Yesterday I made a figure showing how we parameterize quadrangles of stars for the astrometry.net blind system. The parameterization is clever, because it is rotation- and scale-free, but if stars are uniformly (ie, Poisson) distributed on the sky, the parameters uniformly fill a compact, finite, simply shaped volume in four-dimensional parameter space. This is not a requirement for a good system, but it makes a few things much easier.
[for yesterday] Zolotov, Willman, and I specified Zolotov's first project with stellar velocities in the Galaxy halo in models and in data. I then read papers in preparation by Maller (on inclination effects in galaxies), by Sheldon (on using the weak lensing by clusters of background sources to infer the masses of the clusters statistically, and the cluster-mass cross-correlation), and by Quintero (on environmental relationships).
Yesterday, Sheldon and I discussed the recent press about the definition of the word
planet, under discussion at the IAU. We agreed that this press is not good press, because it implies that astronomers are scholasticists, interested more in the definitions of words than the origins of our Solar System. Not all publicity is good publicity!
On the research front, Blanton and I discussed a problem I have been thinking about a lot lately, which came up in a talk by Dan Maoz while I was at MPIA this summer: How do you figure out the distributions of properties of things, separated by type, when your type assignments are imperfect? Maoz faces this for imperfectly typed supernovae, but this comes up frequently (for example when galaxies are placed in redshift bins using noisy photometric redshifts). Unfortunately, the correct methodology is not written down anywhere convenient to the typical astronomer.
In between working on the first paper for astrometry.net there was a lot of discussion here at NYU about the offset between the mass center and the baryon center of the subclump in the
bullet cluster, which got a lot of press because it says some things about dark matter. This is a very hard observation for modified gravity theories to reproduce (at least the ones that want to replace dark matter with a modification to GR). More interesting to me is that the sub-clump is moving incredibly fast (hence
bullet), although the theorists say this is not incredibly unlikely.
During a week in the wilderness, I worked on the paper describing our success with blind astrometry with SDSS data. Thanks to the heroic work of Lang and Mierle, our success rate is over 99.8 percent and we have no false positives after having solved (blind) over 300,000 SDSS r-band fields.
Yesterday Bhardwaj and I measured the average current star formation rates in her samples of elliptical galaxies, using the H-alpha line luminosity, as a function of total galaxy stellar luminosity (stellar mass). Her next job is to compare these estimates to the star-formation rate estimates from GALEX UV imaging, and from the suite of models that explain the total galaxy spectral energy distributions.
Zolotov and I figured out the first-year physics problem of estimating the angular momentum vector for a galaxy disk in a numerical simulation of a forming galaxy, and transformed all the particle positions and velocities into the simulation equivalent of galactic coordinates.
Masjedi, Blanton, and I discussed the current state of redshifts for the PRIMUS prism spectroscopy project. Blanton is taking over for Masjedi while Masjedi gets some papers written. We concluded, as we always do, that calibration is everything.
Zolotov and I worked on preparing simulation outputs so that we can "observe" the stars in simulated galaxies much as the SDSS observes stars in the Milky Way, in preparation for some comparisons.
It was a sad day yesterday, as it was the last day for Quintero at NYU; we will miss him!
Quintero led group meeting; he spoke about a new project we have conceived to find the progenitors of bulge-dominated, red, dead galaxies. We think we will be able to use their statistics to test or constrain the emerging picture that AGN feedback must be involved. We can also refine the limit from Quintero et al (2004) on the birth rate of new bulges.
Yesterday, Willman, Zolotov, and I found a specific focus for our project on stellar velocities in the SDSS.
I also discussed and read a very nice new paper by Marla Geha (OCIW) et al on the gas in dwarf galaxies, to appear on arXiv soon. They find that when you add gas to stars, there is a very straightforward baryonic Tully-Fisher relation, implying that there is very little in the way of mass-dependent expulsion or stripping of baryonic material from low-mass galaxies. They also find a very strong dependence of the gas fraction distribution on small-scale environment; I think it is the strongest environment dependence known for galaxies.
