Virgo, chemical potential

I mailed comments to Wu on her work on re-identifying Virgo Cluster member galaxies within the SDSS (which covers Virgo almost completely). This is a non-trivial project, because we are trying to cross-identify every member of Binggeli's original Virgo Cluster Catalog.

I tried to understand the role (and values) of the chemical potential in early-universe thermodynamics. I guess it should be obvious, but it has been a long time since I have taken a statistical physics class.


relic abundances, CMYK

On the plane I worked out (for the first time since about 1993) how one calculates the relic abundance of an annihilating particle species in the expanding universe. Interactions (creation and annihilation processes) try to keep the species in thermal equilibrium (in abundance and in energy). The expansion of the universe dilutes the abundance and redshifts away the energy. The net creation rate (excess creation rate over annihilation rate) is proportional to the deficit in the abundance relative to the thermal equilibrium abundance. Kolb & Turner derive this with a very complicated argument, but it follows nicely from detailed balance (as they note, to their credit). The net creation rate is also proportional to the net mean effective annihilation cross section (sigma), which depends on the creation and destruction processes, the masses of the species and the decay products, and the energy distribution in the species and decay products (since you need to have sufficient energy to create new particles).

I also worked out some things related to RGB to CMYK conversion (in the context of our galaxy atlas). Most of the "algorithms" in use are wrong, because they don't consider the physics of "subtractive" color systems. I am going to write something up about this.


posting will be spotty

I am off for four weeks of travel; posting may get spotty. I may even violate the rules at right.


mass profiles, HI masses

In the morning, Sheldon showed us that the weak lensing signal from massive clusters is consistent with the NFW profile, and that cluster richness is closely related to mass.

In the afternoon, Navarro showed us that CDM profiles really are (plausibly) shallower than r−1.5 at their centers, with a number of arguments. He also showed us that triaxiality of galaxy halos affects rotation curves significantly; plausibly enough to place currently discrepant measurements in agreement with models.

Katarina Kovac (Groningen) came by and showed us a very nice measurement of the HI mass function. She went from raw data (1400 pointings, each producing a 3 million pixel data cube) to mass function; nice work! She has much better sensitivity than HIPASS and finds a "flat faint end slope".


not much

Letters of recommendation are a necessary part of my job, and it is a pleasure to write good ones, but I can't say I got much research done today.


black holes, lensing

Noam Liebeskind showed us that if black holes suffer large kicks at merging, you can't get a tight black-hole-mass vs bulge-mass relation. Liebeskind's technique was monte-carlo on the mass growth through mergers, like a technique that Alice Shapley and I discussed (in Chicago) for analysis of the width of the red-galaxy color–magnitude relation (when you allow dry merging). But Gruzinov noted that you might get even stronger limits on kicks by looking at the centrality of the black holes, since the relaxation time for an off-center black hole might be long!

Bart Pindor dropped by and discussed lensing with me—or at least he listened when I went on about lensing statistics. He was suspicious that arc statistics could be considered evidence for non-gaussianity.


cluster lensing, pretty pictures

Another great day at New Views, featuring talks (mainly) about galaxies. NYU work got many mentions today, including my polemic (by Dalcanton), Masjedi's LRG–LRG clustering (by Weinberg), and Berlind's halo-occupation stuff (also by Weinberg).

In the course of a long day, I learned things too numerous to mention, but the big shocker for me was Gladders's talk about massive galaxy clusters. He showed, among other things, that the redshift distribution of lensing-arc-selected clusters is totally wrong; the number found per redshift increases with redshift when all predictions (by, eg, Dalal) have it fall with redshift. It ought to fall with redshift because at higher redshifts the clusters are less numerous and less massive and have smaller (angular) Einstein radii (even at fixed mass). The observations are so far from the data, you must either: (1) blame an enormous selection effect (and few can imagine one so strong), (2) postulate a bizarre evolution in the central densities of clusters (opposite to what you would imagine), or (3) seed the early Universe with non-gaussian cluster seeds to derail the evolution in the mass function by gravitational collapse. The last option is clearly insane, but Gladders also noted that there are some huge galaxy clusters observed at substantial redshifts. Nikhil Padmanabhan and I discussed the possibility of a blind arc search in SDSS; it would be shallow but wide.

Before school, as it were, I started making some pretty SDSS images of galaxies for a documentary being produced by the American Museum of Natural History.


New Views

Today I arrived in Chicago for two days at New Views of the Cosmos, a new Kavli conference in honor of Dave Schramm. I saw talks about dark matter (Frieman), neutrinos (Conrad), the composition of cosmic rays (Hoerandel), neutrino dark matter (Abazajian), accelerator production of dark matter (Baltz), positrons at the Galactic Center (Yuksel) and many others.

