#ken75, day 1

Today was the first day of the meeting Galactic Archaeology and Stellar Physics in honor of Ken Freeman (MSSSO). As per usual when I am at a meeting, this blog can't convey the full day of talks, so I will just put here very personal highlights.

Freeman opened the conference, giving his overview of what he wants to know about the Galaxy. He is excited about the revolution happening now in which we might have 6-d phase space and 30-ish chemical abundances for stars all over the Galaxy. He brought up two themes that would be very important in today's talks, the bimodality in the alpha/Fe distribution (and its connection to different disk components), and radial migration. On the former, he uses the bimodality to separate the thin and thick disks; he is so confident that he literally calls a chemically separated component the “thick disk”. On the latter, he showed some results I hadn't seen on velocities as a function of metallicity that he argued make the radial migration clear. I have to figure that out! Relevant to things we worked on in the Gaia Sprint, he asked whether the disk components are different heights because of heating or a big event. I think we now know that at small heights it is heating. But the alpha-rich component might be thick because of an event.

Hekker discussed the SAGE project to get a uniform catalog of masses and ages for stars out of non-uniform inputs. She referenced The Cannon but is taking an opposite tack: She is trying to homogenize the data by making all the data constrain the same physical model.

Ruiz-Dern discussed red-clump stars, and in particular building a data-driven model of the relationships between spectroscopic parameters and photometric colors. She showed very good evidence that we could do a lot of the science we do with spectroscopy with photometry instead! That was not her goal, but it got me thinking in a totally new way about my project with Lauren Anderson.

Bovy discussed his results of dissecting the Galaxy into narrow chemical-abundance slices. Where Freeman had used the differing amplitudes in the alpha/Fe bimodality as a function of position to show how different different parts of the Galaxy are, Bovy used the same data to show how similar different parts are! That's a great property of a good scientific result: It can be interpreted either way! He discussed in detail what aspects of his Galaxy decomposition results are consistent and inconsistent with ideas from radial migration.

Talks by Duong and Chiappini again used chemistry to investigate the thin and thick disks, and Chiappini explicitly warned the audience that we will get different results if we split the Galaxy on chemical or structural lines. This also mirrored comments by Bovy.

Toyouchi looked at explaining the alpha/Fe bimodality with an event in the Milky Way's past. This got me thinking about the question: How can we tell whether the bimodality is a fundamental property of the chemical enrichment of molecular clouds or whether it is just the result of some very specific event in the Milky Way's particular past?

Feuillet showed amazing age-abundance relationships for the 19-ish elements that APOGEE observes. It is a goldmine of empirical results. She finds a few highly problematic elements. Like us, she finds that alpha/Fe is strongly correlated with age, at all alpha/Fe values and at all ages.

Buder talked about the GALAH survey and what has been learned and improved with the Gaia DR1 TGAS release. He announced that GALAH is using The Cannon as part of its data analysis pipeline. He said (and I believe him) that they are using it to speed up the code. I like that; it's good for my brand!

APOGEE and time variability

The day started with a conversation with new NYU graduate student Marc Williamson about a project I would like to do in the APOGEE data, looking for time variability in stellar spectra. There should be variations there, because of convection and star spots; the question is whether we can see it, and whether we learn anything about stars from it. I am optimistic. It also connects to ideas I have about improving radial-velocity measurements.

I had lunch with Jo Bovy (Toronto), with whom we discussed what Gaia and APOGEE will jointly reveal about the Milky Way. I like to say that there is great complementarity, because APOGEE is infrared, and its red giants span much of the disk, while Gaia is optical and its sensitivity is best in the halo. However, Gaia is a very sensitive system overall, so it is a detailed question just how much or little Gaia data we will get on the APOGEE giants. Of course I ended up deciding that even if we do get lots of Gaia information about APOGEE targets, that only strengthens my view!

emission-line galaxy spectra

The day opened with a conversation with Guangtun Zhu (formerly JHU) who has been doing great things with the eBOSS and MaNGA spectra from SDSS-IV. On the former, he has made a composite (average) spectrum and can see many things that haven't been seen before in galaxies like these. He can see fluorescence from the outer ISM (or maybe IGM) and he can see the effects of other extremely weak emission and absorption lines. He can also see that the emission lines are due to outflows, but in great detail: Different lines with different relative amounts of absorption and emission have different profiles and he has a consistent story for all of these.

