Line intensity mapping

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Figure 1. Left panel shows the main Epochs in the evolution of large scale structure in the Universe from Recombination (~370,000 years after the Big Bang) til Today (~13,6 Billion years later). Right panel shows the gap in measurements of the large scale sctructure in the Universe that can only be filled by Line Intensity Mapping Surveys.

Line Intensity Mapping

Line Intensity Mapping (LIM) is expected to revolutionise astrophysics and cosmology during the Epoch of Reionization due to its blind surveys with a combination of large volume cover and redshift precision.

A space mission over the total microwave band could be used to tackle some of the main mysteries in the Universe: the origin of galaxies and large-scale structure and how well does the standard model of cosmology hold in the early Universe.

The microwave band contains emission from strong galaxy spectral lines (such as CO and [CII]), diagnostic spectral lines (FIR lines such as OII, and OIII) and continuum infrared background light (the CIB).

The proposed mission would make it possible to separate the different signals from the maps with precision and accuracy superior to any previous ground-based mission. A unique advantage of a sky survey is that the target emission can be observed over a wide spectral range that will enable to statistically recover the cosmic web over a large fraction of the Universe timeline.

Moreover, this mission would open a window for precision cosmology at the EoR and it will make it possible to determine how the properties of the first galaxies are influenced by they position relatively to the large-scale structure.

Figure 1: Model-independent constraints on the shape of the cosmic expansion history, E(z) =H(z)=H0 (normalised to CDM). For details, see [1]. This illustration is calculated for a LIM experiment measuring CO(1-0) over a 1000 deg2 patch for 10,000 hours [1].

Estimated instrument sensitivities for the deep and full-sky surveys (purple dashed and green dash, respectively), versus models predictions for several lines of interest, with the cluster (orange dot-dash), shot (yellow dotted), and total power (blue) shown separately. A full-sky survey with a space-based instrument would be expected to place tight power spectrum constraints on CO and [CII] emission at z~3 (left panels), over a large range of spatial scales from k= 0.01-10 h/Mpc. A deeper survey on a smaller region of sky (i.e., fsky = 0.01) could achieve a high signal-to-noise detection of the [CII] EoR power spectrum (lower left panel), as well as several fainter line species like the higher-J (Jupper 6) transitions of CO. Such an instrument would also be capable of measuring the [OIII] 88 m line (upper right).


Until today, constraints on the parameters of ΛCDM have come from two main sources, the cosmic microwave background (CMB) formed when the Universe was only 380,000 years (it characterizes structures in the early Universe) and galaxy surveys which probe LSS at late times (z<1 when the Universe was approximately 6-13.7 Billion years). There is a huge time frame in between these tracers, where the picture of our Universe changed drastically, but which has not yet been fully explored for cosmology proposes.

Measurements with Line Intensity Mapping (LIM) across this uncharted volume have the potential to strongly improve the precision of measurements of several cosmological parameters as they hold unique qualitative advantages over the more established cosmology observables. Synergies between line intensity maps of several spectral lines will make it possible to reconstruct a set of independent maps of LSS in the Universe. Ultimately, the combination of experiments targeting different tracers of LSS will be most effective in breaking different degeneracies between the cosmological parameters.

LIM has the potential to test and improving ΛCDM constraints by shedding light on the nature of dark matter, dark energy, and what drives early-Universe inflation. Intensity maps of CO rotational lines at medium redshifts and CII emission towards reionization can fill the gap in measurements of the cosmic expansion history [1], which may be crucial for understanding the growing tension with measurements of the Hubble constant and determine whether it involves time-dependent dark energy. A large number of modes can yield constraints on primordial non-Gaussianity at the level of sigma (fNL <1) [2,3], which is the target threshold for discerning between single and multi-field models of inflation. Mono-energetic dark matter decay can be tested using its correlation with the mass distribution inferred from the cross-correlation of spectral-intensity maps with galaxy or weak-lensing surveys [4]. And probing beyond ΛCDM, powerful constraints can be placed on the number of effective relativistic degrees of freedom and the sum of neutrino masses [5].

Figure 2: Spectral lines detectable by a sky survey in submillimiter to high-frequency radio waves. Shown above are model predictions for the line intensities and the relative sensitivities of the full-sky and "deep" (400 deg2) surveys. The full survey will be used to perform statistical tomography measurements and constrain cosmological models. The deep survey will enable direct imaging of the emission sources, cross-correlations with other surveys, precise foreground subtraction and pinpointing the localisation of galaxy (proto)clusters well into the Epoch of Reionization. The shown signal strengths adopt line luminosities scaled from the IR luminosity based on observational relations. Current constraints on these line ratios are uncertain by up to one order of magnitude.

