superMIGHTEE: Ultra-Broadband Deep Radio Survey

• superMIGHTEE is a collaborative ultra-broadband radio survey that combines MeerKAT’s high-sensitivity L‐band with uGMRT’s low-frequency data to achieve μJy-sensitivity imaging over key extragalactic fields.
• It employs rigorous calibration and advanced imaging techniques to generate precise spectral energy distributions and robust source classifications across a frequency range of 200 MHz to 2.5 GHz.
• The project reveals critical transitions between AGN-dominated and star-forming populations, setting a foundation for future studies of cosmic magnetism, neutral hydrogen, and galaxy evolution.

The superMIGHTEE project is a collaborative, ultra-broadband exploration of the deep radio sky that combines MeerKAT MIGHTEE survey data with upgraded Giant Metrewave Radio Telescope (uGMRT) observations to produce deep, arcsecond-resolution images and radio source catalogues spanning frequencies from 200 MHz to 2.5 GHz. By harmonising MeerKAT’s L‐band sensitivity with uGMRT’s low-frequency reach across key extragalactic deep fields (XMM-LSS, COSMOS, E-CDFS), superMIGHTEE delivers μJy-sensitivity wideband imaging, robust source classification, and spectral analyses that uniquely probe the evolution of star-forming galaxies and active galactic nuclei (AGN) over cosmic time (Lal et al., 11 Sep 2025). The project’s initial release provides a new window into the faint radio Universe, revealing key transitions in source populations and laying the groundwork for detailed studies of cosmic magnetism, neutral hydrogen, and galaxy evolution prior to the SKA era.

1. Scientific Aims and Survey Architecture

superMIGHTEE aims to construct the most comprehensive ultra-deep, ultra-wideband radio survey to date by fusing MeerKAT’s high-fidelity, high-sensitivity L-band (856–1711 MHz) continuum and spectral line data with uGMRT’s wide spectral range (approximately 200 MHz–850 MHz, extending to 2.5 GHz in future releases). The principal scientific drivers include:

• Elucidating the cosmic evolution of AGN and star-forming galaxies through broadband spectral analysis and precise source classification at arcsecond scales.
• Characterising spectral energy distributions (SEDs), with emphasis on the turnover from AGN-dominated populations at high flux densities to star-forming galaxy domination at fainter flux densities, exploiting both spectral indices and curvature.
• Enabling statistical and resolved studies of neutral hydrogen (H I) and OH megamasers, probing the baryon cycle and environmental dependence of galaxy assembly.
• Advancing polarimetric and spectroscopic explorations of cosmic magnetic fields throughout large-scale structures, clusters, and filaments via future spectropolarimetric releases.

Coverage of XMM-LSS, COSMOS, and E-CDFS fields is selected due to the availability of extensive multi-wavelength legacy data and high-quality ancillary redshifts.

2. Observational Strategy and Data Processing

superMIGHTEE’s observational design leverages complementary capabilities of MeerKAT and uGMRT:

• MeerKAT: Primary L-band coverage (856–1711 MHz), with arcsecond (∼6″) resolution and rms sensitivities as low as 2 μJy beam⁻¹, provides the higher-frequency anchor.
• uGMRT: Bands 3 (centered at 400 MHz) and 4 (centered at 650 MHz), each imaged at ∼5–7″ resolution, deliver 9.9 deg² and 6.9 deg² coverage, respectively, at rms sensitivities as low as 4–9 μJy beam⁻¹ depending on field and weighting.

Key processing steps include:

• Observations performed in ∼8 hr tracks with regular calibration (bandpass, delay, gains) and RFI excision (using, e.g., the sumthreshold algorithm).
• Data partitioned into spectral windows, each calibrated and self-calibrated using a variant of the processMeerKAT pipeline (CASA-based, adapted for uGMRT’s feeds).
• Imaging with tclean; $u, v$ cuts applied to avoid poorly sampled baselines.
• Pointings mosaiced with inverse variance weighting and wideband primary beam corrections. Effective frequency mosaics account for spatial variations in the primary beam.
• Astrometric solutions cross-referenced with optical/NIR catalogs, achieving positional accuracy of ∼0.03–0.09″ (Lal et al., 11 Sep 2025).
• Source detection via PyBDSF, including detailed analyses of total-to-peak flux ratios and SNRs to assess the degree of spatial resolution.

This multi-tiered, multi-frequency approach allows seamless integration of MeerKAT and uGMRT data, offering uniform, deep coverage across the fields and enabling robust comparison and cross-calibration of flux scales.

