By: John Vickers - Project Scientist: MilkyWay@Home, Rensselaer Polytechnic Institute.
We have been getting a lot of questions lately about whether or not we are getting real scientific results, why the new sgr runs are getting better likelihoods, what the numbers crunched actually mean, what our scientific goals are, etc. In return we've posted our publications on the front page-- but, like some other BOINC user said: you guys aren't astrophysicists. I'm going to try and summarize the physics portion of this project in layman’s terms-- from the data collection to the current achievements to the future plans.
|These are (l, b) (l and b are galactic longitude and latitude respectively-- the equator being the galactic plane) plots of the amount of sky covered by the Sloan Digital Sky Survey (SDSS1 on the left and SDSS2 on the right).|
Our project begins with the Sloan Digital Sky Survey (http://www.sdss.org/), an ambitious project whose goal is to map out as large a portion of the sky as possible. To this date the SDSS has mapped about a quarter of the sky, including over 300 million objects.
But what really is a list of a couple million points in 3D space other than a massive problem to tackle? Sure, we can plot all these points together and get a gorgeous map of the sky, but once again-- what do these pictures mean?
That’s where research astrophysicists come into play. One of the hot spots in galactic astronomy (astronomy relating to just the Milky Way) at the moment is stellar stream mapping. The general idea is that the Milky Way Galaxy actually has a couple of smaller galaxies mixed in with it, probably from galactic collisions (click here for a simulation of how a galactic collision turns a galaxy into a stream-- simulation by Kathryn V. Johnston at Columbia University) beginning sometime in ancient history and continuing to this day (don't worry, it is very seldom that actual material like stars or planets collide-- there is so much empty space that it is highly improbable). The Sagittarius Dwarf Galaxy is one of the closer galaxies residing in our own and it is our particular area of interest.
In general an astrophysics problem revolves around creating a model on a computer system that will replicate what we see in the sky-- if a model matches exactly then we can leapfrog off the information that model reveals to work on a bigger, more involved problem. Currently, the MW@Home BOINC application is made to model plates of stars. We input a 2.5 degree cross section of data (the shape is commonly called a wedge, or stripe) and the program attempts to create a new, uniformly dense wedge of stars from the input wedge by removing a stream(s) of data. The streams it removes are necessarily cylindrical and their stellar density falls off in a Guassian manner (denser in the middle, sparser at the edges).
|Density map of that cross section of sky|
Each stream removed possesses 6 parameters: weight (% of stars in the stream), mu (a measure of angular position in the stripe, given by the ticks on the circumference of the above plots), r ( a measure of distance, given by the radial ticks above), phi (one 3D angle indicating direction of the removed cylinder), theta (the second required angle), and sigma (a measure of width). And each wedge background possesses 2 parameters: q (a measure of the flatness of the spheroid) and r0 (a measure of the radius of the spheroid core). So every run has 2+6n parameters, where n is the number of streams being modeled.
|Model for the Sagittarius Dwarf Stream|
|3D model of the Sagittarius Dwarf Stream produced by David Law at the University of Virginia.|
|Plot of all of the data point positions and directions found by Nathan Cole|
What we want to do is end up with as many data points as possible from BOINC-- we can use mu and r to plot the location in space and the angles phi and theta to plot the direction of the stream. What we end up with is a picture similar to the above.
Here is a plot of all of the data point positions and directions found by Nathan Cole-- it is exactly the same as the picture just before it, just less artsy.
Here is the corresponding plot in a plane perpendicular to the above. Imagine now that you had the prior plot on a piece of paper and you tilted it until all you see is a line. That line (which represents a plane) is signified by the middle line in this plot. So putting these two plots together would yield a 3D interpretation of the found points and directions of the streams.
|3D interpretation of the found points and directions of the streams.|
First, the separation plot should leave a near uniform background-- if there's still over densities in the output, we are not getting an accurate picture of the 2 spheroid parameters.
Second, the vectors in the plane should be cohesive-- we want the stream to flow rather than zig zag through space as it were.
Third, we want the vectors in the perpendicular plane to be close to parallel to the plane-- again, we want it to flow, not zig zag.
We did all that, Nate wrote his Thesis on it. So what are we doing now? Basically at this point we want to refine our results and get them to be more accurate. To do this we have stitched all the SDSS data together and taken wedges out that are perpendicular to the stream-- the general idea is that a perpendicular cross section is much easier to decipher than a skewed one-- thus our error measurements will be smaller and the likelihoods will be higher. I have just now begun runs on BOINC using this new geometry (all the recent *_sgr_* runs), although I have been working with it since the beginning of last summer on the 88 processor WCL grid here at RPI. For reference, it took me about a week per run on the MPI grid-- now I am getting about 5 runs per day on BOINC, it's amazing.
|Simple diagram illustrating the idea|
Here is a juxtaposition of one of Nate's wedges (left, SDSS stripe 13) and one of mine (right, sgr stripe 35) in the same area of 3D space-- notice how his stream stretches almost all the way across the stripe because it is tilted relative to the stripe while mine is nice and compact. This translates to smaller errors in our reported findings.
|Here is a juxtaposition of one of Nate's wedges|
These are both tremendous topics in modern astrophysics-- first of all, the location and direction of the Sagittarius stream is still somewhat debated. Some people, like Nate, believe that the stream passes by us. Others think that the stream crashes down on top of the Sun. And the spheroid has yet to be accurately modeled. Such a model would make galactic simulations much easier to create as they would require less unknown variables in their simulations; and it could also provide valuable clues to the dark matter problem.
I will reserve some posts below for the purpose of uploading new versions of the plots shown above (vectors and separations) for the stripes most recently crunched so you can check this thread periodically to see the science side of the progress here at MW@Home. On a related topic, the enterprising individual could make a screensaver from these images (perhaps the vectors walking across the screen, or a separation emerging from a parent wedge) --I regret to say that my screensaver-building skills are limited to slide shows and that such endeavors aren't exactly high priority here at the lab haha. If you PM me, then I can possibly find higher resolution images as well.
I hope this helps you to understand what it is you are crunching. And thanks for helping us get this far!
To get involved in the MilkyWay@home project, visit the website here; http://milkyway.cs.rpi.edu/milkyway/