DESI DR2 Results III: The Mass of the Milky Way
Authors: DESI Collaboration, C. Aganze, J. Aguilar, S. Ahlen, C. Allende Prieto, E. Armengaud, A. Aviles, S. Bailey, A. 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
Abstract
We present measurements of the mass of the Milky Way using distant halo stars selected from the Dark Energy Spectroscopic Instrument (DESI) Data Release 2 (DR2). 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. All methods are applied to mock surveys based on cosmological simulations that include generating individual tracer stars, convolving their phase-space coordinates with DESI and Gaia DR3-like uncertainties, and tailored models of the survey selection functions. By comparing the performance on these mocks, we quantify how systematic biases from both methodology assumptions and galaxy disequilibrium impact each estimate. The mock-based systematics, which differ by method and tracer, are used to correct the individual estimates and to derive joint estimates using a rigorous formalism that accounts for per-method biases and residual correlations between methods. 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$_\odot$ and a virial mass estimate of $M_\mathrm{200c} = 1.19^{+0.14}_{-0.12} \times 10^{12}$ M$_\odot$, with similar results when combining only RRL-based or BHB-based measurements. These results provide a roadmap for future DESI efforts, as well as those in other spectroscopic surveys, to reliably constrain the mass of our Galaxy.
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A distribution-function estimate of the Milky Way mass profile from outer halo stars in DESI DR2G. E. Medina, T. Callingham, M. Valluri et al. · 2026 · Not summarized yet
The distribution-function estimate uses the Galactic Mass Estimator on DESI DR2 outer-halo BHB and RR Lyrae stars, one method within the paper’s comparison of mass estimates.
Abstract
We present measurements of the Milky Way mass using blue horizontal-branch (BHB) and RR Lyrae (RRL) stars from the second data release of DESI (DR2), covering its first three years of operations. We employ 279 BHBs and 213 RRLs at $\sim\!50$--$80$\,kpc from the Galactic center, selected from catalogs roughly twice as large as those of DESI DR1, used in our previous mass measurement. We apply the Galactic Mass Estimator (GME), a hierarchical Bayesian framework that assumes power-law forms for the potential and the tracer density, and accounts for distance and velocity uncertainties. Validating the method on 24 AuriDESI mock catalogs (six Auriga halos at four viewing angles), we find that it o…
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The emPDF method estimates the Milky Way mass profile from DESI DR2 halo stars, while the paper compares four methods and calibrates their biases with mocks.
Abstract
We employ a novel data-driven dynamical modeling method, the empirical distribution function (emPDF), to constrain the Milky Way mass profile using a large sample of halo stars from the Dark Energy Spectroscopic Instrument Milky Way Survey (DESI MWS). The method does not assume any functional form of the tracer distribution. Deviations from the steady-state assumption and the effects of observational errors are further tested and calibrated using a number of realistic mock observations (AuriDESI). Applying the calibrated method to two classes of tracers in DESI, we derive a best-constrained virial mass and concentration of $\log M_\mathrm{200c}/M_{\odot}=12.05_{-0.12}^{+0.21}(_{-0.18}^{+0.26…
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Mock catalogues for the DESI DR2 Milky Way mass modeling effortA. H. Riley, A. Bystrom, G. E. Medina et al. · 2026 · Not summarized yet
The mock catalogues reproduce DESI DR2 halo tracers, uncertainties, and selection features to validate the dynamical methods used to estimate the Milky Way’s mass.
Abstract
The Dark Energy Spectroscopic Instrument (DESI) survey is an ongoing campaign to obtain millions of spectra, including for stars within the Milky Way. One of DESI's Key Projects for its second data release (DR2) is to measure the mass of the Milky Way using a variety of techniques applied to six-dimensional kinematics of distant halo stars. We present mock surveys generated from the Auriga cosmological simulations (AuriDESI) that are used to validate these techniques in several companion studies. The AuriDESI tracer catalogues are tailored to reflect key features of the DR2 Key Project dataset, including the selection of RR Lyrae and blue horizontal branch stars, uncertainties on line-of-sig…
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GaiaUnlimited maps the Milky Way’s old stellar disc with red-clump stars, rather than estimating the mass from distant halo tracers.
