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Abstract

• A complete sample of point source emissions in the spiral type galaxies NGC300 and M31 have been catalogued and analysed. This study has identified a total of 72 and 864 point source emissions in both NGC300 and M#!.

• New high-sensitivity and resolution radio-continuum images of NGC300 and M31 at λ=20~cm have been created as part of this study. These new images were produced by merging archived 20~cm radio-continuum data sourced from ATCA and the VLA. Images presented in this thesis are extremely sensitive and feature high angular resolution (<10”).

Outline

• Overview of Instruments• Data reduction process• Results– NGC 300– M31

Electromagnetic Spectrum

Radio-Telescopes

• The quality of the image is directly dependent upon the amount of flux gathered.

• A larger dish will collect more flux, improving the image.

Bigger is not always better…

Radio-Interferometers

• Rather then build bigger, we build more.• A baseline is defined as the distance between

2 individual dishes. • Creates a synthesized single dish telescope.

Effects of Baselines

• Array configurations with longer baselines produce higher resolution images– Intrinsic structure of sources can be studied– Image artifacts may be introduced

• Array configurations with shorter baselines produce lower resolution images– Extended sources are more apparent ie. Extended

Emission

Instruments Used - ATCA

• The Australian Telescope Compact Array • Made up of six 25 meter diameter dishes• Longest baseline supported is 6 kilometers

Instruments Used - VLA

• The Very Large Array• Made up of twenty-seven 22 meter diameter dishes• Longest baseline supported is 36 kilometers

Merging Observations

• Merging multiple observations together is possible

• Observations with different array configurations can produce images with both intrinsic and extended structure to sources

Data Reduction Process

• Two main data reduction software packages were used– Astronomical Image Processing System (AIPS)• Use was limited to preprocessing of VLA only

– Multichannel Image Reconstruction, Image Analysis and Display (miriad)

Miriad - Overview

Actual data reduction is comprised of a number of individual steps.

Miriad - Flagging

Primary Calibrator 1934-638 with data points to be flagged

Miriad - Calibration

• Calibration is necessary to establish a flux density scale and correct for instrument gains over time

• Achieve this by observing significantly strong and are well understood sources

• By comparing results measured to what we expect the results to be, we can deduce certain corrections which need to be made, and apply these corrections to our data

Example - Primary Calibrator

Before Calibration After Calibration

Miriad - Invert

• This task actually produces a “dirty” map from a set of calibrated sources

• A number of additional parameters can be specified to alter processing– How to weigh baselines of varying length– Enable image mosaicing

Example – “Dirty” Map

Miriad - Cleaning

• “Dirty” images suffer from artifacts which degrade the quality of the image

• These artifacts can be removed from these “dirty” images– Miriad tasks CLEAN and MOSSDI both use an SDI

CLEAN algorithmic approach• Most computationally expensive stage of the

imaging process

Miriad - Restor

• Produces a final cleaned image based on the outputs of previous tasks

Results – NGC 300

NGC 300 - Observations Used

• PUT A TABLE HERE SUMMARISING:– PROJECT CODE– VLA/ATCA– RA AND DEC– DATE

NGC 300 - Final Image

NGC 300 - Sources

• A total of 72 distinct sources were identified• This new source catalogue was compare to a

previously defined catalogue (Payne et al., 2004)– 8 previously defined sources, out of 48, were not

present in our catalogue

Results – M31

M31 - Observations Used

• 15 individual VLA Projects were used• Observations conducted between October

1983 and September 1996• Contains 57 individual pointings

M31 – Final Image

M31 - Sources

• Across 17 individual images from 15 VLA projects, 864 unique sources were identified.

• These sources were compared to those listed in Gelfand (2004), which describes sources at λ = 92cm– 118 sources were found to be common to both

lists

Spectral Index Histogram

THANKS!

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