This week on HST

HST Programs: August 5 - August 11, 2013

Selected highlights

GO 12115: A Panchromatic Hubble Andromeda Treasury

M31: the Andromeda spiral galaxy

M31, the Andromeda galaxy, is the nearest large spiral system to the Milky Way (d ~ 700 kpc), and, with the Milky Way, dominates the Local Group. The two galaxies are relatively similar, with M31 likely the larger system; thus, Andromeda provides the best opportunity for a comparative assessment of the structural properties of the Milky Way. Moreover, while M31 is (obviously) more distant, our external vantage point can provide crucial global information that complements the detailed data that we can acquire on individual members of the stellar populations of the Milky Way. With the advent on the ACS and, within the last 2 years, WFC3 on HST, it has become possible to resolve main sequence late-F and G dwarfs, permitting observations that extend to sub-solar masses in M31's halo and disk. Initially, most attention focused on the extended halo of M31 (eg the Cycle 15 program GO 10816 ), with deep imaging within a limited number of fields revealing the complex metallicity structure within that population. With the initiation of the present Multi-Cycle Treasury program, attention switches to the M31 disk. "PHAT" is conducting a multi-waveband survey of approximately one third of disk and bulge, focusing on the north-east quadrant. Observations started in Cycles 19, extended through Cycle 20 and are concluding in Cycle 21. The data provide a thorough census of upper main-sequence stars, open clusters, associations and star forming regions, matching the stellar distribution against the dust and gas distribution.

Artist's impression of the black hole and surrounding torus in an AGN

This year (2013) is the fiftieth anniversary of the recognition that QSOs (quasars, quasi-stellar objects) were extremely luminous objects lying at substantial redshifts. The central engine powering these luminous objects is now recognised as a supermassive black hole, marking the central regions of a galaxy. As such, QSOs are clearly related to (and more luminous than) active galactic nuclei (AGN). Like AGNs, QSOs have been segregated into two categories based on their spectral properties: systems with broad lines are characterised as Type 1 QSOs; systems with narrow lines are classed as Type 2. As with AGN, the underlying cause of these differences is generally believed to reside more in our perspective than on the sources themselves: heavily obscured systems, where the central accretion disk lies behind a thick veil of dust, are observed as Type 2 systems; they are expected to evolve to form Type 1 systems as the dust is ablated and destroyed. The present SNAP program aims to test this scenario by coupling mid-infrared Herschel observations, probing the dust environment, with HST near-infrared WFC3 imaging of targets drawn from two matched samples of Type 1 and Type 2 QSOs.

GO 12961: A Cepheid Distance to NGC 6814

The spiral galaxy, NGC 6814 (NOAO)

Cepheid variable stars have been the prime extragalactic distance indicator since Henrietta Leavitt's discovery of the period-luminosity relation described by Cepheids in the Small Magellanic Cloud. It was Hubble's identification of Cepheids in NGC 6822 that finally established that at least some nebulae were island universes. Cepheids and the extragalactic distance scale figure largely in HST's history, notably through the Hubble Constant Program, one of the initial Key Projects. HST has since observed Cepheids in more than 30 galaxies. The present program aims to extend observations to the Seyfert galaxy, NGC 6814, a face-on spiral lying at a distance of 20-30 Mpc from the Milky Way. The system lies in Aquila, close to the Galactic Plane and therefore subject to significant foreground reddening that complicates distance estimation. NGC 6814 is highly variable at X-ray wavelengths, clealy indicating the presence of a supermassive black hole driving the nuclear activity. This program will use multi-epoch imaging with the WFC3-UVIS and WFC3-IR cameras to identify and monitor Cepheid variables in the system, using the photometric measurements to determine the distance and line of sight reddening. These observations will be coupled with ground-based spatially-resolved spectroscopy of the nuclear regions to determine the mass of the central black hole.

GO 13055: Orbital Evolution and Stability of the Inner Uranian Moons

HST ACS images of the planet, part of the ring system and a number of the smaller moons

All of the gas giants in the outer Solar System possess an extensive entourage of small satellite moons. They all also possess ring systems. Uranus has (as of mid-2013) a total of 27 known satellite companions. The two largest moons, Titania and Oberon (almost all of the Uranian moons have Shakespearean names) have diameters of ~1500 km and were discovered by William Herschel shortly after he discovered the planet. Two other moons (Umriel and Ariel) have diameters of more than 1,100 km, while Miranda is around 472 km in diameter, but almost all of the remaining moons are smaller than 100 km in diametr, with the smallest (Mab, Margaret and Trinculo) closer to 20 km. As such, these smaller objects are likely to be captured KBOs. Nine of the moons are characterised as "irregular" moons, with orbits that lie well beyond that of Oberon. These moons form a dynamically chaotic system, and significant changes in orbital properties have been detected over a matter of 10 years or less. There is also evidence for a faint ring within this system. the major rings in the Uranian system were discovered in 1977, when the planet occulted the relatively anonymous 10th magnitude K1 subgiant, HD 128598. Photometry from the Kuiper Airborne Observatory, which was able to continuously monitor the event, showed a symmetric sequence of ten events (five before occultation, five after); combined with ground-based observations from Perth, Australia, a total of nine rings were identified. Subsequent observations by Voyager 2, both occultations and during its flyby, identified a few additional rings and revealed belts of dust. Particles in both the prominent inner rings, which have been imaged with HST, and the more tenuous outer rings have relatively short lifetimes, indicating that the ring system must be replenished on a regular basis. The present program aims to monitor the Uranian moons over the next three cycles to better establish the dynamics of this active system.