Thursday, August 13, 2009

What I have been doing this summer

So I started out this summer by being a TA which went well (I haven't received any death threats from my students). The rest of my summer has been focused on research, specifically learning how to model things on the computer. As of right now I have two co-advisors, one who focuses on the computation aspect of our project, and the other on the observation aspect. I am supposed to be the one that translates between the two and keeps them both up to speed on what the other is doing. In the process I learn both computation and observation (and maybe some instrumentation). So far I am learning a lot and the project I am working on seems fun.

We are focusing on two galaxies to get a good idea of what is happening in their cores in order to understand AGN and galactic blowout. What is galactic blowout? That can best be explained with a picture. This picture was taken using Hubble Space Telescope by my advisor Gerald Cecil a while back. It is an image of central region of NGC 3079.You can see the expanding bubble of gas in the middle. It is this expanding bubble that we are interested in. Notability the curvature of the filaments the source of the bubble and the circumstances that made it form. My job is to figure out a way to model this as realistically as possible.

For that I have been learning how to use a hydrodynamics code called Athena. There were other codes I could have used (like VH-1 for example) but my other advisor Fabian Heitsch recommended Athena for this type of project. So I downloaded Athena and went to work figuring out how to use it. I would say that it is a very well written code (of the little that I have used it) and that it is very intuitive and easy to learn (it's written in C). Right now I have been making up some toy models to get a good sense of how it works and what I can do with it. Recently (this week) I have been working on getting the data out and into an interesting format (one that I can show people and wow them with). On that note I have a short video for you guys.

What I have here is an extremely simple (emphasis on extremely, and simple) model of the disk of a galaxy (a constant density disk with exponential fall off to halo densities, not very realistic but for now it works). I have insterted a "starburst" in the center with an over-pressure region (luminosity/supernova) along with some wind (i.e. kinetic energy, matter outflow). The computation is only doing 2D, and is small enough that I am running it on my laptop.

Here the x-y plane represents the computaion grid (250 grid points in the x, 500 in the y). The z axis is density. I took the output from Athena and fed it into MATLAB and turned it into a movie that I am posting here. I hope you enjoy. When I have real stuff (with good physical interpretation) I will post about that.


  1. It's always nice when you don't get death threats from students. :) (I've TAd enough times in Grad school to especially find this funny.)

    Your research sounds really interesting Ryan. Galactic physics is really cool and we're blessed to have someone here doing that kind of reseach. (Someone needs to help at least me undersand AGNs better.)

    Also, that is a great video.

  2. It's also really interesting how much good research involves hydrodynamics. This stuff seems to pop up everywhere. Also, the it produces videos like the above are always cool.

  3. Hydrodynamics is in a lot of good research because turbulence is one of the biggest unsolved problems in physics. It may not be as sexy as unification theories, but then again we all can't be narcissistic particle cosmologist, now can we? :)

  4. Wow! Could you explain what the movie is again. Is this a blowout? Is an AGN something that explodes inside a galaxy?

  5. Ouch Nick! Speaking of turbulence, the airline industry could use some help.

  6. So perhaps I should give a little more background to what I am doing here.

    First, galaxies are not very dense. To give you an idea, the gas in the galactic center has a density of ~1000 PARTICLES per cubic centimeter (1 cc = 1 ml) (plus or minus 1 or 2 orders of magnitude). In comparison, water has a density of ~10^22 particles per cubic centimeter (that's 10,000,000,000,000,000,000,000 particles). So galaxies are not very dense compared to what we are used to here on earth, but compared to intergalactic space (that's the space between galaxies, as opposed to interstellar space, which is the space between stars) a galaxy is rather dense. To give a number, the gas inside a galaxy is ~ 1 million times (10^6) as dense as the gas outside a galaxy.

    So the video that I posted shows the density of the gas both inside and outside the galactic disk. At the beginning you can see a "bar" in the middle. This bar represents the galactic disk, where the density is higher. Outside (on either side) of the bar the density is very low, almost zero.

    Now the point of what I call a "blowout" is that something happens (either an AGN or a star formation event) that puts energy into the system and it proceeds to blowout the dense central disk of the galaxy and make the densities in the center go very low. In the process a significant ammount of matter gets thrown out of the galaxy. In this case there was a blowout.

    Second, AGN's. AGN stands for Active Galactic Nuclei. This might be a good thing to explain in a full post as they play a very important role in how a galaxy works. The short answer is that an AGN is a black hole with stuff falling into it. All galaxies have a central black hole (some have suggested that this can be used to determine whether or not something is a galaxy). But the central black holes are not always active, or doing much. Every once in a while something (like a star) gets too close to them and gets ripped apart. In the process of getting ripped apart they put out a lot of energy (A LOT). This energy creates what might be called an explosion inside the galaxy, and if it is powerful enough can puch a hole in the galaxy (i.e. clear out everything around it).

    While I may look into the effects of AGN on galaxies I am focusing on star formations, or star forming regions. A star forming region is a relatively small region of space (50-100 lightyears accross) compared to a galaxy (100,000 lightyears accross), where in a short period of time (1-10 million years) a large number of stars are formed (1,000-100,000). All this activity creats some very strong inter stellar winds (think solar wind only stronger). If the wind is strong enough it will blowout the center of the galaxy (or where ever the star forming region is) and turn the interstellar wind into an intergalactic wind.

    Like I mentioned in the post what I have right now is just a toy model, kind of like the preliminary sketches before someone actually starts painting.

  7. "This might be a good thing to explain in a full post as they play a very important role in how a galaxy works."

    Seriously Ryan, you should explain all of the various physics involved with this over the course of several posts since a.) it is really good stuff and you are in a unique position to explain it and b.) It is complicated enough that to do a really good job would take too much space for a single post. (I'm pretty sure.)

    (Plus, who really understands all of E&M from the 4 lines it takes to tell the whole tell? 2 lines if you use differential forms. The point is good physics takes a lot of time to explain adequately and so is best if it is broken up.)

  8. In addition to Joe's suggestion, you might also post on the computational details of Athena for the computer geeks among us.


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