Abstract We present new Hubble Space Telescope (HST) imaging of the ionised filaments in the brightest group galaxy NGC 5044, providing the first high-resolution view of such structures in a galaxy group. The filaments extend several kiloparsecs from the centre, with widths of ∼50–120 pc. Some strands are as narrow as those in cluster cores, while others are broader, consistent with the weaker confining pressure of the intragroup medium. With our limited sample, we find that the filament width ( W ) roughly scales with ambient pressure ( P ) as W ∝ P −0.4 . Combining HST with molecular and MUSE observations, we measure column densities and magnetic field strengths. Equipartition magnetic fields decline from ∼40 µ G near the centre to ∼20 µ G at 5 kpc, about 2–3 times weaker than in clusters. Dynamical stability arguments require stronger radial magnetic fields (∼10 2 µ G), consistent with simulations and magnetic field lines draping and flux freezing around cavities, though such high values may be difficult to reconcile with Faraday Rotation Measure limits. Turbulence and cosmic rays can also provide complementary support. Filaments are stable against gravitational collapse, and ultraviolet imaging reveals no star formation in NGC 5044 (<10 −3 M ⊙ yr –1 ), confirming that star formation in filaments in both groups and clusters remains largely quenched. NGC 5044 hosts an ionised gas core within its Bondi radius with n e ∝ r –1 and filling factor f ≳ 3 × 10 –3 , that is connected to the extended filaments, suggesting a channel for gas inflow toward the black hole. Our results show that group filaments share the same origin and stabilising mechanisms as cluster filaments, with magnetic fields and AGN feedback preserving filamentary structures with ambient pressure and dust survival as key factors for molecular gas formation and survival. Lower pressure groups favour broader, diffuse filaments with sporadic molecular clumps and less dust shielding, while higher pressure clusters host narrower strands with stronger molecular/ionised gas alignment. We predict that (i) filament widths scale with ambient pressure, (ii) filament-coincident Faraday rotation structures should appear at ≤0.1 kpc resolution, and (iii) molecular/ionised gas co-spatiality is weaker in groups than in clusters.
Tamhane et al. (2026) studied this question.