🔭 astrophysics

Vz-GAL: Probing Cold Molecular Gas in Dusty Star-forming Galaxies at z=1-6

The Vz-GAL survey presents the first high-redshift CO(J=1-0) detections of 90 dusty star-forming galaxies at z=1–6, revealing massive cold gas reservoirs with consistent depletion timescales across cosmic time and providing key excitation corrections that refine gas mass estimates and confirm universal cold gas conditions in star-forming galaxies.

Original authors: Prachi Prajapati, Dominik Riechers, Pierre Cox, Axel Weiss, Amelie Saintonge, Bethany Jones, Tom J. L. C. Bakx, Stefano Berta, Paul van der Werf, Roberto Neri, Kirsty M. Butler, Asantha Cooray, Diana

Published 2026-01-26

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Prachi Prajapati, Dominik Riechers, Pierre Cox, Axel Weiss, Amelie Saintonge, Bethany Jones, Tom J. L. C. Bakx, Stefano Berta, Paul van der Werf, Roberto Neri, Kirsty M. Butler, Asantha Cooray, Diana Ismail, Andrew J. Baker, Edoardo Borsato, Andrew Harris, Rob Ivison, Matthew Lehnert, Lucia Marchetti, Hugo Messias, Alain Omont, Catherine Vlahakis, Chentao Yang

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

The Big Picture: A Cosmic Census of "Baby" Galaxies

Imagine the universe as a giant, bustling city. In the early days of this city (billions of years ago, when the universe was much younger), there were massive construction zones where new stars were being born at a frantic pace. These construction sites are called Dusty Star-Forming Galaxies (DSFGs). They are like giant, dusty nurseries where stars are being crated up faster than anywhere else in the universe.

For a long time, astronomers could see the "smoke" (dust and infrared light) coming from these nurseries, but they couldn't see the "fuel" (cold molecular gas) that was actually making the stars. It was like seeing a car engine revving loudly but not knowing how much gas was in the tank.

The Vz-GAL Project is a massive new survey that finally opened the hood of 92 of these ancient, dusty galaxies to measure exactly how much fuel they have. They used the Karl G. Jansky Very Large Array (VLA), a giant radio telescope in New Mexico, to listen for a specific "hum" made by carbon monoxide gas. This hum is the universal language of cold gas.

The Main Findings

1. The Fuel Tank is Huge (and Full)

The team successfully detected the "hum" (CO emission) in 90 out of the 92 galaxies they looked at.

The Analogy: Think of these galaxies as massive trucks. The researchers found that these trucks are carrying between 200 billion and 20 trillion times the mass of our Sun in fuel (gas).
The Result: They have enough fuel to keep building stars for a long time, but not forever.

2. The Engine is Running Hot (Star Formation Efficiency)

The paper asks a simple question: How fast are these galaxies burning their fuel compared to how much they have?

The Analogy: Imagine two cars. One is a slow, fuel-efficient sedan (a normal galaxy). The other is a drag racer (a starburst galaxy). The drag racer burns fuel incredibly fast.
The Discovery: These ancient dusty galaxies are like drag racers. They are burning their fuel much faster than normal galaxies do today. However, the paper found something surprising: The drag racers of the early universe and the "super-drag racers" (ULIRGs) of our local neighborhood run on the same efficiency.
The Takeaway: Nature seems to have a "self-regulating thermostat." Even though these galaxies are huge and chaotic, they maintain a consistent rate of star-making per unit of gas. They aren't just randomly exploding; they are running on a specific, efficient engine setting.

3. The "Thermostat" of the Gas (Excitation)

To measure the gas correctly, astronomers have to understand how "hot" or "excited" the gas molecules are.

The Analogy: Imagine a crowd of people in a room.
- Low Excitation: Everyone is sitting quietly on the floor (Ground state).
- High Excitation: Everyone is jumping on the furniture (High energy states).
The Discovery: In these dusty galaxies, the gas is a mix. Some is sitting quietly, but a lot of it is jumping on the furniture (warm and dense). The team measured the ratio of people sitting vs. jumping. They found that while the gas is generally "sub-thermal" (mostly sitting), there is a significant amount of "jumping" gas, especially in the most intense star-forming regions.
Why it matters: If you assume everyone is sitting quietly when they are actually jumping, you will miscalculate how many people are in the room. The paper provides a new, more accurate "counting manual" for these galaxies.

