Most Strong Lensing Deflectors in the AGEL Survey are in Group and Cluster Environments

TL;DR

This study catalogs 89 spectroscopically confirmed strong lenses from the AGEL survey to compare deflector scale (via $M(<\theta_E)$) with deflector environment (via the fifth-nearest-neighbor density $\Sigma_5(z)$ derived from photometric redshifts). Using EAZY for $z_{phot}$ and MCMC-based theta_E fitting on DECaLS imaging, the authors classify lenses into galaxy-, group-, and cluster-scale deflectors and environments, and compare these with control fields. They find that a large fraction of AGEL deflectors inhabit group- or cluster-scale halos, while the Einstein masses are often group-scale, yielding only a weak correlation ($r\approx0.38$) between $M(<\theta_E)$ and $\Sigma_5(z)$. AGEL fields are systematically denser than control fields, suggesting selection and line-of-sight biases that impact lens modeling and cosmological inferences. The results advocate incorporating line-of-sight environment information into lens models to reduce systematics, and provide a public data release of photometric redshifts, $r$-band magnitudes, and lensing parameters for the community.

Abstract

The environments of deflectors in strong lensing systems affect our ability to test cosmological models and constrain evolutionary properties of galaxies. Here we measure the deflector scale (Einstein mass) and deflector environment (halo mass) of 89 spectroscopically confirmed strong lenses in the ASTRO3D Galaxy Evolution With Lenses (AGEL) survey. We classify deflector scale by measuring $θ_{\rm{E}}$ to determine the mass enclosed by the Einstein radius, $M(<θ_{\rm{E}})$. We quantify deflector environment by using photometric redshifts to determine the galaxy surface density to the fifth-nearest neighbor $Σ_5(z)$. We find that 47.2% of our deflectors are embedded in cluster environments, whereas only 9.0% have cluster-scale Einstein radii (masses). We measure a weak correlation ($r = 0.38$) between Einstein mass and $Σ_5(z)$, suggesting that the assumption of single galaxy-scale deflectors in lens modeling is overly-simplified. We hypothesize that the weak correlation results from galaxy-scale bias in the original AGEL selection and the observational challenge of detecting faint arcs with large Einstein radii. Comparing number densities, $N_{\rm{gal}}$, between AGEL and control fields, we find that AGEL deflectors are in systematically denser environments. Our study provides a method to identify strong lenses as a function of deflector environment and approximate the impact of large-scale environment in lens modeling. We provide the measured lensing parameters for our 89 AGEL systems as well as $z_{\rm{phot}}$ and $r$-mag (AB) maps of the line-of-sight.

Figures (12)

• Figure 1: The $z_{\text{spec}}$ distribution of AGEL deflectors (in gray) and sources (in orange) with DECaLS photometry and spectroscopic $z_{\text{defl}}$, $z_{\text{src}}$ (89 systems). Our sample has a mean deflector redshift of $\bar{z}_{\text{defl}}= \ 0.55\xspace$ (dashed gray) and source redshift of $\bar{z}_{\text{src}}= 1.87\xspace$ (dotted orange).
• Figure 2: EAZY output for a line of sight (LOS) object in the field of AGEL2303, with fitted redshift $z_\text{phot} = 0.80$. Left:$\log_{10}$ Flux vs. $\log_{10}$ observed wavelength. The black squares are the observed fluxes in six bands, and the magenta circles are the modeled flux measurements; the green distribution is the SED fit. The posterior SED shows continuum features, consistent with an early-type galaxy. Right: the posterior redshift probability distribution, $P(z)$, against $z$.
• Figure 3: $z_{\text{phot}}$ vs. $z_{\text{spec}}$ plot for a sample of 83 central deflectors with measured $z_{\text{phot}}$. Black triangles are deflectors whose SED fits showed blending of photometry with the source, and gray circles are deflectors without obvious signs of blending. Error bars are the $16$th and $84$th percentile values in the $P(z)$ outputs. For unblended systems, we measure $\frac{\Delta z}{1 + z_{\text{spec}}}$ = 0.032, demonstrating that $z_{\text{phot}}$ measurements accurately reproduce deflector redshifts for most AGEL systems. We observe a bias of $0.03$ for these fits. For blended systems, we measure $\frac{\Delta z}{1 + z_{\text{spec}}}$ of $0.07$. The bias in our unblended deflectors is due to $z_{\text{phot}}$ overestimates of early-type galaxies Abdalla_2011; in our blended deflectors, it is due to source light bleeding into deflector photometry.
• Figure 4: Example $z_{\text{phot}}$ measurements of the LOS for (from left) galaxy- (AGEL0003), group- (AGEL0919), and cluster-scale environment candidates. Foreground candidates ($< 0.8 \times z_{\text{defl}}$) are shown with blue squares, background candidates ($>1.2 \times z_{\text{defl}}$) with red triangles, and deflector environment candidates with green circles. $z_{\text{phot}}$ and $r$-mag LOS distributions for all 89 systems are available via the https://doi.org/10.7910/DVN/GYDQQP.
• Figure 5: Toy model of $\Sigma_{5}(z)$. The orange ellipses are galaxies fit to the deflector environment at $z_{\text{defl}}$. Including the central deflector, the distance to the fifth-closest fitted deflector environment member (labeled "5") to the center of the field is $D_{\text{5, Mpc}}$ (dashed red line). $\Sigma_{5}(z)$ is the surface density enclosed by the circle with radius $D_{\text{5, Mpc}}$ (red, dashed circle).