Gas In GalaxiesEdit

Gas in Galaxies

Gas is the raw material from which stars are born and galaxies grow. In the wide range of environments found in galaxies—from the crowded disks of spirals to the tenuous halos that surround them—gas exists in multiple physical states or phases. Each phase is shaped by gravity, magnetic fields, turbulence, and feedback from stars and black holes, and together these processes regulate how galaxies convert gas into stars over cosmic time. Observationally, gas is detected through a suite of emission and absorption lines and through the continuum emission of dust and molecules that accompany it, allowing astronomers to map its distribution, composition, temperature, and motion across a broad range of wavelengths. The study of gas in galaxies connects to broader questions about galaxy formation, chemical enrichment, and the interaction between galaxies and their larger-scale environments galaxy formation.

Gas in galaxies participates in a dynamic cycle: gas accretes from the intergalactic medium, settles into rotating disks, cools and fragments into dense clouds where stars form, and is eventually ejected back into the halo and intergalactic space by stellar winds, supernovae, and active galactic nuclei. This cycle, sometimes described as a baryon cycle, is central to understanding how galaxies sustain star formation, how their chemical composition evolves, and how their gas reservoirs are replenished or depleted over time. The balance of inflows, outflows, and recycling is influenced by a galaxy’s mass, environment, and internal structure, including features such as bars and spiral arms that organize the gas flow circumgalactic medium interstellar medium.

Phases of Gas in Galaxies

Gas in galaxies exists in several principal phases, each with characteristic temperatures, densities, and tracers.

atomic hydrogen gas, HI: This phase is the vast reservoir of cool, neutral gas in galactic disks and halos. It is most commonly traced by the 21 cm line, a radio emission arising from the hyperfine transition. Mapping HI reveals extended gas disks and features such as warps, streams, and high-velocity clouds that extend beyond the bright stellar disk HI 21 cm line.
molecular gas, H2: Star formation occurs in dense molecular clouds. Molecular hydrogen is the most abundant molecule, but because it is difficult to observe directly at typical cloud temperatures, astronomers often use carbon monoxide as a surrogate tracer, via CO lines. The molecular phase is more tightly correlated with star-forming activity than the atomic phase, reflecting the need for high density and shielding from radiation within giant molecular clouds molecular hydrogen CO.
ionized gas, HII regions and warm ionized medium: Young, hot stars ionize surrounding gas, creating bright HII regions that trace recent star formation. A broader diffuse warm ionized medium fills parts of galactic disks and halos and emits in lines such as H-alpha. This ionized component connects star-forming regions to the larger-scale gas dynamics of the galaxy HII region H-alpha.
hot gas, coronal and halo gas: In the halos of galaxies, gas can reach temperatures of millions of kelvin, emitting in X-rays. This hot gas binds the baryons in massive halos and participates in the cooling and accretion of gas onto the disk. The hot phase is central to understanding the long-term gas supply and the interaction between the galaxy and its environment circumgalactic medium X-ray astronomy.

The proportions and distributions of these phases vary with galaxy type, mass, and environment. In many disk galaxies, HI forms a more extended disk than the stars, while H2 concentrates in the inner regions where star formation is most active. The phase balance is also altered by processes such as gravitational instabilities, bar-driven inflows, and feedback from stellar winds and supernovae.

Dynamics, Accretion, and Recycling

Gas dynamics in galaxies are governed by gravity, rotation, turbulence, and feedback. The gas disk generally rotates with the stellar disk, but it can show noncircular motions associated with bars, spiral structure, warps, and inflows toward the center. Gas exchange with the halo and intergalactic medium occurs through several channels:

cold accretion: Simulations and observations indicate that gas can flow along filaments from the cosmic web into galaxy halos, delivering relatively cool gas that can fuel ongoing star formation, especially in lower-mass systems and at earlier cosmic times. This mode of accretion helps explain how galaxies maintain star formation over long periods cold accretion.
hot mode: In more massive halos, gas often shocks to the virial temperature and forms a hot halo that gradually cools and condenses onto the disk. The balance between heating and cooling in these halos influences the rate at which gas becomes available for star formation hot gas.
recycling and galactic fountains: Gas ejected by feedback processes can later cool and fall back toward the disk in a galactic fountain cycle. This recycling can keep gas in circulation within the galaxy for extended times and redistributes metals produced by stars into the halo and disk galactic fountain.
mergers and interactions: Gravitational interactions with other galaxies can trigger gas inflows, tidal stripping, and enhanced star formation, temporarily reshaping the gas content and distribution of a galaxy galaxy interaction.

Observationally, the kinematics of gas—its rotation curve, velocity dispersion, and noncircular motions—provide crucial constraints on the distribution of mass (baryonic and dark matter) and the efficiency with which gas can collapse to form stars. Studies of the gas content in different environments reveal trends such as lower gas fractions in dense environments and in more massive, quenched galaxies, signaling the influence of external processes and internal feedback on gas supply and retention gas fraction.

