This week on HST

HST Programs: June 15 - June 21, 2015

Selected highlights

GO 13657: Probing the Most Luminous Galaxies in the Universe at the Peak of Galaxy Assembly

Hubble ACS image of the GOODS field

Deep imaging with the Hubble Space Telescope and Spitzer have shown that the peak star formation epoch for galaxy evolution lay arond redshift z~2. The most luminous galaxies at that epoch are luminous, ultraluminous and hyperluminous infrared galaxies (LIRGs and ULIRGs). These are systems that have total luminosities that exceed 1011.4 LSun (LIRGs),1011.2 LSun (ULIRGs), or 1013 LSun (HyLIRGs), with most energy emitted at wavelengths longward of 10 microns. Some of these systems are interacting or merging disk galaxies, with the excess infrared luminosity generated by warm dust associated with the extensive star formation regions. There is some controversy, however, regarding whether mergers are the only means of producing such systems, or whether teady processes, such as accretion of cold gas along filaments, might also produce very high luminosity, star-formign systems. The present program aims to tackle this question through high-resolution near-infrared imaging of HyLIRGS with the WFC3-IR camera, searching for unambiguous merger signatures. Those observations will be combined with kinematic data obtained with ground-based Integral Field Units to determine the likely fraction of mergers among these systems.

GO 13713: Observation of Chariklo's rings

The location of the ring system surrounding the centaur, Chariklo

Ring systems are features of all of the gas giants in the Solar System. Saturn's rings are the most prominent, evident to Galileo even if he was unable to discern their nature; the Uranian rings were discovered through stellar occultation measurements in 1977; Neptune's ring system was revealed in a similar manner in the 1980s; and Jupiter's rings were identified by Voyager 1 in 1979. In each case, the rings are believed to be comprised of small particles of rock, ice and dust that are sustained through collisions and maintained in place by gravitational interactions with larger satellites. One other body is known to have a ring system - the asteroid, Chariklo. In contrast to the gas giants, Chariklo is a Centaur, a minor planet orbiting between Uranus and Saturn with a diameter of only ~230 km. Centaurs are believed to originate in the Kuiper belt and are in dynamically unstable orbits that may lead to ejection from the Solar System. Chariklo occulted a background star in 2013, and the observations revealed the presence of two rings, ~7 and ~3 km wide and ~9 km apart. These rings are hypothesised to include at least some water ice particles; previous spectroscopic observations had shown variable water-ice features, together with photometric variability, that could be explained if the rings were edge-on to earth at that epoch. The existence of a ring system in such a small body is extremely unexpected, and the system is likely to have a dynamical lifetime of only a few million years. The present program aims to image the rings directly through deep imaging with the UVIS camera on WFC3.

GO 13790: Frontier Field Supernova Search

Finding chart for the multiply imaged supernova, SN Refsdal, discovered in November 2014 in cluster MACJ1149

The overwhelming majority of galaxies in the universe are found in clusters. As such, these 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. Hubble is currently undertaking deep imaging observations of up to 6 galaxy clusters as part of the Frontier Fields Director's Time program (GO 13495/13496). Those observations have provided a basis for several synergistic programs. The present program is using the Frontier Field observations to search for supernovae at high redshifts, z> 1.5, aiming to set further constraints on dark energy and probing the frequency of supernovae as a function of redshift, the delay time and hence the likely progenitors. Recent observations of the fourth cluster, MACSJ1149.5+2223, resulted in the detection of a particularly unusual object - multiple lensed images of a supernova in a redshift z=1.49 galaxy that is itself multiply lensed. Each of those images results from light following a different path due to the gravitational potential of the foreground cluster and galaxies. Dubbed Supernova Refsdahl, after the gravitational lensing pioneer, the present program is continuing to obtain follow-up observations to monitor the light-curves of each component, hence determining the time-delay for each light path. Those measured delays can be matched against the predictions of gravitational lensing models. Moreover, since the galaxy itself has multiple images it is possible that future observations may detect images of the supernova in other components. Thus, this supernova can (perhaps even will) be discovered more than once - and might even be predicted.

GO 13819: 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.

---

Past weeks:

• Cycle 14 observations (from March 13 2006 to June 30 2006)
• Cycle 15 observations (from July 1 2006)
• Cycle 16 observations (from July 1 2007)
• Cycle 17 observations (from July 13 2009)
• Cycle 18 observations (from August 30 2010)
• Cycle 19 observations (from October 3 2011)
• Cycle 20 observations (from October 1 2012)
• Cycle 21 observations (from October 1 2013)
• Cycle 22 observations (from October 1 2014)

---

page by Neill Reid, updated 11/11/2014
These pages are produced and updated on a best effort basis. Consequently, there may be periods when significant lags develop. we apologise in advance for any inconvenience to the reader.