Detection of lensing substructure using ALMA observations of the dusty galaxy SDP.81

TL;DR

This paper develops a method to detect and quantify dark matter substructure in galaxies using ALMA gravitational lensing data, successfully identifying a subhalo and constraining subhalo abundance down to small masses.

Contribution

The authors introduce a novel interferometric lensing analysis technique to measure subhalo abundance, applying it to ALMA data and demonstrating its effectiveness in detecting low-mass substructures.

Findings

Detected a $10^{8.96} M_{ ext{sun}}$ subhalo with high significance.

Constrained dark matter subhalo abundance down to $2 imes 10^7 M_{ ext{sun}}$.

Results are consistent with $ ext{Lambda}$CDM predictions given current uncertainties.

Abstract

We study the abundance of substructure in the matter density near galaxies using ALMA Science Verification observations of the strong lensing system SDP.81. We present a method to measure the abundance of subhalos around galaxies using interferometric observations of gravitational lenses. Using simulated ALMA observations, we explore the effects of various systematics, including antenna phase errors and source priors, and show how such errors may be measured or marginalized. We apply our formalism to ALMA observations of SDP.81. We find evidence for the presence of a $M=10^{8.96\pm 0.12} M_{\odot}$ subhalo near one of the images, with a significance of $6.9\sigma$ in a joint fit to data from bands 6 and 7; the effect of the subhalo is also detected in both bands individually. We also derive constraints on the abundance of dark matter subhalos down to $M\sim 2\times 10^7 M_{\odot}$, pushing down to the mass regime of the smallest detected satellites in the Local Group, where there are significant discrepancies between the observed population of luminous galaxies and predicted dark matter subhalos. We find hints of additional substructure, warranting further study using the full SDP.81 dataset (including, for example, the spectroscopic imaging of the lensed carbon monoxide emission). We compare the results of this search to the predictions of $\Lambda$CDM halos, and find that given current uncertainties in the host halo properties of SDP.81, our measurements of substructure are consistent with theoretical expectations. Observations of larger samples of gravitational lenses with ALMA should be able to improve the constraints on the abundance of galactic substructure.