Planck-scale physics, galaxy-scale physics

Various members of the CCPP gave talks to visiting NSF Theoretical Physics Program Director Fred Cooper on a one-day site visit. I learned about some of Gia Dvali's new ideas about testing Planck-scale physics with tabletop experiments: He has noticed that if there are Planck-scale black holes created at early times that subsequently evaporate, their evaporation products may have topological charge that could in principle be detected at low energies with some kind of futuristic Aharanov–Bohm-like experiment. Crazy! Also, Andrei Gruzinov gave a nice explanation of Kolmogorov turbulence.

Wu and I discussed error propagation for her Spitzer photometry project, and Quintero and I figured out a great idea for finding post-starburst galaxies at high redshift.



I have to apologize to my loyal reader, because he or she has been waiting for a post for days! But I spent the end of last week preparing for and attending a NASA Extragalactic Database (NED) Users Committee meeting, in which we advised NED on their near-term and long-term priorities. NED is a pretty unique service, because its data ingestion involves a great deal of human scientific judgement. This gives it a crucial role in diversifying the current initiatives for massive data analysis.


gravitational waves from extra dimensions

Lisa Randall (Harvard) gave a nice talk pointing out (and calculating in detail) the fact that some of the generic large extra dimensions theories (in particular Randall-Sundrum 1) have a first-order phase transition at the weak scale and that such theories in general produce gravitational radiation in the LISA bandpass, at LISA-sensitivity levels. So we might discover extra dimensions or other non-trivial properties of the dark sector with LISA. Not bad!



Had a great visit to UC Davis, where I gave a talk to the cosmology group (which is pretty damned large). Spent the rest of the time talking with Chris Fassnacht and Matt Auger about galaxy evolution vs. strong gravitational lensing (Auger says do the former, not the latter, because we will understand the scale factor and galaxy mass distributions better by other means within the next decade or so). Also argued with Lloyd Knox and company about my brave new world fantasy in which we have constrained realization cosmological simulations that actually reproduce all observations, exactly not statistically. They all told me I am crazy; okay it is hard, but isn't science supposed to be hard?



Argued with Gruzinov for a while about statistics, then about noise from extraterrestrials. More on the latter after I return from Cali on Tuesday.


DGP cosmology statistics

Iggy Sawicki (Chicago) gave a very lively talk today about quantitative empirical tests of the leading cosmology-inspired modification of gravity: DGP (named after Dvali, Gabadadze, and Porrati, all here in the CCPP at NYU). The talk was lively because it is basically impossible to do exact calculations of the evolution of fluctuations (it is even hard to do the scale factor!) in this model; the arguments are all about the applicability of various possible approximations. In general, GR calculations are hard; modifications to GR are harder. We have a dangerous possible future in which no substantial competitors to GR ever get worked out. We might end up settling on GR just because it is the only exactly computed model!

After the talk, Blanton and I had a long discussion of the statistics involved in ruling out models. A trivial point, but one that is often misunderstood, is that it is very different to ask the question Is this model consistent with the data? than it is to ask the question Is model A a significantly better fit to the data than model B? Sawicki showed that DGP is a significantly less good fit to the data than GR (ie, Lambda-CDM), but he did not show that either model is consistent with the data. I actually don't think that either is consistent with the data, in the technical sense (chi-squared on the order of the number of degrees of freedom). But of course this just reminds me that we don't make important decisions about the fundamental physics of the Universe on the basis of calculations in gaussian statistics!


anthropic principle

Okay, I admit that discussion of the anthropic principle does not count as research (see rules at right). But really, the people who use the anthropic principle are confused! I agree that you can't observe a universe that cannot contain observers. No duh! But a universe that contains observers arbitrarily different from us is observable. Recall: The anthropic principle is invoked to reduce the range of possible predictions for the properties of the Universe from the vast range allowed by, eg, string theory. You can't use observations of our Universe (such as that it contains galaxies and carbon-based life) to trim down the possible fundamental predictions, because then they aren't predictions. You must only use the fact that the Universe is observed, which requires only observers, not carbon-based observers, not observers in galaxies, not observers around stars, etc. Interestingly, many people who have worked on this problem for many years, including Susskind and Vilenkin (who spoke today at NYU) have got this wrong, and they call the requirement that there be galaxies the anthropic principle.

