#LGAstat, day 3

Today was a great day at #LGAstat. Here is a personal, non-exhaustive set of highlights:

Roth (UCL) gave a great talk about understanding galaxy or galaxy-group properties as a function of cosmological initial conditions. She uses a framework in which she can adjust the initial conditions draw (to, say, have a certain total density, or a ratio of density in one part relative to another, or etc) and see how the simulation results depend on the changes to the initial conditions. Her method permits her to move the ICs smoothly, but also ensure that they remain quantitatively comprehensible as a draw from the true IC prior. She is using this framework to ask causal questions about outcomes for galaxies. The long-term goal is to understand the ICs from with the Local Group (and other groups) formed.

Besla (Arizona) and Olsen (NOAO) spoke about the LMC and SMC, with Besla concentrating on arguments about infall and orbit, and Olsen on arguments about stellar and gas dynamics and the Magellanic stream. Besla's talk was full of rich astrophysical (rather than strictly statistical) arguments, and Olsen brought an actual large data set, which was delivered to every participant by USB key!

Kravstov (Chicago) gave a beautiful talk about the timing argument for dark matter, and its update via importance sampling of simulation-based prior samples of local-group analogs. The timing argument is incredibly powerful and surprisingly accurate, he finds. This, combined with the Besla arguments, suggested a whole host of new projects to be doing with simulations and data.

There were so many great talks today, I can't even think to mention them all, but I should shout out VanderPlas (UW), who told us not to be afraid about having more parameters than data, and also is finding RR Lyrae stars automatically with clever generalizations of the periodogram. Foreman-Mackey told the crowd not to do 1/Vmax weighting of incomplete data (and what they should do instead). Farr (Chicago) blew us away with a new, very simple ensemble sampler, that could be merged beautifully with emcee. Martin (Strasbourg) took us to school with his mixture-modeling of the entire halo of M31, and Lang did more of similar for the PHAT project's (overwhelmingly large) astrometric solution. Martin made a nice point about the comparison of M31 with simulations: The data show a lot of substructure relative to realistic stellar halo simulations for galaxies at similar mass. This led to some very useful discussion.

Spergel and BICEP2

David Spergel (Princeton) came into town and talked about CMB cosmology. He spent a good fraction of his talk throwing doubt on the new, hot BICEP2 / Keck result on B-mode polarization on large scales. He is concerned that the experiments don't seem to satisfy the standard null tests. He laid out a set of requests, which are easy to satisfy. He noted that six new experiments can confirm the result if it is real, so we will know extremely soon. One amusing thing about Spergel's talk was the repeated point (obvious, but often overlooked) that because all CMB experiments are observing the same, single sky, they ought to agree to better than one-sigma, especially on large scales where cosmic variance dominates.

seeing the whole Universe in a single galaxy

In the morning, Andrea Maccio (MPIA) gave a nice talk about making star-formation and AGN feedback in numerical simulations of galaxy formation much more realistic. He is very negative about the possibility that AGN can stop star formation: AGN emit jets, which punch through the central part of the ISM and don't really heat all of the necessary volume. He also showed that the the dark-energy equation of state can affect galaxy evolution, not because the dark energy has any direct effect on how structures form, but because it changes the timing of gravitational collapse and star-formation episodes. That got the audience all in a tizzy: Can we infer the dark-energy equation of state from the radial distribution of stars in a galaxy? The answer is no: It looks like this effect is strongly degenerate with feedback parameters, but it is super-intriguing.

Late in the day, Foreman-Mackey and I checked in with Dawson about the in-transit noise. She has some systems that show a very strong effect of higher noise in transit than out. We suggested improvements to the statistical tests, and Dawson will try to move to smaller and smaller planets tomorrow.

prior on galaxy images, bullet

MJ Vakili (NYU) showed me today what he has been working on to generate a data-driven prior probability distribution over galaxies. It is great work. He finds that he can do a shapelet decomposition, truncate it, and then do a dimensionality reduction (again, as it were), and then fit the resulting distribution of components with a mixture of Gaussians. We have yet to show that the model is good, but when he samples from it, the samples look like actual galaxies. The point is this: If you want to measure a shear map (or anything else, for that matter) from galaxy images, you can't do proper inference if you don't have a prior over galaxy images. So we are playing around with the possibility of making one.

