How the ngVLA Will Uncover the Universe's Molecular Gas Budget
For decades, astronomers have pieced together the cosmic history of star formation, but the story of the fuel that powers it has remained frustratingly incomplete. The next-generation Very Large Array is poised to change that.
Across the vast expanse of cosmic time, galaxies have undergone an extraordinary evolution, birthing trillions of stars. This cosmic drama of construction is powered by a fundamental ingredient: cold molecular gas. This gas—primarily hydrogen molecules—is the raw material from which stars are born. Just as a fire requires wood to burn, star formation requires this cold galactic fuel.
Yet, the history of this material, the "molecular gas budget" of the universe, has been largely unmeasured, obscuring a vital chapter in the story of cosmic evolution. The next-generation Very Large Array (ngVLA), a revolutionary radio telescope in development, is designed to find this missing chapter. By peering into the earliest epochs of the universe, it will act as a cosmic fuel gauge, precisely measuring the reservoirs of gas that set the stage for galaxy formation and the very building blocks of cosmic structure 1 2 .
Cold molecular gas is the primary fuel for star formation, yet its cosmic history remains largely unknown.
The next-generation VLA will measure the molecular gas budget across cosmic time with unprecedented precision.
Astronomers have long been able to chart the "cosmic star formation rate density"—a measure of how many stars were forming in a given volume of the universe at different points in time. This history shows a dramatic peak around 10 billion years ago, a cosmic high noon of star formation, followed by a steady decline to the present day 4 .
The prevailing theory is that this rise and fall must be driven by a corresponding rise and fall in the available fuel: cold molecular gas. However, directly measuring this gas history has been a formidable challenge. Confirming this parallel evolution is crucial, as it would show that the star formation history of the universe is fundamentally gas-supply driven 4 .
Molecular hydrogen itself is notoriously difficult to detect directly. Instead, astronomers use carbon monoxide (CO) as a proxy tracer. By observing the rotational transition lines of the CO molecule, they can infer the presence and quantity of the much more abundant molecular hydrogen.
The most reliable of these tracers is the CO(1-0) transition, also known as the "ground transition." It is this specific line that provides the most direct measure of the total molecular gas mass, as it is not as sensitive to local environmental conditions like temperature and density as other higher-frequency transitions 1 .
Current telescopes, even powerful ones like the Atacama Large Millimeter/submillimeter Array (ALMA), struggle to observe this fundamental CO(1-0) line in distant galaxies. They are often forced to observe higher-frequency transitions, which require significant and uncertain corrections to estimate the total gas mass. This has left our understanding of the cosmic molecular gas budget filled with gaps and uncertainties, especially in the universe's first few billion years 1 .
The next-generation Very Large Array (ngVLA) is being designed as a transformative instrument that will open a new window on the cold, dark universe. Its core strength lies in its unprecedented sensitivity to the radio waves emitted by molecules like CO in their ground state. The ngVLA's design will allow it to observe the CO(1-0) line at virtually any redshift above z > 1.5, which corresponds to the last ~10 billion years—the most active period of cosmic star formation 1 2 .
This capability is a game-changer. By directly targeting the most reliable tracer and circumventing the uncertainties of CO excitation, the ngVLA will provide calibration-quality data on the molecular gas content of galaxies throughout cosmic time.
Furthermore, its order-of-magnitude improvement in sensitivity and resolution over current telescopes will not only allow for the detection of the brightest galaxies but also for a complete census of the more typical, fainter galaxy populations that have so far remained hidden from our view 1 3 . This will move the field from studying a few rare objects to performing robust, statistical analyses of the galaxy population as a whole.
The ngVLA will directly observe the CO(1-0) line, the most reliable tracer of molecular gas mass.
Order-of-magnitude improvement in sensitivity will reveal faint, typical galaxies previously undetectable.
Will observe the most active period of cosmic star formation (last ~10 billion years).
