Galaxies: Elliptical Galaxies

Elliptical galaxies are ellipsoidal stellar entities, supported more by random star motion than by orderly rotational motion. The classical picture of elliptical galaxies is one where the majority of their stars are old (>~ 8 Gyr), they have little dust and gas left between the stars, which allows for no on-going star formation activity, and they are broadly featureless, showing, for example, no spiral structure.

However, recent studies of elliptical galaxies, using data collected from state-of-the-art telescopes, show that these galaxies are far from being the old, dead, uniform structures they were once believed to be. We see hot gas in X-rays, dust features and ionised gas in carefully-examined optical features, molecular gas in radio observations, and evidence of recent star formation. In addition to stars, elliptical galaxies have a complex, multi-phase interstellar medium (ISM), which is very important in the context of reconstructing the evolutionary history of galaxies. In some elliptical galaxies, we see evidence of stellar shells, tails and kinematically distinct cores, suggestive of violent interactions in the galaxy's past.

The majority of bright elliptical galaxies are found in groups and the cores of galaxy clusters. They represent a greater fraction of the galaxy population in rich galaxy systems with high local galaxy densities than in the poor systems. This is the now well-known morphology-density relation.

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NGC 1275: The image NGC 1275 taken by the Wide Field and Planetary Camera 2 (WFPC2) of the Hubble Space Telescope (HST). NGC 1275 lies at the centre of the Perseus Cluster, and is one of the most peculiar galaxies in the sky. It may have undergone a merger in the recent past, which could explain the strong dust lanes. Protoglobular clusters may have formed from the gas in the merger, which can seen as the numerous compact bright sources surrounding the galaxy. It is also a strong X-ray and radio emitter.

The Extragalactic Group at Birmingham is studying various aspects of elliptical galaxies, with the aim of relating the observed diversity of observed characteristics of ellipticals with their environments, to explore how and when this interesting class of galaxies are formed. Projects include:

  • Stellar populations in early-type galaxies 1: we have a powerful technique which allows us to disentangle multiple stellar population components in elliptical galaxies, using spectroscopy and the latest stellar population models. This allows us to identify all the major episodes of star formation in these galaxies, giving us the ages, chemical content, and mass fractions of the dominant populations. We are using this technique on a large sample of near-by early-type galaxies (the Local Early-type Galaxy Study, LEGS), which includes galaxies in cluster environments, as well as less-studied group and field environments. This, in conjunction with multi-wavelength data (X-ray, ultraviolet, optical photometry, radio) and cosmological models, will give us a detailed picture of of the physical histories of these galaxies.

  • Stellar populations in early-type galaxies 2: in addition to the more detailed, individual approach described above, we are also exploiting the huge, homogeneous data set of the Sloan Digital Sky Survey (SDSS). In collaboration with co-workers in the Computer Science Department, we have developed a powerful algorithm (Algorithms) which can rapidly and robustly identify early-type galaxies containing young stellar populations from their SDSS spectra. This allows us to constrain the mechanisms via which elliptical galaxies form, by investigating the environments in which they do so. We find that the majority of present-day elliptical galaxies were most likely formed by major galaxy-galaxy mergers/interactions (Galaxy Mergers).

  • Fossil Groups - These are massive ellipticals embedded in extensive hot gas halos, believe to result from the merger of galaxies in compact groups. We are currently studying the stellar properties of elliptical galaxies in fossil groups.

  • X-ray structure of ellipticals: the hot gas envelopes of elliptical galaxies provide a probe of their chemical and thermal history. We have studied the properties of this gas in a sample of galaxies with ROSAT data, and now have new and forthcoming observations with XMM-Newton and Chandra. One of the issues to be addressed is that of the differences in chemical and thermal history between the brightest galaxy in a group or cluster and its satellites.

  • Origin of X-ray binaries in elliptical galaxies - SOMAK

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

Page written by Louisa Nolan and last updated 28 Nov 2007.