Stellar velocity profiles and line strengths out to four effective radii in the early-type galaxies NGC 3379 and NGC 821

TL;DR

This study uses advanced integral-field spectroscopy to measure stellar kinematics and line strengths out to four effective radii in two early-type galaxies, revealing constant line strength gradients, old and metal-poor halos, and the necessity of dark matter for explaining observed dynamics.

Contribution

It introduces a novel observing technique with SAURON that enables probing faint galaxy outskirts and constructs detailed dynamical models including dark matter halos.

Findings

Line strength gradients remain constant out to 4 Re.

Dark matter constitutes 30-50% of total mass within 4 Re.

Halo models show less radial anisotropy and steeper Mgb - Vesc relation.

Abstract

We use the integral-field spectrograph SAURON to measure the stellar line-of-sight velocity distribution and absorption line strengths out to four effective radii (Re) in the early-type galaxies NGC 3379 and NGC 821. With our newly developed observing technique we can now probe these faint regions in galaxies that were previously not accessible with traditional long-slit spectroscopy. We make optimal use of the large field-of-view and high throughput of the spectrograph: by adding the signal of all ~1400 lenslets into one spectrum, we obtain sufficient signal-to-noise in a few hours of observing time to reliably measure the absorption line kinematics and line strengths out to large radius. We find that the line strength gradients previously observed within 1 Re remain constant out to at least 4 Re, which puts constraints on the merger histories of these galaxies. The stellar halo populations are old and metal-poor. By constructing orbit-based Schwarzschild dynamical models we find that dark matter is necessary to explain the observed kinematics in NGC 3379 and NGC 821, with 30 - 50 per cent of the total matter being dark within 4 Re. The radial anisotropy in our best-fit halo models is less than in our models without halo, due to differences in orbital structure. The halo also has an effect on the Mgb - Vesc relation: its slope is steeper when a dark matter halo is added to the model.