This week on HST

HST Programs: September 2 - September 8, 2013

Selected highlights

GO 12961: A Cepheid Distance to NGC 6814

The spiral galaxy, NGC 6814 (NOAO)

Cepheid variable stars have been the prime extragalactic distance indicator since Henrietta Leavitt's discovery of the period-luminosity relation described by Cepheids in the Small Magellanic Cloud. It was Hubble's identification of Cepheids in NGC 6822 that finally established that at least some nebulae were island universes. Cepheids and the extragalactic distance scale figure largely in HST's history, notably through the Hubble Constant Program, one of the initial Key Projects. HST has since observed Cepheids in more than 30 galaxies. The present program aims to extend observations to the Seyfert galaxy, NGC 6814, a face-on spiral lying at a distance of 20-30 Mpc from the Milky Way. The system lies in Aquila, close to the Galactic Plane and therefore subject to significant foreground reddening that complicates distance estimation. NGC 6814 is highly variable at X-ray wavelengths, clealy indicating the presence of a supermassive black hole driving the nuclear activity. This program will use multi-epoch imaging with the WFC3-UVIS and WFC3-IR cameras to identify and monitor Cepheid variables in the system, using the photometric measurements to determine the distance and line of sight reddening. These observations will be coupled with ground-based spatially-resolved spectroscopy of the nuclear regions to determine the mass of the central black hole.

GO 13297: The HST Legacy Survey of Galactic Globular Clusters: Shedding UV Light on Their Populations and Formation

Hubble image of the metal-poor globular cluster, M15

Globular clusters are members of the Galactic halo population, representing remnants from the first extensive period of star formation in the Milky Way. As such, the properties of the 106 to 107 stellar constituents can provide crucial insight into the earliest stages of galaxy formation. Until recently, conventional wisdom was that these are simple systems, where all the stars formed in a single starburst and, as a consequence, have the same age and metallicity. One of the most surprising disoveries in recent years is the realisation that this simple picture no longer holds. Up until about 5 years ago, the only known counter-example to convention was the cluster Omega Centauri, which is significantly more massive than most clusters and has both a complex main sequence structure and a range of metallicities among the evolved stars. High precision photometric observations with HST has demonstrated that Omega Cen is far from unique, with multiple populations evident in numerous other clusters, including NGC 2808, NGC 1851, 47 Tuc and NGC 6752. Multiple populations have also been discerned in a number of clusters in the Magellanic clouds. Sustaining multiple bursts of star formation within these systems demands that they retain gas beyond the first star forming event, which appears to set a requirement that these clusters were significantly more massive during their epoch of formation; put another way, the current globulars may represent the remnant cores of dwarf galaxy-like systems. That, in turn, implies that the stars ejected from those systems make a significant contribution to the current galactic halo. One of the most effective means of identifying and studying multi-population clusters is combining high-precision photometry over a wide wavelength range, particularly extending to UV wavelengths. Sixty-five globular clusters already have R/I (F606W, F814W) Hubble imaging and photometry thanks to the Cycle 14 program, An ACS Survey of Galactic Globular Clusters (GO 10775). The present program aims to build on those data by adding UV/blue observations using the F275W, F336W and F438W filters on the WFC3-UVIS camera. The colorus derived from these filters enable characterisation of the C, N and O abundances of the component stellar populations in these systems.

GO 13327: Proper Motions at 500 Mpc: Measuring Superluminal motions in Optical Jets with HST

Hubble imaging of the jet in the nearby giant elliptical, M87

Highly collimated jets are common features of active galactic nucli and quasars. The overall consensus is that these features originate in a central supermassive black hole, with material driven to relativistic velocities by the accretion of material from a surrounding disk. The classic example is the jet in the massive elliptical galaxy M87, the second brightest galaxy in the Virgo cluster. Originally discovered by Heber Curtis (of Great Debate fame) as a "curious straight ray" on a photographic plate taken with Lick's Crossley reflector, the M87 jet shows extensive structure, with a number of knots and clouds. That system has been monitored by HST since shortly after its launch, and accurate astrometry of the knots shows that they have velocities that are apparently superluminal. The actual velocities are sub-luminal, but relativistic, giving the appearance of faster than light motion in the plane of the sky. The present program aims to extend observations to three additional systems, the radio galaxies 3C 264 (NGC 3682) and 3C 346, and 3C 273, the first quasar to be identified (celebrating its 50th anniversary this year).All of these galaxies have previous HST observations, giving a baseline of~20 years for proper motion measurements.

GO 13364: LEGUS: Legacy ExtraGalactic UV Survey

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 of those 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, and the present program aims to capitalise on the past heritage of HST observations vby adding near-UV imaging for 50 nearby galaxies. The 50 targets are drawn from a catalogue of 400 systems within ~11 Mpc of the Milky Way, and have been selected to provide a fair sampling of the wide variety of galactic systems within that volume. The program will image star-forming regions in these systems in near-UV and blue wavelengths using the WFC3 UVIS channel with the F275W, F336W and F438W filters, supplemented by F55W and F81wW (V and IK) where necessary.