SNAG: Studying Nearby AGN & Galaxies: Post 5

This week has been the final week of lab work, and as such we have been rounding up our findings and neatening up all the graphs we have made. Jonathan went over the graphs he had made and colour coordinated them to better link the graphs to the data each point came from. 

As you can see this more clearly illustrates where the relationship between the graph and the original data, and as was mentioned in last weeks post there were some clear links showing up between mass and temperature of a galaxy, as well as mass and metallicity as we would expect. John began the same process for the AGN galaxies, splitting the whole subsection down into smaller sections and calculating average values in each section. Below shows the AGN containing galaxies split down into the smaller sections used for analysis (CLOUDY data has not been overlaid in this image).

Now as all three active galaxy types had been analysed Tom was able to produce some graphs which showed trends across them, laid side by side for easy comparison. An example is shown below where you can see that the star forming and star burst galaxies show similar trends, which makes sense as star burst galaxies are very close to star forming, but the AGN galaxies show the complete opposite relation between their masses and star formation rates.

Charlie spent this week redefining how we identify passive galaxies in order to reduce the amount of overlap between galaxies already identified by the SDSS and ones we had identified as passive. She achieved this with much success, meaning that at the end of our investigation, 90% of our data has been classified, 41% was classified by SDSS and a further 52% we have now identified as being passive with an overlap of only 3%. Identification of passive galaxies was achieved using a combination of the error in the flux of the Hydrogen Alpha emission line, and the equivalent width of the emission line which looks at the area covered by the emission line.

 Because of this, it was possible for Pascale and Phoebe to re-plot the fractional percentage graph of the different galaxy types and their mass. This is shown below and follows all the trends we would expect of the different galaxy types. The passive galaxies take up the large portion of the high mass galaxies as these have all become quenched, with the lower masses dominated by star forming and star burst galaxies, with the relatively rare AGN galaxies showing a slight bias towards higher masses. This graph is shown below.

To round our data analysis off, Jonathan plotted the position of the AGN galaxies on the sky with the rest of the data. The AGN for the most part are located in the more densely packed regions of the cosmic filaments (represented by the black dots on the image below), which we observed in the first week of data analysis. This reaffirms what we believe about AGN formation as the denser regions contain more mass causing the local environment to be more volatile. It is both reassuring to have our data continually agreeing with what theories predict, and a nice way to end the investigative part of our project by linking our current knowledge back to what we saw in week one.

Pascale Desmet

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