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 0.7 × 10 11 < M * /M ⊙ < 1.4 × 10 11 , and Sérsic 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 6.9 × 10 −6 Mpc −3 (or 3.5×10 −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 , 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.

Galaxy And Mass Assembly (GAMA): blue spheroids within 87 Mpc

“…Based on their results, Noeske et al (2006) suggested that the majority of BSph galaxies at high-z will evolve into small disk galaxies or low-mass spheroids 2 . They neither find any evidence suggesting inside-out growth scenario which could turn BSphs to large disks, nor do they see any disk growth around BSphs (but see Graham, Dullo, & Savorgnan 2015;Graham, Ciambur, & Savorgnan 2016;Graham et al 2017, for an alternative view). Lopes et al (2016) also investigated a sample of low-z galaxies in various environments.…”

Structural and Star-forming Relations since z ∼ 3: Connecting Compact Star-forming and Quiescent Galaxies

“…Simultaneously, the already existing quenched galaxies may also grow in size (horizontal red arrow) either by minor merging 13 or perhaps by accreting stars (and/or gas) that could rebuild a disk, which in some cases could host some star formation (diagonal up, not shown), and eventually lead to the formation of early-type (passive) disks (e.g., Bundy et al 2010;MargalefBentabol et al 2016), which nonetheless maintain the same stellar density within the central 1 kpc. The disk regrowth scenario could explain the apparent decline in the number of the most compact quiescent galaxies (defined as having small sizes of r 1 2 e < -kpc) with time (e.g., Cassata et al 2013;Damjanov et al 2014Damjanov et al , 2015Trujillo et al 2014;Ferré-Mateu et al 2017) since they would be hidden as the bulges of today's massive early-type disks (Driver et al 2013;Graham & Scott 2013;Graham et al 2015).…”

Galaxy And Mass Assembly (GAMA): $\mathcal {M_\star }\text{--}R_{\rm e}$ relations ofz= 0 bulges, discs and spheroids

“…Are elliptical galaxies simply naked bulges and are bulges related to high-redshift compact galaxies (e.g. Graham et al 2015;Berg et al 2014)? Are the discs of early-types, late-types and irregulars indistinguishable?…”