HI Intermediate Velocity Clouds

Updated 24 October 2025

HI Intermediate Velocity Clouds are neutral hydrogen clouds with LSR velocities between 20–100 km/s, crucial for recycling gas between the Galactic disk and halo.
Systematic HI 21-cm surveys, including GALFA-HI and HI4PI, have revealed detailed properties such as low kinetic temperatures, typical masses around 700 M☉, and a sheetlike spatial distribution.
IVCs arise from both recycled disk material via the Galactic fountain and extragalactic accretion, influencing the ISM’s multiphase structure and star formation fuel cycle.

HI intermediate velocity clouds (IVCs) are a distinct population of neutral hydrogen (HI) clouds in the Milky Way’s halo—characterized primarily by their line-of-sight velocities, which are anomalous with respect to galactic rotation but lower in magnitude than the more extreme high-velocity clouds (HVCs). IVCs are widely observed across the sky, particularly at intermediate Galactic latitudes, and play a crucial role in the disk–halo interface, star-formation fuel cycle, multiphase structure of the interstellar medium (ISM), and the transfer of metals and angular momentum between the disk and halo. Recent advances in surveys and multiwavelength analyses have enabled systematic studies of their physical properties, composition, distribution, kinematic behavior, dynamical evolution, and the processes that govern their formation and survival.

1. Definition, Detection, and Demographics

IVCs are usually identified by their Local Standard of Rest (LSR) velocities, which fall in the range $|v_{\rm LSR}| \sim 20{\text{--}}100$ km/s, with precise boundaries varying across studies (Lehner et al., 2022, Begum et al., 2010, Röhser et al., 2016). IVCs are separated from low-velocity HI clouds (LVCs; $|v_{\rm LSR}| \lesssim 20$ km/s) and from HVCs ( $|v_{\rm LSR}| \gtrsim 90$ km/s; note that slight differences in boundaries are present in the literature).

Systematic HI 21-cm surveys provide the foundation for identifying and cataloguing IVCs. Exemplary among these are the GALFA-HI survey (Begum et al., 2010)—with a velocity resolution of 0.18 km/s and 3.5′ spatial resolution—Parkes GASS (Ford et al., 2010), Effelsberg–Bonn HI Survey (EBHIS), and HI4PI. These surveys reveal the prevalence of compact, isolated clouds with angular sizes down to $\sim$ 5 arcmin, HI column densities around $5 \times 10^{18}$ cm $^{-2}$ , and peak brightnesses typically below 1 K (Begum et al., 2010, Ford et al., 2010).

The all-sky census in (Röhser et al., 2016) identifies 239 candidate molecular IVCs (MIVCs) at high latitudes ( $|b|>20^\circ$ ) by decomposing HI spectra into velocity components and correlating with FIR emission, leveraging methodologies where each sightline’s HI emission is modeled as a sum of Gaussian components (Panopoulou et al., 2020).

The population is not uniform: the sky coverage and abundance of IVCs, and especially MIVCs, differs between hemispheres (e.g., 184 MIVC $^-$ vs. 3 MIVC $^+$ in the northern hemisphere (Röhser et al., 2016)) and reflects dynamical processes at play in the Galactic environment.

2. Physical Properties, Ionization, and Chemical Composition

IVCs are predominantly cold or lukewarm neutral HI clouds, but multi-component line profiles are common (Begum et al., 2010, Smoker et al., 2015). Line widths (FWHM) for compact IVCs can be as narrow as 1–8 km/s, with upper limits on kinetic temperatures typically around 300 K for the coldest components. In some cases, double-Gaussian fits reveal co-spatial cold “cores” ( $T_k \sim 100-300$  K) embedded within broader, warmer envelopes.

Individual cloud physical parameters, as measured for larger disk–halo IVCs, include radius $\sim$ 28 pc, HI mass $\sim$ 700 $M_{\odot}$ , and central densities $n_{\rm HI} \sim 0.1-1$  cm $^{-3}$ (Ford et al., 2010, Lehner et al., 2022). The cloud–cloud velocity dispersion is $\sim$ 16 km/s, which is notably too small to explain their inferred vertical scale heights by turbulent support alone (Ford et al., 2010).

