The dynamics of S0 galaxies and their TullyFisher relation

Abstract

This paper investigates the detailed dynamical properties of a relatively homogeneous sample of disc‐dominated S0 galaxies, with a view to understanding their formation, evolution and structure. By using high signal‐to‐noise ratio long‐slit spectra of edge‐on systems, we have been able to reconstruct the complete line‐of‐sight velocity distributions of stars along the major axes of the galaxies. From these data, we have derived both model distribution functions (the phase density of their stars) and the approximate form of their gravitational potentials. The derived distribution functions are all consistent with these galaxies being simple disc systems, with no evidence for a complex formation history. Essentially no correlation is found between the characteristic mass scalelengths and the photometric scalelengths in these galaxies, suggesting that they are dark‐matter dominated even in their inner parts. Similarly, no correlation is found between the mass scalelengths and asymptotic rotation speed, implying a wide range of dark matter halo properties. By comparing their asymptotic rotation speeds with their absolute magnitudes, we find that these S0 galaxies are systematically offset from the Tully–Fisher relation for later‐type galaxies. The offset in luminosity is what one would expect if star formation had been suddenly switched off a few Gyr ago, consistent with a simple picture in which these S0s were created from ordinary later‐type spirals which were stripped of their star‐forming interstellar medium when they encountered a dense cluster environment.

The slowing down of galaxy disks in dissipationless minor mergers

“…Studies of the ≤ R 50 /2 R 50 /2< r ≤ R 50 R 50 < r ≤ 2R 50 2R 50 < r ≤ 5R 50 5R 50 < r ≤ 10R 50 differences between the Tully-Fisher relation of S0s and spirals, while showing some small discrepancies in the offset and scatter, generally find that the S0 galaxies follow the spiral relationship, albeit with a larger scatter (e.g., Hinz et al 2003;Bedregal et al 2006, but see Mathieu et al 2002. The offset and larger scatter have been interpreted as revealing a generally older stellar population in S0 galaxies than in spirals and a range of times during which the S0 galaxies started to evolve passively (Mathieu et al 2002). However, as noted in Bedregal et al (2006), there is no strong correlation between the age of the S0 galaxies and the offset from the T-F relation for spiral galaxies.…”

Stellar populations in the bulges of S0s and the formation of S0 galaxies

“…Despite the relatively small sample and the considerable technical challenges involved in determining the true rotation velocity V rot from absorption line spectra of galaxies with significant non-rotational support (see Mathieu et al 2002), some very interesting results arise. S0s lie systematically below the spiral galaxy Tully-Fisher relation in both the optical and near-infrared (Figure 1).…”

Formation of S0 galaxies through mergers

“…Some authors consider that an offset would be evidence that S0s derive from spirals through simple fading after the gas is stripped through environmental processes (e.g. Mathieu et al 2002). However, simple truncation of star formation in spirals would not be enough to explain the offset (Bedregal et al 2006;Noordermeer et al 2008) or the higher dispersion of the S0s in the TFR (Hinz et al 2001).…”