Gas and Star Formation

The connection between gas and star formation is central to galaxy evolution. The empirical relationships between gas surface density and star formation rate, often summarized in the Schmidt-Kennicutt law, link the amount of gas to how quickly it forms stars. This link is strongest when focusing on the molecular gas component, which is more directly tied to the sites of star formation within giant molecular clouds. Yet there is ongoing debate about the universality of these relations and the role of local conditions, such as turbulence, magnetic fields, and cloud-scale physics, in regulating star formation efficiency Schmidt-Kennicutt.

Key questions in this area include:

What regulates the efficiency of converting gas into stars, and does this efficiency vary with environment, galaxy mass, or epoch?
How do feedback processes from young stars and active nuclei influence the lifecycle of gas and the suppression or triggering of further star formation?
To what extent does the initial mass function of stars, and its potential variation with metallicity or pressure, impact interpretations of gas observations and inferred star formation histories?

Understanding these issues requires combining high-resolution studies of nearby galaxies with statistical surveys of distant systems, leveraging tracers of gas (HI, H2) and star formation (e.g., UV and infrared indicators) across cosmic time star formation Toomre Q.

Observational Probes of Galactic Gas

Astronomers study gas in galaxies with a broad toolkit:

HI mapping via the 21 cm line: Large surveys and resolved maps reveal the outer gas structure, warps, and kinematics of disks and halos 21 cm line.
Molecular gas via CO lines: CO transitions trace dense molecular regions where stars form; ALMA and other facilities enable high-resolution views of molecular clouds in nearby and distant galaxies CO.
Ionized gas through emission lines: H-alpha and related lines map current star formation and the distribution of HII regions within disks, while line diagnostics reveal gas temperatures, densities, and chemical abundances H-alpha.
Far-infrared and submillimeter tracers: Dust emission and fine-structure lines such as [CII] provide information about the cold neutral and ionized phases and the cooling budget of the interstellar medium CII.
Absorption line studies: Spectroscopy of background sources (stars or quasars) probes gas along the line of sight, revealing column densities, chemical composition, and ionization states, including gas in the circumgalactic medium absorption line.
X-ray observations: Hot halo gas and feedback-driven outflows contribute to the X-ray luminosity of galaxies, shedding light on the energy balance and the heating of the circumgalactic medium X-ray.

Interpreting these probes requires modeling the chemistry, radiative transfer, and excitation conditions in the gas, as well as careful treatment of dust, metallicity, and ionization states. Comparative studies across galaxy types, masses, and environments help isolate the dominant factors that govern gas content and star formation histories galaxy evolution.

Gas in Different Galaxy Types and Environments

Disk galaxies: Rotationally supported gas disks commonly host ongoing star formation, with HI extending beyond the stellar disk and molecular clouds concentrated toward the inner regions. The exact balance of HI, H2, and ionized gas reflects internal structure (bars, spiral arms) and external influences (gas accretion, minor mergers) spiral galaxy.
Dwarf and irregular galaxies: Lower-mass systems often have higher gas fractions and can be more susceptible to environmental effects, such as ram pressure stripping in clusters, which can remove gas and quench star formation over time dwarf galaxy.
Elliptical and lenticular galaxies: Many are gas-poor and show little current star formation, though some retain hot halos or ionized gas from past activity; gas accretion and internal recycling can still occur in some cases early-type galaxy.
Cluster and group environments: Density and interactions in groups and clusters can strip gas from galaxies, alter gas inflows, and influence the global star formation activity through mechanisms such as ram pressure stripping and tidal interactions galaxy cluster.

The Milky Way and Our Local Universe

Within the Milky Way, gas is organized into a complex, multi-phase structure that includes an extended HI disk, rich molecular clouds in the inner Galaxy, and numerous HII regions around young clusters. The local interstellar medium bears the imprint of past supernovae, stellar winds, and the galactic magnetic field, while high-velocity clouds and halo gas probe ongoing exchange with the surrounding environment. Observations of gas in our own galaxy provide a detailed laboratory for understanding the physics of star formation, feedback, and the cycling of material between the disk and halo, which can then be compared to external galaxies to place the Milky Way in a broader context Milky Way.

Theoretical Perspectives and Modeling

Theoretical frameworks for galactic gas combine gravity, hydrodynamics, turbulence, magnetic fields, and chemistry in simulations and semi-analytic models. Key ideas include:

Multi-phase interstellar medium: Gas exists in coexisting phases with different temperatures and densities, continually exchanging mass through cooling, heating, and dynamical processes.
Feedback and regulation: Energy and momentum input from stars and accreting black holes influence gas temperatures, outflow rates, and the ability of gas to cool and form stars.
Chemical enrichment: Stellar nucleosynthesis enriches gas with metals, altering cooling rates and the observable signatures of different gas phases.
Gas accretion and the cosmic web: The supply of fresh gas from the intergalactic medium sustains star formation and drives galaxy growth over time.

These models aim to reproduce observed gas distributions, line emissions, and the star formation histories of galaxies across the mass spectrum, while addressing uncertainties in processes such as the efficiency of cooling, the impact of magnetic fields, and the exact mechanisms of wind loading and recycling galaxy simulations circumgalactic medium.