classical collisions, GALEX/FUV

Neal continued his lunch school on "beyond the standard model". I learned something.

I demonstrated (to myself) the (obvious, apparently) fact that when two hard spheres collide elastically in three-space, the cosine of the angle between the incoming and outgoing momenta for each particle has a flat probability distribution function in the center-of-mass frame. Don't ask me why I did this.

Fiddled with some other astrometry issues, and Schiminovich corrected me (quite correctly) that he didn't say we wouldn't be able to solve the GALEX astrometry blind, he said that he would be impressed if we solved the GALEX/FUV astrometry blind, with no help from the NUV channel. And he said that would be useful to boot. He threw down a serious gauntlet today; we expect to pick it up (soon).


twelve catalogs, LRGs

I started making the 12 catalogs in the new regime, in which we split the USNO-B1.0 catalog into twelve separate catalogs, each of which does 1/12 of the sky. This allows us to operate with less RAM (but requires us to run simultaneously on more CPUs).

I wrote a few words for Morad's nearly-done paper on LRG clustering at very small scales.



Roweis and I spent all day working on getting SDSS fields to solve in astrometry.net, as well as short-term planning for the project.

Roweis also gave a very nice talk about one of his alogorithms for aligning one-dimensional datasets with variable x and y scalings. We might be able to use it to solve, once and for all, robustly, Burles's perennial problems with automatic arc solutions in spectrograph calibration.


GALEX tweak, SDSS index

Woah! The WCS tweak code I wrote this summer worked straight out of the box on the GALEX solution Lang found yesterday.

Roweis and I got set up to run all the SDSS fields; we have a bet on whether the 5 arcmin quad index or 10 arcmin quad index will do better.


review, GALEX, glass

I wrote material in my Marseille review about the LRG–LRG merger rate. We learned last week in group meeting that it is hard to show just how robust is Masjedi's strict upper limit.

Dustin Lang (hey, this isn't Lang's Research, is it?) successfully found the WCS for a GALEX field blind! This is big news, especially since Schiminovich predicted that we wouldn't succeed. Now on to all GALEX fields. I spent some time fiddling with Keir Mierle's new (and fun) wiki for the astrometry.net project.

Burles took delivery of the prism for the PRIMUS project. It looks awesome!


automated gravitational lens search

Yes! The insane idea Phil Marshall and I had yesterday for finding gravitational lenses automatically worked. What we do is (a) subtract smooth models from sources in HST imaging that appear to be massive galaxies at reasonable redshift, producing residuals (including, one hopes, lensed background galaxies), (b) loop over possible parameters for a simple lens model, fixing the center of the lens at the center of the massive galaxy, (c) throw image-plane pixels back to a pixelized "source plane", but (d) taking the minimum image intensity value among the values of the image-plane pixels that hit each source-plane pixel, and (e) measuring the surface brightness inside the high-magnification (principally the multiple-imaging) part of the source plane, as a function of the lens model parameters. This method, because it takes the minimum of the image-plane pixels that hit each source-plane pixels, in effect uses the blank parts of the image plane to "veto" the possible locations on the source plane (and hence models that require that originating location) from which the image-plane intensity might originate.

The amazing thing is, it seems to work, in that it finds a high source-plane intensity (after this min-value filtering) for sources that do look like lenses, and it finds a peak in the intensity at values of lens-model parameters that are consistent with what is found from direct, manual lens modeling. Here's an example plot of the source-plane intensity as a function of the lens (singular isothermal sphere) effective radius for something in Lexi Moustakas's LEGS survey. Note the peak around 20 pixels, which Marshall found previously by modeling this source. Non-lenses show no such peak anywhere.


standard model, dark matter, Hough transforms

Neal Weiner gave a nice pedagogical lunch talk setting up the problems with the standard model. He is doing two lunch talks; the first (today) was the standard model, and the second will be "beyond" the standard model.

In other news, Weiner and I are taking on graduate-student-style projects from each other. Weiner is going to find statistically anomalous galaxy groups in Berlind's group catalog, and I am going to work out some cosmological implications of direct dark-matter searches in the context of a few specific dark matter models. More as these projects get underway.

Phil Marshall (SLAC) is in town for 30 hours or so. We discussed, worked out simplifications of, and planned a pair coding session on a version/modification of the Hough transform that might be used to automatically detect gravitational lenses in massive data sets. Tomorrow we code.


Roweis, Wu, Bernstein

Roweis and I discussed the next steps in the astrometry.net project, some of which Mulin has already done, because he has our new machine up and running and on the net with a static IP. I am pretty confident (on the basis of no data) that we will be able to solve Schiminovich's GALEX field, but that is yet to get underway. We discussed some small non-uniformities in the USNO-B1.0 catalog that appear to be related to the photographic plate mapping pattern on the sky. We made some hypotheses about what they are, none of the hypotheses were good!

