Faint Blue Galaxies as a Probe of the X-ray Background at High Redshift

We present a formalism describing the physical content of cross-correlation functions between a diffuse background and a population of discrete sources. The formalism is used to interpret cross-correlation signals between the unresolved X-ray background and a galaxy population resolved to high redshift in another spectral band. Specifically, we apply it to the so-called faint blue galaxy population and constrain their X-ray emissivity and clustering properties. A model is presented which satisfies the recently measured constraints on all 3 correlation functions (galaxy/galaxy, background/background and galaxy/background). This model predicts that faint galaxies in the magnitude range B=18-23 (cvering redshifts z \lsim 0.5) make up $\sim 22 \%$ of the X-ray background in the 0.5-2 keV band. At the mean redshift of the galaxy sample, $\bar z=0.26$, the comoving volume emissivity is $\rho_X \sim 6-9 \times 10^{38}h$ ergs s$^{-1}$Mpc$^{-3}$ . When extrapolated to fainter magnitudes, the faint blue galaxy population can account for most of the residual background at soft energy. We show how the measurement of the angular and zero-lag cross-correlation functions between increasingly faint galaxies and the X-ray background can allow us to map the X-ray emissivity as a function of redshift.Comment: uuencoded compressed postscript, without figures. The preprint is available with figures at http://www.ast.cam.ac.uk/preprint/PrePrint.htm

The X-Ray Background as a Probe of Density Fluctuations at High Redshift

The X-Ray Background (XRB) probes structure on scales intermediate between those explored by local galaxy redshift surveys and by the COBE Microwave Background measurements. We predict the large scale angular fluctuations in the XRB, expressed in terms of spherical harmonics for a range of assumed power-spectra and evolution scenarios. The dipole is due to large scale structure as well as to the observer's motion (the Compton-Getting effect). For a typical observer the two effects turn out to be comparable in amplitude. The coupling of the two effects makes it difficult to use the XRB for independent confirmation of the CMB dipole being due to the observer's motion. The large scale structure dipole (rms per component) relative to the monopole is in the range $a_{1m}/a_{00} \sim (0.5-9.0) \times 10^{-3}$. The spread is mainly due to the assumed redshift evolution scenarios of the X-ray volume emissivity $\rho_x(z)$. The dipole's prediction is consistent with a measured dipole in the HEAO1 XRB map. Typically, the harmonic spectrum drops with $l$ like $a_{lm} \sim l^{-0.4}$. This behaviour allows us to discriminate a true clustering signal against the flux shot noise, which is constant with $l$, and may dominate the signal unless bright resolved sources are removed from the XRB map. We also show that Sachs-Wolfe and Doppler (due to the motion of the sources) effects in the XRB are negligible. Although our analysis focuses on the XRB, the formalism is general and can be easily applied to other cosmological backgrounds.Comment: 14 pages, 3 postscript figures, available from ftp://cass41.ast.cam.ac.uk/pub/lahav/xrb accepted for publication in MNRA

A failure in consensus or a successful advocacy strategy?

Initiated in 2002 by the World Bank and the Food and Agriculture Organization (FAO), the International Assessment of Agricultural Knowledge, Science and Technology for Development (IAASTD, www.agassessment.org) is an interesting experience of an international expertise process aiming at improving global governance for sustainable development. It aimed to understand how agricultural knowledge, technologies and sciences could contribute to reduce hunger and poverty, improve rural livelihoods and at the same time reach environmental objectives. It involved the large mobilization of international scientific expertise, but also the participation of a diversity of stakeholders, and a validation of reports by an intergovernmental plenary. The design of the process was inspired by other global assessments like the IPCC and the Millennium Ecosystem Assessment. Among these international assessments, IAASTD is particularly important as its focus on agriculture necessarily puts the stress on trade-offs and synergies between social and environmental implications of development. Assessing if and how IAASTD managed to reach its objectives will prove useful for other assessment processes, particularly in order to understand how social and economical controversies at the heart of the debate on sustainable development might be structured and dealt with by international expertise processes. Regarding the initial objectives of this assessment and its participatory approach, many analysts criticize IAASTD because it did not reach a consensus among all stakeholders. In this paper, we propose to consider also the alternative perspective of analysis, where this assessment serves an advocacy strategy for a new approach of global agriculture. In this alternative perspective, IAASTD can be considered successful. We also propose to consider that the difference between the two analytical frameworks can be useful in order to re-analyze recommendations for global assessments, and to reopen the diversity of the roles that expertise might play in global debates about environment and development where controversies are central

Astrometric Microlensing of Quasars : Dependence on surface mass density and external shear

A small fraction of all quasars are strongly lensed and multiply imaged, with usually a galaxy acting as the main lens. Some, maybe all of these quasars are also affected by microlensing, the effects of stellar mass objects in the lensing galaxy. Stellar microlensing not only has photometric effects (magnitude fluctuations of the quasar images), it also affects the observed position of the images. This astrometric effect was first explored by Lewis and Ibata (1998): the position of the quasar - i.e. the center-of-light of the many microimages - can shift by tens of microarcseconds due to the relatively sudden (dis-)appearance of a pair of microimages when a caustic is being crossed. We explore this effect quantitatively for different values of the surface mass density and external shear covering most of the known multiple quasar systems. We show examples of microlens-induced quasar motion and the corresponding light curves for different quasar sizes. We evaluate statistically the occurrence of large shifts in angular position and their correlation with apparent brightness fluctuations. We also show statistical relations between positional offsets and time from random starting points. As the amplitude of the astrometric offset depends on the source size, astrometric microlensing signatures of quasars - combined with the photometric variations - will provide very good constraints on the size of quasars as a function of wavelength. We predict that such signatures will be detectable for realistic microlensing scenarios with near future technology in the infrared/optical (Keck-Interferometry, VLTI, SIM, GAIA). Such detections will show that not even high redshift quasars define a 'fixed' coordinate system.Comment: 11 pages, 13 figures, accepted for publication in A&