This week on HST

HST Programs: February 2 - February 8, 2015

Selected highlights

GO 13668: Deep Search for Small Satellites of Eris and Makemake

Hubble Space Telescope images of the Pluto system

As is now well known, on 2006 the IAU changed pluto's status from the outermost planet in the solar system to a dwarf planet, and the largest currently-known member of the Kuiper. One of the main factors in adopting that revised classification was the discovery of several other large KBOs, at least comparable in size with Pluto. These systems included Eris and Makemake, both discovered in 2005 by Mike Brown using the Palomar Schmidt telescope. Eris is probably slightly smaller than Pluto, with a diameter of ~2300 miles, while Makemake has approximately half that diameter. Deep imaging with the Keck adaptive optics revealed that Eris has a companion, the moon Dysnomia; orbital measurements suggest that Eris is more massive (i.e. denser) than Pluto. In contrast, not satellties have yet been detected around Makemake. Pluto itself has 5 known satellites, including Charon and 4 much smaller moons discovered by Hubble. Building on the lessons learned from pluto, the present program is using deep, "white light" (F350LP) imaging with the WFC3 UVIS camera to search for low-mass satellite companions of both systems.

GO 13773: H-alpha LEGUS: Unveiling the Interplay Between Stars, Star Clusters, and Ionized Gas

UGC 4305 = Holmberg II - Arp 268, one of the star-forming galaxies targeted by LEGUS

Understanding the global architecture of star formation is a key step towards understanding the morphological evolution of galaxies and the characteristics of the underlying stellar populations. HST has devoted extensive resources to observations of nearby galaxies over its lifetime, including detailed surveys of a handful of systems, notably the PHAT survey of M31, with the enhanced imaging capabilities made available following SM4. Most programs, however, have focused on optical, far-red and, to a lesser extent, near-infrared wavelengths. While those observations provide high-quality colour-magnitude data that enable an exploration of relatively mature populations, they are less effective at probing active star-formation sites populated by young, high-mass stars. Those regions are most prominent at ultraviolet wavelengths. The HST LEGUS capitalises on the past heritage of HST observations by adding near-UV imaging for 50 nearby galaxies, drawn from a catalogue of 400 systems within ~11 Mpc of the Milky Way. They have been selected to provide a fair sampling of the wide variety of galactic systems within that volume. LEGUS itself is imaging star-forming regions at near-UV and blue wavelengths using the WFC3 UVIS channel with the F275W, F336W and F438W filters, supplemented by F55W and F814W (V and I) where necessary. The present program builds on that foundation by adding WFC3 narrow-band (F657N) H-alpha imaging of 32 galaxies from the sample.

GO 13790: Frontier Field Supernova Search

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. The persent program is obtaining follow-up observations 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. Thus, this supernova can (perhaps even will) be discovered more than once - and might even be predicted.

GO 13872: The GOODS UV Legacy Fields: A Full Census of Faint Star-Forming Galaxies at z~0.5-2

ACS images of a section of the GOODS fields

The Great Observatories Origins Deep Survey, GOODS, originated as a Spitzer Legacy program coupled with a Cycle 12 HST Treasury program. The program was designed to probe galaxy formation and evolution at redshifts from z~1 to z~6. GOODS covers two ~150 sq. arcminute fields, one centred on the Hubble Deep Field in Ursa Major and the Chandra Deep Field-South in Fornax. Initially, the program combined deep optical/far-red imaging (F435W, F606W, F775W and F850LP filters) using ACS on HST with deep IRAC (3.6 to 8 micron) and MIPS (25 micron) imaging with Spitzer. These two fields have become among the most studied celestial regions. In addition to deep HST data at optical and near-infrared wavelengths (both fields have been covered by NICMOS), the fields have been covered at X-ray wavelengths by Chandra (obviously) and XMM-Newton, and ground-based imaging and spectroscopy using numerous telescopes, including the Kecks, Gemini, Surbaru and the ESO VLT. Part of the GOODS South field was covered by the WFC3 Early Release Science observations (see WFC3 ERS ), and both fields are also covered partially by one of the three Multi-Cycle Treasury programs allocated time in Cycle 18-20. Further observations were obtained in Cycle 17, using the G141 grism on the WFC3 IR camera to identify H-alpha+[N II] emission from galaxies at redshifts 0.7 < z < 1.5, and thereby set constraints on star formation at those redshifts. The present program builds on these multiple datasets by adding WFC3-UVIS imaging with the F275W and F336W. These data will cover the CANDELS sections of GOODs and sample far-UV radiation from galaxies at edshifts z > 0.5, tracing the evolution of the FUV luminosity through the peak epoch of star formation.