inferring evolution, hidden Markov model

Sriram Sankararaman (Harvard) gave a great Computer Science Colloquium today about inferring the evolutionary tree (well, it isn't really a tree) from genetic information, particularly as regards humans and neandertals. He is able to show, using the statistics of DNA variability, that humans and neandertals had intermixing long after they separated (both geographically and as species). He was also able to show that there is statistical evidence for the sterility (infertility) of males after speciation. Awesome stuff, and very related to cosmology in many ways: The models are of two-point statistics of the DNA sequences, not the sequences themselves, and the probabilistic modeling methods (approximate Gaussian likelihood functions and MCMC) are very similar indeed.

Prior to that, in group meeting, McFee and Huppenkothen jointly proposed a plan for clustering black hole timing data using a hidden Markov model: The idea is that the data are generated by a probability distribution that is set by a state, and there are finite probabilities of transitioning from state to state at each time step. This is a well-understood idea in machine learning, but also very close to how we think about the generation of the timing data, fundamentally. Great plan! Huppenkothen's first order of business is to run k-means in a feature space (for initialization of the HMM).

Tri-State Astronomy

I spent the research part of my day at the Tri-State Astronomy meeting, beautifully organized by Geha (Yale) and Maller (CUNY) and others. I won't do it justice; there were five great review talks and lots of great posters (and one-minute poster summaries). Alyson Brooks (Rutgers), in her review talk, beautifully summarized the issues with CDM at small scales, and discussed the idea that baryonic processes might explain all of these. My take on her bottom line (that baryons might explain everything) is that it would be a disappointment if we can't see the predicted properties of the dark matter at small scales! But she did point towards isolated dwarf galaxies (that is, very low luminosity galaxies that are not satellites of any more luminous galaxy) as critical tests of the ideas; isolated dwarfs should differ from satellite dwarfs if CDM is going to stay okay at small scales. She also implicitly made many predictions about outflows from small galaxies at intermediate redshifts. And she gave a few shout-outs to Zolotov, who has been deeply involved in these baryon issues since leaving NYU.

do clusters dissolve? and don't add pdfs

Conversations continued with Fouesneau and Weisz; today they were about the evolution of stellar clusters. Fouesneau is working on using observations of clusters of different masses and ages to constrain models of cluster evolution and dissolution. It looks like at early ages (less than 0.1 Gyr), the cluster population is consistent with cluster conservation, but at old ages (greater than 1 Gyr), there must be cluster destruction or dissolution. We wrote down a probabilistic model for this process and a plan for how it could be inferred at the photometric-catalog level (rather than the inferred masses and ages level). Going to the photometric-catalog level permits inclusion of non-approximate completeness functions.

At one point in the conversation I fired up my don't co-add your posterior pdfs rant. If you have a bunch of posterior pdfs, one per object (one per cluster, in this case, in the mass–age parameter space), what is your best estimate for the true distribution in the parameter space? It is not the coaddition of the posterior pdfs. Perhaps it is counterintuitive, but it is better to histogram best-fit values than it is to co-add pdfs. The Right Thing To Do (tm) is to perform a hierarchical analysis (as in this paper), but that's expensive and non-trivial. Fundamentally, adding up pdfs is never a good idea. I think maybe I need to write a Data Analysis Recipes on this.

Dr Jiang

My PhD student Tao Jiang graduated today. His thesis involved the cross-correlations of various galaxy populations as a function of color and luminosity. He was able to show that the red-galaxy population has been growing since redshift unity mainly from merger activity, and that the growth is dominated by red-red mergers (that is, dry mergers). He also showed that the rate of blue-galaxy mergers is consistent with the rate at which blue galaxies transform into red galaxies (pass into the red population), which is pretty striking. Congratulations, Dr Jiang!

galaxies, homogeneity

In the morning, Jeremy Tinker (Berkeley) led our group meeting with a discussion of information about galaxy evolution from clustering. In the approximation that we know the dark matter model, the relationship between galaxies and dark matter can be parameterized and then the observed galaxy—galaxy clustering puts constraints on how the galaxies could possibly form and evolve. He has some counterintuitive results, from the fact that at intermediate redshift, the large-scale clustering of red and blue galaxies is very similar.