I ended the day by working on the text in the paper on image modeling (image differencing) by Dun Wang (NYU) and in the paper on data-driven galaxy SED models by Boris Leistedt (NYU).

substructures, spots, gender, filtering, and more

Today was the usual action-packed day at the CCA. Before the group meetings, Lauren Anderson (CCA) showed me work on stellar twins in photometric (APASS + Gaia) space, and how similar they are in RAVE-on spectroscopic parameters. It looks like she might be able to put (through a machine-learning-like method) spectroscopic parameters on every star in TGAS. This would be yet another data-driven model of stars.

At stars group meeting, Tjitske Starkenburg (CCA) and Keith Hawkins (Columbia) discussed this paper about chocolates which seems to be the only paper so far about new substructures in the Gaia data. There was some discussion about how they converted parallax to distance (apparently Binney has an opinion), and how the sub-structure is found via a cross-correlation between the data and random realizations. The evidence is a bit weak. However, some of the substructures look worth following up in chemistry and in other stellar samples.

This was followed by Michael Gully-Santiago (formerly Kavli, Beijing) showing his work on figuring out the ages and masses of really young (few Myr) stars, and (more ambitiously) getting the spectra of star spots on their surfaces! His project is very related to our spectroscopic binary work: How to measure a cold spot on a hot star? His approach is to modify the (Czekala et al) Starfish code to also fit for cold patches (at the same metallicity and logg as the main surface). For his particular case (LkCa-something), he finds a preference for a spot temperature of 2700-ish K covering 80-ish percent (!) of the star. This was followed and interrupted by lots of discussion about stellar binaries, long-term evolution, longitude effects, and so on.

Between group meetings, Sandro Tacchella (ETHZ) talked about his paper on gender bias in astronomy. The paper gathers good data, and is (properly) limited in its conclusions. It ends with a relatively sophisticated causal inference that is fairly convincing that women are cited less than men for papers with otherwise similar properties. This involved building a predictive model for citations. That led to a good discussion!

In the cosmology group meeting at the end of the day, Kris Sigurdson (UBC, NYU) spoke about CHIME 21-cm observations and foreground mitigation. Foregrounds should be smooth in the frequency direction, unlike the narrow-band 21-cm emission. We discussed the differences between filtering-like (or K-L-like) techniques that deliver minimum-variance modes, and subtraction-like techniques that try to build an explicit model of the foregrounds. We vowed to continue the discussion.

another data-driven model of stars

My people are tired of hearing endlessly about data-driven models of stars. But today Boris Leistedt (NYU) created a new one, and I am extremely excited about it.

The idea—which is more-or-less my unattempted Gaia Sprint idea—is to build a very flexible model in color–magnitude space, and then generate noisy parallaxes. That is, a hierarchical model for the parallaxes, with a color–magnitude diagram that is learned simultaneously. Today, Leistedt had the breakthrough that this could be done in bins in color and magnitude, with a Dirichlet model. That is out-of-the-box inference; he got it working and it looks nice! This is all on the path to removing physical models from (what you might call) the Gaia distance ladder (which starts at parallaxes, and ends with some kind of distance estimate for everything that can be detected).

(The first sentence of the second paragraph of this post uses all three kinds of dashes: em, en, and hyphen. Bring it on, typography nerds!)

Saturn's rings and moons

My nearly-null research day was saved by a brown-bag talk by Bob Johnson (UVa, NYU) about Saturn's moons and rings, and what we have learned from the Cassini Mission. The spacecraft takes images but also in-situ spectrometer measurements, so it can measure interplanetary plasma. There are many surprises, including oxygen ions all around, which means (for example) that Titan has a lot of chemical diversity. Johnson's research is about understanding the plasma physics given heating, transport, and chemical processes. The system is very seasonal, as the rings get heated from below, then edge-on, and then above. And there are apparent changes in the plasma near the co-rotation of the magnetic field and the rings.

