stellar quads

On the second day of the astrometry.net group meeting, Mierle, Lang, and I realized (or, more properly, bolstered our nascent view) that indexing all stellar quads (recall we are using quads of stars to get first guesses on image positions on the sky) on the entire sky is a very bad idea, for the two related reasons that (a) many stars tend to be involved in many quads, so the loss of a single star in the test image at test time can lead to a large loss in the number of expected matching quads, and (b) small n-tuples (like 5-tuples and 6-tuples) of stars can produce enormous numbers of stars, so we get false positive matches because there are many similar-looking 5-tuples on the sky. Yes, many! So we planned for Mierle and Lang to work tonight working on an index that contains no more than one quad for each star.

SDSS astrometry, Kazin

On the first day of the astrometry.net group meeting, Lang, Mierle, and I worked on the SDSS data demonstration project, with Mierle assigned to get post-mortem code (code for understanding failures and false positives) working again (after a re-write) and Lang assigned to re-run a subsample of SDSS fields with different settings of the tolerance and the number of stars to use in quads.

I discussed merger rate projects with Eyal Kazin, a possible new member of the NYU galaxy astrophysics group!

all-sky images, astrometric WCS tweak

I spent an inordinate amount of time on the weekend tracking down a bug in the (hacked) astrometry.net prototype tweak code (take an image with a close-to-correct WCS and make it beautiful, using either SDSS or USNO-B1.0 stars). I got it, and Willman is now solving her astrometry on a bunch of images. This week is the astrometry.net group meeting, so I hope to get that code re-written by a competent programmer.

I downloaded the Bright Star Catalog, and wrote a reader for it, and plotted it—it contains all stars brighter than about V=6.5 mag—for use in some SDSS demos we make, including our famous zoom, which is very out of date. I am preparing to re-make it, but this time it will show all the constellations!

ISCAP seminar

I gave the ISCAP seminar at Columbia yesterday, on constraining the dark sector with observational constraints on galaxy interactions. My viewgraphs are here.

talk, talk, talk

Sheldon previewed his job talk for UC Davis this morning; his results—which show the cross-correlation between mass and galaxy clusters—are awesome. They strongly confirm the NFW profile and what we know about intermediate-scale clustering.

Padmanabhan (Princeton) and I discussed averaging SDSS spectra, transparency, and future massive spectroscopic surveys. No real project yet, but many on the horizon and in planning.

I prepared slides for my talk at ISCAP at Columbia tomorrow. I am going to concentrate on constraining small-scale dynamics of the dark and light sectors using galaxy interactions, mergers, and accretion.

symmetries, hacking

David Kaplan (JHU) gave the HET talk about imposing severe symmetries to cancel out cosmological-constant-like terms (ie, vacuum energy density) in the standard model. It involved duplicating the entire standard model with a whole negative-energy "ghost" standard model, and thus having enormous numbers of terms cancel out. A nice idea (because the small cosmological constant would be required by the symmetry, broken only by gravity), but the theory has super-strange properties, like a rapidly decaying vacuum (!) and possibly causality issues.

I spent the rest of my research time today hacking on the morphology–color–density figures and the post-starburst environment overlap-region analysis.

morphology–color–density

Worked on figures for the future paper on the morphology–color–density relation. We have been saying for years (eg, here and here) that indirect indicators of galaxy morphology do not correlate directly with environmental density, but that they correlate with galaxy star-formation history, which themselves correlate with density. I realized (very late, I must admit), that we can show this directly by simply showing true-color images of the damned galaxies; everyone can "do" morphology however they want on those images.

referee report

Worked on my response-to-referee for my post-starburst environments paper. And more data management.

data management

Maintaining 15 Tb of astronomical data takes a significant amount of human effort! If I were clever, I would have perl scripts take care of all contingencies. Blanton has written some great stuff that keeps our directory structure (built out of many cross-mounted partitions on many large servers) all consistent and unified even when machines go down or data are moved from one to another. But we still hit problems relating to the finite size of our directories and partitions (and the human-factors value in having related data close in hardware). I spent a good fraction of my research time this weekend cleaning up.

spectrograph constraints, astrometry group, Huterer

I started the process of investigating the effect of spectrograph constraints (the SDSS spectrograph can't simultaneously take spectra of two galaxies closer than 55 arcsec) on our galaxy environments results. I am investigating it empirically, using the fact that a large fraction of the sky is covered by the spectrograph twice (the plates overlap), and hence many of the dropped targets get picked back up again.

Roweis and I talked a bit about the agenda for the full-week astrometry.net group meeting coming up in about 10 days. We are going to focus on getting our alpha system up on the WWW.

Dragan Huterer (Chicago) gave two talks, one about statistical anomalies in the CMB at large angular scales, which are very interesting (despite being very a posteriori). Like most cosmologists, I assume these will turn out to be either flukes or systematic errors. His other talk was about the future of constraining the dark energy with observations.

Chen, public data

Jackie Chen (Chicago) spoke about using lensing and galaxy surveys to look at dark-matter substructure in collapsed dark-matter halos. She sowed quite a bit of doubt about the current lensing detections of substructure.

I continued downloading lots of public imaging data relevant to the PRIMUS project. There are a lot of public multi-wavelength deep imaging data sets.

post-starburst environments, Spitzer

Spent the day re-running post-starburst environment code (because I am responding to referee, after an insanely long delay), and downloading Spitzer and optical data in the SWIRE fields. The latter is in support of the PRIMUS project.

not much!

Uh oh. I am ashamed to admit that teaching and graduate-student recruitment and research politics got in the way of research.

filaments, mergers

Quintero provoked a nice discussion of filaments in group meeting. Evidently filaments do exist; the questions are as follows: (1) Are the galaxies in filaments different from other galaxies not in filaments but that live in the same local mass density environments? (2) Can we understand the evolution of clusters as the infall (and simultaneous evolution) of galaxies along the filaments? Not sure if we will pursue these questions, but there clearly are first steps we could take.

Masjedi showed me brand-new results on the rate of mergers of LRGs (that's luminous red galaxies) with smaller galaxies, as a function of color and magnitude of the smaller galaxy. Of course he really measures the cross-correlation functions, but we can use those to (severely) limit the merger rate. This project will give us the mass spectrum of mergers (or constrain it) and, we hope, limit the total mass accretion rate.

quad statistics

As my loyal readers know (both of them, I hope), the astrometry.net project, among other things, will use an index of quadrangles of stars to identify fields blind, ie, even when the user doesn't remember the image pointing, rotation, or scale. I spent quite a bit of time this weekend working out (and recalling) the statistics of such quadrangles: How many are there on the sky? How many unique quads can we index, given finite positional errors? How many quads do we expect in a finite, rectangular image of given dimensions? What fraction of fields will we fail to solve, given the need for redundant successes? Etc. The goal is to re-solve all of the SDSS images blind.