Present Epoch Plus — an X‐ray survey for cosmology

Jahoda, K.

doi:10.1002/asna.2123190159

Cited by 4 publications

(3 citation statements)

References 17 publications

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“…Probably the best way to achieve this will be through the use of softer band data (to provide spatial parameters) combined with point-by-point spectroscopic information which will allow extrapolation to the harder, less contaminated X-ray bands. A spectroscopy capable all-sky imaging survey, such as that discussed by Jahoda (1998) would be well suited to this (see also discussion in Treyer et al (1998)).…”

Section: Discussionmentioning

confidence: 99%

The 2–10 keV X‐Ray Background Dipole and Its Cosmological Implications

Scharf

Jahoda

Treyer

et al. 2000

ApJ

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The hard X-ray (> 2 keV) emission of the local and distant Universe as observed with the HEAO1-A2 experiment is reconsidered in the context of large scale cosmic structure.Using all-sky X-ray samples of AGN and galaxy clusters we remove the dominant local X-ray flux from within a redshift of ∼ 0.02. We evaluate the dipolar and higher order harmonic structure in 4 X-ray colours. The estimated dipole anisotropy of the unresolved flux appears to be consistent with a combination of the Compton-Getting effect due to the Local Group motion (dipole amplitude ∆ = 0.0042) and remaining large scale structure (0.0023 < ∼ ∆ < ∼ 0.0085), in good agreement with the expectations of Cold Dark Matter models.The observed anisotropy does however also suggest a non-negligible Galactic contribution which is more complex than current, simple models of > 2keV Galactic X-ray emission. Comparison of the soft and hard colour maps with a harmonic analysis of the 1.5 keV ROSAT all-sky data qualitatively suggests that at least a third of the faint, unresolved ∼ 18 • scale structure in the HEAO1-A2 data may be Galactic in origin. However, the effect on measured flux dipoles is small ( < ∼ 3%). We derive an expression for dipole anisotropy and acceleration and demonstrate how the dipole anisotropy of the distant X-ray frame can constrain the amplitude of bulk motions of the universe. From observed bulk motions over a local ∼ 50 h −1 Mpc radius volume we determine 0.14 < ∼ Ω 0.6 0 /b x (0) < ∼ 0.59, where Ω 0 is the universal density parameter and b x (0) is the present epoch bias parameter, defined as the ratio of fluctuations in the X-ray source density and the mass density.

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Section: Discussionmentioning

confidence: 99%

The 2–10 keV X‐Ray Background Dipole and Its Cosmological Implications

Scharf

Jahoda

Treyer

et al. 2000

ApJ

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“…many telescopes) is required to obtain the same precision in the measurement of surface brightness. A collection of imaging telescopes capable of measuring surface brightness to 1% per square degree requires nearly 3 times the geometric collecting area of the proportional counter experiment, but is plausibly within the constraints of NASA Medium Explorer mission (Jahoda 1998). An additional advantage of an imaging experiment is the simultaneous production of a catalogue of sources, useful in their own right as tracers of large scale structure.…”

Section: Discussionmentioning

confidence: 99%

Large‐Scale Fluctuations in the X‐Ray Background

Treyer¹,

Scharf²,

Lahav³

et al. 1998

ApJ

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We present an attempt to measure the large angular scale fluctuations in the X-Ray Background (XRB) from the HEAO1-A2 data, expressed in terms of spherical harmonics. We model the harmonic coefficients assuming a power spectrum and an epoch-dependent bias parameter, b x (z), and using a phenomenological scenario describing the evolution of the X-ray sources. From the few low-order multipoles detected above shot noise, we estimate the power-spectrum normalization on scales intermediate between those explored by local galaxy redshift surveys (∼ 100h −1 Mpc) and those probed by the COBE Cosmic Microwave Background (CMB) measurements (∼ 1000h −1 Mpc). We find that the HEAO1 harmonics are consistent with present epoch rms fluctuations of the X-ray sources b x (0)σ 8 ∼ 1 − 2 in 8 h −1 Mpc spheres. Therefore the observed fluctuations in the XRB are roughly as expected from interpolating between the local galaxy surveys and the COBE CMB experiment. We predict that an X-ray all-sky surface brightness survey resolving sources a factor of 10 fainter than HEAO1, may reveal fluctuations to significantly larger scales and therefore more strongly constrain the large scale structure of the Universe on scales of hundreds of Mpcs.

