Looking for the first galaxies: lensing or blank fields?

Abstract

Context. The identification and study of the first galaxies remains one of the most exciting topics in observational cosmology. The determination of the best possible observing strategies is a very important choice in order to build up a representative sample of spectroscopically confirmed sources at high-z (z > ∼ 7), beyond the limits of present-day observations. Aims. This paper is intended to precisely adress the relative efficiency of lensing and blank fields in the identification and study of galaxies at 6 < ∼ z < ∼ 12.Methods. The detection efficiency and field-to-field variance are estimated from direct simulations of both blank and lensing fields observations. Present known luminosity functions in the UV are used to determine the expected distribution and properties of distant samples at z > ∼ 6 for a variety of survey configurations. Different models for well known lensing clusters are used to simulate in details the magnification and dilution effects on the backgound distant population of galaxies. Results. The presence of a strong-lensing cluster along the line of sight has a dramatic effect on the number of observed sources, with a positive magnification bias in typical ground-based "shallow" surveys (AB < ∼ 25.5). The positive magnification bias increases with the redshift of sources and decreases with both depth of the survey and the size of the surveyed area. The maximum efficiency is reached for lensing clusters at z ∼ 0.1−0.3. Observing blank fields in shallow surveys is particularly inefficient as compared to lensing fields if the UV LF for LBGs is strongly evolving at z > ∼ 7. Also in this case, the number of z ≥ 8 sources expected at the typical depth of JWST (AB ∼ 28−29) is much higher in lensing than in blank fields (e.g. a factor of ∼10 for AB < ∼ 28). All these results have been obtained assuming that number counts derived in clusters are not dominated by sources below the limiting surface brightness of observations, which in turn depends on the reliability of the usual scalings applied to the size of high-z sources. Conclusions. Blank field surveys with a large field of view are needed to prove the bright end of the LF at z > ∼ 6−7, whereas lensing clusters are particularly useful for exploring the mid to faint end of the LF.

Mass and magnification maps for the Hubble Space Telescope Frontier Fields clusters: implications for high-redshift studies

“…The expected number counts at z ∼ 7 demonstrate the advantage provided by cluster lenses compared to a blank field of the same sky coverage. The significant increase of bright sources thanks to gravitational lensing causes a positive magnification bias at observed AB magnitudes < 27, owing to the steep slope of the bright end of the UV luminosity function (Maizy et al 2010). This bright-end boost in the number counts exceeds a factor of 3 at mag < 26, which is the typical limit for spectroscopic follow-up with current 8-10m class telescopes.…”

FINDING HIGH-REDSHIFT DARK STARS WITH THEJAMES WEBB SPACE TELESCOPE

“…The only hope of detecting isolated dark stars with JWST at these redshifts would then be to exploit the gravitational lensing provided by a foreground galaxy cluster. Galaxy clusters at z ≈ 0.1-0.6 can in principle boost the fluxes of high-redshift objects by up to factors ∼100 (e.g., Bradac et al 2009;Maizy et al 2010). As shown in Figure 2, this would be sufficient to lift some of the cooler (T eff < 30,000 K) dark star models above the JWST detection threshold.…”

Evolution of the observed Lyαluminosity function fromz= 6.5 toz= 7.7: evidence for the epoch of reionization?

“…While gravitational amplification of background sources by foreground massive galaxy clusters allows us to probe luminosities that are intrinsically fainter than in the field, this is at the expense of areal coverage due to space distortion. The relative merits of blank and cluster fields depend on the shape of the luminosity function (LF) of the objects that are being searched, and on the properties of the observations such as field of view, integation time and overheads (Maizy et al 2010;Richard et al 2008). From the Lyα LAE LF at z = 6.5 that was available at the time of proposal preparation, we computed that either type of fields should yield approximately the same number of targets, while probing different (unlensed) luminosity ranges.…”