merger rates

I spent the morning re-reading papers about merger rates, to improve the discussion in our merger-rate estimate paper. I was reminded of how stark the difference is between studies that estimate the merger rate morphologically (ie, by identifying probable merging galaxies by tidal features or asymmetries in their appearances) and studes that estimate it by close pairs. The morphological studies all get much higher rates, which says to me that either morphological asymmetries are raised by very small accretion events, or else that they last many dynamical times. The close-pair rates have the virtue that—if done correctly—they can provide a strict upper limit on the merger rate, so the fact that the close-pair studies come in lower means that the morphological studies are biased high.