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Galaxy Zoo: unwinding the winding problem - observations of spiral bulge prominence and arm pitch angles suggest local spiral galaxies are winding

Research output: Contribution to journalArticle

  • Karen L. Masters
  • Chris J. Lintott
  • Ross E. Hart
  • Sandor J. Kruk
  • Rebecca J. Smethurst
  • Kevin Casteels
  • William C. Keel
  • Brooke D. Simmons
  • Dennis Olivan Stanescu
  • Jean Tate
  • Satoshi Tomi
We use classifications provided by citizen scientists though Galaxy Zoo to investigate the correlation between bulge size and arm winding in spiral galaxies. Whilst the traditional spiral sequence is based on a combination of both measures, and is supposed to favour arm winding where disagreement exists, we demonstrate that, in modern usage, the spiral classifications Sa-Sd are predominantly based on bulge size, with no reference to spiral arms. Furthermore, in a volume limited sample of galaxies with both automated and visual measures of bulge prominence and spiral arm tightness, there is at best a weak correlation between the two. Galaxies with small bulges have a wide range of arm winding, while those with larger bulges favour tighter arms. This observation, interpreted as revealing a variable winding speed as a function of bulge size, may be providing evidence that the majority of spiral arms are not static density waves, but rather wind-up over time. This suggests the "winding problem" could be solved by the constant reforming of spiral arms, rather than needing a static density wave. We further observe that galaxies exhibiting strong bars tend have more loosely wound arms at a given bulge size than unbarred spirals. This observations suggests that the presence of a bar may slow the winding speed of spirals, and may also drive other processes (such as density waves) which generate spiral arms. It is remarkable that after over 170 years of observations of spiral arms in galaxies our understanding of them remains incomplete.
Original languageEnglish
Pages (from-to)1808-1820
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
Early online date30 Apr 2019
Publication statusPublished - 1 Aug 2019


  • stz1153

    Rights statement: This article has been accepted for publication in MNRAS ©: 2019 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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