HI-MaNGA Surveys: Gas & Galaxy Evolution

• HI-MaNGA surveys are a comprehensive program of 21 cm HI observations combined with MaNGA’s optical spectroscopy to map neutral gas reservoirs in local galaxies.
• The surveys employ single-dish instruments like the GBT, ALFALFA, and FAST to derive HI masses, rotation widths, and gas fractions with ~70% sky completeness.
• Results link HI content with star formation, metallicity, and kinematics, anchoring scaling relations such as the baryonic Tully-Fisher relation and informing models of galaxy evolution.

The HI-MaNGA surveys constitute a program of 21 cm (neutral hydrogen) follow-up observations for galaxies targeted in the Sloan Digital Sky Survey IV’s Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey. HI-MaNGA provides direct measurements of the cold atomic gas reservoirs in ∼10,000 local galaxies, complementing MaNGA’s spatially resolved stellar and ionized gas spectroscopy. This combined dataset enables empirical studies of how neutral gas, star formation, chemical enrichment, galactic kinematics, and environmental processes collectively drive galaxy evolution. HI-MaNGA leverages single-dish 21 cm observations (primarily with the Green Bank Telescope, and incorporating ALFALFA, FAST, FASHI, RESOLVE, and other HI data), ultimately reaching ∼70% sky completeness for the full MaNGA sample. The resulting HI-stellar mass catalog, along with derived properties such as rotation widths and upper limits for non-detections, anchors a wide variety of extragalactic science—ranging from star formation quenching to the baryonic Tully-Fisher relation, external gas accretion, and gas dynamics during interactions.

1. Scientific Motivation and Integration with MaNGA Survey

The principal goal of HI-MaNGA is to assemble an HI census for the optically selected MaNGA galaxy sample (Masters et al., 2019, Masters et al., 13 Feb 2025). By measuring the cold atomic gas reservoir via the 21 cm line, HI-MaNGA augments MaNGA’s resolved maps of stellar populations, nebular ionization, and kinematics with critical information about the primary fuel for star formation. This enables a multi-phase ISM analysis: stellar structures, ionized/molecular/atomic gas content, and dynamics can be probed with spatial and spectral precision.

Key scientific objectives include:

• Quantifying how HI content correlates with recent and long-term star formation, metallicity, and feedback signatures (Stark et al., 2021).
• Exploring secular and environmental processes, such as external gas accretion, gas-star kinematic misalignment, and the detailed evolution of mergers and quenching (Zhang et al., 15 Sep 2025, Sharma et al., 2023).
• Anchoring scaling relations (e.g., the baryonic Tully-Fisher relation, HI mass–stellar mass relation) for comparison with simulations (Goddy et al., 2023, Stark et al., 2021).
• Elucidating the physical mechanisms regulating gas accretion, depletion, and replenishment.

2. Observational Methodology and Sample Construction

HI-MaNGA obtains 21 cm HI spectra for MaNGA galaxies through a combination of dedicated and archival single-dish observations:

• Major efforts at the Robert C. Byrd Green Bank Telescope (GBT): Over 2,500 hours of “filler” time, typically targeting galaxies at z ≲ 0.05, using the L-band VEGAS backend. Spectra are boxcar and Hanning smoothed to ~10 km s⁻¹ velocity resolution for robust profile characterization (Masters et al., 2019, Masters et al., 13 Feb 2025).
• Archival HI data augmentation: Cross-matching with the ALFALFA survey, as well as other HI data products (e.g., FAST, FASHI, RESOLVE), pushes HI coverage to ~70% of the MaNGA sample by DR17 (Masters et al., 13 Feb 2025).
• Observation and analysis strategy:
  • Standard ON–OFF pointing with per-object integration times tuned to reach an HI mass sensitivity threshold—either a constant mass (e.g., $\log(M_\mathrm{HI}/M_\odot) = 9.4$), or down to fixed gas fractions for galaxies at a range of stellar masses.
  • Derived properties include HI masses, velocity widths (W₅₀, W₂₀), inclination-corrected rotation velocities, and mass upper limits for non-detections.