Abstract
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The study measures local thin- and thick-disc kinematics, rather than the Milky Way’s mass from distant halo stars.
Abstract
We present a detailed analysis of kinematics of the Milky Way disk in solar neighborhood using GAIA DR3 catalog. To determine the local kinematics of the stellar disks of the Milky Way galaxy we use a complete sample of 278,228 red giant branch (RGB) stars distributed in a cylinder, centered at the Sun with a 1 kpc radius and half-height of 0.5 kpc. We determine separately the kinematical properties of RGB stars for each Galactic hemisphere in search for possible asymmetries. The kinematical properties of the RGB stars reveal the existence of two kinematically distinct components: the thin disk with mean velocities ${V_R}, {V_{\phi}}, {V_Z}$ of about -1, -239, 0 km s$^{-1}$ correspondingly a…
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The Evolution of the Circumgalactic Medium after a starburst in DESI Post-Starburst GalaxiesZoe Harvey, Sahyadri Krishna, R. Tojeiro et al. · 2026 · Not summarized yet
The study uses DESI DR2 to trace gas around post-starburst galaxies, not to measure the Milky Way’s mass.
Abstract
We present a statistical study of the cool circumgalactic medium (CGM) around massive post-starburst galaxies, traced by Mg II $\lambda\lambda2796,2803$ absorption in background quasar spectra from the Dark Energy Spectroscopic Instrument (DESI) Data Release 2. Cross-matching ~1.98 million quasar sightlines with luminous red galaxies at $0.4<z<0.76$ with stellar masses between $10.6<\log_{10}(M_*/\mathrm{M}_\odot)<12$ yields ~143 million galaxy-quasar pairs, 50-60 times more per impact-parameter bin than our previous SDSS-based analysis. We stack the Mg II absorption as a function of projected separation and classify the foreground galaxies using mean-field independent component analysis (MF…
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Gaia EDR3 proper motions, energies, angular momenta of Milky Way dwarf galaxies: a recent infall to the Milky Way haloYang Y., Hammer F., Li H. et al. · 2023 · Astrophysics of Galaxies · cited 2 times · AI summary available
The study derives Milky Way dwarf-galaxy orbits from Gaia proper motions and discusses recent infall, rather than estimating Galactic mass with halo tracers.
Abstract
Gaia EDR3 has provided proper motions of Milky Way (MW) dwarf galaxies with an unprecedented accuracy, which allows us to investigate their orbital properties. We found that the total energy and angular momentum of MW dwarf galaxies are much larger than that of MW K-giant stars, Sagittarius stream stars and globular clusters. It suggests that many MW dwarf galaxies have had a recent infall into the MW halo. We confirmed that MW dwarf galaxies lie near their pericenters, which suggests that they do not behave like satellite systems derived from Lambda-Cold-Dark-Matter cosmological simulations. These new results require revisiting the origin of MW dwarf galaxies, e.g., if they came recently, t…
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The DESI Y1 RR Lyrae Catalog. I. Empirical Modeling of the Cyclic Variation of Spectroscopic Properties and a Chemodynamical Analysis of the Outer HaloG. E. Medina, T.-S. Li, Sergey E. Koposov et al. · 2026 · Astrophysical Journal · cited 1 time · Not summarized yet
The DESI Y1 catalogue provides RR Lyrae velocities and atmospheric parameters for outer-halo studies, a tracer type also used in the paper’s mass analysis.
Abstract
We present the catalog of RR Lyrae stars (RRLs) observed in the first year of operations of the Dark Energy Spectroscopic Instrument (DESI) survey. This catalog contains 6240 RRLs out to ∼120 kpc from the Galactic center and over 12,000 individual epochs with homogeneously derived stellar atmospheric parameters. We introduce a novel methodology to model the cyclical variation of the spectroscopic properties of RRLs from single-epoch measurements. We employ this method to infer the systemic velocities and mean temperatures of fundamental- and first-overtone-mode RRLs in our sample (without distinguishing between individual spectral lines). For fundamental-mode pulsators, we obtain radial velo…
Prior work it builds on
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Galactic Dynamics: Second EditionJ. 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.
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Pulsating StarsM. 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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