X-factor (astrophysics)
The X-factor, also sometimes referred to as the CO-to-H2 conversion factor, is a proportionality constant used in astrophysics to estimate the molecular hydrogen (H2) column density in interstellar clouds based on the observed carbon monoxide (CO) emission. Molecular hydrogen is the most abundant molecule in the universe, and a critical component of star formation, but it is notoriously difficult to directly detect in cold molecular clouds at typical interstellar temperatures because it lacks a permanent dipole moment and its lowest rotational transitions are energetically difficult to excite.
Carbon monoxide, on the other hand, is the second most abundant molecule (after H2) in molecular clouds. Its rotational transitions are more easily excited at typical interstellar temperatures and can be readily observed at millimeter wavelengths. However, CO is generally much less abundant than H2.
The X-factor, typically denoted as XCO or X, relates the integrated CO intensity (typically in units of K km/s) to the column density of molecular hydrogen (typically in units of cm-2). The relationship is expressed as:
N(H2) = XCO * ICO
Where:
- N(H2) is the column density of molecular hydrogen
- XCO is the X-factor
- ICO is the integrated CO intensity
The X-factor is not a universal constant, and its value can vary significantly depending on the physical conditions of the molecular cloud, such as density, temperature, metallicity, and radiation field. Deriving an accurate value for the X-factor is crucial for accurately estimating the molecular gas content of galaxies and understanding star formation processes.
Several methods are used to estimate the X-factor, including:
- Virial mass method: Comparing the virial mass of a molecular cloud (estimated from its size and velocity dispersion) to its mass estimated from CO observations.
- Dust emission/extinction: Correlating CO emission with dust emission or extinction, which are independent tracers of total gas column density.
- Gamma-ray emission: Using gamma-ray observations to trace cosmic ray interactions with molecular gas.
The typical value for XCO in the Milky Way disk is around 2 x 10^20 cm-2 (K km/s)-1. However, in environments with low metallicity or strong radiation fields, the X-factor can be significantly higher or lower, making it a significant source of uncertainty in estimating molecular gas masses.