A Search for the Most Massive Galaxies: Double Trouble?

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Abstract
We describe the results of a search for galaxies with large (≥350 km s-1) velocity dispersions. The largest systems we have found appear to be the extremes of the early-type galaxy population: compared to other galaxies with similar luminosities, they have the largest velocity dispersions and the smallest sizes. However, they are not distant outliers from the fundamental plane and mass-to-light scaling relations defined by the bulk of the early-type galaxy population. They may host the most massive black holes in the universe, and their abundance and properties can be used to constrain galaxy formation models. Clear outliers from the scaling relations tend to be objects in superposition (angular separations smaller than 1''), evidence for which comes sometimes from the spectra, sometimes from the images, and sometimes from both. The statistical properties of the superposed pairs, e.g., the distribution of pair separations and velocity dispersions, can be used to provide useful information about the expected distribution of image multiplicities, separations, and flux ratios due to gravitational lensing by multiple lenses, and may also constrain models of their interaction rates.