Quench Boost: A How-To-Guide, Part 4

Now that we’ve been initiated into the cool waters of Tools (Part 1), we’ve compared our *own* galaxies to the rest of the post-quenched sample (Part 2), and we’ve put your classifications to use, looking for what makes post-quench galaxies special compared to the rest of the riff-raff (Part 3), we’re ready for Part 4 of the Quench ‘How-To-Guide’.

This segment is inspired by a post on Quench Talk in response to Part 3 of this guide. One of our esteemed zoo-ite mods noted:

There are more Quench Sample mergers (505) than Control mergers (245)… It seems to suggest mergers have a role to play in quenching star formation as well.

Whoa! That’s a statistically significant difference and will be a really cool result if it holds up under further investigation!

I’ve been thinking about this potential result in the context of the Kaviraj article, summarized by Michael Zevin at http://postquench.blogspot.com/. The articles finds evidence that massive post-quenched galaxies appear to require different quenching mechanisms than lower-mass post-quenched galaxies. I wondered — can our data speak to their result?

Let’s find out!

Step 1: Copy this Dashboard to your Quench Tools environment, as you did in Part 3 of this guide.

  • This starter Dashboard provides a series of tables that have filtered the Control sample data into sources showing merger signatures and those that do not, as well as sources in low, mid, and high mass bins.
  • Mass, in this case, refers to the total stellar mass of each galaxy. You can see what limits I set for each mass bin by looking at the filter statements under the ‘Prompt’ in each Table.

Step 2: Compare the mass histogram for the Control galaxies with merger signatures with the mass histogram for the total sample of Control galaxies.

  • Click ‘Tools’ and choose ‘Histogram’ in the pop-up options.
  • Choose ‘Control’ as the ‘Data Source’.
  • Choose ‘log_mass’ as the x-axis, and limit the range from 6 to 12.
  • Repeat the above, but choose ‘Control – Merging’ as the ‘Data Source’.

The result will look similar to the figure below. Can you tell by eye if there’s a trend with mass in terms of the fraction of Control galaxies with merger signatures?


It’s subtle to see it in this visualization. Instead, let’s look at the fractions themselves.

Step 3: Letting the numbers guide us… Is there a higher fraction of Control galaxies with merger signatures at the low-mass end? At the high-mass end? Neither?

To answer this question, we need to know, for each mass bin, the fraction of Control galaxies that show merger signatures. I.e.,


Luckily, Tools can give us this information.

  • Click on the ‘Control – Low Mass’ Table and scroll to its lower right.
  • You’ll see the words ‘1527 Total Items’.
  • There are 1527 Control galaxies in the low mass bin.
  • Similarly, if you look in the lower right of the ‘Control – Merging – Low Mass’ Table, you’ll see that there are 131 galaxies in this category.
  • This means that the merger fraction for the low mass bin is 131/1527 or 8.6%.
  • Find the fraction for the middle and high mass bins.

Does the fraction increase or decrease with mass?

Step 4: Repeat the above steps but for the post-quenched galaxy sample.

You may want to open a new Dashboard to keep your window from getting too cluttered.

Step 5: How do the results compare for our post-quenched galaxies versus our Control galaxies? How can we best visualize these results?

  • In thinking about the answer to this question, you might want to make a plot of mass (on the x-axis) versus merger fraction (on the y-axis) for the Control galaxies.
  • On that same graph, you’d also show the results for the post-quenched galaxies.
  • To determine what mass value to use, consider taking the median mass value for each mass bin.
  • Determine this by clicking on ‘Tools’, choosing ‘Statistics’ in the pop-up options, selecting ‘Control – Low Mass’ as your ‘Data Source’, and selecting ‘Log Mass’ as the ‘Field’.
  • This ‘Statistics’ Tool gives you the mean, median, mode, and other values.
  • You could plot the results with pen on paper, use Google spreadsheets, or whatever plotting software you prefer. Unfortunately Tools, at this point, doesn’t provide this functionality.

It’d be awesome if you posted an image of your results here or at Quench Talk. We can then compare results, identify the best way to visualize this for the article, and build on what we’ve found.

You might also consider repeating the above but testing for the effect of choosing different, wider, or narrower mass bins. Does that change the results? It’d be really useful to know if it does.

Happy Tooling!

