This week on HST

HST Programs: May 13 - May 19, 2013

Selected highlights

GO 12764: The demographics of dark gamma-ray bursts

An artist's impression of a gamma-ray burst

Gamma ray bursts are events that tap extraordinary energies (1045 to 1047 joules) in remarkably short periods of time. Several thousands bursts have been detected over the last 30+ years, and analyses indicate that they can be divided into two classes with durations longer or shorter than 2 seconds. The short bursts appear to release more high energy radiation, so the two subsets are known as long/soft and short/hard bursts. The long/soft bursts appear to originate in the collapse of very massive stars, while the short/hard bursts are coalescing binary systems (probably pairs of neutron stars or black holes). The first optical counterpart to a gamma ray burst was identified in 1998, allowing confirmation of their extragalactic nature, and, since then, more than 60 bursts have been detected at X-ray wavelengths, and half that number detected at either optical or radio wavelengths; all of these detections are long/soft bursts. In the past decade, optical counterparts have been detected for many of these sources, allowing not only direct study of their characteristics, but also investigations of the properties of the underlying host galaxies. Some sources, however, do not appear to generate optical counetrparts, perhaps because the optical emission is suppressed due to substantial local dust obscuration. The absence of those data has hampered previous attempts to match those sources, and their environments, against GRBs with optical counetrparts. The present program aims to circumvent this issue by using Chandra observations to obtain accurate positions for the X-ray counterparts of such sources, and then match that astrometry against deep visual (F606W) and near-infrared (F160W) HST WFC3 images.

GO 12986: Detecting Isolated Black Holes through Astrometric Microlensing

A rather spectacular version of black hole lensing.

Gravitational lensing is a consequence of general relativity. Its effects were originally quantified by Einstein himself in the mid-1920s. In the 1930s, Fritz Zwicky suggested that galaxies could serve as lenses, but lower mass objects can also also lens background sources. Bohdan Paczynski pointed out in the mid-1980s that this offered a means of detecting dark, compact objects that might contribute to the dark-matter halo. Paczcynski's suggestion prompted the inception of several large-scale lensing surveys, notably MACHO, OGLE, EROS and DUO. Those wide-field imaging surveys have target high density starfields towards the Magellanic Clouds and the Galactic Bulge, and have succeeded in identifying numerous lensing events. The duration of each event depends on several factors, including the tangential motion of the lens and its mass. Long-term events are generally associated with a massive lens. Duration alone is not sufficient to identify a lens as a black hole - a source with very low tangential motion relative to the Sun can produce the same effect. However, microlensing not only leads to flux amplification, but also to small astrometric motions, caused by the appearance and disappearance of features in the lensed light. Those motions serve as a mass discriminant - higher mass lenses produce larger amplitude motions. The expected astrometric signal from a black hole lens is > 1.4 millarcseconds, just measureable with HST. This program aims to capitalise on this fact by searching for lensing by black holes in the Galactic field. The observations target long-duration lensing events in the Galactic Bulge.

GO 13031: Testing Collisional Grinding in the Kuiper Belt

Preliminary orbital determination for the KBO WW31, based on C. Veillet's analysis of CFHT observations; the linked image shows the improved orbital derivation, following the addition of HST imaging

The Kuiper Belt consists of icy planetoids that orbit the Sun within a broad band stretching from Neptune's orbit (~30 AU) to distance sof ~50 AU from the Sun (see David Jewitt's Kuiper Belt page for details). Over 500 KBOs (or trans-Neptunian objects, TNOs) are currently known out of a population of perhaps 70,000 objects with diameters exceeding 100 km. Approximately 2% of the known KBOs are binary (including Pluto, one of the largest known KBOs, regardless of whether one considers it a planet or not).TNOs are grouped within three broad classes: resonant objects, whose orbits are in mean motion resonance with Neptune, indicating capture; scattered objects, whose current orbits have evolved through gravitational interactions with Neptune or other giant planets; and classical TNOs, which are on low eccentricity orbits beyond Neptune, with no orbital resonance with any giant planet. The latter class are further sub-divided into "hot" and "cold" objects, depending on whether the orbits have high or low inclinations with respect to the ecliptic. Cold, classical TNOs show relatively uniform characteristics, including red colours, high albedos and an extremely high binary fraction (>30%). They are believed to have formed in situ, and were therefore in place to experience the range of gravitational interactions as the giant planets migrated to their present location. As that migration occurred, subsets are expected to have been trapped in transitory resonance orbits. The present SNAP program aims to use HST to survey up to 56 cold, classical TNOs, aiming to deermine both the binary frequency and the colour distribution of the sample. Collisional grinding models have been invoked to explain the number-magnitude distribution of these obejcts; if those models are valid, then the expectation is that small binaries should also have been disrupted, and the surface of these eroded by collisions to expose the different-composition (colour) interior.

GO 13048: The First Unambiguous Detection of a Distinct Metal-poor Stellar Halo in a Massive Early-type Galaxy

The lenticular galaxy, NGC 3115. highlighting active star formation within the nucleus

The stars in the Milky Way are generally grouped into stellar populations, building blocks that provide insight into the process of galaxy assembly. The traditional populations are the near-spherical, metal-poor Halo, representing the first significant burst of star formation; the Disk, whose constituents have higher metallicities, a flattened density distribution (which defines the Galactic Plane) and significant angular momentum, suggesting a formation history that includes collapse and dissipation; and the central Bulge, which, with a spheroidal distribution and broad metallicity range, may be something of an amalgam of disk and halo. Over the last five years or so, deep HST imaging of the outer regions of our nearest spiral neighbour, the Andromeda galaxy, have shown that it, too, possesses a metal-poor halo. Expanding observations to other systems becomes proportionately more difficult at larger distances. However, globular clusters can provide pointers to the underlying field star metallicity distribution. Observations of the S0 galaxy, NGC 3115, lying at a distance of ~9 Mpc., show that the clusters exhibit a bi-modal colour distribution, suggestive of the presence of a significant metal-poor component. The galaxy is known to possess a central black hole, with evidence of actiev star formation in the nuclear regions. The present program aims to obtain deep multi-colour optical and near-infrared imaging in three fields, mapping the metallicity distribution as a function of radius.