WALLABY HI Survey Overview

• WALLABY Survey is a large-scale, untargeted HI mapping project using ASKAP’s wide-field, phased array feed technology to catalog gas-rich galaxies.
• The survey employs innovative data reduction and 3D kinematic modeling techniques to derive accurate HI masses, rotation curves, and environmental effects.
• WALLABY’s comprehensive data products enable legacy studies of galaxy evolution, statistical analyses of cosmic structure, and discovery of rare dark or low surface brightness systems.

The Widefield ASKAP L-band Legacy All-sky Blind Survey (WALLABY) is a large-scale, untargeted, extragalactic neutral hydrogen (HI) emission survey utilizing the Australian Square Kilometre Array Pathfinder (ASKAP) radio interferometer. WALLABY aims to provide a near-complete census of gas-rich galaxies in the local Universe, leveraging ASKAP’s wide instantaneous field of view and phased array feed (PAF) technology to achieve high survey speed and sensitivity. By covering approximately 75% of the sky (declinations –90° to +30°) to a redshift of $z \lesssim 0.26$, WALLABY is designed to detect over half a million galaxies, enabling detailed studies of the HI properties, kinematics, environment, and cosmological distribution of galaxies on an unprecedented statistical scale.

1. Survey Strategy, Instrumentation, and Technical Specifications

WALLABY is implemented on the 36 × 12 m ASKAP dishes, with phased array feeds producing 36 simultaneous beams per antenna and a field of view near 30 square degrees. Observations are conducted in a mosaicking scheme, with data cubes generated at $\sim$30 arcsec angular resolution using the core 2 km configuration. A high-resolution mode using the full 6 km baselines (10 arcsec) is available for detailed follow-up. The frequency coverage (1130–1430 MHz) delivers a velocity range of $-2000$ km s$^{-1}$ to $+77,000$ km s$^{-1}$, corresponding to a redshift reach of $z\sim0.26$.

ASKAP is sited in an exceedingly radio-quiet region in Western Australia, minimizing RFI, with a typical $T_\text{sys}/\eta$ of 70 K and an rms noise floor $\sim$1.6 mJy/beam per four km s$^{-1}$ channel in a standard 16 hr track. Data reduction is performed by the ASKAPsoft pipeline, with products (cubes, maps, spectra, catalogs) released via the CSIRO ASKAP Science Data Archive (CASDA).

2. Methodologies: Source Detection, Physical Parameter Extraction, and Modeling

HI sources are cataloged from data cubes with source-finding pipelines such as SoFiA and further processed for kinematic modeling using 3D fitting tools (including 3DBAROLO, FAT, and the dedicated WKAPP pipeline). Integrated HI masses are calculated using: $$M_{\rm HI} [M_\odot] = \frac{2.356\times10^5}{1+z} D^2 F_{\rm HI}$$ where $D$ is luminosity distance (Mpc) and $F_{\rm HI}$ is integrated flux (Jy km s$^{-1}$). Dynamical masses use: $$M_{\rm dyn} [M_\odot] = 2.31\times10^5 (V_{\rm rot})^2 R_{\rm HI}$$ with $V_{\rm rot}$ corrected for inclination and $R_{\rm HI}$ in kpc.

Modeling of galaxy properties such as HI mass, stellar mass, halo mass, and disk size employs semi-analytic prescriptions tied to large-scale cosmological N-body simulations (e.g., Millennium Simulation). The conversion from cold gas to HI mass for simulation–observation mapping uses a broken power-law: $$R = (M_{\rm HI}/M_{\rm cold})_0 \left[ (M_{\rm cold}/M_*)^{-\alpha} + (M_{\rm cold}/M_*)^{\beta} \right]^{-1}$$ with $(M_{\rm HI}/M_{\rm cold})_0 \approx 0.41$, $M_* = 8.8\times10^{10} M_\odot$, $\alpha\approx0.52$, $\beta\approx0.56$.

HI data products include moment-0 (integrated intensity), moment-1 (velocity field), and moment-2 (velocity dispersion) maps. Advanced source characterization deploys automated pipeline products (see (Koribalski et al., 2020, Westmeier et al., 2022)).

3. Survey Scope, Expected Yield, and Statistical Power

WALLABY is designed to cover approximately $3\pi$ steradians of sky, with a shallow, wide-area strategy. Simulations predict detection of $6\times10^5$ HI-rich galaxies (Duffy et al., 2012). About $87.5\%$ of galaxies are expected to be spatially resolved in the core (30″) mode, with $5\times10^3$ galaxies well resolved over >5 beams, enabling kinematic disk studies. Use of the full 6 km array would increase this to $1.6\times10^5$ galaxies at 10″ spatial resolution. The dynamic mass and stellar mass range of detected systems spans $10^{11}$–$10^{15}\,M_\odot$ and $10^5$–$10^{12}\,M_\odot$, respectively.

WALLABY’s mass sensitivity for point sources is: $$M_{\rm HI,lim}\approx5.2\times10^8\left(\frac{D_L}{100\,\mathrm{Mpc}}\right)^2\,M_\odot,$$ and the column density sensitivity for emission filling the 30″ beam is: $$N_{\rm HI,lim}\approx8.6\times10^{19}(1+z)^4\,\mathrm{cm}^{-2}.$$ (see (Westmeier et al., 2022)).

WALLABY’s contiguous wide-area coverage ensures robust sampling of local large-scale structure and reduces cosmic variance, which is critical for cosmological analysis.