In the afternoon, Marc Kamionkowski (Caltech) gave the Big Apple Colloquium about the isotropy and homogeneity of large-scale structure, and in particular the cosmic microwave background. He is building non-natural models that permit anisotropy in the power spectrum while preserving isotropy in the temperature and density and all else. There is a small amount of evidence for this statistical anisotropy situation in the current data; it is a long shot but if it holds up it is extremely important.

no more streams, Cox

T. J. Cox (Harvard) gave a nice seminar at the MPIA today about merging disk galaxies in realistic simulations. He does a nice job of creating a huge diversity of spheroidal galaxies, with gas content and/or dissipation one of the dominant parameters. He (indirectly) confirms my methodologies for constraining the galaxy–galaxy merger rate and growth rate of spheroidal galaxies.

Today Lang and I ruled out the streams we have been finding in 5D this week. There is structure in the data that is almost certainly systematic, not real.

seminars

Didn't get much work done today, but there were nice seminars by Rudnick (Kansas) on the evolution of the mass density on the red sequence as a function of environment, by Carilli (NRAO) on the status and promise of high-redshift 21-cm experiments (he is involved in the one called PAPER), and by Sheth (Penn) on statistical techniques for cross-correlations and deconvolution. My take-home message from the Carilli talk is that there is some awesome work to be done in the software domain.

stellar population synthesis

Charlie Conroy (Princeton) came and spoke about modeling galaxy spectra as linear combinations of stellar populations. He is working towards marginalizing over unknowns and propagating errors, and then finding observations that reduce the magnitudes of the ranges for the unknowns. He shows very convincingly that stellar masses (the simplest thing you might want to get out of such a model for a galaxy) are not secure at the factor-of-two level, because of things like late stages of stellar evolution (which are brief, random, and luminous in many cases).

clustering, shapelets

Alison Coil (UCSD) gave a nice talk about clustering and the things that can be learned about galaxy evolution from a halo occupation picture of galaxy and quasar clustering and cross-correlations. She has some tantalizing results on green valley galaxies.

Along those lines, Phil Marshall and I specified a well-posed problem for doing galaxy morphologies and learning things from them using shapelets, or, better, Kuijken's sechlets. We wrote down some early specs, although we didn't in fact get started.

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.

average image, does environment matter?

Actually started up the average image slash quasar–photon cross-correlation code today. It runs. Does it give good answers? I don't know because it is incredibly slow. Ah well, if only I knew a good programmer!

Schiminovich and I spent time talking about religion and politics and then a bit about galaxy environments, in particular the resolution of the debate between Jacqueline von Gorkom (Columbia) and myself about whether environment matters to galaxy evolution. Being a stickler, Schiminovich objected to that characterization of the issue up-front. But I think we almost agreed that environment has its strongest effects on galaxies near the centers of clusters and merging galaxies. These systems make up only a percent or so of the galaxy population in the last few Gyr, so although environment matters deeply to these galaxies, they are a trace population.

We certainly know about environment effects on the bulk of galaxies, but it is straightforward and relatively uninteresting: At fixed mass (or other properties), the most important environmental effect is on the specific star-formation rate. But we know from statistical tests that environment affects this very slowly or very indirectly, probably by heating the interstellar medium (slowly, not generically in rapid bursts or events, which are extremely rare). So I think there is a consensus position possible: Environment matters, but a small amount to a large fraction of galaxies and a large amount to a small fraction.

Dr Pietro Reviglio

I sat on the (successful and entertaining) thesis defense of Pietro Reviglio (Columbia), who used FIRST, NVSS, and SDSS to investigate the evolution of AGN, and the relationships between radio type and optical spectral type. He finds significant evolution, and some problems for the orientation model for the differences between broad-line and narrow-line AGN. He also finds some evolution in the host galaxy population that is consistent with some structural evolution (as in building bulges from disks), although that conclusion is more speculative, of course.