By Harry Ferguson (STScI)
In 2009, there was a call for ambitious proposals to use Hubble for projects that were beyond the scope of what a typical time allocation could accomplish. Hubble time is usually doled out in “orbits.” One orbit of Hubble takes about 90 minutes yielding 45 minutes to an hour of observing time (because the Earth typically blocks a portion of the sky from view). A typical proposal will be for a few orbits of observing time. In this particular call, proposers were asked to consider projects needing at least 450 orbits.
Two teams responded to this call with very ambitious proposals to observe representative patches of sky to search for the most distant galaxies, study the assembly of galaxies over cosmic time, trace the formation of black holes in the centers of galaxies, and study distant supernovae. The proposals were similar in many respects, and the time allocation committee recommended merging the two teams. Thus the CANDELS collaboration was formed, with participation of nearly 100 astronomers with diverse backgrounds and interest. The time allocation was 902 orbits, which is the largest in the history of the Hubble telescope.
Why did so many astronomers – on the proposal teams and the time allocation committee – think this kind of observation was important? And what have the observations revealed?
The answer to the first question goes back to a fundamental assumption of cosmology – that the universe is basically the same in all directions. Obviously this assumption breaks down on small scales (otherwise there wouldn’t be planets, stars, and galaxies), but it appears generally true when averaging over scales larger than about 10 million light years. The Hubble observations allow us measure the past: to observe galaxies and supernovae that are so distant that their light has taken billions of years to reach us. Any single Hubble image will have both nearby galaxies and galaxies for which the light-travel time more than 13 billion years (the universe itself is 13.8 billion years old). To get a reasonably fair census of the distant universe, we need to point at places that are out of the plane of the Milky Way galaxy. We need to take fairly long exposures to collect enough photons. We should observe these same patches at other wavelengths (from x-ray to radio). All else being equal, we should divide the total area into several patches that are disjoint on the sky to reduce systematic errors due to foreground dust or large-scale cosmic structures. Hence the CANDELS survey: a public Hubble survey of the most-studied patches of sky, coordinated with observations from other major observatories.
The CANDELS observations were completed in 2013 and so far there have been over 200 papers published using the data. It’s possible to give only at taste of the scientific results in this blog article. There are many more summaries on CANDELS blog site.
Ever since the installation of the WFC3 camera on Hubble in 2009, the race has been on to identify the most distant galaxies. It was unclear at the outset which strategy would be most successful: taking very deep exposures over a tiny area, shorter exposures over a wider area, or pointing at galaxy clusters and using gravitational lensing to magnify galaxies in the background. Over the course of several years, Hubble has done all three, and the current record holders are in one of the CANDELS fields and in the background of a cluster of galaxies. Follow-up observations of a bright candidate in the CANDELS GOODS-North field suggest that it is at a redshift z=11.1, about 400 million years after the Big Bang (Oesch et al. 2016).
Aside from the lure of seeing the most distant galaxies, there is much to learn from studying statistical properties enabled by the large survey – with samples now approaching 1000 galaxies within the first billion years and 10000 within the first two billion. (Prior to the installation of WFC3 and the CANDELS survey, there were only a handful of good candidates identified at these early times.) There appear to be enough of these very young galaxies to explain the rather rapid “re-ionization” of the universe. About one billion years after the big bang there was a huge injection of energy that stripped 99.99% of the electrons away from the protons in the hydrogen between galaxies. The observations show that there was enough energy in young galaxies to explain this; although we are not yet certain that enough of the photons at just the right energy to ionize hydrogen can escape, because the gas within the individual galaxies might absorb most of it. Galaxies in the first billion years have bluer colors than their counterparts at later epochs – probably because they have not yet had enough time to build up the heavy elements needed to form large amounts of dust and to lower the temperatures of young stars. Nevertheless, in spite of being bluer, few if any of the galaxies show the very blue signature expected of galaxies forming their first generation of stars. Comparing the evolving numbers and stellar masses of galaxies to the theoretically-predicted numbers of gravitationally-bound dark-matter “halos,” leads to the conclusion that the star-formation rates are almost – but not entirely – governed by the somewhat clumpy inflow of gas as the gravitational pull of the newly formed dark-matter halos draws in more gas from the surrounding intergalactic medium.
