The MAGPI Survey: co-evolution of baryons and dark matter in star-forming disk-like galaxies at $0.1 \lesssim z \lesssim 0.85$

TL;DR

Using spatially resolved kinematics from MAGPI and 3DBarolo forward modelling, this study derives the dark matter fraction $f_{_{ m DM}}$ within the effective and optical radii for 266 rotation-supported disk galaxies at $0.1 \,\lesssim \,z \,\lesssim \,0.85$. It finds a mass-dependent dichotomy: low-mass galaxies are strongly DM-dominated while high-mass systems show reduced central DM fractions, correlated tightly with baryon surface density via $f_{_{ m DM}}(<R) \approx 1-10^{(-0.13+0.55(\,\log_{10}(group ar{ ho})-\alpha))}$ with $\\alpha\approx 8.7$ for $R_e$ and $8.5$ for $R_{opt}$; the intrinsic scatter is $\\sim 0.11$ dex. No significant redshift evolution of $f_{_{ m DM}}$ is observed within MAGPI, though combining MAGPI with GS23 suggests that apparent high-$z$ declines are largely due to selection biases against low-mass systems at $z>1$, implying a co-evolution of baryons and DM that preserves rotation-curve shapes over time. Together, these results support a scenario where disk galaxies build baryons and DM in a coordinated, inside-out fashion, maintaining a consistent mass distribution across cosmic time and aligning with predictions from feedback-regulated galaxy formation models.

Abstract

We present a comprehensive analysis of the dark matter (DM) content and its structural dependence in star-forming disk-like galaxies at intermediate redshifts ($0.1 \lesssim z \lesssim 0.85$), utilizing spatially resolved kinematic data from the MAGPI survey. We report the following: (1) Low stellar mass galaxies ($M_{\rm star} < 10^{9.5}\, M_\odot$) are strongly DM dominated across all radii, with average $\langle f_{_{\rm DM}} \rangle \sim 0.85$, while high-mass ($M_{\rm star} > 10^{10.5}\, M_\odot$) systems exhibit relatively low DM fractions in their inner regions ($\langle f_{_{\rm DM}} \rangle \sim 0.47$) which is equivalent to local massive disk galaxies (e.g., Milky Way and Andromeda). This suggests a mass-dependent structural dichotomy, most-likely governed by a combination of internal galactic processes and environmental influences. (2) A tight inverse correlation between $f_{_{\rm DM}}$ and baryon mass surface density ($Σ_{\rm bar}$), with intrinsic scatter of $\sim 0.11$ dex. This is consistent with an inside-out baryon assembly scenario and suggests that the fundamental structural correlations of galaxies were already established by $z\sim 0.85$. (3) No significant evolution in $f_{_{\rm DM}}$ with redshift across the MAGPI window, and when combined with higher-redshift ($0.6 \leq z \leq 1.5$) data from Sharma et al. 2025, we quantitatively show that the reported decline in $f_{_{\rm DM}}(z)$ is most-likely due to observational biases against low-mass systems at $z > 1$. These results offer empirical evidence for a scenario in which disk-like galaxies evolve through a co-regulated build-up of baryonic and DM components, preserving internal structural regularities (such as the total mass distribution and rotation-curve shape) throughout cosmic time.

Figures (14)

