1st Blog Post: Interpreting CLOUDY Simulation Data

The first week of the internship has been a steep learning curve, but it has also been a great introduction into astrophysics research work. This part of the project concerns interpreting data produced by a computer simulation known as CLOUDY, which simulates the ionization of gas clouds due to luminous sources such as stars, active galactic nuclei (AGN), and star-forming galaxies (SFGs).

TOPCAT (a piece of software designed to handle large tables of astrophysical data) is used to analyse the results of the simulations. CLOUDY incorporates three different stellar evolution models in its simulations; BPASS (Binary Population and Spectral Synthesis), a power law model (to simulate AGNs), and a perfect black body model (to simulate stars). The BPASS model is special since it takes into account binary evolution in simulating stellar populations.

A conventional method in studying the spectra of luminous sources is to compare the emission line ratios of certain elements within the source. By comparing characteristic ratios, we are able to discern different types of sources; plotting a graph of ionization ratios for a stellar population produces regions of the graph that divide stellar populations according to certain physical properties. This allows us to put constraints on density, ionization, gas cloud metallicity, age, and temperature of the sources.

A plot, showing the comparison of two line ratios; OIII/Hβ and NII/Hα. A temperature scale has also been included. From the graph, we can see that higher temperature sources can be found near the top-right of the plot, which correspond to a specific range of emission line ratios. The different “bands” corresponds to sources surrounded by gases with different metallicities.

The analysis of CLOUDY simulation data and other similar studies can be used to infer the physical conditions of observed sources. Other projects in the internship, such as the ALMA and HST study of CR7, can be used to constrain characteristic emission line ratios and allow us to find out more about the physical properties of CR7 and other high-redshift sources.


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