4. Environmental Physics: Gas Removal, Morphological Transformation, and Quenching

Pilot fields targeting clusters (Hydra, Norma) and groups (NGC 4636) are used to quantify environmental effects, including ram pressure stripping and tidal interactions (Wang et al., 2021, Reynolds et al., 2021, Wang et al., 2021). High-resolution HI mapping reveals a diversity of ram pressure stripping stages:

• Strippable HI mass fraction $<0.9$ in most HI-detected cluster galaxies—i.e., only a fraction of HI is vulnerable to instantaneous removal.
• Rapid ($<200$ Myr) removal of strippable HI, but $>600$ Myr required for significant total depletion, establishing a staged depletion/stripping scenario.
• A sharp decline in HI detection fraction of infalling galaxies occurs near $1.5\,R_{200}$, implicating the cluster environment in rapid reduction of extended HI envelopes, with inner star-forming disks initially unaffected (Reynolds et al., 2021).
• For low-mass galaxies ($M_* \lesssim 10^9\,M_\odot$) in groups, tidal interactions reduce HI-to-optical disk size ratios and produce central reddening, supporting the view that "pre-processing" truncates outer gas and suppresses inner disk star formation gradually (Wang et al., 2021).

Resolved scaling relations show that higher-mass, denser galaxies tend to have less extended, lower surface-density HI disks, while star-forming, bluer galaxies often show more extended, higher $\mu_{\rm HI}$ disks (Reynolds et al., 2023, Lee et al., 20 May 2025).

5. Rare, Extreme, and Dark Galaxy Populations

WALLABY’s HI selection reveals a significant sample of low surface brightness galaxies (LSBGs) and optically dark HI sources. Analysis of early pilot data (O'Beirne et al., 7 May 2025, Wong et al., 2021, O'Beirne et al., 18 Jan 2024) demonstrates:

• 17% of HI detections are LSBGs ($\langle\mu_g\rangle >23$ mag arcsec$^{-2}$ within $1\,R_e$), spanning $5\times10^5$–$10^{11}\,M_\odot$ in stellar mass.
• 3% are "optically dark" ($>38$ robust candidates), with HI masses up to $\sim10^9\,M_\odot$. Some exhibit kinematic/morphological signatures of tidal stripping, while others are isolated and may be genuinely dark galaxies or extremely faint LSBGs.
• The majority of LSBGs and all dark sources identified in WALLABY’s pilot fields were previously uncatalogued, highlighting the incomplete nature of optically selected galaxy samples.
• Pre-pilot discoveries of isolated "dark" HI clouds near massive galaxies (e.g., NGC 1395) show properties consistent with both tidal debris and extreme LSBGs following the $M_{\rm HI}$–$D_{\rm HI}$ scaling relation (Wong et al., 2021), suggesting that multiple pathways may form such systems.
• "Almost dark" clouds associated with groups (e.g., Klemola 13) support a scenario in which tidal and/or ram pressure stripping redistribute HI outside galactic disks (O'Beirne et al., 18 Jan 2024).

Ultra-diffuse, gas-rich dwarfs are found to obey the baryonic Tully–Fisher relation and appear dark matter dominated, in contrast to reported dark-matter-deficient UDGs (Dudley et al., 18 Sep 2025).

6. Scaling Relations, Disk Structure, and Statistical Galaxy Evolution

Statistically robust scaling relations are constructed from HI kinematic modeling (Deg et al., 11 Nov 2024). Using $\gtrsim$150 uniformly modeled disks, key results include:

• The HI size–mass relation is tight across $\sim$5 orders of magnitude in mass: $$\log_{10}(D_H/\mathrm{kpc}) = (0.506\pm0.003) \log_{10}(M_H/M_\odot) - (3.293\pm0.009)$$.
• The size–velocity, mass–velocity, and $j_{X,H}$–mass (specific angular momentum–mass) relations exhibit low intrinsic scatter.
• Stellar and baryonic Tully–Fisher relations (with $M_b = M_* + 1.35 M_{\rm HI}$) show that inclusion of HI tightens the fundamental scaling relation for disks; the observed slope and scatter are consistent with external studies (SPARC, LVHIS).
• The atomic gas fraction ($f_{\text{atm}}$) correlates with disk stability parameters ($q_X = (j_{X,H} V_{\text{disp}})/(G M_b)$), corroborating theoretical expectations regarding disk self-regulation.

Spatially resolved analyses of HI mass within the stellar disk ($R_{25}$ or $R_{24}$) show that $\sim$68% (within $R_{25}$) and 54% (within $R_{24}$) of HI typically resides within the optical extent, with the tightest correlations between inner HI surface density and optical color (Lee et al., 20 May 2025). This underscores the connection between inner HI supply and current star formation, whereas global HI measures are more affected by extended, relatively inert outer reservoirs.

7. Legacy, Data Management, and Future Prospects

WALLABY is a cornerstone SKA pathfinder project, producing advanced, globally distributed data products via an asymmetric, multi-centre replication strategy (PostgreSQL+Bucardo), designed to scale as data volumes approach the SKA era’s exabyte regime (Parra-Royon et al., 2023). Data distribution involves regional centres generating and exchanging catalogues, kinematic models, and image products, tested for scalability and high-throughput, near–real time operations.

The breadth, depth, and homogeneity of WALLABY data enable:

• Systematic studies of HI mass function and its environmental variation;
• Constraints on galaxy kinematic scaling relations and disk stability on unprecedented scales;
• Probing the processes of gas accretion, removal, and star formation quenching in diverse environments;
• Measurements of cosmic large-scale structure, BAO, and cosmological parameters using gas-rich galaxies as low-bias tracers;
• Discovery and characterization of rare objects—dark galaxies, ultra-diffuse dwarfs, polar rings, and massive HI tails—informing galaxy formation scenarios.

With all data and catalogs made publicly available via CASDA, and the survey strategy validated in pilot studies, WALLABY is set to be a foundational resource for extragalactic astronomy and cosmology, with a design optimized for statistical power, multi-wavelength synergy, and readiness for the SKA’s transformational capability.