Building Galaxies

Zwicky 18I just finished watching the Universe series episode on “Alien Galaxies.” I have to admit that their constant use of the word “Alien” forced me to look up the word alien in the dictionary (or at least on I have to admit that while it is legitimate to call galaxies alien, it’s probably a bit of a stretch of the definition.

That aside, this episode did a good job high lighting all the different types of galaxies that are our there. What it didn’t discuss (let’s face it, you can only do so much in 50 minutes), is how this zoo of galaxies represents a continuum of evolution.

A paper published by Searle and Zinn in 1978 and substantiated by countless papers since then, states one of the possible explanations for galaxy formation is hierarchical clustering. In this model, small clumps form and merge to form larger clumps, which merge to form even larger clumps. We can see this happening today as we look around our own local group of galaxies. Small dwarf galaxies, like the Sagittarius dwarf spheroidal galaxy, are constantly getting consumed by the Milky Way galaxy. These smaller systems are adding their mass to the Milky Way, and both bringing in new material and shaking things up a bit. Past interactions have caused the disk of our galaxy to bloat up, forming the thick disk. They have brought metal poor stars to the local neighborhood of stars, and they have even placed globular clusters – gravitationally bound clusters of thousands of stars – in orbit the wrong way around the galaxy.

It is hard to know exactly what the original population of galaxies looked like. From extremely deep images of the sky we know that young galaxies in the early universe were distorted and blue with star formation (only very young stars can be blue. While a red star could be any age, only systems with on going or just completed star formation have blue stars). The image above left (credit: Y. Izotov (Main Astronomical Obs., Ukraine), T. Thuan (Univ. Virginia), ESA, NASA) shows an image of a diminutive, 500 million year old galaxy (for reference, the Sun is ~4.5 billion years old). We assume that galaxies formed in a variety of different sizes, with the largest areas of extra mass – the largest over densities – forming larger galaxies while areas with less mass formed smaller galaxies or just clouds of gas.

These early collections of material tended to clump up, with galaxies forming groups and clusters, and these groups and clusters forming super clusters. In the process of clumping up, somehow spiral and elliptical galaxies were formed. How you go from blob of gas and stars to spiral galaxy is a bit of a mystery. Hopefully, the next generation space telescope, James Webb, will allow us to peer at younger galaxies and catch snap shots of galaxies in the process of forming.

At one stage during the universe’s construction process, these larger spiral and sometimes elliptical galaxies began to actively consume gas rich small galaxies in large numbers. While not all active galaxies are spiral galaxies, many of them are, and these spectacular systems have amazingly illuminated cores that contain supermassive black holes. Depending on how much material is being consumed and what angle we are looking at the galaxy from, we classify these busily eating systems as quasars, Seyfert galaxies or just active galaxies. In reality, these systems are all just a continuum of the same object. This different names come from not being able to, well, watch a galaxy grow up. Imagine you stumbled across 3 random humans: a 4 day old baby, a pregnant woman, and a 90 year old crone. Would you recognize them as all belonging to the same species? It takes us time to sort out the evolution of a form, especially when that evolution can take billions of years and humans have only had telescopes pointed at the stars for 400 years. We’re getting there however.

And as out understanding grows, we’re realizing that we are living in a nice calm age. The active galaxies are dieing out as gas rich food becomes scarce. The number of collisions is decreasing, and the amount of local fireworks is minimal.

When talking about galaxies, the dead are red. Without dust, without star formation, the universe grows monochrome.

But we’re not to that stage yet. Things are just slowing.

It’s a middle aged universe out there.


  1. Jake August 14, 2007 at 11:03 pm #

    “Small dwarf galaxies, like the Sagittarius dwarf spheroidal galaxy, are constantly getting consumed by the Milky Way galaxy.”

    SO, I am curious, (and please forgive me for sounding presumptuous…if thats the right word) but when you state that these dwarves are being ‘constantly’ consumed by larger galaxys. Are you referring to a large, both past and future, timeline? Or does this type of event happen rather quickly once it has been set in motion?

    I’m aware that the timespan for galactic events is far grander than that of our own. But, can you actually see these events unfold, or is it theoretical based on many different galaxy clusters and the similarities?

    I suppose in an overall context I was wondering if you could define ‘constanly consumed’. 3 in a billion years, 20?

    Anyways, great articles here. Love the podcast.

  2. Jake August 14, 2007 at 11:05 pm #

    Sorry to add on here, I forgot to ask. Is the monochrome thing a physical attribute or is that just a figure of speech.

  3. Andrew M August 18, 2007 at 12:55 pm #

    Hi Pamela,

    Great post, thanks for the insight. I’m adding your thoughts on intergalactic collisions to my list of things we want to learn from the Webb Telescope program. Hopefully by the time it launches we’ll have a long list of topics to explore.

  4. Lonewolf Astronomer April 19, 2008 at 4:01 pm #


    Thanks for the great explanation and synopsis of the Hierarchical Clumping paradigm. How far have we gone though and how safe is it to say that the other method, Monolithic Collapse is no longer the theory of choice?

    — a former graduate student


  1. Webb Telescope Pictures » Blog Archive » Building Galaxies - Intergalactic Collisions - August 18, 2007

    […] Pamela Gay over at the Star Stryder blog wrote a great post a few days ago that clearly outlines galaxy formation for beginners. She also postulates that the Milky Way is a middle-aged galaxy and, “…as our […]

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