This week on HST

HST Programs: September 10 - September 16, 2012

Selected highlights

GO 12444: Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey -- GOODS-North Field, Middle Visits of SNe Search

Part of the GOODS/Chandra Deep Field South field, as imaged by HST

CANDELS is one of three Multi-Cycle Treasury Program, whose observations will be executed over the next three HST Cycles. It builds on past investment of both space- and ground-based observational resources. In particular, it includes coverage of the two fields of the Great Observatory Origins Deep Survey (GOODS), centred on the northern Hubble Deep Field (HDF) in Ursa Major and the Chandra Deep Field-South in Fornax. In addition to deep HST data at optical and near-infrared wavelengths, the fields have been covered at X-ray wavelengths by Chandra (obviously) and XMM-Newton; at mid-infrared wavelengths with Spitzer; and ground-based imaging and spectroscopy using numerous telescopes, including the Kecks, Surbaru and the ESO VLT. This represents an accumulation of almost 1,000 orbits of HST time, and comparable scale allocations on Chandra, Spitzer and ground-based facilities. The CANDELS program is capitalising on this large investment, with new observations with WFC3 and ACS on both GOODS fields, and on three other fields within the COSMOS, EGS and UDS survey areas (see this link for more details). The prime aims of the program are twofold: reconstructing the history of galaxy formation, star formation and nuclear galactic activity at redshifts between z=8 and z=1.5; and searching for high-redshift supernovae to measure their properties at redshifts between z~1 and z~2. The program incorporates a tiered set of observations that complement, in areal coverage and depth, the deep UDF observations, while the timing of individual observations will be set to permit detection of high redshift SNe candidates, for subsequent separate follow-up. The present observations form part of the SNe detection sequence for the GOOODS-North field.

GO 12479: Low-z analogs of high-redshift Lyman Alpha emitters

Galaxies in the GOODS-North field

High-redshift (z>5) galaxies are generally characterised by strong emission lines, notably at Lyman alpha. This is generally believed to be correlated with substantial star formation activity in these systems. These high redshift galaxies are also generally associated with significant dust absorption, complicating the physical interpretation of the observed parameters. Observations have shown that such systems are first detectable around redshift 1, and these lower redshift systems are subject to much less interpretive uncertainties due to environmental factors. The present program aims to use the WFC3 UVIS G280L grism to survey the full GOODS-North field to search for strong Ly-alpha emitters with redshifts 0.7 , z < 1.8. In order to minimise confusion and limit overlapping spectra, these observations are being taken on 24 pointings, giving a total areal coveage of ~140 square arcminutes. Follow-up observations will be used to expand wavelength coverage for candidate systems, confirming their nature and investigating their intrinsic properties.

GO 12599: The Light Echoes around V838 Monocerotis

HST time-lapse imaging of the V838 Mon light echo

V838 Mon is the designation of a variable star that appeared within the constellation of Monoceros in early January 2002. Initial observations showed that both the spectrum and light curve resembled that of a nova, and the visual brightness peaked in early February of that year. However, in March the star began to exhibit somewhat unusual behaviour, with a significant increase in the near-infrared brightness, followed by a dip, and then another rise is April before the star returned to its pre-outburst level. Current analyses suggest that this behaviour was prompted by the rapid expansion of the outer atmosphere, producing a shell that cooled as it expanded to give the increased infrared radiation. Shortly after the outburst, imaging revealed extensive light echoes of the original explosion produced by surrounding gaseous material. HST has been monitoring the appearance of the system for the past decade. Those observations not only provide a means of mapping the interstellar medium in the vicinity of the star, but have also rendered a direct estimate of the distance, 6.1+/-0.6 kpc. At its brightest, V 838 Mon had an absolute magnitude of MV ~ -9.8, making it one of the brightest stars in the Milky Way. It may well be a local example of the rare class of intermediate-luminosity red transients. The latest observations will use ACS to trace the further progress of the light echoes.

GO 12748: Joint Chandra and HST Monitoring of the Crab Nebula

The Crab Nebula

Messier 1, the Crab Nebula, provides astronomy with one of its iconic images. The remnant of a bright supernova observed in 1054 by Arabian and Chinese astronomers, the Crab was first recorded in 1731 by the English astronomer, John Bevis, thirt-seven years before Messier compiled his catalogue of non-comets. The energy source for the gaseous emission is the neutron star that lies in the centre of nebulosity, and was one of the first pulsars to be identified. The Crab is also a source of high energy emission, including radiation at X-ray and gamma ray wavelengths. Overall, this system plays a crucial role in aiding our understanding of post-supernova evolutionary processes. However, there are still some notable undertainties in the detailed processes within even this system. In particular, in September of 2010 the Crab surprised the astronomial community by producing a powerful flare at gamma-ray wavelengths that persisted for 4 days (see GO 12381 ).A second flare of similar magnitude occurred in May, 2011. Observations taken during the 2010 flare by HST and Chandra provided some insight into the effects of the flare, but analysis was hampered by the absence of a comparison set of pre-outburst images of comparable resolution and depth. The present program aims to address that issue through coordinated monitoring of the Crab at X-ray and optical wavelengths. Tne Advanced Camera for Surveys on HST will be used to take images in the F550M filter at 6 epochs in 2012, with the observations timed to be within 10 days of X-ray images taken by Chandra using the AXAF CCD Imaging Spectrometer. These data will establish a reference set should a further flare occur.