Dragonfly

Today's astro seminar was by Pieter van Dokkum (Yale), about the Dragonfly project and their discovery of large, dark-matter-only-ish galaxies. That is, they have found galaxies that have way fewer stars per unit of dark-matter mass than anything previously. That's exciting. We had lots of discussion during the talk and before about the technical value and also challenges of working with small telescopes, and in particular refractors. van Dokkum put a lot of weight on the existence (in the digital photography world) of nano-scale structured anti-reflection coatings, which are available in (high end) commercial lenses but not in refracting telescopes. He discussed the value of having no supports in the aperture, and no mirrors (yes, mirrors are bad for ultra-low surface brightness). There are many connections between this work and the future coronography we need to do to directly image exoplanets. Dragonfly is a beautiful, sensible, low-cost project that is incredibly successful.

empirical binaries

My only real research today was a conversation with new NYU graduate student Shengqi Yang, who is going to start on a project to look at binary stars in the APOGEE data as linear combinations of single stars. That is, a purely empirical model (for now). We discussed how to find similar stars, and how to decide that they are similar, and what a binary star would look like in the continuum-normalized spectral space. The nice thing about the problem is that, in principle, a search for binaries among N spectra might take N³ time, since any star could be a mixture of any two others! However, we are going to start with the Gaia TGAS overlap, which reduces the numbers an also trims the tree, because there are many triples that—given what we know from Gaia—couldn't be so related.

image differencing and stellar rotation

As always, the stars group meeting at the CCA was a research paradise. We opened (at the suggestion of Ruth Angus) by acknowledging that a terrible thing had happened in the United States and that we were collectively sorry about this. That was necessary.

We moved on to a presentation by Dun Wang (NYU) of his image modeling for the Kepler K2C9 imaging campaign to measure microlensing events in the Galactic Bulge. He described his radical technique for image differencing, which has produced (I think) the most precise image differences in the history of astronomy! They have requirements, however, that not all future time-domain imaging surveys would successfully meet. He has some really impressive movies, and we stared at them like zombies for quite a long time.

After that, Ruth Angus (Columbia) presented the recent paper by Davenport about stellar rotation, informed by the Gaia DR1 TGAS data (and thanking the Gaia Sprint!). The paper confirms and extends a discovery of a gap in rotation-inferred ages of stars, which either points to some kind of non-linearity in the evolution of stellar rotation, or else points to a really non-trivial event in the star-formation history of the Galaxy. It could also be that there are just different classes of stars (in terms of rotation). We argued about all this. Hawkins (Columbia) recommended that we look at the relevant stars in chemical-abundance space; this should resolve some of the possible explanations.

After group meeting, Tim Morton (Princeton), Adrian Price-Whelan (Princeton), and I sat with Semyeong Oh (Princeton) to go over the draft (and especially figures) of her new paper on co-moving stars in Gaia TGAS. Lots of great stuff in there, and a valuable tool for stellar astrophysics.

nothing

Today was a total fail, making it two days in a row. The political events of the United States sure don't make for a conducive environment for getting research done.

not much

In a low-research day, Hans-Walter Rix (MPIA) called me to discuss double-lined spectroscopic binaries, and what we might be able to do in the low-hanging-fruit category. I gave him some of the ideas that we have been playing around with in the context of The Cannon. It was basically a fail today, though, research-wise.

the 21-cm signal from reionization

Adrian Liu (Berkeley) gave today's Astro Seminar. He spoke about the radio instruments PAPER and HERA, which are trying to see the 21-cm signal from reionization. He showed reasonable evidence that this signal will be strongest at around redshift 8. HERA is being built as a pre-cursor mission to the SKA. PAPER is done, and although the final data analyses are not complete, Liu claims that it has the best limits on the 21-cm emission from reionization, and they are a factor of tens higher than the expected signal. So there is work to do. He admitted that they are not doing the best they can on foregrounds: They are avoiding rather than modeling the foregrounds. After the talk we discussed this a bit; it connects to the things I am interested in regarding stage-four cosmology.

Late in the day, Megan Bedell (Chicago) pointed out to me that there are hundreds of stars with reduced, high resolution, high signal-to-noise spectra in the HARPS archive. Almost no-one (to our knowledge) is doing anything with these data. Crazy! We discussed plans to exploit this amazing data set.

how precise is The Cannon?

It was a low-research day! But Hans-Walter Rix (MPIA) called me to discuss the endless problem that we don't have label uncertainties that we believe in the output of The Cannon. The context is Ness's work to measure the abundance variability within open clusters (which are famously close to single-abundance populations).

