MeerKAT Fornax HI Survey

• The MeerKAT Fornax Survey is a comprehensive HI and continuum survey of the Fornax galaxy cluster designed to investigate hydrodynamical and gravitational gas removal processes.
• It employs high spatial (10''–100'') and spectral (1–1.4 km/s) resolution along with sub-mJy column density sensitivities to reveal faint HI tails, extended clouds, and polarization details.
• Findings indicate that ram-pressure stripping and tidal interactions rapidly deplete HI in infalling dwarfs, offering key insights into environment-driven galaxy evolution.

The MeerKAT Fornax Survey is a comprehensive, high-sensitivity neutral hydrogen (HI) and continuum survey of the Fornax galaxy cluster, executed with the SKA precursor MeerKAT radio telescope. Designed to investigate the interplay of hydrodynamical and gravitational processes responsible for cold gas removal and quenching of star formation in galaxies—particularly within low-mass cluster environments—the survey leverages sub-mJy column density sensitivities, high spatial and spectral resolution, and an expansive spatial footprint that covers the cluster core and outskirts. Through novel imaging of previously undetected HI tails, extended clouds, and polarization measurements, the survey provides incisive insights into the mechanisms governing the evolution of galaxies and the intra-cluster medium (ICM) in Fornax, representing a template for future HI studies in group- and cluster-scale environments.

1. Survey Design, Methodology, and Data Characteristics

The MeerKAT Fornax Survey targets a $\sim1\times2$ Mpc$^2$ region centered on the Fornax cluster ($D\sim20$ Mpc), encompassing $\sim12$ deg$^2$ in the sky and extending to the virial radius and beyond toward the NGC 1316 (Fornax A) group (Serra et al., 2017, Serra et al., 2023, Maccagni et al., 24 Jul 2025). Observations employ MeerKAT’s L-band receivers with the following technical specifications:

• Column Density Sensitivity: Down to $\sim10^{18}$ cm$^{-2}$ (3$\sigma$), varying from $8\times10^{17}$ to $5\times10^{19}$ cm$^{-2}$ depending on angular resolution.
• Angular Resolution: From 10$''$ ($\sim$1 kpc) to 100$''$ ($\sim$10 kpc).
• Spectral Resolution: 1–1.4 km s$^{-1}$ velocity channels, enabling precise internal kinematic mapping.
• Mass Sensitivity: Down to $\sim6\times10^{5}\,M_\odot$ for HI (Serra et al., 2023, Serra et al., 2017).
• Polarimetric Data: Broadband continuum and polarization imaging, enabling dense rotation measure (RM) grids (Loi et al., 9 Jan 2025).

A highly automated CASA-based pipeline (CARACal) is used for flagging, calibration, continuum subtraction (by polynomial fitting), and multi-scale CLEAN deconvolution. Self-calibration loops increase dynamic range, while refined emission masks (often from SoFiA) improve artifact removal. Emphasis is placed on proper RFI flagging in the UV domain to maintain faint HI emission fidelity (Serra et al., 2023, Maccagni et al., 24 Jul 2025). Polarimetric calibration robustly accounts for angle offsets and instrumental effects (Loi et al., 9 Jan 2025).

2. Gas Removal Mechanisms: Ram-Pressure Stripping and Tidal Effects

The principal finding is the widespread presence of previously unidentified, extended, one-sided, starless HI tails and discrete clouds in Fornax cluster galaxies. Many HI tails are radially aligned with respect to the cluster center, with lengths extending up to 70 kpc and, in some cases, individual HI clouds detected out to 300 kpc from their parent galaxies (Serra et al., 2023, Serra et al., 12 Jul 2024, Maccagni et al., 24 Jul 2025). The morphology, lack of stellar counterparts, and kinematic signatures of these tails represent the most unambiguous evidence to date that ram pressure is operating in Fornax.

Kinematic analysis reveals systematic velocity gradients along the HI tails, often consistent with predictions of hydrodynamical stripping by the ICM. The canonical ram pressure force is described by:

$P_{\rm ram} = \rho_{\rm IGM} v^2$

where $P_{\rm ram}$ is the ram pressure, $\rho_{\rm IGM}$ the local ICM density, and $v$ the relative galaxy–ICM velocity. These data demonstrate that ram pressure can displace tidally pre-loosened HI from galaxies—even in a cluster where the ICM density and velocity dispersion ($\sim370$ km s$^{-1}$) are lower than, for example, Virgo (Serra et al., 2017, Serra et al., 2023).

Crucially, tidal interactions and mergers act as pre-conditioning—dislodging HI from the stellar disk and thus priming the gas for further removal by ram pressure. Observed tails that begin as tidal features are then stretched and swept by the ICM wind, as evidenced by position–velocity diagrams showing transitions from tidal to ram-pressure driven velocity profiles (Maccagni et al., 24 Jul 2025, Serra et al., 12 Jul 2024). These processes operate in a stochastic, stochastic-dynamical interplay unique to low-mass clusters like Fornax.

3. Quantitative Impact on Dwarf Galaxies and the HI Mass Function

Deep HI imaging reveals rapid removal of HI from infalling dwarf galaxies, especially late-type dwarfs (LTDs). Among 304 dwarfs surveyed, a mere 17 retain detectable HI, and the spatial distribution of HI-rich dwarfs avoids the most massive cluster potentials (Kleiner et al., 2023, Maccagni et al., 24 Jul 2025). Observations of compressed HI on the windward sides of LTDs, along with asymmetric, one-sided HI tails, directly implicate ram pressure and cluster tides. Early-type dwarfs (ETDs) are almost universally HI-deficient.

