This week on HST

HST Programs: December 21 - December 27, 2015

Selected highlights

GO 13862: Measuring the Impact of Starbursts on the Circum-Galactic Medium

ALMA image of the gaseous outflows surrounding the nearby galaxy, NGC 253

Galaxy formation, and the overall history of star formation within a galaxy, clearly demands the presence of gas. The detailed evolution of galaxy assembly therefore is tied closely to how gas is accreted, recycled, circulated through the halo and disk, and, perhaps, ejected back into the intergalactic medium. In particular, starburst-driven outflows may represent an important means of removing gas from and galaxy and quenching star formation, producing the systems that populate the "red and dead" sequence. Tracing the overall history is difficult, since gas passes through many different phases, some of which are easier to detect than others. During accretion and, probably, subsequent recycling, the gas is expected to be reside predominantly at high temperatures. The most effective means of detecting such gas is through ultraviolet spectroscopy, where gas within nearby systems can be detected as absorption lines superimposed on the spectra of more distant objects, usually quasars. The present program is using the Cosmic Origins Spectrograph to probe gas in fifteen starburst galaxies at redshifts between z~0.02 and z~0.15. The galaxy-quasar pairs are drawn from the Sloan Digital Sky Survey. The galaxies have starburst ages spanning the range <100 to ~600 Myrs and the attendant quasars have angular separations corresponding to <250 kpc at the galaxy redshift. The COS observations are capable of detecting highly ionised species such as CIV, OVI and SiIV, as well as Lyman-alpha, OI, CII, SiII and SiIII.

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 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 14192: Hubble Close-Up of the Disrupting Asteroid P/2012 F5

The multiple nuclei of MBC P/2013 R3 as imaged by Hubble in late 2013 and 2014

Recent years have seen the discovery of a new class of asteroid within the main belt in the Solar System - objects that show occasional signs of activity in the form of outgassing or the ejection of dusty debris. These active objects have become known as Main Belt Comets. One of the earliest discoveries in this class is the asteroid Scheila. Discovered by the Heidelberg astronomer August Kopff in 1906, and named after an English student with whom he was acquainted, this is one of the larger known asteroids, with a diameter estimated as ~110 km. Early December 2010, Steve Larson (of Arizona's Lunar and Planetary Laboratory) noted that Scheila had sprouted a coma halo in observations taken by the Catalina Sky Survey. An examination of archival images revealed no evidence for activity throughout October and November. Since then other asteroids, including 1979 OW7/1996 N2 and several objects identified in the Pan-STARRS survey, have exhibited similar behaviour. One object in particular, MBC 2013/R3, was found to have multiple nuclei that are gradually drifting apart at speeds of less than 1 metre/second. The latter object is probably best explained as the result of rotational break-up. P/2012 F5 another such system. Observations with the Keck Telescope in August 2015 revbealed 4 distinct nuclei, and a very short rotation period. The present monitoring program with Wide-Feld Camera 3 will enable measurement of the relative orbits and, through time-resolved photomtery, determination of the rotation periods of the individual fragments.