Astrophysics

Era of the compact disk

Some of the strangest galaxies in the Universe just got stranger. It seems that many galaxies in the early Universe not only packed a huge number of stars into a very small volume, but were also rotating rapidly.

By historical standards the Universe is a boring place. Most galaxies have settled down into the routine of middle age and have all but given up on exciting activities such as forming new stars or fuelling black holes. Studying the light of very distant galaxies with powerful telescopes, astronomers are finding that things were a lot more interesting in the first third of the Universe's history. The most striking aspect of galaxies in those early epochs is their variety1,2. Some were forming stars hundreds of times faster than the Milky Way; others were dominated by accretion flows onto extremely massive black holes in their centres; and still others were crashing into one another to form larger galaxies. At the same time, a class of compact, massive galaxies existed that had already stopped forming new stars. Writing in The Astrophysical Journal, van der Wel et al.3 now argue that the majority of these galaxies were disk-shaped.

These compact, massive galaxies are among the most puzzling objects in the young Universe. They seem to have the same number of stars as fully grown galaxies in the present-day Universe. However, their sizes are four to five times smaller and their densities are a hundred times larger4,5 than their present-day counterparts. The surprising discovery of these galaxies a few years ago spurred a flurry of studies, which aim to understand how these massive, compact galaxies formed and how they subsequently managed to grow into the much puffier galaxies we have today.

The installation of a new, very sensitive camera on the Hubble Space Telescope in 2009 offers astronomers the ability to study these remarkable galaxies in much greater detail than before. In their study, van der Wel and colleagues3 went beyond a simple size measurement and determined the morphologies of 14 compact galaxies at a time when the Universe was only about 3 billion years old. The authors concentrated on the degree of flattening of the galaxies. Today's massive galaxies are essentially big balls of stars and tend to be nearly round, whereas lower-mass galaxies such as the Milky Way tend to be elongated because their structure is dominated by a flat, spinning disk of gas and stars. Confirming earlier studies5,6 the authors find that a subset of the massive compact galaxies appear highly flattened. Taking into account that some of the apparently round galaxies could be flat disks seen from the top rather than from the side, van der Wel and co-authors argue that most compact galaxies in the young Universe could in fact be spinning disks (Fig. 1).

Figure 1: Formation of a compact disk galaxy.
figure1

NASA/ESA/HUBBLE HERITAGE TEAM (STSCI/AURA)

According to van der Wel et al.3, most compact massive galaxies in the young Universe were disk-shaped. During their formation, these galaxies probably resembled the nearby galaxy M82, seen here. The blue light is from young stars in a spinning disk. The red light is from ionized hydrogen gas that is being expelled from the galaxy as a result of supernova explosions. Compact disk galaxies forming in the young Universe may have looked even more spectacular than M82, as they were about the same size but ten times more massive.

The case is not yet conclusively proved, because a much larger sample of galaxies needs to be studied to robustly measure the average elongation of compact galaxies. Furthermore, the galaxies may be cigar-shaped (prolate spheroids) rather than disk-shaped. Van der Wel et al. argue that prolate galaxies are rare in today's Universe, but so are rapidly rotating, extremely compact disks. The real test is whether these galaxies do, in fact, rotate, which would confirm that they are disks. Such a test is just beyond the capabilities of today's telescopes7, but it may be possible to measure the rotation of a similar object that is closer to us, or of a galaxy that happens to be gravitationally lensed — that is, has its light bent and magnified — by a foreground object.

If it is confirmed that these distant galaxies are indeed disks, they would be unlike anything seen in the Universe today. On the basis of the sizes of the galaxies and their masses, the implied rotation speed would be an astonishing 700 kilometres per second. To put this into perspective, the Sun moves around the centre of the Milky Way at a relatively sedate pace of about 230 kilometres per second. This is all the more remarkable given that today's descendants of the compact galaxies are thought to be elliptical galaxies, which do not rotate much at all. It may be that the disks did not survive for long. They may have been destroyed in collisions with other galaxies, which probably occurred frequently during the 10 billion years separating compact galaxies from elliptical galaxies. Some of the disk signatures might be expected to survive this onslaught, and in this context it is interesting that the central regions of elliptical galaxies are often kinematically distinct from the outer regions8,9.

The existence of compact disks of stars in the young Universe would also imply the existence of similarly compact disks of gas at even earlier times — in the first billion years after the Big Bang. Given their densities, these gas disks would mostly have consisted of molecular hydrogen and helium, which would in turn imply that they converted gas to stars at a ferocious rate while feeding rapidly growing black holes in their centres. It may be possible to detect and characterize these truly spectacular early phases of massive galaxy formation with the Atacama Large Millimeter Array, which is under construction in Chile.

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Correspondence to Pieter van Dokkum.

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van Dokkum, P. Era of the compact disk. Nature 473, 160–161 (2011). https://doi.org/10.1038/473160a

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