Telescope/Data	Fraction of MaNGA Sample	Typical rms Sensitivity (mJy)	Channel Width (km s⁻¹)
GBT (new)	~30%	1.5	10
ALFALFA (archival)	~40%	2–3	10
FAST, FASHI, RESOLVE	smaller subsamples	0.5–2	6–10

HI masses are calculated as $M_{\rm HI} = 2.356 \times 10^5 D^2 \int S(v) dv$ ($D$ in Mpc, $S(v)$ in Jy), with self-absorption and inclination corrections applied as necessary (Masters et al., 2019, Yu et al., 2022).

3. Major Scientific Results and Population Studies

HI-MaNGA’s breadth supports statistical investigations of the interplay between cold gas and galaxy evolution. Highlights include:

• HI-to-stellar mass ratio and star formation:
  • A strong anti-correlation between $M_\mathrm{HI}/M_*$ and gas-phase metallicity is observed; HI-rich galaxies are systematically more metal-poor (Stark et al., 2021).
  • The correlation between $M_\mathrm{HI}/M_*$ and instantaneous SFR ($\mathrm{H}\alpha$) is weak, but becomes significant on moderate timescales ($\sim$30 Myr), indicating that HI traces the long-term star formation reservoir rather than rapid fluctuations.
  • Multi-variate fits to $M_\mathrm{HI}/M_*$ using forbidden oxygen line EWs and mean $\mathrm{H}\alpha$ surface brightness achieve reduced scatter:

  $$\log\left(\frac{M_{\mathrm{HI}}}{M_*}\right) = (1.222 \pm 0.079)\log\mathrm{EW}([\mathrm{O\,II}]) + (-0.366 \pm 0.034)\log\mu_{\mathrm{H}\alpha} + (12.655 \pm 1.320)$$ - Passive, HI-rich galaxies represent $\sim$5% of the HI-detected sample; their passivity is consistent across $\mathrm{H}\alpha$, SSP, UV, and IR SFR indicators provided the sSFR threshold is calibrated to $<10^{-11.15}\,\mathrm{yr}^{-1}$ (Salem et al., 15 Sep 2024).

• Star Formation Suppression Mechanisms:

  • Among HI-rich, low-SFR systems, analysis of MaNGA maps reveals signatures of external gas accretion (counter-rotation), AGN feedback (cLIER, “red geysers”), and bar-induced dynamical quenching are present, with no single process dominating (Sharma et al., 2023).
  • Detailed kinematic studies confirm that misalignments between stellar and gas angular momentum often coincide with recent or ongoing external accretion events (Zhang et al., 15 Sep 2025).
• Scaling Relations and Cosmological Benchmarking:
  • The observed baryonic Tully-Fisher relation for the HI-MaNGA/MaNGA sample,

  $$\log_{10}(M_{\rm bary}/M_\odot) = (2.97 \pm 0.18) \log_{10} V_{\rm rot} + (4.04 \pm 0.41)$$

  is precisely matched by mock-observed galaxies from IllustrisTNG, validating the simulation’s mass-rotation law (Goddy et al., 2023). - Analyses of HI line profiles (single-peaked vs. double-horned) indicate central HI enrichment via accretion and redistribution of angular momentum is especially marked in quiescent gas-star misaligned galaxies (Zhang et al., 15 Sep 2025).

4. Gas Accretion, Interactions, and Kinematics

HI-MaNGA data enable multi-faceted tracking of cold gas inflow and dynamical effects:

• Gas-Star Misalignments and External Accretion:

