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2010 CFHT Users Meeting
November 16-18, 2010
ASIAA Auditorium, Taipei, Taiwan

Poster Presentation

A Molecular Hydrogen Nebula in the Central Cd Galaxy of the Perseus Cluster

Author(s): Jeremy Lim (HKU), Youichi Ohyama (ASIAA), Chi-Hung Yan (ASIAA), Dinh-V-Trung (Vietnam Academy of Science and Technology), and Shiang-Yu Wang (ASIAA)

Presenter: Jeremy Lim (University of Hong Kong)

We present a narrowband image of the 1-0~S(1) ro-vibrational transition of
molecular hydrogen (H$_2$) from NGC~1275, the central cD galaxy of the
Perseus Cluster. We find that the H$_2$ gas has a spatial morphology
identical to the peculiar optical emission-line nebula associated with this
galaxy, a total luminosity in H$_2$~1-0~S(1) only an order of magnitude less
than in H$alpha$, and if both are thermally excited a corresponding mass in
H$_2$ gas (at $sim$2000~K) over two orders of magnitude less than that in
(partially ionized) atomic gas (at $sim$10,000~K). We find that the
measured ratio of H$_2$~1-0~S(1) to H$alpha$$+$[NII] intensity has a value
averaged over the entire nebula of $0.054 pm 0.007$, and spans at most the
decadal range $sim$0.01--0.1. Suggestions to explain the peculiar H$_2$~1-
0~S(1) to H$alpha$$(+$[NII]) line ratio and the latest attempt to reproduce
the optical and near-IR spectra of this nebula invoke non-thermal in
addition to thermal excitation. We find the same average line ratio for the
bright inner and visually more filamentary outer regions of the nebula,
arguing against non-thermal excitation by relativistic electrons inferred to
cause inverse-Compton scattering of hard X-ray emission from the core of the
Perseus Cluster. Non-thermal excitation by energetic particles produced
locally by processes related to MHD waves cannot be ruled out, but is
speculative. Irradiation by X-ray photons from the surrounding intracluster
gas, resulting in the release of suprathermal secondary electrons that
excite the nebular gas, may provide a more natural explanation for the
relatively uniform average line ratios from the inner to outer nebula.

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