On the last day of the Oort meeting, the four groups (which were working to answer four questions posed by Blandford at the beginning) made their presentations. The two groups that did publishable calculations were the Hubble Constant group (my group) and the substructure lensing group. Our group concluded that there is a strict 99-percent upper limit on the Hubble Constant from the known lens systems of 78 km s-1 Mpc-1, found by making the maximal assumption that mass follows light. This limit is much stronger than that from the HST Key Project or any other direct method other than perhaps the baryon acoustic feature in the LRGs.
The substructure group took a skeptical view and showed that there is no absolutely clear evidence in multiple imaging systems for truly dark substructure in dark-matter halos, in the sense that all lenses with magnification anomalies can be explained either by giving mass to luminous substructure or else line-of-sight effects like extinction or scattering.
Today Chris Kochanek (OSU) and Paul Schechter (MIT) faced off over microlensing and substructure, but unfortunately, they resolved their differences over breakfast, so no sparks flew. Schechter gave a beautiful, pedagogical description of the effects of micro- and milli-lensing (ie, the perturbations to smooth lens models by substructure and clumps). Kochanek showed that he can fit the detailed microlensing light curves of real gravitational lenses by brute force: He actually creates billions of totally random microlensing histories, and finds the few that fit each lens; from these he infers the masses of the stars, the velocities of the stars, and the sizes of the quasar emission regions.
In the afternoon, the Hubble Constant group worked late getting ready it's presentation for tomorrow: We have a much better upper limit on the Hubble Constant than the HST Key Project
On the third day of the Oort meeting here in Leiden, Gary Bernstein (Penn) told us how to analyze future data sets for weak lensing. It was nice to see some of the nitty-gritty, and in fact Bernstein and collaborators are working on some ideas that I love: For very high-accuracy data analysis, especially when instrumental effects are large and signal-to-noise is low, the best approach is to explicitly model the data (rather than
measure things in it); in this approach your
measurements are just parameters of your reduced-chi-squared-unity model of the data.
Bhuvnesh Jain (Penn) followed Bernstein with a discussion of the limiting systematics for future weak lensing surveys. He identified redshift distribution as a significant issue; this might be an independent motivation for the PRIMUS project. Interestingly, Jain was very confident that the atmospheric distortions to the point-spread function will not be a problem (I didn't agree).
Wayne Hu (Chicago) gave his talk today about
precision cosmology. Blandford forced Hu to slow way down when he was discussing the specific assumptions that go into the determination of the distance to last scattering. Hu's discussion was absolutely great; he argued that there are very few assumptions involved, except maybe adiabaticity (which is tested, but not at the level required). He spent some additional time encouraging us to determine the Hubble Constant. He espoused some heresies including that he considered a small amount of spatial curvature much more plausible than a non-trivial equation of state for the dark energy (I agree); these are degenerate for some experiments.
In the afternoon we continued on the Hubble Constant. I spent time re-reading Eisenstein et al (2005), a paper on which I am proud to be a co-author, though I can't take credit for some of the best parts. The baryon acoustic feature in the LRG correlation function constrains strongly the distance to redshift 0.35, but less strongly the Hubble Constant, because uncertainties in the world model enter. Similarly, I realized, the lensing time delays only measure the Hubble Constant in the context of a specific world model; really each system constrains a combination of cosmological parameters. It's my job to figure that out tomorrow.
Yesterday (Monday) was the first day of the Oort Workshop in honor of Roger Blandford (Stanford) and about gravitational lensing. Blandford opened the meeting with five questions and has encouraged the group to split into teams to answer them by the end of the week. I joined the Hubble Constant group, where the question is
Can we use lensing to determine the Hubble Constant? At first we were all extremely negative, but slowly a consensus emerged, that each lens can, in principle, give you a very strong upper limit on the Hubble Constant. This is because the predicted time delays among images depend on the radial concentration of the mass, and the mass profile is very unlikely to be more concentrated than the stars.
Wayne Hu (Chicago) convinced us that this was pretty important, because the CMB only measures the Hubble Constant if you assume flatness and vanilla cosmological constant. A strict upper limit can rule out strange models (like NYU-born DGP), and a strict lower limit (also possible, we think) can rule out spatial curvature and scalar fields that have different redshift evolution from the cosmological constant.
The upshot is that the Hubble Constant group is working towards writing a paper on the subject, which is ambitious, but not obviously impossible.