Interview with Curtis Struck - Galaxy Collisions

The study of galaxy collisions is a relatively recent field of research. Not until the 1920's was it discovered that previously observed ‘nebular’ thought to be located within the Milky Way were far more distant objects, discrete galaxies in their own right, did the fledgling field begin to take flight. Even then it was some years before we conceived the possibility that these far flung, remote objects could interact with each other and had been doing so since the early ages of the Universe.

A new book from Springer/Praxis, written by Curtis Struck Professor of Astronomy and Astrophysics at Iowa State University, tells two stories, firstly the history of this field nicely interwoven into the main content, the life story of the grandest assemblages of matter in the Universe and how their interactions have shaped, not just the way we see them, but our very existence.

We talked to Curtis about his new book and galaxy interactions in general

Curtis, the world of galaxies and their interactions is vast, beyond the imaginations of most people to even contemplate, how do you cope with such impossibly large concepts?

We cannot truly cope with these vast scales, so we work with human-scale images, mental or real. When I look at a galaxy image on my computer screen, the star clusters within it are maybe pixel scale dots. Perhaps, the observations are good enough that I can zoom in on a dot and see the morphology of that star cluster. When the galaxy and the cluster images are on my screen at the same time I am certainly cognizant of the relationship. However, I cannot at the same time visualize what it would be like to be looking out from the centre of a cluster of a million stars, or what one of the more massive bright blue stars might look like up close. It's just too much for the brain to take in simultaneously. We have to consider the different views and scales, the connections between them one-at-a-time. Taking it all in at once would be like looking at a mountain panorama, while simultaneously trying to look at all the individual outcrops through an array of telescopes. ... and thinking about the many timescales, it is at least this bad.

What was the driving force behind writing Galaxy Collisions?

Of course, I thought there was a need for such a book. Some earlier books discussed galaxy collisions as an aspect of the broad story of galaxy formation and cosmology. This was usually a brief treatment, and I thought there was a lot more to be said. Other books, like a recent one revisiting the Arp Atlas of Peculiar Galaxies, focused on observing those objects with small telescopes. I didn't see another book that told the scientific story of recent discoveries in any detail. Then there was the question of whether I could write the book I wanted. I had already written some pedagogical, more technical review articles on the subject. However, that didn't turn out to be nearly as much of a head start as I had hoped! A book for a more general audience is quite different.

There have been many eminent figures associated with the field of galaxy interactions. You mention names in the book such as Hertzsprung, Hubble and Zwicky. For you, which name(s) stands out as being a shining light?

The two names that stand out the most for me are Zwicky and (Alar) Toomre.

Zwicky was the pioneer who obtained the observations to show that interacting galaxies were not as rare as had been thought. They could not be ignored just because most galaxies seemed to fit well into the Hubble classification of normal galaxies. He also emphasized that those strange morphologies could be put into a regular classification scheme of their own, and he believed that they could be understood as the result of known dynamical principles.

More than any one else, Alar Toomre explained how that worked with his computer models and deep insights. These are two peaks on a broad landscape that has been filled in (though not completely!) by many other people.

What do you think is the most important breakthrough in our understanding of how galaxies interact?

Every decade since the 1950s has seen one or more important breakthroughs. The top 3 that I think really started this field are: 1) the recognition that galaxies can collide, they're not too isolated, 2) the realization that colliding galaxies usually merge except in special environments, 3) the discovery that collisions and mergers can greatly effect the star formation and nuclear activity in galaxies. The second item required the discovery of dark matter halos around galaxies. Verifying the third item has been a long process. The discovery history and science of these breakthroughs is described in some detail in the book.

The book describes different types of galaxy interaction. Which do you find the most interesting?

That is a tough question! However, as a theoretical astrophysicist I am most attracted to the symmetric collisions that produce what are to my eye, simple and elegant forms. The ring galaxies and beautiful two-armed spirals are particular favourites. By the way, this very much a minority opinion in the field. Most are attracted to the spectacular major mergers, and issues like whether they transform spiral galaxies into elliptical galaxies. But these systems are very complex dynamically. Intellectually, I think we are much more likely to figure out all the details of what is going on in the simpler systems.

