On the Continuous Formation of Field Spheroidal Galaxies in Hierarchical Models of Structure Formation

We re-examine the assembly history of field spheroidals as a potentially powerful discriminant of galaxy formation models. Whereas monolithic collapse and hierarchical, merger-driven, models suggest radically different histories for these galaxies, neither the theoretical predictions nor the observational data for field galaxies have been sufficiently reliable for precise conclusions to be drawn. A major difficulty in interpreting the observations, reviewed here, concerns the taxonomic definition of spheroidals in merger-based models. Using quantitative measures of recent star formation activity drawn from the internal properties of a sample of distant field galaxies in the Hubble Deep Fields, we undertake a new analysis to assess the continuous formation of spheroidal galaxies. Whereas abundances and redshift distributions of modelled spheroidals are fairly insensitive to their formation path, we demonstrate that the distribution and amount of blue light arising from recent mergers provides a more sensitive approach. With the limited resolved data currently available, the rate of mass assembly implied by the observed colour inhomogeneities is compared to that expected in popular Lambda-dominated cold dark matter models of structure formation. These models produce as many highly inhomogeneous spheroidals as observed, but underpredict the proportion of homogeneous, passive objects. We conclude that colour inhomogeneities, particularly when combined with spectroscopic diagnostics for large, representative samples of field spheroidals, will be a more valuable test of their physical assembly history than basic source counts and redshift distributions. Securing such data should be a high priority for the Advanced Camera for Surveys on Hubble Space Telescope