truly zero-metallicity stars

All week I have been discussing with Hans-Walter Rix (MPIA) the possibility that we could find the elusive, truly zero metallicity stars in the Milky Way halo. This from an email I wrote today, reacting to the observational point that no-one has ever seen such a star:

How could there be absolutely ZERO zero-metallicity stars in the Milky Way? Here's everything I got:

1. Maybe there are literally no low-mass stars ever made at zero metallicity. Absolutely none, at high precision. This is possible, given how little we know about star formation and the IMF.
2. Maybe stellar evolution is so weird at zero metallicity that low-mass stars go dark by 13 Gyr. I very much doubt that this is a possibility. But maybe low-mass stars at primordial abundances never burn and just slowly collapse into white-dwarf-like condensed objects on a very long cooling timescale. They would be super cold by now, like 100 K maybe??
3. Maybe low-mass stars form slower than high-mass stars in star-forming regions at zero metallicity. This permits a few of the high-mass stars to quickly evolve and explode, polluting the outsides of the just-forming low-mass stars. These low-mass stars would then have low but non-zero surface metallicities, and be very alpha-enhanced. This is possible, although would it really leave NO zero-metallicity low-mass stars behind?
4. Maybe low-mass stars somehow self-pollute at formation (or later in their lives). Maybe the nuclear fusion kicks in slightly before gravitational steady-state or radiative zones are set up, and the first bit of nuclear burning gets mixed in to the stellar envelopes? These stars would appear to have non-zero (but weird, carbon-enhanced maybe?) abundances. Or maybe this happens later in life because of some weird internal mixing. I have literally no idea whether any of this is possible.

the chevron

A research highlight today was the Flatiron Galactic dynamics internal group meeting. We discussed kinematic features in the Milky Way halo that have appeared in ESA Gaia DR3 in maybe this paper. We looked at data and (toy) simulations. I'm interested in whether the features appear in metallicity or abundances. The arguments that Neige Frankel (CITA) and I worked out this summer for The Snail looks like they maybe work for all phase-space overdensities caused by perturbations?

weak-lensing inconsistency

Tocay Alexie Leauthaud (UCSC) gave the NYU Astro Seminar, about various things related to cosmological tests with weak lensing. She showed an impressive result, which is that essentially all galaxy–galaxy lensing projects find a weak-lensing signal that is too low by tens of percent relative to what we expect from the Planck cosmological parameters and simple galaxy–halo occupation models. I am interested in looking into this more with Storey-Fisher and her (new, exploratory) models of galaxy occuption in hydro simulations. I have an intuition that the predictions might be overly naive if halo occupation is slightly more complex than expected. I am particularly interested in this issue because I think the galaxy–galaxy weak-lensing signal has been a very fundamental test of our picture of the dark sector.

every Keck/DEIMOS star, ever!

I met briefly this morning with Marla Geha (Yale). She is completing an impressive project, in which she has re-reduced, from raw data, every (or nearly every) Keck/DEIMOS spectrum of a star in the Milky Way or Local Group. These include ultra-faint dwarfs, classical dwarfs, globular clusters, halo, disk, and so on. It is an absolute goldmine of science. We spent time talking about the technicals, since she has done a lot of creative and statistically righteous things in this project (which is built on the exciting new open-source project PypeIt). But we also dreamed about a lot of science that we could be doing with these data. It will be of order 10⁵ stars.

unwinding a spiral

A lot of conversations in the Dynamics group at Flatiron recently have been about spirals: Spirals in phase space, spirals in the disk, even spirals in the halo. In general, as a perturbed dynamical system (like a galaxy or a star cluster) evolves towards steady-state, it goes through a (or more than one) spiral phase. We've (collectively) had an interest in unwinding these spirals, to infer the initial conditions or meta-data about the events that caused the disequilibrium and spiral-winding. Jason Hunt (Flatiron) discussed these problems with Adrian Price-Whelan (Flatiron) and me today, showing some attempts to unwind (what I call) The Snail. That led to a long conversation about what would make a good “loss function” for unwinding. If something was unwinding well, how would we know? That led to some deep conversations.

is leading-order time dependence spirally?

Independently, Kathryn Johnston (Columbia) and David Spergel (Flatiron) have pointed out to me that if you have a Hamiltonian dynamical system that is slightly out of steady-state, you can do a kind of expansion, in which the steady-state equation is just the zeroth order term in an expansion. The first-order term looks like the zeroth-order Hamiltonian acting on the first-order perturbation to the distribution function, plus the first-order perturbation to the Hamiltonian acting on the zeroth-order distribution function (equals a time derivative of the distribution function). That's cool!

