  We present a detailed analysis of Spitzer-IRAC images obtained toward six
Herbig-Haro objects (HH 54/211/212, L 1157/1448, BHR 71). Our analysis
includes: (1) comparisons in morphology between the four IRAC bands (3.6, 4.5,
5.8 and 8.0 um), and H2 1-0 S(1) at 2.12 um for three out of six objects; (2)
measurements of spectral energy distributions (SEDs) at selected positions; and
(3) comparisons of these results with calculations of thermal H2 emission at
LTE (207 lines in four bands) and non-LTE (32-45 lines, depending on particle
for collisions). We show that the morphologies observed at 3.6 and 4.5 um are
similar to each other, and to H2 1-0 S(1). This is well explained by thermal H2
emission at non-LTE if the dissociation rate is significantly larger than
0.002-0.02, allowing thermal collisions to be dominated by atomic hydrogen. In
contrast, the 5.8 and 8.0 um emission shows different morphologies from the
others in some regions. This emission appears to be more enhanced at the wakes
in bow shocks, or less enhanced in patchy structures in the jet. These
tendencies are explained by the fact that thermal H2 emission in the 5.8 and
8.0 um band is enhanced in regions at lower densities and temperatures.
Throughout, the observed similarities and differences in morphology between
four bands and 1-0 S(1) are well explained by thermal H2 emission. The observed
SEDs are categorized into:- (A) those in which the flux monotonically increases
with wavelength; and (B) those with excess emission at 4.5-um. The type-A SEDs
are explained by thermal H2 emission, in particular with simple shock models
with a power-law cooling function. Our calculations suggest that the type-B
SEDs require extra contaminating emission in the 4.5-um band. The CO
vibrational emission is the most promising candidate, and the other
contaminants discussed to date are not likely to explain the observed SEDs.
