hypothesis comparisons

After Lang gave the Galaxy Coffee talk at MPIA, there were lingering questions about the differences between modeling a little smudge in co-added multi-epoch data as a moving, unresolved star or as a non-moving, extended galaxy. In the co-add, these hypotheses are hard to distinguish, but in the individual images, these hypotheses are very different, even though the object may not be measured with good signal-to-noise at any epoch. We began the work of explicitly making the non-moving galaxy model, so that we can perform clean hypothesis tests and quell the last of our critics.

synthetic image modeling

Lang and I discussed once again the issue of matching up datasets at the catalog level, and learning thereby about the positions and motions of stars on the sky. In each of these discussions we always conclude that the limitations of catalog level are such that we always want the images and to work at image level directly. However, today we realized that we could work with synthetic images, created from the catalogs and our model of the sky. The parameters of the model of the sky could be optimized to create synthetic images that best fit the synthetic images created from the set of catalogs. This got us substantially closer to the scalar objective we seek at catalog level.

saving dollars with astrometry

Lang and I have showed that we could have saved the very-red-objects community quite a bit of telescope time (read: money) by measuring proper motions of faint sources and thereby obviating a bit of spectroscopy. But Rix asked us if we could have saved them not just the spectroscopy but also the infrared imaging. Lang and I spent time on this question today. Not sure that we will be able to be so cocky here.

supernova rates, GAIA

Lang and I sat in on the MPIA GAIA group meeting. We discussed the photometric and spectroscopic identification of stars, binary stars, galaxies, and quasars in the GAIA data stream, and tests on the SDSS data and other related data. The GAIA team is using support vector machines, which also got Schiminovich and I excited last month; the GAIA team may be the main (or only?) users of SVMs in astronomy. It turns out there is work here that is similar to the archetypes project I have been pitching to Bovy.

In the late afternoon, Dani Maoz (Arcetri, Tel Aviv) gave a nice talk on supernova rates, focusing on Type Ia rates. He made a pretty good case that some of the Type Ia supernovae are prompt, and those that aren't prompt occur on the short side of the delay distributions that are discussed in the literature. This makes it interesting that galaxies show such clear alpha-enhancement patterns.

group finding

Lang and I pair-coded some web-based analysis of stars taken from the SDSS imaging sample, looking for groups that are plausibly tidally disrupted structures in the Milky Way halo. We didn't find anything, although we wrote code to automatically name it if we do!

nearby supernova, fast movers

Oliver Krause (MPIA) gave a beautiful talk about the discovery of light echos from the Cas A supernova and its identification as a type IIb. This identification was performed by taking a spectrum of the original supernova, but delayed by 300 years because it is being observed now in reflection from a nearby dust cloud! The identification is remarkable, because as of now it appears that the Local Group is way over-represented in type IIb SNe.

Lang and I worked on the fast-movers in our faint proper-motion paper.

structure in point sets, brown dwarfs

The MPIA was abuzz today with talk about finding structures in point sets with non-trivial error properties. This, of course, relates to the identification of streams and satellites in the Milky Way halo, which has been an industry for the last few years, since the discovery of Willman 1 at NYU in 2005. Lang was consulted, as our computational statistics expert.

Lang and I also made the list of low-mass star (including brown dwarf) candidates from our proper motion work into a LaTeX table for publication.

USNO-B and 2MASS

Lang and I conceived of and started to work out a project to build an all-sky astrometric catalog out of the original USNO-B imaging catalogs and the 2MASS catalog, all tied to Tycho. This would be the first step towards our first Astrometry.net astrometric catalog. This would also be a first shot at doing source matching at the catalog level, a subject about which we have been talking for half a year. The first order of business, we realized, is to make a conceptual data model and a scalar objective function.

last day

Today was a relatively unproductive day, but my excuse is that it was my last day at Columbia. I have had a lot of fun up there, mainly thanks to the collaboration with Schiminovich. Next week I will start a thirteen-week stint in Heidelberg and other points in Europe.

mean DB WD image, transparency

The image below shows a measurement of the cross-correlation between DB white dwarfs taken from the SDSS White Dwarf Catalog and far-ultraviolet photons observed by GALEX.

The central source in the image is effectively the average (mean) image of a DB white dwarf in GALEX; its morphology is (roughly) consistent with a GALEX point-spread function. The other sources in the image are residuals created by random bright FUV sources nearby to individual DBs; there weren't enough DBs to make this all average away; none of the nearby sources seen in the image are significant if you ratio this image to the root-variance image obtained by jackknife resampling.

Schiminovich and I spent a long time today discussing the possible effects that might make the same kind of average image not look like a point-spread function when the central sources are at cosmological distances, because of scattering and correlated sources. We also discussed how measurements of the average image as a function of sky position and redshift could constrain transparency.

average image, does environment matter?

Actually started up the average image slash quasar–photon cross-correlation code today. It runs. Does it give good answers? I don't know because it is incredibly slow. Ah well, if only I knew a good programmer!

Schiminovich and I spent time talking about religion and politics and then a bit about galaxy environments, in particular the resolution of the debate between Jacqueline von Gorkom (Columbia) and myself about whether environment matters to galaxy evolution. Being a stickler, Schiminovich objected to that characterization of the issue up-front. But I think we almost agreed that environment has its strongest effects on galaxies near the centers of clusters and merging galaxies. These systems make up only a percent or so of the galaxy population in the last few Gyr, so although environment matters deeply to these galaxies, they are a trace population.

We certainly know about environment effects on the bulk of galaxies, but it is straightforward and relatively uninteresting: At fixed mass (or other properties), the most important environmental effect is on the specific star-formation rate. But we know from statistical tests that environment affects this very slowly or very indirectly, probably by heating the interstellar medium (slowly, not generically in rapid bursts or events, which are extremely rare). So I think there is a consensus position possible: Environment matters, but a small amount to a large fraction of galaxies and a large amount to a small fraction.

mean image code

Nearly finished my mean image code today. This takes a set of points on the sky, and a set of matched random points, and creates the mean GALEX image of those points, properly background-subtracted.

ultraviolet lenses, pretty pictures

Bolton, Schiminovich, and I determined empirically that it would have been impossible to detect the SLACS lenses (which were detected as double redshifts in the SDSS spectra) using GALEX. GALEX detects some of the lensing galaxies, but it doesn't look like it detects (strongly) the background lensed galaxies (despite the fact that they are detected because they are strongly star-forming). Oh well.

Bolton and I made some pretty pictures (or relatively pretty) of the SLACS lenses. The challenge was to show the lensing clearly, despite the fact that most of his HST images are only single-band.

halo shape, Nature

Zolotov and I discussed how the Milky Way halo's shape is measured, with the configuration and velocity-space distribution of stars. She can repeat these measurements in a simulated Milky Way and see if the measurements match the truth.

We made it into the editorial page and news pages of Nature today, here, here, and here.

SLACS

Adam Bolton (Hawaii) gave the pizza lunch talk today, about the SLACS project, which has found a large fraction of all known strong gravitational lens systems by looking for double redshifts among the SDSS spectra. Later in the day, Schiminovich and I discussed with Bolton the possibility that the second redshift could have been discovered in GALEX, which is insane, but maybe possible.