Stellar populations in the SAMI Galaxy Survey
Keywords: galaxies: stellar content
galaxies: radial gradients
galaxies: formation
galaxies: evolution

We present a set of line index-based measurements of the relative α-element abundance, [α/Fe], for up to 2097 galaxies in the Sydney-Australian-Astronomical-Observatory Multi-object Integral-field Spectrograph (SAMI) Galaxy Survey using aperture spectra normalised to the effective radius R_e. We derive a best-fit linear relation between [α/Fe] and the galaxy effective velocity dispersion σ_e for early-type galaxies (ETGs), [α/Fe] = (0.307 +/- 0.015)log_10(σ_e / km/s) − (0.41 +/- 0.03), observing a strong correlation betwen the residuals and the local environmental surface density. We find evidence for galaxies in high-density environments (HDEs) being α-enhanced over their counterparts in low-density environments (LDEs), and that this offset varies with morphological type, with ellipticals α-enhanced by > 0.06 dex and spirals by ≈ 0.02 dex. We use the offset in ellipticals to estimate that star formation in HDEs was truncated ≈ 1 Gyr earlier than in LDEs, at a fixed velocity dispersion of σ_e = 100 km/s. We also find support for a parabolic relationship between [α/Fe] and σ_e in ellipticals, indicating contributions from both environment and mass-based quenching mechanisms, although there is no evidence for this behaviour in later-type galaxies.
We then refit a global [α/Fe]-σ_e relation, using a subset of 1317 galaxies with measurements of the inclination-corrected spin proxy λ(eo)_R_e. We derive an anti-correlation between the residuals Δ[α/Fe] and the spin proxy, Δ[α/Fe] = (−0.060 +/- 0.009)λ(eo)_R_e + (0.027+/-0.004), which appears to be driven by star-forming galaxies, with a gradient Δ[α/Fe] ~ (−0.14 ± 0.02)λ(eo)_R_e. This indicates that disc-dominated galaxies have an extended duration of star-formation, with an estimated increase in the half-mass formation time of ≈ 0.8 Gyr between our low- and high-λ(eo)_R_e samples. We also observe that slow rotators do not have significantly different stellar populations than other low-λ(eo)_R_e galaxies, concluding that galaxies with λ(eo)_R_e < 0.4 experience a similar range of star formation histories, despite their differing structural parameters.
Next, we introduce a novel application of the contour binning method. We compare this method against the use of Voronoi tessellation and fixed-width annuli, demonstrating that it maximises the available radial information in low signal/noise observations. We apply this method to galaxies drawn from the SAMI Galaxy Survey, deriving seeing-corrected radial stellar population gradients for 1453 galaxies. On average, galaxies in our sample have flat age and [α/Fe] gradients, but negative metallicity gradients. We show the variations of these gradients and radial profiles in the size-mass plane, simultaneously separating galaxies by morphological type and environment. Our results reinforce the earlier analysis using global measurements, in that at all galactocentric radii, ETGs in HDEs are older by ≳ 2 Gyr, and have higher metallicities and α-abundance by ≈ 0.05 dex compared to their counterparts in LDEs. These offsets are not present in late-type galaxies (LTGs).
Finally, we demonstrate that radial population gradients do not have any significant dependence on environment, for fixed morphological type. The exception to this is [α/Fe] gradients in high-mass LTGs, which we show is consistent with existing models of star formation in disc galaxies. We discuss the physical implications of our results, including the support for inside-out quenching mechanisms and comparisons with existing literature, and explore potential avenues for further study.