Our Latest Results
Our most current results are now posted on the HUDF09 project page.
Star Formation History of the Universe
Star Formation Rate Density as a function of the Age of the Universe. The data points show different observational measurements. The star formation rate density increases rapidly from early times to a peak when the universe was about 2 billion years old and then declines. At UCSC, we have provided the most robust measurements of this star formation rate density over the first two billion years of the universe (our measurements are shown here in red)
Credit:Bouwens
Our research group has been one of the leaders in observational efforts to explore galaxies in the first 2 billion years of the universe. The first 2 billion years of the universe is exciting because it is during this time that we expect galaxies to have built up from a very small number of stars to the large galaxies we see today. It is also during this time period that our universe was reionized and we expect galaxies to have played the key role in this process.
We have a number on-going research projects aimed at finding galaxies over this entire interval of time: from the very earliest times where it is a challenge just to find a small number of sources to later times where the goal is to collect large statistical samples, to study these samples in detail, and to understand how the properties of the galaxy population are changing. Our principal technique for finding galaxies at early times has been the dropout technique, and a detailed explanation can be found here.
Summary of Our Current Results
Here is a brief summary of our latest research results on early galaxy formation — ordered in terms of the amount of cosmic time from the Big Bang:
400 million years (Redshift 10)
Ultra Deep IR Image
Deep NICMOS Parallel Image to the Hubble Ultra Deep Field. This Region of the Sky Contains the Deepest Optical and Near-Infrared Images Ever Taken of the Universe and is useful for finding star-forming galaxies at redshifts 7 and 10 (700 and 400 million years after the Big Bang, respectively). At UCSC, we are using these data to better understand the properties of the first galaxies.
Credit:Bouwens
Over the past few years, we have been conducting a fairly comprehensive search for galaxies at these early times using several months of NICMOS data from the Hubble Space Telescope. Three years ago, we thought we had found a few galaxies which might be plausible redshift 10 galaxy candidates because they appeared to dropout at wavelengths bluer than 1300 nm. However, all of these candidate redshift 10 candidates were later detected at optical wavelengths in some very deep images that were subsequently acquired, we now know that none of the sources are such high-redshift galaxies. At present, we know of no sources in HST or ground-based data which we can reasonably assert are z~10 candidates using the dropout selection technique. However, just because we have not found these galaxies does not mean they do not exist. Current thinking suggests that these sources are simply too faint to be seen with current instrumentation on large telescopes, and we will need for more powerful telescopes to find them in large numbers.
700 million years (Redshift 7 and 8)
z~7.4 Drop-Out Galaxy
Images of a galaxy at redshift 7.4 (inside white box) in the Hubble Ultra Deep Field. This galaxy is seen just 700 million years after the Big Bang. The galaxy disappears at optical wavelengths (left), but is seen clearly in the infrared (right), as shown in the image boxes at the bottom.
Credit:Bouwens/Magee
Over the past few years, we have been using a substantial fraction of the deep near-infrared data available from the Hubble Space Telescope and large ground-based telescopes to search for galaxies which appear to have emitted their light at redshift z~7-8. These data are available over 23 square arcminutes, or ~2% of the area of the full moon in the case of the HST data and 250 square arcminutes in the case of ground-based data (or ~25% of the area of the full moon).
We identify candidate z~7-8 galaxies by looking for galaxies which dropout at wavelengths of ~1000 nm and bluer. A very careful search through the HST NICMOS data yield 9 reasonably robust z~7-8 galaxy candidates. Some ~10 other z~7-8 candidates are found in searches with wide-area surveys with near-IR cameras on large ground-based telescopes like Subaru or the Very Large Telescope.
Luminosity Functions of Galaxies at Different Cosmic Times
Luminosity Functions of Galaxies at Redshifts of 7.4 (700 million years after the Big Bang), 6 (900 million years after the Big Bang), and 3 (2000 million years after the Big Bang). The luminosity function tells us the volume density of galaxies (vertical axis) versus their luminosity (horizontal axis). More negative absolute magnitudes (shown on the horizontal axis as "M_1900") correspond to brighter galaxies. The volume densities on the vertical axis are presented in logarithmic units -- so that a more negative number corresponds to a lower abundance. Galaxies at all epochs are much more abundant at lower luminosities than they are higher luminosities. However, at higher redshifts (earlier times), there are significantly fewer bright galaxies than there are at later times. This is exactly what we expect in hierarchical scenarios where the bright galaxies build up gradually from faint ones. The present diagram includes our luminosity function results through 2008.
Credit:Bouwens
We have used our searches for z~7-8 galaxies to estimate the volume density of these sources as a function of luminosity -- a quantity known as the luminosity function. The luminosity function is of significant interest to astronomers and allows us to learn something about how the population of galaxies change as a function of cosmic time. Comparisons of the luminosity function of galaxies at z~7-8 (700 million years after the Big Bang) with those at z~6 (900 million years after the Big Bang) tell us that luminous galaxies at z~7-8 were much less prevalent than they were at later cosmic times. Since we expect the more luminous galaxies to build up gradually from less luminous galaxies, we might have expected to find this deficit at bright galaxies at early times. Yet, despite this qualitative agreement with the models, the value in our measurements is that they provide quantitative constraints on how rapidly this build up takes place.
z~6 Drop-Out Galaxies
Images of 28 bright galaxies from the HUDF at a redshift of 6 (900 million years after the Big Bang). Current samples of redshift 6 galaxies now number over 600.
Credit:Bouwens
900 million years (Redshift 6)
Over the past few years, we have taken advantage of all the deep optical data over the deepest HST fields to compile a sample of over 600 galaxies at redshift 6. This sample allowed to study the properties of galaxies at these earliest times. Comparisons of this sample with galaxies later on the history of the universe show thatgalaxies at early times were much smaller, bluer, and lower luminosity (on average) than galaxies which existed later on in the universe. Each of these findings is consistent with the idea that galaxies build up hierarchically from much smaller pieces. Our sample is still the largest compilation of galaxies at these early times.
Spectrum of Very Bright Redshift 6 Galaxy
Spectrum of the brightest galaxy known at a redshift 6 (900 million years after the Big Bang). This galaxy also has more stellar mass than any known redshift 6 galaxy. The spectrum is plotted as flux versus rest-frame wavelength (after correcting cosmological redshift). Notice that this spectrum shows no emission at Lyman-alpha (at a wavelength of 1215.67 Angstroms).
Credit:Corey Dow-Hygelund