Abstracts, Galactic Center Workshop '98
ABSTRACT

The Mid-Infrared Spectrum of the Galactic Center, a Starburst Nucleus

Simpson, J.P. 1; Witteborn, F.C.1; Cohen, M.2; Price, S.D.3

1SETI Institute and NASA/Ames Research Center, MS 245-6, Moffett Field, CA 94035-1000; 2Radio Astronomy Laboratory, University of California, Berkeley, CA 94720-3411; 3Phillips Laboratory, GPOB, Hanscom AFB, MA 01731-3010;

ABSTRACT: At a distance of 1 Mpc, the Galactic Center (GC) would appear 125 times smaller; thus an estimate of its appearance at 1 Mpc in a typical mid-IR field-of-view (FOV) would require averaging over a region of at least ~ 4' or larger in size. Using the MSX Michelson interferometer, we have taken spectra of many positions in the central 25' of the GC with a 6' * 9' FOV. The spectral coverage was 380 to 1700 cm-1 (6 to 26 micron ) and the resolution was ~ 21 cm-1. The spectra exhibit strong UIR/PAH features at 6.2, 7.7, 8.6, and 11.3 micron , in addition to the ionic lines of [Ne II] at 12.8 micron , [S III] 18.7 micron , and [Ar II] 6.98 micron . (This contrasts with the 14'' * 20'' FOV ISO spectrum, Lutz et al. 1996, A &A, 315, L269, of the GC near Sgr A* in which the PAH features were not detected.) There are deep silicate absorption features at 10 and 18 micron and a cold continuum increasing at the longest wavelengths. Additional weak features are present in the spectra. We discuss the variation of the relative strengths of the features as a function of location in the GC and the variation in the extinction at 10 micron . We compare the GC spectra with the MSX spectrum of the Orion Nebula (ApJ, 508, in press), smoothed to the same resolution and multiplied by the estimated GC extinction. The spectra have similar PAH features, but the Orion Nebula also has strong lines of [Ne III] 15.5 micron , [S IV] 10.5 micron , and [Ar III] 8.99 micron and its 25 micron continuum is stronger (colder). Thus, the GC exhibits the mid-IR spectrum of a low excitation H II region and a nearby molecular cloud with a surface photodissociation region (PDR). This is in excellent agreement with the canonical model of a starburst nucleus (such as M82 or NGC 253) in which the hot stars and molecular clouds are randomly distributed. The outer surfaces of the clouds are photodissociated and ionized by the photons from the stars located outside the clouds. The PAH molecules are transiently heated by the stellar photons. Since the exciting stars are located well outside the clouds, the radiation field is dilute compared to a newly-formed blister H II region like Orion; this dilute radiation field causes the relatively low excitation of the ionic lines.

LINKS: Program, Author, simpson@cygnus.arc.nasa.gov