2017-10-12

age-velocity; finishing

I had a great, long call with Jonathan Bird (Vandy) to discuss his nearly-finished paper on the age–velocity relation of stars in the Gaia DR1 data. We discussed the addition of an old, hot population, in addition to the population that shows the age–velocity relation. That's a good idea, and accords with our beliefs, hence even gooder.

I spent the rest of my research time today working through the text of Dan Foreman-Mackey (Flatiron) and my MCMC tutorial. We are trying to finish it this week (after five-ish years)!

2017-10-11

WDs in Gaia, M33, M stars, and more

In our weekly parallel-working Gaia DR2 prep meeting, two very good ideas came up. The first is to look for substructure in the white-dwarf sequence and see if it can be interpreted in terms of binarity. This is interesting for two reasons. The first is that unresolved WD binaries should be the progenitors of Type Ia supernovae. The second is that they might be formed by a different evolutionary channel than the single WDs and therefore be odd in interesting ways. The second idea was to focus on giant stars in the halo, and look for substructure in 3+2-dimensional space. The idea is: If we can get giant distances accurately enough (and maybe we can, with a model like this), we ought to see the substructure in the Gaia data alone; that is: No radial velocities necessary. Of course we will have radial velocities (and chemistry) for a lot of the stuff.

In the stars group meeting, many interesting things happened: Anna Ho (Caltech) spoke about time-domain projects just starting at Caltech. They sure do have overwhelming force. But there are interesting calibration issues. She has accidentally found many (very bright!) flaring M stars, which is interesting. Ekta Patel (Arizona) talked about how M33 gets its outer morphology. Her claim is that it is not caused by its interaction with M31. If she's right, she makes predictions about dark-matter substructure around M33! Emily Stanford (Columbia) showed us measurements of stellar densities from exoplanet transits that are comparable to asteroseismology in precision. Not as good, but close! And different.

In the afternoon I worked on GALEX imaging with Dun Wang (NYU), Steven Mohammed (Columbia), and David Schiminovich (Columbia). We discussed how to release our images and sensitivity maps such that they can be responsibly used by the community. And Andrina Nicola (ETH) spoke about combining many cosmological surveys responsibly into coherent cosmological constraints. The problem is non-trivial when the surveys overlap volumetrically..

2017-10-10

a day at MIT

I spent the day today at MIT, to give a seminar. I had great conversations all day! Just a few highlights: Rob Simcoe and I discussed spectroscopic data reduction and my EPRV plans. He agreed that, in the long run, the radial-velocity measurements should be made in the space of the two-d pixel array, not extracted spectra. Anna Frebel and I discussed r-process stars, r-process elements, and chemical-abundance substructure in the Galaxy Halo. We discussed the immense amount of low-hanging fruit coming with Gaia DR2. I had lunch with the students, where I learned a lot about research going on in the Department. In particular Keaton Burns had interesting things to say about the applicability of spectral methods in solving fluid equations in some contexts. On the train up, I worked on the theoretical limits of self-calibration: What is the Cramér–Rao bound for flat-field components given a self-calibration program? This, for Euclid.

2017-10-09

Euclid and MCMC

I did some work on the (NYC) long weekend on two projects. In the first, I built some code to make possible observing strategies for the in-flight self-calibration program for ESA Euclid. Stephanie Wachter (MPIA) contacted me to discuss strategies and metrics for self-calibration quality. I wrote code, but realized that I ought to be able to deliver a sensible metric for deciding on dither strategy. This all relates to this old paper.

On Monday I discussed our nearly-finished MCMC paper with Dan Foreman-Mackey (Flatiron) and we decided to finish it for submission to the AAS Journals. I spent time working through the current draft and reformatting it for submission. There is lots to do, but maybe I can complete it this coming week?

2017-10-06

dust-hidden supernovae

In my weekly parallel-hacking, I re-learned how to use kplr with Elisabeth Andersson (NYU).

This was followed by a nice talk by Mansi Kasliwal (Caltech) about the overwhelming force on time-domain astronomy being implemented by her and others at Caltech. One of their projects will be imaging more than 3000 square degrees an hour! There isn't enough solid angle on the sky for them. She is finding lots of crazy transients that are intermediate in luminosity between supernovae and novae, and she doesn't know what they are. Also she may be finding the (long expected) fully-obscured supernovae. If she has found them, she may be doubling the observed supernova rates in nearby galaxies. Great stuff.

The day ended with lightning talks at the CCPP, with faculty introducing themselves to the new graduate students.

2017-10-05

uncertainty propagation

I started the day with a long discussion with Ana Bonaca (Harvard) about how to propagate uncertainties in Galactic gravitational potential parameters into some visualization about what we (in that context) know about the acceleration field. In principle, the acceleration field is more directly constrained (by our dynamical systems) than the potential. What we want (and it is ill-posed) is some visualization of what we know and don't know. Oddly, this conversation is a conversation about linear algebra above all else. We both admitted to each other on the call that we are both learning a lot of math in this project!

