Galaxy formation and evolution

Abstract:

Evidence has been found for optical emission from the northern radio jet of 3C 31, the radio source associated with NGC 383. The jet emerges from the dust disc and ring of emission at a radius of 5 arcsec, and within 10.4 arcsec is measured to have a B-band flux of 2.1 μJy and an R-band flux of 2.3 μJy. The radio-to-optical spectral index of this region is 0.78. A second connected region, 11.8 arcsec along the jet in position angle 340°, found to have similar optical colours and radio-to-optical spectral index may also be jet emission. We combine our new data with recent radio and X-ray results to conclude that the emission of the jet is synchrotron from the radio to the X-ray.

Abstract:

We present a mid-infrared (MIR) spectroscopic survey of starburst galaxies as an example of analysis of scientifically focused samples selected from the ISO Data Archive (IDA). We use fine structure lines (FSL) ratios of Ne, Ar and S to construct diagnostic excitation diagrams and, in combination with hydrogen recombination lines (HRL), we determine their elemental abundances. For Ne and Ar, we find that excitation indicators are positively correlated with each other and negatively with abundance. On comparison with a complementary sample of galactic H II regions we find that starbursts are generally of lower excitation. Starbursts exhibiting Wolf-Rayet (WR) features are separated both in excitation and abundance from the remaining starbursts. Most surprisingly, S is found to be underabundant by a factor of ∼ 3 in our low excitation starbursts with respect to the Ne and Ar, contrary to expectations from nucleosynthesis theory. Our results are combined with those of a related sub-sample of Seyfert galaxies (Sturm et al. 2002) to derive diagnostic diagrams discriminating the two types of activity on the basis of excitation traced by MIR lines. The data presented will be useful as a reference for observations of fainter and/or higher redshift sources with future IR observatories such as SIRTF, SOFIA and Herschel.