[Paper Review] Detection of a filament connected to CL0016 with weak gravitational lensing

Yuichi Higuchi, Masamune Oguri+2 more|arXiv (Cornell University)|Mar 22, 2015

Galaxies: Formation, Evolution, Phenomena3 citations

TL;DR

This study presents a weak gravitational lensing detection of a cosmic filament connecting two galaxy clusters, CL0016 and RX J0018, using deep Subaru/Suprime-Cam imaging. By applying two independent background subtraction methods to isolate the filament's lensing signal, the authors measure a surface mass density of $\langle\Sigma\rangle = (3.20 \pm 0.10) \times 10^{14}h\,M_\odot\,\text{Mpc}^{-2}$, confirming the filament's presence with $\geq 2\sigma$ significance and offering a new probe of large-scale structure formation.

ABSTRACT

We report on the weak lensing detection of a filament between two galaxy clusters at $z=0.55$, CL0015.9+1609 and RX J0018.3+1618. We conduct weak lensing analysis of deep multi-band Subaru/Suprime-Cam images with $Lensfit$. The weak lensing signals from the filament are contaminated by signals from the adjacent massive clusters and we statistically subtract the cluster component using two different methods. Both methods yield consistent shear profiles on the filament with $\gtrsim2σ$ significance and the average surface mass density of the filament is $=(3.20\pm0.10) imes10^{14}h$ M$_\odot$Mpc$^{-2}$, which is in broad agreement with previous studies. On-going surveys such as Hyper Suprime-Cam will identify more filaments, which will serve as a new probe of structure formation in the Universe.

Motivation & Objective

• To detect a cosmic filament connecting two massive galaxy clusters, CL0016 and RX J0018, using weak gravitational lensing.
• To overcome contamination from the lensing signals of the massive foreground clusters by developing and applying two statistical background subtraction techniques.
• To measure the projected surface mass density of the filament and compare it with theoretical predictions and previous studies.
• To validate the filament's detection significance using tangential and cross shear profiles across multiple excluded region definitions.
• To demonstrate the feasibility of weak lensing as a robust probe for studying filamentary structures in the large-scale universe.

Proposed method

• Utilized deep multi-band imaging from the Subaru/Suprime-Cam to select background galaxies using photometric redshifts.
• Applied the $\text{Lensfit}$ software to measure galaxy ellipticities and estimate the reduced shear $\mathbf{g} = \gamma / (1 - \kappa)$.
• Constructed a convergence map from the lensing shear to reconstruct the projected mass distribution of the filament.
• Employed two background subtraction methods: (1) subtracting best-fit NFW profiles of the clusters, and (2) subtracting shear profiles from the opposite side of the filament.
• Defined excluded regions around the clusters using $r_{\text{vir}}$ to minimize cluster contamination in the filament signal.
• Fitted the tangential shear profiles with an analytical model assuming a constant convergence profile, fixing $\theta_c = 2$ arcmin for simplicity.

Experimental results

Research questions

• RQ1 Can a filament connecting two galaxy clusters at $z = 0.55$ be robustly detected via weak gravitational lensing despite contamination from massive clusters?
• RQ2 How do different background subtraction techniques affect the reliability and significance of the filament lensing signal?
• RQ3 What is the projected surface mass density of the filament, and how does it compare to predictions from $\Lambda$CDM simulations and prior weak lensing studies?
• RQ4 To what extent do the measured shear profiles deviate from a null hypothesis, indicating a statistically significant detection?
• RQ5 Can weak lensing of filaments serve as a viable probe for testing structure formation models in the $\Lambda$CDM framework?

Key findings

• The filament's lensing signal is detected with $\geq 2\sigma$ significance across all excluded region definitions and both background subtraction methods.
• The average surface mass density of the filament is measured as $\langle\Sigma\rangle = (3.20 \pm 0.10) \times 10^{14}h\,M_\odot\,\text{Mpc}^{-2}$, consistent with previous weak lensing studies of filaments.
• The virial mass and concentration of CL0016 are estimated as $M_{\text{vir}} = (2.09^{+0.10}_{-0.21}) \times 10^{15}h^{-1}M_\odot$ and $c_{\text{vir}} = 5.56^{+0.63}_{-0.79}$, respectively.
• The virial mass and concentration of RX J0018 are estimated as $M_{\text{vir}} = (4.51^{+1.13}_{-1.05}) \times 10^{14}h^{-1}M_\odot$ and $c_{\text{vir}} = 4.61^{+1.90}_{-1.35}$, respectively.
• The tangential shear profiles of the filament are consistent with simulated expectations and show no significant cross-shear signal, confirming the alignment of the lensing signal with the filament axis.
• The convergence map reveals a clear mass bridge between the two clusters, supporting the existence of a filamentary structure connecting them.