Spitzer spectral line mapping of supernova remnants.: I.: Basic data and principal component analysis

We report the results of spectroscopic mapping observations carried out toward small (1' x 1') regions within the supernova remnants W44, W28, IC 443, and 3C 391 using the Infrared Spectrograph (IRS) of the Spitzer Space Telescope. These observations, covering the 5.2-37 mu m spectral region, have led to the detection of a total of 15 fine-structure transitions of Ne+, Ne++, Si+, P+, S, S++, Cl+, Fe+, and Fe++; the S(0)-S(7) pure rotational lines of molecular hydrogen; and the R(3) and R(4) transitions of hydrogen deuteride. In addition to these 25 spectral lines, the 6.2, 7.7, 8.6, 11.3, and 12.6 mu m PAH emission bands were also observed. Most of the detected line transitions have proven strong enough to map in several sources, providing a comprehensive picture of the relative distribution of the various line emissions observable in the Spitzer IRS bandpass. A principal component analysis of the spectral-line maps reveals that the observed emission lines fall into five distinct groups, each of which may exhibit a distinct spatial distribution: (1) lines of S and H-2(J > 2); (2) the H-2 S(0) line; (3) lines of ions with appearance potentials less than 13.6 eV; ( 4) lines of ions with appearance potentials greater than 13.6 eV, not including S++; (5) lines of S++. Lines of group 1 likely originate in molecular material subject to a slow, nondissociative shock that is driven by the overpressure within the supernova remnant, and lines in groups 3-5 are associated primarily with dissociative shock fronts with a range of (larger) shock velocities. The H-2 S(0) line shows a low-density diffuse emission component and, in some sources, a shock-excited component.