SNAG: Studying Nearby AGN & Galaxies

Welcome to our Research group page!

SNAG group members:

Charlie Alexander, Coordinator; Ciara Lithgow, Administrator; Tom Measey, Communications; John Pollard, Report Lead; Phoebe Stainton, Theory Lead; Pascale Desmet, Coding Lead; Jonathan Dixon, Data Analysis Lead


Over the past two weeks we have been assigning roles and planning our actions; this week we began working with the data for our project. We are SNAG, Studying Nearby AGN and Galaxies, a group of 7 undergraduate physicists from Lancaster University.

Over the next 7 weeks, we will report on our progress in our project. Through spectroscopic analysis, we aim to distinguish between active galaxies and passive galaxies. Using data from the SDSS we will look at the trends shown to distinguish between these types.

Our first challenge to overcome was data reduction. During our first lab session we began with a raw data set, some 850,000 data points from the SDSS data release 8. With some discussion we came to realise that taking data from all redshifts would be inherently biased towards the brighter galaxies, as the larger redshift galaxies would need to be much larger than an average galaxy to have the luminosity to be detected at the distance they are from the Earth. We decided to limit data to the range 0.09 < z < 0.11. By limiting the redshift range to nearby galaxies we ensured that the sample would not be biased against smaller, dimmer galaxies. The narrow band also solved our data reduction problem, we now had only 85,000 data points, a much more manageable number.

Using our new dataset, we plotted the galaxies onto a sphere corresponding to the right ascension and declination. Upon looking closer at the graph we saw much of the fine structure of our universe. On our graph you can clearly see the galaxy filaments that run throughout the universe represented by the strings of darker red where there is a heavier concentration of galaxies.


Our next aim was to determine some of the properties of the various types of galaxy. Included in our data set were approximately 30,000 galaxies which have already been defined as either star-forming, starburst, broadline or AGN galaxies. We decided to start at metallicity, by plotting a graph showing the ratios of the strength of the OIII [5007Å] and Hβ lines and the NII [6584Å] and H­­α lines.


Looking at this graph, we can see the galaxies conform to distinct regions. With this graph, in future, we will be able to take a galaxy that is not already classified into one of these groups and match it to the graph to give us an understanding of the physical processes going on inside of it.

We are excited to see more progress as our project continues in the coming weeks!

Tom Measey

To read more about our project, click here for our subsequent blog posts.