• Figure 1: Offset from the star-forming main sequence (MS) as a function of stellar mass ($M_{\rm star}$). The vertical axis shows the logarithmic deviation from the MS, defined as $\delta\log(\mathrm{MS}) = \log(\mathrm{SFR} / \mathrm{SFR}_{\rm MS;}(z, M{\rm star}))$, where $\mathrm{SFR}_{\rm MS}$ is the redshift- and mass-dependent main sequence relation from speagle14. Full primary MAGPI sample ($N=434$) sample is represented by circles color coded for redshift, and the gray stars highlight the number of Unique IDs selected after applying kinematic modelling quality cuts. The gray shaded area represents the $1\sigma$ scatter of speagle14 MS relation. The majority of galaxies lie within the $\pm 2-3\sigma$ range of the speagle14 MS relation, i.e., final sample is consistent with a population of star-forming galaxies.
• Figure 2: Distributions of key galaxy properties for the full sample used in this study. Each panel shows histograms for the primary sample (blue; 434 galaxies) and subsets based on available emission line measurements: H$\alpha$ (red; 161), [OIII]$\lambda\, 4969\,\text{\AA}$ (green; 62), and [OIII]$\lambda\, 5008\,\text{\AA}$ ( sky-blue; 211). The kinematically selected sample (397) shown by orange histograms, and the black hatched histogram shows the Unique IDs (266) in this sample. Panels show (from left to right, top to bottom): redshift ($z$), inclination angle ($\theta_i$; degrees), integrated signal-to-noise ratio (SNR), effective radius $R_e$ (kpc), stellar mass ($M_{star}$; $M_\odot$), star formation rate (SFR; $M_\odot\,{\rm yr}^{-1}$), number of resolution elements ($N_{rings}$ per galaxy), circular velocity computed at $R_{opt}$ ($V_c;\ km\sec^{-1}$), and velocity dispersion ($\sigma;\ km\sec^{-1}$). The final kinematic sample and UIDs are representative of the primary sample in all fundamental properties, confirming the absence of any selection biases in the working sample.
• Figure 3: Examples of kinematic modelling outputs. Row 1 & 2, Columns 1--3: moment-1 and moment-2 maps data, best-fit model, and residuals, respectively. The grey dashed line indicates the position angle, the black cross marks the central $(x, y)$ position, and the green line denotes the plane of rotation. Column 4: major axis position-velocity diagram, where the black shaded area with blue contours represents the data, red contours show the model, and orange squares with error bars indicate the best-fit line-of-sight rotation velocity derived by 3DBarolo. The yellow and blue vertical dashed lines mark the effective radius ($R_{\rm e}$) and optical radius ($R_{\rm opt} = 1.89\,R_{\rm e}$), respectively. Column 5: velocity dispersion profile as a function of radius. Error bars on the rotation and dispersion curves correspond to $1\sigma$ uncertainties. The PSF size is shown by gray hatched shaded region.
• Figure 4: Row 1-2: Example rotation curves derived from multiple emission lines. The circular velocity profiles are shown for H$\alpha$ (orange), [O III]$\lambda$5008 (light blue), and [O III]$\lambda$4960 (dark blue). The vertical yellow dashed line indicates the effective radius $R_{\rm e}$, and the shaded gray region marks the beam-smearing-affected region $R_{\rm PSF}$. Row 3: Comparison of velocities traced using H$\alpha$ and [O III]$\lambda$5008 within effective radius and optical radius, left and right panels, respectively.
• Figure 5: Mass decomposition properties of the MAGPI galaxy sample. Left panel: cumulative mass profiles for an example galaxy (MAGPI1509286279, $z = 0.4202$, $\log(M_{star}\, M_\odot) = 9.50$). The total dynamical mass (blue squares), baryonic mass (red line), stellar disk mass (orange line), bulge mass (brown line), atomic gas mass (green dotted line), molecular gas mass (pink dotted line), and DM mass (black line) are shown. The vertical gray shaded area and dashed lines in orange, sky blue, and blue indicate the PSF, effective radius ($R_{\rm e}$), optical radius ($R_{\rm opt}$), and outer radius ($R_{\rm out}$), respectively. Right panel: Corresponding DM fraction as a function of radius. The solid blue line shows the cumulative DM fraction $f_{_{\rm DM}}(<R)$, with the horizontal black and orange dashed lines indicating 100% DM (top) and 100% baryons (bottom), respectively. Stars indicate the DM fraction within the $R_{\rm e}$ (orange), $R_{\rm opt}$ (cyan), and $R_{\rm out}$ (blue).