linear models for nuisances

The day started with Rodrigo Luger (UW) and Dan Foreman-Mackey (Flatiron) and me discussing a range of projects. They endorsed my general idea of looking for planets by searching resonances! Which is good. We tentatively decided to try to write one of the new ApJ Research Notes about our systematics models for Kepler and other projects. There are a lot of unifying good ideas there; let's spread the Good News. The idea is that it is possible to simultaneously fit a linear model and marginalize it out with a simple linear-algebra move. Work on that started almost immediately.

Lauren Anderson (Flatiron) visualized the proper motions of stars in the Galaxia model as a function of sky position and distance, to see if proper motions can be used to infer distances by methods that are more clever than reduced proper motion. It looks like they can be! We discussed further improvements to the visuals, with help from Vasily Belokurov (Cambridge).

stellar age–velocity relation

Jonathan Bird (Vandy) and I spent the morning working together on his paper on the age–velocity relationship in the Milky-Way disk. He has absolutely beautiful results, from APOGEE red-clump stars and Gaia DR1 transverse kinematics. The thing that is new is that (thanks Martig and Ness) he has actually useful age estimates for many hundreds of stars. And we will have the same for tens of thousands in the overlap with Gaia DR2. Indeed, we commented in the paper that SDSS-V will make this possible at scale. The great thing about the ages is that even with hundreds of stars, we get a comparable measure of the age–velocity relation to studies that involved orders of magnitude more stars.

We discussed the final presentation in the paper. We worked through the figures and drew a simple graphical model to illustrate the project. We then went, very carefully, through the assumptions of the project, so we can state them explicitly at the outset of our methods section, and then use them to structure the discussion at the end. It's a fun intellectual exercise to go through these assumptions carefully; somehow you only understand a project substantially after it is finished!

self-calibrating pulsar arrays, and much more

I had a great conversation with Chiara Mingarelli (Flatiron) and Ellie Schwab (AMNH) today about pulsar-timing arrays and gravitational-wave sources. We are developing some ideas about self-calibration of the arrays, such that we might be able to simultanously search for coherent sources (that is: not just stochastic backgrounds) and also precisely determine the distances to the individual pulsars to many digits of accuracy!. It is futuristic stuff, and there are lots of ways it might fail badly, but if I am right that the self-calibration of the arrays is possible, it would make the arrays a few to tens of times more sensitive to sources! We started with Mingarelli assigning us some reading homework.

In the Stars group meeting, we had a productive discussion led by Megan Bedell (Flatiron), Andrew Mann (Columbia), and John Brewer (Yale) about things learned at the recent #KnowThyStar conference. There are some new uses of machine learning and data-driven models that I might need to spend some time criticizing! And it appears that there are some serious discrepancies between asteroseismic scaling relations for stellar radii and interferometric measurements. Not bigger than those expected by the stellar experts, apparently, but much bigger than assumed by some of the exoplanet community.

Prior to that, in our weekly Gaia DR2 prep working session, we discussed the use of proper motion as a distance indicator in a post-reduced-proper-motion world. That is: The assumptions underlying reduced proper motion are not great, and will be strongly violated in the DR2 data set. So let's replace it with a much better thing!

Adrian Price-Whelan (Princeton) showed some incredible properties of (flowing from beautiful design of) the astropy coordinates package. Damn!

WISE parallaxes, binary AGN accretion

The WISE mission took two passes over the sky. So it is nearly pessimal for parallax determination; parallaxes and proper motions are all mixed up. That said, at each pass there are multiple epochs, so maybe there is a glimmer of hope. Today, in a heroic move, Lang coded up simultaneous parallax and proper motion determination in the un-coadded WISE individual frames, a la this old paper. As expected, parallaxes are badly determined. However, we can use MCMC to explore the full degeneracy between parallax and proper motion conditioned on the WISE data, combine external information, get ready for NEOWISE (or whatever comes next), and apply any prior knowledge. I don't know if it counts as research to just cheer while someone else codes?

