HerMES: CANDIDATE GRAVITATIONALLY LENSED GALAXIES AND LENSING STATISTICS AT SUBMILLIMETER WAVELENGTHS

Abstract

We present a list of 13 candidate gravitationally lensed submillimeter galaxies (SMGs) from 95 deg 2 of the Herschel Multi-tiered Extragalactic Survey, a surface density of 0.14 ± 0.04 deg −2. The selected sources have 500 μm flux densities (S 500) greater than 100 mJy. Gravitational lensing is confirmed by follow-up observations in 9 of the 13 systems (70%), and the lensing status of the four remaining sources is undetermined. We also present a supplementary sample of 29 (0.31 ± 0.06 deg −2) gravitationally lensed SMG candidates with S 500 = 80-100 mJy, which are expected to contain a higher fraction of interlopers than the primary candidates. The number counts of the candidate lensed galaxies are consistent with a simple statistical model of the lensing rate, which uses a foreground matter distribution, the intrinsic SMG number counts, and an assumed SMG redshift distribution. The model predicts that 32%-74% of our S 500 100 mJy candidates are strongly gravitationally lensed (μ 2), with the brightest sources being the most robust; this is consistent with the observational data. Our statistical model also 1

Dusty star-forming galaxies at high redshift

“…Once corrected for magnification, the source has an intrinsic luminosity consistent with a ULIRG at z ∼ 2 with a SFR of around 200 M /year. The observed flux density at 500 µm of 325 mJy (Ivison et al, 2010d) puts this source among the brightest of all the lensed DSFGs discussed in the literature with Herschel-Spire Wardlow et al, 2013;Harris et al, 2012). Moreover, the high magnification of 32, compared to a magnification of about 10 for most lensed DSFGs found with Herschel , which are for galaxy-galaxy lenses, also more prone to differential lensing uncertainties), allows spatial resolution down to 100 parcsec scales within the galaxy.…”

“…However due to small sample size it was not possible to conclusively establish whether all of the extragalactic sources with S 500 > 100 mJy are gravitationally lensed or whether there are intrinsically luminous, but possibly rare, DSFGs at such high flux densities. With 13 DSFGs with S 500 > 100 mJy, Wardlow et al (2013) addressed the same issue and found >93% of the sample was confirmed to be gravitational lensed DSFGs implying <7% of the sources would be intrinsically bright. At z > 1, the non-lensed sources would have luminosities with L > 10 13 L (HyLIRGs).…”

“…Second, due to negative K-correction, longer wavelength observations probe higher redshift galaxies where the optical depth to lensing is rapidly increasing (see Figure 6 of Weiß et al, 2013). At wavelengths shorter than 500µm, while lensed DSFGs do exist, the bright-end counts are dominated by star-forming galaxies at low red-shifts where the lensed counts do not make up an appreciable fraction (Negrello et al, 2007;Bussmann et al, 2013;Wardlow et al, 2013).…”

“…Observationally, this has yet to be properly tested but there are tentative results indicating that the lensed fraction is below 100% due to rare sources such as DSFG-DSFG mergers . Existing followup results show that the sources that are neither associated with local galaxies nor radio blazars are gravitationally lensed with an efficiency better than 90% at 500µm Wardlow et al, 2013). At 1.4 mm a similar (or better) success rate at identifying lensed DSFGs is clear with bright sources (S 1.4 > 60 mJy) in the arcminute-scale cosmic microwave background (CMB) anisotropy maps made with the South Pole Telescope (SPT) Mocanu et al, 2013).…”

“…In this context, gravitational lensing has played a critical role because the spatial enhancement associated with magnification provides a way to investigate the internal structure of distant, faint galaxies to levels unattainable with the current generation of instrumentation for typical DSFGs. A second important fact here is that large samples of gravitationally lensed galaxies can be efficiently selected by searching for bright sources in wide area submillimeter surveys (Blain, 1996;Perrotta et al, 2002;Negrello et al, 2007Negrello et al, , 2010Hezaveh & Holder, 2011;Wardlow et al, 2013). While the early submm surveys were limited to smaller sky areas, the advent of wide-field surveys, especially with Herschel and SPT, at sub/millimeter wavelengths have allowed detections of large samples of lensed DSFGs that can be followed up with a host of radio and submm/mm-wave interferometers.…”