4. The "Atomic Carbon" Check

The team also looked at a different tracer called Atomic Carbon ([CI]).

The Analogy: If CO is the main language of the gas, [CI] is a secondary dialect. If both languages tell the same story, you know the story is true.
The Discovery: The ratio of CO to [CI] in these ancient galaxies is almost identical to the ratio found in normal galaxies today.
The Takeaway: This suggests that the basic "ingredients" and conditions of the gas in these ancient, chaotic galaxies are surprisingly similar to the gas in our own Milky Way, despite the massive differences in size and activity.

5. No "Two-Tier" System

For a long time, scientists wondered if these ancient, chaotic galaxies needed a completely different set of rules (a different "conversion factor") to measure their gas compared to normal galaxies.

The Discovery: The paper finds no evidence for two different sets of rules. The same "conversion factor" (a mathematical tool to turn radio signals into gas mass) works for both the wild, ancient galaxies and the calm, local ones.
The Metaphor: It's like realizing that a dollar bill has the same value whether you spend it in a quiet village or a chaotic, high-speed casino. You don't need a different currency for the casino.

Summary of the "Vz-GAL" Legacy

This paper is essentially a massive census. Before this, we had a few snapshots of these ancient galaxies. Now, we have a high-definition, wide-angle view of 92 of them.

We found the fuel: We confirmed these galaxies have massive gas reservoirs.
We checked the engine: We confirmed they burn fuel efficiently, similar to the most intense star-forming regions in our local neighborhood.
We fixed the math: We proved that we don't need special, complicated math to measure their gas; the standard rules apply.

The authors conclude that while these galaxies look wild and different from our own, the physics of their gas reservoirs is surprisingly universal. They are not aliens; they are just the universe's most intense construction sites, running on the same fundamental principles as everything else.

Note on Limitations: The paper notes that while they have the gas mass, they still need to measure the stars in these galaxies to know exactly how far they are from the "main sequence" (the normal path of galaxy evolution). Also, the gas in these extreme environments might be so dense that even the best tools (CO lines) aren't perfect, and future studies will need to combine multiple "languages" (different gas tracers) to get the perfect count.

Technical Summary: Vz-GAL: Probing Cold Molecular Gas in Dusty Star-forming Galaxies at z = 1 – 6

Problem and Motivation
Dusty star-forming galaxies (DSFGs) at high redshifts ( $z = 1 - 6$ ) are critical to understanding the peak epoch of cosmic star formation. While these systems are often identified via their intense infrared emission, characterizing their cold molecular gas reservoirs ( $H_2$ ) remains challenging. Historically, gas mass estimates in high- $z$ galaxies have relied on mid- to high- $J$ CO transitions (e.g., $J=2-1$ to $J=5-4$ ) observed by facilities like NOEMA. However, converting these higher-energy lines to total gas mass requires assumptions about the excitation conditions of the gas (the CO spectral line energy distribution, or SLED) and the CO-to- $H_2$ conversion factor ( $\alpha_{CO}$ ). The ground-state CO( $J=1-0$ ) transition is the most robust tracer for total molecular gas mass as it is emitted by gas at densities $\gtrsim 10^2$ cm $^{-3}$ and avoids uncertainties associated with excitation corrections. Prior to this work, systematic CO($1-0$) observations of high- $z$ DSFGs were limited to a small number of individual targets, preventing a comprehensive statistical understanding of gas conditions across the population.

Methodology
The authors present the first results from Vz-GAL, a large survey conducted with the Karl G. Jansky Very Large Array (VLA). The sample consists of 92 Herschel-selected, infrared-luminous DSFGs ( $L_{8-1000\mu m} > 10^{12}-10^{13} L_\odot$ ) at redshifts $1 < z < 6$ . These targets were selected from the NOEMA $z$ -GAL spectroscopic survey, which provided robust redshifts and mid/high- $J$ CO data for the sample.