To understand how the ngVLA will unlock these secrets, let's imagine a flagship project: the "ngVLA CO Legacy Survey." This ambitious experiment is designed to make a precision measurement of the dense gas history of the Universe.
The survey will use deep optical and infrared imaging from telescopes like the James Webb Space Telescope (JWST) to identify thousands of galaxies across a wide range of redshifts, from z=1.5 to z=6 and beyond. This ensures a representative sample from different cosmic epochs.
Instead of targeting one galaxy at a time, the ngVLA will perform a "spectral scan" or "line scan" of a deep, blank field. This involves observing the same patch of sky for hundreds of hours across a wide frequency range, collecting the faint radio signals from all the galaxies in that field.
Within this vast dataset, astronomers will search for the characteristic signature of the CO(1-0) emission line. Because the universe is expanding, this line is shifted to a predictable frequency for each galaxy based on its redshift. This allows researchers to not only measure the gas content but also use CO as a redshift beacon, providing highly accurate distances 5 .
For each detected galaxy, the intensity and width of the CO(1-0) line are measured. Using a well-established conversion factor, this line luminosity is then translated into the total mass of molecular hydrogen gas present in the galaxy.
The ngVLA CO Legacy Survey is expected to produce an order-of-magnitude increase in the number of CO detections at high redshift compared to any previous campaign 1 . This vast dataset will allow scientists to construct, for the first time, a precise timeline of how the average molecular gas density in the universe has evolved.
The expected outcome is a curve that rises and falls in sync with the cosmic star formation history, providing direct, observational proof that the availability of fuel is the primary regulator of star formation. By comparing the gas history with the star formation history, astronomers will be able to measure the star formation efficiency—how efficiently galaxies convert their gas into stars—across billions of years, a key test for models of galaxy evolution 4 .
| Redshift (z) | Look-Back Time (Billion Years) | Predicted Gas Density | Cosmic Era |
|---|---|---|---|
| 6+ | >12.5 | Rising | Epoch of Reionization |
| 3 | ~11.5 | At Peak | Cosmic Star Formation Peak |
| 2 | ~10 | Declining | Decline Era |
| 1 | ~8 | Significantly Declined | Recent Universe |
| Tracer Molecule | Transition | What It Probes |
|---|---|---|
| Carbon Monoxide (CO) | J=1-0 | Total molecular gas mass |
| Atomic Carbon ([CΙ]) | 1-0 | Diffuse molecular gas |
| Hydrogen Cyanide (HCN) | J=1-0 | Dense gas in star-forming cores |
To carry out this groundbreaking research, astronomers rely on a sophisticated suite of tools and methods. The following details the key "research reagents" in the hunt for cold gas.
Molecules like HCN, HNC, HCO+ whose emission requires high densities to excite. They probe the inner, dense cores of molecular clouds where stars are actively forming 3 .
An observational technique where a telescope observes a patch of sky across a wide range of frequencies simultaneously, allowing for a blind, comprehensive search for line-emitting galaxies 5 .
A statistical measure showing the number of galaxies with a given CO line luminosity in a volume of the universe. Its evolution reveals changes in the galaxy population's gas content.
The quest to measure the molecular gas budget of the universe is more than an academic exercise; it is a fundamental step toward understanding our cosmic origins. The ngVLA represents a leap in our ability to conduct this census with precision. By directly tracing the primary fuel for star formation across the vast majority of cosmic history, it will answer the pivotal question of what drives the rise and fall of galaxy building.
The ngVLA will do more than just confirm a theory; it will perform a "precision measurement of the dense gas history of the Universe," shedding light on the physical conditions from the epoch of cosmic reionization to the formation of the very first galaxies 2 4 .
In doing so, it will not only fill in a critical missing piece of the cosmic puzzle but also undoubtedly reveal new surprises, challenging our current understanding and opening new frontiers in the exploration of the cold, dark universe from which all visible structure emerged.
Fill critical gaps in our understanding of cosmic evolution
Precisely quantify the molecular gas budget across time
Reveal how gas availability drives star birth