The nature of the interstellar medium in the starburst of NGC 253

Abstract

This thesis contains several complementary sets o f observations of the starburst in the nucleus o f NGC 253. The observations probe the interface between ionised and molecular gas, where UV photons from young stars dominate the heating and chemistry of the gas. The thesis describes how observations o f the emission from these photon-dominated regions (P D R s), particularly in the near-IR to millimetre range of the electromagnetic spectrum , can be used to constrain the dominant energy inputs, chemistry and geometry of the star-formation process in a starburst.

The near-IR shows several lines of H2. Observations of excited H2 in Galactic PDRs indi­cates that the ortho to para (o/p) ratio o f H2 is ~2 whereas in shocked regions the o/p ratio is observed to be 3. Towards NGC 253, the o/p ratio of H2 is observed to be ~ 2 across the entire starburst and so this is direct evidence that PDRs produce the bulk o f the H2 emission in the starburst region. Furthermore, the ratio of Bry/l-O S(1) shows a maximum on the nucleus. With the knowledge that the H2 emission arises in PDRs, the most plausible way to explain the observed Bry/l-O S(1) ratio is for a large fraction of the O&B stars to be clustered into groups. Away from the nucleus, it appears that the H2 emission is arising from PDRs that are bathed by a relatively diffuse FUV radiation field. There seems to be a clear difference between the geometry o f OB stars and PDRs in the starburst to that of the geometry away from the starburst.

Observations of isotopic CO show that the bulk of 13CO emission arises from warm gas whereas the bulk of C18O emission appears to arise from cold gas. It thus appears that some process is at work in NGC 253 that removes C18O from the warm gas associated with PDRs. A proposed mechanism is selective photodissociation of CO and it’s isotopomers.

Observations of the fine-structure transitions of atomic carbon show a significant overabun­dance, a factor of 10 or more, with theoretical models of PDRs. Considerations of the size of the compact starburst, traced by the mid to far IR radiation from dust, indicates that the starburst is centrally concentrated within the central 100 pc of NGC 253. The luminosity coming from such a region, ~ 1 010 L0. , indicates that the UV field is very high within the starburst, possibly greater than 105 times the local value. If all the C+ flux is generated within this region, as is likely, then the ionised carbon emission is also a factor o f 10 or more brighter than is predicted by PDR models. The most likely explanation for the discrepancy between the brightness of C° and C + and the models is that the elemental abundance of carbon is higher in NGC 253 than it is in the PDR models with which we compare our observations. There is no obvious reason why carbon is over-abundant in the gas-phase o f NGC 253.