Hiding in plain sight: An abundance of compact massive spheroids in the local Universe

Abstract

It has been widely remarked that compact, massive, elliptical-like galaxies are abundant at high redshifts but exceedingly rare in the Universe today, implying significant evolution such that their sizes at z ~ 2+/-0.6 have increased by factors of 3 to 6 to become today's massive elliptical galaxies. These claims have been based on studies which measured the half-light radii of galaxies as though they are all single component systems. Here we identify 21 spheroidal stellar systems within 90 Mpc that have half-light, major-axis radii R_e < ~2 kpc, stellar masses 0.7x10^{11} < M_*/M_Sun < 1.4x10^{11}, and Sersic indices typically around a value of n=2 to 3. This abundance of compact, massive spheroids in our own backyard - with a number density of 6.9x10^{-6} / Mpc^3 (or 3.5x10^{-5} / Mpc^3 per unit dex in stellar mass) - and with the same physical properties as the high-redshift galaxies, had been over-looked because they are encased in stellar disks which usually result in `galaxy' sizes notably larger than 2 kpc. Moreover, this number density is a lower limit because it has not come from a volume-limited sample. The actual density may be closer to 10^{-4} / Mpc^3, although further work is required to confirm this. We therefore conclude that not all massive `spheroids' have undergone dramatic structural and size evolution since z ~ 2+/-0.6. Given that the bulges of local early-type disk galaxies are known to consist of predominantly old stars which existed at z ~ 2, it seems likely that some of the observed high redshift spheroids did not increase in size by building (3D) triaxial envelopes as commonly advocated, and that the growth of (2D) disks has also been important over the past 9-11 billion years.