Observational Tests for Competing Theories of Galaxy Formation and the Nature of Dark Matter

The Universe is about 13.8 billion years old. The oldest stars in our Galaxy have estimated ages approaching 13.2 billion years. These stars cannot be among the first formed in our Galaxy because they contain elements that must have been synthesized by nuclear reactions either in the cores or during the explosive deaths of even earlier generations of stars. Both stars and interstellar gases comprise but a small fraction of the mass of galaxies, which are dominated by a mysterious form of matter than emits no light (hence referred to as dark matter). When did gas in bodies comprising primarily dark matter first turn into stars, making galaxies visible for the first time? How did the different stellar components of galaxies — in the case of galaxies like our own, central bulge, disk (in which our Sun resides), and surrounding halo — assemble over time? What is dark matter, which dominates not only matter in galaxies but also matter in the space between galaxies?

Addressing the questions posed above, at the cutting edge of modern astrophysics, requires overcoming two challenges: (i) finding young and therefore distant galaxies, which are faint not just because of their vast distances but also because they are just beginning to form stars; and (ii) spatially resolving these galaxies, which have small angular sizes not just because of their vast distances but also because they are intrinsically small. In this proposal, we describe our ongoing efforts to confront competing theories for galaxy formation and dark matter using the deepest images of the Universe available, obtained with the Hubble Space Telescope. By employing massive galaxy clusters as gravitational lenses to magnify background galaxies, we are able to detect and spatially resolve galaxies having lower luminosities and at greater distances than would otherwise be feasible. The data we use, made publicly available upon release, comes primarily from the Hubble Frontiers Fields (HFF) program, which began in 2013 and is slated for completion in 2016. Targeting six of the most massive clusters known, HFF aims to provide the deepest images of clusters and their lensed galaxies ever obtained. We augment this unique dataset with data from the Cluster Lensing And Supernova survey with Hubble (CLASH) program, the predecessor to HFF. Targeting twenty-five clusters at shallower depths, CLASH was completed in 2013 and the final data products released.

Two main challenges arise when using gravitational lensing to find and deduce the intrinsic properties of strongly lensed galaxies: (i) securely identifying the multiple images belonging to a single lensed galaxy; and (ii) accurately inferring the magnification of the individual lensed images and hence the intrinsic properties (e.g., luminosity, size, and morphology) of the lensed galaxy. Meeting both challenges requires the construction of accurate lens models for galaxy clusters, currently the greatest barrier for fully utilizing galaxies detected through gravitational lensing to test Cosmological theories. We have used our established free-forrn method to construct a lens model for the first galaxy cluster completed in HFF. Our lens model reproduces the observed positions of the lensed images, as well as the appearances of their individual counter-images. Furthermore, our model shows consistency between the redshifts of the lensed galaxies derived from their colors and the redshifts derived geometrically from the lens model, allowing us to clarify and correct a number of lensed images that were previously ambiguous or unidentified. We also demonstrated, for the first time ever in any cluster, that the lens model accurately predicts the relative brightnesses of the multiply-lensed images. With this reliable correction for lensing, we are able to obtain the intrinsic properties of all the lensed galaxies, including those more weakly lensed and do not produce multiple images. The lensed galaxies reach typically 2-3 magnitudes fainter than those found in the deepest blind-field surveys. As we show, these galaxies provide the crucial leverage for distinguishing between competing theories for galaxy formation that incorporate different forms of DM. For example, does the observed galaxy space density increase with decreasing luminosity as might be expected if dark matter is cold and collisionless, or is there an abrupt decrease (turnover) at a particular luminosity indicative of a lower limiting mass for dark matter halos imposed by its wavelike nature?