This week on HST

HST Programs: February 15 - February 21, 2016

Selected highlights

GO 13731: The Real Impact of Extragalactic Jets on Their Environments: Measuring the Advance Speed of Hotspots with HST

A WFPC2 image of the jet in M87

Highly collimated jets are common features of active galactic nuceli and quasars, generally believed to be generated by accretion onto supermassive black holes at the centres of these galaxies. The most famous example is the central jet in the giant elliptical M87, the second brightest galaxy in the Virgo cluster. M87 is a strong radio source, originally detected as Virgo A in 1947. The jet itself was first seen as long ago as 1918, when the lick astronomer Heber Curtis commented on a strange linear feature, a "curious straight ray", in the central regions of the galaxy. Hubble has been observing M87 for more than 20 years, and the high spatial resolution offered by those observations has allowed astronomers to measure the relative motions of features in the jet, The present observations build on that past history for both M87A and the radio galaxy Pictor A. The Advanced Camera for Surveys will be used to obtain moderately deep images of both galaxies, probing the motions of the extreme regions of the jet, the terminal hot spots where the relativistic material in the jet impacts the intergalactic medium.

GO 13847: Determining the Mass of Proxima Centauri through Astrometric Microlensing

An AAO image centred on the nearby red dwarf, Proxima Centauri

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, but duration alone is not sufficient to characterise the lens since a slow-moving source with low mass can mimic a fast-moving high-mass lens. 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 present program aims to capitalise on this fact by measuring the positional deflection of a background stars induced by the close passage of Proxima Centauri, the late-type, low-luminosity M dwarf tertiary companion of Alpha Centauri and the nearest star to the Sun. As a nearby star, Proxima has a well-defined proper motion and parallax, and it will pass close (within 1.5 arcsecnds) to two 18th magnitude stars in May 2015 and June 2015, respectively. The expected signals during the encounters (i.e. the deflection of the background stars) are expected to be approximately 0.5 millarcseconds, and therefore within HST's astrometric capabilities.

GO 14038: HST Frontier Fields - Observations of Abell 370

HST observations of the Frontier Fields cluster, Abell 370

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. 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. in addition, the observations identified several lensed galaxies at redshifts that enter the JWST domaine, with the most distant object lying at a redshift z~11, within a few hundred million years of the Big Bang. The Frontier Fields program is a large-scale Director's Discretionary program that capitalises on the latter characteristic by targeting 4-6 strong-lensing galaxy clusters for very deep optical and near-infrared imaging. WFC3 and ACS will be used to observe the clusters, with simultaneous imaging obtained in parallel of a nearby "blank" field. Since the observations need to made at a specific orientation, they are being taken in two sets, ~6 months apart, alternating between detectors. Abell 370 at z=0.375 is the sixth and final target. The present observations will provide the first epoch data, with ACS covering the cluster and WFC3 centred on the parallel field.

GO 14076: An HST legacy ultraviolet spectroscopic survey of the 13pc white dwarf sample
GO 14077: The frequency and chemical composition of rocky planetary debris around young white dwarfs: Plugging the last gaps

Artist's impression of a comet spiralling in to the white dwarf variable, G29-38

During the 1980s, one of the techniques used to search for brown dwarfs was to obtain near-infrared photometry of white dwarf stars. Pioneered by Ron Probst (KPNO), the idea rests on the fact that while white dwarfs are hot (5,000 to 15,000K for the typical targets), they are also small (Earth-sized), so they have low luminosities; consequently, a low-mass companion should be detected as excess flux at near- and mid-infrared wavelengths. In 1988, Ben Zuckerman and Eric Becklin detected just this kind of excess around G29-38, a relatively hot DA white dwarf that also happens to lie on the WD instability strip. However, follow-up observations showed that the excess peaked at longer wavelengths than would be expected for a white dwarf; rather, G 29-38 is surrounded by a dusty disk. Given the orbital lifetimes, those dust particles must be regularly replenished, presumably from rocky remnants of a solar system. G 29-38 stood as a lone prototype for almost 2 decades, until a handful of other dusty white dwarfs were identified from Spitzer observations within the last couple of years.In subsequent years, a significant number of DA white dwarfs have been found to exhibit narrow metallic absorption lines in their spectra. Those lines are generally attributed to "pollution" of the white dwarf atmospheres. Given that the diffusion time for metals within the atmospheres is short (tens to hundreds of years), the only reasonable means of maintaining such lines in ~20% of the DA population is to envisage continuous accretion from a surrounding debris disk. The Cosmic Origins Spectrograph (COS) is an ideal instrument for probing the abundance of trace elements in white dwarfs atmospheres: more than 70 systems have been observed, with detection rates running at around 50%. The two programs highlighted here are using COS to refine the statistics for such systems. GO 14076 targets a volume-limited sample of 37 white dwarfs within 13 parsecs of the Sun, sufficient to provide an estimate of the overall occurence of accreting systems. GO 14077 builds on previous investigations on younger white dwarfs, targeting white dwarfs with cooling ages of 5-25 Myrs and 100-300 Myrs: models suggest that planetary collisions (and debris) should be more frequent at the younger ages, when the parent stars have just completed extensive mass loss on the AGB.