dwarf galaxies in the mid-infrared

Ronin and I finished her thesis chapter / paper on the ultra-low-luminosity dwarf galaxies observed with SDSS and Spitzer. Our co-author has one week to respond and then we submit it! Congratulations Ronin.

abstracts, image modeling

I worked on abstracts for the Koposov and Wu papers that are being finished now in Heidelberg. Abstracts are the hardest parts of papers and therefore should be written first and constantly tinkered with as the paper is written and revised.

Marshall and I developed a plan to start image modeling, with the goal of finding faint lensing galaxies under the flux of bright, multiply-imaged quasars. This problem is a hard one in ground-based data, but essential if the lenses from PanSTARRS and LSST are going to be properly mined. The project is related to the image modeling projects of Lang and Bovy and myself; Marshall and I are hoping that we can join forces somehow. Among other big issues: Can we believe the surveys' pipeline-output PSF models, or do we have to fit for a set of perturbations or modifications around or away from those outputs? If we do have to fit, what basis functions make most sense?

foreground and background modeling

I added text to the fitting-a-line document about foreground–background mixture modeling for robust fitting. This is a big favorite of mine because it is fast, simple, and very close to the right thing to do. It also resolves some of my issues of a few days ago.

hypothesis confirmation

Phil Marshall showed up in Heidelberg today; he and Kasper Schmidt (MPIA) and I had lunch and discussed (among other things) the confirmation and rejection of hypotheses, which rarely—in the real world—goes according to either the Bayesian or the frequentist methodology. For example, the WMAP-1 paper was taken to be an awesome confirmation of the standard CDM model (and it was!) even though it had a bad chi-squared value (so frequentists were wrong to be excited) and it wasn't being competed against any serious alternative model (so Bayesians have nothing to say at all). I think this all comes down to message length, but I certainly haven't worked it all out yet.

robust fitting and density estimation

After my binary programming failures of yesterday, Bovy implemented a Gibbs sampler and solved my problem overnight. But then in discussing the issues, we concluded—as we always do—that the only principled way to deal with bad data or outliers is to model them. This means performing density estimation or modeling of the distribution function for the outliers simultaneously with performing the fit on the inliers, so all the data can be generatively modeled as a two-component mixture. One component is the inliers, with model parameters fitting those inliers. The other is the outliers, with model parameters describing the distribution of outliers. I think we may have to switch to that in the robust fitting section of the now-infamous fitting a straight line document.

bad data

I worked into the wee hours on robust fitting with arbitrary assignments of the binary classification "good" or "bad" to each data point. Robust fitting methods like this are beautiful, but exponential in the number of data points (this is binary programming, discussed on this blog previously in a different context). I attempted to make use of the magic of sampling but with little success.

cold stream paper, transit timing

I spent most of the day reading carefully and commenting on Koposov's paper on modeling the cold GD-1 stream in a (flexible) Milky Way potential. The paper puts some strong constraints (fully marginalized, of course) on Galaxy and gravitational-potential parameters; for some parameters these are the only constraints at this length scale.

I also attended a nice short talk about transit timing by Monika Lendl (MPIA). It turns out that exoplanets that transit their parent stars produce timing sequences that are extremely sensitive to resonant perturbers. A Jupiter-mass transiting exoplanet can produce observable timing residuals when perturbed by even an Earth-mass (or smaller) perturber, if that perturber is interacting resonantly. This is potentially extremely sensitive, but it has the great problem that for a given timing residual pattern, while it may be easy to detect the perturber, it looks very difficult to understand the perturber uniquely.

orthogonal charge transfer

I spent the day commenting on papers by various co-authors. Over coffee, Eric Bell got onto the subject of orthogonal charge transfer, which is one of the methods by which PanSTARRS is going to maintain good image quality. In OCT, the idea is to monitor bright stars on short timescales, and shift the charge on the CCD east, west, north, or south as the point-spread function shifts, to keep the light falling in the most compact PSF possible. This technology has been developed and tested well at Hawaii. I got interested, because it is a perfect problem (if and how to move the charge, when) to cast into the form of Bayesian decision theory, one of my current pet methodologies.

extragalactic intensity

I revived the project with Schiminovich to measure the extragalactic intensity falling on the Milky Way from quasars as a function of redshift. As expected, as you approach Lyman-alpha, it comes from z<3 quasars, and from a broad redshift range within that.

Wu thesis

I spent my high-quality time today reading very carefully the first science chapter of Ronin Wu's thesis. Her thesis will be on the star-formation and radiative properties of galaxies, and the first chapter is on the mid-infrared properties of extremely low-luminosity galaxies from the SDSS.