The Leo Ring
In 1983, astronomers discovered a large intergalactic cloud of hydrogen gas several dozen megaparsecs away. It was extremely large and massive, for a gas cloud: Initial measurements showed it to be twice the diameter of the Milky Way’s disk, and containing one billion solar masses of gas. It wasn’t a nebula, or a normal gas cloud, or a galaxy. What could it be?
Today, we call this cloud the Leo Ring — a nod to both its shape and its location, in the constellation Leo. It’s been studied quite a lot, and found to be twice as large as originally thought and comparable to a dwarf galaxy in mass. It’s been mapped in more detail over the past 35 years, and remarkable theories have been created as to how it was formed. The story of the Leo Ring is the story of those theories, and their implications for its evolution.
Neutral hydrogen and the 21-centimeter line
The Leo Ring is made up of neutral hydrogen gas, where each proton comes with an electron attached. It resembles in some sense a giant H I region — an interstellar cloud of similar composition. This is in contrast to H II regions, gas clouds that have been ionized, often by the winds of nearby stars. Now, the question of whether or not atoms in a cloud of gas are ionized may seem unimportant for astronomy, but it has major implications for how these clouds can be detected.
In an HII region, light is produced when protons and electrons recombine into neutral atoms. Ionized by photons from a star, their recombination sometimes places the electron in a high-energy state, where it can release a photon and fall to the ground state — the lowest energy level. If the photon isn’t absorbed by another electron — usually, if it’s stronger than needed to ionize an atom — then it will escape the gas cloud. In regions of neutral hydrogen, however, there’s no ionization and recombination. However, there’s a way for the atoms to radiate: the spin-flip transition.
Subatomic particles have discrete amounts of angular momentum, called spin. When an electron in a hydrogen atom is in the ground state, its spin is aligned with the proton’s spin in one of two ways: parallel (pointing in the same direction) or anti-parallel (pointing in the opposite direction). There’s a difference in energy between these two spin states, and if an electron in the high-energy state spontaneously changes its spin to enter the low-energy state — in a spin-flip transition — it can emit a photon, with a wavelength of 21 centimeters.
This transition is known as a forbidden transition, meaning that the odds of it happening are quite small. Therefore, it’s highly unlikely that any single hydrogen atom will undergo a spin-flip transition within a short timeframe. However, if you have an object with a large amount of hydrogen atoms — like an HI region, or the Leo Ring — enough atoms will undergo this transition that the object’s spectrum will have a strong emission line at the relevant frequency. This is the origin of the 21-cm line.
Discovering the Leo Ring
The Leo Ring was discovered by accident. A group of astronomers (Schneider et al. (1983)) were observing the 21-cm line in a galaxy in a neighboring portion of the sky and happened to find an additional signal. When it became apparent that it was a separate source of interest, they explored the region in more detail, still using the 21-cm line. Their first map showed an amorphous mass of no apparent shape.
Using the data, and treating it like a large HI region, the team found that the cloud contained about 1.6 billion solar masses. Its motion corresponded roughly to other galaxies in the vicinity. Assuming that it belonged to a specific galactic subgroup, GWa, it appeared that the cloud must be 10 megaparsecs away — which, in turn, meant it had a radius of about 50 kiloparsecs, making it about two times the size of our galaxy. This enormous size implied that it had a very low mean density — which would explain why no star formation was observed.
Furthermore, the fact that the cloud had not quickly dispersed indicated that there was more mass inside than just hydrogen gas — from 20 billion to 100 billion solar masses. Assuming a more conservative estimate on the lower end of that scale, the cloud couldn’t be a significant component in the motion and evolution of the galactic subgroup. Nonetheless, it was much larger and more massive than a normal HI region or tidal feature. The authors didn’t propose any explanations for the formation of the ring, but suggested monitoring for other clouds or gas in the area, or star formation.
The collision hypothesis
The first suggestion as to the origin of the cloud came in a paper published the following year (Rood & Williams (1984)). The authors pointed out that gaseous structures of similar mass and size had been found in two separate groups of galaxies: Stephan’s Quintet and Seyfert’s Sextet. The galaxies in these groups are interacting with each other gravitationally, and may collide and merge. It appears that neutral hydrogen gas has been stripped from individual galaxies in these groups through tidal forces, producing clouds somewhat similar to the Leo Ring.
Assuming that the gas cloud was the product of an interaction between two galaxies, those galaxies should still be nearby it. The astronomers searched the region and came up with a potential pair: NGC 3384, an elliptical galaxy, and NGC 3386 (also called M96), a spiral galaxy. Both are members of the M96 Group, and appear to lie approximately equidistantly to the cloud. Additionally, there were unexpected asymmetries in the HI distribution of NGC 3386, suggesting it had lost gas.
By measuring HI abundances in both galaxies and comparing them to expected abundances, the authors estimated that each galaxy could have lost roughly half a billion solar masses worth of neutral hydrogen, which coalesced to form the Leo Ring. By looking at the relative velocities of the galaxies, they determined that this collision should have taken place about 500 million years ago. The interaction also formed ring-like structures in both galaxies, which matched previous simulations of collisions. The close matches between the simulations and the observations provided strong evidence for the collision hypothesis.
Primordial or recent?
The debate over the ring’s origin was not finished. A second hypothesis — that the ring was a cloud of primordial gas from the early universe — rose in popularity. The argument was that if the gas had previously been part of the galaxies, it should have been enriched by heavier elements produced by stars through nuclear fusion. Therefore, attempts to detect other emission in the cloud could indicate the presence of stars or stellar by-products. Schneider et al. (1989) performed more complex mapping over a variety of frequencies.
Their HI observations confirmed the ring-like structure of the neutral hydrogen, which appears to encircle NGC 3384 and a neighboring galaxy, M105. NGC 3386, the other culprit behind the collision, lies on the other side of the ring. In addition to the HI mapping, they searched dense clumps in the cloud ni an attempt to find forming stars. Further emission measurements turned up little evidence for molecules of heavy elements. The astronomers concluded that the gas was a remnant of the galaxy group’s formation, stopped from collapsing by interactions with the galaxies.
20 years later, these null results appeared to be substantiated by new observations. Incredibly, a group of astronomers detected ultraviolet light coming from regions of the cloud (Thilker et al. (2009)). The fact that the emission was in the ultraviolet appeared to indicate that it came from young, blue, newly-formed stars. They also determined that the cloud had low metallicity, i.e. a small proportion of heavy elements. The team suggested that this was an indication that the gas was indeed primordial, and possibly in the process of forming a dwarf galaxy. Without previous star formation, there would have been virtually no heavy elements in the cloud.
However, there was one final twist in the saga. One year later, Michel-Dansac et al. (2010) announced the results of optical observations of the Leo Ring. Using the Canada-France-Hawaii Telescope (CFHT), they found optical emission coming from several HI “knots” in the cloud previously known to be the source of UV radiation. Analysis of these clumps showed them to be compositionally similar to structures formed in a ring in NGC 5291, two colliding galaxies. Additional simulations by the team showed that a collision between NGC 3384 and NGC 3386 was the most likely scenario for the formation of a giant HI cloud with these properties. The discovery of an optical counterpart to the other observations seemed to settle the debate.
The Leo Ring has been studied for 35 years, and remains perhaps the strangest and most mysterious HI structure in the known universe. Given its turbulent observational history, there might be more twists around the corner. However, the optical observations of 2010 appear to have swung the debate in favor of a collisional origin. As always, there remains more to study and learn about the cloud, which could provide insight into galaxy interactions, star formation, and the every-changing universe.
