Saturday, August 17, 2013

Were Galaxies Really Mature 11 Billion Years Ago?

Nostra Culpa

Yesterday, the European Space Agency put out a press release on CANDELS research that has been widely covered by various media. Sometimes the scientific message gets garbled in press releases, in spite of best intentions all around. In this case, the wording of the press release is pretty good about conveying the key message (although some of the subtleties get lost). But the gorgeous picture that accompanies the release is more wrong than right. Alas, there was a bit more "artistic license" taken with the press-release artwork than there should have been, and we failed to put the brakes on before the press release went out. The pictures of the galaxies marked as "11 billion years" were not taken from the CANDELS images, and are not at the right redshift (redshift z=2.5 corresponds to a lookback time of 11 billion years). If you would like to see what the images really look like, read on.

This is the gorgeous image that accompanied the ESA CANDELS press release. If only it were closer to the truth! The images further down in this blog post show what galaxies 11 billion years ago really look like in the CANDELS images.

What did galaxies look like 11 billion years ago?

Cutouts from Lee et al.
Galaxies on the right are
forming stars rapidly.
Galaxies on the left are
The basic conclusion of the papers on which this story was based is that we can start to see the dichotomy between star-forming galaxies being "disk like" and non-star-forming galaxies being "spheroid like" already being set into place 11 billion years ago. That's the central message and is what we are finding in the CANDELS survey in the various samples studied by Tao Wang, Eric Bell, BoMee Lee, Alice Mortlock, Victoria Bruce and others.

We wish we could see galaxies 11 billion years ago with the sort of clarity shown on the press-release image, but unfortunately even with Hubble we can't see that level of detail. To the extent that we can distinguish detail they look (a) smaller and (b) generally bluer, and (c) less well-ordered than present-day galaxies.

So -- yes we can see spheroids and disks emerging as separate sequences -- but it is not the case that the galaxies look like their present-day counterparts, which is the impression that you get from looking at the press-release image.  Arjen van der Wel's quote in the press release that they look "remarkably mature" is reasonable when you consider that they might have all looked like train-wreck mergers-in-process.  But some of the news stories based on the press release are taking that to mean they are completely mature, which is certainly not the case.

Illustration of the morphologies of massive galaxies at redshift z~2 using CANDELS cutouts from Wang et al. (2012). The upper-left insert color codes galaxies on the same axis scale, with blue being the star-forming galaxies and red being the less-star-forming galaxies. You can easily see that the less-star-forming ones tend to look more spheroidal.

The light reaching us from galaxies at a redshift z=2.5 took about 11 billion years to get here. So we are looking back in time to when the universe was only about 2.7 billion years old. Below and to the right are some figures from some recent papers by CANDELS and other teams that show images of galaxies at about this redshift. Perhaps this gives a more accurate impression of what we can say and can't say about the comparison between nearby galaxies and their distant progenitors.
A montage of "Milky-Way progenitor" candidates from the CANDELS images put together by Pieter van Dokkum and the 3D-HST team. 


  1. Hi,
    Are the images of the galaxies resized so we can compare their real sizes with each other in the montage?

    Thank you and congratulations for the great work.

  2. Only for the last figure. There the images have been resized so that they are the the same physical scale.

  3. Very glad for the correction and explanation. In the original ESA press release, then, where do the images marked as "11 billion years" old come from, if not from CANDELS observations?

  4. I think they were mostly (if not all) taken from the Hubble Ultra-deep field, which is actually included in the CANDELS "footprint" on the sky, but had a much longer exposure time. The galaxies in the "11 billion year" category in the collage are mostly at redshift z=0.6 to 1 rather than z=2.5 --- so we are seeing light that left 6-8 billion years ago rather than 11 billion years ago. We'll be making a new collage using galaxies at the correct redshifts and will post it on the blog when it's ready.

  5. Harry, I'd like to thank you for posting the correction in the forum. The licence taken in press releases can do more damage then good in engaging the public and your clarification has gone a long way towards correcting the false impression given.

    Thank you

  6. How long till the point of no return?

  7. All this data is great. So much accessible to so many. Its like we're in a space race to understand the universe, and we're almost at the finish line. Now we've got to bring that knowledge to the people and help them see what discoveries lie just beneath the surface of popular culture. And then there's all them X-files to be.declassified and packaged into cinematic travelogues or something. Some day we'll get there.

    All this data posted all over the internet about the universe and planetary discoveries reminds one of the Victorian enlightenment's studies about all the ideas they didn't quite grasp, yet were beginning to finally piece together Somehow all this data must coalesce into some sort of super museum or online mall for all the data models and scientific researchers. There are some of the old guard that resist the currents of the tides of change, yet we are headed in the direction of universal revelation with such a critical mass of data real, change is imminently on the horizon.

    These are exciting times to be living in, and we should all be grateful to witness all this scientific advance, considering how far we have now advanced, despite the numerous setbacks of the Cold War.

    It is as if we are about to get off the beach of evolution and back into the garden of divine knowledge.

  8. Question? concerning the galactic plane, are there any signs of alignment or predisposition for alignment of the galactic plane in regards to all the galaxies? thank you and your colleagues for the fantastic research! ty