How Galaxies Without Dark Matter Sift the Invisible ✛

Original: "Dark Matter Deficient Galaxies as Probes of Dark Matter"

arXiv:2607.26085v1 · 2026-07-27 · CC BY 4.0 · 1 min

General Relativity Galaxies HEP Phenomenology HEP Theory

Some galaxies are almost devoid of [tag:dark_matter]dark matter[/tag] — and that lets us peek into its nature.

In the galaxy NGC 1052 DF2, stars move very slowly — just 8 kilometers per second. That happens when there's almost no invisible mass, which we call dark matter, around. Usually, like glue, it holds galaxies together, but here it seems to have been shaken out.

Galaxy collisions act like a sieve. Ordinary matter slows down and clumps together on impact, while the invisible stuff slips through. If dark matter stuck strongly to baryons, it wouldn't separate. But since we see 'empty' galaxies, the stickiness must be very weak. Scientists calculated: even if just a tiny fraction of dark matter (about 0.5%) could cling, that's already ruled out by observations.

Key number: the interaction cross-section (a measure of 'stickiness') is no more than 0.50 cm²/g. For comparison, if you took matter with the density of air and accelerated it to cosmic speeds, even such a tiny resistance would leave a trace. Here, everything points to an even weaker contact.

Such galaxies are not just cosmic rarities. They are natural laboratories where nature itself has set up an experiment to separate substances, and all we have to do is measure what passed through the sieve.

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In the galaxy NGC 1052 DF2, the stellar velocity dispersion is only about 8 km/s, as if all the mass were provided by the stars themselves, with no dark matter halo.

f_{\mathrm{int}}(1 - e^{-D_{\chi b}}) + (1 - f_{\mathrm{int}}) f_{\mathrm{grav}} \le \delta

The inequality links the fraction of interacting dark matter f_int, the drag depth D_χb, and the gravitational capture efficiency f_grav to the required threshold δ.

\frac{\sigma_0}{m_\chi + m_b} \le \frac{-\ln(1-\delta)}{\Sigma_b} \left(\frac{v_0}{v_{\mathrm{col}}}\right)^n

Upper limit on the cross-section normalization σ₀/(m_χ+m_b) as a function of the exponent n, the baryon column density Σ_b, and the velocity ratio v₀/v_col.

Scientists

Fritz Zwicky Rashid Alievich Sunyaev Augustin-Louis Cauchy Christiaan Huygens Emmy Noether Isaac Newton

Concepts

dark matter deficient dwarf galaxies dark matter detection axion detection axion galaxy formation mass ratio mass distribution neural network galaxy mapping dust extinction metal poor galactic halo composition dust obscuration rare event search hidden particle decay weak lensing black hole formation mechanism galactic halo gravitational lensing analysis alternative gravity tests high redshift galaxies

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How Galaxies Without Dark Matter Sift the Invisible ✛