[My day ended early because: NYC Comic Con!]

2017-10-04

Gaia and exoplanets

At our weekly Gaia DR2 prep workshop, a bunch of good ideas emerged from Megan Bedell (Flatiron) about exoplanet and star science. Actually, some of the best ideas could be done right now, before DR2! These include looking at our already-known co-moving pairs of stars for examples with short-cadence Kepler data or known planetary systems. There is also lots to do once DR2 does come out. In this same workshop, David Spergel (Flatiron) summarized the work that the Gaia team has done to build a simulated universe in which to test and understand it's observations. These are useful for trying out projects in advance of the data release.

In the afternoon, everyone at the Flatiron CCA, at all levels, gave 2-minute, 1-slide lightning talks. It was great! There were many themes across the talks, including inference, fundamental physics, and fluid dynamics. On the first topic: There is no shortage of people at Flatiron who are thinking about how we might do better at learning from the data we have.

2017-10-03

systematizing surprise; taking logs

I had a substantial conversation with Kate Storey-Fisher (NYU) about possible anomaly-search projects in cosmology. The idea is to systematize the search for anomalies, and thereby get some control over the many-hypotheses issues. And also spin-off things around generating high-quality statistics (data compressions) for various purposes. We talked about the structure of the problem, and also what are the kinds of limited domains in which we could start. There is also a literature search we need to be doing.

I also made a Jupyter notebook for Megan Bedell (Flatiron), demonstrating that there is a bias when you naively take the log of your data and average the logs, instead of averaging the data. This bias is there even when you aren't averaging; in principle you ought to correct any model you make of the log of data for this effect, or at least when you transform from linear space to log or back again. Oh wait: This is only relevant if you are not also transforming the noise model appropriately! Obviously you should transform everything self-consistently! In this case we have nearly-Gaussian noise in the linear space (because physics) and we want to treat the noise in the log space as also linear (because computational tractability). Fortunately we are working with very high signal-to-noise data, so these biases are small.

2017-10-02

exploration vs exploitation

I met with Lauren Anderson (Flatiron) first-thing to figure out how we can munge our hacky #GaiaSprint projects into real and cutting-edge measurements of the Milky Way. We looked at the VVV infrared survey because it ought to be better than 2MASS for mapping the inner disk and bulge. We looked at using SDSS photometry to map the halo. On the latter, the dust modeling is far simpler, because for distant stars, the dust is just a screen, not an interspersed three-dimensional field. We also discussed the ever-present issue for a postdoc (or any scientist): How much time should you spend exploiting things you already know, and how much exploring new things you want to learn?

In the morning I also discussed the construction of (sparse) interpolation operators and their derivatives with Megan Bedell (Flatiron).

At lunch, Yacine Ali-Haimoud (NYU) gave a great brown-bag talk on the possibility that black holes make up the dark matter. He showed that there are various different bounds, all of which depend on rich astrophysical models. In the end, constraints from small-scale clustering rule it out (he thinks). Matt Kleban (NYU) and I argued that the primordial black holes could easily be formed in some kind of glass that has way sub-Poisson local power. Not sure if that's true!

2017-09-30

LIGO noise correlations

I spent some weekend science time reading this paper on LIGO noise that claims that the time delays in the LIGO detections (between the Louisiana and Washington sites) are seen in the noise too—that is, that the time delays or coincidence aspects of LIGO detections are suspect. I don't understand the paper completely, but they show plots (Figure 3) that show very strong phase–frequency relationships in data that are supposed to be noise-dominated. That's strange; if there are strong phase–frequency relationships, then there are almost always visible structures in real space. (To see this, imagine what happens as you modify the zero of time: The phases wind up!) Indeed, it is the phases that encode real-space structure. I don't have an opinion on the bigger question yet, but I would like to have seen the real-space structures creating the phase–frequency correlations they show.

2017-09-29

the life stories of counter-rotating galaxies

Today was the third experiment with Friday-morning parallel working in my office. It is like a hack week spread out over months! The idea is to work in the same place and build community. During the session, I worked through a multi-linear model for steallar spectra and tellurics with Bedell, based on conversations with Foreman-Mackey earlier in the week. I also worked through a method for generating realistic and self-consistent p(z) functions for fake-data experiments with Malz. This is a non-trivial problem: It is hard to generate realistic fake data, it is even harder to generate realistic posterior PDFs that might come out of a probabilistic set of analyses of those data.

