Galaxies: Merging Galaxies


A sequence of optical images of merging spiral galaxies arranged in an approximate order from top left to bottom right: approaching, interacting and finally merging into a single elliptical galaxy. We have studied the way in which hot gas is produced by supernova explosions triggered during this process - see discussion below.

Galaxies were once thought of as `island universes', evolving slowly in complete isolation. It is now clear that many galaxies interact in a variety of ways with satellite and neighbouring galaxies, and collisions and mergers of galaxies are now believed to be key evolutionary mechanisms. There are probably very few galaxies in the universe that were not shaped by interactions or even outright mergers, and the position of a galaxy in Hubble's morphological sequence may depend mainly on the number and severity of merger events in its past history. Pure disc systems, formed from relatively isolated protogalactic gas clouds, appear at one end of the sequence, giant ellipticals, possibly produced through mergers of similar spirals, appear at the other. In between, mergers between galaxies of differing mass produce galaxies with a wide range of bulge to disc ratios.

The image sequence above shows the sample that we used in the first study of the X-ray properties of an evolutionary sample of merging galaxies, using data from the ROSAT X-ray observatory. During the merger of two disc galaxies, the conversion of orbital to internal energy during a close tidal encounter causes the two progenitor systems to sink together and coalesce violently into a centrally condensed system, disrupting any pre-existing discs, and largely randomising the stellar motions. Gas flows into the centre of the galaxies during this process, where it fuels massive bursts of star formation, leading to multiple supernova explosions which heat gas to very high temperatures.

The hot gas and point-like X-ray sources produced in merging galaxies are now being studied using new data from the Chandra and XMM-Newton observatories, as are the preferred locations for such mergers / interactions to take place.

Topics being addressed by our work include:

  • Chandra and XMM studies of a sample of mergers - The higher quality X-ray data now available from Chandra and XMM, allows us to explore the energetics of the hot gas produced by starbursts, and to study the populations of very luminous point sources, which may indicate the presence of massive black holes. The image below shows contours of X-ray emission from a Chandra observation of the early-stage merger Arp 270 (also seen at the top left in the merger sequence illustrated above), overlaid on an optical image from the Palomar observatory. It is clear that the production of hot gas has already started in this system, and there is a surprisingly large population of ultra-luminous point sources (green diamonds).


  • The Antennae: a deep Chandra study - The pair of merging galaxies known as "The Antennae" (NGC4038/4039), which is the third in the sequence shown at the top of the page, is the closest classic merging galaxy system. This is the subject of a very long exposure (114 hours in total) with Chandra, by an international team led by Pepi Fabbiano at the Harvard-Smithsonian Center for Astrophysics, of which our group is a member. The image below shows the most sensitive X-ray image ever taken of a merging galaxy system. It shows a ring of bright X-ray sources associated with the star-forming regions in the northern galaxy, and near the nucleus of the southern one. Also visible are two huge bubble-like structures, each 10 kpc across, which extend to the south. Our group is studying the properties of these structures, which were probably blown by multiple supernova explosions, and appear to contain an energy in their hot gas equivalent to about 100,000 supernovae.


  • E+A galaxies - the missing link? The spectra of E+A galaxies are characterised by strong hydrogen Balmer absorption lines, and O[II] emission, although they have broadly the shape of early-type galaxy spectra. These spectra are typical of a galaxy with recent (~< 1 Gyr), but not on-going, star formation. There has been much debate about i) what induced the recent star-formation and ii) what has turned it off. In view of the recent star-formation, which requires a reservoir of cold gas in the parent galaxy, these galaxies are likely to represent an important link between disc-dominated, rotationally-supported gas-rich galaxies, and spheroid-dominated, pressure-supported systems. We have developed a robust and rapid novel algorithm, which we have applied to the huge number of early-type spectra now available from the Sloan Digital Sky Survey (SDSS). Hence, we have been able to ascertain the environments in which early-type galaxies with young stellar populations reside, and can conclude that the majority of these objects have most likely undergone a recent major galaxy-galaxy merger or interaction. We are currently comparing our results with those from cosmological simulations, and intend to extend this analysis to higher-redshift environments, to study the evolution of galaxy-galaxy merging.


    Colour-magnitude diagram, showing SDSS (0.06 < z < 0.122, M_R < -21) galaxies (black), early-types (red), early-types with young stellar populations found using our novel algorithm (green) and E+A galaxies (blue). The early-types with young stellar populations are situated in the so-called 'green valley', the relatively sparse region between the blue cloud of star-forming galaxies, and the red sequence of early-types. This strongly suggests that the green valley galaxies are a transition stage between star-forming and passive galaxies.

Researchers: Trevor Ponman, Louisa Nolan, Nicola Brassington

Page written by Trevor Ponman and Louisa Nolan, and was last updated on Fri 11 Nov 2005