Tuesday, March 20, 2012

the end of MICE

I'm about to finish running GALFIT on the HST F606 MICE image (preview here).
I've used GALFIT before, while writing my MSc thesis, but oh, those were faint, redshifted and unremarkable SDSS galaxies. That was simple -- I used 5 components once, because a galaxy had a LINER AGN.
The MICE are monsters. They've apparently already passed through each other once, and have highly disturbed tidal features, clumps of active star formation, dust lanes, you name it. I've never used GALFIT to model spiral arms, too, so that was an interesting experience.
Here are they:

I started simple, first masking out each galaxy and fitting the other in turn, then fitting both simultaneously (that's a better way to do it anyway). After running about 150 models I decided to mask out the dark dust lanes in galaxy A -- I couldn't fit them using truncation functions, as GALFIT crashes (on my 64bit SUSE, at least) while attempting that and I don't think the science goal required this. I have some reservations about the decomposition, as I think the image has some flat-fielding issues, besides, the galaxies take up a large part of the overall image, so I have doubts about the determined sky value as well.
In the end I had 91 free parameter, and I think more might have made sense, but that was past the point of diminishing returns. I stopped at Chi^2 = 1.877, and guess the main reason why the model would fit the image so poorly were the bright and irregular cores of the galaxies -- or possible higher order flat-field curvature. The other objects were masked out.
Here is the model -- at ds9's 99.5 scale and heat colourmap (this is handy: ds9 -scale mode 99.5 -cmap Heat -medatacube ~/workspace/MICE/galfit/subcomps.fits). The long, bright tails are not well visible with these settings, but they weren't the goal.
I am not really happy with the residual picture, as it tells me there are some big, obscured components lurking underneath. But that'll have to do for the moment, as I have other assignments. I've been doing this for other people in the collaboration, so I don't think I can publish the fit logs here, unfortunately.

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