The fact that the Universe contains galaxies is an observation; the requirement that it is in principle observable is properly the anthropic principle. Of course we don't know how to compute observability, given a stated theory (indeed, we cannot even predict the proton from the standard model, let alone observers!). Until observers are predictable, the anthropic principle is not useable for science, in practice.


Tully–Fisher relation

Jim Pizagno (Stony Brook) gave a great and very lively group-meeting talk about his work on the Tully–Fisher relation—the narrow power-law relationship between galaxy stellar mass or luminosity and disk rotation velocity—and the scatter about the relation. He did the very striaghtforward thing of attempting to measure rotation curves for a complete sample of galaxies, not selected by subjective morphology. After subtracting out estimates of observational uncertainties in quadrature, he gets a scatter of about 0.4~mag, which is small by any theoretical comparison (and the theoretically predicted scatters don't include scatters in dust content). His talk brought up a lot of side issues too, relating to the metallicities and star-formation histories of the galaxy disks.

After his talk, Pizagno and I discussed his project to make stellar-mass images of SDSS galaxies and re-analyze his spectra in the context of these two-dimensional maps. Unfortunately, the ball is in my court!


server room

Does it count as research that Blanton and I discussed the HVAC needs for our server room?


evolution of red galaxies

Much of the debate at the merger conference in Baltimore last week was about the evolution of the red, old, early-type galaxies; what we usually call (stupidly) the red sequence. Today Masjedi, Blanton, and I discussed Masjedi's job applications and dissertation, which all relate to the growth of galaxies on the red sequence. Most studies of the luminosity function of red galaxies suggest an increase of between tens of percent and a factor of two in the total stellar mass on the red sequence since a redshift of unity. As I have mentioned here many times, Masjedi can see this process in action among the SDSS LRGs and can quantitatively limit the mass accretion rate at the most massive end.

Unfortunately, it is at the most massive end that there is the most disagreement about the evolution of red galaxies; Masjedi points out that this can be partially resolved with photometrically selected red galaxies below the spectroscopic limit in the SDSS. He is also working on extending his work on mass accretion to lower-luminosity red galaxies, where the community is in agreement that there has been substantial evolution.


WCS tweak

Mierle and I walked through the linear tweaking of the WCS of a small portion of a huge NOAO KPNO 4-m Mosaic camera image. My IDL implementation works just fine; now we are trying to get Mierle's much better C implementation to work. The migration from IDL to C makes the code more portable, and independent of closed-source products.



I continued working on the automatic detection of satellite trails in astronomical imaging, which is formally trivial, but quite non-trivial in the face of real data issues. In the Hough transform image below, the white trapezoidal dot in the upper left is the signal of a satellite trail.

This is a clear detection, but the transform involves doing of order one million medians of a thousand values each. That's not fast.


galaxy mergers, day 3

Masjedi gave a great talk on his close-pair merger rate estimates and the growth of luminous red galaxies at the conference today. He is one of the few participants in the meeting who committed to very specific numbers on the merger or accretion rate, and, though there are caveats, it is a beautiful peice of work.

There was a amusing, lively, and chaotic summary session to which I contributed. I think there is a picture emerging—for the growth of red galaxies—but very few components of the picture have hard numbers on them.


galaxy mergers, day 2

Another great day at the meeting on galaxy–galaxy mergers at STScI. Among the many contributions was a nice discussion of the role of dissipation in shaping merger remnants by Rothberg (STScI), who cited some nice old papers by Carlberg, Gunn, and Kormendy that I have not previously seen; they discuss the issue I go on about: you can't make (high density) ellipticals from the mergers of (low density) spirals. Along the same lines, Novak (UCSC) showed that realistic simulations of disk–disk mergers tend to produce oblate galaxies in prolate halos, with a perpendicular alignment of the symmetry axes. This is testable right now by weak lensing.

