This week on HST

HST Programs: February 22 - February 28, 2016

Selected highlights

GO 14112: Monitoring ice plumes of Europa

The HST imaging of a potential water plume around Europa's south pole superimposed on an image of the satellite

Europa is the smallest, and the most intriguing, of the four Galilean satellites of Jupiter. With a diameter of 3139 km, Europa is almost twice the size of Earth's moon and significantly larger than Mercury. In 1957, Gerard Kuiper commented that both infrared spectroscopy and the optical colours and albedo suggested that Jovian satellite II (Europa) is covered "by H2O snow". Images taken by the Voyager space probes in the late 1970s (see left) reveal a smooth surface, with only a handful of craters larger than a few kilometres. These features are consistent with a relatively young, icy surface. Subsequent detailed investigations by the Galileo satellite strongly suggest that a substantial body of liquid water, heated by tidal friction, underlies a 5 to 50 km thick icy crust. The presence of this subterranean (subglacial?) ocean clearly makes Europa one of the two most interesting astrobiology targets in the Solar System. Most recently, analysis of observations taken by the Space Telescope imaging Spectrograph (STIS) on Hubble indicated the presence of an extended cloud of Lyman-alpha emission near the polar regions while Europa was furthest in its orbit from Jupiter, strongly suggesting that Europa's oceans may be vaporising into space.Follow-up observations on two further occasions earlier in 2014 failed to detect any emission, suggesting that the emission is either sporadic or periodic; in the latter case, the emission might be related to the location of Europa within its orbit and the consequent tidal strain imposed by Jupiter. Over the past year, Hubble has monitored Europa' activity through several programs. The present program is using UV imaging and spectroscopy with the ACS Solar Blind Camera is searching for fluorescence. with most observations taken while Europa is in eclipse. Those observations will be matched against reference data taken out of eclipse.

GO 14116: The stellar origins of supernovae

A recent supernova in M100

Supernovae mark the (spectacular) evolutionary endpoint for a subset of stellar systems. Standard models predict that Type II supernovae originate from relatively massive stars, while Type Ia arise from interactions between close binaries that include a compact (WD, neutron star) component. There are, however, still some questions over whether we fully understand the range of possible progenitors.The present program focuses on probing the progenitors of nearby supernovae. The last decade has seen the development of a number of large-scale programs, usually using moderate-sized telescopes, that are dedicated to monitoring (relatively nearby galaxies, searching for new supernovae. Over the past 25 years, Hubble has acquired images of numerous galaxies, and some of the newly discovered supernovae fall within those galaxies. The research team associated with this project will scour those data for evidence of a point-source that matches the astrometric position of the supernova derived from ground-based data. Should such a candidate be detected, Hubble observations are triggered to refine the supernova position and obtain a more definitive match with a optential progenitor in the pre-supernova hubble image. The present observation targets the type IIb supernova AT2016ADJ, discovered in Centaurus A on February 8.

GO 14140: Using UV-bright Milky Way Halo Stars to Probe Star-Formation Driven Winds as a Function of Disk Scale Height

HST images of the Galactic star-forming region, S106

Star formation is energetic process. Massive stars in high-mass, high density star clusters can generate Galactic-scale winds that play a strong role in sculpting subsequent star-formign events, and hence affect the ovrall evolution of the parent galactic system. Such effects are likely to be present in, and influencing, galaxies over a wide range of redshift, extending back to at least z~6. The detailed density profiel and spatial extent of these winds remains difficult to assess. The present prorgam aims to probe this question through ultraviolet spectroscopy of blue horizontal branch stars within the Galactic halo. The targets are old, evolved, metal-poor, low-mass stars that have evolved off the main sequence and through the giant branch phase. Long separated frm their own star-forming regions, they serve as hot, blue, background beacons against which the hot gas generated by current star-forming regions can be seen in silhouette. The Cosmic Origins Spectrograph (COS) will be used to obtain spectra at far-UV wavelengths, enabling the search for the characteristics absorption features of C IV, Si II, Fe II and Si IV. measurement of the line profiles will allow determination of the kinematics and densities of the intervening ionised materials.

GO 14199: Refsdal Redux: Precise Measurements of the Reappearance of the First Supernova with Multiple Resolved Image

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, providing information on the mass distribution within the lensing cluster. Over the past three cycles, Hubble has been undertaking deep imaging observations 6 galaxy clusters as the Frontier Fields Director's Time program (GO 13495/13496). Those observations have provided a basis for several synergistic programs. In particular, the observations enabled a 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. In 2014, 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 original detections were followed over the course of their fading. But, more spectacularly, models of the cluster potential and the consequent light paths led to a prediction that the supernova should appear in one of the other lensed images of the parent galaxy in late 2015. The present program set out to check those predictions by re-imaging the cluster. Update: the supernova was not present in observations obtained on November 14, 2015, but has been detected in the December 11 observations, thus representing the first time that a supernova has been "predicted" successfully.Further observations are being obtained to monitor the light curve.