What is the life story of typical galaxies in the universe? What processes shape their formation and evolution?
In the Bouwens group, we pursue the study of galaxies from the earliest epochs in the universe where galaxies are first observable. While the first stars are thought to have formed in the first 100 million years of the universe, galaxies are thought to have formed somewhat later, i.e., at 200 million years and on. Observing galaxies back to these early epochs is challenging due to the large distances, large redshift factors, and intrinsic faintness of the low-mass galaxies that formed at early times. In the most sensitive images with the Hubble Space Telescope, we are only capable of observing galaxies back to ~400 million years after the Big Bang, but with the James Webb Space Telescope, we should be capable of observing sources at even earlier times, perhaps to ~250 million years after the Big Bang. [Click here to read more about the first galaxies and the XDF effort.]
In the Bouwens group, we are pursuing the observational study of galaxies using a variety of different facilities and state-of-art data sets. A survey of the new and exciting data sets becoming available for scientific pursuits will now be presented.
Sensitive Observations of Lensing Fields with HST+Spitzer: One particularly exciting set of observations that has been pushing the envelope is the ambitious Hubble Frontier Fields program. This program has obtained extremely deep exposures with both the Hubble and Spitzer Space Telescopes over massive galaxy clusters in the distant universe demonstrated to gravitationally lens large volums of the distant universe. Observations with this program plausibly allow for the identification of galaxies some 10-40 times fainter than observed from the deepest observations over other sight lines on the distant universe. This allows for interesting constraints on the physical properties of the very low-mass galaxies in the early universe. In addition, astronomers can gain the best possible insight into the evolution of the faint-end slope to the luminosity function, as well as the presence of a possible turn-over in the luminosity function at the faint end.
Spitzer/IRAC: Another very exciting data set are especially sensitive observations with the IRAC instrument on the Spitzer Space Telescope at 3-5 microns over the two GOODS fields. Observations over the GOODS fields were obtained as part of the ambitious GREATS program (PI: Ivo Labbe), probing to an unprecedented depth of 200 hours over an area of 200 arcmin2. This should allow us to probe the build-up of stellar mass in galaxies with cosmic time with unprecedented sensitivity, as well as probing the strength of nebular emission lines at rest-frame optical wavelengths. Strong nebular line emission appears to be ubiquitous in star-forming galaxies at early times and have a huge impact on the measured HST+IRAC spectra. While once these lines represented a nuisance for estimates of the stellar mass in distant star-forming galaxies, these lines can increasingly be used as a tool to refine our understanding.
In parallel with our obtaining extremely sensitive observations with Spitzer over wide areas, very sensitive observations are also being obtained over the very wide-area legacy fields from the SMUVS (PI: Caputi) and Spitzer-Legacy-over-COSMOS programs (PIs: Labbe/Caputi). The wide area observations are valuable since in combination with the very sensitive optical and near-IR imaging observations and other multi-wavelength observations can be used to locate large samples of rare sources, which are much more difficult to study in narrower fields like the GOODS fields and especially the Hubble Ultra Deep Field (HUDF). Rare sources can include especially massive galaxies at high redshift, i.e., z>7, and galaxies showing extreme line emission. Follow up of such sources, both with spectroscopy and additional imaging observations, resulted in some record-breaking spectroscopic redshift confirmations, i.e., at z=7.73 and z=8.68, and will likely represent an important focus of activity in the future with the James Webb Space Telescope.
ALMA: Another area in which the Bouwens group is involved is the exploitation of sensitive observations at mm wavelengths with the Atacama Large Millimeter Array (ALMA) to probe dust emission and the molecular gas content of distant galaxies. Astronomers can probe both quantities in galaxies at many different places on the sky, but arguably the most interesting place to conduct this probe in the distant universe is the HUDF, where we have the most sensitive spectroscopic and imaging observations available at any place on the sky. Not surprisingly, several sensitive programs have already been obtained over th
e HUDF, but these programs will be dwarfed by an ambitious 150-hour ASPECS (ALMA SPECtroscopic Survey in the HUDF) program designed to obtain sensitive observations at both 1.3 mm and 3 mm. These ALMA observations will be obtained over the next yea and give us our most detailed look at dust emission and gas in normal galaxies in the z>=2 universe. Our Leiden-based team is a cre member of that effort, having already led a prominent paper on dust emission from normal star-forming galaxies in the z>=2 universe.
Optical Spectroscopy with MUSE: Another state-of-the-art data set the Bouwens group in Leiden is using to pursue the study of galaxies at the highest redshift involves spectroscopic observations with the MUSE instrument as part of the GTO (guaranteed-time-observations) effort. MUSE is a revolutionary state-of-the-art integral field unit spectrograph capable of obtaining spectra of every single 0.2″x0.2″ pixel over an entire 1 arcmin**2 field. As a result, MUSE has proven to be a very efficient probe of line emission. This is allowing the GTO team to derive > 1000 spectroscopic redshifts over the HUDF area, ~5-10x more than previously known. Precise knowledge of the redshifts for individual sources is valuable, since it indicates both the position of nebular lines in the Spitzer/IRAC observations and the position of CO+CII lines in ALMA observations. This allows for a much more thorough use of the Spitzer/IRAC + ALMA data in probing both line emission and gas masses in distant star-forming galaxies.
In addition to our use of MUSE data for redshift information, the MUSE spectra have an abundance of information on the physical properties of distant star-forming galaxies using the extensive set of emission lines present in the spectra. The revealed lines in the MUSE GTO data set include most Balmer line series and [OIII] lines at z<1 and high-ionization UV lines out to z~5. The systematic study of these lines in lower-mass, lower-metallicity in galaxies at modest redshift is interesting, since such sources can be observed at moderately high S/N in current observations and likely represent realistic analogues to similar galaxies at much earlier epochs in the universe. The exploration of such sources in the MUSE GTO data should be quite interesting and significant given the availability of adaptive-optics observations in the future GALACS-I and proprietary access to 260 nights of data from the MUSE GTO consortium.
UV Observations with HST: The Bouwens group is also interested in studying star-forming galaxies at later points in cosmic time, i.e., at z~1-3, 2-5 billion years after the Big Bang. Studying galaxies in this epoch are valuable, since much higher S/N observations can be obtained on galaxies and the lessons learned are generally relevant for galaxies at earlier times which are much more difficult to study. Our Leiden-based team is working with other members of the HDUV team (PI: Oesch) to exploit the new sensitive wide-area observations
that have been obtained over 100 arcsin2 GOODS-North and GOODS-South fields (indicated with the purple squares in the inset figure shown to the left). The HDUV survey is much larger than previous surveys of its kind and will allow us to make great progress in answering long-standing issues which have been puzzling for scientists in the community. We are us ing the data to ascertain the fraction of high-energy ionizing photons that escape from galaxies, while looking in detail at how the physical properties of galaxies change as a function of their stellar mass. In addition, we are quantifying the prevalence, luminosity, and stellar masses of galaxies at z~1-3 and comparing the derived quantities with the derivation of similar quantities at earlier times.
Rychard J. Bouwens 