globulars, planetary nebulae, galaxy mass

Eric Peng (DAO, Victoria) gave a great group meeting talk about globular clusters, which separate into metal-poor and metal-rich populations. He has suggestive evidence that the metal-poor globular clusters (which probably formed at very early times) may trace the dark matter better than galaxies or stars! This would be wonderful if true, and the hypothesis makes many testable predictions.

Patrick Huggins (NYU) gave the astro seminar, about planetary nebulae. He showed that they show strong evidence for (time variable) jets, and he discussed the mysteries associated with that. He noted that a lot could be explained if all of the AGB stars that go through the PN phase are in binaries, but this seems very hard to explain in terms of the binary fraction.

Jim Pizagno (Stony Brook) talked to me about his mass images of galaxies (that is, transforming optical images into mass images using relationships between color and stellar mass-to-light ratios). He is close to a very robust system that will be of very wide applicability.


nothing for two days; electrostatics

For two days now I have done nothing but NSF proposing. I think it may be done, but by the rules that is not research! Today I went to a nice talk by Eric Dufresne (Yale) about electrostatics and entropy in ion solutions. Evidently there is a lot that isn't understood; what I am hoping is that this area of soft condensed matter produces some very general principles, the way that, for example, thermodynamics did at the turn of the century.



In between paragraphs in my NSF proposal, Mierle and I discussed combinatorics. I can't really claim that was research.


data reduction, fundamental galaxy observables

[I missed a few days on travel.]

On Thursday, Burles, Bolton, and I had a data analysis pow-wow at MIT. We are facing some crazy data for the PRIMUS project and they are not yet tamed. Burles feels that a lot of the problem is fitting the sky through the slits, with non-zero instrument resolution, and then tracing and extracting the object spectra from the two-dimensional spectral images. I hope so, but it is going to take some real work to do it right.

Today, Moustakas gave a nice overview at lunch (on the blackboard) of the fundamental observables in galaxy evolution: luminosities, colors, and emission line strengths. All of these are just measurements of the spectral energy distribution with different levels of crudeness, but they are differently sensitive to age, star-formation history, dust, and chemical composition, all of which are themselves inter-related. To figure out how galaxies form, and how stars form within galaxies, a consistent picture must be drawn for all these observables, that is also consistent with what we know (and we know a lot) about the dark sector.


blind astrometry failures

Interspersed between NSF proposal paragraphs, I followed up some failure fields in our blind astrometry demo project (in which we throw away all the astrometry information for every SDSS field, randomize their order, and solve them all again, totally blind). Almost all of the failures can be attributed to holes in the USNO-B1.0 catalog (not our problem), I believe, but for those few failures that can't, some are bad SDSS data where, eg, the telescope jumped or fell out of focus. Even these are very rare: Our success rate is more than 99.8 percent.


not much but tweak

Today was all teaching and all grant proposal writing, so it would be against the rules for me to point out that I submitted this pedagogical note on air resistance to the arXiv.

Mierle and I agreed on a short-term plan for getting our astrometric WCS tweak system on the web, and making sure that our code produces files that our customers can use.



According to my own rules, teaching and proposal-writing don't count as research!

Gruzinov, Moustakas, and I spent some time talking about the physical plausibility of the bullet cluster, the subject of our colloquium on Thursday (which I, unfortunately, will miss).


William & Mary

I spent Friday at William & Mary, where I gave the physics colloquium. I didn't get any research done, but spent an enjoyable day hanging out with Josh Erlich and his colleagues.


big computer projects, backups

Today was all computers all the time. Piet Hut (IAS) came in for the day to have lunch and discuss ambitious coding problems, including the development of enormously complicated code, and the realization of incredibly complex constraints in constrained realizations. Hut is working on some ideas for collaborative coding which, if they pan out, could transform open-source and multi-developer projects. He is also doing some very ambitious simulations that combine stellar evolution with stellar dynamics in dense environments (such as the centers of globular clusters).

I also worked on the backups for the astrometry.net code repository. We have multi-site, off-site backups. If you don't, you are making a big mistake (trust me, I know from direct experience).


gas fractions in clusters, image test-bed

I started to put together the image test-bed for the astrometry.net pre-alpha testing. I will be contacting the alpha users (no doubt my most loyal readers) soon for example images.

At coffee we discussed a recent paper on cluster gas fractions which claims that x-ray observations of clusters were in (weak) disagreement with WMAP results on cosmological parameters. The odd thing is, they find that the discrepancy is a strong function of the radius at which you do the measurement, and that the discrepancy gets small at large radius; perhaps there is no discrepancy at all when extrapolated to the cosmic mean density?


simplexy, galaxy emission lines

Mierle and I worked through some small remaining issues in Blanton's simplexy code, which is a very simple peak-finding and measuring code, suitable for finding stars (or other sources) in astronomical imaging, with very few free parameters. The hope is that we can hard-code simplexy so it works on almost all possible input images without much modification. Checking that it does is the next order of business; so far we have only tested it on a single (real) image.

Blanton, Moustakas, and I discussed the things you can learn from galaxy emission lines, in particular star-formation rates and the electron densities and temperatures in ionized regions in galaxies. Moustakas believes that he can rule out a suggestion in the literature that the electron densities in star-formation regions are a strong function of redshift.


source detection

Mierle and I checked out the results of Blanton's super-simple simplexy code that detects and measures the positions of sources in astronomical images. It seems to work pretty well. Now Mierle is packaging Blanton's code into something that reads a (possibly multi-HDU) FITS file, runs simplexy, and writes the output to a (possibly multi-HDU) FITS binary table.

metallicity evolution, astrometry testbed

Moustakas gave group meeting, about the evolution in the mass–metallicity relation. Last year he came and gave a job talk on the same subject, and one very interesting thing has changed: If he factors in reasonable luminosity evolution for galaxies, his evolution in the luminosity–metallicity relation is severely diminished. His results are now (nearly) consistent with no chemical evolution whatsoever for massive galaxies. On the other hand, he does find that at high redshifts there is a (rare) population of massive but low-metallicity galaxies. Maybe these are proto-galaxies?

Mierle assigned me the job of building the testbed for the astrometry.net tweak and online systems. I have started, and soon I will be contacting the alpha users for sample images.


tweak archaeology

In preparation for Mierle's clean room of tweak to C, I dug up all my various versions of tweak in IDL and tried to run them on some basic data. Damn they are ugly. But they almost work.



Keir Mierle (Toronto) just showed up for two months of hard work on astrometry.net. We spent a long time going through the gory details of tweak, which is the process by which we take a pretty-good astrometric WCS solution for an image and make it precise (and, we hope, accurate) and encode it in the image header in a standards-compliant way. None of this is very difficult; all of the difficulty comes in dealing with the heterogeneity of input images and desired output WCS formats.

Mierle's main project, if we can stabilize tweak quickly, will be to work on the general blind camera calibration problem, in which a large number of images from a single camera are used to figure out the optical properties, and variations of those properties with focus, temperature, gravitational loading, etc., just by looking at the images themselves. This is the problem astronomers working on deep imaging call astrometry, and frankly we are not competitive if we can't do this very well.



John Moustakas arrived to start his postdoc this week! It was great to spend some time chatting about mergers, galaxy spectra, and the Spitzer Space Telescope. He told us that he and Tremonti (Arizona) have evidence that high-redshift star-burst/AGN galaxies have very high velocity outflows, presumably driven by star formation or black-hole accretion. It is going to be a fun time having Moustakas in the group.

The school year is just starting, so I didn't get much research done, but I did return the proofs to ApJ on this article.