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

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Video file (mp4)

The gist

The MAGPI Survey presents a comprehensive analysis of dark matter content and its structural dependence in star-forming disk-like galaxies at intermediate redshifts (0.1 ≲ z ≲ 0.85) using

In short

The MAGPI survey analyzed dark matter content in star-forming disk galaxies at intermediate redshifts (0.1 to 0.85) using kinematic data to study baryon and dark matter co-evolution. Findings show low-mass galaxies are strongly DM dominated, while high-mass systems have lower DM fractions in their centers, suggesting mass-dependent structural differences driven by internal processes.

Key concepts

Dark Matter Fraction (fDM)
This measures the ratio of dark matter mass to the total dynamical mass (stars plus gas) within a specific region of a galaxy. It helps determine how much of a galaxy's gravity comes from invisible dark matter versus visible components.
Structural Dichotomy
The study found that galaxies are not structurally uniform; their dark matter content varies significantly based on stellar mass. Low-mass galaxies are strongly DM dominated, whereas high-mass galaxies show different DM fractions in their inner versus outer parts, indicating a mass-dependent structural pattern.
Inside-Out Assembly
The inverse correlation between the dark matter fraction and baryon surface density suggests that galaxies build up their structure from the inside out. This means that the fundamental structural relationships observed in galaxies were already established by redshift 0.85, implying a continuous evolutionary process.
MAGPI Survey Sample
The survey selected 397 star-forming disk galaxies based on strict criteria: confirmed redshifts, specific emission lines (H$\alpha$, [OIII]), and inclination angles. This rigorous selection ensured the data used for analyzing dark matter content was robust and statistically independent.

Terminology used across episodes

This episode discusses

The paper

The MAGPI Survey: co-evolution of baryons and dark matter in star-forming disk-like galaxies at 0.1 z 0.85 · Read on arXiv

Gauri Sharma, Andrew J. Battisti, Emily Wisnioski, J Trevor Mendel, Sabine Bellstedt, Claudia Del P Lagos, Caroline Foster, Adriano Poci, Katherine E Harborne, Ryan Bagge, Stefania Barsanti

University of Strasbourg, CNRS UMR 7550, Observatoire astronomique de Strasbourg University of the Western Cape Department of Physics and Astronomy Institute for Fundamental Physics of the Universe International Centre for Radio Astronomy Research Australian National University ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions Sub-Department of Astrophysics University of Oxford School of Physics University of New South Wales Sydney Institute for Astronomy School of Physics Sydney

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 z 0.85), utilizing spatially resolved kinematic data from the MAGPI survey. We report the following: (1) Low stellar mass galaxies (M star < 10 9.5, M) are strongly DM dominated across all radii, with average f DM about 0.85, while high-mass (M star > 10 10.5, M) systems exhibit relatively low DM fractions in their inner regions (f DM about 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 DM and baryon mass surface density (Σ bar), with intrinsic scatter of about 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 about 0.85. (3) No significant evolution in f DM with redshift across the MAGPI window, and when combined with higher-redshift (0.6 at most z at most 1.5) data from Sharma et al. 2025, we quantitatively show that the reported decline in f 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.

Transcript

Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Hi, I'm Vera.

Jocelyn: And I'm Jocelyn, here with Subrahmanyan.

Vera: We're looking at the paper: "The MAGPI Survey".

Jocelyn: Here's the gist of the paper. The MAGPI Survey presents a comprehensive analysis of dark matter content and its structural dependence in star-forming disk-like galaxies at intermediate redshifts (0.1 ≲ z ≲ 0.85) using spatially resolved kinematic data, which is crucial for understanding the co-evolution of baryons and dark matter in these systems.

Vera: How it works The study utilizes 3DBarolo to model gas kinematics, obtaining moment maps, surface brightness profiles, rotation velocity profiles, and velocity dispersion profiles The observed rotation velocity profiles are corrected for inclination and gas pressure support to derive intrinsic rotation curves By modeling these intrinsic rotation curves and decomposing the total dynamical mass profile into the baryonic and the DM components without employing parametric halo models, they estimate the DM fraction (fDM) within both the effective radius (Re) and the optical radius (Ropt) The sample selection process involved several stringent criteria to ensure robust kinematic constraints ((1) confirmed spectroscopic redshift and detection of either (or all) of these emission lines: Hα (6563 ˚A),

OIII: (4969 ˚A), and

OIII: (5008 ˚A)) ((2) inclination angles within the range of 25◦ ≤ θi ≤ 75◦) ((3) S/N > 3) ((4) log(Mstar

