so much Gaia; #renameJWST

I spent the day working on ESA Gaia data, parallel to Kate Storey-Fisher (NYU). She was working on the quasar catalog and the correlation with the CMB convergence maps; I was working on estimating stellar luminosities from low-resolution spectral coefficients. We are too much in the thick of it to report how it's going yet. But stellar luminosities are hard to predict from spectra!

Late in the day I sent this letter to NASA:

I served on the US Astronomy and Astrophysics Advisory Committee (AAAC) for several years; I served on the NASA Spitzer Space Telescope Oversight Committee for many years; I served on the NASA Extragalactic Database User Committee for several years; and my research has been funded by NASA for my entire career (since I was a PhD student in the 1990s). I currently do research with HST, Kepler, TESS, 2MASS, WISE, and WMAP data, and now I'm getting ready for JWST and SPHEREx.

I am writing to say that I think it would serve NASA's interests, and the interests of NASA science especially, to rename JWST. There have been plenty of discussions of the name; it is clear that many scientists (and especially those who are part of the LBGT community or who have concerns for the LBGT community) feel disrespected by the name. I also personally think that the evidence is clear that some of the career activities of James Webb did direct harm to patriotic Americans who were gay.

I want to emphasize, however, that I think the important argument about the name goes beyond the question of any individual historical facts: The LBGT communities are of great importance to all of us. These voices must be heard, and the legitimate concerns must be addressed.

Because NASA is a forward-looking agency, and working towards a more equitable, better world, especially for people working in science and engineering in the US, I think it is time that the spacecraft be renamed. I think this could be done easily and without any trouble; many spacecraft have either officially or effectively changed names at or around first light, two examples that come immediately to mind are WMAP and Spitzer.

Gaia quasar redshift distributions

Today I worked with Kate Storey-Fisher on the ESA Gaia quasar sample. We looked at the redshift distribution as a function of magnitude and as a function of color cuts or selection. Right now we are getting an odd angular correlation function for the sample, which we think is because our random catalog (or completeness model) is wrong in important but subtle ways. Interestingly, there are probably terms coming from both the dust map (extinction) and from the Gaia internal completeness (scanning law) and maybe both contribute enough to change the answer? But it sure looks like dust dominates.

Doppler shifts and radial velocities

I am a big believer that there is a difference between a Doppler shift and a radial velocity. For one, they have different units! For another, the former is measurable (sometimes) and the latter is not (or rarely). But today I agreed with Megan Bedell (Flatiron) that we should write our paper on the subject in terms of the words “radial velocity” and not “Doppler shift”. After all, we are talking to a community with a common language! I spent some time on the train from Vienna to Heidelberg editing the figures for our paper on this subject.

is it ever scientifically conservative to use machine learning?

I gave a talk Is machine learning good or bad for science? in Vienna today (slides here). I spent a lot of time on the ontology and epistemology of it all. One thing that led to some debate afterwards is my claim (at the end of the talk) that using extremely flexible machine learning methods can be extremely conservative in some cases: If you are modeling a nuisance that possibly interferes with your signal of interest, and you used a very flexible model, you have a strong argument that you tried as hard as you could (in some sense) to dilute your signal of interest with that nuisance. My talk was followed by interesting discussion with many, and a lovely dinner with Viennese (not just Austrian, but Viennese) wine.

Dr Ratzenböck

It was my pleasure to be a part of the PhD committee for Sebastian Ratzenböck (Vienna), who wrote a dissertation in computer science but as applied to astrophysics. He had three advisors, in statistics, in computer science, and in astronomy, and he beautifully bridged the three worlds. His research was on finding members of stellar clusters, and on finding new stellar clusters. He showed (pretty convincingly, I think) that star-forming regions break up into many individual star-forming events with different ages and different kinematics. One of his conclusions is that all star formation happens in clusters or groups! He also made a nice technical advance, which was to build a tool to select clustering hyper-parameters in the space of physical quantities one cares about, instead of in the space of arbitrarily-defined clustering-method parameters. It was a great thesis, a beautiful defense, and a fun time drinking afterwards. Congratulations Dr Ratzenböck!

is machine learning good or bad for science?

I took the train to Vienna today, for a PhD defense and to give a talk about machine learning. My talk is interdisciplinary so I looked at how to generalize my arguments about astrophysics to all of the natural sciences. It turns out that this isn't as easy as I'd like, since it is hard to be specific outside of astrophysics! I'm going to learn a lot getting this talk ready.

