[Paper Review] Detecting low-mass haloes with strong gravitational lensing II: constraints on the density profiles of two detected subhaloes

Giulia Despali, Felix M. Heinze+4 more|arXiv (Cornell University)|Jul 17, 2024

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study re-evaluates two low-mass subhalo detections in strong gravitational lensing systems (SDSSJ0946+1006 and JVASB1938+66) by modeling their density profiles with four different functional forms, finding that steep inner slopes—close to or steeper than isothermal—are required. The results indicate that the NFW profile fails to explain the observations without extreme concentrations, and the data constrain the enclosed mass within a characteristic radius more robustly than the inner slope, suggesting baryonic effects or non-standard dark matter models may be needed to reconcile the data with CDM predictions.

ABSTRACT

Strong gravitational lensing can detect the presence of low-mass haloes and subhaloes through their effect on the surface brightness of lensed arcs. We carry out an extended analysis of the density profiles and mass distributions of two detected subhaloes, intending to determine if their properties are consistent with the predictions of the cold dark matter (CDM) model. We analyse two gravitational lensing systems in which the presence of two low-mass subhaloes has been previously reported: SDSSJ0946+1006 and JVASB1938+66. We model these detections assuming four different models for their density profiles and compare our results with predictions from the IllustrisTNG50-1 simulation. We find that the detected subhaloes are well-modelled by steep inner density slopes, close to or steeper than isothermal. The NFW profile thus needs extremely high concentrations to reproduce the observed properties, which are outliers of the CDM predictions. We also find a characteristic radius within which the best-fitting density profiles predict the same enclosed mass. We conclude that the lens modelling can constrain this quantity more robustly than the inner slope. We find that the diversity of subhalo profiles in TNG50, consistent with tidally stripping and baryonic effects, is able to match the observed steep inner slopes, somewhat alleviating the tension reported by previous works even if the detections are not well fit by the typical subhalo. However, while we find simulated analogues of the detection in B1938+666, the stellar content required by simulations to explain the central density of the detection in J0946+1006 is in tension with the upper limit in luminosity estimated from the observations. New detections will increase our statistical sample and help us reveal more about the density profiles of these objects and the dark matter content of the Universe.

Motivation & Objective

• To assess whether the observed properties of two low-mass subhaloes in strong lensing systems are consistent with the predictions of the cold dark matter (CDM) model.
• To test the validity of the NFW profile in describing the density structure of these subhaloes, given that previous detections assumed a Pseudo-Jaffe profile.
• To determine which aspects of the subhalo mass distribution—inner slope or enclosed mass—can be robustly constrained by lensing data.
• To evaluate whether simulated subhaloes from the IllustrisTNG50-1 simulation, including effects of tidal stripping and baryons, can reproduce the observed lensing signatures.
• To assess whether the luminosity constraints from observations are compatible with the stellar content required by simulations to explain the central density of the J0946+1006 subhalo.

Proposed method

• The authors model the lensing systems SDSSJ0946+1006 and JVASB1938+66 using four different density profile models: NFW, Pseudo-Jaffe, Hernquist, and power-law.
• They compare the lensing predictions of these models against high-resolution HST and Keck adaptive optics data to constrain subhalo mass and structure.
• They use the IllustrisTNG50-1 simulation to identify analogues of the detected subhaloes and assess their structural properties, including inner slope and concentration.
• The Einstein radius is calculated both from the analytical profiles and from the particle distributions in the simulation to assess stability and noise in the measurements.
• They derive scaling relations between subhalo mass and two key observables: projected enclosed mass (M2D) and Einstein radius (Rein).
• They evaluate the robustness of constraints on the inner slope versus the enclosed mass within a characteristic radius (Req), using both simulated and observed data.

Experimental results

Research questions

• RQ1 Can the observed lensing distortions in SDSSJ0946+1006 and JVASB1938+66 be explained by standard NFW profiles without requiring unphysically high concentrations?
• RQ2 Which subhalo structural parameter—inner density slope or enclosed mass—can be more robustly constrained by strong lensing observations?
• RQ3 Do simulated subhaloes from the IllustrisTNG50-1 simulation, including baryonic and tidal effects, reproduce the steep inner slopes observed in the two detections?
• RQ4 Is the stellar content required to explain the central density of the J0946+1006 subhalo consistent with the observed upper limit on luminosity?
• RQ5 Can isolated dark haloes along the line of sight provide a viable alternative explanation to the detected subhaloes, especially given their potential differences from subhaloes within galaxies?

Key findings

• The two detected subhaloes are best explained by steep inner density slopes, close to or steeper than isothermal, indicating a highly concentrated mass distribution.
• The NFW profile fails to reproduce the observations without requiring extremely high concentrations, which are outliers in the CDM prediction distribution.
• The enclosed mass within a characteristic radius (Req) is constrained more robustly by lensing data than the inner slope, suggesting that mass within a specific radius is the key observable.
• Simulated subhaloes from TNG50, which include effects of tidal stripping and baryons, can reproduce the observed steep inner slopes, somewhat alleviating tension with CDM predictions.
• The stellar content required to explain the central density of the J0946+1006 subhalo exceeds the observed upper luminosity limit, indicating a potential inconsistency with standard baryonic models.
• New detections will be essential to increase the statistical sample and improve constraints on the true density profiles of low-mass subhaloes and the nature of dark matter.