Congratulations to Masjedi, who passed his PhD defense this morning. I didn't get much else done today, but that was fun.
Spent time on the weekend refining the argument about merger rates and small-scale clustering. It relates to my long-term goal of advocating considerations of continuity in the interpretation of cosmological data. I also prepared for Masjedi's thesis defense, which is on this subject.
I wrote the following abstract, for a paper I will probably never write! Note how confidently I can tell you the conclusion before doing the research.
Abstract: Any galaxy–galaxy merger event must be preceded by a period in which the two pre-merger galaxies formed a close pair, therefore any estimate of the merger rate puts a constraint on some galaxy–galaxy cross- or auto-correlation function at small scales. Because the timescales for merging are not known exactly, and because galaxy correlations arise from processes not always related to merging, these constraints are not precise; nonetheless, when made with conservative assumptions, they are still very informative. Here we review published galaxy–galaxy merger rates and use them to put conservative constraints on various galaxy–galaxy cross-correlation functions, with different assumptions about merging timescales. We find that many merger rates make easily falsified predictions for galaxy correlations. Present-day measurements of galaxy clustering on small scales are only consistent with the lowest published merger rates.
I spent a small amount of time today working on the possibility of evolving astrometry.net into a precision astrometry system, a system that can measure proper motions and parallaxes, and that can build standards catalogs; right now it just does rough work.
I spent the morning at Columbia, discussing GALEX, galaxy environments, and related matters with Wu and Schiminovich, and describing the current state of astrometry.net at the relaxed "pizza lunch". Also Schiminovich showed me his lab, which is cool.
Mondays are my worst days, but I did get time to closely read the conclusion to Masjedi's most important thesis chapter and the concluding chapter to the thesis as a whole. He has shown that the growth of luminous red galaxies by merging is very slow at the present day (less than 2 percent, on average, of growth per Gyr), and dominated by
dry mergers with galaxies more luminous than L-star. This is not inconsistent with either theoretical or observational bounds on the merger rate, but it is definitely lower than most other estimates. Masjedi's measurements are unique in that they are very firm upper limits; he has basically made the maximal reasonable assumptions about the mergers; any more conservative or realistic assumptions would lower his inferred rate.
The PRIMUS project has had the bad judgement to put some of my code in the critical path for extracting 1-d spectra from the 2-d images. These images are amazing; they contain huge numbers of tiny spectra! Our philosophy is that all parts of spectral extraction must be automated, including finding the spectra on the image. My code to do this hit its limits this month; I spent today analyzing and fixing it.
I spent a lot of Thursday (sorry, forgot to post) and today interacting with our new alpha testers for astrometry.net. The alpha users are certainly finding a lot of
issues with the service, which is great. Internally, we are debating what new features are highest priority; different alpha users want very different things.
I also spent a lot of time working through draft chapters of Masjedi's thesis, which is a pleasure.
Jim Condon (NRAO) gave the seminar today; he showed us that he can (in principle, and probably in practice) measure the Hubble Constant to few-percent accuracy using maser galaxies. If he can, he will nail things like flatness and the nature of dark energy. This is nicely complementary to the baryon acoustic feature stuff we do here at NYU.
With Zolotov I pitched some projects to undergrads yesterday, including a project to look at galactic substructure with NYU undergraduate Kathy Zhang and to look at the formation of disk-galaxy spiral structure with NYU undergraduate Bob Ma.
Research time today was spent reading closely Masjedi's nearly complete dissertation (he defends his PhD in 12 days), and working with Zolotov on the research techniques that will lead to the start of a dissertation. Zolotov is having fun learning about SQL. Masjedi is having (perhaps less?) fun summarizing four years of hard work.
Pizagno got me back to thinking about our mosaic images of SDSS galaxies, because he is making stellar mass maps. He requires correct inverse variance maps, and we were creating pretty good, but not perfect, maps. (We record our uncertainties in images in units of inverse variance, not traditional root-variance; this is because the inverse variance is what you use for weighted means, and also because then badly measured pixels get values in the uncertainty map of zero, not infinity.) I made them much closer to correct today.
At the afternoon astro seminar, Ann Zabludoff (Arizona but on sabbatical at NYU) told us about galaxy groups at intermediate redshift, in the context of lensing (where there are groups contributing significantly to many of the strong lens systems, something I used to work on back in graduate school), and in the context of the baryon census, where she finds a large fraction of the baryons in stars in the intergroup but intragalactic space (about thirty percent of the stars, and between a few and tens of percent of the total baryonic content, depending on mass).
We decided that we can begin to go alpha with astrometry.net next week!