In the brown-bag today, Craig Lage (NYU) showed detailed simulations he is doing of the "Bullet Cluster". He is doing by-hand likelihood optimization, with an hours-long simulation inside the loop! But the results are gorgeous: He can reproduce all the large-scale features, and a lot of the small-scale details. He says it isn't a challenge to CDM, but it is a challenge to theories in which there is no dark matter. One of his goals is to test dark-matter interactions; it looks very promising for that.

MDM, SAMSI, MCMC, WISE, FA

Jasper Hasenkamp (NYU) gave the brown-bag, about fixing anomalies between large-scale-structure cosmology results and cosmic-microwave-background cosmology results using mixed dark matter—the standard CDM model plus a small admixture of a (possibly partially thermalized) neutrino-like species. The model seems to work well and will make new predictions, including (in principle) for accelerator experiments. Mark Wyman (NYU) has also worked on similar things.

At the "No More Tears" phone-con (about Kepler planet-searching), we talked about wavelets with Bekki Dawson (Berkeley) and other exoSAMSI participants. In our MCMC meeting, we worked on finishing Hou's nearly finished paper on nested sampling, and we quizzed Goodman about mixing the stretch move (the underlying engine of emcee) with Metropolis-Hastings to capitalize on the observation that in most likelihood functions there are "fast" and "slow" parameters, where the "fast" parameters can be changed and the likelihood call re-made quickly, while the "slow" parameters require some large, expensive re-calculation. This is generic, and we came up with some generic solutions. Some of them are even permitted mathematically. Foreman-Mackey is thinking about these in the context of running n-body simulations within the inference loop

In other news, Lang delivered a draft paper about his work on the WISE imaging, and Fadely had some ideas about finding nails for our factor-analysis hammer.

lensing cross-correlations, Gaussian Processes

In Galaxy Coffee, Myers (Wyoming) talked about cross-correlations of quasars with the CMB convergence (lensing) map. There are now large-scale convergence maps available, so it is possible to more-or-less automatically generate object-lensing cross-correlations. These are pretty straightforwardly interpretable in terms of halo occupation or mass correlations. This is a great new capability for cosmology, and permits testing of a lot of halo-occupation ideas that have been kicking around for a few years.

In the afternoon, Foreman-Mackey talked about our work with Kepler. He made various important points, but what I learned most clearly is that the biggest advantage we gain over our competitors by using Gaussian Processes is in what we call "search": We can do hypothesis tests or parameter estimation quickly but still marginalize out the stochastic variations of the stars.

theories and measurements of galaxies

Annalisa Pillepich (UCSC) gave a seminar in the morning about the effects of baryons on galaxy formation, comparing dark-matter-only and more realistic simulations. She finds (as does Zolotov and collaborators) that baryons can have a big effect on the structure of dark-matter halos. She finds that baryons have a big enough effect to resolve many of the remaining issues in the comparison of simulations and observations of low-mass galaxies in the Local Group. Really her conclusion (and my reading of her and Zolotov's work) is that—though there are large uncertainties—the magnitudes of the effects of baryons are large enough that they could resolve most current issues, and other things might help resolve them too. So once again the dark matter could be vanilla CDM. That's bad!

In the afternoon, Stephane Courteau (Queens) argued similarly that theories of galaxy evolution have enough uncertainties in them that they don't really predict the properties of the galaxy population well, but from the opposite perspective: He argued that no simulations get all the observed properties of the disk-galaxy population right at the present day. He argued that there needs to be very strong feedback to explain the population in current semi-analytic-style models. He also showed some amazingly good scaling relations for disk galaxies, measured carefully and lovingly by his group.