Chemical composition is diverse. While absorption-line studies previously indicated near-solar metallicities for many IVCs (in contrast to HVCs, which often exhibit 0.1–0.3 solar metallicities), sub-mm dust emission analyses across the sky have revealed that a substantial fraction of IVCs ( $\sim$ 20%) are dust-poor and likely metal-poor, with dust-to-HI ratios as low as $\sim$ 0.1–0.3 of the local ISM (Hayakawa et al., 2022). This heterogeneity hints at a dual origin: some IVCs are recycled disk material (Galactic fountain), while others are infalling, low-metallicity gas possibly supplied from extragalactic sources or the Magellanic system (Fukui et al., 2017, Hayakawa et al., 2022).

Elemental abundance ratios, notably Ca II/Na I and Ca II/O I from absorption-line spectroscopy, show significant variation ( $-0.45$ to $+1.5$  dex in Ca II/Na I; $0.2$–$1.5$ dex below solar for Ca II/O I) (Smoker et al., 2015). These trends, along with systematics in Ca II/HI, support the importance of depletion onto grains and ionization corrections in the IVC regime.

3. Kinematic Behavior, Dynamics, and Spatial Distribution

IVCs exhibit kinematics that deviate from pure Galactic rotation by $\sim$ 20–30 km/s (Begum et al., 2010). The spatial distribution is best described as oblate/sheetlike with a strong concentration towards the Galactic plane; vertical scale heights of $h = 1.0 \pm 0.3$ kpc have been derived (Lehner et al., 2022), implying they are confined to the thick disk or disk–halo interface ( $|z| \lesssim 1.5$ kpc). This contrasts with HVCs, whose distribution extends to $h = 2.8 \pm 0.3$ kpc and beyond.

The covering factor of IVCs is high ( $f_c = 0.90 \pm 0.04$ at $|z| \gtrsim 1.5$ kpc (Lehner et al., 2022)). Kinematic modeling demonstrates that IVCs cannot be understood as a monolithic velocity class: velocity–distance analysis (Marasco et al., 2022) reveals that a two-component inflow/outflow model (both with $v_{\phi} \sim v_{\odot}$ and $|v_z| = 50$ –100 km/s) best describes the data, where inflows are diffuse and widespread in the halo, while outflows are collimated in biconical regions.

Within the Fermi Bubbles, a population of $\sim$ 200 entrained HI clouds (including velocity extrema characteristic of the IVC/HVC regime) show acceleration with latitude: outflow speeds rise from $150$–$200$ km/s near the Galactic center to $330$ km/s at $z \sim 2.5$ –$3.5$ kpc, with cloud lifetimes estimated at 4–10 Myr—far longer than basic cloud-crushing models predict (Lockman et al., 2019, Bordoloi et al., 29 Apr 2025).

4. Formation Pathways, Evolution, and ISM Role

Multiple formation mechanisms are implicated for IVCs:

Galactic Fountain: Stellar feedback (supernovae, stellar winds) expels gas from the disk, which cools, condenses, and eventually rains back to the disk. Observational support includes near-solar metallicities in some IVCs, disk-like rotation, high covering factors near the disk, and the presence of both diffuse inflows and collimated outflows (Lehner et al., 2022, Marasco et al., 2022, Röhser et al., 2016).
Accretion of Extragalactic Gas: Dust-poor, metal-poor IVCs, such as IVC 86-36 with metallicity $\lesssim 0.2$ solar (Fukui et al., 2017), likely represent infalling gas from the halo or Magellanic system (Hayakawa et al., 2022).
Turbulent Compression and Shock-Induced Condensation: MHD simulations and observations of filamentary structures, even in the absence of strong gravitational confinement, reveal shocks and supersonic turbulence ( $M_s \sim 3$ ) are capable of organizing the warm neutral medium (WNM) into velocity-coherent filaments (Liu et al., 15 Feb 2025). In molecular IVCs, ram pressure during descent toward the disk triggers rapid cooling, grain-surface H $_2$ formation, and the transition to dense, shielded molecular clumps (Röhser et al., 2014, Röhser et al., 2016).

IVCs contribute to the structure and thermal balance of the multiphase ISM as small-scale, dense pockets (bridging the regime between the classic CNM and diffuse clouds) (Begum et al., 2010, Röhser et al., 2016). Furthermore, the population at the disk–halo interface serves as both a reservoir and recycling channel for fueling star formation; estimates suggest the combined atomic and molecular IVC inflow could account for a substantial fraction of the mass required to sustain the global Galactic star formation rate (Röhser et al., 2016).

