Uncovering the hidden underbelly of a starburst galaxy

In a new publication, a team of astronomers from Curtin University, with CAASTRO co-authors Dr Emil Lenc (University of Sydney) and Prof Steven Tingay (Curtin University), have used radio observations spanning 21 years to uncover the processes driving intense star formation in the nuclear regions of the starburst galaxy NGC 253. The nuclear region is obscured by gas and dust when observed at optical wavelengths but is revealed at radio wavelengths. Utilising the Australian Long Baseline Array (LBA), the highest-resolution instrument in the southern hemisphere, Curtin PhD student Hayden Rampadarath and colleagues were able to detect and resolve individual compact sources in NGC 253.

By combining the LBA results with previously published observations at higher radio frequencies the team modelled the spectra of 20 sources (see image) to measure the line-of-sight ionised gas content. Foreground ionised gas causes a drop-off in source intensity at lower frequencies (highlighted in red in the image). Spectral modelling also enabled source categorisation. Supernova remnants, relics left behind after massive stars end their lives with a bang, have spectra dominated by synchrotron emission so their brightness decreases steeply with frequency (marked “S” in the image). Whereas HII regions, clouds of hot gas in which star formation has recently taken place, have spectra dominated by thermal emission so their brightness is flat or increases with frequency (marked “T”).

Additionally, using improved modelling of radio supernovae, the team were able to place limits on both the supernova rate and star formation rate within the nuclear region of the galaxy. The results, which suggest a supernova rate of less than one every five years and a star formation rate of less than 4.9 solar-masses per year, are consistent with studies utilising very different techniques. Furthermore, they point to the possible existence of a small population of undetected supernova remnants and suggest a low rate of radio supernovae production in NGC 253.

by E. Lenc

Publication details:

H. Rampadarath, J. S. Morgan, E. Lenc, S. J. Tingay in AJ (2014) “Multi-epoch Very Long Baseline Interferometric Observations of the Nuclear Starburst Region of NGC 253: Improved Modeling of the Supernova and Star formation Rates”