Wednesday, July 29, 2009

Willman I Gradient

I've spent the last two weeks messing around with finding the best fit slope for Willman I's velocity distribution along its major axis.

The best fit has a slope of -.484 km/s/arcminute.

But I wanted to figure out how likely this is, so ran a couple of simulations. Based on Willman I's mean velocity, position, and spread in spatial points, I simulated 100,000 fake galaxies with specified velocity dispersions.

With a velocity dispersion of 1 km/s, an absolute slope of greater than .484 is never (or almost never) found.

A velocity dispersion of 5 km/s has a slope higher than that value 17.01% of the time and a velocity dispersion of 10km/s has a greater slope 48.75% of the time.

The velocity dispersion of Willman I is 3.52, and just for kicks, the probability of it having that slope with that dispersion is 5.40%.

Wednesday, July 15, 2009

Results! (sort of)

Well this week I've actually accomplished several things.
I sent out a paper to several of Beth's collaborators detailing my results for the kurtosis of the various dwarf galaxies' velocity distributions.
For the most part, their small star numbers mean that their negative kurtosis falls in line with the simulated results for a normal distribution with the same number of stars.

However, Canes Venatici II has a random statistically significant high kurtosis, which is probably due to its having several far off outliers (making thicker tails, which result in higher kurtosis, remember?).
But for the rest of them we'll need more stars to determine anything else through that method.

But I also decided that there is almost definitely some rotation going on in Willman I. I plotted the systematic velocities in various sectors of the data, which seem to show that the "left" side is moving towards us faster than the average velocity, and the "right" side is relatively moving away from us. When I chopped out the stars with high metallicity, the probable outliers, I saw a really apparent gradient in the systematic velocity distribution. It's pretty neat!

It's probably evidence for streaming motions, although I'm still not entirely sure what that means, other than the fact that it is a result of tidal disruption.

To Do:
1) Beautify my kurtosis/skewness plots per Josh's suggestion.
2) Figure out what's going on with Nmix/correspond with Jay.
3) Look for rotation in the other galaxies.
4) Take a nap! The midnight Harry Potter viewing was probably not the brightest idea, despite being unexpectedly funny and entertaining.

Wednesday, July 8, 2009

Longer Time No Blog

It's been a while, but I've basically been doing the same things or variants on them.
I made a bunch of sweet figures of simulated data and its kurtosis, skewness, and Nmix results. I then plotted real data to see if they matched up at all, which wasn't really the case. But they're still potentially useful figures.

The main problem was that my simulations show that kurtosis is rather strongly negative for simulated Gaussians with small numbers of stars. But most of our data sets have small numbers of stars, so a negative kurtosis won't really tell us anything.

Also, I randomly discovered that running Nmix on the Segue I data set with and without the 5 outlier points yields drastically different results. With the points, the probability of a single component is close to 70%, without them it is only 4%. Something is clearly wrong with Nmix.

But now I'm trying to determine if there are "streaming motions" in the Willman I kinematics, which basically mean rotation. I'll divide the data into chunks based on position and then calculate the average velocity, seeing if some are negative or positive which would indicate spinning. I'm not really sure what that would mean, but since it's not a spiral galaxy, it would probably mean definite tidal disruption.

Well that's all for now and hopefully I'll keep up the blogging.