Because of dust extinction

Because of dust extinction - Because of dust extinction,...

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Because of dust extinction, one always wants to use lines at the longest possible wavelength. As Susan Kleinmann once said, "I don't care if it's plastic, as long as it's in the infrared and you can see it". For hydrogen, this has driven work into the Paschen and Brackett lines in the near-IR, although instrumental and background limitations have made this slow going. A NICMOS filter survey was especially fruitful in unveiling star formation deep in the dusty centers of galaxies. Seeing line emission does require a source of energy input, so that hydrogen is ionized only where 13.6 eV per atom is readily available. This may come from OB stars, from hot evolved stars (as in planetary nebulae), or from interstellar shocks (as in supernova remnants). A few ions, like S + , have lower ionization potentials, and since radiation longward of the Lyman limit at 912 Angstroms isn't absorbed by neutral hydrogen, there's plenty of starlight available to ionize these species. Also, collision excitation of neutral atoms by electrons released during hydrogen
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Because of dust extinction - Because of dust extinction,...

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