= Solution
If the gas can cool appreciably below the <virial temperature>, <radiative cooling> removes thermal energy and pressure support. In a <dark-matter halo> the gas then contracts, dissipating more energy as it falls. Efficient condensation requires the <radiative gas cooling time> to be short enough compared with the relevant dynamical or assembly time; merely having an available low-temperature transition does not guarantee that the gas reaches it quickly.
The collisionless <dark matter> cannot lose comparable energy through radiation and remains extended. Gas with appreciable conserved <angular momentum> stops radial collapse when rotation supports it, often forming a disk; lower-angular-momentum gas reaches a more compact central region. Cold dense gas can fragment into self-gravitating clouds and form stars if its gravitational instability overcomes remaining support. <Stellar feedback> subsequently reheats or expels gas and regulates the conversion. \b[Efficient cooling enables central baryonic condensation and star formation; angular momentum and feedback determine the resulting galaxy.]
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