The Local Merger Rate of Disk Galaxies

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Session 20 -- Mergers, Masers and Megamasers
Display presentation, Wednesday, January 12, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[20.02] The Local Merger Rate of Disk Galaxies

W.C. Keel, W. Wu (U. Alabama)

We consider a sample of galaxy mergers, defined solely from morphological criteria without bias as to IR or UV properties. The statistical propertis of the sample itself can be used to give an effective magnitude limit and solid-angle coverage, so that we may derive the luminosity function. The local merger rate results from comparison of the luminosity function of mergers and their remnants to that either of double galaxies (the predominant parent population), or for normalization purposes, the total luminosity function of galaxies. We include the effects of changing relaxation timescale with luminosity, by including dynamical and photometric data where available. The current merging rate for galaxies in bound pairs is estimated at 1.3 per Hubble time per galaxy. Normalized to all galaxies, the rate is currently about 0.1 per galaxy per Hubble time. Together with recent HST results, this shows that the merger rate is decreasing with time faster than the galaxy density - that is, the merger rate per galaxy in the universe was much higher than we see now. A two-phase model for the role of merging may be appropriate, where galaxies in bound pairs merge rapidly and non-paired galaxies are almost immune to the process. The HST counts show, in this case, that most of the primorial pair population has now been lost to merging.

Using this sample, we can also address the typical intensity and duration of merging-induced starbursts. Remarkably, much of the star-formation signature is hidden by dust when most intense, so that the optical luminosity is not strongly affected. This can simplify calculations of the effects of merging at moderate redshift.

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