post-starburst environments, computers

I worked on responding to referee on our post-starburst environments paper, the one that shows that among IGM interactions, tides from nearby neighbors, and mergers as plausible triggers for the starburst, mergers is preferred, statistically. The paper will be substantially improved in clarity when I am done.

We ordered a bunch of computers for astrometry.net.


clustocentric distances

With pressures from teaching and recruiting all high, the only research I was able to get done today was to give comments on Quintero's response-to-referee on his paper on the dependences of galaxy property distributions with clustocentric distance.


GRBs and SNe

Andrew MacFadyen (IAS) gave a nice talk about collapsars—relativistic outflows from newly forming black holes punching through their massive stellar envelopes and appearing as gamma-ray bursts. He had some simulations which suggest that the relativistic jet can bore a hole through the envelope to make a channel for a clean (ie, not mass-loaded) jet. One piece of evidence for this general picture is the association of GRBs and SNe, which has gotten very good recently. In related news, he doubts that all massive stars go supernova, and thinks the supernova properties might depend strongly on metallicity and other factors. This is good news for those of us who are suspicious about using chemical abundances to trace formation histories.


astrometry statistics, wind power

I figured out on the plane to CA that it is impossible (well, hard) to do analytic estimates for success and failure rates for our quad system of blind astrometry without a simulation. I will write that next.

I tried to estimate the amount of power available to the Earth from wind power. I got about 5 watts per square meter, assuming strong winds but inefficient generators. That ain't much power. What does the industry say?



Sam and I reminded ourselves of some near-term goals for astrometry.net. I started to work on one of my "tickets": to compute theoretically expected success and failure rates, under simple assumptions.


transparency, Padmanabhan, Ye

More discussions with various people about transparency; in particular the Tolman test, which I had not realized is a great test of transparency. It is a fundamental prediction that the luminosity distance be (1+z)2 times the angular diameter distance; any departure indicates a lack of transparency (or a modification of extremely fundamental physics; I think this emerges pretty much directly from Liouville's theorem).

Nikhil Padmanabhan (Princeton) gave a great talk about measuring large-scale structure with purely photometric (ie, not spectroscopic) data. He showed reasonable evidence for the baryon oscillations, and the power spectrum for LRGs, as measured with 1.5 (Gpc/h)3 (the largest volume ever used for such studies). During his talk I realized that we can never have an enormously significant measure of the baryon feature, since we have a few sigma in SDSS, and the SDSS, in effect, measures of order 3 percent of the entire Universe (present day). The square root of 30 is not a big number, and, more-or-less, that is about as much better as we can do.

Jun Ye (JILA) gave a great talk showing incredibly stable, incredibly precise frequency, time, and wavelength standards, using frequency combs. His viewgraphs showed frequencies like 429,228,004,229,900.0 Hz, with uncertainties less than 1 Hz. In time, this kind of work will require an update in the definition of the second (which is currently based on microwave, not optical, frequencies), and permit insane fundamental tests of physical quantities and systems.


morphology, Kaloper

I started making figures for my future paper on the morphology–density relation, showing that it is subordinate to the color–density relation.

Kaloper (Davis) showed that w<−1 can be mimiced by a w=−1 universe plus some photon decay or transformation into axions. It inspired me to think about the transparency of the Universe, which I discussed with Weiner and Blanton.


Ménard, Quintero, Wu, HST

Brice Ménard (IAS) gave a nice talk on the galaxy–mass cross-correlation function as determined by lensing magnification, not shear (as Sheldon is determining it here at NYU). It is an ambitious project with good future prospects. In addition, he gets for free a measure of the dust extinction associated with galaxies, the galaxy–dust cross-correlation, as it were. He discussed publication bias in the previous literature on magnification bias, and gave some evidence that the set of observational results published is, on average, higher than expected because those with null results didn't publish! In other news, he also showed what I think is a galaxy light–gas cross-correlation function, made by stacking quasar images where MgII absorption is present at lower redshift. This is a promising technique for measuring gas contents and extents of large samples of galaxies, though only statistically.

Quintero is nearly done with his response to referee, which is quite a bit of hard work. Blanton, Berlind, and I discussed with him and promised him comments on what he has so far.

Wu and I discussed the problem cases in her match of Binggeli's Virgo Cluster Catalog to the SDSS catalog.

I remembered that HST proposals are due next week and panicked.



I am back at work after a great trip. Today I wrote code to convert CMYK to RGB and back according to the physical principles I lay out in the great conversion document I am writing. Yes, this counts as research.


thermodynamics, color

I reminded myself that when a rapidly creating and annihilating particle species is in thermodynamic equilibrium with its decay products, its abundance is proportional to the momentum space volume available to it, but supressed by a Boltzmann factor on its mass relative to the temperature. For highly relativistic situations (temperature much greater than mass), the equilibrium abundance goes like T3 (because the momentum is the temperature and there is no significant Boltzmann suppression). For non-relativistic situations, the abundance goes like (mT)3/2, (because the momentum is proportional to the geometric mean of the mass and kinetic energy) times the Boltzmann factor exp(−m/T). Duh!

I started to write up my ground-breaking and earth-shattering treatise on RGB to CMYK conversion. Get ready, world!


southern sky

I learned from Christina Hood how to find the Southern Cross, and the Large and Small Magellanic Clouds. I also took some photos of the dark sky with a digital camera, in the hopes of putting them together into a deep image. Not sure if this really counts as research, but hey, I'm on vacation.



We've now written two papers (here and here) that indicate that the morphology–density relation is merely a product of the morphology–color relation and the color–density relation, but which contains little over and above that product. These papers were limited by our blunt morphological tools; ie, we have quantitative measures of galaxy properties that are closely related to classical morphological classification, but not identical with it.

I realized that I can demonstrate the point directly with images of galaxies, as we did for our contribution to AAS 205, and let our readers do the morphological classification any way they like. I started to write this up.