large surveys

Blanton and I spent a good chunk of time discussing large surveys. Most of the big, upcoming astrophysics surveys want new participants to buy in with cash plus work. The question is: Where's the biggest bang for the buck? (And, I guess: Can we get some bucks?)

anomalous galaxies, anthropics, halo shapes

In group meeting, Ben Weiner (UMD) spoke about galaxies with anomalous rotation curves—really anomalous! Later in the day, he, Blanton, and I tried to find low-redshift (ie, SDSS) analogs to his "diffuse red galaxies" that he finds in substantial numbers at redshift of unity. We found a few among a clean sample of 30,000 nearby galaxies. Ben has been remarkably adept at finding galaxies at redshift unity that don't seem to exist today.

At lunch, Lenny Susskind (Stanford) gave a very nice talk about the large-scale structure bound on the shape of the inflaton potential. He calls it the anthropic bound, but I call it the observation that there are galaxies. Somehow, Susskind has convinced himself that there can't be astronomers if there aren't galaxies. Don't ask me how! But despite this problem of interpretation/philosophy (don't count on great physicists to be great methodologically), his talk was great and reminded me of all the cosmological physics connected to inflation.

After lunch, Andrew Zentner (Chicago) gave a great talk about the shapes (ie, triaxiality) of galaxy halos in CDM, and bounds thereon. He finds that the Milky Way halo is unusually spherical relative to what we expect of dark-matter-only CDM, but not so anomalous when you consider the effect of the baryon cooling on the halos (it makes them more spherical). Nonetheless, he predicts a strong Holmberg effect (concentrations of MW satellite galaxies along the axis perpendicular to the galaxy disk); we (Masjedi, Willman, and others) do not think there is a very strong effect around our own Galaxy or any others, although the literature is filled with claims and counter-claims. A rule of thumb: When the literature is filled with claims and counter-claims, and they are all a few sigma, then probably the real punchline is that there isn't much effect.

clusters

[For yesterday:]

Mike Gladders (OCIW) gave a very nice talk on the use of optically selected clusters in cosmology, showing that the results of surveys like RCS-2 and its successors will be great for cosmological tests. From my point of view his most interesting result is that though the bivariate distribution of mass and redshift for clusters is consistent with standard-model cosmology, the same distribution for clusters selected to have strong-lensing arcs is totally wrong. There are issues in interpretation, but the effect is so strong, it suggests that the solution will be important and interesting.

I also worked on some pretty pictures.

tidal interactions, physics education

Boring day today, writing text to distinguish non-merger tidal interactions of galaxies from the kind of strong tidal interactions that occur during major mergers.

More interesting was the NYU Physics Colloquium by Wieman (Colorado) on teaching undergraduate physics. He showed the main research results (use peer interactions, focus on concepts and beliefs, discuss the organizational structure, connect to the real world, force the students to interact and participate) that I already know, but also one I didn't: In some cases (building circuits was his example), students learn to build real circuits in the lab more quickly if they learn from a simulation than if they learn from building real circuits! This only works where the simulation is carefully constructed to be realistic but also free of irrelevant distractions (apparently students new to physics often spend time worrying about things like the color of the insulation on the wires).

pretty pictures

I worked on things related to our nascent galaxy atlas, including some code and some general planning. No new pretty pictures to report.

In other (non-research-related) news, tonight, after my class, from the roof of 715 Broadway, Quintero and I took this picture:

ultra-faint MW satellites

As my loyal readers both know, in 2005 Willman discovered the least luminous galaxy in the Universe: it is a tiny companion to the Milky Way, discovered initially as an over-density of about eight red-giant stars in a small patch of the sky in Ursa Major. Willman's objective, statistical search for such objects has put very strong limits on what else can be out there. Blanton, Willman, and I discussed some of the technical details of her forthcoming paper on these limits. This is a difficult project, but very important, given that dark-matter models predict enormous numbers of companions in the dark sector, and galaxy-formation models have very different mechanisms for explaining which are observable as galaxies.

LHC, clustering, and weak lensing

There were two talks on the LHC today, one by Jenni (CERN) on the ATLAS experiment, in which NYU is about to become a partner, and the other by da Costa (Harvard) about the top. I embarassed myself by asking about the uncertainty principle: Unlike astronomers with photons, high-energy experimentalists do not try to detect particles at the fundamental limits of precision. Of course such precision would be insanely expensive (and these machines are expensive enough already). But in both talks, the LHC came across very well; I am very excited about it.

In group meeting, Molly Swanson (MIT) discussed her work on mangle (a toolkit for the description of piecewise constant functions on the sky using spherical polygons) and some very nice results on linear relative bias among different galaxy populations. She showed that linear bias is a pretty damn good model, even on few-Mpc scales!

Sheldon, Masjedi, and I discussed features in the weak lensing signal around LRGs that Sheldon is convinced (probably rightly) are evidence of significant systematic errors. But the error is hard to find and I got to wondering if the result could be real. It could be if a significant fraction of LRGs are tens of kpc from the centers of their dark-matter halos. Is that insane? Surely galaxy formation models would tell us if it were true? Time to call in the theorists.

code bugs, LRG clustering, cosmic rays

The NYU Computer Science Colloquium was by Xie (Stanford) on automatic detection of bugs in code, through direct boolean modeling of the source code. He noted some remarkable statistics, eg: that it is believed that most commercial code has of order 10 bugs per thousand lines of code; that bugs cost society on the order of 0.6 percent of the GDP; that GM vehicles are expected to have 100 million lines of code by 2010 (they already have a million). Xie showed that security-critical bugs (eg, for buffer overflow) can be found automatically with few false positives, and he is responsible for finding hundreds of bugs in open-source software, such as openSSH, mySQL, and the Linux kernel.

