Our Latest Results
Additional results are posted on the HUDF09 project page.
Wide Field Camera 3
Absolutely essential to our latest results on high-redshift galaxies is the Wide Field Camera 3 (WFC3) which is a new instrument that was installed on the Hubble Space Telescope during HST servicing mission #4. The servicing mission was launched on May 11, 2009 and was a complete success. Though this camera is useful for many areas of astronomy, the near-infrared capabilities of this instrument are particularly exciting for high-redshift science. The near-infrared capabilities of WFC3 significantly exceed the capabilities of the NICMOS instrument on board the Hubble and have increased our capacity for discovery by a factor of 40. Among the improvements are a much larger field of view (a factor of 6 improvement: covering ~4.4 arcmin2 on the sky vs. ~0.8 arcmin2 covered by NICMOS), higher near-infrared sensitivity (improving the quantum efficiency by a factor of 2-3), and a better overall spatial resolution (improving the sharpness of the images by a factor of 3 over NICMOS' wide-area camera). Putting each of these factors together, we effectively gain a factor of roughly 20-40 in the efficiency with which we are able to find sources at high redshifts.
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 UC Santa Cruz and Leiden University (Netherlands), we have provided arguably 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 black).
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:
~500 million years (Redshift 10)
Ultra Deep IR Image
The Hubble Ultra Deep Field WFC3/IR Image. 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 8 and 10 (650 and 500 million years after the Big Bang, respectively). At UCSC and Leiden, we are using these data to better understand the properties of the first galaxies.
Since the installation of the WFC3/IR instrument on the Hubble Space Telescope during the fourth servicing mission (SM4), we have been conducting a fairly comprehensive search for galaxies at these early times using the existing WFC3/IR observations. Already a wide variety of deep and ultra-deep WFC3/IR observations are available and now cover well known fields like the Hubble Ultra Deep Field as well as the shallower fields like the GOODS fields and the UKIDSS Ultra Deep Survey field. It is remarkable given the volumes of WFC3/IR data in existence now that we know of so few plausible z~10 candidates. In fact, at present we know of only one source in the HST observations we have examined so far we are suggesting is a plausible z~10 galaxy. Of course, despite the small numbers, it is remarkable that we can use the Hubble Space Telescope to push so far back in cosmic time!
650 million years (Redshift 8)
z~7.4 Drop-Out Galaxy
The first five z~8 candidates identified in the first few weeks of science observations with WFC3/IR on the Hubble Ultra Deep Field. All the candidate galaxies appear prominently in the redder wavelength bands, but disappear in the bluest wavelength (< 1 micron) bands.
We have also been using the new WFC3/IR observations 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~8. z~8 galaxies are star-forming galaxies whose spectra cut off abruptly at ~1.1 microns due to the neutral hydrogen forest and IGM.
Amazingly, we were able to discover 5 such sources in the first few weeks of science observations with the WFC3/IR camera with HST. We now know of more than >47 z~8 candidate galaxies in the current WFC3/IR observations. This is in significant contrast to 2.5 years ago -- where no good z~8 candidates were known. Interestingly enough, the first z~8 galaxy discovered was from a z=8.2 gamma-ray burst and reported in Nature by two teams: Tanvir et al. and Salvaterra et al.
750 million years (Redshift 7)
z~7.4 Drop-Out Galaxy
Images of the first robust redshift z>=7 galaxy discovered (inside white box). This galaxy was identified with the NICMOS camera on HST in the Hubble Ultra Deep Field but has been confirmed to be robust with WFC3/IR. 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.
z~7 galaxies were first discovered in HST observations over the Hubble Ultra Deep Field with the near-IR NICMOS camera. z~7 galaxies were identified by looking for sources that dropout at wavelengths of ~1000 nm and bluer. However, searches for these sources with NICMOS were not very efficient due to its small field of view and limited sensitivity. As a result, just discovering a small number of candidates required enormous amounts of observing time with HST.In total, only ~10-20 viable z~7 candidates were identified with NICMOS and other ground-based telescopes required like Subaru or the Very Large Telescope.
The new WFC3/IR observations are much more useful for finding large numbers of z~7 galaxies. At present we know of almost 100 probable z~7 galaxies.
Luminosity Functions of Galaxies at Different Cosmic Times
Luminosity Functions of Galaxies at Redshifts of 10 (500 million years after the Big Bang), 7 (750 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_1600,AB") 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. At all epochs galaxies are much more abundant at lower luminosities than they are at higher luminosities. What is interesting here is the evolution observed in the luminosity function from early times (z~10) and much later times (z~3). Galaxies at later times are much brighter and numerous than they 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 early 2011.
We have used our searches for z~4-10 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~10 (500 million years after the Big Bang) with those at z~8 (650 million years after the Big Bang), z~7 (780 million years after the Big Bang), and z~4 (1600 million years after the Big Bang) tell us that galaxies at z~7 and z~10 were much less luminous and 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 such a change 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.
900 million years (Redshift 6)
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.
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).