5. Molecular Content, Small-Scale Structure, and Cloud Evolution

The transformation from atomic to molecular gas in IVCs is a dynamic, pressure-driven process. Observations show a pronounced transition as environmental pressure increases (typically from ram pressure as clouds descend towards the disk), allowing H $_2$ to form on dust grains. FIR-bright IVCs, with clear CO and dust signatures, evidence large H $_2$ columns ( $N_{\rm H_2}$ ) and dust optical depths; FIR-dim IVCs track the earlier, atomic-dominated stage of the transition (Röhser et al., 2014). Detailed multiwavelength campaigns reveal significant variation in the X $_{\rm CO}$ conversion factor within individual clouds, but when averaged over the cloud, the values approach the canonical Milky Way disk average (Röhser et al., 2016).

Not every IVC is molecular: a subset of candidate IVMCs (IVMCs, Editor's term) show no detectable OH or CO emission, indicating low H $_2$ abundances or highly clumpy molecular structure below the detection limit of single-dish pointings (Smith et al., 2023). Known molecular IVCs are rare and often exhibit localized, high column density clumps embedded in diffuse HI envelopes.

Three-dimensional mapping techniques reveal that IVCs contribute to the line-of-sight complexity of the ISM, often introducing additional dust and HI layers at high latitudes, affecting both the spectral energy distribution and polarization properties relevant for CMB studies (Panopoulou et al., 2020).

6. Feedback, Galactic Environment, and Evolutionary Context

Large-scale environmental factors influence both the distribution and fate of IVCs. Surveyed quadrants differ: the first Galactic quadrant (QI), located at the tip of the Galactic bar, exhibits three times as many disk–halo HI clouds as the symmetric fourth quadrant (QIV), as well as a scale height twice as large. This is attributed to enhanced star formation and supergiant shell (superbubble) activity in QI, reinforcing the intimate link between cloud population and feedback-driven dynamics (Ford et al., 2010).

Clouds entrained in outflows—from the Fermi Bubbles or star-forming disks (e.g., in external galaxies)—show velocities, morphologies, and lifetimes governed by the balance of wind ram pressure, radiative cooling, magnetic field support, and hydrodynamic instabilities. Survival mechanisms (magnetic pressure, cooling, and continual fragment reformation) are still under investigation, but observations of cloud acceleration and resilience challenge the minimal survival times predicted by collisional disruption scenarios (Lockman et al., 2019, Bordoloi et al., 29 Apr 2025).

7. Theoretical Challenges, Observational Prospects, and Open Problems

Modeling of IVCs and their analogs in cosmological simulations (e.g., TNG50) demonstrates large variance among galaxies in cloud masses ( $\sim$ 10^6M_\odot $), sizes ($ \sim$10 kpc—though smaller clouds may be unresolved or undercounted at current resolution), metallicity (from sub-solar to super-solar), and kinematic substructure. Clouds are thermally under-pressurized with respect to their environment but buffered by magnetic fields (with magnetic:thermal $\beta \sim 1$). Many clouds originate in close association with satellite galaxies, suggesting an origin via ram-pressure stripping or tidal interaction; in situ formation from hot phase cooling is also possible (Ramesh et al., 2023). Simulated line-of-sight velocity distributions and clustering properties are broadly consistent with observations but also predict a large population of low-velocity (difficult-to-detect) clouds.

Remaining controversies and research challenges include:

The relative contributions of the Galactic fountain and external accretion to the total IVC population (Hayakawa et al., 2022).
The precise pathways of atomic-to-molecular transition in low-density environments.
The role and fate of magnetic fields in cloud survival (Ramesh et al., 2023).
The ultimate evolution of clouds in outflows and the fate of molecular fuel for star formation in the Milky Way and other galaxies (Schneider et al., 24 Apr 2024, Gim et al., 2021).
The prevalence and impact of dust/metal-poor IVCs on the global ISM metal budget (Hayakawa et al., 2022).

Advances in high-resolution, high-sensitivity radio surveys and three-dimensional ISM mapping, along with sophisticated multiphase numerical modeling, are expected to refine the census of IVCs, clarify their internal dynamics and chemical diversity, and illuminate their role in the Galaxy’s baryon cycle.