Distribution Of Far-infrared Emission Research Articles

The distribution of far-infrared emission from edge-on galaxies

We use resolution-enhanced IRAS images (HiRes) to study the distribution of far-infrared (FIR) emission in 20 edge-on, disc galaxies. Half of our sample are selected as infrared-bright galaxies which emit most of their energy at wavelengths longer than a few mum. The remaining 10 objects are `normal' (quiescent) galaxies. The distribution of FIR emission along the major axis of the infrared-bright galaxies tends to be far more centrally concentrated than that detected in quiescent discs. Indeed, many of the infrared-bright objects are dominated by a luminous, compact or unresolved source (<3 kpc) located in the nuclear region. It is known from emission-line studies carried out in the past (Hα imaging and long-slit spectroscopy) that most of the galaxies that we have selected as infrared-bright possess an energetic outflow or `superwind', which is powered by enhanced, circumnuclear star formation. We believe, therefore, that the compact FIR emission that we have detected arises from dust enclosing the central starburst. Low-resolution radio continuum maps for the same galaxies show a striking morphological similarity to the IRAS images, confirming the presence of enhanced, recent star formation at the centre of the infrared-bright galaxies. The wider availability of higher resolution FIR images, such as HiRes, also invites an investigation into whether edge-on discs are resolved along the minor axis. In particular, we explore the possibility that the starburst-driven superwinds, present within many of our infrared-bright galaxies, are transporting dust grains out of the disc. None of the objects that we have examined (infrared-bright or quiescent) exhibits structure that can be reliably interpreted as dust outflow. Various artefacts of the HiRes data, specifically those which produce faint, spurious structure in the vicinity of bright FIR sources, ultimately constrain the sensitivity of our images to about 1 MJy sr^-1 at 60 mum. Thus, if the grains entrained in the outflow are too cold (<=15 K), we cannot expect to detect them with the present study.

Spatial distribution of far-infrared emission in spiral galaxies. I. Relation with radio continuum emission.

We use high resolution IRAS and 20 cm radio continuum (RC) images of a sample of 22 spiral galaxies to study the correlation between the far infra-red (FIR) and RC emissions within the galactic disks. A combination of exponential and gaussian profiles rather than a single exponential profile is found to be a better representation of the observed intensity profiles in the two bands. The gaussian component, which we show is not due to the effects of limited beam-resolution, contains more than 60% of the total flux in majority of the galaxies. The dominance of the gaussian component suggests that the nuclear star forming regions and the bulge stars are more important contributors to the emission in the two bands, rather than the outer exponential stellar disks. The RC profile is flatter compared to the FIR profile, resulting in a decrease of their ratio, Q60, away from the center. However, the Q60 increases in the extreme outer parts, where the dispersion in the FIR and RC correlation is also higher than in the central regions. The global Q60 and its dispersion match those in the inner parts of the galaxies. These results imply that the observed tight correlation in the global quantities reflects processes in the inner regions only where OB stars and the associated Type II supernovae control the FIR and RC emission. In the outer parts heating of very small dust grains by the old disk stars provides a secondary component in the FIR emission, without associated RC emission. The edge-on galaxy NGC3079 shows extended FIR and RC emissions along its minor axis, probably associated with the nuclear starburst activity.