Disc cloaking: Establishing a lower limit to the number density of local compact massive spheroids/bulges and the potential fate of some high-z red nuggets

The near-absence of compact massive quiescent galaxies in the local Universe implies a size evolution since z similar to 2.5. It is often theorized 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 less than or similar to z less than or similar to 2, thereby cloaking the nugget as the extant bulge/spheroid component of a larger, now old, galaxy. We have performed detailed, physically motivated, multicomponent decompositions of a volume-limited sample of 103 massive (M-*/M-circle dot greater than or similar to 1 x 10(11)) galaxies within 110 Mpc. Many less massive nearby galaxies are known to be 'fast-rotators' with discs. 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, n(c,) (Sph) similar to (0.17-1.2) x 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 multicomponent 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 (1 x 10(10) < M-*/M-circle dot (sic) 4 x 10(10)) 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 less than or similar to z less than or similar to 2.

Automated summary

This paper investigates the presence and evolutionary pathways of compact massive quiescent galaxies in the local Universe. Through detailed decomposition and classification, it is found that some existing elliptical galaxies are actually galaxies with rotating discs. The paper also reports indicators such as the bulge-to-total flux ratio and volume number density of local galaxies. The study suggests that disc-cloaking is an important alternative for galaxy evolution.