HIDING IN PLAIN SIGHT: AN ABUNDANCE OF COMPACT MASSIVE SPHEROIDS IN THE LOCAL UNIVERSE

• Published in: The Astrophysical journal 2015-05, Vol.804 (1), p.1-15
• Main Authors: Graham, Alister W., Dullo, Bililign T., Savorgnan, Giulia A. D.
• Format: Article
• Language: English
• Subjects: Abundance; Casing (process); Density; Galaxies; galaxies: bulges; galaxies: evolution; galaxies: formation; galaxies: fundamental parameters; galaxies: high-redshift; Spheroids; Stellar mass; Three dimensional; Universe
• ISSN: 0004-637X; 1538-4357; 1538-4357
• DOI: 10.1088/0004-637X/804/1/32

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.