Two-Dimensional Kinematics and Dynamical Modelling of SLACS Lenses: Insights from Deep MUSE Observation

Abstract

Early-type galaxies (ETGs) hold a wealth of information regarding galaxy evolution and the processes shaping the large-scale structure of the Universe. Within the CDM cosmological framework, their observed properties serve as crucial tests for the hierarchical merging paradigm and, as the end products of this process, ETGs are sculpted by successive mergers of increasingly massive objects, offering a window into the entirety of galaxy formation and evolution. While lacking the grand, ordered disk and spiral arms characteristic of their more prominent counterparts, ETGs offer unique opportunities to study stellar populations, probe the nature of dark matter (DM), and constrain models of galaxy formation and interaction through their distinct morphologies and stellar kinematics.

The unification of mass and structure constraints from kinematic and dynamical modelling with the larger scale mass information from strong lensing allows great insight into the intrinsic properties of ETGs and the nature of DM. Access to a diverse range of spatial scales makes combined lensing and dynamical studies powerful in disentangling the stellar and the DM mass distributions of lens galaxies at their characteristic radii, and thus breaking the degeneracies between these two components.

We present results from the first spatially resolved kinematic and dynamical modelling analysis of the unique SDSSJ0946+1006 (‘Jackpot’) triple-source lens system, where a single massive foreground galaxy multiple-images three background sources at different redshifts. Deep IFU spectroscopic data were obtained using the MUSE instrument on the VLT, which, compared to previous single-slit observations, provides full azimuthal area coverage, high sensitivity (5 hour integration) and high angular resolution (0.5arcsec FWHM). To account for the strong continuum contributions from the source, a multiple-component stellar template fitting technique is adopted to fit to the spectra of both the lens galaxy and the bright lensed background arc simultaneously. Through this, we robustly measure the first and second moments of the two-dimensional stellar kinematics out to about 10kpc from the centre of the lens, as well as resolving the inner profile inwards to 1kpc. The two-dimensional kinematic maps show a steep velocity dispersion gradient and a clear rotational component. We constrain the characteristic properties of the stellar and DM mass components with a sufficiently flexible parameterised dynamical model and an imposed lensing mass and find a DM density slope of , i.e. significantly steeper than an
unmodified NFW profile () and consistent with a contracted DM halo. Our fitted models have a lensing-equivalent density slope of , and thus we confirm most pure lensing results in finding a near isothermal profile for this galaxy.

In a direct extension of this work, these kinematic and dynamical modelling techniques are generalised and applied to a broader sample of 9 additional SLACS early-type lens galaxies. The gravitational lens systems within our sample span a lens and source redshift range of and respectively. This broader analysis yields a robust determination of the first and second moments of the two-dimensional stellar kinematics for the entire sample.
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Among the galaxies in this sample, J1250-0135 is a particularly noteworthy case. Previously excluded from sample studies due to the presence of bright spiral arms, which significantly hinder any accurate lensing analysis, this complex lens system now benefits from the wealth of information provided by MUSE IFU data. The intricate structured light is definitively confirmed to be at the redshift of the background source, allowing the 2D stellar kinematics of the foreground elliptical lens galaxy to be measured for the first time.
Furthermore, we detect a clear signature of a kinematically-decoupled core in J1250, characterised by two counter-rotating components at the lens redshift.
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We employ the anisotropic Jeans modelling framework and sample the posterior PDF for the model parameters under the assumptions of an NFW-like DM halo and a rigidly enforced lensing mass at the Einstein radius.
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We additionally explore models in which the imposed lensing mass is no longer rigidly enforced at the Einstein radius. Instead, it is permitted to vary, scaled by a free parameter, . The inferred for a significant fraction of lenses in our sample is greater than unity at about the 10 level, with an ensemble average across the galaxies of ranging between 1.00 and 1.88.
A significant deviation of from unity might indicate systematic uncertainties in the lensing or dynamical model, or limitations imposed by the quality of the available data. Thus, we discuss the implications of this finding and comment on possible deficiencies in the modelling techniques commonly used for analysing early-type lens galaxies.