Galaxies are gravitationally bound systems of stars, gas, dust, metals, and dark matter. Understanding the physical properties of the interstellar medium (ISM) is therefore crucial, as they regulate star formation and ultimately drive galaxy evolution.

The ISM, composed of gas, dust, cosmic rays, and magnetic fields, is a dynamic environment that regulates galaxy evolution through a continuous cycle. (A) Diffuse, warm gas cools to form the cold neutral medium and (B) further condenses into molecular clouds dominated by H₂. During this phase, dust grains enable cooling, shield the gas from radiation, and catalyze molecule formation. (C) Stars form in the cold, dense cores of molecular clouds and (D) synthesize heavy elements through nuclear fusion over their lifetimes. (E) Stellar winds and supernova explosions return metals and dust to the ISM, enriching the gas from which subsequent generations of stars form and driving the chemical evolution of galaxies.

Galaxies in the observable universe span a wide range of sizes, luminosities, and physical properties, including stellar, gas, and dust masses, as well as metallicity. Based on their morphology, galaxies are commonly classified as early-type (elliptical), late-type (spiral), or irregular systems. The Milky Way (MW), which hosts our Solar System, is a massive, metal-rich spiral galaxy with prominent spiral arms and a central bar. Late-type galaxies typically have high stellar masses, while their ISM accounts for only ~10-20% of the total baryonic mass. They are generally metal-rich (Z ~ 0.5–1 Z_☉) and contain dust at roughly 1% of the gas mass, corresponding to a dust-to-gas ratio (DGR) of ~0.01. Their relatively high molecular gas content sustains ongoing star formation. In contrast, dwarf irregular galaxies have lower stellar masses but much higher ISM fractions, reaching up to ~50% of the baryonic mass. The Small Magellanic Cloud (SMC), a satellite of the MW, is a well-studied example of this class. Such systems are typically metal-poor (Z < 0.5 Z_☉), exhibit low molecular gas content, and contain little dust, with DGR values below 0.01.