Quench Boost: A How-To-Guide, Part 3

I’m very happy to be posting again to the How-To-Guide. We’ve made a number of updates to Quench data and Quench Tools. Before I launch into Part 3 of the Guide, here are the recent updates:

  1. The classification results for the 57 control galaxies that needed replacements have been uploaded into Quench Tools.
  2. We’ve applied two sets of corrections to the galaxies magnitudes: the magnitudes are now corrected for both the effect of extinction by dust and the redshifting of light (specifically, the k-correction).
  3. We’ve uploaded the emission line characteristics for all the control galaxies.
  4. We’ve uploaded a few additional properties for all the galaxies (e.g., luminosity distances and star formation rates).
  5. We corrected a bug in the code that mistakenly skipped galaxies identified as ‘smooth with off-center bright clumps’.

In Part 1 of this How-To-Guide to data analysis within Quench, you learned how to use Tools and were introduced to the background literature about post-quenched galaxies and galaxy evolution.

In Part 2 you used Tools to compare results from galaxies *you* classified with the rest of the post-quenched galaxy sample.

In Part 3 we’re going to use the results from the classifications that you all provided to see if there’s anything different about the post-quenched galaxies that have merged or are in the process of merging with a neighbor, and those that show no merger signatures.

The figure below is of one of my favorite post-quenched galaxies with merger signatures. Gotta love those swooping tidal tails!


Let’s get started!

Step 1: Because of the updates to Tools, first clear your Internet browser’s cache, so it uploads the latest Quench Tools data.

Step 2: Copy my starter dashboard with emission line ratios ready for play.

  • Open my Dashboard and click ‘Copy Dashboard’ in the upper right. This way you can make changes to it.
  • In this Dashboard, I’ve uploaded the post-quenched galaxy data.
  • I also opened a Table, just as you did in Part 2 of this How-To-Guide. I called the Table ‘All Quench Table’.
  • In the Table, notice how I’ve applied a few filters, by using the syntax:

filter .’Halpha Flux’ > 0

  • This reduces the table to only include sources that fulfill those criteria.
  • Also notice that I’ve created a few new columns of data, just as you did in Part 2, by using the syntax:

field ‘o3hb’, .’Oiii Flux’/.’Hbeta Flux’

  • That particular syntax means that I took the flux for the doubly ionized oxygen emission line ([0III]) and divided it by the flux in one of the Hydrogen emission lines (Hbeta).
  • This ratio and the ratio of [NII]/Halpha are quite useful for identifying Active Galactic Nuclei (AGN).
  • It’d be really interesting if we find that AGN play a role in shutting off the star formation in our post-quenched galaxies. A major question in galaxy evolution is whether there’s any clear interplay between merging, AGN activity, and shutting off star formation.

Step 3: Create the BPT diagram using the ratios of [OIII]/Hb and [NII]/Ha.

  • BPT stands for Baldwin, Phillips, and Terlevich (1981), among the first articles to use these emission line ratios to identify AGN. Check out the GZ Green Peas project’s use of the BPT diagram.
  • Click on ‘Tools’. Choose ‘Scatter plot’ in the pop-up options.
  • In the new Scatterplot window, choose ‘All Quench Table’ as your ‘Data Source’.
  • For the x-axis, choose ‘logn2ha’. For the y-axis, choose ‘logo3hb’.
  • Adjust the min/max values so the data fits nicely within the window, as shown in the figure below.
  • Remember that you can click on the comb icon in the upper-left of the plot to make the menu overlay disappear.
  • Do you notice the two wings of the seagull in your plot? The left-hand wing is where star forming galaxies reside (potentially star-bursting galaxies) while the right-hand wing is where AGN reside. Our post-quenched sample of galaxies covers both wings.


Step 4: Compare the BPT diagram for post-quenched galaxies with and without signatures of having experienced a merger.

  • To do this, you’ll need to first create two new tables, one that filters out merging galaxies and the other that filters out non-merging galaxies.
  • Click on ‘Tools’. Choose ‘Table’ in the pop-up options.
  • In the new Table window, choose ‘All Quench Table’ as the ‘Data Source’. Notice how this new table already has all the new columns that were created in the ‘All Quench Table’. That makes our life easier!
  • Look through the column names and find the one that says ‘Merging’. Possible responses are ‘Neither’, ‘Merging’, ‘Tidal Debris’, or ‘Both’.
  • Let’s pick out just the galaxies with no merger signatures.
  • Under ‘Prompt’ type:

filter .Merging = ‘Neither’

  • If you scroll to the bottom of the Table, you’ll notice that you now have only 2191 rows, rather than the original 3002.
  • Call this Table ‘Non-Mergers Table’ by double clicking on the ‘Table-4’ in the upper-left of the Table and typing in the new name.
  • Now follow the instructions from Step 3 to create a BPT scatter plot for your post-quenched galaxies with no merger signatures. Be sure to choose ‘Non-Mergers Table’ as the ‘Data Source’.
  • You might notice that this plot looks pretty similar to the plot for the full post-quenched galaxy sample, just with fewer galaxies.

What about post-quenched galaxies that show signatures of merger activity? Do they also show a similar mix of star forming galaxies and AGN?

  • To find out, create a new Table, but this time under ‘Prompt’ type:

filter .Merging != ‘Neither’

  • The ‘!=’ syntax stands for ‘Not’, which means this filter picks out galaxies that had any other response under the ‘Merging’ column (i.e, tidal tails, merger, both). Notice how there are 505 sources in this Table.
  • Now create a BPT scatter plot for your ‘Mergers Table’.
  • Make sure this plot has a similar xmin,xmax,ymin,ymax as your other plots to ensure a fair comparison.
  • You might also compare histograms of log(NII/Ha) for the different subsamples.

What do you find? Do you notice the difference? What could this be telling us about our post-quenched galaxies?!

Before you get too carried away in the excitement, it’s a good idea to compare the post-quenched galaxy sample BPT results against the control galaxy sample.

This comparison with the control sample will tell you whether this truly is an interesting and unique result for post-quenched galaxies, or something typical for galaxies in general. You might consider doing this in a new Dashboard, as I have, to keep things from getting too cluttered. In that new Dashboard, click ‘Data’, choose ‘Quench’ in the pop-up options, and choose ‘Quench Control’ as your data to upload. Now repeat Steps 1-4.

Do you notice any differences between your control galaxy and post-quenched galaxy sample results? What do you think this tells us about our post-quenched galaxies?

Stay tuned for Part 4 of this How-To-Guide. I’d love to build from your results from this stage, so definitely post the URLs for your Dashboards here or within Quench Talk and your questions and comments.

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3 responses to “Quench Boost: A How-To-Guide, Part 3”

  1. Chris Lloyd says :

    OK, have followed and completed the analysis but I can’t see any difference between the Merging and Non-Merging scatterplots or histograms.

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  1. Quench Boost: A How-To-Guide, Part 4 | Galaxy Zoo - September 15, 2013

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