This week on HST

HST Programs: July 4 - July 10, 2016

Selected highlights

GO 14080: LyC, Ly-alpha, and Low Ions in Green Peas: Diagnostics of Optical Depth, Geometry, and Outflows

A montage of green pea galaxies discovered by the Galaxy Zoo project

Understanding the galaxy formation and galaxy evolution has been a strong focus of astronomical research since thne 1970s and remains a key issue for 21st century astrophysics. Since we cannot follow an individual galaxy through time, tackling these questions requires a statistical approach, and, as a result, large-scale surveys have played a crucial role in the field. In particular, the Sloan Digital Sky Survey, conducted from Apache point Observatory from the 1990s through the early years of this century, has provided a treasure trove of information of galaxies at low and moderate redshifts. One of the interesting discoveries that originated from SDSS was the discovery of so-called "green pea" galaxies - compact objects whose combined images have a greenish hue, originally uncovered by citizen scientists as part of the Galaxy Zoo project. Closer inspection shows that these are gas-rich galaxies lying at relatively low redshifts, 0.1 < z < 0.36, with the green-tinged hue partly stemming from the presence of strong oxygen ([O III]) emission. These characteristics indicate that the galaxies are undergoing strong star-forming episodes. The present program aims to take advantage of HST's unparalleled resolution and extraordinary sensitivity at ultraviolet wavelengths, and will use the Cosmic Origins Spectrograph to obtain spectra are far-UV wavelengths. Those observations will probe optical depth, the geometry of neutral gas and radiative transfer within these systems, revealing the physical structure of the underlying star forming regions and potentially offering insight into the nature of high redshift lyman-alpha emitters.

GO 14096: RELICS: Reionization Lensing Cluster Survey

Hubble image and mass map for the cluster ACT-CL J0102-4915, one of the clusters included in the RELICS program

The overwhelming majority of galaxies in the universe are found in clusters. As such, those systems offer an important means of tracing the development of large-scale structure through the history of the universe. Moreover, as intense concentrations of mass, galaxy clusters provide highly efficient gravitational lenses, capable of concentrating and magnifying light from background high redshift galaxies to allow detailed spectropic investigations of star formation in the early universe. Hubble imaging has already revealed lensed arcs and detailed sub-structure within a handful of rich clusters. At the same time, the lensing characteristics provide information on the mass distribution within the lensing cluster. The present program builds on the highly successful CLASH program,which used 17-colour ACS/WFC3 images to map 25 galaxy clusters, tracing the mas profile and the dark matter distribution, and the Frontier Fields program, targeting six clusters for deep multi-colour imaging. RELICS is focused on using massive galaxy clusters as gravitational telescopes, searching for strongly lensed background galaxies drawn from the high redshift universe. Imaging 46 fields in 41 galaxy clusters, this program aims to identify galaxies with redshifts in the range 9 < z < 12. By targeting strongly-lensing clusters, standard models for galaxy evolution suggest that the program can deliver ~100 galaxies in that redshift range, together with more than 150 galaxies at z~8. A significant number of these galaxies should be brighter than H~25.5, and therefore accessible to more detailed follow-up observations. Conversely, the actual number of galaxies detected will set constraints on the galaxy number-redshift distribution, and the overall formation and assembly history.

GO 14269: Just the BASICs: Linking Gas Flows in the Circumgalactic Medium to Galaxies

A computer simulation of galactic gas accretion and outflow

Galaxy formation, and the overall history of star formation within a galaxy, clearly demands the presence of gas. The detailed evolution therefore depends on how gas is accreted, recycled, circulated through the halo and, perhaps, ejected back into the intergalactic medium. Tracing that evolutionary history is difficult, since gas passes through many different phases, some of which are easier to detect than others. During accretion and, probably, subsequent recycling, the gas is expected to be reside predominantly at high temperatures. The most effective means of detecting such gas is through ultraviolet spectroscopy, where gas within nearby systems can be detected as absorption lines superimposed on the spectra of more distant objects, usually quasars. Extensive observations of galaxies at modest redshift (0.15 < z < 0.35) have shown that material extends to radii of hundreds of kpc, with a total mass in metals that is at least comparable with the mass in the central galaxy. The structures appear to show a bimodal metallicity distribution, with a metal-rich (~40% solar) component that may reflect winds and outflows, and a metal-poor (3% solar) component that may trace cold accretion streams. The present program aims to use ACS to image 14 QSO fields that harbour well-characterised circum-galactic medium absorbers, with the aim of measuring the geometry and morphology of the parent galaxies and searching for correlations with the gaseous properties.

GO 14337: Dynamical Masses for Free-Floating Planetary-Mass Binaries

Epsilon Indi Bab, the binary brown dwarf companion of the nearby K dwarf

Brown dwarfs are objects that form like stars, but lack sufficient mass to drive the central temperature above a few million degrees, and therefore never succeed in igniting core hydrogen fusion. Discovered in the mid-1990s, these objects initially have surface temperatures of ~3,500K, but cool rapidly and move through spectral types M, L, T and Y. Following their discovery, considerable theoretical attention has focused on the evolution of their intrinsic properties, particularly the details of the atmospheric changes in the evolution from type L through T to Y and beyond. The initial transition marks the emergence of methane as a dominant absorber at near-infrared wavelengths, while ammonia becomes increasingly apparent in the coolest dwarfs identified by the WISE mission. Current models suggest that the L to T transition occurs at ~1400-1200K, while the T to Y transition occurs around 600K. The spectral changes are at least correlated with, and perhaps driven by, the distribution and properties of dust layers ("clouds"). The overall timescales associated with this process remains unclear. Mass is a crucial factor in mapping those changes, but mass is also the most difficult quantity to measure in a reliable fashion. The present proposal tackles this issue through astrometry of ultracool binary systems, deriving the orbits and hence dynamical masses. The four targets in this multi-year program include two late-T/Y dwarf binaries, whose masses are expected to lie in the range 5 to 15 jupiter masses.