Intensity Mapping

In cosmology, Intensity Mapping (IM) is the name given to mapping the intensity fluctuations of a given tracer of mass on large angular scales.

‌Proposed tracers include the 21cm line of atomic hydrogen and the rotational lines of CO. Rather than detecting individual galaxies, which takes a long time and requires very high sensitivities, IM is based on mapping the large-scale fluctuations, using the integrated emission from many galaxies rather than the individual galaxies by themselves.

This is much more efficient and means that a modest single-dish telescope can be used. Using a well-known emission line also gives the added advantage of knowing the redshift automatically, since the frequency/wavelength gives the redshift directly. A number of IM experiments have been proposed over recent years. You can learn more about them below:

  • BINGO
    Forecasted power spectrum measurements from the complete BINGO survey (1 year on-sky total integration).
    Forecasted power spectrum measurements from the complete BINGO survey (1 year on-sky total integration). If systematics are controlled, including foreground removal and calibration, then BINGO will provide very accurate measurements of the HI power spectrum across a range of scales and redshifts. It also has the sensitivity to detect the BAO wiggles at z=0.1-0.5 (shown in the inset panel), and therefore constrain models of dark energy.

    The Baryon acoustic oscillations in Integrated Neutral Gas Observations (BINGO) experiment is a project to build a special purpose radio telescope to map redshifted neutral hydrogen emission between z = 0.13 and 0.48. It is an international project with collaborators in Brazil, Saudi Arabia, Switzerland, United Kingdom and Uruguay. It is the only radio telescope that aims at mapping neutral gas, as traced by the 21cm line, on large angular scales and at redshift z~0.3. We call this method HI intensity mapping.

    Using the Baryon Acoustic Oscillations (BAOs) as a standard ruler allows us to measure the expansion of the universe as a function of redshift and so, to constrain the properties of dark energy. The telescope will have no moving parts and consist of a primary mirror of about 40 m diameter and a secondary a bit smaller. It will have around 50 "pixels" (detectors). With this design, the accuracy on the measurement on the acoustic scale will be ~2% for one year of integration time, by performing a drift scan survey of 15 deg x 200 deg. This will be achieved by employing a static 40m dual-dish radio telescope with a resolution of 40 arcmin at 1 GHz.

    The plan is to build the telescope in a disused open-caste gold mine in northern Uruguay where radio interference is minimal. The original BINGO concept is discussed in the paper by Battye et al., 2013 MNRAS, 434, 1239 (Cornell University Library). You can also view pages with more recent information on the BINGO telescope and simulating single-dish intensity mapping experiments on the same site. BINGO is also a pathfinder for using the SKA as an instrument for ultra-deep large-scale IM surveys, particularly for understanding systematic errors and the data analysis challenges for extracting such small cosmological signals. 

  • COMAP

    The Owens Valley Radio Observatory, near Bishop, California, U.S.A. The observatory, run by Caltech, contains several radio astronomy facilities including several ex-CARMA 10.4 m dishes, seen above. One of these dishes is being used for the COMAP phase 1 path-finder instrument.

    COMAP is a CO intensity mapping experiment based at the Owens Valley Observatory (California, U.S.A.) and is led by a team at Caltech, but includes collaborators from JPL, Stanford, Maryland, Miami, Oslo, and Manchester. COMAP is a multi-phase project, with an initial pathfinder 30 GHz instrument installed on a re-commissioned 10.4-m CARMA telescope. This will probe the CO J=0-1 line integrated over many galaxies for a redshift range of 2 < z < 3. The aim is to make the first high S/N detection of the CO power spectrum to constrain star-formation and galaxy models. The second phase of the COMAP project will attempt to probe large-scale structure associated with CO emission during the epoch of reionisation (z > 6) using a larger array at frequencies ~15 GHz.

     The COMAP pathfinder will provide new constraints on galaxy formation theories during the epoch of galaxy formation, which is close to the peak in cosmic star formation. Critically, as the COMAP pathfinder is an intensity mapping experiment, it will give a unique insight into the properties of lower mass galaxies that cannot be realistically measured using traditional galaxy survey techniques. The pathfinder phase is currently fully funded by a NSF award and contributions from the collaborating institutes. 

  • SKA

    The Square Kilometre Array (SKA) has also been proposed to carry out an IM survey. If SKA-MID dishes can be used as total-power devices (rather than working as an interferometer), it has been shown that it would be hugely powerful, comparable to the Euclid satellite mission (Bull et al. 2015 ). However, to achieve this, one will need to have unprecedented control of systematic errors such as 1/f noise, sidelobe contamination, cross-polarization, interference etc. We are developing detailed end-to-end simulations to quantify to what level systematics will be an issue (as seen discussed in the following paper entitled 'Simulated Effects of 1/f Noise on an SKA Intensity Mapping Survey'.) The BINGO experiment will be an important pathfinder instrument for the SKA in determining which systematic errors are a limiting factor.

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