Something that interested me greatly was what Jedamzik said about discrepancies (relative to big-bang nucleosynthesis) in the 7Li and 6Li abundances relative to predictions (7Li is too low by a factor of 3 to 4, and 6Li is too high by a lot). This used to be thought to be a problem with stars (maybe 7Li is somehow depleted or destroyed) and cosmic rays (maybe 6Li is produced entirely by CR spallation), but now neither mechanism seems to be working well, especially since both abundances show little system-to-system scatter and a weak dependence on overall metallicity. This might be a window into new physics, such as energy injection at MeV at BBN (decaying SUSY particles?) to spallate 7Li and 4He to 3He (which then makes 6Li). As Jedamzik noted, Dave Schramm would have liked the idea that BBN anomalies could be used to discover new physics. (Later, Dave Tytler also discussed this a bit).

At lunch, Blandford and I discussed finding cosmic strings in HST data by searching for patterns of multiple imaging. Nice, and possible!


Moustakas's metallicities

Today's talk (it has been a seminar-filled week) was by John Moustakas (Arizona), who gave us unparalleled evidence for evolution in the mass–metallicity relation. Good, hard evidence for evolution is not easy to find. Unfortunately, he can not yet show direct evidence for non-closed-box evolution (as I am always on about), such as merging or accretion or outflows.

John has put a lot of time into taking integrated spectra of a large variety of galaxies in the local Universe; he showed very nice relationships between line strengths, chemical abundances, and dust. His sample of low-redshift galaxies provides a great baseline for high-redshift studies; oddly the low-redshift part is often the hardest to get (and to get right).


Zaldarriaga's telescope

Matias Zaldarriaga (Harvard) gave a colloquium about directly observing atomic hydrogen between the recombination (should be combination) epoch and reionization, in (redshifted) 21-cm radiation. At redshifts 100 to 30, it is visible in absorption (because the spin temperature is locked to the gas temperature, which is colder than the CMB (assuming no stars or AGN have formed). At redshifts around 10, it is visible in emission, because the spin temperature becomes hotter than the CMB (although this doesn't make it very bright, since both the CMB temperature and the spin temperature exceed the characteristic temperature of the transition, about 0.07 K). Anyway, it appears to be a huge store-house of information for fundamental cosmology (better than the CMB because it is 3-d, not 2-d, and it doesn't suffer from Silk damping at small scales), and it will be measured (if all goes well) this decade.


Navarro's galaxies

Julio Navarro (UVic) showed us that when small galaxies merge with large, if the large is oblate, the small's orbit is pulled into the "disk plane" of the oblateness by anisotropic dynamical friction. This opens up the possibility that there is structure and accretion history encoded in the disk, especially since the radius at which stuff settles is generally related to the central density of the incoming satellite. Note the contradiction—or at least tension—with the discussion last week.


Waxman's neutrinos

Eli Waxman (Weizmann) gave a great lunch-time talk on the ultra-high energy neutrinos, the neutrino counterparts of ultra-high energy cosmic rays. He gave a nice argument that the CRs must come from incredibly luminous sources, based on their energies. My summary of this argument is that for any energy/charge, or potential, there is a corresponding power, or luminosity, V2/R. In CGS units, resistance has units of velocity, so the speed of light is a fundamental resistance (ie, the impedance of free space). Of course, Waxman gives a physical explanation of this, not my mystical one, but the upshot is the same: Very high energy particles means very high potentials, and very high potentials mean very luminous sources or engines. Waxman, pretty convincingly, argues that the engines are probably gamma-ray bursts.

Waxman and the late John Bahcall (IAS) showed that this CR flux requires an ultra-high energy neutrino flux, and places a strong upper limit on the extragalactic neutrino flux over a wide range of energies. Interestingly, these ultra-high energy neutrinos will be detected by the next generation of detectors (Ice Cube and the like). If they come from GRBs, they will point back to their sources and they will be prompt, ie, simultaneous with the photon burst (both niceties are true because neutrinos don't suffer magnetic deflections). Then Waxman will deserve a big prize.


SDSS catalog access

Thanks to Schlegel and others for writing very useful building blocks, I was able to hack together a piece of code that (relatively) quickly provides all SDSS sources within an angle d of any sky point. This is for fast solving of HST image astrometric WCS, of course.



On the last day of Willman's workshop, Raja Guhathakurta (UCSC) led a very nice discussion of what we know about M31 structure, substructure, and ongoing mergers. The age and metallicity of the infalling stream, apparently, is very similar to that of the bulge. He also showed that the bulge of M31 looks more like a n=2 Sérsic profile than a de Vaucouleurs, confirming my suspicions about bulge/disk decomposition for more distant galaxies. Indeed, he showed that the separation of bulge, disk, and halo is not at all easy for M31.


ANITA, astro-ph

Steven Barwick (Irvine) gave a nice talk about ANITA, a project to measure radio Cherenkov light from the energetic neutrinos related to ultra-high energy cosmic rays. It occurred to me that it might be possible to make a long-wavelength telescope in Antarctica that measures, simultaneously, radio emission from the sky and radio emission from neutrinos in the ice.

I posted my Marseille review to astro-ph, and Quintero's paper appeared too.