A new X - ray mission to measure the power spectrum of fluctuations in the Universe

We propose a new, simple, dedicated X-ray mission to measure the power spectrum of density fluctuations in the Universe, by accurately mapping the X-ray background on the whole sky on scales of ∼ 1 deg 2. Since the method relies on the detection of excess fluctuations produced by the clustering of X-ray sources at intermediate redshifts above the confusion noise produced by bright foreground sources, counting noise and systematic noise need to be kept to a minimum. We propose a proportional counter with collimated fields of view of various sizes as the optimal instrument for this mission. 1. Introduction Recent progress in the optical identification of faint X-ray sources discovered in ROSAT deep fields shows that the bulk of the soft (0.5-2 keV) X-ray background (XRB) originates at intrmediate redshifts z ≈ 1 − 2 (Boyle et al 1994, McHardy et al 1997, Hasinger et al 1997, Schmidt et al 1997). The X-ray volume emissivity of the Universe in the soft band peaks at these intermediate redshifts. This is particularly interesting, since it is at these redshifts where the process of galaxy formation culminates its non-linear growth and where the bulk of star formation appears to happen (Madau et al 1996). Studies of the angular structure of the XRB should be able to map the lumpiness of the Universe at these redshifts. The distribution of density fluctuations in the Universe on different scales is described in terms of the power spectrum (PS) of the fluctuations P(z, k c) (z is the redshift and k c the comoving wavenumber), which is the Fourier transform of the two-point correlation function. The current information on the PS comes from both local galaxy surveys and from studies of anisotropies in the microwave background radiation (MBR). The former are most powerful on small scales < 50 h −1 Mpc (h is the Hubble constant in units of 100 km s −1 Mpc −1) and the latter emphasise large scales (∼ 1000 h −1 Mpc) at very high redshifts (z ∼ 1500) when the amplitude of the PS was very small. Overall fits of the PS can be reproduced by Cold Dark Matter models where the PS presents a broad peak at comoving wavevectors k c ∼ 0.01 − 0.1 hMpc −1 (see Peacock 1997 for recent parametrisations). In this paper we propose a new X-ray mission dedicated to measure the PS near its maximum at …