Wednesday, October 2, 2013

An Update on Mature Galaxies in the Early Universe

One of the goals of CANDELS is to document how galaxy shapes evolved over about 13 billion years of history.  Avid readers of this blog might recall our posting about the presence of mature galaxies 11 billion years ago. In that post, we explained that a beautiful montage of galaxies that accompanied a recent European Space Agency press release gave a misleading impression of the scientific finding.

We're delighted to say that the ESA press officer understood our concerns immediately and offered to produce a version with images of galaxies that are from the CANDELS data themselves. That image is shown below. The leftmost panel is identical with the original press release, but the center panel now uses images of galaxies at redshifts between z=0.3 and 0.7 and the rightmost panel uses galaxies between z=2 and 2.7.

An updated image of the evolution of the Hubble sequence over the past 11 billion years. From ESA.

At each epoch, the illustration is intended to show the Hubble tuning fork. The four leftmost postage-stamps show early-type galaxies, known as ellipticals and lenticulars (or S0 galaxies). The tuning fork then splits to show the late-type spiral galaxies. The top four images show normal spiral galaxies while the bottom four panels show barred spirals (galaxies with a bar-like feature in the middle).

While this is still not a perfect representation of what is happening to galaxy shapes, there are several things to notice.

  • The galaxies 4 billion years ago and 11 billion years ago were smaller. Alas, this image still doesn't have the relative scales exactly correct between the epochs, but the sense of the evolution from smaller to larger is consistent with detailed measurements.
  • The early-type galaxies 11 billion years ago look sort of similar to those today, albeit smaller. They had about the same round, ball-like shape with nearly uniform color. They tend to be redder than the late-type galaxies.
  • The late-type galaxies 4 billion years ago look pretty similar to today. They have spiral features and you can find examples that have bars in their centers.
  • The late-type galaxies 11 billion years ago look a bit different. It is very hard to find any convincing examples of barred spirals, and the ones that don't have bars look more disordered than their present-day counterparts.
  • The greenish tint of the galaxies in the center panel isn't real. These images are composites through different filters, and the color balance has not been tweaked in a meaningful way. When measured in detail, galaxies in the past tended to be bluer than today, which is a result of the fact that they contained more hot, young stars and less dust.
While the illustration here gives a better illustration of how galaxies are transforming their shapes, it doesn't illustrate some of the more interesting features like bright clumps or mergers. Also it doesn't tell you anything about the relative numbers of galaxies of different shapes. It turns out that the early-type galaxies were quite rare 11 billion years ago, while they are much more common today, for example. 

Finally, in spite of our quibbles about the artwork, we would like to give a shout-out to the Hubble outreach team at ESA for phenomenal work over the years. Most recently, they were recognized for their efforts with a Parsec award "Fact behind the Fiction" award for their brilliant Hubblecast series. 

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