Kazin and I discussed the possibility of explaining Majedi's correlation function by looking at the separation distribution of the two most massive subhalos inside halos massive enough to contain two LRGs. Wechsler's results (discussed here earlier) suggest that this project will work well.

Sigl (Paris 7) gave a nice overview of the current and future of very high-energy cosmic rays, which is bright, given the start of the Auger Observatory in Argentina. Auger is almost certain to see anisotropies at very high energies, because the GZK cut-off means that they come from nearby, and there just aren't that many possible sources, even if they come from gamma-ray bursts!

SDSS sky

Blanton and I worked for a while on sky. It is interesting that such a simple problem (subtract the smooth, additive foreground) is so hard! We decided that the way you subtract sky is a strong function of the scientific question you are asking, and you make different choices in different cases. So to make a sky determination, you have to have a very specific question in mind. This may seem strange, but it is because the sky has structure on all scales, and sometimes you want to photometer a star in a dense field, and sometimes you want to detect low surface-brightness emission behind a field of Galactic stars. There is no one sky that gives good signal-to-noise for both of these cases. For measuring the properties of huge, well-resolved galaxies, we were using a hack, but now we are getting close. Here's a comparison (before is on top, after is below):

SDSS sky

Blanton got very close to solving the SDSS "sky" problem today (the problem that the SDSS images of large galaxies or large patches of sky show non-astronomical features resulting from our inadequate tracking of sky levels). Unfortunately, I don't quite yet have side-by-side comparisons to show, but it looks very promising. I still think there are issues but we are much closer.

post-starburst environments, SDSS sky

I nearly finished the response-to-referee for the post-starburst galaxy environments paper. Nearly is a relative word.

Blanton and I had some back-and-forth (and Blanton did a lot of work) on fitting the sky in SDSS imaging for very large, very low-redshift galaxies. Our current page of nearby RC3 galaxies shows many data artifacts, many of which relate to sky-level tracking. The problem is not easy to solve. This is annoying, because the SDSS is drift-scanned, which greatly improves the consistency and simplicity of the sky!

cluster influences, environments, clustering of mass and galaxies

In group meeting, Berlind talked about reactions to NYU work at the SDSS meeting last week. This led to a discussion of Quintero's current project of looking for signs of the direction to the cluster in galaxies in the infall regions, the results of which were very well received at the meeting, apparently. We figured out that because Quintero is looking at broadband light, he won't be extremely sensitive to some of the effects claimed in the literature.

Berlind mentioned that Park (Seoul) is working on some environmental relations similar to those measured by Blanton and me. But he is using some kind of Nth-nearest neighbor statistic that has variable length scale (and a large median length-scale, and we have shown that only very small scales—Mpc or so—matter).

After group meeting, Berlind gave a short talk in the brown-bag about confirming some non-trivial relationships between cluster ages, masses, and clustering (as measured by the autocorrelation function). Age is positively correlated with clustering amplitude at masses below the nonlinear scale, but negatively above! Wechsler (Chicago) predicted this from simulations and Berlind has confirmed.

Sheldon, Masjedi, and I talked about ratios of cluster-mass and cluster-galaxy or galaxy-mass and galaxy-galaxy cross-correlations to measure the mass-to-light ratio as a function of scale. The observed power-law galaxy autocorrelation functions can only be explained in conventional CDM cosmology if these cross-correlation ratios have very specific shapes with radius. I hope this discussion was the birth of a new (and fast) project on this.

python cartography

This picture of the celestial sphere (below) may look familiar, but it is brand new: It is the output of my first-ever Python program. I even use (for the first time in my life) object-oriented features. I spent quite a bit of time on planes, trains, and hotel rooms working on this. The only connection this has to my research is that I plan to start working in Python from here on, if possible.

My astute readers will notice Orion there near the meridian (the NYC horizon is shown as a thick great circle). My very astute readers will notice that this is a perspective projection of the sphere, not orthogonal (or orthographic).

correlated point sets, talk

I gave this talk (PDF) at the IAP in Paris, to which we moved my LPNHE talk because of the student strike on in Paris. Morad will (a) notice its disturbing similarity with my ISCAP talk, and (b) be upset that I did not update the correlation function figure to its most recent version.

I also wrote a stupid code that iteratively modifies a point set in N dimensions until it has a given two-point correlation function, something Michael Joyce assigned me to do as homework. Now my homework is to make a point set that has the same correlation function as the LRGs (the Masjedi et al plot in the talk PDF file above).

[ps: I will be en vacance until Monday. I know that all two of my loyal readers will be disappointed.]

two-point statistics

I spent the last two days arguing about two-point statistics with Michael Joyce (Paris LPNHE); especially on the remarkable power-law correlation function over five orders of magnitude in separation shown in Masjedi et al, 2006 and our determination of the scale of homogeneity of the Universe (well, of LRGs in the Universe) in Hogg et al, 2005. The integrated "fractal dimension" plot in the latter paper also shows a power-law over a large range of scales, and yet it is a different power than what you would expect naively from the correlation function in the former paper. So there are multiple, unexpected, large-scale power-laws to explain in the clustering in the Universe. In other news, we both lamented the fact that we have no general ways to make (ie, set up in the computer or algorithmically) distributions of points with arbitrary correlation functions.