The probability of galaxy-galaxy strong lensing events in hydrodynamical simulations of galaxy clusters

Abstract

Context. Meneghetti et al. (2020) recently reported an excess of galaxy-galaxy strong lensing (GGSL) in galaxy clusters compared to expectations from the ΛCDM cosmological model. Theoretical estimates of the GGSL probability are based on the analysis of numerical hydrodynamical simulations in the ΛCDM cosmology. Aims. We quantify the impact of the numerical resolution and AGN feedback scheme adopted in cosmological simulations on the predicted GGSL probability, and determine if varying these simulation properties can alleviate the gap with observations. Methods. We repeat the analysis of Meneghetti et al. ( 2020) on cluster-size halos simulated with different mass and force resolutions and implementing several independent AGN feedback schemes. Results. We find that improving the mass resolution by a factor of ten and twenty-five, while using the same galaxy formation model that includes AGN feedback, does not affect the GGSL probability. We find similar results regarding the choice of the gravitational softening. On the contrary, adopting an AGN feedback scheme that is less efficient at suppressing gas cooling and star formation leads to an increase in the GGSL probability by a factor between three and six. However, we notice that such simulations form overly massive subhalos whose contribution to the lensing cross-section would be significant while their Einstein radii are too large to be consistent with the observations. The primary contributors to the observed GGSL cross-sections are subhalos with smaller masses, that are compact enough to become critical for lensing. The population with these required characteristics appears to be absent in simulations. Conclusions. Based on these results, we reaffirm the tension, previously reported in Meneghetti et al. (2020), between observations of GGSL and theoretical expectations in the framework of the ΛCDM cosmological model. The GGSL probability is sensitive to the galaxy formation model implemented in the simulations. Still, all the tested models have difficulty reproducing the stellar mass function and the internal structure of galaxies simultaneously.

Galaxies in the central regions of simulated galaxy clusters

“…Finally, we find that the V max − M SH at M SH 10 11 h −1 M , which is the most relevant mass-range in GGSL, shows tension between observations and simulations of different resolutions and feedback parameters. Meneghetti et al (2022) found that all our simulations have problems in recovering GGSL probability, in accordance with this work. In fact, while some simulations can produce an integrated GGSL in agreement with observations (as shown in their Fig.…”

Accelerating Early Massive Galaxy Formation with Primordial Black Holes

“…In general, the recent discovery of potentially very early massive galaxy formation by JWST (Labbé et al 2022) hints at faster structure formation in the high-z universe, above the ΛCDM baseline (Boylan-Kolchin 2022; Lovell et al 2022), which can be achieved if a small fraction (∼10 −6 -10 −3 ) of dark matter is composed of massive (10 9 M e ) PBHs, although more work needs to be done to check whether such fast structure formation is consistent with other observations of the high-z Universe. A similar trend is also seen in observations of (proto-) galaxy clusters that show an excess of stronglensing sources (Meneghetti et al 2020(Meneghetti et al , 2022 and of star formation (Remus et al 2022) that are difficult to explain in ΛCDM. Strikingly, this trend for accelerated structure formation at high z goes in the opposite direction to that of invoking the suppression of fluctuations to account for the well-known small-scale problems of ΛCDM.…”

Accelerating early massive galaxy formation with primordial black holes

“…In general, the recent discovery of potentially very early massive galaxy formation by JWST (Labbé et al 2022) hints at faster structure formation in the highz universe, above the ΛCDM baseline, which can be achieved if part of dark matter is composed of massive ( 10 9 M ) PBHs. A similar trend is also seen in observations of (proto-) galaxy clusters that show an excess of strong-lensing sources (Meneghetti et al 2020(Meneghetti et al , 2022 and of star formation (Remus et al 2022) that are difficult to explain in ΛCDM. Strikingly, this trend for accelerated structure formation at high-z goes in the opposite direction of invoking the suppression of fluctu-ations to account for the well-known small-scale problems of ΛCDM.…”