#streams2015, day 3

Today was the half-day at the meeting (hike in the afternoon). The highlight for me was the talk by Carl Grillmair (IPAC), who talked about finding and tracing streams, observationally. Grillmair is the discoverer or co-discoverer of more than half of all the known Milky-Way streams. He showed us some of the visualizations that help him find them. He said that we should replace his by-eye work with automated methods, but (to my knowledge) no-one is close right now.

After Grillmair, Sesar (MPIA) showed a beautiful analysis of the new Ophiuchus stream, where he measures everything via probabilistic inference. Kallivayalil (Virginia) showed the evidence for the LMC proper motion measurement. I am a believer! She then showed noisier data from other Local-Group dwarfs and streams. Slater (Michigan) showed properties of the Milky Way's stellar halo at large radius. At large radius, the halo is consistent with being entirely disrupted satellites.


#streams2015, day 2

Today was a great day at the meeting; many good and lively talks and discussions. This is a healthy field! Here are some personal highlights for the day:

In his opening talk, Wyn Evans (Cambridge) gave credit to Koposov, Hogg, & Rix for fitting the GD-1 stream with a (trivial, wrong) orbit model (that is, not a physically correct stream model): The orbit model seems to return the correct potential. The authors of the paper refused this credit, since it appears to be luck rather than skill that obtained this coincidence. A few talks later, Bovy disagreed with Evans; he showed that as the potential is made more realistic, the stream model departs more from the orbit model.

Sanders (Cambridge), Bovy, Binney (Oxford), and McMillan (Lund) all talked about using actions and angles for modeling streams. The math is beautiful, but the computation is expensive. It is not clear to me that this is the right approach, long term. It also requires a strong constraint on the potential and the populated orbits: Many orbits are likely to be chaotic!

Continuing on that theme, Pearson (Columbia) and Price-Whelan coined the term "stream fanning" for streams on chaotic orbits, which show very different morphologies from those on regular orbits. Price-Whelan showed that the fanning happens earlier and more prominently than you would expect from standard chaos measures like Lyapunov time; he presented an argument in terms of frequency-space diffusion. In the question period it came up that if we take the long streams to be highlights of regular orbits, then this puts some kind of very complex (or even fractal) posterior on potential models. Crazy!

In the discussion between sessions, many great ideas came up. One is to use The Cannon to obtain more precise spectroscopy-informed stellar absolute magnitudes for red-giant stars. Another is to find the fountain of high-velocity stars above the bulge in Gaia DR2; we might be able to find A and F stars (most known high-velocity stars are B stars, I think).


#streams2015, day 1

I am at the Stellar Streams in the Local Universe meeting at Ringberg Castle. The meeting is small and workshop-like, so we are trying various experiments. One is to keep a running, completely public web document where everyone is encouraged to take notes on the talks and sessions. The first day was extremely lively. Here are some unfair and impressionistic thoughts:

Bovy opened the meeting, with his summary of the scientific subject of stellar streams in the Milky Way Halo, which range from dynamical modeling of the Milky Way to inferring the dark-matter and star-formation properties of low-mass galaxies.

I always think of the dynamical modeling, and rarely the galaxy evolution, so it was great to have Kathryn Johnston back up Bovy with the point that while we think of the Milky Way as being its own unique and bespoke object, there are enough collapsed, star-hosting overdensities disrupted in our halo that they probably constitute a relatively fair sample of small objects in the Universe. That is, we can do cosmology in our own halo (duh!). She also showed that the stream-to-shell ratio can be a cosmological test and speak to these questions, and she included a quantitative definition of "shell" and "stream"!

In the question period after a talk by Andrew Cooper (MPA) about simulations, he was asked whether in the outer halo the distribution of stars would follow the distribution of dark matter. This is a great question and idea, since both the stars and the dark-matter particles in those outer reaches are just test particles coming in. He said that they don't have an answer yet, but it looks likely that the stellar distribution is only a weakly biased version of the dark-matter distribution. That would be awesome.

Brendan Griffen (MIT) showed the Caterpillar suite of cosmological simulations of Milky-Way-like galaxies. This project is very simple, very well conceived, and producing very valuable tools.


fast likelihood evaluations for the CMB?

