Eight years and 8 billion years of cosmic history

Next up in our series of eight blog posts celebrating eight years of Galaxy Zoo is this post from Tom Melvin, who was the lead author of the the first publication from Galaxy Zoo: Hubble, which looked at how the fraction of barred disk galaxies has evolved over the last eight billion years. Tom is also the first person to write a PhD thesis substantially based on Galaxy Zoo classifications, which he is in the process of completing final corrections for.

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Barred disc galaxies at high redshift identified by Galaxy Zoo Hubble. The redshift (‘z’) and the fraction of volunteers identifying a bar (‘Pbar’) are noted in each image.

This was the first time the Galaxy Zoo volunteers had been asked to classify galaxies taken by the Hubble Space Telescope, which provided beautiful images of galaxies whose light has taken up to eight billion light years to reach us!

With your classifications, we were able to select a sample of disk and barred disk galaxies, as shown above in Figure 1, and explore how the fraction of disk galaxies that are barred has evolved over the last eight billion years. We found that this bar fraction has been increasing as the Universe has grown older, doubling from 11% eight billion years ago to 22% four billion years ago, which is shown below in Figure 2. We also know from Galaxy Zoo 2 that this continues to increase, with around one third of disks having a bar in our local Universe. We were able to expand on this by showing that it was the most massive disk galaxies that were the driver of this evolution.


Redshift evolution of the fraction of barred disc galaxies. Each point represents the observed bar fraction in a 0.3 Gyr bin, with the number of barred disc galaxies and total number of disc galaxies indicated. The grey shaded region indicates the error on the measurement. We show the mean bar fraction for the whole sample (fbar = 13.3 ± 0.7%) as the horizontal dot-dashed line, as well as a linear relationship between the bar fraction and the lookback time which is shown by the solid line.

As bars tend to only form in disk galaxies that are settled and relaxed, or ‘mature’, our results showing an increasing bar fraction over the last eight billion years tells us that the disk galaxy population has matured as the Universe has aged. As this evolution is being driven by the most massive disk galaxies, we were able to conclude that the most massive disk galaxies become mature sooner than their lower mass counterparts.

In addition to these results, we were able to identify a population of ‘red spiral’ galaxies thanks to your classifications. These red spirals’ would typically be omitted from other disk samples, as they would be classified as elliptical galaxies – but as you can see below, these are clearly beautiful red spiral galaxies! What is interesting about this population of disks is that their bar fraction of 45% is much higher than the bar fraction of the whole disk sample, which is roughly 14%.


Images showing 3 unbarred (images a − c) and 3 barred (images d − f) “red spiral” galaxies from Galaxy Zoo Hubble.

So, thanks to your help classifying the amazing images from the Hubble Space Telescope, we were able to track the evolving bar fraction of disk galaxies over the last eight billion years. There is plenty more to be done with this sample of galaxies, so keep an eye out for future results looking at how galaxies have evolved over the past eight billion years!

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About karenlmasters

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

One response to “Eight years and 8 billion years of cosmic history”

  1. Ahmad says :

    Thank you Karenlmasters and for all the team at zooniversere for the great efforts you do in enhancing the way that discoveries are going by accumulating people’s opinion to reach more reliable theories about this great universe. I was interested on how you were able to link the structure of the galaxies especially their bars to the age of the galaxies.
    I was wondering if the dark matter and the dark energy could also be an effect of the structure of the galaxies as they are filling the gabs in the space of the galaxies and the gravitational forces between the components of the galaxies!
    your opinion and you reply will be appreciated.
    many thanks

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