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Intensity mapping is the measurement of some signal over the celestial sphere, producing an intensity map. A frequent goal is determination of cosmic matter density, in particular, the density of matter through time and space, which helps lay out the history of the universe and its parameters. Common is line intensity mapping (sometimes abbreviated LIM), surveys mapping the intensity of one or more spectral lines (intensity mapping surveys), including 21-cm experiments, and [CII] mapping, as well as H-alpha and Lyman-alpha forest surveys, both of which are examples of hydrogen intensity mapping.
Molecular lines are also of interest, such as carbon monoxide lines, e.g., to map out potential star-forming regions, both within the Milky Way and star formation over cosmological time and space. The 21-cm line, [C II], Hα, and Lyα are also star formation signs that can be viewed at a distance, e.g., to detect the presence of distant star-forming galaxies.
The intent of intensity mapping and some other surveys is coverage of all or a significant portion of the sky. Issues include the field of view, scan speed and the scan pattern for efficient coverage and for scheduling time. In some cases, the Earth's rotation is used, e.g., for surveys using cylindrical telescopes. For space-borne surveys, rotation of the satellite might be used.
Intensity maps may be of emissions made at a range of redshifts (e.g., a tracer indicating star formation) or may be from an early emission that is influenced during its travel history through redshifts, such as studies of the CMB or 21-cm surveys. In the latter case, anisotropy is analyzed with interest to see whether it was in the original source (e.g., quantum fluctuations in the CMB) and if not, whether it is an indication of large scale structure at substantial redshifts, or is the result of some phenomena within the Milky Way, and if it is distant, whether it is due to some particular object (e.g., a known galaxy cluster) or can be ascribed to unresolved distant objects, e.g., faint galaxies. Thus such studies contribute to all these research areas. The influences on the signal that are not of interest are sometimes termed interlopers. Cross-correlations between intensity maps and other types of surveys, e.g., those covering the large scale structures and/or distant objects such as quasars are means of performing such analysis.
Intensity mapping projects covering a substantial range of redshifts require the ability to recognize the signal (while taking into account background and foreground signals in the same range), and for a very large redshift range, inevitably requires more than one kind of instrument or telescope.