Hubble's Universe Unfiltered

  • September 16, 2016

    Thousands of Galaxies in the GOODS/ERS2 Field

    by Frank Summers

    Looking deeply into the cosmos with Hubble, astronomers can see all sorts of galaxies. Some are large and nearby, with the familiar spiral and elliptical shapes. Others are small and distant, with dotted, elongated, and/or irregular structures. This long exposure, from the Great Observatories Origins Deep Survey (GOODS), showcases the wide variation in galaxy shapes, sizes, and colors seen across the universe.

    One of the reasons this image is so colorful is that it goes beyond the wavelengths that the human eye can see (visible or optical light). This survey was also done in the shorter wavelength ultraviolet light, as well as the longer wavelength infrared light. Astronomers are studying these galaxies across all the wavelengths Hubble can observe in order to get a fuller picture and fuller understanding of their structures.

    The change of structures with distance is extremely important, as it also indicates how galaxies change over time. The light from a galaxy five billion light-years away has taken five billion years to traverse the intervening space, and thus we see that galaxy as it was five billion years ago. Looking out into space is also looking back into time.

    The history of galaxy development is contained in these images. Nearby galaxies show the familiar patterns of what I'll call adult galaxies. More distant galaxies are, on average, younger and show the development of galaxies through their teen years. The most distant galaxies seen by Hubble are the child galaxies, with very incomplete development and only the indications of their future potential. The GOODS survey and others have systematically imaged, categorized, and tracked the changes in galaxies over time to learn how our Milky Way and its brethren went from tiny stellar and gaseous clumps to vast galactic swarms.

    But the story is not written plainly in these images. The expansion of the universe stretches the light waves that travel across this expanding space. Hence, light that starts out as visible light can be stretched to infrared wavelengths by the time we observe it. Similarly, ultraviolet light can be stretched to visible light, or even beyond to infrared light. This cosmological redshift adds in a color shift for each galaxy according to its distance from us. The deconvolution of color information is an important consideration in comparing images and observed structures. It is also a major motivation for doing such multiwavelength studies.

    Finally, I'll note that cosmological redshift pushes the most distant galaxies outside of Hubble's reach. The light from the most distant galaxies, the baby galaxies if you will, has had their light redshifted so far into the infrared regime that Hubble cannot observe them. NASA's current infrared observatory, the Spitzer Space Telescope, does observe at those wavelengths, but lacks the angular resolution to discern such tiny sources. To see these galaxies requires a telescope with the same keen resolution of Hubble, and extending to the infrared wavelengths of Spitzer. That is the James Webb Space Telescope, which will launch in October 2018.

    Images like this can be mesmerizing for astronomers. The diversity of shapes and sizes and colors are cool to explore for most anyone. Now add on top of that an exploration of galaxy development across both space and time. The visual and intellectual adventure can be truly intoxicating.

    A high-resolution version (18 megapixels) of this image is available on the HubbleSite press release pages. Enjoy!