Now couple that idea with the fact that a steady-state Hamiltonian system is a set of phase-mixed orbits nested in phase space (literally a 3-torus foliation of 6-space). Isn't this first-order equation the equation of a winding-up spiral mode? I think it is! If so, it might unite a bunch of phenomenology, from cold stellar streams to spiral structure in the disk to The Snail. I discussed all this with Adrian Price-Whelan (Flatiron).

how clustered is the DM in the Milky Way halo?

Today David Spergel (Flatiron) came by to discuss the following question: How much do we know—empirically—about fluctuations or clustering in the dark matter distribution in the Milky Way halo? Spergel's idea is maybe to use old or metal-poor halo stars: Since stars form in the centers of their DM halos (we think), the clustering or fluctuations in phase space of the old stars should always be larger than the clustering or fluctuations in the dark matter. I bet that's true! And it's easy to test in standard simulations, I think.

getting ready for EDR3: cold streams

ESA Gaia EDR3 is next week! I have been trying to get ready in various ways. Today Ana Bonaca (Harvard) showed me remarkable evidence that the cold stellar streams in the Milky Way halo are clustered in kinematic space, maybe also with the halo globular clusters! We discussed things to do in this area. It's perfect for EDR3 because if the clustering is real, it should improve with the improved precision of the EDR3 data.

splitting exposures after the fact

[It's been a hard 2020. Apologies for my violations of the Rules to the right. I can assure you that it has been for a good set of reasons.]

I spent part of my research day listening to Adrian Price-Whelan (Flatiron) talk out a few different job talks. His challenge is to explain why we can learn things about the dark matter with streams. And explain why we can't! That is, streams are complicated.

In the latter part of our conversation I asked him if he could find APOGEE exposures of accelerating stars that are accelerating so strongly (from a binary orbit, say) that we could measure the velocity difference between the first half of the exposure and the second half. Why? Because the APOGEE observations are taken “up the ramp”; this makes it possible to split them after the fact. Any up-the-ramp imager takes data that can be sub-framed after the fact, which leads to all sorts of possible time-domain projects! Let's figure that out.

In Stars & Exoplanets Meeting today, we had a discussion about getting ready for ESA Gaia EDR3. What should we be doing? And Megan Ansdell (Flatiron) told us about using shallow-ish convolutional neural networks to find stellar flares in the presence of astrophysical noise.

disk–halo (and halo–halo) mean-velocity differences

I had a meeting with Emily Cunningham (Flatiron) to discuss any projects of mutual interest. She has been looking at simulations of the Milky Way (toy simulations) in which the LMC and SMC fall in. These simulations get tidally distorted by the infall, and various observational consequences follow. For example, the disk ends up having a different mean velocity than the halo! And for another, different parts of the halo move relative to one another, in the mean. Cunningham's past work has been on the velocity variance; now it looks like she has a project on the velocity mean! The predictions are coming from toy simulations (from the Arizona group) but I'm interested in the more general question of what can be learned from spatial variations in the mean velocity in the halo. It might put strong constraints on the recent-past time-dependence.

substructure, phases, EPRV

Nora Shipp (Chicago) has been in town this week, working with Adrian Price-Whelan to find halo substructures and stellar streams around the Milky Way. The two of them made beautiful animations, paging through distance slices, showing halo stellar density (as measured by a color-magnitude matched filter). There are lots of things visible in those animations! We discussed the point that what makes overdensities appear to the human eye is their coherence through slices.

That made me think of things that Bill Freeman (MIT) and his lab does with amplifying small signals in video: Should we be looking for small overdensities with similar tricks? Freeman's lab uses phase transforms (like Fourier transforms and more localized versions of those) to detect and amplify small motions. Maybe we should use phase transforms here too. That led Price-Whelan and me to hack a little bit on this image pair by Judy Schmidt, which was fun but useless!

Late in the day, Megan Bedell (Flatiron), Lily Zhao (Yale), Debra Fischer (Yale), and I all met to discuss EXPRES data. It turns out that what the EXPRES team has in terms of data, and what they need in terms of technology, is incredibly well aligned with what Bedell and I want to do in the EPRV space. For example, EXPRES has been used to resolve the asteroseismic p-modes in a star. For another, it has made excellent observations of a spotty star. For another, it has a calibration program that wants to go hierarchical. I left work at the end of the day extremely excited about the opportunities here.

nothing

Today was almost all admin and teaching. But I did get to the Astronomical Data Group meeting at Flatiron, where we had good discussions of representation learning, light curves generated by spotted stars, the population of planets around slightly evolved stars, and accreted stellar systems in the Milky Way halo!

setting the scope for the next GD-1 paper

Today (and really over the last few days as well) I had a long discussion with Ana Bonaca (Harvard) about the results of our spectroscopy in the GD-1 stellar-stream fields. As my loyal reader knows, Bonaca, Price-Whelan, and I have a prediction for what the radial velocities should look like in the stream, if it is a cold stream that has been hit by a massive perturber. Our new velocity measurements (with the Hectochelle instrument) are not the biggest and best possible confirmation of that prediction!

