Thursday, August 15, 2013

What Did Galaxies Look Like at Cosmic High Noon?

The rate of star formation was highest about 10 billion years ago, a period that CANDELS astronomers call “Cosmic High Noon.” The redshift then was about 2, which means that the universe has expanded by a factor of three since then in each of the three spatial dimensions, so it was 3x3x3 = 27 times denser back then. The universe was also much brighter, with so many galaxies so much closer together forming lots of short-lived massive stars, which shine much more brightly than lower-mass long-lived stars like our sun.

What did those early galaxies look like, and how did they evolve into the galaxies we see around us today? Answering that question is one of the most important goals of the CANDELS survey. The infrared capability of the Wide Field Camera 3 (WFC3), installed in the last astronaut visit to Hubble Space Telescope in 2009, gives us the ability to see galaxies at redshift 2 in the wavelengths of visible light. Visible light, with wavelengths ranging from blue at 0.4 to red at 0.7 microns (a micron is a millionth of a meter), gives us crucial information about the long-lived stars in galaxies. The wavelengths of light emitted at redshift 2 expand by a factor of 3, just as space does, so visible wavelengths expand to 1.2 to 2.1 microns. WFC3 allows us to make images at wavelengths as long as 1.7 microns, while WFC3 and other HST cameras make images at shorter wavelengths that allow us to trace recent star formation because such ultraviolet light is emitted by short-lived massive stars.

Simulated galaxy at redshift 2.1 from a high-resolution cosmological simulation.  Top: rest-frame optical
image from the Sunrise computer code, taking in account stellar evolution and the scattering and absorption
of light by dust and subsequent dust re-radiation.  Bottom: The same simulated galaxy, as seen by Hubble
Space Telescope in V (visual light) and H (1.5-1.7 micron infrared wavelengths) bands.  Because of the
redshift of the radiation from this galaxy, what HST sees as V-band light was emitted as ulraviolet in the
galaxy rest frame, which mainly traces new star formation, while what HST sees as H-band light was emitted
as red light, which traces the older stellar population in the high-redshift galaxy.  Note that the V-band image
is clumpy, which is also often the case for real galaxies at these redshifts. Image Credit: Joel Primack
One of the things that we have found is that star-forming galaxies at redshift 2 were often rather clumpy, unlike the rather smooth Milky Way and other nearby galaxies. My colleagues and I have been simulating the formation and evolution of galaxies, and our simulations often also look rather clumpy, with giant star-forming regions in their disks. The clumps occur partly because the galaxies have so much gas in their disks that the disks become gravitationally unstable and break up into clumps of gas that rapidly form stars. We have been comparing the observed and simulated galaxies systematically, and we have been gratified to find that they appear fairly similar in their sizes and shapes, as well as their clumpiness.

Big galaxies today are a combination of stellar disks and spheroids. The Milky Way is a disk galaxy, with a relatively small spheroidal bulge. The most massive nearby galaxies are giant ellipticals, essentially all spheroidal. Most stars appear to form in galactic disks, but today most stars are in galactic spheroids. How did that transformation occur? The standard answer is that when comparable-size disk galaxies merge, their beautiful disks are turned into spheroids as the disk stars go every which way.

In addition to simulating galaxies, my colleagues and I have also run the Bolshoi simulation, the highest resolution large cosmological simulation yet. Using the Bolshoi simulation, we have found that there are not nearly enough galaxy mergers to account for the large number of galactic spheroids. But we have also found that the clumps can rapidly merge onto the galactic centers, which may well be the most important mechanism for forming galactic spheroids. In order to see if this true, and to discover other secrets of galaxy formation, we are running many more galaxy simulations and comparing them ever more systematically with observations. We have also just finished running a new version of the Bolshoi simulation based on the precise new cosmological parameters released by the Planck cosmic microwave background satellite team on March 21, 2013.

Key collaborators on this work: Most of the galaxy simulations were run by Daniel Ceverino, now a postdoc in Madrid. At UCSC my grad student Chris Moody converted them to realistic images taking into account stellar evolution and dust, using our Sunrise computer code; Priya Kollipara measured the sizes and shapes of these images; and Mark Mozena compared them systematically with CANDELS observations under the direction of David Koo and CANDELS Co-PI Sandra Faber.  Avishai Dekel -- with his postdocs Adi Zolotov and Dylan Tweed, his student Nir Mandelker, and others -- has analyzed these simulations and clarified the nature of the disk instabilities that lead to clumps merging to form spheroids. Anatoly Klypin ran the Bolshoi simulation; Peter Behroozi, Sebastian Trujillo-Gomez, and I helped analyze it; and Lauren Porter and Rachel Somerville worked with me on using the Bolshoi simulation to make galaxies semi-analytically.

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