Discovery of PAHs in the halo of NGC 5907

Abstract

We have used sensitive archival data from the Infrared Space Observatory (ISO) to make maps of the edge-on low SFR galaxy, NGC 5907, in 6 different MIR bands: LW2, LW5, LW6, LW7, LW8, and LW10, covering the spectrum from 6.5 to 15.0 μm and including several narrow bands that isolate the infrared aromatic spectral features commonly referred to as PAHs. Most of the MIR emission is dominated by PAHs and it is likely that emission from VSGs contribute only negligibly except in the broad IRAS-equivalent band. The flux ratios are typical of galaxies with low SFRs or quiesent regions within galaxies (e.g. M 83) and a very high PAH/continuum ratio is observed. The PAH emission follows the CO distribution and also shows some correlation within the disk with the λ850 μm distribution. However, the PAH emission also reaches larger galactocentric radii than the CO and other correlations suggest that the PAHs are also more widespread. A significant new discovery is the presence of PAHs in the halo of the galaxy. In the narrow bands that isolate single PAH features, the emission shows structure similar to high latitude features seen in other galaxies in other tracers. The features extend as far as 6.5 kpc from the plane but scale heights of 3.5 kpc are more typical. The λ11.3/λ7.7 ratio also appears to increase with distance from the major axis. To our knowledge, this is the first time PAHs have been seen in the halo of an external galaxy. Just as significantly, they are seen in a low SFR galaxy, suggesting that strong SNe and winds are not necessary for these large molecules to reach high latitudes.