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 that 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 kpc, stellar masses , and Sérsic indices typically around a value of n = 2-3. This abundance of compact, massive spheroids in our own backyard - with a number density of Mpc-3 (or 3.5 × 10-5 Mpc-3 per unit dex-1 in stellar mass) - and with the same physical properties as the high-redshift galaxies, had been overlooked because they are encased in stellar disks that 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, 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 that existed at z ∼ 2, it seems likely that some of the observed high-redshift spheroids did not increase in size by building (three-dimensional) triaxial envelopes as commonly advocated, and that the growth of (two-dimensional) disks has also been important over the past 9-11 billion years.