Wu and I discussed her GALEX-SDSS-Spitzer multi-wavelength study of very low-luminosity galaxies. They have a huge range in properties, from objects with specific star-formation rates that are among the highest I have ever seen, to post-starburst objects, and they have anomalous 8-micron properties. They are very low redshift, so they are generally resolved in GALEX and have non-trivial morphologies.
Roweis and I spent an inordinate amount of time on the phone (thank you unlimited calling plans) working through the astrometric tweak code. Despite our initial skepticism, we decided that almost everything works. Recompile and test, let's go alpha!
I also spent quite a bit of time going through one of Masjedi's thesis chapters, in which he shows at very high confidence that luminous red galaxies are not distributed in their dark matter halos according to an NFW-like or Moore-like radial profile, but something with a much
steeper inner profile. This result is at very high significance, and either means that baryons have a lot of influence on the dark matter distribution, or else that galaxies separate dynamically from the dark matter for dissipational reasons. The latter is more likely, but, quantitatively, both effects are interesting.
Friday was all talk, with Tim McKay (Michigan), Sheldon, and I discussing SDSS measurements of intercluster light in the morning. At group meeting, McKay spoke about his very rapid follow-up of GRBs with ROTSE-III, and Blanton spoke about the Tully-Fisher relation for S0 ("ess zero") galaxies (about which he learned in New Zealand). At lunch, Schiminovich and I looked at some PRIMUS spectra of UV-luminous galaxies; I am not sure he was impressed! Before the seminar, Pizagno and Tatjana Vavilkin (Stony Brook) discussed with me more physical approaches to creating mass images out of the SDSS data, using stellar population synthesis images. At our weekly astro seminar, McKay worked through cosmology with
optically (I think he meaned
visually) detected galaxy clusters, from soup to nuts.
This weekend, I blew large amounts of
testbed data into the pre-alpha astrometry.net engine. All of the images I sent were examples of images that previously failed to solve. This time, almost all of them solved, so I think we are close. Most of these testbed images came from our future
alpha users; I learned that some of them gave us some pretty mean images to solve!
Zolotov and I discussed how we should go about locating the Sagittarius stream in the four-dimensional space of sky position, magnitude, and velocity. This involves making a very large number of plots and browsing them in some sensible way, or else four-dimensional visualization!
Wu and I discussed bad redshifts in SDSS. One of her low luminosity galaxies may have a wrong redshift, which is incredibly rare in the SDSS data. That would be fun! We also discussed backups, since Wu nearly lost all of her work in a minor computer failure here.
The astrometry.net team started discussions of how to archive, serve, back up, and protect our data. None of our current systems scale, even in the short term.
Spent some time arguing with Sheldon about photometric redshifts. He has photometric redshifts that are somewhat degenerate (redshift vs galaxy spectral type), possibly biased, and with large errors, for a very large number of SDSS galaxies. How to determine the true redshift distribution given these data? The right answer is not just to histogram the determined redshifts, but to do something more conservative, given that each individual redshift can be significantly wrong, but there is nonetheless a lot of aggregate information. It is a very nice problem, ill-posed as all good problems are.
Tuesday was a big day for planning the
After Sloan 2 surveys, which are projects to discover extra-solar planets, to take imaging and spectra of stars in the Galaxy, and to measure the baryon acoustic feature at redshifts of about 0.3, 0.6, and 2.0, all with the 2.5-m telescope at Apache Point Observatory (the telescope used for the SDSS). My principal involvement is likely to be in the acoustic feature project, though I am also very interested in the stellar spectroscopy. Because the planning for the survey is largely at the quasi-proposal-writing level, this might not qualify as bloggable research (see
rules at right), but the scientific issues that arise in the description of this project are deep and interesting.
One of the things I am most excited about is the enormous range of possible ancillary science that can be done with the baryon acoustic feature data. These data will include spectra for many hundreds of thousands of luminous red galaxies, densely sampling an enormous volume to redshift of 0.7 or so, and spectra for hundreds of thousands of quasars at redshifts around 2.5. The challenge (though it isn't that challenging!) is to design highly informative measures of galaxy evolution that can be executed with these data even when the project is fully optimized for baryon acoustic feature science (as it will and should be).
Roweis and I spent some time working on the
tweak phase of astrometry, which follows rough determination of pointing, rotation, and scale, and which refines a close solution to a precise solution.
We also both worked on the text of paper zero, which we are drafting with a computer-science focus and in the flashy style of Science or Nature. We hope to finish and submit while we are in alpha. We still aren't in alpha, but we are very, very close (and our alpha testers are getting, well, testy).