# Has the gaseous cosmic web ever been directly observed?

For a couple of decades now we've had maps of the cosmic web as traced by galaxy positions from redshift surveys like the 2dfgrs:

Are there any similar maps which rely on gaseous tracers? I guess this should be impossible with current observatories in neutral hydrogen (21-cm emission) because it is too faint, and ionized gas in emission is also difficult in this regime, I think? There's always the Ly-$\alpha$ forest in absorption, but is there a sufficiently dense population of background sources to make maps of specific sky patches, or are only statistics of the web based on relatively isolated sight lines possible?

In the end, what is the best map of the cosmic web directly tracing gas (emission or absorption) currently available?

• The Caltech Cosmic Web Imager has made observations in specific areas, but I don't think they're close to producing a map. caltech.edu/news/… – D. Halsey Aug 26 '18 at 21:56

I think the Lyman-alpha forest is the best tool for this. The Lyman-$\alpha$ forest is a series of absorption lines in the spectra of high redshfit ($5>z>2$) quasars:
where the top panel is a standard low redshift quasar spectrum, and the bottom panel is a Lyman-$\alpha$ forest quasar. The absorption lines are caused by the light from the quasar passing through gas clouds of neutral hydrogen in the IGM, and the lines contain information about properties such as temperature, size and redshift of the gas clouds, which means these spectral lines contain information about the distribution of gas in the high redshift universe.
In general, the individual lines of sight you get from Ly$\alpha$ forest observations are far enough apart that we mainly do statistics such as correlation functions and power spectra, but recently a study has been done by KG Lee et al using a high density set of observations to reconstruct a tomographic map of the IGM gas, which I think is exactly what you're after: