🔭 astrophysics

DINGO/GAMA /WAVES: HI-halo mass relation

Using a novel combination of DINGO, GAMA, and WAVES data to extend stacking analyses beyond spectroscopic limits, this study establishes a double power-law HI-halo mass relation with a turnover at $10^{11.2} M_\odot$ , revealing that while central galaxies dominate the neutral hydrogen budget in lower-mass halos, gas-rich satellites become the primary contributors in massive groups and clusters, accounting for a significant increase in measured HI content that cannot be explained by low-surface brightness or intra-group gas alone.

Original authors: Ajay Dev, Martin Meyer, Simon P. Driver, Jonghwan Rhee, Trystan S. Lambert, Paul Nulsen, Richard Dodson, Tobias Westmeier, Matthew Whiting, Sabine Bellstedt, Aaron Robotham, Jochen Liske, Elmo Tempel

Published 2026-04-30

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CC BY 4.0

Original authors: Ajay Dev, Martin Meyer, Simon P. Driver, Jonghwan Rhee, Trystan S. Lambert, Paul Nulsen, Richard Dodson, Tobias Westmeier, Matthew Whiting, Sabine Bellstedt, Aaron Robotham, Jochen Liske, Elmo Tempel, Ivan Baldry, Jon Loveday, Luke Davies, Barbara Catinella, Michael J. I. Brown, Kristine Spekkens, Benne W. Holwerda

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, invisible web made of dark matter. These webs are called halos, and they act like cosmic gravity wells. Inside these wells, galaxies form and live. For a long time, astronomers have known that galaxies are made of stars, but they've been trying to figure out exactly how much "fuel" (cold gas) they have to keep making new stars. This fuel is mostly neutral atomic hydrogen (H i).

This paper is like a massive census of how much gas is hidden inside these dark matter "houses" (halos) of different sizes.

Here is the breakdown of what the researchers did and found, using simple analogies:

1. The Challenge: Finding the Invisible

Imagine trying to count the people in a city.

The Problem: If you only look at the people wearing bright, loud shirts (bright galaxies), you miss the people in dark clothes (faint galaxies). In astronomy, we can easily see bright galaxies, but the faint ones are hard to spot with our telescopes.
The Old Way: Previous studies tried to count the gas in galaxy groups by looking only at the "bright shirts" (spectroscopic data). They suspected they were missing a lot of gas from the "dark clothes" (faint satellites) that live in the same group but are too dim to be seen individually.

2. The New Tool: The "Stacking" Trick

To solve this, the researchers used a clever trick called spectral stacking.

The Analogy: Imagine trying to hear a single person whispering in a noisy room. You can't hear them. But if you have 100 people whispering the same thing at the same time, and you line them up perfectly, their whispers add up to a loud, clear shout.
The Method: The team took the radio signals from hundreds of galaxies that were too faint to see individually, lined them up perfectly, and added them together. This allowed them to "hear" the average amount of gas in a group, even if they couldn't see the individual gas clouds.

3. The Innovation: Adding the "Ghost" Guests

This is where the paper gets really clever.

The Setup: They used data from a survey called GAMA (which has detailed info on bright galaxies) and combined it with a new catalog called WAVES (which has positions for millions of fainter galaxies, but no detailed speed data).
The Trick: They took the faint galaxies from the WAVES list and said, "You live in the same neighborhood as this bright galaxy we know about." Even though they didn't know the exact speed of the faint galaxies, they assumed they were moving with the group.
The Result: By adding these "ghost" guests (faint satellites) to the stack, they found 1.5 to 3 times more gas in the largest galaxy groups than previous studies had found. It turns out, the "dark clothes" crowd was holding a massive amount of the fuel all along.