At lunch, Brian Farris (NYU) gave a nice talk about simulations of accretion onto merging black holes. He proposes that we might be able to see late-stage (0.1 pc ish) merging black-holes by looking for periodicities in the accretion luminosity. Oddly, the periodicity is not at the orbital frequency but instead at some lower frequency that has to do with how the disk reacts to the torques from the orbiting pair of holes.

lunch

Fergus, Schölkopf (MPI-IS, visiting us for three months, now counting as a Camp Hogg regular), Brewer, Fadely, Foreman-Mackey, and I went to lunch together. What a pleasure! Long (unfortunately expensive) lunches are an important part of how we get things done here at Camp Hogg. We discussed further the Kuiper Belt problem, and many other things. One beautiful idea that came up is that outer Solar System objects always have (nearly) zero proper motion, whereas extra-Solar Galactic sources always have proper motions that are as large as (or larger than) the parallax amplitude. Late in the day we discussed (over drinks I am afraid) with Brewer possible engineering improvements to nested sampling and the future of complex, high performance samplers that adaptively take advantage of everything that is now known about the huge class of sampling problems. It was a great day.

M5 proper motion

After a few minutes of conversation, Blanton demonstrated to Willman and I that even though the SDSS data are not designed to deal with crowded fields well, they can be used to measure the proper motions of halo globular clusters. Koposov, Rix, and I demonstrated that the proper motions can be used statistically when we looked at the GD-1 Stream, but then I left this subject behind, even though globular clusters are actually much easier to measure than the GD-1 Stream. This relates to my repetitive rant that there are many great measurements waiting to be made in existing data.

moving groups, spectroscopic reductions

On the weekend I spent time reading Bovy's next huge tome about the moving groups, in which he demonstrates that they are far more likely to be dynamically created than to be fossils of past star-formation events, and that within the dynamical options they are more likely to be generated by transient dynamical complexity (from, for example, spiral arms and similar inhomogeneities) than from long-lived dynamical resonances. This supports suggestions by Tremaine. We hope to finish the paper within days.

I also finally worked through and sent my comments on spectro perfectionism; in short I complained about their treatment of exposure time, darks, and biases; their restriction to invertible matrices; the fact that the final output they advocate has no Bayesian generalization and is not a point estimate of any simple statistical property of the data or spectrum; their timidity about huge matrices and huge least-squares problems; and the issues of consistently modeling cosmic rays or data with non-Gaussian noise. I never have this many complaints about a paper unless I love it (with one notable exception); don't get me wrong: I love the paper.

proper motion measurements

Among other things, Bovy and I discussed the first steps towards measuring proper motions in the USNO-B and SDSS raw data streams. This is a big project, but we have decided to get closer to the data.

moving groups demoted

Bovy finished another draft round on our first Hipparcos papers, one an immense method paper and one with some results, using Geneva-Copenhagen Survey to externally vet any Hipparcos-inferred structure in the velocity distribution. We find that this conservative method disfavors some of the less-significant structures claimed in the literature, and that there are no moving groups strongly evident in the Hipparcos data that weren't known prior to Hipparcos. Bad news for Hipparcos, but it does mean that we have something useful to contribute to this literature.

the disk, AEGIS

Spent the day with Bovy at the IAS speaking with Tremaine (and others) about inferring dynamics from kinematics. We decided that the best place to start is the disk, because (a) there are lots of data available right now, (b) we already have results on the velocity distribution and the significance and origins of structure therein, (c) there are other straightforward projects to work through there (most of them much easier than what Bovy has already made happen), and (d) we need to build up some experience before we start thinking about the Gaia problem, which is huge.

Sandy Faber (UCSC) gave the IAS talk today; she spoke about the stellar mass and star-formation evolution results from DEEP2, but especially the multi-wavelength AEGIS data. She argued for a very simple picture in which halo growth is simple, and galaxy growth within those haloes is also simple (depending only on halo mass, with a slight adjustment with redshift). She showed that even this very simple picture explained most of the observations adequately. I don't disagree.

predicting radial velocities

Most of the day was lost to an NSF proposal, but Bovy and I got in some quality time on his prediction of radial velocities in the Solar Neighborhood. He has a model of the velocity field from the transverse velocities; that model plus the measurement of each star's transverse velocity makes a prediction for each star's radial velocity, in the form of an error-convolved probability distribution function. Bovy can search these for the most discrepant values among those stars that do have radial velocities (these are stars from the halo, probably), find the stars for which the radial velocity prediction is most informative or constraining (these provide critical tests of the model), and find the stars for which the radial velocity prediction is the least informative (these provide the most valuable follow-up observations for improving the precision of the model).

submitted!