McIntosh (Amherst) can show that the merger probability is relatively insensitive to environment (where environment means here the mass of the hosting dark-matter halo). This relates strongly to results in preparation here at NYU by Blanton and Ignarra. He also quipped that we have A growing but dead red sequence and a star-forming but static blue sequence. But we all knew that McIntosh is a poet.

van Dokkum (Yale) showed his evidence for dry merging from very low surface-brightness tidal features in red galaxies. He gave a wonderful presentation of the material, and drew out the tension with Masjedi's work, and Brown's (NOAO). He nicely set up Masjedi for his talk tomorrow! There is a lot to do with the inter-comparison of Bell, Brown, van Dokkum, and Masjedi, because they all have slightly different things to say about the merger rate.

Just like yesterday, there were good talks too numerous for me to mention, but one highlight was Puech's (Paris) result on the near-infrared Tully–Fisher relation at redshift 0.7: When he trims his sample to galaxies with very clean two-dimensional velocity maps, he gets exactly the local relation. So it isn't just the luminosity function of blue galaxies that is static!


galaxy mergers, day 1

Today was the first day of the Galaxy Mergers: From the Local Universe to the Red Sequence meeting at STScI. The many very good talks today included one by Schiavon (Virginia) in which he did very detailed analyses of the mean galaxy spectra we produced for red galaxies in this paper. He finds non-trivial age and metallicity variations with galaxy mass and environment, as we expected (but were unable to do ourselves, since models are complicated).

Bell (MPIA) argued that any galaxy evolution scenario that cannot be ruled out observationally at the order-of-magnitude level is, by definition, plausible. This position is ascetic in its conservatism, but I love it.

Fall (STScI) showed evidence that the formation of star clusters is very regular (statistically) and depends primarily on the local (two-dimensional, don't ask me why) gas density. He noted a conspiracy that the mass function happens to be very similar to the luminosity function; this is not generic for sources like these because their luminosities change very quickly with time, as I wrote once in this paper. I love a conference that gives me opportunities like this to toot my own horn!


WCS explanations

Mierle and I learned today that all of the papers describing the WCS conventions are pretty difficult! None of them give the complete formulae for converting pixel location to sky coordinates or the reverse. They describe parts of the transformations in words. None of them give fully worked-out examples against which code could be checked.

Furthermore, the WCS conventions themselves are ill-defined, in that there are mutiple different parameters/keywords that mean the same thing, and potentially conflict. So it is a mess. We decided to work internally in astrometry.net with a simplified, well-defined, minimal WCS parameterization, and push all this ugliness to the original reading and writing of WCS at the beginning and the end. We started a polemical and pedagogical paper on the subject.

digital galaxy atlas

On Monday, I rewarded myself for weeks of work on that NSF proposal with a day of work on my digital galaxy atlas project. I worked on two things, both pretty technical:

I worked on the problem of producing, for each galaxy image, a noise map that gives approximately the correct pixel-to-pixel noise or uncertainty in the intensity. This is relatively trivial, in that we have a noise model (even an implemented noise model), but it requires some decisions (a) where I have smoothed the data either explicitly or implicitly, and (b) where overlapping fields contribute to the image.

I also worked on automatic identification, fitting, and subtraction of all satellite trails in the images. For this project I am using a Hough transform, in which the space of x and y (pixels in the original image) gets transformed into the space of angle and offset for linear trails (each of these is a pixel in the Hough transform image). I think I need to do some kind of median filtering because in the dumbest possible implemetation (recall that I always start with this!) pairs and triples of stars dominate the signal.