photometric redshifts

Back in Seattle, Bovy, Hennawi, Myers, and I discussed the most general extreme deconvolution approach to photometric redshifts, with quasars as the sources of interest. Since it has been a whole week, Bovy has, of course, implemented fully what we had in mind! It performs incredibly well, naturally incorporates GALEX and UKIDSS data where we have them, can be marginalized along directions other than the photometric directions to give fully empirical (hierarchical) prior PDFs over quasar spectral energy distributions, and improves quasar target selection. It is pretty sweet! And XD is so fast now, he was able to optimize the (more than) ten-thousand parameters in an hour or two. Have I mentioned that I love my job?

fitting on the celestial sphere

Lang and I got the Tractor working today in celestial coordinates. Usually source detection, photometry, and morphological fitting is done in images in pixel space (x-y position on the CCD). But if you want to properly combine data from multiple images (and especially images in different bands or with different PSFs), you have to do your fitting simultaneously in both images, and use some world coordinate system to convert positions in one image to positions in another. We gave the Tractor this capability today, in a very one-sided pair-coding session (Lang worked, I chatted).

proposal, Tractor

Rob Fergus and I finished the proposal today. In the process, I think I got him psyched about (some aspects of) astronomy. We vowed to meet next week to talk about cosmic-ray modeling, which is absurdly naive right now, and key to getting good image models. Last night, after I posted here (but for a break from all this proposal-writing), Lang and I discussed the Tractor, our image-modeling vaporware. We figured out a minimal-scope paper to write, and code to release. Tomorrow we hope to get some traction (as it were).

more proposal...

...and more and more. Damn I spend a lot of time writing proposals.

proposal writing

Friday and all weekend was taken up with proposal writing, which is still on-going. A lot of the proposal is about replacing astronomical single-point estimates (that is, arithmetic operations on the data that produce a single estimate of a quantity, like a maximum-likelihood estimate of an object's position) with a probabilistic treatment (that is, a full likelihood function or a posterior PDF over the position).

Cyber-what?

On the plane home, I tried to sharpen up what the point is for Rob Fergus (NYU) and my attempt to get some funding for research bridging computer vision, computational photography, and astronomy. Interdisciplinarity (is that a word?) is hard: The languages are different, and the assumptions are even more different.

I also read and critiqued this amusing paper but my critique doesn't meet the criteria for inclusion in this exclusive blog.

AAS 217, day three

Over lunch, Bovy, Hennawi, Myers, and I discussed the next projects for the XDQSO project. We figured out how to do XD photometric redshifts, which is an amusing problem on which we might get excellent performance (by my definition of performance). We also discussed variability and the production, from our probabilistic results, a hard-cut catalog.

I went to the Milky Way session in the afternoon, which included very nice talks by Kevin Schlaufmann (UCSC) and, of course, Zolotov. Schlaufmann showed that there is significant substructure in radial-velocity space, and that the overdensities in radial velocity also show chemical-abundance differences from the smooth component. Some of his ECHOS (what he calls the substructures) did not look to me at first glance to be statistically significant on their face, but then they do show chemical-abundance offsets from the field that do, taken as a whole, look highly significant. So I guess Schlaufmann is right and I am wrong!

Zolotov followed with a highly relevant theory talk, showing that you expect chemical-abundance differences between halo stars that fell in and halo stars that were kicked out of an early central bulge or disk. In the question period, it came up that the in-situ stars (as she calls them) should be on nearly-radial orbits. Furthermore, if they are kicked out in a set of short-duration events, there could be very strong non-trivial structure in phase-space for these stars. Gotta think about that, and then convince Zolotov and her team to find the strongest signatures in the simulations.

AAS 217, day two

I gave my talk in a session that—being built around the NASA ADAP funding program—contained lots of nice discussion of engineering and code matters. John Moustakas (UCSD) spoke about PRIMUS, which is so impressive I am very proud to have played the tiny, tiny part I did. Gordon Richards (Drexel) showed that you can't reliably measure the accretion rate of a quasar without a lot of multi-band data; every quasar has oddities that make its optical accretion-rate estimate unreliable at the factor-of-few level. He also showed some nice systematics on the broad emission lines, which might be relevant to (or re-discovered by) my work with Hennawi and Tsalmantza on quasar redshifts.

In the afternoon, I met up with Christopher Stumm (Microsoft), in part to catch up and in part to lobby for writing up (for publication in the astrophysics literature) his integration of Astrometry.net into flickr. By performing this integration, he has built one of the most remarkable citizen-science platforms in existence. And he is a team of one!

talk preparation

On the airplane to Seattle for AAS 217, I prepared my talk slides. It is a 15-minute talk (number 206.04 if you are at the meeting), with the following summary:

• Catalog matching is not a good way to combine imaging data sets
  • (the Right Thing To Do^TM is build a simultaneous model of everything).
• Forced photometry is a great approximation when one data set is better
  • (one data set is deeper and higher in angular resolution).
  • SDSS has this property relative to GALEX.
• A typical quasar in SDSS is not clearly detected in GALEX
  • but nonetheless GALEX helps us find them,
  • GALEX photometry at < 5 σ can provide photometric redshifts,
  • and GALEX measures at high signal-to-noise their average properties.
• We use this framework to measure the extragalactic UV background.

Now will the speaker ready room accept my home-built PDF presentation compiled from LaTeX?

ultraviolet interpolation

Schiminovich came down again and we got a bit more done on the ultraviolet background, working on the distribution of intensity as a function of quasar (source object) redshift. We haven't yet faced the need to K-correct the intensity information we have collected, but we will have to face that soon. We confirm, disturbingly well in some respects, models of the intergalactic ultraviolet intensity as a function of redshift.

UV radiation from quasars

Schiminovich came down for the day and we worked on the ionizing radiation in the Universe from quasars, as a function of redshift. We are making a direct measurement using GALEX, but also using SDSS to tell us where to photometer the GALEX data, which is too shallow to detect many of the quasars of interest. We discussed—and Schiminovich made some progress on incorporating—incompleteness information from (imperfect) quasar selection.

exoplanet radial velocity data

Debra Fischer (Yale) and Matt Giguere (Yale) came down to NYU today to discuss radial velocity data with Hou and me. We discussed many things, but the most interesting to me was the possibility that we could insert improvements into the spectral extraction and fitting pipelines. Even small improvements in these pipelines could lead to large scientific advances. Fischer handed us some data in different states of rawness.

a new Atlas of Galaxies?

I spent the day at Princeton, working with Lang on PHAT and the Tractor. In the afternoon I spent a short period discussing with Dave Spergel (Princeton) and Robert Lupton (Princeton) the possibility of producing a published atlas of galaxies from the SDSS. They were filled with good ideas for making such a book, and making it the de facto replacement of all previous atlases. They also suggested ways to connect to citizen science, which seems like a great opportunity. This all in preparation for AAS, where I hope to make a decision on this: Go or no go?

hierarchical star–galaxy classification

Ross Fadely and I co-wrote a document today laying out the basic hierarchical framework for performing star–galaxy classification in multi-band imaging. It involves a large set of stellar models and a large set of galaxy models. Then a hierarchical approach puts properly normalized and properly informative prior probabilities on all the models given the total data set. Each object then is classified using the prior information inferred from everything, and prior information and classifications are inferred simultaneously. Will it work? I have faith in the Church of Bayes!

the Tractor

Lang and I worked today a bit on generalizing the Tractor, our probabilistically justified source detection system. It runs on SDSS imaging, but we want to make it work on any imaging. We are not close.