction and for a given WHIM ionization fraction, being proportional to the product of the mass column ρr of one clump and the number of intercepted clumps ∼ nr 2 R, will then ∼ ρ WHIM R, i.e. a constant for any sightline of length ∼ R that penetrated our large volume.
There are several ways of constraining this electron column. The Sunyaev-Zel’dovich effect (of the WHIM) will be at a very low level, as will reionization, with δT CMB /T CMB ∼ σ Th n e ℓkT /(m e c 2 ) ∼ 10 -7 for characteristic WHIM parameters at the minimum (homogeneous) overdensity of δ = Ω WHIM /Ω c ≈ 0.02, viz.
where in obtaining n e we assumed a fully ionized pure hydrogen plasma. For comparison, the Sunyaev-Zel’dovich effect of a typical massive cluster (T = 10 8 K, n e = 10 -3 -10 -4 cm -3 , l ∼ 2 Mpc) is δT CMB /T CMB ∼ 10 -4 -10 -5 (see Hernández-Monteagudo et al. 2008 for a discussion of the SZE from the WHIM). Another limit might be afforded by the realization that such a column can cause frequency dependent delay on the timescale of minutes to hours in the arrival of ∼ 100 MHz emission from distant quasars, except quasars with so rapid a variability as to avail themselves for this test are those very ones affected by plasma scintillations in our local interstellar medium (Dennett-Thorpe and de Bruyn 2002). Angular broadening of quasars caused by WHIM-like scintillation was assessed by Lazio et al 2008, who concluded that the only way of securing a useful observational limit is if an AGN is found to ’twinkle’ at a position close to that of a pulsar, as data about the latter will enable us to take out the interstellar effects of our Galaxy. Thus, the situation regarding these ’tangential’ probes is that they too do not deliver any useful verdict.
Here we take a step backwards by returning to the prospect of direct WHIM filament detection at the outskirts of clusters, for reasons that would soon beco
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