HI in galaxies

The reservoir of cold neutral hydrogen (HI) gas in the interstellar medium of galaxies acts as the intermediary phase between accrediting ionised gas in the intergalactic medium, and the molecular gas from which stars form. It is the HI content that plays a vital role in galaxy formation. The evolutionary history of galaxies can be traced through studies of the HI disk, which is disturbed through e.g tidal interactions and mergers, as it typically extends beyond the stellar disk.

Image credit: NRAO

Many observations of interstellar neutral hydrogen are based on the 21-cm line, originating in the hyperfine splitting of the ground state of the hydrogen atom. The transition can be observed in emission (typically to redshifts of z < 0.2; although this can be extended through stacking), or in absorption to higher redshifts, towards bright radio sources such as active galactic nuclei (AGN).

Intensity observations give information about the amount and the distribution of HI, while we can explore galaxy rotation curves through velocity fields or spectroscopic studies, or search for evidence of galaxy interactions through HI studies. 

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