Mergers Paper submitted

This is from Dan Darg, a graduate student at Oxford, who’s been working on the mergers:

merger_title.jpg

The mergers paper is finally out and will be quite a tour de force. We are confident Galaxy Zoo is the largest visually examined parent sample from which any merger sample has ever been derived and we were able to put together ~3000 merging systems. By contrast, a decade or so ago, a sample with 20 mergers would have been considered a `large’ sample. Galaxy Zoo has thus enabled us to examine several of the key properties of merging galaxies. These include their colours, (stellar) masses, environment, star-formation rates and AGN activity.

The paper is quite long but should be fairly readable to a general audience. I begin in section 1 by an overview of the issues that concern mergers in modern astrophysics and previous methods to find them. This gives us a means to contrast and compare the Galaxy Zoo method which we believe has several advantages.

In section 2 I describe the construction of our catalogue of ~3000 mergers. Here you can see exactly how your votes were used to find the most robust merging systems and is well worth a read (especially if you want to see what issues arose and what we’ll try to overcome in the Galaxy Zoo 2 project).

In section 3 we start to look at results, starting with colour-magnitude digrams since these are the most direct things we detect when we look at pretty much anything in astronomy, i.e. how much and what colour light is coming from mergers. We compared the light with that from a randomly select “control” sample of galaxies taken from SDSS and found that our mergers have a wider spread in colour! In particular, we found a lot of very “blue” looking galaxies in mergers. This can be interpreted to mean that mergers involve (or bring about) new star formation.

We then found some useful results in section 4. Firstly, we estimated the fraction of galaxies in the local universe involved in a major merger. This sort of thing has been sought a lot in modern research and our figure, 1-3%, is very much in the range of expectation. We also were able to estimate something new though, namely, the fraction of spiral to elliptical galaxies in mergers. No other empirical study to date has been able to do this. Interestingly, we found more spirals in mergers than ellipticals compared with the global population!

In section 5 and 6 we studied the environments and stellar masses of our merging galaxies and found that our merging galaxies tended to occupy slightly denser environments and, ellipticals in particular in mergers, seemed to be more massive than their control counterparts.

In section 7 we studied the spectra of our mergers in order to figure which ones had Active Galactic Nuclei, which ones are producing lots of new stars and which ones are inactive. All of these processes and properties are important to our understanding of how galaxies form and evolve and our paper will hopefully provide the impetus for lots of new projects that seek to answer these questions.

Many thanks to all you all for pressing that “merger” button! Lots of interesting science is coming out of it!

Dan

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9 responses to “Mergers Paper submitted”

  1. Waveney says :

    Well done.

    Note: There is a reference made here to the Merger Hunting, but the main use of your results will be in the second Mergers paper.

  2. Alice says :

    That’s great – I’ll use that as a reference while I’m ploughing through the real thing, Dan! Thanks for writing it up for us.

  3. Joseph K. H. Cheng says :

    Congratulations, Dan ! Glad that our Galaxy Zoo data have made more contribution science. I hope this trend will carry on in the future.

  4. Thomas J says :

    Well done Dan and thre GZ team.

  5. Graham Dungworth says :

    Congratulations Dan et al.

    Those somewhat denser environments and the 1.5/1 ratio for spirals to ellipticals in mergers could be due to preferential stripping of Dark Matter, leading to depletion in gas poor ellipticals; the more massive the system the more probable the merger. Might help to reveal better and crucial abundance and distribution data for Dark Matter.

    All in all an accomplished feat for you all.

  6. LankyYankee says :

    Great news! Is the text of the paper posted where Zooites could read it?

  7. Alice says :

    Usually it’s made available after a week or two, at which point they’ll update with a link 🙂

  8. Mark says :

    Could Dark Energy be an area of intergalatic space propogating the effect because of the lack of matter’s impact on space itself?

    To put it differently, we know that frame dagging is caused by matter. But could the distance away from matters frame dragging effects make space itself act differently? If so then this could explain why we can’t see dark matter, why we can’t actually see the energy, but we would see it’s effects and we do.

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