Willman, Ronin Wu, and I discussed Wu's project to re-determine the properties of Virgo Cluster galaxies. Her plots look great and she is starting to write up the method into a proto-paper. I think there is a lot to do with this data set. Willman mentioned vanGorkom's beautiful HI data on Virgo; I should go up to Columbia and ask for an audience!

Rebecca Bernstein gave a great talk about chemical abundances in globular clusters. She explained how chemical abundances are related to formation history of galaxy stellar populations. More impressively, she explained in detail how chemical abundances can be determined with observations of stars and (harder) with observations of compact populations of stars. All this work was done with a spectrograph she built herself, so it was an impressive show as well as being informative.


review, optics

I got another page written in my Marseille review.

Rebecca Bernstein (Michigan) is visiting. We discussed galaxy clusters, in particular intracluster light, fossil groups, and environments. We discussed the Dark Energy Survey. Most interestingly, we discussed the process by which she designed the camera for the DES, all the hard and soft requirements, and how to improve an okay design to a good design incrementally.


group inpsection

I wrote code to make full-color images of Berlind's groups, so we can inspect the outliers in the magnitude gap distribution I discussed yesterday. The code isn't quite ready (because Blanton has to give me the Rvir code), but it is close. We find that the gap distribution is slightly larger than expected from the pure independent, Poisson sampling distribution (there are more groups with big gaps); the question is, do these large-gap clusters all have correctly computed magnitude gaps?


magnitude gaps

Inspired by some results shown by Milos Milosavljevic in his talk yesterday, Blanton and I tried to do a single-afternoon project on the distribution of magnitude gaps between first-ranked and second-ranked and between first-ranked and third-ranked galaxies in Berlind's groups. The distributions are barely different from what you would expect if galaxy groups are simply random, Poisson samplings of the relevant luminosity functions. We are trying to figure out just how different. The big picture is: Are the groups with very large magnitude gaps old "fossil groups" where the original members have all merged into the central galaxy, or are they just the random occurences you get from sampling the luminosity distribution randomly? We didn't finish, but we got everything ready for the quantitative analysis.


dissipationless galaxy formation, binomial probabilities

Milos Milosavljevic gave a talk elucidating the role and consequences of dissipationless clustering and merging for massive galaxies, under the assumption (well justified observationally) that there is no new star formation in these objects.

I nearly convinced Neal Weiner to do our "binomial outliers" project: looking for groups of N galaxies from Berlind's group catalog that are very unlikely—given binomial probability—of containing m galaxies that have some unusual or remarkable property. An easy project, but certain to produce a few LPU.


constrained realizations, Wiktionary

I continued to work on my review for Marseille. In working on the end part, where I exhort my colleagues to work on certain crucial problems, I have decided that I am going to say something about constrained realizations. There is nothing—in principle—to stop us from having simulations not just of the cosmological model, but of our actual Universe, constrained by all the multi-wavelength things we know about all lines of sight we have observed (including the actual galaxy properties, their positions, the structures in which they reside, and, in principle, our actual CMB).

There are huge degeneracies, both because the observations are highly incomplete and because the model is extremely uncertain (where it comes to galaxy formation especially). But it is possible—in principle, and soon in practice—to integrate over these degeneracies statistically with Markov chains or Gibbs samplers and the like. The chain of allowed realizations would make predictions (those things that were common to all models in the chain but not yet observed) and direct decisive new observations (those things that varied enormously among models in the chain). And each link in the chain would give specific initial conditions and formation history for every observed object, and all the not-yet-observed ones too! It is hard to imagine this future, but it is neither impossible nor ridiculous. Indeed, once simulations get a bit faster and more user-friendly, I will start to pursue it myself!

I also adjusted Wiktionary entries on physics subjects (I don't think this counts as research, but hey, it's the weekend).


Thompson, knowledge, star catalogs

Todd Thompson (Princeton) gave a great, thought-provoking talk (with lots of lively audience participation) on the possibility that all star-formation (especially in starburst galaxies) is limited by radiation pressure acting on the dust mixed in with the cold gas. Interestingly, the hypothesis is hard to rule out directly, and gets a lot of the order-of-magnitude calculations for galaxies right.

Quintero and I virtually decided that his paper is about the information that galaxies know about their nearest cluster; we adjusted some of the wording to get this accross.

After a conversation with Sam about the memory limitations on his side of the astrometry project, I re-wrote the USNO subsampler to rank its output such that the user (Sam and Dustin and Keir) can simply truncate the catalog I give them and they will still get a uniform, all-sky catalog. This was not hard, of course. I am re-building the catalogs (yet again).