Figure 1: The left panel shows the number of very distant galaxies identified by the CANDELS survey (red) and deeper surveys (blue) since the WFC3 camera was installed on Hubble. The right panel shows the estimate of the “cosmic star-formation rate” – the number of stars formed per year in a fixed volume of the universe – as a function of time since the Big Bang.
The addition of infrared wavelengths – both from Hubble and from the Spitzer and Herschel observatories at longer wavelengths – has been essential for searching for galaxies that are either full of dust or shutting off their star formation. Such galaxies are red enough that they are difficult to pinpoint as distant-galaxy candidates in the Hubble images alone or entirely invisible in the Hubble images. Massive dusty or “quenched” galaxies are expected to be extremely rare in the early universe because there simply hasn’t been time for them to form. Nevertheless, there are dozens of interesting candidates found in the CANDELS fields when inspecting the infrared images. These will high-priority targets for spectroscopy with JWST and ALMA, which will be able to confirm their distances.
Cosmic High Noon
The overall cosmic rate of star formation peaked at a redshift z ≈ 2, when the universe was about 3-4 billion years old. The CANDELS observations provided the first large samples of galaxies with high-resolution images spanning wavelengths from the rest-frame ultraviolet to the optical. The longer wavelength data from Spitzer helps to pin down the total stellar masses of the galaxies, by providing extra sensitivity to some of the oldest, reddest stars. Using samples of tens of thousands of galaxies, we are able to assess the successes and failures of our current theoretical understanding of galaxy evolution, and provide some clues to guide future developments. The observations tell us that something is “quenching” the star-formation in massive galaxies as early as 2-3 billion years after the Big Bang. These quenched galaxies emerge as very compact “red nuggets,” which must grow substantially in size and over the next ten or so billion years, increasing in mass mostly by merging with neighboring galaxies rather than forming new stars in situ. The compact star-forming progenitors of these galaxies (blue nuggets) appear to be present in sufficient numbers to account for the red nuggets, but we do not yet entirely know how or why star-formation is shutting down. The blue nuggets have a somewhat higher incidence of active nuclei: central black holes that are accreting gas at a high rate, and perhaps heating the gas that would otherwise cool to form stars. Quenched galaxies have higher central densities of stars than most star-forming galaxies, so the thought is that when sufficiently large amounts of gas collect in the center, this triggers a burst of star formation and perhaps also feeds an active nucleus. The energy feedback from the star formation and the nucleus are sufficient to shut off subsequent star formation. High-resolution computer simulations of forming galaxies suggest that the trigger for this gas funneling is a mix of gravitational instabilities within a star-forming disk of gas and mergers with surrounding galaxies. When dust is included in these simulations, they look remarkably like the galaxies we see, but differ enough in their statistical properties (for example their colors) that we know that some aspects of the physical models are not quite correct.
Figure 2: Computer simulations vs. observations. The bottom panels show some of the highest-resolution hydrodynamical simulations of galaxies that have yet been constructed on supercomputers. The images in the middle show the same galaxies viewed from two different camera directions and placed at a large distance from the telescope so that our view matches what we might see from Hubble. The top panels show galaxies selected from the CANDELS survey. Qualitatively, the computer simulations doing a very good job of matching what we see in deep observations.
Towards the present day
CANDELS has provided us with large enough samples of galaxies that it is possible to try to find examples of what the Milky Way galaxy might have looked like in the past. We can attempt to match progenitors to descendants in the overall population of galaxies by isolating galaxies that are at about the same rank in the overall ranking of galaxies by stellar mass (from biggest to smallest). Figure 3 shows a visual summary of the results of this kind of effort – in what might be considered to be a family tree of the Milky Way. The progenitors are smaller, bluer, and generally do not have the familiar spiral-plus-bulge structure that we see in present-day galaxies. The same study provides a way to infer the amount of cold gas that ought to be present as fuel for star formation, and these predictions are being tested with ongoing observations from the ALMA observatory.
Figure 3: Examples of progenitors of a Milky-Way-mass galaxy taken from the CANDELS survey. Redshift and time (in billions of years since the big bang) run along the horizontal axis. The figure has been divided into three panels for convenience; the earliest times are at the bottom and the latest times are at the top. The galaxies are shown to the same physical scale and the colors are a fair representation of their rest-frame colors. The position along the vertical direction illustrates how blue (or equivalently, hot) the galaxy is, with red toward the top and blue toward the bottom.
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