Galaxy Zoo Literature Search

Dear volunteers,
Here at Galaxy Zoo we know that some of you are looking for ways to be more involved in the entire process of making science from your clicks.

So we had an idea…..

The team are currently in the process of writing a paper which in its introduction discusses some of the current assumptions/errors/approximations common among our fellow astronomers when thinking about galaxy morphology and classification. As such we’d like to collect as many papers as possible which do the following things:

  1. Claim that colour and morphology are equivalent
  2. Define “early-type” galaxy as any galaxy without visible spiral arms (e.g. our “smooth” category, which can include elliptical galaxies, and smooth disks), rather than as a galaxy that isn’t a disk.
  3. Define “early-type” galaxy as including Sa spirals as well as lenticular and ellipticals.
  4. Define “late-type” galaxy as only late-type spirals (e.g. excluding Sa spirals)
  5. Use colour or spectral type to split galaxies into “early-“ or “late-“ types (or “elliptical” and “spiral”)
  6. Use the bulge-to-total ratio (or some proxy for it like concentration, or the SDSS “fracDeV” parameter) to place spiral galaxies in a sequence.

The current draft text in the paper which talks about these assumptions is:

“The morphology of a galaxy encodes information about its formation history and evolution through what it reveals about the orbits of the stars in the galaxy, and is known to correlate remarkably well with other physical properties (e.g. Roberts & Haynes 1994). These correlations, along with the ease of automated measurement of colour or spectral type, have resulted in a recent trend for classification on the basis of these properties rather than morphology per se (e.g. Weinmann et al. 2006, van den Bosch et al. 2008, Zehavi et al. 2011). Indeed the strength of the correlation has led some to authors to claim that the correspondance between colour and morphology is so good that that classification by colour alone can be used to replace morphology (e.g. Park & Choi 2005, Faber et al. 2007). Meanwhile the size of modern data sets (e.g. the Main Galaxy Sample of the Sloan Digital Sky Survey, SDSS, Strauss et al. 2002) made the traditional techniques of morphological classification by small numbers of experts implausible. This problem was solved making use of the technique of crowdsourcing by the Galaxy Zoo project (Lintott et al. 2008, 2011). One of the first results from the Galaxy Zoo morphological classifications was to demonstrate on a firm statistical basis that colour and morphology are not equivalent for all galaxies (Bamford et al. 2009, Schawinski et al. 2009, Masters et al. 2010) and that morphology provides complementary information on galaxy populations useful to understand the processes of galaxy evolution. “

and later when talking about the spiral sequence:

“Modern automatic galaxy classification has tended to conflate bulge size alone with spiral type (e.g. Laurikainen et al. 2007, Masters et al. 2010a), and automatic classification of galaxies into “early-” and “late-” types, referring to their location on the Hubble Sequence and based on bulge-total luminosity ratio (B/T ) or some proxy for this through a measure of central concentration, or light profile shape (e.g. Sersic index, as reviewed by Graham & Driver 2005) has become common (e.g. van der Wel et al. 2011). Indeed, Sandage (2005) says this is not new, claiming ”the Hubble system for disk galaxies had its roots in an arrange- ment of spirals in a continuous sequence of decreasing bulge size and increasing presence of condensations over the face of the image that had been devised by Reynolds in 1920.””

We’d like to ask for your help in searching for more examples of these behaviours. We have made a simple Google form, and we ask that you submit any examples you find in the next few weeks.

Some of the papers you find might end up cited in the Galaxy Zoo team paper (please be aware there are rules/guidelines about the appropriate number – we don’t want to have too few;  it doesn’t make the point about how widespread this is, and we don’t want to single out specific astronomers,  but the journal won’t accept too many either). If there are more papers found than we can use, they will be kept in a list on the Galaxy Zoo website (and we can continue to add to them if needed).

I want to reassure you that helping with this does not mean you have to read the entire extragalactic astronomy literature, or even the entirety of a paper! The best place to look for this information in a paper will be the “Sample Selection”, or “Data” sections. Modern online PDF papers also have excellent search facilities – so searching the text for key words (e.g. “spiral”, “early-type”, “colour/color-selected”) may work extremely well.

We’re happy for you to do this however you like (e.g. Google Scholar is fine), but we’d like you to return the NASA ADS (Astrophysics Data System) URL for the paper you find. You can search ADS here: http://adsabs.harvard.edu/abstract_service.html, and I give examples below of the URL I mean. This makes it easy for us to get the full bibliographic data to add the reference to the paper.

One tip – there are some papers in extragalactic astronomy which are cited by most/many results. A good place to start looking through recent papers would be the citation and reference lists of such papers, which can be found in ADS.

For example:

Strateva et al. 2001 “Color Separation of Galaxy Types in the Sloan Digital Sky Survey Imaging Data”
http://adsabs.harvard.edu/abs/2001AJ….122.1861S  is cited by 926 papers, and references 25 – this would be an excellent starting place, and the more papers you read the more mentions you may find other other papers doing similar things.

Other good starting places:
Strauss et al. 2002: “Spectroscopic Target Selection in the Sloan Digital Sky Survey: The Main Galaxy Sample” http://adsabs.harvard.edu/abs/2002AJ….124.1810S

Ironically the papers which cite some of our Galaxy Zoo papers where we demonstrate there are galaxies which are not in the normal correlation between colour and morphology may also be good starting points (some citations to these are along the lines of saying things like: “most galaxies fall into blue=spiral; red=elliptical, a few don’t (cite Galaxy Zoo here), but we’re going to use this definition anyway”.

The initial papers on colour not being the same as morphology are:

Bamford et al. 2009 (281 citations): http://adsabs.harvard.edu/abs/2009MNRAS.393.1324B
Schawinski et al. 2009 (81 citations): http://adsabs.harvard.edu/abs/2009MNRAS.396..818S
Masters et al. 2010 (125 citations): http://adsabs.harvard.edu/abs/2010MNRAS.405..783M
We hope that if several of you take up the challenge, you’ll find different paths through the literature and find lots of different examples for us. Again here’s the link to our Galaxy Zoo Literature Survey.

Thanks for your help!

Karen Masters (Galaxy Zoo Project Scientist)

About karenlmasters

Astronomer at Institute of Cosmology and Gravitation, University of Portsmouth. Project Scientist for Galaxy Zoo. Spokesperson for the Sloan Digital Sky Survey. Busy having fun with astronomy!

One response to “Galaxy Zoo Literature Search”

  1. Jean Tate says :

    This is a very cool, and worthy project! 🙂

    May I ask, to what extent are you also interested in distinguishing lenticulars (S0 galaxies) from early type spirals (Sa) and late type ellipticals (E+)? Especially when it comes to transition types, E/S0 and S0/a.

    In my experience, this is an even more confused area of classification than the proxies used to distinguish spirals from ellipticals, and early type spirals from late type ones.

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