4. The Big Discovery: The "Gas Curve"

When they mapped out how much gas exists in halos of different sizes, they found a specific shape, like a rollercoaster:

Small Halos (The Cottages): Small groups of galaxies have a lot of gas relative to their size.
Medium Halos (The Mansions): As the halo gets bigger (around the size of our Milky Way's neighborhood), the amount of gas peaks. This is the "sweet spot" for gas.
Large Halos (The Skyscrapers): In the biggest, most massive groups (clusters), the gas starts to disappear. The environment is so harsh that the gas gets stripped away or heated up, preventing new stars from forming.

The Tipping Point: They found that in smaller groups, the main galaxy (the "host") holds most of the gas. But once the group gets huge, the satellite galaxies (the smaller neighbors) hold most of the gas. It's like in a small family, the parents hold the money, but in a massive corporation, the employees hold the bulk of the assets.

5. What About "Ghost Gas"?

Some scientists wondered if there was a lot of "dark gas" floating in space between galaxies—gas that isn't attached to any galaxy at all.

The Finding: The researchers compared their "stacked galaxy" numbers with previous studies that looked at the entire group at once (including the space between galaxies).
The Conclusion: The numbers matched almost perfectly. This means there isn't much "ghost gas" floating around. Almost all the gas in these groups is locked up inside the galaxies themselves, even the faint ones.

Summary

This paper is a better census of the universe's gas supply. By using a new method to include faint, hard-to-see galaxies, they discovered that massive galaxy groups hold much more fuel than we thought. They also confirmed that the gas is mostly inside the galaxies, not floating freely in the dark space between them. This helps us understand how galaxies grow, starve, and evolve over time.

1. Problem Statement

Understanding the distribution of neutral atomic hydrogen (H i) within dark matter halos is critical for constraining theories of galaxy formation, feedback mechanisms, and the baryon cycle. While the Stellar-to-Halo Mass relation (SHMR) is well-established, the H i-to-Halo Mass (HIHM) relation remains less constrained, particularly in group and cluster environments.

Key challenges addressed in this work include:

Sensitivity Limits: Direct H i detections are biased toward gas-rich galaxies, missing the fainter, gas-poor populations that dominate high-mass halos.
Spectroscopic Incompleteness: Previous studies relying on spectroscopic group catalogs (e.g., GAMA) miss faint satellite galaxies below the spectroscopic magnitude limit ( $r \approx 19.8$ ), leading to potential underestimates of total H i mass in massive halos.
Diffuse Gas: It remains unclear whether a significant fraction of H i exists as "dark" clouds or low-surface-brightness (LSB) structures not associated with individual galaxies, or if the H i budget is fully accounted for by bound galaxies.

2. Methodology

The authors combine multi-wavelength data to construct a robust HIHM relation across a wide halo mass range ( $10^{10.5} \lesssim M_h/M_\odot \lesssim 10^{14.5}$ ).

Data Sources

H i Data: 100-hour pilot survey data from DINGO (Deep Investigation of Neutral Gas Origins) using the ASKAP (Australian SKA Pathfinder) interferometer. This provides high spatial resolution ( $30''$ beam) and deep sensitivity ( $\sim 0.67$ mJy beam $^{-1}$ ).
Spectroscopic Data: The GAMA (Galaxy and Mass Assembly) survey provides a highly complete spectroscopic sample ( $r < 19.8$ ) and a robust group catalog (G3C) with velocity dispersion-based halo masses.
Photometric Data: The WAVES (Wide Area VISTA Extragalactic Survey) photometric catalog provides positions and photometric redshifts for fainter galaxies ( $Z < 21.1$ ) that fall below the GAMA spectroscopic limit.

