The Universe is full of rogue stars, rogue planets and galaxies that were tough for older telescopes to see

Millions of overlooked galaxies

When researchers took a closer look at surveys of galaxies in the local universe, they found many had been mischaracterised. More careful analysis of images revealed that 21 galaxies that originally looked like big 3D clouds of stars – "giant elliptical galaxies" – were actually flat 2D disc galaxies with bulges in the middle.

Those bulges have "exactly the same physical mass and compact size as the galaxies in the early universe," Graham says. This suggests that the vast majority of compact spheroids that were thought to have disappeared aren't actually missing – they've just grown a disc, possibly by gathering hydrogen gas and stars from smaller galaxies but without major mergers.

The results suggest that there are 1000 times more of these galaxies in the local universe than previously thought – roughly as many as there were in the early universe.



Astrophysical Journal - Hidig in Plain Sight: An Abundance of Compact Massive Spheroids in the Local Universe

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. This abundance of compact, massive spheroids in our own backyard 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.

Read more »