1.6.1
Pertinent scientific questions to be addressed
Mapping the the small scale structure of the ISM in order to determine the small scale structure of the ISM has the potential to answer some important questions. It can give a global picture, an overview of small scale structure. Creating the maps using different tracers can reveal detail and then standing back to look at the whole picture can suddenly make things obvious that before had gone unnoticed because study has been too concentrated in one small area. The global picture that mapping can give also highlights areas of interest to study in detail.
Knowing what type of environment is where and how that changes on small scales is key to understanding how some molecules may be able to survive the harsh inter- stellar space. By mapping the abundances of the Diffuse Interstellar Bands, sodium
and calcium we can put constraints on conditions that are needed for DIBs to survive, for the environment the need to form or in what type of environment they may be destroyed. It is the the question that has been asked since their discovery and to date we still do not know the answer.
Specific questions that are still to be answered include the following:
1. What is the structure of the Interstellar Medium, in particular of the Local Interstellar Medium (LISM)?
2. What is the Earth going to run into next?
3. What is or are the carriers of the Diffuse Interstellar Bands (DIBs)?
To try to answer these questions I have used absorption-line spectroscopy of back- ground stars to obtain spectra that include DIBs, I have observed different regions, the low metallicity of the Magellanic Clouds and the higher metallicity of the local Bubble. Does metallicity make a difference? I have observed nearby stars for the Local Bubble to see the detail and get the local picture. I have observed the distant LMC and SMC that would also trace the environment though the Local Bubble and the Galactic Halo. With the SMC and LMC I am the distant observer and I observe the global picture. Are there areas where there are similarities between the LB and MC - are there differences?
1.6.2
Methodology (absorption-line spectroscopy of back-ground
stars)
Spectroscopy is the study of the interaction between matter and radiation as a function of wavelength or frequency and is used in astronomy mainly to determine the chemical composition and physical properties of astronomical objects or to measure the velocity of objects from the Doppler shift of their spectral lines. Spectral lines are observed either in absorption or emission and are the result of a photon carrying a specific
amount of energy that allows a change in the energy state of the atomic or molecular system it encounters. Absorption lines occur when the detector is in a direct line with the photon source and the gas; a decrease in the intensity of light at the frequency of the incident photon is seen as the re-emitted photons from the gas are generally in random directions different to the original direction. Emission lines occur when the detector is not in line with the photon source; in this case the detector sees the pho- tons that are randomly emitted from the gas, see Figure 1.5 which was obtained from http://www.atnf.csiro.au/outreach/education/senior/astrophysics/spectra astro types. html on 03 May 2010.
As spectral lines are specific to a type of atom they are used to identify the chemical compositions of the medium the photons encounter and are re-emitted from (emission); the stars that emit the original photon (absorption); and the interstellar medium (absorption). As interstellar absorption lines are produced by atoms in the interstellar medium and seen in absorption against the spectrum of a background star, they are observed in stellar spectra but have no connection with the star. There are two major advantages of absorption-line studies over emission-line studies when studying the ISM. Firstly, the angular resolution of a telescope and the Earth’s atmospheric seeing limit the spatial scales that are probed by emission-line studies; whereas those two problems do not affect absorption-line studies and the spatial scales probed are as narrow as the angular size of the background source. As stars subtend very small angles on the sky the scales probed by absorption-lines studies are three or more orders of magnitude finer than the scales probed by emission-line studies. This means that even if a star only moves a milliarcsecond per year across the sky a different portion of the ISM will be encountered when a star is re-observed the following year. Secondly, emitted photons are spread isotropically and so line emission is much weaker than the corresponding absorption of background light. Therefore, absorption-line studies are vastly more sensitive to small amounts of interstellar matter than emission-line studies are.
Figure 1.5: Adapted from a diagram by Kaler (1989) by Australia Telescope National Facility.
1.6.3
Overview of this work
The remainder of this work describes the contributions I have made to the study on the Interstellar Medium during my Ph.D studies. Chapter two describes in detail the code I had to write in order to be able to reduce the set of data from the New Technology Telescope in Chile and how I fitted the profiles to the data to take the measurements I wanted from the data. Chapter 3 presents the maps I created of the Local Bub- ble and discusses the findings. Chapter 4 presents the maps I created of the Small Magellanic Cloud and the Large Magellanic Cloud together with Galactic foreground maps that I could create using the same observation as the Galactic component of the DIBs and sodium lines were well separated from the Magellanic Clouds because of the Doppler shifts of the Clouds. Chapter 5 discuses the two projects together, what are the similarities and differences between the local ISM, the SMC and the LMC as well as considering the implications for the nature of the carriers of the Diffuse Interstellar Bands.
2
Data collection and processing
“I don’t see the logic of rejecting data just because they seem incredible” – Fred Hoyle