Late in the day, he argued with me that doing galaxy photometry by fitting simple parameterized models is always wrong. That's what I am doing with my Atlas project with Mykytyn and Patel. I don't agree with Courteau here—my view is that all photometry is in effect model fitting—but his points are good and need to be addressed. Translated into my language, effectively he argues for very flexible photometric models of galaxies; non-parametric models if you will. I agree but only on the condition that those flexible models are regularized so that in effect galaxies observed at different angular resolutions are being photometered equivalently. Well, at least for my purposes I need that (I want to find similar galaxies at different distances).

other universes

Stephen Feeney (UCL) gave a nice talk on finding bubble collisions by performing model fits and Bayesian hypothesis tests on the cosmic microwave background maps. He had to make some approximations to do the Bayes integrals. He concludes that the rate of bubble collisions is low (I guess we would be suspicious if he concluded otherwise), but because of resource limitations and decisions made early in the project, he and his team did not test for large-angle signatures, which are expected to be the most common. The good news is that such tests are going to be easy in the near future.

extremely faint tidal features

David Martinez-Delgado (MPIA) gave a beautiful seminar today about his work obtaining extremely flat and senstitive images of nearby galaxies, showing that many have faint, tidal remnants of past accretion events. The data are beautiful, thanks to his exquisite flat-fielding. He and his team made most of the images with 10 or 20 cm (yes, cm) telescopes!

During the rest of the day, Lang and I worked on source detection in multi-exposure imaging; we are pretty sure we can beat all of the standard methods, but we don't have a clear demo yet.

brute-force modeling, exoplanet evolution

Two great seminars today. The first was by Ross Fadely (Haverford), who had done full brute-force modeling of strong gravitational lensing in a CDM-substructure context. He lays down CDM-compatible substructure in an enormous outer loop of realizations, and then does lens model selection within that loop. Brute force modeling that warms the heart. He finds, unfortunately, that the lenses don't obviously contradict CDM.

The second was by Josh Winn (MIT), who talked about very clever measurements of star-spin vs orbital angular momentum mis-alignment in exoplanet systems. The data show some beautiful regularities that are not implausibly explained by a combination of few-body effects driving inward migration (of hot Jupiters) followed by tidal damping of inclinations and eccentricities on pretty short time-scales. He argued, effectively (though not explicitly) that exoplanets measure the tidal dissipation timescales (or tidal quality factor) of convective stars much better than models can predict it. In the question period, the subject of free-floating planets came up. Mental note to self: Discover these!

star-galaxy separation, substructure

Ross Fadely and Beth Willman (Haverford) came in for two days, making my visitor list pretty long! We discussed Fadely's early-days results on star–galaxy separation. He has some strange objects that are obviously stars but are better fit by galaxies. We tasked him with giving more detail on a few of these cases to see if there is something simple wrong (or too general) with the galaxy models we are using, or too restrictive about the star models we are using.

In the afternoon, Willman gave the physics colloquium about the ultra-faint galaxies. What a rich and successful field this has been! And (in my humble view) it all started here at NYU with a pair (here and here) of papers. Now these galaxies are found to be incredibly numerous, observed to contain dark matter (Willman focused on the velocity measurements of Simon & Geha), possible sites for observable dark-matter annihilation (though Willman didn't discuss that at all), and promise to increase in observed number dramatically in the next decade.

clumpy streams from a clumpy halo, CVs

I went up to Columbia today to see Kathryn Johnston and David Schiminovich. I took the local train, so that I would have plenty of time to read and comment on the latest draft of Joo Yoon's paper (with Johnston and me) on tidal streams. In the paper we use scaling relations—dimensional analysis and order-of-magnitude arguments—to describe the dynamical effects of halo substructure on cold stellar streams; then we compare with numerical simulations and observations. The result (against my wishes) is that the rattiness of the Pal-5 and GD-1 streams is consistent, at least roughly, with what is expected in a CDM halo. Argh! But Yoon has done a great job.

At Columbia's Pizza Lunch, Christian Knigge (Southampton) gave a nice explanation of CVs, which are accreting WDs in close binaries with red dwarfs. Because they are Roche-lobe-filling, they have orbital evolution (mass-transfer) times equal to the thermal (Kelvin-Helmholz) times in the dwarf stars. So the non-main-sequence-ness of the dwarf star gives a semi-direct measure of the instantaneous mass-transfer rate. Nice! He also argued that at some periods, there should be many non-CV binaries; Schiminovich and I might be able to detect those in our time-domain GALEX stuff.

galaxies like our own

My break from proposal-writing was a nice talk by Risa Wechsler (Stanford) about finding galaxy systems like that of the Milky Way plus Magellanic Clouds, both in the low-redshift SDSS sample and in huge n-body (dark-matter only) simulations. In both cases, having two massive satellites is rare, but she goes way beyond this, looking at what you can infer about the Milky Way from the presence of the Magellanic Clouds alone. Pretty neat stuff, though in the end she has a depressingly strong endorsement of the standard Lambda-CDM model.