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“…The XRB is a unique probe of fluctuations on intermediate scales between those of local galaxy surveys and COBE [40,41,42,18,43,44,45], although the interpretation of the results depends somewhat on the nature of the X-ray sources and their evolution. The rms dipole and higher moments of spherical harmonics can be predicted [18] in the framework of gravitational instability and assumptions on the distribution of the X-ray sources with redshift.…”

Section: The X-ray Backgroundmentioning

confidence: 99%

The large-scale smoothness of the Universe

Lahav

Rees

1999

Nature

184

164

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The Universe is inhomogeneousÐand essentially fractalÐon the scale of galaxies and clusters of galaxies, but most cosmologists believe that on larger scales it becomes isotropic and homogeneous: this is the`cosmological principle'. This principle was ®rst adopted when observational cosmology was in its infancy, and was then little more than a conjecture. The data now available offer a quantitative picture of the gradual transition from small-scale fractal behaviour to large-scale homogeneity.A fractal is a distribution or shape that is not homogeneous (in general), but possesses the property that each part is a simulacrum of the whole. In other words it`looks' the same on all scales. Fractals abound in nature: for example, the coastline of a small peninsula drawn on paper could equally well depict a large continent; and fractals have long been studied as manifestations of scaling laws in solid-state physics 1,2 . The clustering of galaxies (see Fig. 1) lends itself to a fractal description 1±7 , because the clumpiness prevails over a wide range of scales. The big question for cosmology is whether the distribution of matter continues to be a simple fractal beyond the scale of clusters of galaxies: if it does, then the cosmological principle is invalid.Past attempts to determine the distribution of matter have confronted two important obstacles. Most of the mass in the Universe is dark; we can detect it only by its gravitational effect on objects that we can see, and it is still unclear how to relate the distributions of light and mass. In particular, we have not known how to match the clustering of galaxies with the cosmic microwave background (CMB) anisotropies, which tell us about the mass uctuations in the early Universe. In addition, little has been known about¯uctuations in the distribution of matter on scales intermediate between those of local galaxy surveys (( 100 h -1 Mpc, where h is the Hubble constant in units of 100 km s -1 Mpc -1 ) and scales probed by the Cosmic Background Explorer (COBE) satellite () 1,000 h -1 Mpc).Recent observations of radio galaxies and the X-ray background (XRB, which probably arises from distant active galactic nuclei), at median redshift Å z < 1, can now effectively probe these intermediate scales.Moreover, experiments to measure¯uctuations in the microwave background on smaller angular scales than those probed by COBE are also helping to bridge the gap. Future surveys of more than one million galaxies will probe to a median redshift Å z < 0:1 (which roughly corresponds to a co-moving distance of ,300 h -1 Mpc). Current data, however, already strongly constrain any non-uniformities in the galaxy distribution (as well as the overall mass distribution) on scales ) 300 h -1 Mpc.In the language of fractals (see Box 1), fractal dimensions are used to characterize the degree of clustering: these generalize our intuitive concepts of dimension and can take any positive value less than or equal to 3. On scales below ,10 h -1 Mpc, galaxies are distributed with the correlation dimension...

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Present Epoch Plus — an X‐ray survey for cosmology

Cited by 4 publications

References 17 publications

The 2–10 keV X‐Ray Background Dipole and Its Cosmological Implications

The 2–10 keV X‐Ray Background Dipole and Its Cosmological Implications

Large‐Scale Fluctuations in the X‐Ray Background

The large-scale smoothness of the Universe

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