Tag: Astronomy

Spiral galaxies are undoubtedly one of the most beautiful structures in the universe. Yet, their simple elegance belies a complex nature. How do such structures not “wind up” and what causes them in the first place? The answers to these questions is a long standing challenge. Under one model, spiral structure is created by spiral density waves. In another, they are induced by tidal interactions. It is this approach that is explored in a new paper by Dobbs et al., accepted for publication in the Monthly Notices of the Royal Astronomical Society. Specifically, the authors attempted to use modeling of tidal forces to recreate the structure of the spiral arms on the grand design spiral, M51.

To model the interaction, they began with a model of a simple galaxy with a mass distribution (broken into a disc, bulge, and halo) similar to that for M51. Their initial galaxy was initially free of spiral structure, but “gravitational instabilities in the stars [Note: as opposed to the galactic gas. Not in individual stars.] produce a multi-armed” and patchy spiral structure (known as a flocculent spiral). This flocculent nature was first predicted in a 1964 paper by Toomre and has been simulated numerous times since then. Dobbs’ team then introduced a point source to represent the smaller galaxy (NGC 5195) along the orbital parameters derived by previous simulations of Theis and Spinneker in 2003.

For the first 60 million years, significant new structure was not evidence. The disc showed some perturbation due to the approaching companion, but no new spiral structure arose. However, by 120 million years from the start of the simulation, hints of a spiral arm on the side of the galaxy closest to the companion begin forming and by 180 million years, two pronounced “grand design” spiral arms dominate the face of the galaxy, spanning over 15,000 light years.

But the arms were too good to last. By 240 million years, the arms only stretch to a mere 6,500 light years as the gravitational forces from the companion seem to shepherd the galaxy’s gas as it is pulled around in its orbit. By 300 million years, the spiral arms have grown again and the pair looks remarkably similar to the present state of the M51/NGC 5195 system.

The authors note several features their simulation has in common with the observed galaxy. On the side where the companion first approached the galaxy, they note a “kink” in one arm (labeled as A in image to left). Another similarity is a splitting of one of the spiral arms although, again, the exact positioning is different (labeled B).

Another comparison the authors made was to the strengths (or amplitude) of various arm patterns (1 arm, 2 arm, 3 arm, etc…) over time. They found that the two armed pattern was the most predominant, but from the mechanics, they determined there were underlying higher armed structures that never fully took hold. However, these higher armed patterns did come close to the strength of the 2 arm spiral. The authors note that this is consistent with the observational findings of another group studying M51 in a work that has yet to be prepared for publication.

However, there are also some differences. A plume of gas extended from the simulated M51 which has no counterpart in actual observations (labeled C). Actual observations show large amounts of gas in front of the companion galaxy which are not present to the same degree in the simulation (labeled D). Lastly, real observations show a noticeable flattening of M51’s arms closest to the companion. Again, these do not appear in the simulation. The authors suggest discrepancies may be due to the over simplistic modeling of NGC 5195 as a point source instead of an extended body, or slight differences in initial parameters when compared to the actual system.

Even with these differences, the authors suggest that their modeling of the interaction shows that spiral structure, at least in this case, is most likely the result of the tidal interaction on M51 by NGC 5195. They also note that spiral density waves are likely not the culprit since other studies have not been able to determine a consistent “pattern speed” for the galaxy (the pattern speed is the angular speed at which the arms would rotate if viewed as a coherent structure). Instead, observations showed that the arms should have different pattern speeds at different radii.

Although their work does not suggest that all spiral structure is formed by tidal interactions with companions, this work makes a strong case for the possibility in many galaxies which would have such companions and M51 in specific. Furthermore, the simulations also reveal that these tidally induced arms are a temporary phenomenon. Since they do not have a fixed speed, they will slowly wind up and as the interaction progresses, the galaxies will be further distorted and eventually merge.

(Thanks to Claire Dobbs for permission to reproduce images from the paper as well as clarification on a few points.)

As galaxies evolve, many lose their gas. But how they do this is a point of contention. One possibility is that it is used to form stars when the galaxies undergo intense periods of star formation known as starburst. Another is that when large galaxies collide, the stars pass through one another but the gas gets left behind. It’s also possible that the gas is pulled out in close passes to other galaxies through tidal forces. Yet another possibility involves a wind blowing the gas out as galaxies plunge through the thin intergalactic medium in clusters through a process known as ram pressure.

A new paper lends fresh evidence to one of these hypotheses. In this paper, astronomers from the University of Arizona were interested in galaxies that displayed long gas tails, much like a comet. Earlier studies had found such galaxies, but it was unclear whether or not this gas tail was pulled out from tidal forces, or pushed out from ram pressure.

To help determine the cause of this the team used new observations from Spitzer to look for subtle differences in the causes of a tail following the galaxy ESO 137-001. In cases where tails are known to be pulled out tidally (such as in the M81/M82 system), there “is no physical reason why the gas would be preferentially stripped over stars.” Stars from the galaxy are pulled out as well and often large amounts of new star formation are induced. Meanwhile, ram pressure tails should be largely free of stars although some new star formation may be expected if there is turbulence in the tail which causes regions of higher density (think like the wake of a boat).

Examining the tail spectroscopically, the team was unable to detect the presence of large numbers of stars suggesting tidal processes were not responsible. Furthermore, the disk of the galaxy seemed relatively undisturbed by gravitational interactions. To support this, the team calculated the relative strengths of the forces acting on the galaxy. They found that, between the tidal forces acting on the galaxy from its parent cluster, and its own centripetal forces, the internal forces where greater, which reaffirmed that tidal forces were an unlikely cause for the tail.

But to confirm that ram pressure was truly responsible, the astronomers looked at other parameters. First they estimated the gravitational force for the galaxy. In order to strip the gas, the force generated by the ram pressure would have to exceed the gravitational one. The energy imparted on the gas would then be measurable as a temperature in the gas tail which could be compared to the expected values. When this was observed, they found that the temperature was consistent with what would be necessary for ram stripping.

From this, they also set limits on how long gas could last in such a galaxy. They determined that in such circumstances, the gas would be entirely stripped from a galaxy in ~500 million to 1 billion years. However, because the density of the gas through which the galaxy would slowly become denser as it passed through the more central regions of the cluster, they suggest the timescale would be much simpler. While this timescale say seem long, it is still shorter than the time it takes such galaxies to make a full orbit in their cluster. As such, it is possible that even in one pass, a galaxy may lose its gas.

If the gas loss occurs on such short timescales, this would further predict that tails like the one observed for ESO 137-001 should be rare. The authors note that an “X-ray survey of 25 nearby hot clusters only discovered 2 galaxies with X-ray tails.”

Although this new study in no way rules out other methods of removing a galaxy’s gas, this is one of the first galaxies for which the ram stripping method is conclusively demonstrated.

Source:

A Warm Molecular Hydrogen Tail Due to Ram Pressure Stripping of a Cluster Galaxy