HST view of NGC 5044: Constraints on Filament Widths, Magnetic Support, Multiphase Structure, and Comparison with Cluster Environments

Abstract

We present new Hubble Space Telescope (HST) imaging of ionised filaments in the brightest group galaxy NGC 5044. These filaments extend several kiloparsecs and have widths of $\sim$50--120 pc, with some as narrow as those in cluster cores and others broader, reflecting the lower confining pressure in groups. Filament width ($W$) scales with ambient pressure ($P$) as $W \propto P^{-0.4}$. Combining HST, ALMA, and MUSE data, we measure column densities and magnetic field strengths. Equipartition fields decline from $\sim$40 $μ$G at the centre to $\sim$20 $μ$G at 5 kpc, about 2--3 times weaker than in clusters. Dynamical stability requires stronger radial fields ($\sim$10$^2$ $μ$G), consistent with simulations and magnetic draping, though such high values exceed Faraday Rotation Measure limits. Turbulence and cosmic rays also contribute support. Group and cluster filaments are stable against gravitational collapse, and ultraviolet imaging reveals no star formation in NGC 5044 ($<$10$^{-3}$ M$_\odot$ yr$^{-1}$). NGC 5044 hosts an ionised gas core within its Bondi radius with $n_e \propto r^{-1}$ and filling factor $f \gtrsim 3 \times 10^{-3}$, that is connected to the extended filaments, suggesting a channel for gas inflow toward the black hole. Group and cluster filaments likely share a common origin, with magnetic fields and AGN feedback preserving their structure. Ambient pressure and dust survival regulate molecular gas formation. Lower-pressure groups favour broader, more diffuse filaments with sporadic molecular clumps and weaker dust shielding, whereas higher-pressure clusters host narrower strands with stronger molecular-ionised gas alignment. We predict that (i) filament width scales with ambient pressure, (ii) filament-coincident Faraday rotation structures emerge at $\leq 0.1$ kpc resolution, and (iii) molecular/ionised gas co-spatiality is weaker in groups than in clusters.

Figures (12)

• Figure 1: Left: RGB composite of NGC 5044 combining HST F300X (blue), F665N (green), and F814W (red) images. The F665N filter is centred on the H$\alpha$ line. The images are modified to enhance the dust filaments in the left panel and H$\alpha$ filaments in the right panel. Right: Same image after subtracting the galactic continuum from each filter before combining, highlighting the inner filaments and showing no young blue star clusters.
• Figure 2: F665N residual image showing extended H$\alpha$+[N ii] filaments in NGC 5044. The image is smoothed to enhance the visibility of the filaments. Red rectangles mark the regions where filament surface-brightness profiles were extracted in the unsmoothed image. The extracted profiles (blue) and their Gaussian fits (orange) are shown in the accompanying panels. We also show the ALMA CO(2-1) contours based in the analysis presented in tamhane22 in magenta. The Giant Metrewave Radio Telescope 380 MHz contours from rajpurohit25 are shown in blue. The magenta regions show CO contours at 0.17 Jy km s$^{-1}$ in moment 0 maps which corresponds to 2.5-$\sigma$ detection.
• Figure 3: F660N-Pol residual image showing extended H$\alpha$+[N ii] filaments in M87. The image is smoothed to enhance the visibility of the filaments. Red rectangles mark the regions where filament surface-brightness profiles were extracted in the unsmoothed image. The extracted profiles (blue) and their Gaussian fits (orange) are shown in the accompanying panels. The Very Large Array L band radio contours are shown in blue.
• Figure 4: F665N residual image showing extended H$\alpha$+[N ii] filaments in NGC4696. The image is smoothed to enhance the visibility of the filaments. Red rectangles mark the regions where filament surface-brightness profiles were extracted in the unsmoothed image. The extracted profiles (blue) and their Gaussian fits (orange) are shown in the accompanying panels. Additional narrow peaks in panels 1 and 4 are point sources. Magenta contour shows the ALMA CO(1-0) detection in moment 0 map of tamhane22 at 0.06 Jy km s$^{-1}$. The Very Large Array L band radio contours are shown in blue.
• Figure 5: Maps of H$\alpha$ flux (left), velocity (centre) and velocity dispersion (right) generated from MUSE datacube. North is up and East is to the left.