M⊙: ) = 8 − 11.5) The final kinematic sample consists of 397 galaxies, which are referred to as UIDs This procedure ensures that each galaxy contributes a single statistically independent data point to the population-level analysis while optimally incorporating information from multiple emission-line tracers The circular velocities within Re and Ropt are used as characteristic circular velocity (Vc) of a galaxy This Vc is adopted as the characteristic circular velocity (Vc) of a galaxy The total dynamical mass is decomposed into the contributions from the stellar disk MDisk(R), bulge Mbulge(R), atomic gas (HI) MHI(R), molecular gas (H2) MH2(R), and DM MDM(R) The total dynamical mass (Mdyn(R)) is decomposed into the contributions from the stellar disk MDisk(R), bulge Mbulge(R), atomic gas (HI) MHI(R), molecular gas (H2) MH2(R), and DM MDM(R) The DM fraction is given by fDM(R) = MDM(R)/Mdyn(R) The region within Re is defined as the inner region, and the region from Re to Ropt as the outskirts The circular velocity measured at Ropt—where the rotation curve is typically flat—is adopted as the characteristic circular velocity (Vc) of a galaxy The DM fraction is computed by comparing the total dynamical mass to the baryonic mass budget within a given radius Finally, the DM fraction is given by fDM(R) = MDM(R)/Mdyn(R) The circular velocity measured at Ropt—where the rotation curve is typically flat—is adopted as the characteristic circular velocity (Vc) of a galaxy The DM fraction within Re and Ropt is presented for 181 and 246 galaxies, respectively The results are presented and discussed in Section 4 & 5 The key conclusions of this work are as follows In Figure 6, we show that the low-mass galaxies (Mstar < 109.5 M⊙) are DM dominated with average ⟨fDM⟩ ≈ 0.85 within both the Re (inner regions) and Rout (outskirts) In high-mass (Mstar ≥ 1010 M⊙) galaxies the DM fraction is low within the inner regions compared to their outskirts At fixed circular velocity low-mass galaxies (Mstar < 109.5 M⊙) exhibit about ∼ 40% higher DM fraction than their high-mass counterparts within Re On the other hand, outskirts of both high- and low-mass systems show no significant change (less than 10%) in DM fractions This suggests an inside-out disk growth In Figure 8, the fDM as a function of baryon mass surface density, measured within the effective radius (left) and out to the optical radius (right) Both panels highlight the inverse correlation between DM fraction and baryon surface density Galaxies with higher central baryon surface densities Σbar ≳ 108.0 M⊙ kpc−2 cannot be less than the total observed baryonic mass In contrast, systems with low baryonic surface densities Σbar < 108.

Paper summary: Vera: So to recap this MAGPI Survey paper, they’ve performed a comprehensive analysis of dark matter content and its structural dependence in star-forming disk-like galaxies at intermediate redshifts between zero point one z and zero point eight five z > <ref:2601.12315#pg1,in star-forming disk-like galaxies at>

Jocelyn: The core thesis is that the dark matter fraction isn't uniform across all galaxies; it depends heavily on their stellar mass, with low-mass systems being much more DM dominated overall >

Subrahmanyan: And they also found a tight inverse correlation between this dark matter fraction and the baryon mass surface density, suggesting an inside-out growth pattern for these structures >

Vera: They claim that the structural correlations of galaxies were already established by redshift zero point eight five, meaning we’re looking at a snapshot where those fundamental relationships are pretty stable > <ref:2601.12315#pg1>

Jocelyn: Plus, they addressed the evolution with redshift and found that while there's no strong evolution in the dark matter fraction within their own MAGPI data range, higher-redshift data suggests some apparent decline is likely an observational artifact >

Subrahmanyan: This matters because it constrains our understanding of how these structures assemble; it supports a picture where baryons and dark matter are co-evolving through a regulated process >

Conclusion: Vera: Thinking about the title, "The MAGPI Survey: co-evolution of baryons and dark matter in star-forming disk-like galaxies at zero point one z zero point eight five," it really captures the essence of what they found > <ref:2601.12315#pg1,The MAGPI Survey: co-evolution of baryons and dark matter in star>

Jocelyn: It’s about showing that these galaxies aren't just existing in isolation; they are actively co-evolving, building their dark matter and baryonic parts together in a regulated way across time >

Subrahmanyan: The implication is that the total mass distribution and the shape of the rotation curve remain regular even as cosmic time marches on, which puts a strong constraint on cosmological models like CDM regarding structure formation dynamics >

Vera: Basically, they show that galaxies grow in a way that respects these internal structural patterns, regardless of whether you’re looking at low-mass or high-mass systems >

Jocelyn: It means the building blocks of cosmic structure follow a set of rules for how dark matter and gas are incorporated into the visible structures we see today >

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