Visualize me doing that NFL-style end-zone post-touchdown dance. The faint-source proper motion paper Measuring the undetectable (link should come live on 2008-09-01) is submitted to The Astronomical Journal and to the arXiv. Lang, Jester, and Rix all did some last-minute pulling together to get that done. Thanks, team! Now, does anyone want to follow up our brown dwarfs with infrared spectroscopy?

SDSS Southern Coadd Catalog

We realized today that we were slightly mis-using the SDSS Southern Stripe Coadd Catalog, and the parent sample for our faint-source proper motion paper went from 1500 sources to less than 100. But that's good, because now some of our statistics make much more sense.

colors and proper motions

Lang computed infrared colors (from UKIDSS and SDSS-II) for the very faint, fast-moving sources to compare with the less-fast-moving sources at the same magnitude. Although all are very red in i−z, they vary hugely in z−J, with the faster-moving redder. This is good.

facilitating science

Jester, Rix, and I had various conversations today about how we can demonstrate that Lang and my method for measuring the astrometric variations of extremely faint sources can be used to speed, cheapen, and facilitate upcoming science projects, especially PanSTARRS. I am now working on paper modifications based on these discussions.

Rix comments

Rix "bled" all over the faint proper-motion paper this weekend (think: lots of red ink). I got started mopping it up today.

poster

I poster-ified Lang and my paper faint-source proper motions for the SDSS Symposium next week in Chicago.

writing

I worked on completing final edits and to-do items on the faint proper-motion paper

finished writing

I finished the faint-source proper-motion paper. It still needs to be vetted by collaborators, but I am stoked. Here is the abstract:

The near future of astrophysics involves many large solid-angle, multi-epoch, multi-band imaging surveys. These surveys will, at their faint limits, have data on large numbers of sources that are too faint to detect at any individual epoch. Here we show that it is possible to measure in multi-epoch data not only the fluxes and positions, but also the parallaxes and proper motions of sources that are too faint to detect at any individual epoch. The method involves fitting a model of a moving point source simultaneously to all imaging, taking account of the noise and point-spread function in each image. By this method it is possible—in well-understood data—to measure the proper motion of a point source with an uncertainty (found after marginalizing over flux, mean position, and parallax) roughly equal to the minimum possible uncertainty given the information in the data, which is limited by the point-spread function, the distribution of observation times, and the total signal-to-noise in the combined data. We demonstrate our technique on artificial data and on multi-epoch Sloan Digital Sky Survey imaging of the SDSS Southern Stripe. With the SDSSSS data we show that with this technique it is possible to distinguish very red brown dwarfs from very high-redshift quasars and from resolved galaxies more than 1.6 mag fainter than by the traditional technique. Proper motions distinguish faint brown dwarfs from faint quasars with better fidelity than multi-band imaging alone; we present 16 new candidate brown dwarfs in the SDSSSS, identified on the basis of high proper motion. They are likely to be halo stars because none has a significantly measured parallax.

post-starbursts in GALEX, writing

Continued writing on the faint-motion paper. It is funny how much there is left to do when a project is done!

Wu began working on the GALEX properties of post-starburst galaxies: They aren't detected at the MIS depth. That is good, because star-forming galaxies are detected. So a GALEX selection is likely to work, at some level. The question is, how good will it be? We would like to have a GALEX-based selection of the post-starburst galaxies so we can perform emission-line studies without worrying about the fact that the post-starbursts are selected on the basis of emission lines. Fortuitously, Christy Tremonti (Arizona) showed up at the MPIA today for a month, so she may be able to help out.