Candidate gravitationally lensed submillimeter galaxies in Herschel-ATLAS associated with WISE elliptical counterparts

TL;DR

The study introduces an independent lens-candidate selection method using H-ATLAS SMGs and AllWISE elliptical counterparts within $18$ arcsec, targeting $1.2 < z < 4.0$ sources to reveal fainter, lower-magnification lenses. Through two-component SED fitting with $\text{CIGALE}$ across UV–submm bands, the authors derive physical properties for both the foreground lenses and background SMGs and estimate modest lensing magnifications ($\mu \approx 1$–$1.5$) for most candidates. The result is a new catalog of 68 lens candidates, including 15 high-probability lenses, with no overlap with known strong-lens catalogs, suggesting a distinct population accessible for individual follow-up. The work demonstrates how combining wide-field submillimeter and mid-IR data can complement statistical weak-lensing studies by enabling case-by-case investigations of lower-magnification lensing events and their impact on SMG properties, while highlighting the need for high-resolution imaging to confirm lensing and refine models.

Abstract

We present a new and independent methodology for identifying gravitational lens candidates using data from the H-ATLAS and AllWISE surveys. Unlike previous approaches, which are typically biased toward bright, strongly lensed submillimeter galaxies (SMGs), our method uncovers fainter systems with lower magnifications. This enables the identification and individual study of lensing events that would otherwise only be accessible through statistical weak lensing analyses. Our approach focuses on high-redshift SMGs from H-ATLAS in the range $1.2 < z < 4.0$, and searches for associated AllWISE sources within an angular distance of 18 arcsec. Candidate lenses are selected based on their WISE colors ($0.5 < \mathrm{W2} - \mathrm{W3} < 1.5$ mag), consistent with those of elliptical galaxies, and further filtered using $J-\mathrm{W1}$ color and photometric redshift cuts to reduce stellar contamination. This conservative selection yields 68 new lens candidates. We then performed SED fitting with CIGALE across UV to submillimeter wavelengths to estimate the physical properties of both the lenses and the background SMGs, and to assess the lensing nature of these candidates. Despite the uncertainties, we were able to constrain key parameters such as stellar and dust masses, infrared luminosities, and star formation rates. In addition, the estimated magnifications for most candidates are modest, consistent with the weak lensing regime ($μ\simeq 1{-}1.5$), although a few sources may require more precise modeling. Future efforts could refine this methodology to recover potential candidates outside our selection, and high-resolution follow-up observations will be essential to confirm the lensing nature of these sources and to further investigate their physical properties.

Figures (14)

• Figure 1: Color-color diagram based on WISE magnitudes (in the Vega system), showing the positions of SMGs from H-ATLAS (in gray), and the elliptical galaxy selection (black box), defined by $0.5 < \mathrm{W2}-\mathrm{W3} < 1.5$ and $|\mathrm{W1}-\mathrm{W2}|<0.3$ mag. The blue box corresponds to a sample of WISE-selected starburst galaxies used to test our fitting and cross-matching procedure. Confirmed lens candidates from the catalogs of Negrello2017 and Bakx2024 are shown as plus signs and crosses, respectively.
• Figure 2: Angular separations of the AllWISE lenses and their best counterparts in other surveys with respect to their parent catalog positions (i.e., Herschel and WISE, respectively for the top and central panels). For comparison, the top and bottom panels show the angular separations of the subset of WISE-selected SMGs, which are more likely to be true counterparts of the Herschel sources.
• Figure 3: Rest-frame composition of the best-fit models created with CIGALE, normalized at $\lambda=1\ \mu$m. The solid blue, red, and black curves represent the median SED models of the lens candidates, their lensed SMGs, and the sample of unlensed SMGs, respectively, while the shaded areas indicate the range between the 16th and 84th percentiles to the median values. We note that the lensed sources are somewhat fainter than the unlensed ones. Additionally, the SED template of SMM J2135-0102 Ivison2010Swinbank2010 is plotted in purple for comparison purposes.
• Figure 4: Color-color diagram used to distinguish stars and galaxies. The black dashed rectangle indicates our selection of lens candidates ($J-\mathrm{W1}>-0.35$, $g-i>0.4$), while stars are located in the $J-\mathrm{W1}\lesssim-0.5$ region (in AB system, extinction-corrected magnitudes). Photometric redshifts derived from SED analysis are shown as a color map, as additional redshift constraints were applied to exclude sources with unreliable photometric redshifts, particularly at $z<0.1$ and $z>0.6$. Best candidates (A lenses) were further selected based on the $\chi^2_r$ of the models and the proximity of the submillimeter sources.
• Figure 5: Example of the best-fit models for a type A candidate (left), a type B candidate (center), and one of the unlensed SMGs selected. In the first two panels, the elliptical and SMG components are shown respectively in blue and red, together with their $\chi^2_r$ values and redshifts in matching colors. In the third panel, red indicates the fit to the far-IR component of the SED, while the fit including all available filters is shown in gray. In addition, the dashed gray curve represents an alternative fit including a modest AGN contribution. The complete set of best-fit models for the candidates can be found in Appendix \ref{['sec:full_catalog']}.