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AI summary · arXiv: 2611.05518v1 · astro-ph.GA

DESI DR2 Results III: The Mass of the Milky Way

DESI Collaboration, C. Aganze, J. Aguilar, S. Ahlen, C. Allende Prieto, E. Armengaud, A. Aviles, S. Bailey,
and 108 more authorsA. Bault, J. Behera, L. Beraldo e Silva, J. R. Bermejo-Climent, F. Beutler, D. Bianchi, S. Brieden, A. Brodzeller, D. Brooks, A. Byström, T. M. Callingham, R. Canning, A. Carnero Rosell, F. J. Castander, E. Chaussidon, J. Chaves-Montero, X. Chen, Yingtian Chen, A. P. Cooper, A. Cuceu, Tamara M. Davis, K. S. Dawson, A. de la Macorra, J. Della Costa, Arjun Dey, Biprateep Dey, Z. Ding, P. Doel, G. M. Eadie, H. Ebina, V. A. Fawcett, S. Ferraro, A. Font-Ribera, J. E. Forero-Romero, C. S. Frenk, C. Garcia Palau, E. Gaztañaga, Oleg Y. Gnedin, Satya Gontcho A Gontcho, A. X. Gonzalez-Morales, G. Gutierrez, J. Guy, Jiaxin Han, H. K. Herrera-Alcantar, C. Holm-Hansen, K. Honscheid, D. Huterer, M. Ishak, S. Juneau, T. Karim, S. Kent, D. Kirkby, N. Kizhuprakkat, S. E. Koposov, A. Kremin, A. Krolewski, O. Lahav, C. Lamman, M. Landriau, L. Le Guillou, A. Leauthaud, M. E. Levi, Q. Li, S. Li, Ting S. Li, Z. Li, K. Lodha, Y. Luo, M. Manera, P. Martini, G. E. Medina, A. Meisner, R. Miquel, J. Moustakas, A. D. Myers, S. Nadathur, J. Najita, G. Niz, E. Paillas, N. Palanque-Delabrouille, W. J. Percival, A. Porredon, F. Prada, I. Pérez-Ràfols, J. Ratajczak, C. Ravoux, N. Rehemtulla, A. H. Riley, A. Rocher, C. Rockosi, G. Rossi, E. Sanchez, C. Saulder, D. Schlegel, M. Schubnell, A. Shafieloo, R. Sharples, J. Silber, M. Siudek, T. Tan, G. Tarlé, R. Vaisakh, M. Valluri, W. Wang, B. A. Weaver, R. H. Wechsler, J. Yu, Y. Zhou
Original paper Submitted to arXiv on 03 Nov. 2026 This paper is a DESI Collaboration Key Project publication. 25+12 pages, 14+2 figures, 5+1 tables. Submitted to ApJ License: CC BY 4.0 Also in: fr

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The essentials

Using DESI DR2 halo stars and mock-calibrated dynamical methods, the study estimates the Milky Way’s mass within 80 kpc and its virial mass.

Based on the abstract only: check the details in the original paper.

Question

How massive is the Milky Way, as inferred from distant halo tracers, and how can biases in different dynamical methods and the Galaxy’s disequilibrium be accounted for?

Method

The study analyzes 10,036 RR Lyrae and blue horizontal branch stars with six-dimensional phase-space measurements, spanning approximately 10–95 kpc from the Galactic center. Four dynamical methods are tested on cosmological mock surveys that incorporate tracer stars, DESI- and Gaia DR3-like measurement uncertainties, and survey selection functions; mock-derived biases and residual correlations are then used to combine the estimates.

Findings

  • Result The joint estimate of the Milky Way’s mass within 80 kpc is 6.36 × 10^11 solar masses, with reported asymmetric uncertainties.
    See the passage in the paper
    We obtain a joint estimate of the mass within 80 kpc to be $M(r < 80 \mathrm{\,kpc}) = 6.36^{+0.53}_{-0.48} \times 10^{11}$ M$_\odot$
  • Result The estimated virial mass is 1.19 × 10^12 solar masses, with reported asymmetric uncertainties.
    See the passage in the paper
    and a virial mass estimate of $M_\mathrm{200c} = 1.19^{+0.14}_{-0.12} \times 10^{12}$ M$_\odot$
  • Result The combined mass estimates are similar when using only RR Lyrae or only blue horizontal branch measurements.
    See the passage in the paper
    with similar results when combining only RRL-based or BHB-based measurements.
  • Authors’ interpretation The authors use mock-survey performance to quantify and correct biases associated with method assumptions and Galactic disequilibrium.
    See the passage in the paper
    By comparing the performance on these mocks, we quantify how systematic biases from both methodology assumptions and galaxy disequilibrium impact each estimate.

