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.



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.


cross-correlation and average image

I started on my quasar–photon cross-correlation code. If you preserve the azimuthal information, this cross-correlation is what astronomers would call the average or stacked image of the quasars in the GALEX imaging. That's a nice talking point, about which I may wax poetic soon.


photon—quasar correlations

Schiminovich and I figured out that one possible method for working out the sources of the metagalactic ionizing radiation impinging on the Milky Way (local extragalactic intensity field at 912 Å) is to cross-correlate the far-ultraviolet (and also near-ultraviolet) photons recorded with GALEX with quasars at various redshifts. I have waxed poetic about cross-correlations before, because they can be measured at very high signal-to-noise in common situations, and often contain all the information you can possibly have (see yesterday's post, for example, and posts on statistical gravitational lensing previously).



Brice Ménard (CITA) gave a nice seminar at Columbia about angular correlations between background (redshifts one and higher) quasars and foreground (redshifts one third) galaxies. The correlations are dominated by lensing, but also have a small color term which is consistent with absorption by dust. His results on dust compare favorably to my results with Jo Bovy and John Moustakas, although he worked entirely in angular units; it is somewhat easier to interpret and model in projected transverse distance units (a simple modification to their current strategies). He had not done any de-projection or halo modeling of the results, so he couldn't precisely say what dust is associated with galaxies of each individual type.

After the talk Lam Hui (Columbia) and I discussed various matters transparent, including tests for the monopole (unclustered) term in attenuation, and possible other explanations for chromatic effects in photon propagation. For example, if the dark matter is made of substantial millilensing or microlensing lumps, and if quasars have a wavelength–size relation, there might be chromatic effects in the angular correlations.


nonlinear dynamics

Bovy and I spent the morning discussing places chaos (as in nonlinear dynamics) enters into astrophysics, since he is interested in both subjects. I identified several places: In the Solar System, the Lyapunov time is much shorter than the time over which there has been and will be macroscopic stability. There are also many issues in planetary system formation. In numerical models of globular clusters and similar N-body systems, some of the relaxation times seem to disagree with analytic/scaling expectations. In the disk of the Milky Way, there is very complicated velocity-space structure, possibly caused by caustics and other nonlinear structures in phase space. In the halo of the Milky Way, phase-space structure is expected to be extremely rich because the accretion history is extremely rich. In cosmology, there is my insane idea of constrained realizations of the entire observable Universe, which has a nonlinear dynamics or control theory aspect to it.


archetypes, luck, outreach

Spoke with Jo Bovy about finding spectral archetypes among the SDSS spectra; Roweis has code to find (an approximation to) the minimal set of archetypes that represent all of the SDSS spectra, if we give him the graph of which galaxies represent which others. This set of archetypes would be the ultimate in non-parametric descriptions of spectrum space; far preferable to things like PCA, which assume that the galaxies come from (and fill) a linear subspace; and it would permit new kinds of galaxy modeling and fitting.

At Pizza lunch, Arlin Crotts (Columbia) spoke about lucky imaging they are doing with a small telescope on Kitt Peak. He described a good night in which they could use two percent of the data for a diffraction-limited (or nearly) stacked image. This led to long philosophical discussions with Schiminovich about whether you can or should use all of the data. I took the position—that you might imagine—that each image must contribute information; there is no way that adding in information from a new image could reduce the information; if adding in less-good images is making the stack worse, then those images are being added in wrongly. Of course adding them in rightly might be damned difficult, since it probably involves something akin to deconvolution (or, as the astronomers say, forward modeling).

In the evening, I spoke to the Amateur Astronomers Association of Princeton about Astrometry.net and the future Open-Source Observatory. Extremely enjoyable!


metagalactic radiation field, healpix

I wasted much of the day building IDL routines to make pretty pictures from healpix maps. This was a waste both because there are better programmers than me, and better languages than IDL.

Over lunch, Schiminovich and I discussed our project of estimating the intensity of the ionizing radiation impinging on the Milky Way from extragalactic sources, and the contribution from different kinds of sources. We discussed the issue that most of the ionizing radiation comes from significant redshift; this is both because quasars are more abundant and luminous at higher redshift, and because galaxies tend to be self-shielding. Hopefully we can refine our understanding of the radiation with GALEX.


Dr Pietro Reviglio

I sat on the (successful and entertaining) thesis defense of Pietro Reviglio (Columbia), who used FIRST, NVSS, and SDSS to investigate the evolution of AGN, and the relationships between radio type and optical spectral type. He finds significant evolution, and some problems for the orientation model for the differences between broad-line and narrow-line AGN. He also finds some evolution in the host galaxy population that is consistent with some structural evolution (as in building bulges from disks), although that conclusion is more speculative, of course.


more writing, warm and hot

More writing yesterday and today. Lang made some figures for the nascent paper. I also spent some time at the Warm and Hot Universe meeting at Columbia. Several nice talks on clusters, and some good stuff on new missions.


proper motion paper, archetypes

Worked on writing up the proper motion measurement paper with Lang, and discussed the objective generation of spectral archetypes with the PRIMUS team.


brown dwarfs and quasars separated again

My loyal reader will recall that I spent the summer writing code to separate quasars and brown dwarfs using proper motions, measured in very low signal-to-noise data (that is, multi-epoch data in which the source is not detectable at any individual epoch), and that I spent the spring re-writing that code with the help of Lang. Today we finally closed the loop and showed that indeed we can separate the two populations using angular kinematics:


convex hull

Among many things which were—believe it or not—more boring than this, I spent part of today getting a geometric picture of the convex hull for stellar colors. An unresolved binary star system is composed of two stars; a galaxy is composed of many stars. Stars do not fill all of color space, but live on some restricted set of loci in the multi-dimensional space of astronomically observed colors. Combinations of stars, such as binaries and galaxies, must therefore also live in a subspace of the whole space. This is a no-brainer, but it is not trivial to visualize the situation, because color space is not linear in any sense. Some of the results were a bit surprising to me, which is embarrassing.


GALEX, tech report

Chris Martin (Caltech) was in town, and we discussed many things GALEX. In particular he got me excited (only I could get excited about this) about re-calibrating the sensitivity map, which is a non-trivial function of detector position. This all came up because Schiminovich and I have some ideas about using the time-tagged photon stream; this stream has residual artifacts in it from imperfect measurements of the sensitivity map, which is sampled in a well-defined way as the spacecraft dithers during the exposure.

In the morning, after a long bureaucratic mission at City Hall and before a power lunch with Masjedi on Wall Street, I worked on what the Astrometry.net team calls the tech report, the paper about the system that is written in the style of an Astronomical Journal submission, summarizing success to date of our blind astrometry and data recovery system.