The interacting-galaxy pair NGC 5752/4 in Bootes, shown as a color composite from HST WFPC2 images. The inset is half-size; click for the full pixel scale, covering a region 91 by 108 arcseconds. This pair comprises half of the Arp 297 system, which looks like a compact group but consists of two galaxy pairs at quite different redshifts (NGC 5752/4 lie about 60 megaparsecs or 200 million light-years away). I illustrate this below with a diagram taken from a paper with Kirk Borne, overlaying the full field of the HST image and redshift information on a red-light picture from the Kitt Peak 4-meter telescope:
The large spiral is NGC 5754. Its internal structure barely shows disturbance, including a quite normal selection of star clusters for a bright spiral. The outer tidal structure (especially obvious in the 4-m picture above) does show tidal features, as does the symmetry of the inner spiral pattern and the kinked arms just beyond its inner ring. In contrast, the small companion NGC 5752 has undergone a starburst event, with a rich population of massive and luminous star clusters clumping around the nuclear region and intertwined with the intricate dust lanes. The energy release from the massive stars is driving an outflow of gas (much as we see in M82). The different responses of the two galaxies owe much to their very different masses and sizes. The structure of the system can be nicely fit from n-body simulation, as seen in this diagram from the same paper:
The lower sequence shows time steps in the simulation, while the right panel is an enlargement of the best fit tilted to match the viewing geometry of the real galaxies. We are seeing the galaxies about 250 million years after closest approach, in which NGC 5752 crossed the disk plane of NGC 5754 at an angle of about 60°. The weak star-formation response of NGC 5754 indicates that formation of the massive clusters often found in strongly interacting and merging galaxies requires a certain threshold of dynamical disturbance; a weak but long-lasting perturbation won't do the trick.
Image usage note: by NASA policy, all Hubble images are not subject to copyright (including those where the visualization comes from somewhere else than STScI). If you want to use this one and cite the source, the credit line would read W. Keel (Univ. Alabama), K. Borne (George Mason Univ.), and NASA.
Last changes: 11/2003 © 2003