Mike O'Neil and I worked on the real-space covariance function for the cosmic microwave background radiation. He summed the Legendre polynomials to convert the C_ell spectrum into an angle-space kernel or covariance matrix. He plotted the real-space covariance and it is very pretty; it shows the baryon acoustic feature and oscillates at huge angle. He then looked at some symmetries of the matrix: It can be expressed as a sparse sum of outer products. Because of this, it is possible that he can develop a fast method for matrix factorization, linear-algebra solve operations, and (log) determinant evaluations.


momentum-driven winds, enfastening stellar models

At Galaxy Coffee, Jonathan Stern (MPIA) showed an analysis of quasar momentum-driven winds in the context of AGN feedback for galaxy evolution. He finds that the quasar radiation is not sufficient to drive the wind. In the discussion of the point Joachim Bestenlehner (MPIA) pointed out that Wolf–Rayet stars can have momentum-driven winds that exceed the naive radiation pressure L/c by a factor of five-ish. It relates to the fact that there are multiple scatterings of the radiation. Andrea Macció's (MPIA but soon NYU-AD) showed some nice work on simulations of small galaxies that are infalling in to the halos of larger galaxies.

Mike O'Neil came down to HD for two days (from Frankfurt) and we spent part of the afternoon understanding how to run and interpret PICO, which computes CMB power spectra. We started towards converting its output (which is in "ell" space) into a real-space covariance function. When we left work, O'Neil was looking at recurrence relations for Legendre polynomials!

We took advantage of O'Neil's presence to have a discussion with various MPIAers about interpolating grids of models (so that you can have a model defined everywhere in parameter space, but only compute it in full at a sparse set of points). He had various extremely useful points of advice. However, his reaction to the problem of interpolating 1-D LTE spectral models was not “do this kind of interpolation” but rather “I bet we can make the models run much, much, much faster!” So we downloaded the original Kurucz model paper [warning: huge PDF file] and began exegesis.


star ages, exoplanet spectroscopy

At Milky Way group meeting, various projects were discussed. Marie Martig (MPIA) and Maria Bergemann (MPIA) are both looking at how we might extract stellar ages from stellar spectra. Martig is looking at the dredge-up of elements from the core into the stellar exterior. Bergemann is looking at how extremely weak chromospheric emission might adjust (minutely) the shapes of the Balmer lines. Both of these projects are directly relevant to what Ness and I are doing with The Cannon.

After lunch, the PSF coffee had two impressive presentations. In the first, Nestor Espinoza (PUC Santiago/Chile) showed very impressive transmission spectroscopy of transiting planets. He has some spectral features that are hard to explain, but he notes that they might be caused by some systematic problems with the pipelines. They are taking reasonably high resolution (few thousand) spectra, but binning them down for analysis; it is interesting to think about what could be done with non-trivial binnings of the spectra, that are customized to particular questions.

In the second presentation, Florian Rodler (MPIA) took time-domain spectra in the red side of the K band to find the forest of molecular CO lines in the inner planet in the Upsilon Andromeda multi-planet system. This system is amazingly complicated, with two super-Jupiters with huge mutual inclination, large eccentricity, and a (circular) hot Jupiter. It sure doesn't look like it could be stable! The system properties are known from a combination of radial-velocity monitoring (of the main star), astrometric monitoring (of the center of light), and direct detection (of the molecular lines from the hot Jupiter).


debugging outliers

I spent the day debugging with various students. Eddie Schlafly (MPIA) and I helped Nina Hernitschek (MPIA) debug her Gaussian-process (plotting) code, I helped Kopytova debug her spectral fitting code, and Rix and I helped Anna Ho debug her LAMOST models. On the latter, I have a hypothesis that perhaps bad data (strange, corrupted, or physically odd stars) are distorting the data-driven model of The Cannon. In particular, if there are outliers, we will get bad results for the intrinsic scatter parameters in the model. This all came up because the goodness-of-fit measurements are coming in “too good”.


my first Monday at MPIA

[Very few posts for the last few days, because I was on a mini-vacation and getting ready for the annual move to Heidelberg.]

My research day started in the garden with Rix, looking at Anna Ho's paper using The Cannon to transfer APOGEE-like stellar parameter labels onto stars with LAMOST spectra. We worked through the figures and made comments.

At the weekly PanSTARRS meeting, Nina Hernitschek (MPIA) showed beautiful samples of QSO candidates and RR Lyrae candidates found with time-domain analysis. Her method involves Gaussian-process modeling multi-band time-domain data, and then a supervised classification. The results are very convincing: For example, on a map of the sky with the MW dwarf galaxies and star clusters circled, there are clear overdensities of RRL stars in many of the circles. Also, the RRL stars clearly trace the MW halo, bulge, and disk. She also has incredible 3π maps of QSOs, which are amazingly isotropic near the Galactic poles, but also have many QSOs in (or near) the disk plane and even behind the Galactic center. I bet she has the best low-latitude QSO sample ever.

