SPHEREx data

Dustin Lang (Perimeter) and I spoke today about many things, but the conversation got de-railed when Lang showed me his visualizations of the brand-new NASA SPHEREx data. Oh. My. Goodness. First of all, the data are being released daily, before the team has done its analysis, so that anyone in the world can do anything with it. Talk about open! Talk about international! Talk about everything I believe about astrophysics! Also, the data are well documented, high in signal to noise, and released with good and useful metadata. This is killer. Of course Lang has already made a viewer for it and can do all comparisons to the DESI data. Get in touch with him if you want tools.

stars don't orbit at their guiding radii!

A subject of conversation all week has been about stellar orbits in the Milky Way disk, driven by some visualizations made by Adrian Price-Whelan (Flatiron). We often describe the azimuthal action or the z component of the angular momentum (L_Z or J_Phi) of a disk star in terms of the guiding radius, or the radius of a circular orbit of the same azimuthal action. The idea is: If the star has radial action, it will oscillate around the guiding radius as it orbits. Wrong!! If the vertical action is comparable to or larger than the radial action (and that’s typical), the star will orbit outside the guiding radius, always. The trivial picture is simply incorrect.

Gothamfest

Once a year (and differently every year), we get together as much of the astronomical community in New York City as we can and have them give fast talks. Today was great! I learned a huge amount, and no highlight reel would do. But here are some examples: Amanda Quirk (Columbia) has great data on M33 stars that maybe we could use to build images of the orbital toruses using technology that Price-Whelan and I developed over the last few years? Marc Huertas-Company (Paris) said (confidently?) that many of the star-forming galaxies found by JWST at very high redshift are likely prolate. Michael Higgins (CUNY) and Keaton Bell (CUNY) have a beautiful system to separate sources of variability out in NASA TESS data using structure in frequency space. Kate Storey-Fisher (NYU) showed results from Giulio Fabbian cross-correlating her ESA Gaia quasar sample with the ESA Planck lensing map, with better error bars than any previous survey! Ben Cassese (Columbia) showed a moving-object pipeline with NASA TESS imaging that detects outer Solar System objects, much like old work by Dustin Lang and myself.

halo mass assembly

On Fridays, Kate Storey-Fisher (NYU) organizes a small meeting to discuss her projects on dark-matter halos using equivariant scalar objects constructed from n-body simulation outputs. Today we included Yongseok Jo (Flatiron), who has worked on building tools to paint galaxies onto dark-matter-only n-body simulations. We discussed joint projects, and conceptual issues about mass-assembly histories. In particular, I am interested in how we can predict formation histories of dark-matter halos from the galaxy contents alone, or infer the dark matter distribution in phase space from the stellar distribution in phase space. I love these projects, because they combine growth of structure, gravitational dynamics, galaxy formation, and machine learning.

dust and star formation

Julianne Dalcanton (Flatiron) gave a great talk at NYU today about star formation, interstellar medium, stellar ages, and dust in Local Group galaxies. She showed that the standard star-formation indicators from infrared emission from dust are way wrong. But she also showed lots of interesting detail in the interstellar medium and star-formation history in M33 and M31. M31 really does seem to have a ring which is not just over-dense in star formation; it's actually over-dense in stars. That's odd, and interesting.

the symmetries of the observed universe are different from the symmetries of the latent universe

Kate Storey-Fisher and I spent a long time today talking about how to build a project that is about cosmological observables, built from the concepts in her projects on applying coordinate-free geometric forms to theoretical objects in cosmology. The idea could be: Find geometric scalars that exist in the theoretical (or latent) universe, find geometric scalars that exist in any observational survey of the observable universe, and learn the relationships between these; construct cosmological tests and tests of the dark-matter model. The big issue (from my perspective) is that the symmetries that apply to the 6-dimensional phase space of the Universe are different from the symmetries that apply to the observed 3-dimensional redshift-and-angle-space of the (galaxy or quasar, say) observations. Some might say that there are no symmetries in this observational space, since there are window functions and selection functions, but this is not correct: Coordinate symmetries still exist there, it is just that these other functions must also be tracked, in the same space. Anyway, it's a nice research program to figure all this out.

regularities of dark-matter halos

There is a regular dynamics meeting (maybe Galactic dynamics meeting?) at Flatiron. I went today and I learned a lot, from Ivana Escala (Princeton) and Danny Horta-Darrington (Flatiron). I briefly presented Kate Storey-Fisher's project of describing dark-matter halos with coordinate-free nonlinear geometric scalars, which isn't really a dynamics project but it could be, because these scalars could be part of a canonical transformation of the dark sector. Anyway, the crowd had interesting things to say. In particular, the idea came up that the subspace in which the dark-matter halos live (subspace of the space of these scalars) is likely to be very compact (or low-dimensional, or both) and that the susbspace probably depends on the dark-matter model. That's a great idea, and suggests that maybe we can construct new tests of gravity.

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!

extragalactic stellar stream

Sarah Pearson (NYU) is working on modeling a stellar stream (disrupted satellite galaxy) around an external galaxy. The goal is to figure out what observables are most critical, and what properties of the host galaxy are most strongly constrained by a good model. That is, information theory. Pearson showed beautiful results today to Adrian Price-Whelan (Flatiron) and me: She can show that the mass of the galaxy's dark-matter halo is covariant with velocity gradients along the stream. Those would be hard to measure but not impossible. One high-level objective is to understand what would be the scientific merit of a big program with new imaging data and follow-up spectroscopy.

me reading?

