Spectroscopic bulge-disc decomposition: a new method to study the evolution of lenticular galaxies

TL;DR

This paper introduces a spectroscopic bulge-disc decomposition method that separates galaxy components to analyze their stellar populations, revealing insights into S0 galaxy evolution and the role of dust in observed color gradients.

Contribution

A novel wavelength-by-wavelength spectroscopic decomposition technique enabling separate analysis of bulge and disc spectra in S0 galaxies.

Findings

Bulges have higher metallicities and younger stars than discs.

Color gradients are present in both components, likely due to dust, not age or metallicity variations.

The method provides high-quality spectra for detailed stellar population analysis.

Abstract

A new method for spectroscopic bulge-disc decomposition is presented, in which the spatial light profile in a two-dimensional spectrum is decomposed wavelength-by-wavelength into bulge and disc components, allowing separate one-dimensional spectra for each component to be constructed. This method has been applied to observations of a sample of nine S0s in the Fornax Cluster in order to obtain clean high-quality spectra of their individual bulge and disc components. So far this decomposition has only been fully successful when applied to galaxies with clean light profiles, consequently limiting the number of galaxies that could be separated into bulge and disc components. Lick index stellar population analysis of the component spectra reveals that in those galaxies where the bulge and disc could be distinguished, the bulges have systematically higher metallicities and younger stellar populations than the discs. This correlation is consistent with a picture in which S0 formation comprises the shutting down of star formation in the disc accompanied by a final burst of star formation in the bulge. The variation in spatial-fit parameters with wavelength also allows us to measure approximate colour gradients in the individual components. Such gradients were detected separately in both bulges and discs, in the sense that redder light is systematically more centrally concentrated in all components. However, a search for radial variations in the absorption line strengths determined for the individual components revealed that they are absent from the vast majority of S0 discs and bulges. The absence of gradients in line indices for most galaxies implies that the colour gradient cannot be attributed to age or metallicity variations, and is therefore most likely associated with varying degrees of obscuration by dust.