At lunch-time I had a conversation with Laura Inno (MPIA), Rix, Eddie Schlafly (MPIA), and others about Cepheid stars in the Milky Way disk. There should be tens of thousands but hundreds are known. Rix gave some very good reasons for finding them all: Cepheids are very young stars but they are cool (5000 K) not hot, so you can measure their abundances. They are also luminous, so you can see them at large distance and through the dust. We discussed how to find them with PanSTARRS and WISE data, perhaps starting from what Hernitschek has already done.


GALAH data!

Today I got an unexpected treat when Melissa Ness showed me some new GALAH data from the HERMES instrument. The project is intending to take R 28000 spectra of about a million stars, and determine up to 30 element abundances, or something insane like that. Ness and I are looking at whether The Cannon could be used for basic parameter estimation and to assist in the long-run goals of the project. The data look absolutely beautiful and the first attempts with the data make it look like The Cannon will Just Work (tm). We are a bit concerned about continuum normalization, but aren't we always?


#eprv, day 2

I spent the day at Extreme Precision Radial Velocities at Yale. It is a great meeting, because it is very focused on the instrumentation and code that underly radial-velocity planet search and characterization. Today was a stats-heavy day, with me, Eric Ford (PSU), and Tom Loredo (Cornell) leading off with pedagogical talks. I gave an entirely new (for me) talk about noise modeling, and it was followed by absolutely excellent questions (every question pointed out a talk slide I should have made). Loredo made a nice point, which is that statistics is not a method or tool, it is a language or framework for communicating about quantitative questions. I couldn't agree more!

At lunch, Ana Bonaca organized a gathering of probabilistic reasoners to discuss asteroseismology with Sarbani Basu (Yale). This gave us an opportunity to feel out some of the issues if we try to build a probabilistic model (a forward model of the time-domain data) to replace the standard practice of Fourier transformations (or periodograms or the like). That was productive and useful.

In the afternoon, one talk that particularly stood out was by Xavier Dumusque (CfA) about The Keplerian Fitting Challenge. He made fake radial-velocity data, filled with difficult but realistic noise sources, and challenged groups to find and characterize the injected signals. He did a great job describing the successes and failures of the different groups, and even awarded nice bottles of wine to the two top-performing teams. This project, like the GREAT projects for weak lensing, are important community-building and critical-review projects for difficult data-analysis challenges.


#eprv prep

I spent my research time today preparing my slides for the Extreme Precision Radial Velocity meeting at Yale. I am talking about noise models, arguing for creating and taking advantage of great flexibility, and then controlling complexity with priors or regularization or hierarchical inference. I also want to give some ideas about how, technically, we do what we do.


the probabilistic properties of The Cannon

I wrote text in the “Methods and data” section of the mass-and-age paper I am writing with Ness. I emphasized particularly the point that The Cannon is a probabilistic model: It is a likelihood function, which is optimized at training time, and then again at test time. The only difference between training and test is which parameters are varied. In the former, it is the spectral expectation and variance parameters, at fixed label values. In the latter, it is the label values, at fixed spectral expectation and variance parameter values. The cool thing about this likelihood formulation is that it makes it trivial to account for heteroskedastic noise variances and missing data (in both the training data and the test data).


writing about ages

I wrote a paragraph in the paper Ness and I are writing about inferring stellar ages from spetroscopy.



It is the summer, and it feels like I am getting lots done! That said, I only got in a small amount of serious research time today. I spent most of it commenting on a draft manuscript from Price-Whelan on chaos in the halo of the Milky Way. Yes, dynamical chaos. This is the coming to fruition of a pretty old idea at CampHogg: What is the difference between stellar streams on regular orbits and those on chaotic orbits? It turns out that the differences are bigger than we expected!


star-formation history of the Milky Way

I read (or skimmed, really) some classic papers on the star-formation history of the Milky Way, in preparation for re-asking this question with APOGEE data this summer. Papers I skimmed included Prantzos & Silk, which infers the SFH from (mainly) abundance distributions and Gizis, Reid, and Hawley, which infers it from M-dwarf chromospheric activity. I also wrote myself a list of all the possible ways one might infer the SFH. I realized that not all of the ways I can think of have actually been executed. So now I have a whole set of projects to pitch!