Limitations

  • The analysis is limited to the two selected halo tracer populations and their sampled radial range, approximately 10–95 kpc from the Galactic center.
    See the passage in the paper
    We apply four dynamical modeling methods to 10,036 RR Lyrae (RRL) and blue horizontal branch (BHB) stars with 6D phase space coordinates that span from $\sim$10 to 95 kpc from the Galactic center, the largest samples of their kind.

Why it matters

The paper presents a mock-calibrated framework for combining Milky Way mass estimates while accounting for method-specific biases and correlations. The authors argue that this offers a roadmap for future DESI analyses and other spectroscopic surveys.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: Milky Way mass Galactic halo RR Lyrae stars blue horizontal branch stars dynamical modeling

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  • Comprehensive Summary
  • Layman's Summary
  • Blog article
  • Original abstract
This paper addresses a central question in Galactic astronomy: how much mass does the Milky Way contain? The authors use distant stars in the Galactic halo to study the gravitational field governing their orbits. The study draws on the second data release of the Dark Energy Spectroscopic Instrument (DESI) and focuses on two types of halo stars: RR Lyrae variables and blue horizontal branch stars. The sample contains 10,036 stars with measured positions, distances, and velocities, enabling analysis of their six-dimensional phase-space coordinates. Described as the largest sample of these tracers assembled for this purpose, it spans distances of approximately 10 to 95 kiloparsecs from the Galactic center, probing both the inner halo and the Milky Way’s outer regions. The team applies four dynamical modeling methods to estimate the Galaxy’s mass. Before combining the results, they test each method using mock surveys based on cosmological simulations. These mocks include simulated tracer stars, measurement uncertainties similar to those in DESI and Gaia DR3, and survey selection functions tailored to the observations. The tests quantify biases arising from modeling assumptions and departures from dynamical equilibrium. The authors find that these systematic effects differ across methods and between the two stellar tracers. Using the mock-survey results, the authors correct the individual mass estimates and combine them with a formalism that accounts for both method-specific biases and residual correlations between methods. This approach aims to produce a more reliable estimate than a simple average of results that may share systematic errors. The combined estimate of the Milky Way’s mass within 80 kiloparsecs is 6.36 × 10^11 solar masses, with uncertainties of +0.53 and −0.48 × 10^11 solar masses. The reported virial mass, defined as M200c, is 1.19 × 10^12 solar masses, with uncertainties of +0.14 and −0.12 × 10^12 solar masses. The authors also find similar results when combining measurements based exclusively on RR Lyrae stars or blue horizontal branch stars, providing a consistency check between the two tracer populations. The study demonstrates how mock-based calibration and careful combination of multiple methods can improve Milky Way mass estimates. It also offers a roadmap for future work with DESI and other spectroscopic surveys. However, the available material includes only the title, author list, and abstract, so the precise assumptions of the four modeling methods, individual method results, selection-function construction, and fuller discussion of limitations cannot be assessed. The abstract identifies modeling assumptions and disequilibrium as sources of systematic uncertainty but does not quantify their effects or describe other caveats.
Created on 10 Nov. 2026

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Weekly letter Presented in "Astronomy and space this week" of 9 November 2026. Astronomy and space this week: Weighing galaxies, tracing particles and auditing cosmic evidence
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Prior work it builds on

  1. Related* Groundwork for this paper
    Galactic Dynamics: Second Edition
    J. Binney, S. Tremaine · 2008 · cited 3,953 times · Not summarized yet

    Galactic Dynamics presents general methods and concepts relevant to the paper’s dynamical modeling of the Milky Way’s mass.

  2. Related* Groundwork for this paper
    Pulsating Stars
    M. Catelán, H. Smith · 1942 · Science · cited 225 times · Not summarized yet

    Pulsating Stars provides background on the variable stars that include the RR Lyrae tracers used in the paper.

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