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 scenarios. 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 definitive for precise conclusions to be drawn. In this paper we discuss whether more detailed considerations involving the resolved internal colours of morphologically selected galaxies of known redshift from the Hubble Space Telescope can assist in making progress. With the limited resolved data currently available in the Hubble Deep Fields, we find popular hierarchical models produce as many spheroidals with highly inhomogeneous colours as observed, but they underpredict the proportion of homogeneous, passive objects at redshifts z ≃ 1. This suggests that while the star formation rate in spheroidals at low redshifts (z ≲ 1) is predicted correctly, the formation rate at higher redshifts is underestimated. Although more detailed tests of galaxy evolution inevitably involve additional complexities compared to the classical tests based on counts and integrated colours alone, we argue the time is right for embarking on more detailed comparisons, particularly in view of the resolved data expected for large numbers of distant spheroidals from the recently commissioned Advanced Camera for Surveys on the Hubble Space Telescope.