Surveys: LEGS


legslogo The Local Elliptical Galaxy Survey (LEGS) is a survey of the nearest ~200 early-type galaxies (ellipticals and S0s).The purpose is to constrain theories of early-type galaxy evolution across a range of environments, by studying a statistically significant sample of early-types which are close enough for us to obtain good quality, multi-wavelength data.

It used to be believed that all early-type galaxies formed the bulk of their stellar mass in the early universe, and then evolved passively until the present day. Present-day early-types were therefore thought to be simple systems, containing a single mature stellar population. However, as the instruments we use to study the universe have improved, we have discovered a wealth of complex phenomena in early-types, which suggests that this simple picture cannot be the whole story. We now believe that many, if not all, early-types were formed from the low-redshift merging of disk galaxies, with the merging event giving rise to an associated burst of star-formation. However, the effects of environment, the influence of the 'parent' galaxies, the role of feedback from stars and active nuclei, and the effect of multiple mergers on our present-day (local) early-types are still not well-understood. We therefore take advantage of the availability of multi-wavelength data to take a multi-parameter approach to building a complete and detailed picture of the present state and history of early-type galaxies. we aim to uncover the relationships between observed phenomena and the formation and evolution routes of early-type galaxies. In particular, we are interested to see whether specific formation scenarios (e.g. low-redshift major mergers, high-redshift assembly) give rise to different classes of early-type galaxies, identified by their observed characteristics (e.g. age /chemical composition of stellar populations, X-ray gas characteristics, radio emission, photometric structure).

Our sample consists of all the early-types out to a distance of 25 Mpc, brighter than an absolute B magnitude ~ --16.5, with Galactic latitude higher than 20 degrees. Dwarf early-types with a lower distance cut-off will also be added to the sample. We are using archived and new observations (X-ray, ultraviolet - near-infrared spectra, optical and infrared photometry, millimetre, sub-millimetre and radio), to study the interplay between gas (hot, warm, cold), star formation history, stellar and gas distributions and dynamics, active nuclei and environment. The proximity of the galaxies in our sample means that we can obtain good-quality, and hence usefully detailed, multi-wavelength observations.

Our preliminary results, relating star-formation history to isophotal shape in 55 of our sample galaxies, show that the most recently-formed stellar populations in early-types with boxy isophotes are in general older that those in early-types with disky isophotes. This is consistent with the results of N-body galaxy-galaxy merger simulations, which produce boxy galaxies from gas-poor mergers, with no associated star-formation. This suggests that the youngest stars in boxy galaxies result from an earlier, and more gas-rich, merger than the most recent one, which produced the boxy isophotes.

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Four different views of the large early-type galaxy NGC 4697. Top left: Chandra X-ray image, showing hot diffuse gas, plus many X-ray point sources. Top right: The UV-optical spectrum of NGC 4697 (thick black line). Fitting a three-component stellar population model (thin black line, single stellar population components are blue, red and green) to the spectrum of this galaxy reveals that the majority of its stars are old (14 Gyr, 72% by stellar mass), but there is also a significant intermediate-age component (5 Gyr, 26% by stellar mass). A small old, metal-poor population is also required to give an adequate fit. Bottom left: an I-band image of NGC 4697, showing its disky isophotes. Bottom right: I-band surface brightness profile. The cuspy nature of the I-band surface brightness profile, together with the disky isophotes suggest that this galaxy may be the remnant of the major merger of two gas-rich galaxies. The age of the intermediate stellar population suggests that this event took place 5 Gyr ago. NGC 4697 resides in a group environment, where we expect merging events to be relatively frequent.

Researchers: Louisa Nolan, Trevor Ponman, Somak Raychaudhury, Alan Whiting

This page was written by Louisa Nolan, and was last updated on