Robertson, Magnasco

Brant Robertson (Harvard) gave a nice (informal) talk on the influence of black holes on galaxy formation, in the context of galaxy–galaxy mergers. His disk–disk mergers do seem to produce galaxies that look remarkably like early-type galaxies, and he finds that the feedback from black-hole accretion is extremely important. Unfortunately he has lots of parameters, many of which are going to be hard to explain. But it was very convincing that the properties of ellipticals and lenticulars are not, by themselves, a problem for the dominant paradigm of cosmogony.

Marcelo Magnasco (Rockefeller) gave an interesting talk in which the first half was about how the ear works mechanically and the second half was speculation about how it might work from an information processing point of view.


review, wikis, XMM

I worked on my review paper for the Marseille meeting.

Phil Marshall and I discussed the possibility of making a web-based user interface or wiki to get community or student feedback on whether individual extracted HST images of massive galaxies are gravitational lenses. This is similar to things Blanton and I have discussed to get the SDSS galaxies classified morphologically. The idea is crazy, but interesting.

With help from Quintero, I inspected the Juett's XMM targets by eye.


merging and the correlation function, XMM

Today, we discussed Morad's results on the correlation function with Milos Milosavljevic (Caltech). Milos said that he has a theoretical result that the inflow of galaxies (subhalos) into clusters (halos) is linear with time, ie, the galaxies flow in at a constant linear velocity. This relatively naturally leads to a 3-space correlation function that goes like r−2.

I gave K+A galaxy targets to Adrienne Juett (Virginia) for an XMM proposal on neutron star binaries (ie, to find binaries with A and later-type stellar companions).


standard catalog, editing

I started (and nearly finished) the job of making the uniform (via healpix) version of the GALEX-optimized astrometric standards catalog (Schiminovich has given us a challenge, if you recall). I gave Morad comments on the second half of his paper.


statistics, ultra-low-mass galaxies, numerical simulations

Sam tells me that his student Dustin Lang has confirmed numerically my (trivial) prediction that the number of valid quads for the astrometry project goes as the sixth power of the angular separations considered. Apparently Dustin has made it possible to quickly assemble the set of all possible (allowed) quads, given a star catalog.

At group meeting we discussed Beth's work on ultra-low-mass companions to the Milky Way: discovery, follow-up, theory, etc. She has enough to give three talks on the subject, at least! Her limits on detections are awesome; it is surprising, frankly, that she finds so few objects, because, eg, she could find things 10 times fainter than Draco, at distances up to a few times further away (ie, in ten or more times the volume).

Romeel Davé (Arizona) gave a characteristically great talk about numerical simulations, where they fail, where they succeed, and prospects for the future. As mentioned previously in this blog, he makes quite a bit of there being two modes of accretion: hot (passing through an accretion shock) and cold (radiating gravitational potential energy as it accretes). There are hints that this may produce the "bimodality" in galaxy properties, or the red sequence. But even sophisticated models do not yet produce the red sequence "naturally". Morad and I got Romeel to agree to make and publish some specific theoretical predictions related to Morad's current and future papers on the galaxy–galaxy merger rate.


fabulous, galaxies as vectors, sterile neutrinos

I worked more on my review paper.

Blanton, Quintero, and I discussed a project on the vector field described by galaxy dipole moments (eg, the vector offsets between the centroid of the bulge and disk, or old stars and new). It is interesting, because some investigators, including Vogt et al, find that galaxy asymmetries are related to the direction to the nearest rich cluster.

Mikhail Shaposhnikov gave a nice talk on a dark matter model employing very weakly interacting sterile neutrinos. Interestingly, large parts of this parameter space could be ruled out by very high (spectral) resolution x-ray spectroscopy, because generically sterile neutrinos will have decay pathways to the standard neutrinos, emitting a photon.


large redshift surveys, clustocentric

Today Blanton and I discussed what we would do with 100 nights on a good 2 m telescope. What's the point of doing yet more redshift surveys at redshift of about 2/3? As I see it, you got (a) baryon feature, (b) precision tests of the growth of large-scale structure, or (c) evolution of galaxies and star-formation activity. I can't think of much else, except rare objects, lensing, mergers, etc., most of which fall into category (c). So if you are going to do a new survey, you better be able to show that you can do one of these better than anyone else.

Quintero's paper is nearly ready for prime time.



A combination of meetings, refereeing, and code bugs (yes, bugs in the code to make the damned USNO-B1.0 subset!) kept me from significant research progress today.


uniform sky catalog

Thanks to Fink, I got healpix working and started making Sam his perfectly uniform sky catalog for automated astrometry. I made a healpix grid with Nside=64 (49152 pixels), and am keeping the brightest 1024 USNO stars (subject to R>14 mag) in each pixel. I return the full, all-sky list in brightness order, from brightest to faintest. So Sam will get a very uniform all-sky catalog with a full brightness ranking.

It would be nice, philosophically, if we could operate astrometry.net (both server-side and client-side) with no reference to specific brightnesses beyond ranking. This would relieve our users of anything approximating calibration (or even nonlinearity correction).