Isolated elliptical galaxies

This thesis presents a detailed study of a well defined sample of isolated early-type galaxies. We define a sample of 36 nearby isolated early-type galaxies using a strict isolation criteria. New wide-field optical imaging of 20 isolated galaxies confirms their early-type morphology and relative isolation. We find that the isolated galaxies reveal a colour-magnitude relation similar to cluster ellipticals, which suggests that they formed at a similar epoch to cluster galaxies, such that the bulk of their stars are very old. However, several galaxies of our sample reveal evidence for dust lanes, plumes, shells, boxy and disk isophotes. Thus at least some isolated galaxies have experienced a recent merger/accretion event which may have also induced a small burst of star formation. Using new long-slit spectra of 12 galaxies we found that, isolated galaxies follow similar scaling relations between central stellar population parameters, such as age, metallicity [Z/H] and α-element abundance [E/Fe], with galaxy velocity dispersion to their counterparts in high density environments. However, isolated galaxies tend to have slightly younger ages, higher metallicities and lower abundance ratios. Such properties imply an extended star formation history for galaxies in lower density environments. We measure age gradients that anticorrelate with the central galaxy age. Thus as a young starburst evolves, the age gradient flattens from positive to almost zero. Metallicity gradients range from near zero to strongly negative. For our high mass galaxies, metallicity gradients are shallower with increasing galaxy mass. Such behaviour is expected in the remnants of multiple mergers. The metallicity gradients are also found to be correlated with the central age and metallicity, as well as to the age gradients. We generally find flat [E/Fe] gradients. We also examine the Fundamental Plane in both traditional Re, μe and σ space and κ-space. Most isolated galaxies follow the same Fundamental Plane tilt and scatter for galaxies in high density environments. However, a few galaxies notably deviate from the plane in the sense of having smaller M/L ratios. This can be understood in terms of their younger stellar populations, which are presumably induced by a gaseous merger. In conclusion, our results are compatible with an extended merger/accretion history for most isolated elliptical galaxies. However, for those galaxies which show no fine structures nor any young stellar populations, an early formation epoch followed by passive evolution is more probable.