Lyman-continuum (LyC) emission is everything blueward of 912 Angstroms (so it includes extreme UV photons, X-rays and gamma rays). There are many low-redshift astrophysical plasmas such as the diffuse halo gas around galaxies that are hot enough ($\sim10^6$ K) that we expect them to produce copious UV/X-ray emission (either continuum or metal line emission). On the other hand, neutral hydrogen can absorb any LyC photon (no matter how energetic, as long as the wavelength is $<912$ Angstroms) in order to become ionized.

Since there is tons of neutral hydrogen in the interstellar medium between Earth and other galaxies (and even between us and gas in our own Milky Way's hot corona), I'm struggling to understand how we can ever detect LyC emission from extragalactic objects at $z\sim0$. Forgetting about the low density of such diffuse, faint X-ray-emitting gas and assuming we use space-based telescopes to avoid extinction by the Earth's atmosphere, under what conditions would we expect to detect LyC emission at $z\sim0$? The only thing I can think of is that there might be patches of the sky that are relatively underdense in neutral atomic hydrogen in our ISM (maybe it's ionized or molecular hydrogen, or no hydrogen at all), so in those parts of the sky we may be able to detect LyC from other galaxies.

There is also the question of high-redshift sources of LyC emission. Given that their light will cumulatively encounter much more intergalactic gas on the way to us, I also find it hard to imagine how we can ever measure LyC emission from them. I guess at $z<6$ the Universe is reionized, so there may not be much neutral atomic hydrogen in the IGM to cause significant absorption/scattering. But it's confusing because the IGM was reionized by LyC photons yet we cannot measure them directly...?

Lyman-continuum (LyC) emission is everything blueward of 912 Angstroms (so it includes extreme UV photons, X-rays and gamma rays). There are many low-redshift astrophysical plasmas such as the diffuse halo gas around galaxies that are hot enough ($\sim10^6$ K) that we expect them to produce copious UV/X-ray emission (either continuum or metal line emission). On the other hand, neutral hydrogen can absorb any LyC photon (no matter how energetic, as long as the wavelength is $<912$ Angstroms) in order to become ionized.

Since there is tons of neutral hydrogen in the interstellar medium between Earth and other galaxies (and even between us and gas in our own Milky Way's hot corona), I'm struggling to understand how we can ever detect LyC emission from extragalactic objects at $z\sim0$. Assume that we are perfectly sensitive to the diffuse, faint X-ray-emitting gas at low density and further assume we use space-based telescopes to avoid extinction by the Earth's atmosphere -- under what conditions would we then expect to detect extragalactic LyC emission? The only thing I can think of is that there might be patches of the sky that are relatively underdense in neutral atomic hydrogen in our ISM (maybe it's ionized or molecular hydrogen, or no hydrogen at all), so those parts of the sky might be more "transparent" to extragalactic LyC photons.

There is also the question of high-redshift sources of LyC emission. Given that their light will cumulatively encounter much more intergalactic gas on the way to us, I also find it hard to imagine how we can ever measure LyC emission from them. I guess at $z<6$ the Universe is reionized, so there may not be much neutral atomic hydrogen in the IGM to cause significant absorption/scattering. But it's confusing because the IGM was reionized by LyC photons yet we cannot measure them directly...?