Ideally, there would be a single set of orbit parameters to describe the paths the two galaxies take when flying past each other, and eventually towards their ultimate merging into one galaxy. It is difficult for researchers to know for sure if they have found the single best set of parameters. Is there a better set of orbit parameters? Are their multiple sets of equally good parameters? Are there no good sets? We can call this problem the issue of determining uniqueness. The volunteers for Merger Zoo have achieved an unprecedented level of study for each of these 54 systems. Typically researchers will look at a few dozen to a few hundred simulations of interacting galaxies and pick the best orbit from that sample. Together, we’ve reviewed on average over 50 thousand simulations for each pair and selected over 1000 simulations for further study. We’ve taken the multiple sets of orbit parameters identified for each system and examined them to see how well they’ve identified a single, best-fit orbit. When we look at the entire sample, we don’t see a single orbit. However, if we begin to exclude some of the sample by filtering out the low fitness simulations, we see the range of orbit parameters becomes smaller. If we increase the fitness value used in that filter, we continue to see smaller and smaller ranges of values. In this manner, we can say that we see convergence towards a small range of values for each of the orbit parameters. Arp 82, the image from the top of the post, is a good example of this convergence.    For each population we show a box plot describing the distribution of the parameter. The box represents the range of data from 0.25 to 0.75 of the population. The horizontal line is the median, and the thin whiskers show the outliers. The populations shown are all states viewed by the users, all states selected by the users, and then several populations filtered by fitness to include the to top 50%, 25%, 10%, 5%, 2%, and finally the top 1% of simulations by fitness. The distribution of values describing the time of closest approach demonstrates some convergence. We see that the applet sampled a range of orbits that had times of closest approach ranging between 60 to 600 million years ago. By the time we filter to just the top 1%, we see the range is now only 100 to 250 million years ago with the likely range of 120 to 162 million years ago. Hancock et al find a time since closest approach of around 200 – 250 million years.  The ratio of masses between the two galaxies converges in the same way.

Your Help is Needed-

The big thing we need help with is figuring out THE very best model of each system and comparing models of different systems.    The Merger Wars site has a couple of new interfaces that are now posted.   They include:

• Pick the Best: find the best model for each system
• Simulation Showdown: comparing the simulations from two different galaxies to find out which systems have the best models
• Merger Wars – HST :  the last batch of Merger Wars results from non-Sloan images
• My Mergers: This is a new update that shows your contributions to the project.  [Note: you need to be logged in for this link to work.]

After we get this final set of data, we will be archiving the site and writing a set of papers based on this work.  However, your help is really needed with this final part of our analysis.

A Zoo of Mergers-

The image below is a combined image of 54 thumbnails showing all of the SDSS galaxy pairs studied in detailed by our Merger Zoo volunteers. Clicking on the image below will take you to our updated Gallery. From that page you can click on each individual thumbnail to see the top simulation results.    You have done an amazing job with this project.   Thanks so much for your help.

Galaxies Modeled in Merger Zoo

John and Anthony, The Merger Zoo guys