Master Thesis Defense by Hao Zhang
Title: Spectral analysis of the CASSOWARY 31 lensing system
Abstract: We analyzed the five objects in the CASSOWARY 31 lensing system based on the MUSW UV spectrum and studied the properties of the object with the highest signal-to-noise ratio through simulations, and found it to be a dwarf galaxy with low metallicity, and its spectrum is probably derived from the joint action of stellar population and shock in the galaxy. Firstly, we remeasured the redshifts of 4 of the lensed objects, paying particular attention to two objects with good signal-to-noise ratios: S0 and S3. We measured the column densities of some of these elements through their unsaturated absorption lines. Their absorption features show a clear blue shift, suggesting the presence of outflows. The outflow is further evidenced by the existence of resonant scattering Fe II* emission lines in S0 and its spatial extension in the corresponding narrow band. Next, we used various diagnostics to estimate the properties of S3. We finally got that the stellar mass of S3 is around 109 M⊙, the value of the metallicity of the stellar population is about 0.2 - 0.3 Z⊙, and the star formation rate is between 1.5 - 4.5 M⊙ yr−1. The ionization parameter is about log(U) = -2.14, and the value of log(sSFR) is between -8 and -9. According to these properties, we think S3 is a dwarf galaxy without a starburst phenomenon. Next, we used BPASS and CLOUDY to simulate the output spectrum of S3. We finally concluded that the pure photoionization model could not reproduce the spectrum of S3, and adding AGN to the model would lead to an excessively high He II intensity that does not match the observed situation. The stellar population + shock model can well reproduce the UV line ratios we observed in the spectrum. Despite questionable reliability, we think that the lower metallicity stellar population + shock model with shock contribution below 50% in incident radiation was the closest to the observation results of S3.
Supervisor:
- Lise Bech Christensen, University of Copenhagen, Niels Bohr Insitute
Censor:
- Thomas Greve, DTU Space