As my collaborators and friends know, if there is one thing I hate to do, it is spend all day reading the literature. I love and respect the literature! But don't make me actually read it. But today I sucked it up and read some 20-ish papers about characterizing dark-matter halo shapes, to find out if the coordinate-free shape measurements that Kate Storey-Fisher (NYU) and I are measuring are new. I think they are! In almost every paper I read, the word “shape” translated to eigenvalues of the positional variance tensor, or maybe ratios of those. Am I wrong?

shapes of dark-matter halos

I had a very very long meeting today with Kate Storey-Fisher (NYU) in which we talked through every aspect of our current project, at every level of abstraction. It was great! And at the end of it, I had a way simpler description of our project than I think I could have articulated even yesterday: We are asking whether high-order, coordinate-free measurements of dark-matter halo shapes can predict galaxy contents.

how can linear regression be hard?

Maybe I'm known in astronomy for being both a machine-learning developer and a machine-learning skeptic. I hope so! Anyway, I love linear regression, because it has a lot of the power of bigger ML models, but it's easy to implement and to understand. And yet!

Today Kate Storey-Fisher (NYU) and I looked at her code to predict galaxy properties given dark-matter-halo properties in a set of n-body simulations. We are doing very simple regressions but the condition numbers of the matrices are blowing up and some of our answers don't look great. And this is generic: Many linear-regression models are messed up by condition numbers and numerical linear algebra, and it is hard to diagnose, and it is hard to treat. And if linear regression is hard—and hard for us—why do I believe anything that inovolves 42 layers of fully-connected RELU network?

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.

aluminium abundances

A very good conversation broke out in our weekly Gaia DR3 & SDSS-V prep meeting today, about Aluminum abundnaces in stellar photospheres, which are the key tool in a new paper about the Milky Way being drafted by Vasily Belokurov (Cambridge). Keith Hawkins (Texas) is in town, and he also happens to be the discoverer of very interesting relationships between Fe, Al, Mg in low-metallicity stars. The ratio [Al/Fe] increases with [Fe/H] at low metallicities and decreases at high metallicities. That has something to do with the different timescales for different kinds of supernovae and different rates of star formation. This all might explain why Christina Eilers (MIT) and I are finding weird issues when we try to fit [Al/Fe] as a function of stellar evolutionary state and dynamical actions in the Galaxy.

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.

Dr Dou Liu

Today I had the pleasure of sitting on the PhD defense of Dou Liu (NYU), who has been working on AGN in the centers of galaxies, using MANGA data from SDSS-IV. The part of Liu's thesis that is most exciting to me (perhaps not surprisingly) is the technical chapter, in which he finds a new method for combining irregularly dithered integral-field-unit spectroscopy exposures into a full data cube, with sky coordinates on two axes, and wavelength on the third. In this final data cube, his method gets much better final resolution, and lower pixel-to-pixel covariances in the noise, relative to the standard pipelines. His trick? He has generalized spectro-perfectionism (invented for spectral extraction) to the multi-dimensional spectral domain. It's beautiful stuff, and has implications for all sorts of imaging and spectroscopy projects going forward. Congratulations Dr. Liu and thank you!

abundance calibration and abundance gradients

Today Christina Eilers (MIT) updated Hans-Walter Rix (MPIA) and me on our project to self-calibrate the element-abundance measurements in APOGEE. We are looking at self-consistency of the abundance distribution as a function of actions; in a well-mixed Galaxy this could be used to calibrate the biases of the abundance measurements with surface gravity (a known effect in the data) and spectral resolution (a possible effect). Eilers has beautiful results: The abundances get better and the abundance gradients in the Galaxy (with radius or azimuthal action, and with vertical height or vertical action) become more clear and more sensible. So we have a paper to write!

streams in external galaxies

Sarah Pearson (NYU), Adrian Price-Whelan (Flatiron), and I met today to discuss fitting tidal streams (and especially cold stellar streams) discovered around external galaxies. We are starting with the concept of the stream track, and therefore we need to turn imaging we have of external galaxies into some description of the stream track in some coordinate system that makes sense. We spent time discussing that. We're going to start with some hacks. This isn't unrelated to the work I have been discussing on microscopy of robots: We want to make very precise measurements, but in very heterogeneous, complex imaging.

Dr Sicheng Lin

Today Sicheng Lin (NYU) defended his PhD dissertation on the connections between galaxies and the dark-matter field in which they live. He worked on elaborations of what's known as “halo occupation”, “abundance matching” and the like. At the end, I asked my standard questions about how the halo occupation fits into ideas we have about gravity, and the symmetries of physical law. After all, “haloes” aren't things that exist in the theory of gravity. And yet, the model is amazingly successful at explaining large-scale structure data, even down to tiny details. That led to a very nice and very illuminating discussion of all the things that could matter to galaxy clustering and dark-matter over-densities, including especially time-scales. An important dissertation in an important area: I learned during the defense that the DESI project has taken more than one million spectra in it's “science verification” phase. Hahaha! It makes all my work from 1994 to 2006-ish seem so inefficient!

Dr Shengqi Yang

I've had the pleasure of serving on the PhD committee of Shengqi Yang (NYU) who defended her PhD today. She worked on a range of topics in cosmological intensity mapping, with a concentration on the aspects of galaxy evolution and galaxy formation that are important to understand in connecting the intensity signal to the cosmological signal. But her thesis was amazingly broad, including theoretical topics and making observational measurements, and also ranging from galaxy evolution to tests of gravity. Great stuff, and a well-earned PhD.