Authors: Dexter S. H. Hon, Alister W. Graham, Benjamin L. Davis, Alessandro Marconi
Abstract: The near-absence of compact massive quiescent galaxies in the local Universe implies a size evolution since z∼2.5. It is often theorised that such `red nuggets' have evolved into today's elliptical (E) galaxies via an E-to-E transformation. We examine an alternative scenario in which a red nugget develops a rotational disc through mergers and accretion, say, at 1≲z≲2, thereby cloaking the nugget as the extant bulge/spheroid component of a larger, now old, galaxy. We have performed detailed, physically-motivated, multi-component decompositions of a volume-limited sample of 103 massive (M*/Msolar≳1011) galaxies within 110 Mpc. Among our 28 galaxies with existing elliptical classifications, we found that 18 have large-scale discs, and two have intermediate-scale discs, and are reclassified here as lenticulars (S0) and elliculars (ES). The local spheroid stellar mass function, size-mass diagram and bulge-to-total (B/T) flux ratio are presented. We report lower-limits for the volume number density of compact massive spheroids, nc,Sph∼(0.17–1.2)×10−4 Mpc-3, based on different definitions of `red nuggets' in the literature. Similar number densities of local compact massive bulges were reported by de la Rosa et al. using automated two-component decompositions and their existence is now abundantly clear with our multi-component decompositions. We find disc-cloaking to be a salient alternative for galaxy evolution. In particular, instead of an E-to-E process, disc growth is the dominant evolutionary pathway for at least low-mass (1010≲M*/Msolar≲4×1010) red nuggets, while our current lower-limits are within an alluring factor of a few of the peak abundance of high-mass red nuggets at 1≲z≲2.