energy injection, spectral extraction

Padmanabhan (LBL) gave the astro seminar on the consequences of dark-matter annihilation on the CMB. He showed that the energy injection can extend the recombination epoch and reduce the observed power on small scales. It is almost exactly degenerate with the slope of the primordial perturbation spectrum, so if you marginalize over energy injection at redshift 1000, you find that even WMAP doesn't say anything interesting about the tilt of the power spectrum (at least if you allow energy injections that are large like in these new Sommerfeld-enhanced scenarios).

Bolton (Hawaii) and I discussed optimal spectral extraction. Bolton and Schlegel have taken this to a new level, working out optimal methods even in the face of extremely non-trivial trace, and wavelelengh-dependent resolution. He also worked out (maybe for the first time) a truly geometry-independent meaning of the word "resolution" for a spectrograph or imager (it depends not on the size of the PSF directly, but only indirectly through correlations in the sensitivities of independent pixels).


generative model

Fergus and I talked for a while this morning about the Lang project of a generative model of the sky. Fergus opined that it would be an interesting project for computer vision, because it has a hope of being a complete model that generates all of the data, not just certain features or parts. On the other hand, it might become intractable fast, especially if we want the billions-by-billions covariance matrix. We discussed some of these issues.



Schiminovich and I got our plan off the ground to look at an all-sky catalog built from GALEX and USNO-B data. So far, it doesn't look pretty, but I am hoping that is a bug in my code, and not the photometric issues with USNO-B.


data-intensive computation

My research time today was spent writing about data-intensive computation, for various audiences. This is dangerously close to not research. In the evening I discussed it, among other things, with Lang.

[Today marks the 4th anniversary of this research blog. It has been a great tool for me (encouraging me to get my work done) and it has been a less-great tool for my colleagues (slowing down those who mistakenly added it to their RSS readers).]


cold streams

Rix called today and we discussed, among other things, inferring the shape of the Galaxy potential using the shapes of cold stellar streams from disrupting clusters. We agree that there are some almost model-independent things you can infer, just from the orbital plane of the stream. I have an intuition that those model-independent things can be put together to fully model the Galaxy potential, if you have enough streams, but Rix was suspicious about my argument that there will be thousands of streams detected in the next decade. I have a good argument (coming soon).


dark-matter annihilation

I have been on about dark-matter annihilation recently. Today, Michael Kuhlen (IAS) gave a very nice talk about it in the context of dark-matter simulations of galaxy formation and the Fermi (GLAST) satellite. There is a shot that dark-matter substructure in the Galaxy might be detected in the next few years by the annihilation signal in the Fermi maps.


catalog matching

I tried to get more specific—on paper—about what you can and can't learn by matching catalogs, and why simultaneous modeling of imaging is so much better. It turns out that it is better not really because it overcomes all the disadvantages of catalog matching (though it does overcome many of them), but mainly because it has different (and better) explicit and implicit assumptions. Unfortunately, this is getting a bit subtle.


decadal survey, stream computation, high-resolution spectroscopy

This may qualify as not research, but this morning Schiminovich and I discussed the pros and cons of making a pitch to the decadal survey, the astronomy-wide priority-setting exercise.

At lunch, Kathryn Johnston (Columbia), Joo Heon Yoon (Columbia), and I discussed Yoon's simulations of satellites disrupting tidally in a galaxy in the presence of substructure. This work complements my analytical work on the subject and there may be a collaboration possible.

Wu, Schiminovich and I spent the afternoon looking at the high-resolution (for Spitzer, anyway) Spitzer spectroscopy of normal galaxies from our S5 project. Many of the galaxies have clearly visible narrow lines, even with our current quick-and-dirty reductions.


catalog matching

I reviewed projects with Bovy this afternoon. This conversation renewed my excitement about the discovery Lang and I made this summer: Catalog matching is something close to NP-hard, or perhaps even ill-posed, even in the best of circumstances. That is, it is impossible to create a matched catalog from two catalogs, one coming, say, from GALEX and one from HST without making assumptions that are unjustified (perhaps even demonstrably wrong) and that will affect significantly the output. This is true even when the two catalogs are without imperfections! The only exceptions are cases in which one catalog is considered ground truth and we desire only probabilistic information from the other catalog, but even here there will be exceedingly ambiguous catalog entries.

We conceived of a solution to this problem, which involves image modeling, where that modeling can be of the original imaging data on which the catalogs were based, or else synthetic imaging created from the catalogs in question. I resolved to write up this solution, with Bovy performing an example analysis to demonstrate its feasability.


writing, thesis priorities

[Back at work]

Most of the day was spent preparing for the semester. In research time, I spoke with Lang for a while about prioritizing the remaining work for his thesis. We decided that no more fun is permitted until the thesis is done. That's no more fun for Lang and no more fun for me. My job is to give detailed comments on the introduction to the main paper on Astrometry.net.

In the week since my last post, I have not done much research except working on long-term writing projects, and thinking about issues in stellar dynamics.


perturbation theory

I re-did some perturbation theory calculations from this summer today, to check and bolster my substructure-and-streams project.


substructure and streams

Wrote the introduction to my paper on detection and localization of dark-matter substructure in the Milky Way with cold stellar streams. More details as I figure out what the paper is actually about.



[Still (mostly) on vacation.]

In what research time I have found in the last few weeks, I have mainly worked on some long-term writing projects, one of which is about the empirical basis of astronomy and cosmology. This is an interesting subject, because the data are so indirect (photons only), from objects that are so remote, in a context where there is no experimental activity, only passive observation. Not entirely research, but not irrelevant either.