The formation of S0 galaxies: evidence from globular clusters

Abstract

Aims. We devise a simple experiment to test the theory that lenticular (S0) galaxies form from spirals whose star formation has been shut down. An individual galaxy's fading is measured using the globular cluster specific frequency (S N ), defined as the number of globular clusters normalised by the galaxy luminosity. This is compared with a spectroscopically-derived age estimate. Methods. We make NTT/EMMI long-slit spectroscopic observations of 11 S0 galaxies at z < 0.006. We measure the absorption-line indices, Hδ, Hγ, Mgb, Fe5270 and Fe5335 within the central r e /8. By inverting single-stellar population models, luminosity-weighted mean ages, metallicities and α-element abundance ratios are derived. We estimate the amount of fading a galaxy has undergone by comparing each galaxy's S N with its deviation from the mean spiral S N . Results. Galaxies with higher S N have older stellar populations. Moreover, we find that the zero-point and amount of fading is consistent with a scenario where lenticulars are formed by the quenching of star formation in spiral galaxies. Our data also rule out any formation method for S0s which creates a large number of new globular clusters. We confirm that previous results showing a relationship between S N and color are driven by the S N −Age relation. Five galaxies show detectable Hβ,lines. However, only in the two youngest galaxies is this emission unambiguously from star formation. Conclusions. Our results are consistent with the theory that S0 galaxies are formed when gas in normal spirals is removed, possibly as a result of a change in environment. The on-going star formation in the youngest galaxies hints that the timescale of quenching is 1 Gyr. We speculate, therefore, that the truncation of star formation is a rather gentle process unlikely to involve a rapid burst of star formation.

Physical Properties and Environments of Nearby Galaxies

“…In this manner, the S0s may form a continuum with the Sa galaxies or other red spirals, which as noted in §3.9 are also offset from Tully-Fisher by about 0.5 mag (at high V c at least). Similar conclusions result from the study of globular cluster populations, which are substantially more frequent per unit luminosity in S0s than in spirals of similar mass; this trend would result if the stellar population faded while the globular cluster population remained unchanged (Barr et al 2007). …”

Formation of S0 galaxies through mergers

“…Many studies support other environmental-driven processes (such as gas stripping and strangulation) as the most important drivers of the observed transformation of spirals into S0s in clusters since z ∼ 0.8 (see Poggianti et al 2001Poggianti et al , 2009Barr et al 2007;Desai et al 2007). However, recent observational and theoretical studies suggest that cluster galaxies have undergone a "pre-processing" through mergers in groups and filaments before falling into a cluster, and that even the gravitational binding of the group is preserved during the infalling for up to one orbital period (this is known as galaxy "postprocessing").…”

Bars, Ovals, and Lenses in Early-Type Disk Galaxies: Probes of Galaxy Evolution

“…This deviation is explained by the luminosity evolution of spiral galaxies: the transformation from spirals to S0s occurred at various times in the past, and the galaxies have been passively fading ever since. The globular cluster frequency (the number of globular clusters per unit V-band luminosity) has been used as an independent estimate of the degree to which the luminosity of S0s has faded relative to that of their spiral progenitors (Aragón-Salamanca et al 2006;Barr et al 2007). This estimate is based on the assumption that the frequency of globular clusters is constant during the transformation process.…”