Intracluster light is a close tracer of the dark matter halo shape

Abstract

We investigate whether the intracluster light (ICL) can serve as a reliable tracer of the shape of the underlying dark matter (DM) haloes in galaxy clusters. Using the cosmological Hydrangea cluster simulations, we measure the 3D and projected shapes of both components with a shape tensor computed in concentric ellipsoidal shells, out to the virial radius $R_\mathrm{200c}$ for each cluster. The ICL and DM are closely aligned, with their major axes typically offset from each other by $\lesssim$10 degrees. Their axis ratios also match closely, with a typical difference of only $\approx\! 0.07$ for both the major-to-minor and major-to-intermediate axes, the DM being slightly rounder than the ICL. These trends are consistent across 2D and 3D measurements and agree well with results from isophotal fitting of mock images. In detail, the axis ratio offset is sensitive to the method used to remove satellites, and may also depend on the choice of subgrid physics models. We demonstrate that the ICL traces the DM shape better than the distribution of satellite galaxies, which exhibits larger scatter in the axis ratio and misalignment angle and is overall more elliptical. Together, these results indicate that the ICL can act as a useful proxy for DM halo ellipticity and orientation.

Figures (10)

• Figure 1: Left: surface density map of the stars in cluster CE-7 G-87 from the Hydrangea simulation at redshift $z = 0$, projected along the $\mathit{xy}$ plane. Middle: surface density map of the BCG+ICL stars, defined by removing stars bound to satellite galaxies, with ellipses indicating the shape measured in several radial bins. Right: surface brightness map in the $H_{\mathrm{E}}$ band of the corresponding Euclid-like mock image of the isolated BCG+ICL component (excluding satellites), produced following the methods in martin_preparing_2022.
• Figure 2: Radial profiles of 3D axis orientations and ratios for BCG+ICL and DM. Left column: (absolute) angle between the BCG+ICL and DM major axes, $|\Delta \phi_{\mathrm{major}}|$ (top), and minor axes, $|\Delta \phi_{\mathrm{minor}}|$ (bottom), respectively. Middle column: median axis ratios $p = b/a$ (major to intermediate; top), and $q = c/a$ (major to minor; bottom) for the BCG+ICL (green) and DM (purple). Right column: median difference in axis ratios $p$ (top) and $q$ (bottom) between the BCG+ICL and DM. All three quantities are plotted against the normalized (elliptical) radius, $a/R_{\mathrm{200c}}$. Solid lines trace the medians, shaded regions indicate the 1$\sigma$ scatter. Both the orientation and axis ratio agree closely between the BCG+ICL and DM, albeit with a slightly more elliptical shape for the former.
• Figure 3: Radial profiles of orientations and axis ratios of BCG+ICL vs. DM, analogous to Fig. \ref{['fig:3D results']} but in 2D projection. Left: absolute angle difference, $|\Delta \phi|$, between the position angles of the DM and ICL major axes against $a/R_{\mathrm{200c}}$. Shaded regions represent the 0--25$^\mathrm{th}$, 25$^\mathrm{th}$--50$^\mathrm{th}$, and 50$^\mathrm{th}$--75$^\mathrm{th}$ percentiles. Middle: major-to-minor axis ratio, $p=b/a$, of the BCG+ICL ($p_{\rm{BCG+ICL}}$, green) and DM ($p_{\rm{DM}}$, magenta). Solid lines trace the medians, shaded regions the 1$\sigma$ scatter. Right: axis ratio difference $\Delta p = p_{\rm{DM}} - p_{\rm{BCG+ICL}}$. As in 3D, position angles and axis ratios agree closely, with the tightest correspondence at ICL-dominated radii.
• Figure 4: Radial profiles of projected axis ratios in low- and high-mass clusters. Left: axis ratios $p$ of the BCG+ICL, as a function of $a/R_{\mathrm{200c}}$, for Hydrangea clusters below (blue) and above (green) the median mass of the full sample ($1.8\times 10^{14}\,\mathrm{M}_\odot$). Middle: the same, but for DM (pink and purple for low- and high-mass clusters, respectively). Right: difference between the BCG+ICL and DM axis ratios for high- (brown) and low-mass (orange) clusters. In all three panels, solid lines represent medians and shaded regions the 1$\sigma$ scatter. The offset between BCG+ICL and DM is very similar in both subsamples, despite clear trends in either component individually with halo mass.
• Figure 5: Difference in BCG+ICL axis ratio derived from the shape tensor of star particles, $p_{\rm ST}$, and from isophote fitting of mock images with AUTOPROF applied to corresponding mock images, $p_{\rm{iso}}$. The solid line shows the median, the shaded region the 1$\sigma$ scatter, and the error bars show 1$\sigma$ uncertainties estimated via bootstrap resampling. Beyond the very centre ($a \gtrsim 0.01\,R_\mathrm{200c}$), both methods give near-identical results.