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Register a new userThe Effect of Large-Scale Structure on the Magnification of High-Redshift Sources by Cluster-Lenses
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Cluster gravitational lensing surveys like the Hubble Space Telescope Frontier Fields survey will detect distant galaxies 10-50 times fainter than any yet discovered. Using these surveys to measure the luminosity function of such faint, distant galaxies, however, requires that magnification maps built from the constraints of strongly-lensed images be accurate. For models that assume the cluster and nearby (correlated) structures are the only significant sources of lensing, a potential source of error in these maps comes from the fact that light rays also suffer weak deflections by uncorrelated large-scale structure along the line-of-sight, i.e. cosmic weak lensing (CWL). To demonstrate the magnitude of this effect, we calculate the magnification change which results when the same cluster-lens is placed along different lines of sight. Using a simple density profile for a cluster-lens at z~0.3-0.5 and the power spectrum of the matter density fluctuations responsible for CWL, we show that the typical magnifications of ~5(10) of sources at z=6-10 can differ by ~10-20(20-30)% from one line-of-sight to another. However, these fluctuations rise to greater than order unity near critical curves, indicating that CWL tends to make its greatest contribution to the most magnified images. We conclude that the neglect of CWL in determining the intrinsic luminosities of highly-magnified galaxies may introduce errors significant enough to warrant further effort to include this contribution in cluster-lens modeling. We suggest that methods of modeling CWL in galaxy-strong-lensing systems should be generalized to cluster-lensing systems.
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Many distant objects can only be detected, or become more scientifically valuable, if they have been highly magnified by strong gravitational lensing. We use EAGLE and BAHAMAS, two recent cosmological hydrodynamical simulations, to predict the probability distribution for both the lens mass and lens redshift when point sources are highly magnified by gravitational lensing. For sources at a redshift of two, we find the distribution of lens redshifts to be broad, peaking at z=0.6. The contribution of different lens masses is also fairly broad, with most high-magnification lensing due to lenses with halo masses between 10^12 and 10^14 solar masses. Lower mass haloes are inefficient lenses, while more massive haloes are rare. We find that a simple model in which all haloes have singular isothermal sphere density profiles can approximately reproduce the simulation predictions, although such a model over-predicts the importance of haloes with mass <10^12 solar masses for lensing. We also calculate the probability that point sources at different redshifts are strongly lensed. At low redshift, high magnifications are extremely unlikely. Each z=0.5 source produces, on average, 5x10^-7 images with magnification greater than ten; for z =2 this increases to about 2x10^-5. Our results imply that searches for strongly lensed optical transients, including the optical counterparts to strongly lensed gravitational waves, can be optimized by monitoring massive galaxies, groups and clusters rather than concentrating on an individual population of lenses.
We introduce a technique to measure gravitational lensing magnification using the variability of type I quasars. Quasars variability amplitudes and luminosities are tightly correlated, on average. Magnification due to gravitational lensing increases the quasars apparent luminosity, while leaving the variability amplitude unchanged. Therefore, the mean magnification of an ensemble of quasars can be measured through the mean shift in the variability-luminosity relation. As a proof of principle, we use this technique to measure the magnification of quasars spectroscopically identified in the Sloan Digital Sky Survey, due to gravitational lensing by galaxy clusters in the SDSS MaxBCG catalog. The Palomar-QUEST Variability Survey, reduced using the DeepSky pipeline, provides variability data for the sources. We measure the average quasar magnification as a function of scaled distance (r/R200) from the nearest cluster; our measurements are consistent with expectations assuming NFW cluster profiles, particularly after accounting for the known uncertainty in the clusters centers. Variability-based lensing measurements are a valuable complement to shape-based techniques because their systematic errors are very different, and also because the variability measurements are amenable to photometric errors of a few percent and to depths seen in current wide-field surveys. Given the data volume expected from current and upcoming surveys, this new technique has the potential to be competitive with weak lensing shear measurements of large scale structure.
Measurements of the galaxy number density in upcoming surveys such as Euclid and the SKA will be sensitive to distortions from lensing magnification and Doppler effects, beyond the standard redshift-space distortions. The amplitude of these contributions depends sensitively on magnification bias and evolution bias in the galaxy number density. Magnification bias quantifies the change in the observed number of galaxies gained or lost by lensing magnification, while evolution bias quantifies the physical change in the galaxy number density relative to the conserved case. These biases are given by derivatives of the number density, and consequently are very sensitive to the form of the luminosity function. We give a careful derivation of the magnification and evolution biases, clarifying a number of results in the literature. We then examine the biases for a variety of surveys, encompassing optical/NIR, 21cm galaxy and 21cm intensity mapping surveys.
We present observations of redshifted CO(1-0) and CO(2-1) in a field containing an overdensity of Lyman break galaxies (LBGs) at z=5.12. Our Australia Telescope Compact Array observations were centered between two spectroscopically-confirmed z=5.12 galaxies. We place upper limits on the molecular gas masses in these two galaxies of M(H_2) <1.7 x 10^10 M_sun and <2.9 x 10^9 M_sun (2 sigma), comparable to their stellar masses. We detect an optically-faint line emitter situated between the two LBGs which we identify as warm molecular gas at z=5.1245 +/- 0.0001. This source, detected in the CO(2-1) transition but undetected in CO(1-0), has an integrated line flux of 0.106 +/- 0.012 Jy km/s, yielding an inferred gas mass M(H_2)=(1.9 +/- 0.2) x 10^10 M_sun. Molecular line emitters without detectable counterparts at optical and infrared wavelengths may be crucial tracers of structure and mass at high redshift.
A short overview is given on the development of our present paradigm of the large scale structure of the Universe with emphasis on the role of Ya. B. Zeldovich. Next we use the Sloan Digital Sky Survey data and show that the distribution of phases of density waves of various scale in the present-day Universe are correlated. Using numerical simulations of structure evolution we show that the skeleton of the cosmic web was present already in an early stage of the evolution of structure. The positions of maxima and minima of density waves (their phases) are the more stable, the larger is the wavelength. The birth of the first generation of stars occured most probably in the central regions of rich proto-superclusters where the density was highest in the early Universe.
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