This week on HST

HST Programs: December 22 - December 28 , 2014

Selected highlights

GO 13677: Fellowship of the Andromeda Dwarf Galaxies: A Census of their Extended Star Formation Histories

M31, the Andromeda spiral galaxy, and its companiosn - from the PANDAS survey

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. Like the Milky Way, Andromeda has a number of satellite galaxy companions. At present, some 31 systems have been identified, the majority from the Pan Andromeda Archaeological Survey (PANDAS), a wide-field survey utilising ground-based imaging data. The four most prominent satellites are the dwarf ellipticals NGC 147, NGC 185, NGC 205 and M32; most of the remaining systems are dwarf spheroidals, with absolute visual magnitudes between MV<\sub> =-10 and -6 and masses less than 108 MSun. The present program aims to probe the star formation history of these systems. The Advanced Camera for Surveys (ACS) will be used to obtain deep V (F606W) and I(F814W) imaging of the 19 dwarf spheroidals that currently lack any HST data; the structure of the red giant branch and horizontal branch in the resultant colour-magnitude diagrams will map the metallicity and age of the component stellar populations.

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. The present program aims to address this question through UV imaging and spectroscopy with the ACS Solar Blind Camera. The majority of the observations will be taken while Europa is in eclipse, with the goal of detecting fluorescence by the ice plumes. Those observations will be matched against reference data taken out of eclipse.

GO 13816: High-Resolution Imaging of Active Galaxies with Direct Black Hole Mass Measurements

Schematic diagram of an active galactic nucleus

Active galaxies are characterised by bright, compact nuclei that are the source of strong emission lines due highly ionised material. These phenomena are generally believed to arise in hot gas in an accretion disk, centred on a massive (>106 solar mass) hole; indeed, detailed kinematics for a number of objects have confirmed the presence of a compact, massive object in the core. Most active galactic nuclei (AGNs) are found in spiral galaxies that possess at least a moderately prominent bulge. These systems appear to show a correlation between radius of the boad-line region (in light days) and the total luminosity; in addition, there is an observed correlation between the mass of the black hole and the bulge luminosity. These correlations set constraints on the mechanisms underlying the formation and evolution of central black holes. As observational phenomena, they also depend critically on our ability to separate the relative contributions of the black hole system and starlight. The present proposal aims to address the latter task by obtaining high resolution HST WIFC3 images of 14 AGNs that have BLR radius measurements and back hole mass determinations from reverberation mapping. The WFC3 data will provide a moe effective method of deconvolving the relative contribution of starlight in the cenrtgal regions of the systems.

GO 14041: Classifying and Following a Strongly Lensed Likely Supernova with Multiple Images

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. At the same time, the lensing characteristics provide information on the mass distribution within the lensing cluster. Hubble is currently undertaking deep imaging observations of up to 6 galaxy clusters as part of the Frontier Fields Director's Time program (GO 13495/13496). Those observations have provided a basis for several synergistic programs. The present program is using the Frontier Field observations to 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. Recent 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. Follow-up observations are being obtained to monitor the light-curves of each component, hence determining the time-delay for each light path. Those measured delays can be matched against the predictions of gravitational lensing models. Moreover, since the galaxy itself has multiple images it is possible that future observations may detect images of the supernova in other components. An extensive imaging program is underway, monitoring the detailed light curve of the object. The present DD program is using the WFC3 G141 grism to obtain spectra of the images, with the goal of determining unequiviocally the type of supernova. If the object is a Type Ia, then the observed magnitude can be matched against the expected intrinsic luminosity as a function of time, providing direct measurement of the lensing amplification at specific points within the system.The timing observations will also provide direct measurements of the Hubble constant at this epoch.