Gaia quasar redshifts

Today Kate Storey-Fisher (NYU) arrived in Heidelberg and we worked on improving the ESA Gaia DR3 quasar redshifts, using NASA WISE and IRAS information. We were trying to reproduce experiments performed by Hans-Walter Rix (MPIA) last week. It looks like we can get clean-ish samples, where more than 90 percent of the redshifts are correct (as compared to ground-based spectroscopic surveys), provided that we stick to bright quasars. The question is: Is this good enough for our cosmological goals? We discussed how to empirically evaluate the selection function, which is our next task.

bouncing ball

I worked on the weekend on making a toy problem for testing physics-related machine-learning methods: A ball bouncing off an elastic surface, under gravity. Both the surface and the gravity vector break the symmetry; this problem is not at all invariant with respect to rotation, translation, or boost. And yet the laws of physics can be written in a coordinate-free form. I am trying to figure out whether we can make this distinction usefully in the literature: The distinction between coordinate-free and equivariant. I think they are different concepts, even though the mathematics of them are identical.

local linear regressions

For some reason, even though I dislike deep learning, I love local linear regressions. My friends tell me that RELU networks are locally linear, so I am really just a hypocrite. Anyway, today Adrian Price-Whelan and I built a regression in which we find nearest neighbors (among the training-set objects) in the space of ESA Gaia DR3 Bp/Rp spectral coefficients and, among those neighbors, we fit a locally linear model to predict the parallax of the test object. Technically we use a clever trick called the ”schmag“ but which should probably be called the reduced parallax, in which we correct the parallax into the inverse of the square root of the luminosity. Why? It's so we can use the parallax errors fairly, and include training-set objects with negative parallaxes.

Hill I will die on: If you cut your sample to high SNR parallaxes or positive parallaxes, you will bias any regressions you do to predict parallaxes or distances or distance moduli!

Gaia Hike, day 5

Today was day 5 of the Gaia Hike. Jason Hunt and I looked at a file of kinematic information prepared by Adrian Price-Whelan (Flatiron) to look at the possibility that the Milky Way has a low-amplitude counter-rotating disk of stars. We found nothing. This was in contrast to what Claudia Bielecki and Federico Sestito were finding—with a similar file created by George Kordopatis. By the time I had to leave the meeting, we hadn't resolved the discrepancy. It's interesting either way!

Gaia Hike, day 4

On day 4 of the Gaia Hike, Neige Frankel (CITA) and I tried to look for the signature of the Snail (the vertical phase spiral in the Milky Way stellar kinematics) in metallicity. It's visible in the Gaia Collaboration chemical cartography paper. But it's not trivial to find it. We got a tiny hint of it using RVS metallicities, and we resolved to try some more tomorrow. We also figured out that it should be there even if the Snail is a late production of a late interaction: Abundance gradients FTW.

Gaia Hike, day 3

Today was the literal hike day of the Gaia Hike. I couldn't go, for uninteresting technical reasons. So instead I spent my time preparing hack projects for those who are looking for straightforward hack ideas and want to learn. I got stuck many times and Adrian Price-Whelan (Flatiron) helped me un-stick. I guess straightforward isn't straightforward! Anyway, I produced this document which contains hack ideas. It is just a start, just a stub, but maybe it will be useful?

Gaia Hike, day 2

Today was day two of the Gaia Hike at UBC. The results of yesterday's discussions and hacking were discussed in the morning and then we moved to tutorials about how to use the ESA Gaia data responsibly. From my own personal perspective, the highlight was a big tutorial from George Seabroke (UCL) on the high-resolution RVS spectra. His presentation went through all the properties and issues with the data, including things like overlapping spectra on the focal plane. It was really impressive, incredibly useful, extremely detailed, and an amazing representation of how complex and challenging a mission like Gaia is. Congratulations to the entire DPAC for pulling this off!

Gaia Hike, day 1

Today was the first day of the Gaia Hike hosted at UBC and led by Neige Frankel (CITA). The day started with business cards (short intro talks from everyone), followed by an attempt to find common themes across participants. Once the themes were identified, we split into groups to talk about what we might do this week with the ESA Gaia DR3 data. I ended up in a mapping and visualization group, which was fun, and (of course!) we closed out the day hacking on a piece of the data on stellar parameters, trying to figure out why the stellar parameters don't look exactly as we expect.

coordinate freedom?

I spent the weekend recovering from the Gaia Fete. During my recovery day, I worked on very long-term projects. For example, I spent some time working on how to express the following issue in my work (with Villar) on exact symmetries:

The mathematics and computer-science communities call these exact symmetries “equivariances” and they are imagining that the data or the laws of physics are precisely equivariant in the sense that if you (say) rotate all the inputs, you get a rotated output. But this is not the main reason that we write the laws of physics in terms of exact symmetries! We write the laws of physics in terms of invariants because we want our laws of physics to be coordinate free. This is required even when the laws aren't equivariant! But I have trouble making this distinction clearly, since the mathematical implementations of the two symmetries are identical. There's some cool philosophy here: Does coordinate freedom enforce symmetries? What would it even look like for the laws of physics to be asymmetric but coordinate free?