Fluorescent H_2 Emission Lines from the Reflection Nebula NGC 7023
  Observed with IGRINS
  We have analyzed the temperature, velocity and density of H2 gas in NGC 7023
with a high-resolution near-infrared spectrum of the northwestern filament of
the reflection nebula. By observing NGC 7023 in the H and K bands at R ~ 45,000
with the Immersion GRating INfrared Spectrograph (IGRINS), we detected 68 H2
emission lines within the 1" x 15" slit. The diagnostic ratios of 2-1 S(1)/1-0
S(1) is 0.41-0.56. In addition, the estimated ortho-to-para ratios (OPR) is
1.63-1.82, indicating that the H2 emission transitions in the observed region
arises mostly from gas excited by UV fluorescence. Gradients in the
temperature, velocity, and OPR within the observed area imply motion of the
photodissociation region (PDR) relative to the molecular cloud. In addition, we
derive the column density of H2 from the observed emission lines and compare
these results with PDR models in the literature covering a range of densities
and incident UV field intensities. The notable difference between PDR model
predictions and the observed data, in high rotational J levels of v = 1, is
that the predicted formation temperature for newly-formed H2 should be lower
than that of the model predictions. To investigate the density distribution, we
combine pixels in 1" x 1" areas and derive the density distribution at the
0.002 pc scale. The derived gradient of density suggests that NGC 7023 has a
clumpy structure, including a high clump density of ~10^5 cm^-3 with a size
smaller than ~5 x 10^-3 pc embedded in lower density regions of 10^3-10^4
cm^-3.
