moving groups demoted

Bovy finished another draft round on our first Hipparcos papers, one an immense method paper and one with some results, using Geneva-Copenhagen Survey to externally vet any Hipparcos-inferred structure in the velocity distribution. We find that this conservative method disfavors some of the less-significant structures claimed in the literature, and that there are no moving groups strongly evident in the Hipparcos data that weren't known prior to Hipparcos. Bad news for Hipparcos, but it does mean that we have something useful to contribute to this literature.

spiral patterns

In the astro seminar today, Rix gave disturbing evidence that the disk spiral patterns have a fixed (or close-to-fixed) pattern speed. This was not his primary goal, but it was the most remarkable part of it all, from my point of view. One clever thing about the research is that they were able to make this argument without definitively identifying the spiral pattern or spiral arms themselves.

stream directions continued

Bovy, Rix, and I further specified the stream-direction question, and gave it to Bovy as homework. He completed his homework by late in the evening. In potentials with high symmetry, the stream direction will be in the orbital direction. But it remains to see if it will in, say, an oblate logarithmic potential (or worse).

stream directions

Rix arrived for two weeks while I was out on a mental-health break (hence no posts for a few days). Today, he, Bovy, and I spent a good chunk of the morning going over the Rix-related on-going projects, including the Solar System dynamics, Galactic Black Hole, Galactic masers, and cold stellar stream projects. In regards the latter, Rix encouraged me to write up my concerns about the direction of the tidal stream in a short note. I started that late in the day. I will say more soon, but my concern is not about the fact that there is an action gradient along the stream, my concern is that even in the limit of zero action gradient, the stream will not obviously (to me, anyway) stretch out in the orbital direction. That said, it usually does in the simulations!

halo star formation

I spent my research time today working through the latest draft of Zolotov's paper on the origin of Milky-Way halo stars, as indicated by Milky-Way-like simulations.

LSST, ionizing radiation, units

After a frustrating time looking for a units bug (there wasn't one) in Wu's code, I went to Zeljko Ivezic's (UW) talk at the Columbia Astronomy Colloqium. He spoke about LSST and the relationship to SDSS, intellectually, technically, and scientifically. It was great (though I had to leave a few minutes early).

Prior to that, Schiminovich and I got close to a measurement of the ionizing radiation incident on the outskirts of the Milky Way from optically selected quasars. It looks from the data (and from models too) like we can straightforwardly extrapolate to 912 A (observed frame) despite the intergalactic medium.

I failed...

...to do any research today. Today was catch-up day after sacrificing the last two weeks of my life to this amusement. So that's two Rules-breaking posts for two this week. Not a good start.

automated source lists

I spent all day at the Spitzer Science Center, reviewing their plans (still tentative) for a heritage-archive source list, or set of heterogeneous catalogs built from the imaging to support object-based and statistical searches of image contents. If they do this, they have to do it very much hands free because the data set is huge. That wouldn't be such a problem (SDSS was done hands free) but of course they also have to deal with the fact that there are many different observing programs, exposure times, dither patterns, and field characteristics, including diffuse emission, and etc. The plans so far are excellent; they really impressed me, as they always do at the SSC. It got me thinking once again about my polemic on probabilistic catalogs and image modeling. (I think this post violates The Rules.)

re-done dynamical inference paper

Iain Murray de-bugged some of our old, wrong, and rejected methods in our dynamical inference project. This convinced us to re-try and we have a method and an answer we like much better. We frantically changed figures and text, added Murray as a co-author, and now we are done.

done dynamical inference paper

Bovy and I finished the first draft of the dynamical inference paper. We hope to put it on the arXiv on Monday night.

ultraviolet photons, infrared lines

Schiminovich and I realized that we have a few very simple things we can work out using our GALEX photometry of SDSS photometric quasars, not the least of which is an estimate of the extragalactic ionizing radiation impinging on the outskirts of the Milky Way, broken down by redshift (and quasar luminosity, and other things). Schiminovich was appointed to draft paper 1.

With Ben Johnson (Cambridge), Wu, Schiminovich, and I discussed the physical meanings of infrared lines. Johnson has run a large grid of models and can make plots that put Wu's line measurements into some kind of physical context.

writing

While Bovy implemented as fast as he could, I wrote as fast as I could. Our deadline approaches rapidly, but we are still at sea. Useful comments came in from Iain Murray (Toronto), a colleague of Lang's and a super Bayesian.

maximum entropy

Bovy and I realized that one way to cast the Bayesian orbital roulette problem is as a maximum entropy problem: If we really think the phases ought to be well mixed, we seek the solution for which there is maximum entropy remaining in the phase distribution. This is tantamount to saying that all phases are equally likely, it turns out.

100 percent success!

For the Astrometry.net project, Lang built an index of quads and stars from the 2MASS survey, to act as a comparison for the indices we built from USNO-B 1.0. He ran the 114 (out of 180,000) SDSS fields that failed on the USNO-B indices through the 2MASS indices and every single one of them calibrated successfully. This means that for SDSS r-band fields (55 sec exposures on a good 2.5-m telescope), the Astrometry.net system has (naively) a better than 99.999 percent success rate with no false positives. Although this is Lang's research and not mine, exactly, I am so stoked I don't know what else I would post about.

roulette prior

It being Spring Break here at NYU, the plan was to get one paper done per day! So far: Four days, zero papers. Damn.

Bovy and I spent the day hacking and talking on the dynamical inference problem: Determine the parameters of the gravitational potential given a snapshot of test particle positions and velocities. As I have noted previously, this problem is ill-posed in general; it all comes down to priors. Today we decided to revive our old idea of the roulette prior, or the prior that leads to a flat posterior distribution for particle phases (in the language you can use for integrable potentials; there are generalizations for non-integrable). We got talked out of that prior by Tremaine, but I now think for the wrong reasons; we certainly didn't understand the problem as well back then.