Tracing Polycyclic Aromatic Hydrocarbons and Warm Dust Emission in the Seyfert Galaxy NGC 1068

Abstract

We present a study of the nearby Seyfert galaxy NGC 1068 using mid- and far-\ninfrared data acquired with the IRAC, IRS, and MIPS instruments aboard the\nSpitzer Space Telescope. The images show extensive 8 um and 24 um emission\ncoinciding with star formation in the inner spiral approximately 15" (1 kpc)\nfrom the nucleus, and a bright complex of star formation 47" (3 kpc) SW of the\nnucleus. The brightest 8 um PAH emission regions coincide remarkably well with\nknots observed in an Halpha image. Strong PAH features at 6.2, 7.7, 8.6, and\n11.3 um are detected in IRS spectra measured at numerous locations inside,\nwithin, and outside the inner spiral. The IRAC colors and IRS spectra of these\nregions rule out dust heated by the AGN as the primary emission source; the\nSEDs are dominated by starlight and PAH emission. The equivalent widths and\nflux ratios of the PAH features in the inner spiral are generally consistent\nwith conditions in a typical spiral galaxy ISM. Interior to the inner spiral,\nthe influence of the AGN on the ISM is evident via PAH flux ratios indicative\nof a higher ionization parameter and a significantly smaller mean equivalent\nwidth than observed in the inner spiral. The brightest 8 and 24 um emission\npeaks in the disk of the galaxy, even at distances beyond the inner spiral, are\nlocated within the ionization cones traced by [O III]/Hbeta, and they are also\nremarkably well aligned with the axis of the radio jets. Although it is\npossible that radiation from the AGN may directly enhance PAH excitation or\ntrigger the formation of OB stars that subsequently excite PAH emission at\nthese locations in the inner spiral, the orientation of collimated radiation\nfrom the AGN and star formation knots in the inner spiral could be\ncoincidental. (abridged)\n