The Universality of Dark Matter Density Profiles for Milky Way Analog Galaxies

TL;DR

The paper addresses the challenge of characterizing the Milky Way's dark matter halo by testing the universality of DM density profiles across MW-like systems. It combines 11 observed MW analogs with 127 IllustrisTNG-50 halos, deriving rotation curves from HI kinematics using 3D-Barolo and performing mass decomposition to isolate the DM component, then fitting NFW halos to obtain DM densities. A key result is the local DM density at the Solar position, found to lie in the range $0.17$–$0.46$ GeV cm⁻³, with a median around $0.39$ GeV cm⁻³ for the simulated sample, consistent with MW estimates in the literature and suggesting universality in MW-like DM halos despite diversity in halo properties. The study also reveals a bimodal stellar-to-halo mass relation and demonstrates how baryonic processes and halo concentration influence LDMD, offering important context for direct detection experiments and galaxy formation theories. Together, the external MW analogs and high-resolution kinematic data provide a robust framework for inferring DM distributions in MW-like systems and guiding future surveys with next‑generation radio facilities.

Abstract

The structure, extent, and mass of the Milky Way's (MW) dark matter (DM) halo are observationally challenging to determine due to our position within the Galaxy. To overcome this limitation, we study a combined sample of 127 MW analogs from the IllustrisTNG-50 cosmological simulation with observations of 11 nearby galaxies. Using both spatial and spectral high-resolution data from VLA and GMRT telescopes, we employ the 3D-Barolo algorithm to derive precise kinematic maps and rotation curves (RCs). To perform a careful analysis of the stellar component, we use Spitzer mid-IR imaging at 3.6 and 4.5um. We decompose the RCs into their different mass components, enabling the construction of a DM radial profile for each galaxy. By using a MCMC-based routine, we account for the DM contribution for the observed RCs. For our simulated sample, we obtain DM radial profiles directly from the TNG50 database. We probe for the universality of the DM profiles by deriving and comparing the equivalent local DM density (LDMD), a critical parameter linked to DM direct detection experiments on Earth. We calculate the DM density at the corresponding location of the Sun in each of the analogs. Our analysis yields a final LDMD range of 0.17-0.46 GeV cm^-3. Finally, by leveraging our mass estimates (M200}, Mgas and Mstar), we contextualize our findings with the efficiency of star formation in MW analogs and with the diversity of galaxies inhabiting similar halo masses.

Figures (8)

• Figure 1: Sample selection from the S$^4$G catalog. Gray points represent the full S$^4$G sample. The light red shaded region indicates our selection criteria in the parameter space. Selected MW analog galaxies are highlighted with colored circles (this work) and green triangles deisídio2024. The Milky Way reid2016 and Andromeda Carignan_2006 are shown for comparison as red and blue diamonds, respectively.
• Figure 2: Rows 1 $\&$ 3: S$^4$G mid-infrared images sheth2010 of our six MW analog galaxies with overlaid HI intensity contours, ordered by total observing time. The contours are based on the 0th moment map from $^{3D}$Barolo (see Section \ref{['subsec:barolo']} for details about the kinematic modeling). Contours are shown at 1, 2, 3, and $5\sigma$ levels for VLA sources, and at 2 and $5\sigma$ levels for GMRT sources. Rows 2 $\&$ 4: HI line spectra for the observed galaxies, corrected for optical redshifts. While the characteristic double-horn profile (indicated by the blue vertical lines) of a rotating disk is clearly detected in all galaxies, the GMRT data exhibit higher noise levels. See text for details.
• Figure 3: Velocity maps (1st moment) for the observed galaxies: (a) NGC 7479, (b) NGC 2775, (c) NGC 7606, (d) NGC 0779, (e) NGC 5678, and (f) NGC 5878. First column: HI velocity field derived from the radio data cubes. Second column: 2D projection of the 3D kinematic model generated by $^{3D}$Barolo. Third column: Residuals (data $-$ model). The dashed black curve traces the kinematic major axis for each galaxy, as determined by the best-fit inclination and position angle parameters from the $^{3D}$Barolo modeling. The solid green line indicates where the kinematic transition occurs between the receding (redshifted) and approaching (blueshifted) sides.
• Figure 4: Rotation curve (RC) decomposition and best-fit NFW parameters for the observed galaxies. Rows 1 $\&$ 3: Decomposed RCs for (a) NGC 7479, (b) NGC 7606, (c) NGC 2775, (d) NGC 0779, (e) NGC 5678, and (f) NGC 5878. Each panel shows the HI-derived circular velocity (black points; $v_{\mathrm{circ}}$), the total best-fit model (red dash-dotted line; $v_{\mathrm{mod}}$), and contributions from gas (green), stars (blue), and the NFW halo (orange). The vertical dark red line marks the equivalent Solar neighborhood at $R = 1.8 \times R_{\mathrm{eff}}$licquianewman2015. The lower panel displays the normalized residuals $(v_{\mathrm{mod}} - v_{\mathrm{obs}})/v_{\mathrm{obs}}$. Rows 2 $\&$ 4: Posterior distributions for the best-fit NFW parameters ($M_{200}$, $c$) from our MCMC analysis, corresponding to the RCs directly above them. These results are essential for the stellar-to-halo mass (SHMR) and concentration-mass ($c-M_{200}$) relations discussed in Section \ref{['sec:results']}.
• Figure 5: Rotation curves (RCs) for 127 Milky Way (MW) analogs systems from the Illustris TNG-50 cosmological simulation. Curves are color-coded by stellar mass, spanning a range of $1$–$9 \times 10^{10}~M_{\odot}$. We also add the derived RC for the MW in Eilers_2019, in black data points. All radii are normalized by the stellar effective radius ($R_{\mathrm{eff}}$), taken directly from the simulation.