Galaxies and Large Scale Structure

When the Universe was just 0.1-1 billion years, the first stars emitted high energy radiation which heated and reionized the gas in the intergalactic medium. At the same time, these stars converted a fraction of the primordial hydrogen and helium into heavier atoms (metals), which then formed molecules and dust particles that emit radiation at (sub) mm wavelength through thermal and line emission. Their detection out to high redshift, across large patches of sky, in hundreds of frequency bands, opens the path to a census of baryons in these various forms across cosmic time, and hence to galaxy evolution and star formation history. Spatial fluctuations in stellar emission trace the cosmic web at high redshift, and thus structures over a large fraction of the Hubble volume.

The study of the high-redshift Universe as described above combines the detection of three types of radiation: i) emission from compact sources (continuum and lines) at high redshift; ii) spectral line emission from atoms and molecules in unresolved galaxies with Line Intensity Mapping (LIM) (mapping large scale structure and its cosmic evolution); diffuse dust emission from the background of unresolved galaxies constituting the Cosmic Infrared Background (CIB).

i) Compact sources: Detection of individual high redshift objects requires the best possible angular resolution (to reach a high sensitivity); hence, it is associated with target surveys or restricted to small volumes. Several current and future traditional galaxy surveys aim to detect and resolve bright galaxies well into the Epoch of Reionization (time of formation of the first galaxies; when the Universe was 0.3 to 1 billion years of age). These missions will probe the different gas phases of individual bright sources, therefore, constraining their main properties. However, high redshift galaxy surveys will usually miss the low luminosity galaxies characteristic of the early Universe and blind surveys will be restricted to small volumes.

ii) Line Intensity Mapping: A key advantage of Line Intensity Mapping is that even with a resolution of a few arcminutes a survey with a sensitivity of the order of 0.03 μK would be able to detect the overall emission from a set of sub-millimetre spectral lines from high-z galaxies. With such a low spatial resolution and a frequency cover in the 30-1000GHz range, such a survey could map the cosmic web over a large fraction of the sky and over most of its timeline (starting when the Universe was just 500 Myr old; a redshift of z~10). The brightest voxels in a three-dimensional map pinpoint the location of many high-redshift protoclusters and low-redshift galaxy clusters.

Our main target lines are CII (at a redshift of z>5) to trace star formation rate during the EoR and the four first lines of the CO ladder which traces molecular gas the main fuel for star formation (see Fig. 1). We aim to observe CII at 200 to 400 GHz with a sensitivity of 0.03 μK and CO between 50 and 200 GHz with sensitivity 0.2μK. We require detecting at least two CO lines at each redshift bin together with CII so that we can separate the different lines.

By detecting several CO lines originating from the same emitting structures, we can measure the average CO ladder and use it to constrain the first galaxies interstellar medium (temperature and density distribution). As the level of these line emissions is uncertain, we require that the space mission have the capability, in addition to the full sky survey, to map a deep patch of sky of a few hundred square degrees.

Figure 3: Spectral energy distribution of the extragalactic background light from [7]. The cosmic infrared background in red is the second brightest background after the CMB (in grey). The CIB is a major component of the extragalactic background light, with a spectrum that spans wavelengths from the millimeter regime down to the mid-infrared [8,9].

iii) The Cosmic Infrared Background: The CIB is sourced by emission from young stellar populations which is absorbed by the surrounding dust and re-emitted at longer wavelengths; quantifying the infrared output from a galaxy will make it possible to recover galaxies intrinsic energy output.

Measurements of both the mean intensity and the spatial fluctuations of the CIB can yield important scientific output. Reaching high accuracy in absolute measurements is particularly challenging. Current constraints from the combination of absolute photometry from FIRAS with relative photometry from Planck are only at the 10% level [6]. A future space mission with an absolute spectrometer will allow more than an order-of-magnitude improvement in precision and enable the detection of extended intergalactic dust emission (or emission from more exotic sources such as dark-matter decay [4], by separating out the signal coming from known galaxy populations.

The CIB fluctuations signal can be fully exploited in cross-correlation with either spectroscopic galaxy surveys at low redshifts or line-intensity maps (e.g., of CO and [CII] emission) at medium and high redshifts. Tomographic CIB maps that trace galaxy evolution across time can be achieved with moderate angular resolution and wide frequency coverage (∼5 arcmin over 100 to 1000 GHz). Such an observation will make it possible to determine the timeline for the buildup of the CIB and to identify its sources.


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