It’s a matter of time: Empirical Constraints on Supernova Yields and Delay Times from Dwarf Spheroidal Galaxies

Mairéad E. Heiger, Alexander P. Ji, Joshua S. Speagle, et al.

What I liked: This paper puts a vague idea I had about chemical evolution into practice, and the results are far more exciting than I expected. The idea is that in dwarf galaxies, we measure both star formation histories and chemical abundances, but chemical evolution models impose parametric assumptions that can dominate the qualitative results (e.g. the shape of the alpha knee). Mairéad developed a way to greatly reduce these assumptions by using the star formation histories and metallicity distributions as input rather than forward modeling. She then applies this to study the Type Ia SN delay time distribution, where — surprisingly — she finds a t⁻² DTD instead of the usually assumed t⁻¹. This seems like a qualitative change in our understanding of dwarf galaxy chemical evolution, and her work has me all excited about chemical evolution again.

ELVES-Field I: A Sample of Nearby Isolated Dwarf Galaxies: A First Look at the Mass Function of Field Dwarfs

Scott Carlsten, Jiaxuan Li, Jenny Greene, Alex Drlica-Wagner, Shany Danieli

ELVES-Field II: Isolated Dwarf Galaxy Quenched Fractions Rise Below M★ ≈ 10⁷ M☉

Scott Carlsten, Jiaxuan Li, Jenny Greene, Alex Drlica-Wagner, Shany Danieli

What I liked: These two ELVES-Field papers are a heroic effort to search for and confirm low-mass dwarf galaxies (down to M★ ~ 10⁶ M☉) in the local volume. The key scientific takeaway is that field dwarf galaxies around M★ ~ 10⁶ M☉ are a bit smaller and more quenched (~30%) than higher-mass galaxies above 10⁷ M☉, and it’s not clear why — reionization probably doesn’t kick in until M★ < 10⁵ M☉. Scott visited UChicago at the start of this project, and I got to see him develop parts of the search and confirmation pipeline. These papers represent a crazy amount of invested work, and it will be interesting to see how they can be extended to LSST.

A Chemodynamical Census of the Milky Way’s Ultra-Faint Compact Satellites. I. A First Population-Level Look at the Internal Kinematics and Metallicities of 19 Extremely-Low-Mass Halo Stellar Systems

William Cerny, et al.

What I liked: This paper is the result of 5 years of observations on the smallest stellar systems currently known in the Milky Way halo — M★ < 10³ M☉ and r½ < 10 pc. The field had largely ignored these objects, thinking they were “uninteresting” low-mass star clusters, but increasing interest in star clusters combined with the realization that some might actually be dwarf galaxies led Will to revisit everything. This is the first spectroscopic census of this fascinating population. Annoyingly, despite the huge amount of telescope time invested, we are unable to definitively classify nearly all of the objects. It’s a harbinger of things to come in LSST.

Evolution of Low-Mass Population III Stars: Convection, Mass Loss, Nucleosynthesis, and Neutrinos

Thiago Ferreira, Earl P. Bellinger, Ebraheem Farag, Christopher J. Lindsay

What I liked: This paper studies the detailed stellar evolution of low-mass Pop III stars — something I have wanted to see for a long time (often pitching it to undergrads and grads without getting a bite). Two top-level results stand out: low-mass Pop III stars live slightly less time than more metal-rich counterparts, and you can get CNO enrichment during the RGB phase. A follow-up paper by the same authors on asteroseismology of these objects — to see if you can detect differences in core structure — is now very much on my radar. Working through this paper with MESA will be one of the suggested projects for the NFC seminar this spring.

The Milky Way’s circular velocity curve measured using element abundance gradients

Danny Horta, Adrian M. Price-Whelan, Sergey E. Koposov, et al.

What I liked: A clever paper using abundance gradients to measure the rotation curve of the Galaxy — Danny gave a talk on it at a SDSS-V telecon. As I have learned more about why these abundance gradient methods work, I have come to appreciate just how clever a way this is to constrain galactic dynamics.

The LBT Yₚ Project IV: A New Value of the Primordial Helium Abundance

Erik Aver, Evan D. Skillman, Richard W. Pogge, et al.

What I liked: I didn’t read this paper, but we had a colloquium from Keith Olive on Yₚ, and this is the culminating paper. One key takeaway: Yₚ is the best BBN constraint on Neff. This is a very nice, high-quality observational program with a great result. It also illustrates a general principle — that doing cosmology is often the best way to get an amazing dataset. Noah Rogers is at CIERA right now and is thrilled at how many high-quality spectra he gets to study, which would have been hard to obtain otherwise.