Techniques

Direct Detections: 408 H i detections with optical counterparts were analyzed to trace the upper envelope of the HIHM relation.
Spectral Stacking: To probe below detection limits, the authors employed two stacking strategies:
- GAMA-only Stacking: Stacking spectra of spectroscopically confirmed group members.
- GAMA+WAVES Stacking (Novel Approach): Extending the sample by associating faint photometric galaxies from WAVES to GAMA group centrals.
  - Method: Photometric satellites are assigned to a group if they lie within the projected $R_{200}$ of a spectroscopic central.
  - Redshift Handling: For satellites lacking spectroscopic redshifts, the spectrum is extracted at the central galaxy's spectroscopic redshift. This assumes true satellites share the halo's velocity dispersion, causing their signal to cluster coherently in the stack, while projected interlopers contribute primarily as noise.
Halo Mass Estimation:
- Dynamical Mass: Based on velocity dispersion for groups with $N_{FoF} > 4$ .
- Abundance Matching (AM): Used to assign halo masses to centrals based on stellar mass, allowing the inclusion of isolated galaxies and low-richness groups.

3. Key Contributions

Extension Beyond Spectroscopic Limits: The paper introduces a novel stacking technique that incorporates photometric members (WAVES) into spectroscopic groups (GAMA). This recovers H i content from faint satellites otherwise missed by spectroscopic surveys.
High-Resolution Interferometry: Utilizing ASKAP/DINGO data ( $30''$ resolution) rather than single-dish data (e.g., ALFALFA at $3.5'$ ), allowing for better separation of sources and reduced confusion in crowded group environments.
Comprehensive HIHM Mapping: Provides the first HIHM relation derived from interferometric data that spans from dwarf galaxies to massive clusters, separating the contributions of central and satellite galaxies.

4. Key Results

The HIHM Relation

Double Power-Law Form: The HIHM relation exhibits a double power-law shape with a turnover (peak efficiency) at $M_h \sim 10^{11.2} M_\odot$ .
- Below the turnover, H i mass scales positively with halo mass.
- Above the turnover, H i efficiency declines due to environmental quenching and the transition from cold to hot gas halos.
Comparison with Previous Studies: The results are consistent with previous group-stacking studies (e.g., Guo et al. 2020; Dev et al. 2023) and direct detection studies (Hutchens et al. 2023), validating the use of stacking techniques.

Impact of Photometric Members

Significant Mass Recovery: Including WAVES photometric members increases the measured H i content in halos above $10^{13} M_\odot$ by a factor of 1.5 to 3.
Satellite Dominance: This highlights that a substantial portion of H i in massive groups and clusters resides in faint satellites below the spectroscopic detection threshold.
Low-Mass Halos: No significant difference was found between GAMA-only and GAMA+WAVES stacks for $M_h < 10^{13} M_\odot$ , as satellite abundance is low in these systems.

Centrals vs. Satellites

Transition Mass: Central galaxies dominate the H i budget for $M_h \lesssim 6 \times 10^{12} M_\odot$ . Above this mass, satellites dominate the total H i content.
Central Quenching: The H i fraction in central galaxies declines sharply at high halo masses, consistent with the suppression of star formation in massive cluster centrals.

Diffuse/Unaccounted H i

Consistency Check: The total H i mass derived from galaxy stacking (summing individual galaxies) is consistent with total group spectral stacking measurements.
Conclusion: This implies that diffuse H i clouds and low-surface-brightness galaxies contribute only a minor fraction to the total halo H i mass. The H i budget is largely accounted for by gas bound to individual galaxies.

5. Significance

Methodological Advancement: The "GAMA+WAVES" stacking technique demonstrates a powerful pathway for future surveys (e.g., combining LSST/Euclid with SKA/MeerKAT) to recover the full baryonic content of halos without waiting for deep spectroscopy of every faint galaxy.
Environmental Physics: The results confirm that environmental processes (ram-pressure stripping, strangulation) effectively suppress H i in massive halos, but the reservoir of gas is maintained by a population of faint, gas-rich satellites.
Cosmological Context: The study provides a precise empirical constraint on the HIHM relation, essential for calibrating semi-analytic models and hydrodynamical simulations of galaxy evolution.

In summary, this work establishes that the H i content of dark matter halos is dominated by galaxies, with a distinct transition from central to satellite dominance at $M_h \sim 6 \times 10^{12} M_\odot$ , and that accounting for faint photometric satellites is crucial for an accurate census of gas in massive galaxy groups.