Just before lunch, Tjitske Starkenburg (Flatiron) gave the NYU Astro Seminar. She mainly talked about counter-rotating galaxies. She took the unusual approach of following up, in the simulations she has done, some typical examples (where the stars rotate opposite to the gas) and figure out their individual histories (of accretion and merging and movement in the large-scale structure). Late in the day, she and I returned to these subjects to figure out if there might be ways to read a galaxy's individual cosmological-context history off of its present-day observable properties. That's a holy grail of galaxy evolution.

2017-09-28

what's the point of direct-detection experiments?

In the morning I spoke with Ana Bonaca (Harvard) and Chris Ick (NYU) about their projects. Bonaca is looking at multipole expansions of the Milky Way potential from an information-theory (what can we know?) point of view. We are working out how to visualize and test her output. Ick is performing Bayesian inference on a quasi-periodic model for Solar flares. He needs to figure out how to take his output and make a reliable claim about a flare being quasi-periodic (or not).

Rouven Essig (Stonybrook) gave a nice Physics Colloquium about direct detection of dark matter. He is developing strong limits on dark matter that might interact with leptons. The nice thing is that such a detection would be just as important for the light sector (new physics) as for the dark sector. He gave a good overview of the direct-detection methods. After the talk, we discussed the challenge of deciding what to do as non-detections roll in. This is not unlike the issues facing accelerator physics and cosmology: If the model is just what we currently think, then all we are doing is adding precision. The nice thing about cosmology experiments is that even if we don't find new cosmological physics, we usually discover and measure all sorts of other things. Not so true with direct-detection experiments.

2017-09-27

Gaia, EPRV, photons

In our Gaia DR2 prep workshop, Stephen Feeney (Flatiron) led a discussion on the Lutz–Kelker correction to parallaxes, and when we should and shouldn't use it. He began by re-phrasing the original LK paper in terms of modern language about likelihoods and posteriors. Once you put it in modern language, it becomes clear that you should (almost) never use these kinds of corrections. It is especially wrong to use them in the context of Cepheid (or other distance-ladder) cosmology; this is an error in the literature that Feeney has uncovered.

That discussion devolved into a discussion of the Gaia likelihood function. Nowhere in the Gaia papers does it clearly say how to reconstruct a likelihood function for the stellar parallaxes from the catalog, though it does give a suggestion in the nice papers by Astraatmadja, such as this one. Astraatmadja is a Gaia insider, so his suggestion is probably correct, but there isn't an equivalent statement in the official data-release papers (to my knowledge). There is a big set of assumptions underlying this likelihood function (which is the one we use); we unpacked them a bit in the meeting. My position is that this is so important, it might be worth writing a short note for arXiv.

In Stars group meeting, Megan Bedell (Flatiron) showed her current status on measuring extremely precise radial velocities using data-driven models for the star and the tellurics. It is promising that her methods seem to be doing better than standard pipelines; maybe she can beat the world's best current precision?

Chuck Steidel (Caltech) gave a talk in the afternoon about things he can learn about ionizing photons from galaxies at high redshift by stacking spectra. He had a number of interesting conclusions. One is that high-mass-star binaries are important! Another is that escape fraction for ionizing photons goes up with the strength of nebular lines, and down with total UV luminosity. He had some physical intuitions for these results.

2017-09-26

machine learning

The day started with a somewhat stressful call with Hans-Walter Rix (MPIA), about applied-math issues: How to make sure that numerical (as opposed to analytic) derivatives are calculated correctly, how to make sure that linear-algebra operations are performed correctly when matrices are badly conditioned, and so on. The context is: Machine-learning methods have all sorts of hard numerical issues under the hood. If you can't follow those things up correctly, you can't do correct operations with machine-learning models. It's stressful, because wrongness here is wrongness everywhere.

Later in the morning, Kilian Walsh (NYU) brought to me some ideas about making the connections between dark-matter simulations and observed galaxies more flexible on the theoretical / interpretation side. We discussed a possible framework for immensely complexifying the connections between dark-matter halos and galaxy properties, way beyond the currently-ascendent HOD models. What we wrote down is interesting, but it might not be tractable.

2017-09-25

thermal relics

In a low-research day, I discussed probabilistic model results with Axel Widmark (Stockholm), a paper title and abstract with Megan Bedell (Flatiron), and Gaia DR2 Milky Way mapping with Lauren Anderson (Flatiron).

The research highlight of the day was an excellent brown-bag talk by Josh Ruderman (NYU) about thermal-relic models for dark matter. It turns out there is a whole zoo of models beyond the classic WIMP. In particular, the number-changing interactions don't need to interact with the visible sector. The models can be protected by dark-sector layers and have very indirect (or no) connection to our sector. We discussed the differences between models that are somehow likely or natural and models that are somehow observable or experimentally interesting. These two sets don't necessarily overlap that much!