3rd Blog Post: Interpreting CLOUDY Simulation Data

The third week of the internship was a slow week for the CLOUDY project, but nonetheless progress has been made. Improving on last weeks efforts to forecast the physical conditions in CR7, work has been done to further constrain the sample of CLOUDY models that best represent CR7 .

Looking at the plot of the CLOUDY BPASS models that compares the UV emission line ratios (CIII/HeII and CIV/HeII), we find an interesting distribution that has a butterfly shape. If we look at the intensity of HeII emitted, we find that lower intensities occupy the left part of graph, while higher intensities occupy the right, as shown below.

A plot showing the UV line ratio tests for the CLOUDY BPASS models, including the HeII intensity of the sources. Red indicates higher intensities while blue indicates lower intensities. The population of sources seems to be separated by their intensities, with high intensity sources occupying the right area of the graph.

In real observations, many of the fainter (blue-green) sources would not be detected by telescopes. Hence, we can discard sources that have intensities lower than two orders of magnitude of the source with maximum intensity. In doing so, we eliminate all of the sources that appear on the left side of graph. It is also interesting to look at the metallicity of the gas clouds surrounding the remaining sources, shown below.

A plot showing the remaining models that would actually be observable. The colour scale on the right shows metallicity of gas clouds surrounding the models, in units of Solar Metallicity. It can be seen that as we move up the graph, metallicity increases, and depends strongly on the CIII/HeII ratio.

Now that we have further constraints on the BPASS models, we can combine them with the CR7 constraints. This brings down the total fraction of models that best represent CR7 to 9% of the original population. We can visualize these constraints on the BPT diagram, illustrated below.

A BPT diagram, showing the new constraints applied to the BPASS models, showing the demarcation curve for discriminating AGNs (above the curve) from SFGs (below the curve) proposed by Kauffmann and Kewley.

The BPT diagram indicates that the ionizing sources in CR7 are not powered by AGNs, instead suggesting star formation as the main source of ionization. However, the BPT diagram looks at the optical line ratios. UV line ratio tests seem to disagree with this result, instead placing the CR7 models in regions that are populated by AGNs, suggesting that AGNs are the ionizing sources in CR7. The reason for this is not yet known, and is thought to be a result of the computer modelling.

This project will be put on hold as more effort will be allocated to the HST Grism project.

2nd Blog Post: Interpreting CLOUDY Simulation Data

The second week of the internship went a lot smoother than the first, after becoming accustomed to using new software and getting to know the team better. After reading a few papers on emission line ratio studies of galaxies, things in the CLOUDY project don’t seem so cloudy anymore.

Using data from the 2015 CR7 discovery paper by David Sobral and his team, we apply constraints on the simulation data to determine the physical properties of CR7. This is done by looking at two emission line ratios: HeII/OIII and HeII/CIII. From the CLOUDY simulation data, we obtain a range of values for the physical conditions of CR7 including temperature, density and metallicity of the gas clouds relative to the solar metallicity.

Results from the Blackbody Model. Temperature is shown in Kelvin, along with other parameters. Parameters worth noting are temperature, and the metal line ratios; carbon-oxygen (C/O) and oxygen-hydrogen (O/H). SD stands for standard deviation.

Halfway through the week, a more complete CLOUDY simulation using the BPASS model was completed by Lara Alegre, another student studying the CLOUDY models in detail. We create a BPT diagram (devised by Baldwin, Phillips & Terlevich to discern different types of ionization sources) for the new simulated data.

BPT diagram for the new CLOUDY simulation data. The two curves shown are the criteria proposed by Kauffmann (dotted line) and Kewley (solid line) to discern between star-forming galaxies (SFGs) and active galactic nuclei (AGN). AGN sources would sit in the region above the curves. The red dots represent sources that have HeII/OIII and HeII/CIII ratios which fit the constraints set in the CR7 discovery paper.

From the BPT diagram, all sources simulated by CLOUDY lie in the region below the BPT demarcation curve, indicating none of them are AGN-like sources. All CR7-like sources are well below the curve, suggesting that CR7 is not powered by AGNs. Using the new data, we can also find further constraints on the physical conditions within CR7.

Results from the Blackbody model, using the new BPASS model data to further constrain CR7 parameters.

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.