Galactic Hydrodynamics

Grant Proposal

Figure 1: A Hubble Space Telescope view of an elliptical galaxy, on the left, and a spiral galaxy to its right, both within the Coma Cluster. Credit: STScI/GSFC.

Figure 2: Starting configuration for a galaxy simulation using a method known as smoothed particle hydrodynamics (SPH). Each dot represents a gas particle carrying information about position, velocity, acceleration, internal energy, entropy, pressure, and density. There are 200,000 dots in this particular simulation.

"The goals of this project are to elucidate the hydrodynamic histories of elliptical galaxies, and to constrain physical nature of feedback, from star formation, supernova, and AGN, using the new fundamental correlations revealed by Chandra, coupled with detailed hydrodynamical simulations. These goals will be achieved through a program of simulations of progressively increasing complexity, designed to isolate the effects of, in turn, quiescent AGN feedback, merger-induced AGN feedback, and merger-induced star formation and supernova feedback."

- Tom Statler & Chris Freyer, "The Hydrodynamic Histories of Elliptical Galaxies" (grant proposal)

What does that mean?

Let's start with the big picture. There are several different kinds of galaxies. You are probably familiar with pictures of the Milky Way galaxy where we live, characterized by curling spiral arms around a yellow bulge, kind of resembling an egg yolk. This is called a spiral galaxy, and it is NOT what I'm studying. Instead, I simulate what are known as elliptical galaxies, which look basically like large blobs of gas, roughly shaped like ellipses. I'm interested in how these galaxies evolve over time. A LONG time.

Galaxies change with time because there are forces which act on them. The most familiar force is gravity, or the attraction between masses. The first step to understanding why the universe evolves is to remember that everything pulls on everything else through gravity. Every cluster of galaxies pulls on every other cluster of galaxies. Every individual galaxy within the cluster pulls on every other galaxy within the cluster. Every star within the galaxy pulls on every other star within the galaxy. Every atom pulls on every other atom, and so on and so forth.

The magnitude of the gravitational force (i.e. how strong is it) depends on two things. First, the masses of the objects involved. The more massive the objects are, the greater the gravitational attaction. Your weight is based on the gravitational attraction between the earth's mass and your mass. You weigh more than a hamster because you are more massive than a hamster. You weigh less than an elephant because you are less massive than an elephant.

The other factor in determining the strength of gravitational pull is the distance between the gravitating objects. The closer they are, the stronger the gravity. In some respects, gravity is analogous to magnetism (another force, but not as important for galaxy evolution). Imagine you have two magnets. Far away, they don't seem to be attracted to each other much, but bring them close together, it's hard to keep them apart. Gravity works the same way...but on much larger distance scales.

Now that we've established the important properties of gravity, let's ask what might deter the force of gravity, i.e. why might two objects NOT want to pull together? The answer to this question has to do with ENERGY. Suppose you are babysitting two small children, and you want them to go to bed. The more energy they have, the more difficult it is settle them down. However, if you allow them to run around and expend their energy, it becomes much easier to put the tired children to bed. Similarly, it is more difficult to pull two very energetic (speedy or hot) objects together through gravity than it is to attract two sluggish, cool objects together.

So why does this matter for galaxies? Remember, each atom pulls on every other atom. Conceivably, there could be a pocket of gas where the atoms are closer together than in the rest of the galaxy. Then we say the density of this pocket is higher than elsewhere in the galaxy (density = how much stuff is compacted into how much space). Also suppose that the temperature of this dense pocket of gas is relatively low compared to the rest of the galaxy. Then this cool, dense gas gravitates together much more easily than the surrounding gas. We say this gas pocket collapses further in on itself. When a certain critical density is reached, we've just described the ideal conditions for star formation. The phenomenon where gas collapses to a certain density and forms stars is called COOLING.

Remember that energy hinders gravitational collapse. When gas collapses into stars, it ignites through nuclear fusion, and the star begins to "burn." Thus, the star begins to radiate away its energy, returning that energy to the surrounding gas. The gas, now heated by the star, is no longer cool enough to continue collapsing into stars. This process where energy is returned back to the gas is called FEEDBACK. Normal stellar radiation is not the only source of feedback; energy is also returned to the gas when stars undergo a violent explosive death known as a supernova. Feedback is also evident in radiation from accreting super-massive objects near the centers of galaxies, known as active galactic nuclei, or AGN.

The idea is that cooling should balance feedback, such that the galaxy maintains a state of quasi-equilibrium over a long time (think hundreds of millions of years) but not necessarily over short time periods.

Equilibrium is a term describing a state of stability; a system in equilibrium tends to stay in equilibrium unless disrupted. A good example of equilibrium might be a very still and quiet pond - you disrupt its equilibrium state by dropping a big rock in the middle. In the case of elliptical galaxies, the equilibrium described here is one of hydrostatic equilibrium, meaning the gravitational forces pulling inward are balanced by the pressure forces pushing outward. In reality, ellipticals are not in a state of perfect hydrostatic equilibrium, but in most cases this is a reasonable approximation.

I study x-ray emission from hot gas in normal elliptical galaxies. I'm interested in the hydrodynamical history of this gas, as it cools through bremssrahlung and line emission and flows inward, or is reheated through various forms of feedback (i.e. from stellar wind, supernovae, AGN activity). The balance between cooling and feedback is a major point of interest.

The technique I use is called SPH, or smoothed particle hydrodynamics - a Lagrangian code in which fluid attributes are modeled as individual particles. Then information about position, speed, acceleration, temperature, density, etc. is encoded in each particle.

Specifically, I want to constrain the nature of feedback processes by running simulations using SPH, and discard those models which fail to match x-ray observations of morphological asymmetry, temperature gradient, x-ray isophotal ellipticity, and x-ray luminosity.