I worked on the galaxy evolution synthesis this morning. I think I decided that
predictions of the galaxy luminosity function are too dependent on ad-hoc models of star formation and feedback for the luminosity function itself to provide a good test of the CDM model. In principle, the observed luminosity function and the theoretical mass function from CDM (for DM halos) produces a prediction for the Tully–Fisher relation (see recent work by Blanton and Geha) or the fundamental plane, but it seems that even this is loose in the sense that a departure or violation would be viewed by very few as evidence against the fundamental assumptions of CDM. It would be viewed as evidence that dark matter and stars can have different velocity dispersions (which would by no means present any paradox). Maybe with the addition of weak lensing (galaxy–galaxy) constraints it would become a falsifiable prediction of the fundamental CDM model.
I worked on the galaxy evolution synthesis this morning. I think I decided that
On the trains connecting Paris to Heidelberg, I wrote a very specialized (but therefore fast and efficient) expectation maximization code for use in astrometry tweak. I wrote the unit tests first and then wrote and debugged my code until it satisfied the unit tests.
I spent part of the day working through Peebles's latest proto-draft of our synthesis of the evidence for and against the cold-dark-matter paradigm. Peebles nicely laid out (in a philosophical introduction) the difference between
falsification. These are two approaches when one has a dominant model and a lot of phenomenology that is only tied to that model through indirect links, as with evolution in biology and cosmology in physics. Adapters try to learn about the indirect links by adjusting them until there is agreement between the fundamental theory and the data; adapters believe the fundamental model. We, of course, are falsifiers: we believe that you make most progress in this situation by trying to construct empirical tests that are capable of falsifying the fundamental hypotheses.
My only research today (Fete de la Musique here in Europe) was some thought and a conversation with Roweis about scaling up our astrometry.net interface to an industrial scale, possibly with some of the existing image management web services already in place at Google. We already have a way to use Flickr, which is being worked out by Stumm. The challenge is to make this all work with a user who has FITS or RAW files or equivalents, but we think there are some straightforward solutions.
NYU undergraduate Bob Ma accomplished something nice today. He measured the distances of a set of HII regions from the center of M51 (literally with a ruler, working on the HST image), and also their angles relative to a reference angle (ie, their polar coordinates). What did he find? That the angle depends linearly on radius. This is exactly what you expect if the spiral arm pattern in M51 was begun as an m=2 distortion that wrapped up under the action of a flat rotation curve. What raised the m=2 distortion? The interaction with the companion, of course. The slope of the line on the radius–angle plot gives you the time since the raising of the distortion. This is all part of my evil plan to demonstrate that spiral patterns are transient, and to identify their causes, individually, in all sufficiently well-observed galaxies.
I spent the early morning hours learning how the GALEX and 2MASS missions do their astrometry. They are both impressive, both possibly better than any standard astrometric catalog, though both have possible remaining systematics. It is hard to tell from the documentation of either mission what systematics have been explored to date. Certainly the people working on these projects did not see themselves as producing astrometric standards, even though that is exactly what they have produced, beautifully.
I spoke about our efforts to clean up the USNO-B Catalog using computer vision techniques at Rix's group meeting at MPIA yesterday, and worked a bit on the paper. Today was spent in transit to Paris. Gotta love traveling by train at a rate of 1 km every 10 seconds!
Sergey Koposov (with great wizadry) obtained quickly a sample of halo M-giants and their SDSS-USNO proper motions. A very large fraction of halo M giants live in the observed Milky Way substructure, presumably because the substructure is higher in metallicity than the general halo population (and higher metallicity giants are redder/cooler than typical halo K giants). The histograms of proper motions for these halo giants are not promising for statistical proper motion work, although Rix and I have no fear.
I spent time discussing with Rix and then reading Sergey Koposov's (MPIA) draft paper on the luminosity distribution of Milky Way satellites. It is a nice paper because he re-discovers all of the satellites automatically, and can therefore do an objective completeness analysis as a function of luminosity, size, and distance. His results still depend on the (unknown) radial distribution; I argued that he can determine that, with significant uncertainty, from his data directly, or in conjunction with the luminosity distribution.
Spent what little time I had on the weekend writing figure captions and incorporating figures into our USNO-B paper.
I arrived in Heidelberg today and spent some quality time after dinner catching up with Rix on proper motions measured statistically.
I got a lesson today from Mierle on "test-driven development" in which you write unit tests for each code function before you write the code function, you run all your tests each time you commit new code changes, and you thereby never write wrong code. Of course, you need your unit tests to be powerful!
[I was out Tuesday and Wednesday on travel.]
Spent a great day with Ben Weiner (Arizona) visiting. We discussed recent published and unpublished work on outflows from star-forming galaxies at redshifts 0.5 to 1.5. It looks like these are confirming the general view that feedback, and maybe AGN feedback, is important to shaping galaxies.
Weiner, Moustakas, Blanton, and I spent some time discussing measurements of the [O II] 3727 luminosity function, which I measured years ago. This could be done much better now, with far more galaxies and much better redshift coverage. Whenever this comes up, we ask "what's the point?", but Weiner and Moustakas both gave very convincing arguments that it is worthwhile. Now would someone please measure it?
I have been having the realization lately that in almost all cases in which we astronomers use iterated sigma clipping, we should be fitting a mixtures of gaussians model with something like the "expectation maximization" algorithm. It has all the advantages of sigma clipping, plus a clear interpretation in terms of Bayesian statistics. I spent the morning working on that.
I spent my morning nerd-out working on Peebles's and my synthesis of galaxy evolution observations. I was reminded of my philosophical position on all this, with which I have no-doubt bored my legions of blog fans before:
The primary goal of observational astrophysics—as distinct from, say, pure astronomy—ought to be to rule out physical models. Note that all important physical experiments have the property that they ruled out—or could have ruled out—one of the fundamental, dominant theories of their day. In physical cosmology, we have a fundamental, dominant theory for the growth of structure (CDM). Our role as observers is not to bolster this model, or find ad-hoc parameters we can add to the model that make it consistent with the data. Our role is to perform experiments that have the power, even in the face of uncertainties (about how galaxies form, for example), to rule out or substantially modify the fundamental assumptions of this theory. If an experiment does not have the power to rule out the theory, then it can hardly be said to provide substantial support when the results end up in agreement! Thus my primary experimental design criterion is that my observations be capable of falsifying CDM, even after marginalization over unknowns.