Extragalactic Rydberg atoms

The cold neutral medium (CNM) is one of the most enigmatic components of interstellar space. Conditions that prevail in the low-temperature, low-density CNM are such that carbon atoms are more likely to be ionized than the more abundant hydrogen atoms. And when those atoms recombine with ambient electrons, they can form fragile, high-quantum-number Rydberg states whose initial deexcitations entail the emission of low-energy photons. Photons from those deexcitations have now been seen outside our galaxy for the first time. Leah Morabito of Leiden University in the Netherlands and her collaborators found them in the nearby star-forming galaxy M82; they used LOFAR, an array of about 20 000 radio dipole antennas centered in a field outside the Dutch village of Exloo. LOFAR can barely detect individual Rydberg transitions in M82. But by stacking the spectra of 22 transitions from quantum numbers 468–508 down to the next lowest state, Morabito and her colleagues obtained an 8.5-standard-deviation detection. For this proof-of-concept observation, only LOFAR’s central 2-km core was used (see figure). Although the core couldn’t resolve M82’s structure, the stacked spectrum’s profile suggests that the origin of the carbon absorption lies in the vicinity of the galaxy’s star-forming nucleus. Future observations with the full array will pinpoint the carbon’s location. The discovery of extragalactic Rydberg absorption opens a new window on the CNM, which LOFAR’s more sensitive successor, the Square Kilometer Array, is poised to exploit when it debuts next decade. (L. K. Morabito et al., Astrophys. J. Lett. 795, L33, 2014, doi:10.1088/2041-8205/795/2/L33 .)