Observations: The survey utilized the VLA in its most compact D-configuration (and C-configuration for specific targets) to maximize sensitivity to extended emission. Observations covered the Ku, K, Ka, and Q bands to target the redshifted CO($1-0$) line ( $\nu_{rest} = 115.271$ GHz). Ten targets at $z > 3.6$ were also observed in CO($2-1$).
Data Reduction: Data were processed using CASA v6.4.1. The authors performed rigorous flagging, calibration (using standard flux calibrators, with corrections for a flaring 3C138), and imaging. Continuum subtraction was applied, and spectral cubes were generated.
Analysis: The team extracted spectra using peak-pixel methods for unresolved sources and $2\sigma$ contour-based extraction for resolved systems. They derived integrated line luminosities ( $L'_{CO}$ ) and molecular gas masses ( $M_{H_2}$ ) assuming a Milky Way-like conversion factor of $\alpha_{CO} = 4.0$ $M_\odot$ (K km s $^{-1}$ pc $^2$ ) $^{-1}$ . The study also calculated CO line brightness temperature ratios ( $r_{J1}$ ), gas depletion times ( $\tau_{dep}$ ), gas-to-dust mass ratios ( $\delta_{GDR}$ ), and compared CO($1-0$) luminosities with atomic carbon CI luminosities from the $z$ -GAL survey.

Key Contributions
This work quintuples the existing sample of CO($1-0$) detections for Herschel-selected bright DSFGs, creating the largest homogeneous sample of high- $z$ CO($1-0$) observations to date (106 galaxies when including the pilot sample). By anchoring the analysis in the ground-state CO($1-0$) line, the study provides a direct calibration of gas masses that is independent of the excitation assumptions required for higher- $J$ lines.

Results

Detection Rates: CO($1-0$) emission was detected at $>2\sigma$ significance in 90 out of 92 targets (57 at $>5\sigma$ ). CO($2-1$) was detected in 10 targets.
Gas Masses and Depletion Times: The derived apparent gas masses (uncorrected for lensing magnification $\mu$ ) range from $\mu M_{H_2} = (2 - 20) \times 10^{11} (\alpha_{CO}/4.0) M_\odot$ . The corresponding gas depletion times ( $\tau_{dep}$ ) fall between 50 and 600 Myr.
Star Formation Efficiency (SFE): The $L_{IR}/L'_{CO(1-0)}$ ratio, a proxy for SFE, shows that high- $z$ DSFGs and local Ultra-Luminous Infrared Galaxies (ULIRGs) exhibit similar efficiencies, significantly higher than local main-sequence (MS) galaxies. The data suggest a self-regulatory mechanism maintaining a consistent SFR per unit gas mass in starbursts across redshifts.
CO Line Ratios: The study reports median CO line brightness temperature ratios: $r_{21} = 0.88 \pm 0.25$ , $r_{31} = 0.61 \pm 0.22$ , $r_{41} = 0.49 \pm 0.15$ , $r_{51} = 0.47 \pm 0.13$ , and $r_{61} = 0.28 \pm 0.13$ . These values indicate mostly sub-thermal excitation for mid/high- $J$ lines, confirming that using higher- $J$ lines without correction leads to significant underestimation of gas masses.
Gas-to-Dust Ratios: The median gas-to-dust mass ratio is $\delta_{GDR} \approx 100$ , consistent with values found in metal-rich, dusty galaxies in the local universe and other high- $z$ samples.
[CI]/CO Correlation: For a subsample of 23 sources, the median log ratio $\log(L'_{[CI]}/L'_{CO(1-0)}) = -0.71 \pm 0.12$ . This is consistent with local ULIRGs and MS galaxies, suggesting a common baseline state for the interstellar medium (ISM) across different star formation modes and redshifts.

Significance and Claims
The authors claim that their findings support the use of a single, Milky Way-like $\alpha_{CO}$ factor for these populations, finding no evidence for a bimodal distribution of conversion factors. The consistency of SFE and depletion times between high- $z$ DSFGs and local ULIRGs implies that the physical conditions driving star formation in these extreme environments are self-regulated and similar across cosmic time, despite differences in dust temperatures and gas densities.

The paper emphasizes that while CO($1-0$) is the most robustly calibrated tracer for total gas mass, it may not be the optimal column density tracer in highly excited environments where the line is optically thick. Consequently, the authors conclude that the "best calibration" for gas mass and column density in these extreme systems will require semi-empirical modeling of multiple CO lines and continuum data, potentially incorporating [CI] as a complementary tracer. The Vz-GAL dataset serves as a foundational resource for such future modeling efforts to characterize the multi-phase ISM of the early universe.