A simple exponential "toy model" [Editor's term] characterizes the HI mass decay:

$$M_{\rm HI}(t) = M_{\rm HI,0} \cdot \exp\left(\lambda \frac{t}{t_{1/2}}\right),\quad \lambda = \ln(0.5)$$

where $t_{1/2}$ is the characteristic "halving time" for HI mass. Empirically, a $10^{8}\,M_{\odot}$ dwarf is stripped of HI within $\sim$240 Myr—much shorter than the typical cluster crossing time ($\sim2$ Gyr). Lower mass dwarfs ($<10^7\,M_\odot$) are likely stripped in a single event (Kleiner et al., 2023).

These rapid removal mechanisms lead to a suppressed HI mass function in the cluster environment, with long HI tail features (carrying up to 15% of a galaxy’s HI) and extended HI clouds contributing to the intra-cluster HI budget (Serra et al., 2023, Maccagni et al., 24 Jul 2025). Environmental removal thus shapes both the baryonic content and evolutionary trajectory of low-mass galaxies in Fornax.

4. Multi-phase Gas, Nuclear Activity, and Cluster-scale Feedback

Joint analysis with optical (MUSE), millimeter/sub-millimeter (ALMA), and spectro-polarimetric (MeerKAT continuum) data allows a detailed census of the interstellar and circumgalactic medium. Gas-rich dwarfs display disturbed, offset, and kinematically complex atomic, molecular, and ionized gas phases (Zabel et al., 5 Nov 2024). HI is generally more extended and disrupted than CO-traced molecular gas or H$\alpha$-traced ionized gas, suggesting sequential gas-phase removal, with HI stripped first.

Star formation efficiency in dwarfs is strongly suppressed; measurements of the resolved molecular Kennicutt–Schmidt relation

$\Sigma_{\rm SFR} \propto \Sigma_{H_2}^{N}$

yield values systematically below those for field counterparts ($N\sim1.4$ canonical), and HI (but not H$_2$) deficiencies exceeding 1 dex are common. This supports a scenario of ongoing star formation quenching via environmental processing.

In Fornax A, deep L-band continuum imaging resolves a double-shell morphology in the radio lobes, indicating multiple AGN activity episodes, each coupled to the gas supply and merger history within the dense cluster environment (Maccagni et al., 2019). Diffuse HI filaments ($\sim100$ kpc) and ionized gas structures discovered in the intra-group medium further illustrate the co-existence and survival of multi-phase gas via magnetic and pressure confinement mechanisms, with intermittent AGN fueling potentially sustained by environmental gas flows (Kleiner et al., 2021).

5. Cluster Magnetic Fields, Polarization, and Rotation Measures

MeerKAT delivers the densest rotation measure (RM) grid currently available, detecting 508 polarized sources over 6.35 deg$^2$ with a density of 80 polarized sources deg$^{-2}$ (Loi et al., 9 Jan 2025). The polarization imaging (900–1400 MHz, %%%%36$''$37%%%% resolution) achieves sensitivities down to $\sim$3 $\mu$Jy beam$^{-1}$ in Stokes Q/U.

The RM grid enables detailed mapping and statistical analysis of the Fornax cluster’s magnetic field structure. The radial RM profile decreases with increasing cluster-centric distance, as expected from a declining magnetic field and electron density. However, local enhancements ("high-RM stripes" and mean profile increments at $\sim$300 kpc) are detected, potentially due to magnetic field compression by shocks or accretion flows. The majority of detected sources are "Faraday simple," showing linear $\lambda^2$ rotation:

$\Delta\psi = {\rm RM}\cdot \lambda^2$

${\rm RM} = \int_0^L n_e\,B_{||}\,dl$

Polarized source counts, normalized for Euclidean geometry, exhibit a tentative upturn at $p\sim9\,\mu$Jy, consistent with a significant AGN population even at sub-$\mu$Jy fluxes (Loi et al., 9 Jan 2025).

6. Cluster Membership, Kinematic Modeling, and Environmental Diagnostics

The survey’s spatial and kinematic fidelity enables discrimination between true cluster members and foreground/background galaxies. For example, the dwarf ESO 358-60, while close in systemic velocity to Fornax, displays a symmetric, undisturbed HI disk atypical in the cluster core (Kamphuis et al., 7 Mar 2025). Application of 3D tilted ring models and the baryonic Tully–Fisher relation:

$$M_{\rm bar} = D^2 \bigg[ (M/L)\cdot 10^{-0.4(m_{3.6}-M_{\odot,3.6})} + 2.36\times10^{-7} F_{\rm HI}\cdot 1.33 \bigg]$$

demonstrates this galaxy lies at $\sim$9.4 Mpc—far in the foreground relative to Fornax ($\sim$20 Mpc). Accurate HI kinematic modeling is therefore essential for cluster membership assignment, impacting interpretation of environmental processes.

7. Implications for Galaxy Evolution in Low-mass Clusters

The MeerKAT Fornax Survey establishes that, even in relatively low-mass clusters ($M_{\rm vir}\sim5\times 10^{13}\ M_\odot$), environmental processing efficiently removes cold gas reservoirs and truncates star formation in galaxies, particularly dwarfs. The combined pre-conditioning by tidal forces and subsequent ram-pressure stripping provides a pathway for morphological transformation, the build-up of the red sequence, and regulates the gas available for AGN fueling and cluster medium enrichment (Serra et al., 2017, Loni et al., 2023, Maccagni et al., 24 Jul 2025).

The survey’s methodologies and findings contribute major observational constraints to models of baryon cycling, environment-driven quenching, and the role of the ICM in cosmic gas dynamics. These results set a benchmark for forthcoming wide-field HI surveys with next-generation instruments such as the Square Kilometre Array (SKA), enabling statistical studies of cluster assembly and galaxy transformation across cosmic time.