The SLUGGS survey: using extended stellar kinematics to disentangle the formation histories of low-mass S0 galaxies

Abstract

We utilise the DEIMOS instrument on the Keck telescope to measure the widefield stellar kinematics of early-type galaxies as part of the SAGES Legacy Unifying Globulars and GalaxieS (SLUGGS) survey. In this paper, we focus on some of the lowest stellar mass lenticular galaxies within this survey, namely NGC 2549, NGC 4474, NGC 4459 and NGC 7457, performing detailed kinematic analyses out to large radial distances of ∼ 2−3 effective radii. For NGC 2549, we present the first analysis of data taken with the SuperSKiMS (Stellar Kinematics from Multiple Slits) technique.To better probe kinematic variations in the outskirts of the SLUGGS galaxies, we have defined a local measure of stellar spin. We use this parameter and identify a clear separation in the radial behaviour of stellar spin between lenticular and elliptical galaxies, thereby reinforcing the physically meaningful nature of their morphological classifications.We compare the kinematic properties of our galaxies with those from various simulated galaxies to extract plausible formation scenarios. By doing this for multiple simulations, we assess the consistency of the theoretical results. Comparisons to binary merger simulations show that low-mass lenticular galaxies generally resemble the spiral progenitors more than the merger remnants themselves, an indication that these galaxies are not formed through merger events. We find, however, that recent mergers cannot be ruled out for some lenticular galaxies.

Kinematics of simulated galaxies II: Probing the stellar kinematics of galaxies out to large radii

“…Foster et al (2018) uses direct integral-field observations with a radial coverage out to typically ∼ 1.5-2.5R e . Bellstedt et al (2017) utilises the SKiMS technique which uses DEIMOS slit observations to capture the underlying stellar velocity field and apply a novel interpolation method to obtain a continuous velocity map with a radial coverage out to ∼ 2-3R e . A similar approach is used by Pulsoni et al (2018), however using planetary nebulae as discrete tracers, which allow for a significantly larger radial coverage out to typically ∼ 6R e .…”

The SLUGGS Survey: Understanding Lenticular Galaxy Formation via Extended Stellar Kinematics

“…For each of the galaxies in the SLUGGS survey (both elliptical and lenticular), we measured the stellar spin λ R within 1 R e within [1], given by…”

“…However, since each measurement is flux-weighted, the stellar spin measured in the outer regions of the galaxy will be more highly weighted towards the spin in the central regions, making it difficult to distinguish changes in spin with greater radii. As described in [1], we rather make these measurements over isophotal annuli.…”

“…Using this technique, a number of galaxies in the SLUGGS survey were noted [1] to have strongly downturning stellar spin profiles at larger radii. For such galaxies, the traditional λ R − diagram only conveys the behaviour in the central regions.…”

“…The four galaxies from the SLUGGS survey identified as having downturning stellar spin profiles are NGC 821, NGC 3377, NGC 4278, and NGC 4473 [1]. What the spin-ellipticity tracks have in common for each of these galaxies is that they rise in the central regions, and then turn back on themselves at larger radii.…”