  • Galaxies with significant gas-star kinematic misalignment ($\Delta \mathrm{PA} \gtrsim 30^\circ$) are identified as prime sites of external HI acquisition (Zhang et al., 15 Sep 2025, Sharma et al., 2023).
  • In quiescent galaxies, external accretion transforms HI line profiles from double-horned (disk-dominated) to single-peaked (centrally enriched), as quantified by parameters like the concentration $C_V$ (ratio of 85% to 25% cumulative velocity widths) and “$K$” (area between normalized CoG and diagonal) derived from curve-of-growth profile modeling.
  • SF misaligned progenitors are already HI-rich; further central enrichment produces only minor spectral changes.
• Mergers and Galaxy Pair Dynamics:
  • Merger-stage analysis (via kinematic asymmetry, velocity decomposition, and line profile asymmetry) shows that HI gas fractions decrease moderately ($\sim$15%) at pericentric passage, accompanied by significant enhancements in SFR and star formation efficiency (Yu et al., 2022).
  • Time-resolved comparison of HI and $\mathrm{H}\alpha$ kinematics (via global line profiles from FAST and MaNGA, respectively) shows that initially large HI-$\mathrm{H}\alpha$ velocity offsets ($>100$ km s⁻¹, pre-merger) diminish as systems coalesce, indicative of gas mixing and eventual kinematic alignment (Jin et al., 13 Jan 2025).

Physical Process	HI-MaNGA Evidence
External gas accretion	Misaligned gas-star kinematics; centrally-enriched HI profiles in QS systems (Zhang et al., 15 Sep 2025)
Mergers/interactions	Mild HI depletion, boosted SFR/SFE at pericenter; disturbed HI profiles (Yu et al., 2022, Jin et al., 13 Jan 2025)
Bar quenching	Enhanced bar fraction in HI-rich low-SF galaxies (Sharma et al., 2023)
AGN feedback	cLIER/“red geyser” occurrence in passive HI-rich galaxies (Sharma et al., 2023)

5. Data Products, Technical Implementation, and Data Releases

HI-MaNGA releases (“DR” nomenclature; DR3 as of SDSS DR17) include:

• HI mass and upper limit catalogs for MaNGA galaxies, enabling population and statistical analyses.
• Rotation widths (W₅₀, W₂₀), HI mass fractions, and derived properties for follow-up science.
• Open-access Value Added Catalogs, cross-linked to all MaNGA optical IFU datasets, facilitating integration with environmental, morphological, and ancillary data (Masters et al., 13 Feb 2025, Abdurro'uf et al., 2021).
• Incorporation of additional HI data from ALFALFA, FAST, RESOLVE, and other surveys to maximize completeness and dynamic range.

Data analysis protocols employ robust pipeline procedures: RFI excision, automated profile leveling and smoothing, formal noise determination, stringent quality flagging, and propagation of mass/width errors according to the adopted radiometer and mass estimator equations (Masters et al., 2019, Masters et al., 13 Feb 2025).

6. Future Directions and Theoretical Context

HI-MaNGA’s scope and depth make it a benchmark for galaxy evolution studies in the pre-SKA era. Prospective research directions include:

• Spatially Resolved HI Imaging: Next-generation arrays (ASKAP/WALLABY, MeerKAT, SKA) will extend beyond single-dish detections to deliver HI surface density profiles, necessary to directly link radial atomic gas structure with star formation thresholds and angular momentum transport (Bland-Hawthorn, 2014, Giovanelli et al., 2015, Zhang et al., 15 Sep 2025).
• Synthesis with Multiwavelength Data: Ongoing efforts use MaNGA, deep optical/IR imaging (DESI, WISE), molecular gas surveys (e.g., ALMaQUEST), and environmental catalogs to disentangle the drivers of star formation efficiency and quenching (Arora et al., 2021, Sharma et al., 2023).
• Simulations and Mock-Observation Tools: Forward-modeling suites (e.g., iMaNGA from IllustrisTNG) enable direct comparisons between theory and data, bolstered by identical reduction, calibration, and profile extraction pipelines (Nanni et al., 2022, Goddy et al., 2023).
• Accretion/Quenching Mechanisms: Systematic studies of kinematic misalignments, HI profile shapes, and star formation suppression mechanisms in extended samples will be key for constraining feedback, mixing, and environment-induced effects.

HI-MaNGA’s architecture—comprehensive mass-limited HI coverage matched with high-fidelity optical IFU data—positions it as a linchpin for empirical and theoretical advances in the understanding of the baryon cycle and galaxy transformation in the local universe.