First Result from Galaxy Zoo Hubble

Posted on behalf of Tom Melvin:

Hello everyone, my name is Tom Melvin and I’m a 3rd year PhD student at Portsmouth University. I have been part of the Galaxy Zoo team for over two years now, but this is my first post for the Galaxy Zoo blog, hope you enjoy it!

I’m very happy to bring you news of the latest paper based on Galaxy Zoo classifications, and the first paper based on Galaxy Zoo: Hubble classifications. Galaxy Zoo: Hubble was the first Galaxy Zoo project to look at galaxies beyond our local universe, using the awesome power of the Hubble Space Telescope. These images contained light from galaxies which have taken up to eight billion years to reach us, so we see them as they appeared eight billion years ago, or when the universe was less than half its current age! So what is the first use of this data? Well, we combine our Galaxy Zoo: Hubble classifications with Galaxy Zoo 2 classifications to explore how the fraction of disk galaxies with galactic bars has changed over eight billion years.
Here’s the title…..

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Our work is based on a sample of 2380 disk galaxies, which are from the Cosmic Evolution Survey (COSMOS), the largest survey Hubble has ever done. To see how the bar fraction varies over such a large time-scale, we look at the number of disk galaxies and what fraction of them have bars in 0.3 Gyr (300 million year) time steps. In Figure 1 we show that eight billion years ago only 11% of disk galaxies had bars. By 4 billion years ago this fraction had doubled, and today at least one third of disk galaxies have a bar.

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Figure 1: The evolving bar fraction with cosmic time (Figure 7 in the paper).

We know that bars tend to only form in disk galaxies which have low amounts of atomic gas and are in a relaxed state, or what we call ‘mature’. Combining this knowledge with our observations, we can say that, as the Universe gets older, the disk galaxy population as a whole is maturing. To see whether this is true for all disk galaxies, we split our sample up into three stellar mass bins, allowing us to look at the evolving bar fraction trends for low, intermediate and high mass disk galaxies.

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Figure 2: Mass dependent evolution of the bar fraction with cosmic time (Figure 8 in the paper)

The results for this are shown in Figure 2, where we observe an intriguing result. The bar fraction increases at a much steeper rate with time for the most massive galaxies (red), compared to the lower mass galaxies (blue). From this we can say that the population of disk galaxies is maturing across the whole stellar mass range we explore, but it is predominantly the most massive galaxies which drive the overall time evolution of the bar fraction we observe in Figure 1.

At the end of the paper we offer an explanation as to why the time evolution of the bar fraction differs for varying stellar mass bins. We can make the reasonable assumption that, by eight billion years ago, the majority of massive disk galaxies have formed, and have been, and continue to form bars up to the present day – hence the steeply increasing bar fraction we observe. However, the same assumption is not true for the low mass galaxies. There are some which are ‘mature’ disk galaxies eight billion years ago, but not all are ‘mature’ enough to be classified as disks. As with the most massive galaxies, these low mass disks are forming bars at a similar rate up to the present day, but the difference with this low mass sample is that there are still low mass disks forming up to the present day as well – leading to the much shallower increase in the bar fraction with time we observe.

In addition to these results, we are also able to present an interesting subset of disk galaxies. Your visual classifications has allowed our work to include a sub-sample of ‘red’ spiral galaxies (like those found from Galaxy Zoo 2 classifications). This sub-sample is generally omitted from other works that have explored this topic, as their way of identifying disks is based on galaxy colours. This means that these ‘red’ galaxies would have been classified as elliptical galaxies! Figure 3 shows a few of these ‘red’ disk galaxies (with the full sample of 98 here), so why don’t you take a look and decide for yourself! Not only is it very cool that you are able to identify these ‘red’ disks, but they also influence the results we observe. Just like in our local universe, these ‘red’ disks have a high bar fraction, with 45% of them having a bar! Could this be a further sign that bars ‘kill’ galaxies, even at high redshifts?

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Figure 3: A sample of ‘red’ disk galaxies found by Galaxy Zoo volunteers (Figure 10 in the paper).

So that is a summary of the first results from Galaxy Zoo: Hubble. If you want more detail have a read of the paper in full here and take a look at the press release too! Thanks for all your hard work and help in classifying these galaxies!

Posted on behalf of Tom Melvin.

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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!

13 responses to “First Result from Galaxy Zoo Hubble”

  1. Roger Griffith says :

    Awesome 🙂 congrats

  2. Peter Dzwig says :

    For Karen OR Tom, whioever cares to reply! 😉

    I have spent the afternoon looking at fig 2 and have skimmed the last couple of pages of the paper itself and as a result I have a couple of questions.

    Firstly at an LBT of 7-8 Gyr everything appears to be much of a muchness in terms of fraction of BSGs for each of your bins – in fact they are pretty much indistinguishable. What mechanism do you invoke in order to explain the sudden separation of the “massive” BSGs from the rest and the rapid growth in fraction over a period of a couple of Gyr?

    Secondly how do you explain the sudden drop off in the fraction of BSGs at 4 – 5 Gy?

    • Tom Melvin says :

      Hi Peter,

      Thanks for having a read! I will start by answering your second question – why the sudden drop off of the bar fraction in the most massive BSGs at 4-5 Gyr. This is because of the volume limit of the sample. The COSMOS field, where all of these galaxies are found, has a small area (2 square degrees), and so at this lookback time there are only so many massive disk galaxies that can fit in such a volume. So this drop off is not physical, just a limitation of our observations.

      Your first question is an excellent one! The bar fractions here are all pretty similar (especially within the errors!). The reason why the most massive ones increase at a faster rate is because they ‘mature’ sooner than the lower mass disks. They have lower gas fractions too, and these are both ideal situations for forming and sustaining bars. The lower mass disks may be less stable with high gas fractions, so bars may not form, or those that do may be less stable. So the mechanism for this is just that more massive galaxies have the required conditions for bars to form whilst not all low er mass disk galaxies do.

      The interesting thing is why do all of these mass bins have a similar bar fraction at tlb=8Gyr? For lookback times greater than this, it is expected that the universe (being much smaller in volume) was a more violent place, where galaxies are more likely to have had a recent merger and where gas fractions of galaxies were much higher. This combination means that even the most massive disks would not have the required conditions to form and sustain bars. It is a little surprising that as many as 10% of all disks have bars at this age, and we are continuing to explore this to see why!

      Tom

  3. Peter Dzwig says :

    …to clarify: that last bit refers to the massive BSGs.

  4. Jean Tate says :

    Wonderful! Well done!

    I’ve started a thread on this, in the GZ forum: Questions about the GZH paper (Melvin et al. 2014)

    If you, dear reader, have questions or comments, why not come and join in the discussion?

  5. Tom Freethesouls Zolotor says :

    When will more data be coming out from GZ Hubble? Very good press release. Thanks.
    Tom Zolotor

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