However, our velocities are not inconsistent with our predictions either. The question is: What to say in our paper about them? We listed the top conclusions of the spectroscopy, and also discussed the set of figures that would bolster and explain those conclusions. Now to plotting and writing.

Along the way to understanding these conclusions, I think Bonaca has found a systematic issue (at extremely fine radial-velocity precision) in the way that the Hectochelle instrument measures radial velocities. I hope we are right, because if we are, the GD-1 stream might become very cold, and our velocity constraints on any perturbation will become very strong. But we will follow up with the Hectochelle team next week. It's pretty subtle.

visualizing substructure in large data

Today Doug Finkbeiner (Harvard), Josh Speagle (Harvard), and Ana Bonaca (Harvard) came to visit me in my undisclosed location in Heidelberg. We discussed many different things, including Finkbeiner's recent work on finding outliers and calibration issues in the LAMOST spectral data using a data-driven model, and Speagle's catalog of millions of stellar properties and distances in PanSTARRS+Gaia+2MASS+WISE.

Bonaca and I took that latter catalog and looked at new ways to visualize it. We both have the intuition that good visualization could and will pay off in these large surveys. Both in terms of finding structures and features, and giving us intuition about how to build automated systems that will then look for structures and features. And besides, excellent visualizations are productive in other senses too, like for use in talks and presentations. I spent much of my day coloring stars by location in phase space or the local density in phase space, or both. And playing with the color maps!

There's a big visualization literature for these kinds of problems. Next step is to try to dig into that.

Dr Taki

Today was a beautiful and accomplished PhD defense at NYU by Anna-Maria Taki (NYU). Taki is a particle phenomenologist who is looking at signatures of dark matter in the ESA Gaia data. She is concentrating on methods that relate to gravitational lensing: In addition to magnification changes, lensing can induce artificial proper motions and artificial accelerations in the stars. Indeed, Jupiter and Saturn have huge gravitational-lensing signatures at Gaia precision, and they are calibrated out. But if there are dark-matter substructures (say) between us and the SMC or LMC, we could see them in principle as anomalies in the Gaia data. Taki has developed matched filters and statistical techniques for finding the signatures. No detections yet! But there is a hope that an end-of-mission Gaia search could be very interesting.

In the discussion over champagne, I discussed with various people the idea that Taki's work could inspire a new small-explorer class NASA mission. If you could show that such a mission could definitively rule out the main predictions of lambda-CDM, that would be a competitive proposal, I think. And a beautiful experiment.

The day ended with a great and fun PhD candidacy exam by Paul McNulty (NYU). He is using data science and information theory to understand how neural activity relates to motor function in fruit-fly larvae. We discussed the sense in which such work is physics. It is, of course! But it's interesting how interdisciplinary physics has become.

where is the dark mass? April Fools

The day began with a call with Ana Bonaca (Harvard), in which she showed me that she can take her models of the GD-1 stream perturbation and predict the present-day location of the substructure (or dark mass) that created the perturbation. Because the model space is broad, the “error box” is large, but the fact that we have such a prediction is fun, and interesting. All this progress flows from the fact that we now have some radial-velocity data on the stream and the spur (which is the feature we think was raised by a dark-matter interaction).

On the arXiv today were the annual set of April Fools papers. My loyal reader knows that I love papers in this category when they are silly or funny but in fact contain an interesting or important calculation or inference. There were two in this category today with Flatiron origins. One was Luger et al, inferring the mean cloud cover on Earth from systematic effects in the NASA TESS imaging! Another was Burns et al, showing that instead of “cubing the sphere” (what climate modelers do to avoid spherical coordinate singularities in discretization) you can “sphere the cube” (embed a cubical simulation volume in a natively spherical-representation simulation). This latter project was ridiculous, but it showed very dramatically that they have a representation for simulating spherical domains with no singularity anywhere (and especially not at the center of the sphere, and at no angular position on the surface).