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8 responses to “Quench Boost: A How-To-Guide, Part 4”

  1. zutopian says :

    I am not familiar with this GZ side project and Tools.:
    I didn’t classify and I haven’t done posts in Talk. Besides I have technical problems with Tools.: I use IE10. I can view shared dashboards/view a galaxy from Talk in Tools, but I can’t create dashboards.: When clicking Data, nothing happens.
    Actually, I would like to comment concerning merging galaxies.:
    At your dashboard “4blog Merger Fraction” there is given “298 total items” in “Control Merging”, but in your blog post however you cite “Control mergers (245)”. I wonder, why there is a difference? Do I miss something? Well, I found another different number in the control sample data, which I had downloaded.: 276 Mergers (127 tidal and 149 merging). That’s confusing!
    Besides I checked a few IDs and I found following one, which is however classified as merging in both dashboards and also in the downloaded data, but actually it isn’t a merger, because the 2nd object is a star!http://quench.galaxyzoo.org/#/examine/AGS00002e5

    • Michael Zevin says :

      I’ve heard of some others having issues with tools when using IE, I’d recommend using another browser when using tools – I use Chrome and it works great with tools.

      As far as the discrepancies with the number of merging control galaxies, it probably has to do with the filter you applied on the data. During classification, there were 4 options you could select to identify if the galaxy showed signs of merging: Merging, Tidal Debris, Both Merging and Tidal Debris, or Neither. Since we are looking at galaxies that show any signs of a merging event, we want to include the galaxies that were identified as ‘Merging’, ‘Tidal debris’, and ‘Both’.

      You can do this by applying just 1 filter. Instead of filtering all 3, you can just filter out the galaxies that do not show any signs of merging. The prompt for the filter should look like:

      filter .Merging != ‘Neither’

      which is just saying ‘filter only galaxies that have some sort of merger signature. When I did this I got 298 of the control galaxies with merger signatures.

      You are definitely right about the false-positive merging galaxy, it sure is a star in the foreground field and not part of a galaxy merger. Unfortunately, things like this are bound to happen (hopefully not too often) when using galaxy zoo – most people saw it as a merging event and classified it as such. During classifications, people also cannot click on the objects (as you can in the SDSS navigation) to confirm whether something is a ‘star’ or a ‘galaxy’. The power of galaxy zoo though is the number of classifications – even though a few samples may be classified wrongly we will have many many more that are classified correctly!

      • zutopian says :

        Thank you for your reply. While I was sending a comment (at 8:54 pm) about another similiar case, you replied.: I picked again a wrong classifaction and I have just checked a few IDs so far! I am skeptical concerning the reliability of the classifications. I think, that especially all merger classifications should be checked by the science team.

      • zutopian says :

        So I forgot to add the option “both (merging and tidal)”. I reviewed the downloaded control data.: 13 both +127 tidal+ 148 merging= 288
        (In my 1st comment I wrongly added “149” merging)
        There is however still a difference! 288 versus 298
        Well, I know the answer.: There are 9 star/artifact classifications in the downloaded control data. However, they are also in the “control galaxies with merger signatures” sample! This “Merger sample” should be actually without “star” classifications. I think, that the science team should check, if these are really stars!
        Finally: 13 both+ 127 tidal+ 148 merging+ 9 STARS= 297 (+1 ???)

      • zutopian says :

        Jean find out, that “+1 ???” is AGS00004n1 =”null” classification.
        Here is the related Talk discussion, where she refers to my comments.:

      • Jean Tate says :

        I checked the latest version (v3, by my count) of the QC catalog; zutopian is right: the prompt “filter .Merging != ‘Neither’” selects objects/records for which the primary classification is ‘Star/artifact’. It also selects the object with AGS ID (‘uid’) AGS00004n1; the value of ‘merging’ for this object is ‘null’ (it’s a duplicate record that should have been removed but was not) … this is the “+1 ???” :-;

        I wrote this up in a post in Quench Talk, here

  2. zutopian says :

    In addition to my previous comment:
    The following one is from the Q-Sample and classified as merging, but actually it isn’t a merger, because the 2nd object is a star according to the spectrum chart! http://quench.galaxyzoo.org/#/examine/AGS000006k

  3. zutopian says :

    I continued to check further classifications.: Here is a case, which is actually cleary a merger, but however it is classified as “Neither”.
    I am astonished! (Please confirm, that there is given “Neither”: QS data).

    So here is a merger, which had been missed and curiously it is an obvious one, but in the previously mentioned cases, the classifications as “merger” are wrong!

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