How do galaxies turn gas into stars, regulate their growth, and assemble their mass across cosmic time? At the heart of this question is the baryon cycle, the continual exchange of matter and energy between stars, the interstellar medium (ISM), and the large-scale circumgalactic environment. Gas cools and gravitationally collapses to form stars, while massive stars return energy and newly synthesized elements through radiation, winds, and supernovae, disrupting their natal clouds and recycling material within galaxies and into their surroundings.
MISTY is designed to establish a resolved view of this cycle across the low-metallicity regime, where the physical conditions that govern galaxy formation and evolution differ fundamentally from those in massive, near-solar-metallicity systems. Metal-poor massive stars emit harder ionizing radiation; reduced dust and metal content alter shielding, cooling, and chemistry; and shallow gravitational potentials allow stellar feedback to redistribute or expel gas more efficiently.
MISTY is designed around two defining features of the baryon cycle: it operates across a wide range of spatial scales and links stars, multiphase gas, and dust. Understanding it therefore demands a multi-scale, multi-wavelength approach. The formation and evolution of young star clusters, H II-region structure, cloud dispersal, metal enrichment, and turbulence injection occur on scales of approximately 10–100 pc, while their cumulative effects on gas flows, ionizing-radiation escape, galaxy-wide star formation, and chemical evolution must be measured across entire galaxies. Characterizing stars, multiphase gas, and dust further requires observations across the electromagnetic spectrum.
Design principles
A representative dwarf-galaxy population
MISTY targets nearby dwarf galaxies selected to span the diversity of the low-mass, low-metallicity population. The survey includes systems with a broad range of star-formation activity, gas-phase metallicity, and large-scale environment, from isolated field dwarfs to satellites of massive hosts and interacting dwarf–dwarf systems.
Each of these properties is expected to drive major variations in the low-metallicity baryon cycle. Star-formation activity sets the ionizing-photon budget; metallicity regulates cooling, chemistry, and shielding; and environment influences gas supply and dynamical history. Sampling this parameter space allows MISTY to determine which physical processes are widespread among dwarf galaxies and which depend on particular conditions.
Because the MISTY targets are drawn from a statistically defined nearby population, the resolved measurements will also provide a physical calibration set for interpreting the far larger dwarf-galaxy samples expected from DESI, Rubin, and Euclid in the local Universe.
MISTY targets galaxies within 30 Mpc, where seeing-limited ground-based optical integral-field spectroscopy can resolve physical scales smaller than approximately 150 pc, comparable to the turbulence injection scale from supernovae, and sufficient to isolate individual star-forming complexes. At these distances, near-infrared adaptive optics observations trace scales of 20 pc. Space-based imaging reaches scales of ≤10 pc, sufficient to isolate individual star clusters and resolve their surrounding nebulae. At the same time, the survey is designed to cover each galaxy in its entirety. Full-galaxy maps allow MISTY to link the physics of star formation and feedback on cloud scales to their consequences on galaxy-wide scales.
The baryon cycle couples young stars to multiphase gas and dust, each of which requires different observational tracers. HST and JWST imaging identify young star clusters from their deeply embedded phases, observed in the infrared, to their exposed phases, observed at ultraviolet and optical wavelengths. HST + JWST narrowband Hα and Paα imaging maps ionized nebulae, while JWST imaging of the 3.3 μm PAH feature traces photodissociation regions associated with neutral gas.
Optical integral-field spectroscopy maps ionized gas across entire galaxies. Its medium-spectral-resolution mode, with wide wavelength coverage, provides nebular diagnostics of gas temperature, density, ionization state, and chemical abundances, including direct abundance measurements from faint auroral lines. Its high-spectral-resolution mode resolves ionized-gas motions at approximately 10 km s⁻¹, enabling measurements of turbulence, expanding structures, and outflows.
Survey Science Goals
Feedback before the first supernova
How is star formation regulated by pre-supernova feedback?
The low efficiency with which galaxies convert molecular gas into stars suggests that feedback rapidly disrupts star-forming clouds. In near-solar-metallicity disks, the brief spatial overlap between young stars and their natal gas indicates that cloud dispersal is dominated by pre-supernova feedback. Does this hold, or become more pronounced, at low metallicity, where dust shielding is weaker and radiation fields are harder?
MISTY will measure:
- Cluster ages and embedded-to-exposed timescales from HST + JWST photometry
- Gas temperature, density, and metallicity around young clusters from the IFU data
- Expansion velocities of H II regions (~10 km s−1 resolution)
- The relative contributions of thermal, radiation, and wind pressure to cloud disruption
How does supernova feedback reshape dwarf galaxies?
Supernovae stir the interstellar medium, disrupt star-forming regions, and can drive metal-enriched gas out of the shallow potentials of dwarf galaxies, shaping the gas reservoir available for future star formation. How much mass and metal content actually escapes, and how does that depend on star-formation rate, metallicity, and environment?
MISTY will measure:
- Expanding shells around individual H II regions (~10 km s−1 resolution)
- Galaxy-scale outflows and potential galactic fountains
- Outflow energetics: ionized-gas mass, mass-outflow rate, kinetic power, and mass-loading factor
- A feedback census spanning star-formation rate, metallicity, and large-scale environment
How does ionizing radiation escape and shape the surrounding gas?
Photons that leak from their natal H II regions travel hundreds of parsecs through a porous interstellar medium, altering the thermal, chemical, and ionization state of gas far from where they were produced. Radiation that escapes galaxies altogether is thought to have played a central role in cosmic reionization. What sets the escape fraction at low metallicity?
MISTY will measure:
- Ionizing-photon output of thousands of young clusters from HST + JWST imaging
- Ionizing photons absorbed by the surrounding gas from narrowband Hα and Paα imaging
- Local escape fractions as a function of cluster and gas properties
- Diffuse ionized gas across entire galaxies from deep IFU data cubes
How are metals mixed and recycled in low-mass galaxies?
Chemical abundance patterns record the production of elements in stars, their mixing through the interstellar medium, their ejection in outflows, and the accretion of pristine gas. Resolving these patterns has been limited by the faintness of the auroral lines needed for direct, temperature-sensitive abundances; MISTY’s depth makes those lines measurable across large areas of each galaxy.
MISTY will measure:
- Auroral lines from multiple ionized-gas species across 3500–10,000 Å
- Oxygen, nitrogen, sulfur, and argon abundances on approximately 100–150 pc scales
- The spatial scales over which newly synthesized elements are redistributed
- Signatures of inflow, outflow, and local mixing in the abundance field
How do PAHs form, evolve, and survive at low metallicity?
Polycyclic aromatic hydrocarbons (PAHs) and other grains dominate photoelectric heating in neutral gas: far-ultraviolet photons eject electrons from their surfaces, and those electrons thermalize and heat the surrounding gas, setting the balance between the cold and warm neutral medium. PAHs may also contribute to molecular-hydrogen formation on grain surfaces. Their mass fraction falls sharply with metallicity, yet PAH emission is still detected in some metal-poor galaxies.
MISTY will measure:
- 3.3 μm PAH emission from narrowband JWST imaging—the first systematic sample, as the feature has so far been measured in only a handful of dwarf galaxies
- PAH emission relative to the locations and properties of young massive clusters and the structure of neutral and ionized gas