Observations of Millimetre-Wavelength Hydrogen
Recombination Lines in the Galaxy NGC253
P. J. Puxley and C. M. Mountain Gemini 8m Telescopes Project, 950 N. Cherry Ave., Tucson AZ 85719
P. W. J. L. Brand Institute for Astronomy, University of Edinburgh, Blackford Hill,
Edinburgh, UK, EH9 3HJ
T. J. T. Moore Astrophysics Group, Liverpool John Moores University, Byrom St.,
Liverpool UK L3 3AF
N. Nakai Nobeyama Radio Observatgory, Minamimaki, Minamisaku, Nagano 384-13,
Japan
Abstract.
The millimetre-wavelength hydrogen recombination lines
H40,
H53
and H66
have been observed in the nucleus of the nearby starburst galaxy
NGC253. The line fluxes have been compared with existing radio recombination
line measurements and are found to be consistent with spontaneous emission
from optically thin HII regions. The inferred hydrogen ionization rate is
(3.7±0.8) × 10 s. This value is up to an order of magnitude larger than
those inferred from the recombination lines Br and Br implying substantial
extinction due to dust even at near-infrared wavelengths. Comparison with
existing 3.3mm continuum measurements implies an electron temperature
for the gas of (5000±900)K if the continuum at this wavelength is entirely
due to free-free emission.
,
H53
s
. This value is up to an order of magnitude larger than
those inferred from the recombination lines Br
and Br