Large-scale multi-line spectroscopy of the galactic center: origins of the far-infrared [CII] line deficit

Takao Nakagawa ¹, Haruyuki Okuda ¹, Hiroshi Shibai ², Yasuo Doi ³, Kenji Mochizuki ², & Takashi Onaka ⁴

¹ Institute of Space and Astronautical Science, Japan ² Department of Physics, Nagoya University, Japan

³ Department of Earth Science and Astronomy, University of Tokyo, Japan

⁴ Department of Astronomy, University of Tokyo, Japan

We made multi-line spectroscopic observations of selected areas in the Galactic center region on a large scale (ten degrees) to reveal the origins of the conspicuous [CII] 158 micron line deficit in the Galactic center (GC).

Our previous balloon observations revealed that the [CII] line is surprisingly weak toward the GC, and the ratio of the [CII] line emission to far-infrared continuum ( ) shows a large dip at the central several hundred parsecs of the GC. The results indicate the low abundance of the C+ ions in the GC area. We hypothesized that this low abundance is mainly due to soft UV radiation with fewer C-ionizing photons. This soft radiation field, together with the pervasively high molecular gas density, makes the molecular self-shielding more effective, and raises temperature of molecular gas at the C⁺/C/CO transition zone. Hence our hypothesis can naturally explain not only the weak [CII] line but also the pervasive existence of warm molecular clouds in the GC.

The [CII] line alone is not sufficient to determine the physical conditions and to check our hypothesis, we made multi-line spectroscopic observations by ISO. ISO observations covers various points (HII regions, molecular clouds, diffuse clouds) in the Galactic center region (longitude = +6 to -6 degrees). We also observed two points in the general Galactic plane for comparison. Main results are: 1. The average [OI]/[CII] line ratio is slightly higher in the Galactic center region than in the general Galactic plane.

2. The most important difference between the two regions is that ubiquitous existence of OH absorption in the Galactic center region. We see OH absorption feature at any observed points in the Galactic center but not in the Galactic plane.

The second result suggests the pervasive existence of cold molecular gas toward the Galactic center. But the first result suggest that [CII]-emitting gas is only slightly denser in the Galactic center than in the Galactic plane. Hence we conclude that it is the existence of dense molecular clumps that makes interstellar gas in the Galactic center unique and the [CII] line is emitted not from these dense clumps but mainly from diffuse inter-clump clouds.