Our formal uncertainties with The Cannon are tiny, but under-estimated because they don't properly account for the choices we made in optimizing the internals. Our cross-validation uncertainties are much better, but still over-estimates because they effectively include systematic terms that go beyond precision. That is, if we only care about precision in a single cluster, the cross-validation is an over-estimate. And we can see that empirically, because with a single-abundance fit we get chi-squared values that are much smaller than the number of degrees of freedom.

My view is that we should use the open clusters themselves to set the uncertainties. This sounds circular: How can we estimate the intrinsic abundance spreads if we set our observational uncertainties assuming that the spreads are zero? But it isn't: For one, different open clusters are different in their abundance spreads. For two, there are long tails of abundance differences even in the best clusters. For three, even if there were neither of these effects, we would still get great upper limits!

The long-term solution is to go fully Bayesian. I became motivated to work on this now. I owe ideas about this to various people, including Jonathan Weare (Chicago).

group meetings awesome

My goodness Wednesdays are good research days! As per usual, I spent the day at the CCA, where the research activity was dominated by my two group meetings. Here are some notes I took at each:

Sarah Pearson (Columbia) introduced herself at the stars group meeting; she talked about stellar streams and using them to constrain the shape of the MW dark-matter halo. With Pal 5 stream morphology alone, she can show that the Law & Majewski potential (used to explain Sagittarius) must be wrong, because it would create very strong stream fanning (which happens in chaotic potentials). Pearson is also thinking about finding thin streams in external galaxies, because morphology alone is so potentially constraining.

After Pearson, I asked about the projects from the Gaia Sprint, and their statuses. Leistedt (NYU) said things about modeling the color-magnitude diagram. He complained about the difficulty of fitting mixtures of Gaussians, especially because of initialization-dependence. Morton (Princeton) talked about his code to fit stellar properties given photometry and parallaxes. He complained about working with extinction in the broad G bandpass; this required a structural change to his code. (He mentioned also a high-resolution spectroscopic survey of transiting planet hosts, that could be relevant to us). Morton also brought up the interesting and fun issue that if you have a sample of transiting planet hosts, it is hard to get a control sample, because in no sense does “no transiting planet” mean &ldquo:has no planet”. This is worth more discussion! Angus (Columbia) talked about her work on rotation for stars with parallaxes; she is computing rotational ages. She has looked at the age–dispersion relation and also the ages of stellar pairs, to test age estimators and check that the wide pairs have small ages. In this discussion, the idea came up to compare photometric rotation periods with spectroscopic v sini measurements. Starkenburg (CCA) talked about RAVE+Gaia data on known stellar cluster members, to observe the kinematic escape of stars. We discussed a possible self-consistent model, accounting for the observed stellar mass. Anderson (CCA) spoke about photometric twins. These should be spectroscopic twins, asteroseismic twins, and (as Angus pointed out) even photometric light-curve twins! Leigh (AMNH) talked about numerical scattering experiments he is doing to look at the hypervelocity stars coming from the Galactic Center. He is comparing competing models for the scattering mechanism at the SMBH. Hawkins (Columbia) spoke about his hierarchical model of the population of red clump stars. He can show that they are standard candles to 0.08-ish mag, depending on which sample he chooses. His hierarchical model de-noises the parallaxes and makes very strong predictions for future Gaia data releases!

In the cosmology group meeting, Francisco Villaescusa-Navarro (CCA) spoke about HI gas and intensity mapping, in the context of large-scale structure with the SKA. There are strong trade-offs between resolution for angular and line-of-sight measures of the BAO. We discussed how to suppress the noise and foregrounds in practice for future measurements. He gets percent-level expectations for the Hubble Law at redshifts 0.5<z<2.5, which is promising, but will it satisfy our demands? That might be beat by Euclid! But he is considering only radial modes, not transverse modes. The discussion included lots of talk about beams, foregrounds, estimators, deconvolution, calibration, ancillary data, and self-calibration, with heavy participation by Kris Sigurdson (UBC).

the bento-box MCMC sampler

In a low-research day I did get in a conversation with NYU student Neelang Parghi about an old idea (code-named “bento box”) I have had (with my MCMC collaborators) about an MCMC method that would identify multi-modality, split the space, and recursively sample simpler and simpler problems until it is sampling much smaller, simpler problems that are technically trivial. Parghi is going to try to make this happen as part of his Computational Physics class term project.