This week on HST

HST Programs: October 19 - October 26, 2014

Selected highlights

GO 13352: WISP - A Survey of Star Formation Across Cosmic Time

A region of massive star formation

Star formation is the key astrophysical process in determining the overall evolution of galactic systems, the generation of heavy elements, and the overall enrichment of interstellar and intergalactic material. Tracing the overall evolution through a wide redshift range is crucial to understanding how gas and stars evolved to form the galaxies that we see around us now. The present program builds on the ability of HST to carry out parallel observations, using more than one instrument. While the Cosmic Origins Spectrograph is focused on obtaining ultraviolet spectra of unparalleled signal-to-noise, this program uses the near-infrared grisms mounted on the Wide-Field Camera 3 infrared channel to obtain low resolution spectra between 1 and 1.6 microns of randomly-selected nearby fields. The goal is to search for emission lines characteristic of star-forming regions. In particular, these observations are capable of detecting Lyman-alpha emission generated by star formation at redshifts z > 5.6. A total of up to 40 "deep" (4-5 orbit) and 20 "shallow" (2-3 orbit) fields will be targeted in the course of this observing campaign.

GO 13407: COS Gas Flows: Challenging the Optical Perspective

A computer simulation of galactic gas accretion and outflow

The detailed history of galaxy formation and assembly depends on the availability of gas for star formation, and on how processes engendered by that star formation affects the distribution and intrinsic properties of that gas. Feedback in the form of winds and ionising radiation from either young stellar associations or massive black holes may be capable of interfering with, and even interrupting, the overall assembly process, while accretion of gas from external sources may drive active star formation. Gas flows in these galaxies are traced using emission lines, notably the optical Balmer series and ultraviolet Lyman series due to neutral hydrogran. These lines show broad profiles, traditionally ascribed to velocity broadening due to the gas being ejected from the parent galaxy. However, recent observations of ultraluminous starburst galaxies with Hubble's Cosmic Origins Spectrograph strongly suggest a different picture; detailed modeling of the emission lines suggests that the broad profiles arise partly from Rayleigh scarrering within infalling gas rather than in cosmic outflows. The present program aims aims to investigate this hypthesis through observations of four ultra-luminous infrared galaxies,using COS to obtain spectra of the Lyman-beta lines. Detailed modeling of the velocity profiles of those lines in conjunction with data for Lyman alpha and the Balmer H-alpha lines will establish the appropriate gas flow mechanisms in these systems.

GO 13663: Precise Orbit Determination for a New Horizons KBO

Hubble Space Telescope images of the Pluto system, including the recently discovered moons, P4 and P5

The Kuiper Belt lies beyond the orbit of Neptune, extending from ~30 AU to ~50 AU from the Sun, and includes at least 70,000 objects with diameters exceeding 100 km. Setting aside Pluto, the first trans-Neptunian objects were discovered in the early 1990s. Most are relatively modest in size, with diameters of a few hundred km and photometric properties that suggest an icy composition, similar to Pluto and its main satellite, Charon. In recent years, a handful of substantially larger bodies have been discovered, with diameters of more than 1000 km; indeed, one object, Eris (2003 UB13), is slightly larger than Pluto (2320 km) and 25% more massive. We know the mass for Eris because it has a much lower mass companion, Dysnomia, which orbits Eris with a period of 16 days (see this recent press release ). Pluto itself has at least 5 companions: Charon, which is about 1/7th the mass of Pluto, and the much smaller bodies, Hydra, Nix, P4 and P5 discovered through HST observations within the last few years. The New Horizons Mission was launched on January 19th 2006 with the prime purpose of providing the first detailed examination of Pluto. The Pluto encounter represents the first phase of the originally-proposed mission. Following the fly-by, set for Bastille day in 2015, the aim is to re-direct New Horizons towards one or more smaller members of the Kuiper Belt, with the goal of providing a closer look at these icy bodies. However, New Horizons needs to identify an appropriate target - a KBO with orbital parameters such that New Horizons can use its modest complement of remaining fuel to reach the target. Adding a further complication, Pluto happens to lie within 5 degrees of the Galactic Plane and the consequent high star density has proven a barrier to deep ground-based searches. As a consequence, the New Horizons team was awarded Hubble time to search an area roughly the size of the full moon for a suitable target. Those observations have succeeded in detecting at least two KBOs that should be accessible to New Horizons. Follow-up observations over the next few months will lead to refined orbital parameters and a final choice of a target for a potential extended mission.

GO 13675: Comet Siding Spring at Mars: Using MRO to Interpret HST Imaging of Comets

A composite image showing Hubble imaging of Mars and Comet Siding Spring 2013/A1

2013 was an interesting year for Hubble and comets: Comet ISON was tracked as a potential great comet until its breakup, passing through perihelion in late November; PanSTARRS and other surveys are turning up more evidence of residual cometary activity within objects in the asteroid belt, main belt comets; and an Oort cloud comet discovered in early January 2013 was identified as having a close encounter with Mars in October 2014. The last-named object is Comet 2013/A1 (Siding Spring), originally found by Robert McNaught in the course of the ongoing Siding Spring Survey for near-Earth asteroids and comets. Pre-discovery images have been located among observations by the Pan-STARRS survey and the Catalina Sky survey, and it quickly became apparent that the orbit will take the comet extremely close to Mars in late 2014. More detailed calculations showed that the comet would pass within 133,000 km of the planet on October 19, 2014. As an Oort comet, potentially on its first pass through the inner Solar System, Comet 2013/A1 has a substantial component of icy, volatile substances, generating an extensive coma and tail of gas and dust that will envelope the planet and its surrounding, perhaps leading to observable effects in the interaction with the tenuous Martian atmosphere. Given this possibility, astronomers developed a coordinated observational campaign to study the encounter utilising terrestrial ground-based observatories, space-based missions such as Hubble, and NASA probes in orbit and on the surface of Mars. The present program is part of the Hubble campaign, and uses the UVIS channel on Wide-Field Camera 3 to image the cometary nucleus and surrounding coma.