3. Source Catalogue Construction and Spectral Analyses

The initial superMIGHTEE data release provides:

Band Center	Area (deg²)	Source Count	Angular Res. (″)	Median RMS (μJy/beam)
650 MHz	9.9	27,101	5–7	4–9
400 MHz	6.9	10,946	5–7	4–9

Sources extracted from mosaics are cross-matched with MeerKAT 1.284 GHz catalogues for spectral analysis. Redshift distributions, determined via optical/NIR association, extend to $z \sim 4$ with a median $z = 1$.

Spectral properties are characterised using both traditional two-point spectral indices (e.g., $\alpha_{400-650}$, $\alpha_{650-1280}$, with $S_{\nu} \propto \nu^{\alpha}$) and broadband SED fits:

$$S_{\nu} = S_0 \left(\frac{\nu}{\nu_0}\right)^{\alpha} \exp\left[q (\ln \nu)^2\right]$$

where $S_0$ is reference flux, $\alpha$ is the spectral index, and $q$ quantifies curvature. Systematic flux scale uncertainties are below 5%.

A color–color plot of spectral indices delineates four populations: falling, peaked, rising, and V-shaped spectra. The median spectral index steepens with increasing flux density, confirming the transition from star-forming galaxies (flatter spectra at lower fluxes) to AGN-dominated sources (steeper spectra at higher fluxes). For example, at $S_{650} > 10$ mJy, $\alpha_{400-650} \approx -0.57$, $\alpha_{650-1280} \approx -0.75$. The fraction of sources with peaked spectra rises toward lower flux densities.

4. Astrometric and Flux Calibration Consistency

Astrometric corrections are rigorously enforced through cross-matching with high-quality optical/infrared catalogs. The resulting offsets are $<0.1″$, ensuring robust counterpart identification and spectral reliability.

Total-to-peak flux ratio diagnostics are used to separate compact from extended sources and to identify effects of confusion, blending, and image fidelity. The consistency of flux scales across MeerKAT and uGMRT bands is validated by SED model fitting, with deviations limited to a few percent. Systematic comparison with external surveys and several internal weighting schemes supports the reliability and homogeneity of the superMIGHTEE catalogues at the sub-5% level.

5. Insights into Galaxy Evolution and Populations

Broadband superMIGHTEE data reveal that the median spectrum transitions as a function of flux density:

• Bright regime ($S_{650} >$ few mJy): Dominated by AGN, with optically thin synchrotron spectral indices ($\alpha_{650-1280} \approx -0.75$).
• Faint regime ($S_{650} <$ few mJy): Increasing contribution and then dominance from star-forming galaxies, with a flatter spectral index ($\alpha_{650-1280} \approx -0.51$) consistent with supernova-driven synchrotron emission.
• Spectral curvature: Enhanced at lower frequencies (flattened spectra), interpreted as a signature of increased free-free absorption or altered cosmic ray populations in faint sources.

The redshift distribution extending to $z \sim 4$ enables studies of evolutionary effects in faint radio populations, particularly the low-luminosity AGN and high-redshift star-forming systems that are inaccessible to shallower or higher-frequency-limited surveys. The increase in peaked-spectrum fractions at low fluxes may trace young radio sources or high-redshift compact AGN activity.

6. Forthcoming Data Releases and Expansion

Planned future releases by the superMIGHTEE collaboration will:

• Deliver spectropolarimetric hypercubes and high-spectral-resolution continuum-subtracted spectral line cubes, facilitating comprehensive studies of cosmic magnetic fields and neutral hydrogen/OH maser populations.
• Incorporate ongoing uGMRT band 2 (130–260 MHz) observations, broadening frequency leverage for studies of spectral turnovers, free–free absorption, and complex SED curvature.
• Enable stacking analyses of H I and OH in emission at intermediate redshifts, leveraging sample sizes and homogeneous coverage for statistical constraints on cosmic baryon cycles.

A plausible implication is that extended spectral and polarization datasets from superMIGHTEE will uniquely complement the forthcoming SKA programs by providing pilot-scale results on source characterization, clustering, and dust-unbiased evolution well in advance of full SKA operations.

7. Significance in the Context of Extragalactic Radio Surveys

superMIGHTEE, as a joint MeerKAT-uGMRT program, represents a paradigm shift in the paper of faint radio source populations by combining wide area, ultra-deep sensitivity, and true broad-bandwidth coverage. This is the first survey to achieve $\mu$Jy sensitivities from 400–1300 MHz at arcsecond resolution over ∼10 deg², matched with robust source catalogs and validated calibration.

The comprehensive multi-frequency, multi-resolution approach provides direct constraints on the evolution of the radio luminosity function, cosmic star formation, and AGN feedback signatures. In providing detailed source spectra, cross-identifications, and physical classifications, superMIGHTEE has established an essential resource for the extragalactic radio community and constitutes the definitive pre-SKA broad-band extragalactic radio survey template (Lal et al., 11 Sep 2025).