New Directions in Modern Cosmology, day 3

Another good day in Leiden. One highlight was work by Kunz (Geneva), who showed simple models for generating CDM-like inhomogeneities in the CMB without inflation. He gets very close, but in the details at large scale, causality requires some inflation-like activity. His arguments were very general. More was said about this by Magueijo (Imperial), who uses a variable speed of light to do inflation's work. In general these causality arguments come from the problem that it is very hard to set up the initial conditions.

Lachièze-Rey (Paris) described nice detectors for the baryon acoustic feature that don't require construction of the full correlation function or power spectrum. This led nicely into an afternoon discussion of homogeneity, where my results, Sylos Labini's (Rome), and Kazin's (NYU) were batted around. I didn't fare as well as I would have liked in part because we have not closed all the loopholes remaining for some kinds of technical inhomogeneities. I certainly think that it is well established that the Universe has a mean density, but Sylos Labini and others have a good point that if you can do your data analysis without assuming the mean density then you should, if for no reason other than that it is rarely well measured (so it adds uncertainty to your results). Don't get the wrong idea from my measured tone here: The universe is not a fractal on large scales!

New Directions in Modern Cosmology, day 2

Today was my first day at the meeting in Leiden, named above. It is an eclectic group, because the idea was to bring together outsiders and insiders in the whole cosmology thing and have people hash it out: A great idea. The talks were good, but I hope I will be forgiven for saying that (for my geeky self) I learned the most from the talk given by Hao Liu (Beijing), who is re-analyzing the WMAP data starting at the time stream. He is finding many very interesting things out about the calibration, attitude model, and map-making. It appears that the quadrupole amplitude is very soft. It is all about attitude and configuration modeling, just like Hipparcos, which is not surprising. Plus there are sidelobe issues. It appears that no-one, not Hou or the WMAP team, is building the full likelihood including all calibration parameters and then marginalizing out everything but the map (or the cosmological parameters). That is, there still is not an honest Bayesian analysis (if that is even possible!).

perturbed streams

Kathryn Johnston appeared in my office today and we spent a few hours discussing simple calculations of perturbations to tidal streams by CDM-substructure-like satellites. She has greatly simplified the confused thoughts I had about them a year or two ago.

fractal on very large scales?

Matt Kleban (NYU) and I discussed the fractal Universe today, in the context of my polemic. He questioned a number of things, including whether the fractal dimension could be less than three but only very slightly, and what would that mean? Also, if you believe some forms of inflation, the Universe sort-of has to be a fractal on super-super-horizon scales; is that an issue? Of course I don't think either of those issues is so important, because neither conflicts with CDM (the theory I would love to be wrong), but he brought up some very relevant points. One of these is whether the fact that there are always order-unity density fluctuations possible on all scales means that it might be hard to define or understand the mean density in any single realization. But it remains true that there are no GR calculations of any observables in any expanding Universe with a distribution of matter with fractal dimension significantly different from three on large scales. That lack is a substantial hindrance to the success of any model that is fractal in that sense (for the reasons I try to give in the polemic).

dark galaxies

Barry Madore (OCIW) gave a very provocative talk about the possibility that there might be dark galaxies and then showed that after some pretty hard looking (using galaxy–galaxy interaction signatures), he can't find any. So they are rare, at the near-L-star level. Nice and fundamental!

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.

merger origin of the halo, GALEX

Adi Zolotov—spending this year at Haverford with Willman—was back in the city today, and we spent some hours talking about projects. Her big project is about the signatures of hierarchical galaxy formation in the straightforward observables available to us from halo stars, such as spatial distributions, metallicities, and kinematics. She is developing a theory-motivated set of observational tests that will be sensitive to merger history.

Chris Martin (Caltech) came and gave the Physics Colloquium. He spoke about the GALEX satellite, with a focus on technical and hardware aspects. It was beautiful stuff.