If we only had resources to study one interacting system, which one would you choose? Which system has the most potential to increase our understanding in the field?

Would you let me count the Milky Way, and do I get to wait a few hundred million years or so when we're merging with M31 the Andromeda galaxy? Ok, I'm being facetious. There are several ways to approach that difficult question. The choice will be inevitably biased to nearby systems, because we can't see enough detail in distant systems to judge them. Granted that, which types are the most scientifically interesting? Well, it depends on what process you want to study. Right now, I might pick the Arp 82 system, for the variety of processes that can be studied in it. It has one of the longest tidal tails, which contains young star clusters. There are interesting waves and more star clusters in the disk. There is a bridge to the companion with more interesting features. Models suggest that this system has been interacting for a considerable time (even in galaxy time, measured in many millions of years). So there has been plenty of time for interaction-induced effects, but it is not yet merging with its companion. It has a little of everything. It's similar to the M51, the Whirlpool galaxy, which is another favorite. However, no beautiful Hubble image has been made of this one, in contrast to the Cartwheel and the Antennae (still other favourites). It's a bit of a researcher's secret, though there are a couple of images in the book.

Galaxy Collisions describes how dark matter makes up a large component of a galaxy’s mass. Are we any closer to solving the dark matter puzzle?

Maybe! Word is trickling into the scientific literature of tentative, but not statistically reliable, indications of detections in underground particle detectors. Although astronomical observations are good for revealing the distribution of dark matter, it's difficult to use them to reveal its nature. If dark matter consists of a new type of elementary particle, then the underground detectors may have the advantage. An astronomical solution is not impossible, for example, if dark matter consists of self-annihilating elementary particles that can produce observable photons. But even if this is the case, the observations are very difficult. I am hopeful that the dark matter puzzle will be solved before too many years, but if not, I am certain that more possibilities will be eliminated.

Galaxy Collisions include images from the HST and other telescopes. In your opinion, does one instrument stand out above the rest in terms of increasing our understanding of the large scale structure of the Universe?

Yes, the Hubble Space Telescope does stand above the others in what it has accomplished in the last couple of decades. However, as is apparent in the book, I have soft spots for a few less well known instruments. One is the VLA radio observatory, made famous in the movie Contact. It's ability to map hydrogen gas in the outer parts of galaxies makes it an especially sensitive detector of interaction dynamics. The VLA has recently completed a major upgrade (it's now called the eVLA), and so is even more sensitive.

The NASA GALEX (Galaxy Evolution Explorer) has given us some beautiful ultraviolet imagery over the last decade. It is a small space telescope, with only two imaging channels (now only one). However, ultraviolet observations are very sensitive to young stars, and GALEX has discovered a number of star-forming regions in tidal tails. Unfortunately, it is in the process of being shut down. The only way to make space-based UV observations will be with HST, but the demand for HST observations is so great that only a small fraction of those proposed can be carried out.

As is apparent in the book, the NASA Spitzer Space Telescope and the small optical telescope used to make the Sloan Digital Sky Survey are other favourites.

Galaxy Collisions talks a great deal about modelling galactic interactions with computers, today we can do an internet search and find java scripts that will model interactions reasonable well. How do these on-line scripts compare with the early models available to researchers?

These on-line scripts are indeed as good as the early models. The Galaxy Zoo Mergers project http://mergers.galaxyzoo.org/how_to_take_part has one such script that was derived from an old research code by a group led by my former student John Wallin.

What is next on the drawing board? Do you have plans for another book along the same lines of Galaxy Collisions? If so, when could we expect to see it published?

I'm primarily a researcher and college teacher. The book was an excursion off the regular path for me. I've re-immersed myself in teaching and research for the time being, and right now there are no more books on the drawing board. However, at the rate research in this field is going I could see a need to update Galaxy Collisions some years down the road, but we'll see how it goes.

Curtis, thank you for your time and the opportunity to chat

Paul Rumsby
For Best Astronomy Books, September 2011

Read Our Review of Galaxy Collisions