Building Blocks of The Universe
Galaxies are the visible building blocks of the universe imbedded in a more intriguing, complex, and mostly invisible cosmos. Theory and observations, including from HST, are used to disentangle the relationship between galaxies and the weakly interacting dark matter – seemingly comprised of non-baryonic particles. While these two constituents make up about 32% of the universe (only about 4 –5% visible), 68% is the dark energy component. To understand the interplay between all these constituents, many HST observations have been used to piece together a sketchy but consistent understanding of the universe, as well as the physics of galaxy assembly, which is an observable tracer and a result of the workings of the cosmos.
The Local Group
A comfortable notion of how galaxy assembly proceeds is that small objects may have formed first in the universe and built up larger structures. The process can be quite dynamic. Numerous studies of Local Group galaxies (Milky Way, M31, M33, LMC, SMC and many dwarf galaxies, such as the Sagittarius Dwarf out to roughly 1Mpc) demonstrate up close how such processes have occurred.
For example, HST data analysis of the “Smith Cloud” indicates this object is boomeranging back towards the Milky Way, having previously been launched from the galactic disk about 70 million years ago. Observations of the Milky Way and M31 demonstrate that these two major galaxies may eventually merge, while HST data probed the gravitational interplay between the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC) which has produced a very large-scale structure called the Leading Arm. This latter structure appears to be residual material torn out of the SMC by the LMC.
Within each of these galaxies and the additional companion dwarf galaxies in the Local Group, HST high resolution spatial imagery and spectroscopy yield the star- and star-cluster-formation rates which varies from galaxy to galaxy. A deeper understanding of such differences sheds light on the formation of the immediate vicinity of the universe, but also provides a bench mark for studies of more distant structures. Numerous observational programs, small and large, including Treasury programs probe these parameters in Local Group galaxies.
The Local Volume
HST observations of numerous nearby galaxies beyond the Local Group are aimed at understanding the characteristics such as star formation, metallicity, and interactions of galaxies close enough to study in detail. One example is LEGUS (Legacy ExtraGalactic UV Survey), the investigation of 50 galaxies close enough to resolve them into their constituents using the high resolution of HST. Star cluster formation is being traced spatially as well as chronologically in these galaxies, which have quite a range of morphological type. This study is especially enhanced through the UV capability of HST that allows examination and calibration of the UV-star-formation rates in these objects. Many other factors such as the number and types of star clusters, the characteristics of the interstellar medium, and supernovae environments are being used to paint a more complete picture of the dynamical history of the relatively nearby universe.
Other studies including ACS Nearby Galaxy Survey Treasury (ANGST) program, sampled nearly 14 million stars in nearly 70 galaxies at distances out to approximately 4Mpc, demonstrate that galaxies are quite diverse. These galaxies exhibit varying mixtures of young and old populations, a variety star formation region morphologies, and complex assembly histories. Interacting galaxies are common, as well as clusters of galaxies with objects of different morphological types. The understanding of the composition and role of gas and dust in these galaxies is being probed.
More Distant Universe
At greater distances, HST observations are crafted to investigate early stages of galaxy evolution and the fundamental questions regarding the distribution and evolution of galaxy morphology, as well as the incidence of galaxy merging and clustering. Star formation is seen to be triggered and quenched in different galaxies, but the precise parameterization of these processes is still murky. A variety of factors must contribute, such as the gas-and-dust content of a galaxy, interactions, and activity in the galactic nucleus. The role of low-mass galaxies in galaxy assembly, for example, by merging with larger galaxies appears to trigger active star formation. An example of a large program designed to research such factors in galaxy evolution is the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) through imagery of more than 250,000 galaxies. Another aim of that study is also to search for Type Ia Supernovae beyond a redshift of 1.5 to document the accuracy of their use as cosmological standard candles.
An equally important issue to understand the interstellar medium within galaxies and as well as the diffuse gas surrounding galaxies and permeating clusters of galaxies. These studies are accomplished with spectroscopy using sight lines that pass from distant QSOs through the target diffuse material revealing the density and metallicity throughout. At these distances, peculiarities such as ionized mass ejections, and disruptive events and collisions appear to be important factors in galaxy formation.
Approaching the First Galaxies
At the distance and scale of the farthest reaches of the universe, the structure and composition of the cosmos is intertwined with galaxy formation. Dark matter and dark energy may play significant roles. By using Deep Field observations along with gravitational lensing data (e.g., the Cluster Lensing and Supernova study with Hubble [CLASH] and Frontier Fields programs) these topics are addressed with HST's enhanced panchromatic imaging capabilities. Observations of galaxy clusters across the UV, visible, and IR probe the presence of dark matter and unveil strong- and weak-lensing phenomena. At the large distances probed with lensing, some of the youngest and most distant galaxies are being characterized. It is even possible to examine the morphological structure of those distant objects with sophisticated gravitational lens models. HST observations continue to probe the distant universe and will work hand in hand with JWST investigations.