The Comet Galaxy’s mysterious blue knots
What’s the final fate of these blue clumps?
For the time being, the Milky Way is a pretty stable place to be. Sure, in several billion years, it will collide with our neighbor, Andromeda, but for now, its shape and structure are relatively unperturbed. The Milky Way is one of two major components of the Local Group of galaxies, along with Andromeda, and so it dominates all intergalactic interactions in its vicinity. This includes stripping stars from some of its satellite galaxies, including the Sagittarius Dwarf Spheroidal Galaxy, a reminder that galaxy clusters can be remarkably dynamic places.
Not all galaxies are so lucky; many interact with their neighbors, which can lead to mergers or merely the stripping of gas and dust. Famous examples are the Whirlpool Galaxy, the Mice Galaxies, and the Antennae Galaxies. I’ve talked about the dramatic results in previous posts; the Leo Ring and Hoag’s Object are notable examples. What’s interesting about all of these cases is that they feature a galaxy interacting with just one companion, and not their parent galaxy cluster as a whole.
When a galaxy clashes with a larger group, the effects can be even more dramatic. Galaxy clusters aren’t simply a bunch of galaxies thrown together; they include intracluster gas and more complicated dark matter halos than those found around individual galaxies. Therefore, cluster-galaxy interactions can be highly complex, and produce fascinating results. One excellent example is the Comet Galaxy, situated in the cluster Abell 2667. The Comet Galaxy is moving towards the center of the cluster at speeds between 1000 and 1730 kilometers per second; in the process, it encounters extremely hot gas in the intracluster medium. The pressure from the collision strips gas and stars from the galaxy, forming a tail several times as long as the Milky Way.
Initial measurements
The key to the collision is something called ram pressure, which is a common phenomenon in astrophysics, especially in the study of gas and dust. When an object moves through a fluid, the collision causes a drag on the object, which in turn applies a pressure equal to the density of the liquid times the squared velocity of the object relative to the fluid. In short, the faster the incoming object is moving, and the denser the medium, the more pressure will be applied. This means that for objects with low speeds relative to their surroundings experience little ram pressure, while objects with high speeds — such as certain galaxies — are strongly affected.
The Comet Galaxy was discovered by a team (Cortese et al. (2007)) looking at images of massive galaxy clusters taken by the Hubble Space Telescope. It was one of two examples exhibiting strange “knots” of blue stars, and designated 235144−260358, along with 131124−012040 (found in the cluster Abell 1689). The blueness indicated that the stars were young, as blue stars — usually O-type and B-type — are typically short-lived main sequence stars that live for at most a few tens of millions of years. Therefore, collections of blue stars indicate recent or ongoing star formation, and the fact that blue stars are young indicated that they must still be near their places of birth — in other words, the blue knots were places of star formation.
The strange thing was that in both cases, the clumps were offset from the associated galaxy, implying some very recent or ongoing interactions. Star formation is often triggered by interactions with a neighboring galaxy, so the astronomers decided to observe the systems in more detail. In addition to the optical data from Hubble, they procured data at near-infrared wavelengths from the Very Large Telescope (VLT), radio wavelengths from the Very Large Array (VLA), and x-ray wavelengths from the Chandra X-ray Observatory. Spectroscopic data from the VLT in optical bands also proved useful.
Analysis and evolutionary models
After reducing the data, they were able to determine certain properties of the systems. They found the Comet Galaxy’s parent cluster, Abell 2667, to have a redshift of 0.2265, implying a distance of over 3 billion light-years. They also found several peculiarities in the galaxy itself, including a disrupted spiral arm, stripping of gas inside the galactic disk, and possible star formation near its center. The latter phenomenon manifested itself as a sharp increase in brightness near the galactic nucleus, and low x-ray emission ruled out an active galactic nucleus as an alternate source of luminosity. The star formation rate was quantified as 53 stellar masses worth of stars per year, much higher than that in the Milky Way, and leads to extreme emission at infrared wavelengths; the galaxy was therefore categorized as a Luminous Infrared Galaxy (LIRG).
The knots of stars were found to extend for 70 to 140 kiloparsecs, several times the diameter of the Milky Way. Tantalizingly, they were found to have magnitudes similar to low-mass dwarf galaxies and high-mass super star clusters; their location, opposite to the Comet Galaxy’s direction of motion, implied that they had been stripped from the galaxy by ram pressure. The peculiar features inside the galaxy supported this hypothesis. Additionally, gas was observed between the knots consistent with a “tail” trailing behind the galaxy, showing that the knots were part of this larger structure. Spectroscopic measurements indicated that the star formation must have begun anywhere from 5 million to 1 billion years ago, depending on the star formation model.
In both cases, the group constrained the velocity of the galaxies relative to the cluster to be between 1000 and 1730 kilometers per second, which is quite fast — several times larger than the Milky Way’s escape velocity and about 50 times Earth’s speed around the Sun. Tidal forces from the cluster likely drove gas into the center of the galaxy, triggering star formation there. Farther out, ram pressure was in the process of stripping gas from the galactic disk; the high speed of the galaxy makes this an efficient process. Stripping by ram pressure was much more apparent in 131124−012040 than in the Comet Galaxy; the former has had most of its gas stripped, while the latter is still in the early stages of gas loss. This is largely because the stripping is more effective on low-mass systems than high-mass systems, and so low-mass systems, like 131124–012040, will not experience the central star formation seen in the Comet Galaxy.
Blue knots and ultra-compact dwarfs
The authors investigated the blue knots in more detail. The knots had only been seen before in one other cluster, Abell 2125 (in the galaxy C153), which contains massive gas clouds. Their shape, structure and luminosity made them particularly interesting, as they were all similar to the characteristics of dwarf galaxies. While it wasn’t possible to tell precisely when star formation began, the stuctures formed between 50 and 150 million years ago; they couldn’t have formed from the intracluster medium itself because other galaxies in the cluster would have destabilized them.
Assuming the knots remain stable, it appears possible that they could be the progenitors of ultra-compact dwarf galaxies (UCDs), a class of small, high-density dwarf galaxies that have been discovered and studied in recent years. Several hypotheses for their formation exist: tidal stripping of dwarf galaxies (Bassino et al. (1994)), primordial perturbations in regions of dark matter (Drinkwater et al. (2004)), and mergers of super star clusters (Fellhauer & Kroupa (2002)). The astronomers argued that these knots have the potential to merge in the future to form an ultra-compact dwarf, supporting the third theory of formation.
Since 2007, a large number of jellyfish galaxies like the Comet Galaxy have been discovered, which exhibit cometary tails, often with accompanying star formation. Ram pressure remains the most likely mechanism for tail formation (see Ebeling et al. (2014)). Hopefully, as more and more UCDs and jellyfish galaxies are discovered, more blue knots will be found, and we’ll be able to piece together a better picture of UCD formation and evolution.
