A shot in the dark: searching for dark substructures in the RX J0437+00 galaxy cluster

TL;DR

The paper investigates the presence of low-mass dark matter subhalos in the core of the galaxy cluster RX J0437.1+0043 by exploiting the exotic Hyperbolic-Umbilic (HU) lensing configurations. Combining HST imaging and VLT/MUSE IFU spectroscopy with Lenstool-based lens modelling, the authors identify a potential DM subhalo of mass $m_{ m halo} = (2.25 \pm 0.94)\times 10^{9}\,M_\odot$ near HU Image 1.2, though the detection is preliminary due to shallow data and model degeneracies. They discuss the reliability of the signal and the possibility of line-of-sight perturbers, and propose JWST follow-up and expansion to a HU-lens sample to enable statistical constraints on the low-mass end of the subhalo mass function and DM properties.

Abstract

Obtaining a census of dark matter structures at low mass ($\leq 10^9 M_\odot$) can provide strong constraints on the nature of dark matter, though identifying such structures remains difficult. In this work, we study the galaxy cluster RX J0437.1+0043, taking advantage of its powerful "exotic" Hyperbolic-Umbilic (HU) lensing configuration to search for substructure candidates. Using a combination of high resolution imaging, IFU spectroscopy, and gravitational lensing modelling, we report on a tentative detection of a dark matter subhalo ($m_{\rm halo} = 2.25 \pm 0.94 \times 10^9 M_\odot$) near the vicinity of one of the largest HU images. We stress that this result is still preliminary and that deeper data and more advanced modelling techniques are needed to ultimately confirm this detection. Nevertheless, this work outlines the first steps towards understanding subhalo properties in dense cluster environments, developing HU cluster lenses as a potential new tool for investigating dark matter.

Figures (9)

• Figure 1: Examples of rare, "exotic" lens configurations. The left column displays the critical curves in the (observed) image plane, with the characteristic image formations and image parity shown by the multicoloured regions. The right column shows the corresponding caustics in the (demagnified, intrinsic) source plane. [Adapted from Figs. 1-3 in meena2020.] Lenses in exotic configurations are more sensitive to the effects of nearby substructures than more common "stable" lensed images, making them ideal probes to look for low-mass subhalos. The Hyperbolic-Umbilic (HU) lensing configuration seen in the middle panels are the subject of this work.
• Figure 2: Spectroscopically-confirmed multiply-imaged sources in the RX J0437 field, overlaid on multi-band HST imaging (F140W/F110W/F814W). Sources identified as Hyperbolic-Umbilic (HU) systems appear in cyan, while all other systems are shown in red. Numbering convention of all systems follows that presented in lagattuta2023.
• Figure 3: Multiple images of HU System 1 ($z = 2.97$), seen in several data sets. In the top left panel, we see a greyscale image of the complete system, observed in HST/F814W imaging, the highest resolution image available. A zoom-in of one image (Image 1.2) is seen in the bottom left cutout. From this data we have identified seven distinct morphological features ("clumps", labelled 1-7), which we identify with coloured apertures. A 3-band colour image of the same system is shown in the bottom middle panel. This adds colour information, but individual clumps appear less distinct, due to the lower resolution of the HST/F110W and F140W imaging. The top right panel shows how the system appears in VLT/MUSE imaging, in this case, a complete ring structure composed of Lyman-$\alpha$ emission. A zoom in (of the same individual image) is shown in the bottom right panel. Contrasting with the HST imaging, the MUSE view of the system shows a broad, smooth feature, rather than distinct clumps.
• Figure 4: Multiple images of HU System 2 ($z = 1.98$), seen in several data sets. The layout and composition of these images follows that of Fig. \ref{['fig:sys1']}, though the zoomed-in image this time is the combined merging images of Images 2.1 and 2.4. In System 2, we identify three distinct clumps in the high-resolution imaging, which, due to their larger sizes are still distinct in the colour panel. The bright central component is readily identified in MUSE data (here CIII emission), while the two edge components are less bright.
• Figure 5: Multiple images of HU System 10 ($z = 6.02$), seen in several data sets. The layout and composition of these images follows that of Fig. \ref{['fig:sys1']}, though the zoomed-in image this time is the combined merging images of Images 10.1 and 10.2. Unlike Systems 1 and 2, System 10 is not seen in any broadband images, having been solely identified in deep MUSE observations as a faint Lyman-$\alpha$ emitter. Future, deeper imaging may yet reveal continuum emission and more complex morphological features for this system.