The star-formation histories of elliptical galaxies across the fundamental plane

We present the first results from a study designed to test whether, given high-quality spectrophotometry spanning the mid-ultraviolet–optical wavelength regime, it is possible to distinguish the metal content and star-formation history of individual elliptical galaxies with sufficient accuracy to establish whether their formation history is linked to their detailed morphology and position on the Fundamental Plane. From a detailed analysis of ultraviolet-optical spectrophotometry of the ‘cuspy’ elliptical galaxy NGC 3605 and the giant elliptical NGC 5018 we find that: 1) optical spectra with λ > 3500 Å may not contain sufficient information to robustly uncover all the stellar populations present in individual galaxies, even in such relatively passive objects as elliptical galaxies, 2) the addition of the ultraviolet data approachingλ = 2500 Å holds the key to establishing well-constrained star-formation histories for these galaxies, from which we can infer a formation and evolution history which is consistent with their photometric properties, 3) despite the superficial similarity of their spectra, the two galaxies have very different ‘recent’ star-formation histories – the smaller, cuspy elliptical NGC 3605 contains a high-metallicity population of age ≃ 1 Gyr, and has a position on the fundamental plane typical of the product of a low-redshift gas-rich merger (most likely at z ∼ 0.08), while the giant elliptical NGC 5018, with a sub-solar secondary population, appears to have gained its more recent stars via mass transfer / accretion of gas from its spiral companion, 4) despite these differences in detailed history, more than 85% of the stellar mass of both galaxies is associated with an old (9-12 Gyr) stellar population of near-solar metallicity. This pilot study provides strong motivation for the construction and analysis of highquality ultraviolet-optical spectra for a substantial sample of ellipticals spanning the fundamental plane.