galactic center dynamics

In my infinitesimal research time today, Bovy and I discussed the dynamical measurements of the black hole at the Galactic Center. It appears that received wisdom is that by far the best measurement of its mass comes from the one closest star that has completed a full orbit in the 15-or-so years in which it has been observed, and all other measures are deprecated to that one. It also appears that the central mass estimate has creeped upwards throughout that time, so there are unresolved discrepancies among different data sets. Joint analyses of all the data available have not been attempted, to our knowledge.

HST, Astrometry.net, the evening

Lang and I, working towards the Astrometry.net main paper, built an SDSS-based quad index and verification system (using the command-line tools of the Astrometry.net codebase), optimized to calibrate (blind) images as small as those produced by the Hubble Space Telescope. We succeeded in calibrating some images.

We spent the evening—into the wee hours—working on computational photography (treating the output of the Astrometry.net digital camera as scientific data). That's Friday night at Astrometry.net world headquarters! While we hacked, Stumm was being wined and dined by the museum set at the American Association of Museums Annual Meeting for his work on web-2.0 museum interactions with Flickr and Astrometry.net.

sources real

Today we inspected, in the context of the deepest imaging we have, the sources we harvested from the Flickr group images yesterday. They look real, so we are emboldened. We realized that between this and the stuff Lang did on Comet Holmes (he can determine the orbit from the footprint of Flickr-submitted images!), we have some good evidence that Flickr and amateur astrophotographers can have a big impact on science.

evidence for sources

Stumm, Lang, and I gathered some (heuristic) evidence for sources in the region of M42 (the Orion Nebula) using images submitted to the Flickr pool. The image below shows in transparent blue symbols the detected sources in the Flickr images in a tiny patch in the nebula. We take it from this that the amateur data do have highly redundant (and therefore reliable) information about many sources.

magnification bias

I had a great time at the thesis defense of Marilena Loverde (Columbia) today. Her thesis was on magnification bias effects on precise cosmology observables, and also signatures of non-gaussianity. She took a theory-centric view, which permitted her to investigate many precise effects, all of which will be either confusing or useful to precise surveys in the near future.

tons of bad data?

Christopher Stumm (Microsoft) and Lang are in town this week, to see if we can do reliable science with unreliable data. Stumm designed, wrote, and operates our Flickr pool in which we calibrate amateur astrophotography just as if it were professional data. Now we want to use those data as if they were professional data, since they certainly contain tons of information. But the challenge is accounting for the fact that data of unknown provenance will, in general, contain unknown artifacts, errors, and noise. We started on project specification this morning.

Astro2010 town hall

I spent much of today at the New York area town hall meeting for the NAS Decadal Survey. This was fun, although it was not research.

Reid masers

I can't say I did much today, but Bovy made progress understanding how we might use the Reid (CfA) maser measurements in the Milky Way to get a better mass estimate.

SDSS-3 target selection

The plot below (made by Schiminovich and me today) shows the GALEX–SDSS colors of quasars with photometric redshifts between 2 and 3.5. The white points have spectroscopic redshifts in that same range; the cyan points don't. The vertical axis is a GALEX–SDSS color that separates them well. This might have implications for SDSS-3 quasar target selection, which takes place in regions of the sky with good GALEX coverage.

decision theory

I got all fired up about decision theory this past weekend, and wrote something about it today, in the context of the Astrometry.net verify step. Here is the first paragraph:

All science is really decision theory, not inference! After all, the end result of an inference is a paper, and what one writes in the paper depends on the result of the inference, and not at all trivially. That is, the content of the paper is not simply a statement of the posterior distribution function (obviously). Despite the obviousness of this, most discussions of scientific data analysis end at (or even before) the inference step, and don't give quantitative guidelines for anything that follows, except perhaps for the construction of confidence intervals.

bayesian vs frequentist

Many assume that I am a religious or dogmatic Bayesian, because I use Bayesian statistics. I am not; I use them because (a) they are not ad-hoc, (b) they are justified in the context of probability theory, and (c) for many problems I do they perform better than the alternatives (see, for example, our April Fools' paper). Bovy and I spent some time today doing battle with Kyle Cranmer (NYU) on this point, which was productive inasmuch as we understood the differences between a Bayesian and frequentist approach, but unproductive in that Cranmer thinks he could do much better with a frequentist method (to our Fools' problem). We asked him to put up or shut up, as Gruzinov says. While we were arguing, who should walk in but Yuri Levin (Leiden), the author of the frequentist orbital roulette paper.

dark galaxies

Barry Madore (OCIW) gave a very provocative talk about the possibility that there might be dark galaxies and then showed that after some pretty hard looking (using galaxy–galaxy interaction signatures), he can't find any. So they are rare, at the near-L-star level. Nice and fundamental!

Oort problem

Rix, Bovy, and I discussed re-doing the famous Oort problem on the vertical structure of the Milky Way disk; that is, infer the density of the disk by simultaneously fitting for the position and velocity distribution of stars in the vertical direction. The differences now relative to the 1930s is that we have far more stars, far better measurements, and we can do the test over a finite fraction of the Milky Way disk. We might be able to see gradients and look for signatures of departure from axisymmetry, which would be qualitatively new.

Milky Way masers

Masers are incredibly bright, compact emission regions in the Galaxy that permit measurements of full six-dimensional phase-space position. Reid (Harvard) has a sample of 18 that he used to measure the mass of the Milky Way. Rix, Bovy, and I spent an inordinate amount of time criticizing the Reid method, although we are not sure we can do better. We also involved Johnston and Weinberg (UMASS) in our discussions.

Grillmair stream

Koposov may have evidence that the velocity of the Grillmair stream is not exactly parallel to the configuration direction of the stream. Since the stream is cold and very near pericenter (by any reasonable model), this could have important implications for either the potential of the Milky Way or else how the stream formed. Unfortunately, it is hard to tell which. Had many confusing conversations about this today, but Bovy worked out that there are large classes of potentials in which the misalignment can't happen unless there is either a strong action gradient in the stream or else a time-dependence (or other nastiness) in the potential.