#GaiaSprint, day 5

Today was the last day and wrap-up for the 2019 SB Gaia Sprint. It was quite a week! A few highlights from the wrap-up (for me, very subjective, not fair or complete) were: Schwab Abrahams (Berkeley) showed that stars which are flagged in certain ways in the Gaia data are reliably variable stars, by looking at TESS light curves. Coronado (MPIA) showed that stars with small orbital-action differences tend to also have small element-abundance differences. Brown (Leiden) and others worked on making “Gold” samples in Gaia data that make it easy for people to look at or follow up spectroscopically. Mateu (UdelaR) improved her catalog of, and meta-data on, stellar streams in the halo. El Badry (Berkeley) convincingly showed us that there is an excess of very precisely equal-mass binary stars even at very large separations. Widrow (Queen's) showed first attempts at trying to perform a regression that can be used to infer the Galactic bar density from velocity fields. Hunt (Toronto) showed velocity and density maps of a simulated disk that look very much like the features that Eilers (MPIA) and I see in the data! And Laporte (UVic) showed a great movie of the data in the phase spiral (The Snail!) that shows its beautiful and informative dependence on azimuthal action (or really vertical frequency I think!). It was a great week with great people doing great things in a great location. I'm exhausted! The wrap-up slides are available here.

#GaiaSprint, day 4

Each day at the Sprint, we have a check-in, in which daily results are discussed. Today Cecilia Mateu (UdelaR) showed improvements she has made to the database or list she maintains of known or reported stellar streams in the Milky Way halo. With the encouragement of Ana Bonaca (Harvard) and the help of Adrian Price-Whelan (Princeton), she made an astropy-compatible data file that delivers coordinate transformations into the stellar stream reference frames (great-circle coordinates). This will make it much, much easier for people to perform analyses on streams and compare new detections to known objects.

At lunch, a subset of the group that discussed the ESA Gaia selection function yesterday met again to discuss the possibility of putting together a large funding proposal to create what's needed. Many interesting things came up in this discussion. One is that many more projects are enabled by the selection function. So a small investment here greatly increases the impact of Gaia. Another is that we need to have a set of clearly defined example problems that illustrate the relevant issues. Another is that many of these possible example projects need not just an observational selection function but also a 3-d dust map in the Milky Way. Is that the same project or a different one? Another is that there aren't a lot of possible funding avenues that would be appropriate in both scale and international scope. It was a valuable discussion, but I don't know where we are at the end.

The highlight of the day was a long discussion of the kinematics of the Milky Way bar with Larry Widrow (Queen's) and Ortwin Gerhard (MPE) and Christina Eilers (MPIA) and Sarah Pearson (Flatiron). We almost became convinced that we are seeing the bar at the center of the Galaxy kinematically. It appears as a quadrupole in the velocity field. But if we are seeing it, we are seeing it at the wrong angle! So there is work to do. And many of the simple ideas about what we see depend on some kind of steady-state assumption, when in practice the bar evolves on a time-scale comparable to it's rotation period. More soon!

GD-1 spur velocities

Early in the morning I spoke with Ana Bonaca (Harvard) about the amazing velocity data she has taken for stars in the GD-1 stellar stream in the Milky Way halo. As my loyal reader knows, this stream has a spur of stars off the main branch that are consistent with being perturbed away by a massive perturber that flew by. Now she has precise velocity information about stars in the main body of the stream and in the spur. Contrary to our naive predictions, the stream and spur have very similar velocities. But the spur appears to be far lower in velocity dispersion. Is this real? And is this what we expect? We didn't predict it in our theoretical paper on the subject, but then again we didn't look! I can see some arguments that it might be true. Bonaca also sees many other things in the data, like that the GD-1 stream membership is improved dramatically when we have metallicity information.

dark matter as a latent-variable field

It was a light research day today! But I did get in a conversation with Ana Bonaca (Harvard) about dark matter and information. She has written a nice paper on what information cold stellar streams bring about the gravitational force field or potential in static models of the Galaxy. We have a bit of work on what information perturbed streams (perturbed by a compact dark-matter substructure) bring. We have ideas about how to think about information in the time-dependent case. And on a separate thread, Bonaca has been thinking about what information the stellar kinematics and chemistry bring.

In some sense, the dark matter is the ultimate latent-variable model: The observations interact with the dark matter very weakly (and I'm using “weak” in the non-physics sense), but ultimately everything is driven by the dark matter. A big part of contemporary astrophysics can be seen in this way: We are trying to infer as many properties of this latent-variable field as we can. Because of this structure, I very much like thinking about it all in an information-theoretic way.