This week on HST

HST Programs: December 26 - January 1, 2017

Selected highlights

GO 14080: LyC, Ly-alpha, and Low Ions in Green Peas: Diagnostics of Optical Depth, Geometry, and Outflows

A montage of green pea galaxies discovered by the Galaxy Zoo project

Understanding the galaxy formation and galaxy evolution has been a strong focus of astronomical research since thne 1970s and remains a key issue for 21st century astrophysics. Since we cannot follow an individual galaxy through time, tackling these questions requires a statistical approach, and, as a result, large-scale surveys have played a crucial role in the field. In particular, the Sloan Digital Sky Survey, conducted from Apache point Observatory from the 1990s through the early years of this century, has provided a treasure trove of information of galaxies at low and moderate redshifts. One of the interesting discoveries that originated from SDSS was the discovery of so-called "green pea" galaxies - compact objects whose combined images have a greenish hue, originally uncovered by citizen scientists as part of the Galaxy Zoo project. Closer inspection shows that these are gas-rich galaxies lying at relatively low redshifts, 0.1 < z < 0.36, with the green-tinged hue partly stemming from the presence of strong oxygen ([O III]) emission. These characteristics indicate that the galaxies are undergoing strong star-forming episodes. The present program aims to take advantage of HST's unparalleled resolution and extraordinary sensitivity at ultraviolet wavelengths, and will use the Cosmic Origins Spectrograph to obtain spectra are far-UV wavelengths. Those observations will probe optical depth, the geometry of neutral gas and radiative transfer within these systems, revealing the physical structure of the underlying star forming regions and potentially offering insight into the nature of high redshift lyman-alpha emitters.

GO 14269: Just the BASICs: Linking Gas Flows in the Circumgalactic Medium to Galaxies

A computer simulation of galactic gas accretion and outflow

Galaxy formation, and the overall history of star formation within a galaxy, clearly demands the presence of gas. The detailed evolution therefore depends on how gas is accreted, recycled, circulated through the halo and, perhaps, ejected back into the intergalactic medium. Tracing that evolutionary 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. Extensive observations of galaxies at modest redshift (0.15 < z < 0.35) have shown that material extends to radii of hundreds of kpc, with a total mass in metals that is at least comparable with the mass in the central galaxy. The structures appear to show a bimodal metallicity distribution, with a metal-rich (~40% solar) component that may reflect winds and outflows, and a metal-poor (3% solar) component that may trace cold accretion streams. The present program aims to use ACS to image 14 QSO fields that harbour well-characterised circum-galactic medium absorbers, with the aim of measuring the geometry and morphology of the parent galaxies and searching for correlations with the gaseous properties.

GO 14648: A New Threshold of Precision, 30 micro-arcsecond Parallaxes and Beyond

HST WFPC2 image of NGC 4639, one of the Cepheid-rich spiral galaxies used to calibrate SNe Ia

The cosmic distance scale and dark energy are two key issues in modern astrophysics, and HST has played a vital role in probing both. On the one hand, HST has been involved in cosmic distance measurements since its inception, largely through the H0 Key Project, which used WFPC2 to identify and photometer Cepheids in 31 spiral galaxies at distances from 60 to 400 Mpc. On the other, HST is the prime instrument for investigating cosmic acceleration by searching for and following Type Ia supernovae at moderate and high redshift. These two cosmological parameters are directly related, and recent years have seen renewed interest in improving the accuracy of H0 with the realization that such measurements, when coupled with the improved constraints from the Cosmic Microwave Background, provide important constraints on cosmic acceleration and the nature of Dark Energy. Previous HST programs have focused on identifying and measuring light curves for cepheids in external galaxies (eg GO 10802 , GO 11570 ) or quantifying the effects of variations in intrinsic stellar parameters, such as metallicity (eg GO 10918 , GO 11297 ). The present SNAP program is part of a suite of HST programs focusing on the Galactic Cepheids that form the foundation for the whole distance ladder. These programs employ a revived version of an old technique to determine both accurate astrometry, hence trigonometric parallaxes and reliable distances, and accurate photometry, hence flux emasurements. The technique is drift-scanning - tracking HST during the observation so that stars form trails on the detector. This mode of observations was available in the early years of HST's operations, and has been revived primarily as a means of obtaining high signal-to-noise grism spectroscolpic data of stars hosting transiting exoplanets. However, the same technique can be used in imaging mode, and the extended trails allow not only multiple measurements of position differences for stars in the field but also extremely high signal-to-noise photometry. The latter is crucial in obtaining direct photometry of tghe local calibrations on the same HST system, the same system that is being used for photometry of Cepheids in the external galaxies that serve as the basis for the distance scale. Observations have been obtained for more than 20 such stars. The present program aims to refine the distance estimates by obtaining four additional epochs for 9 core Cepheids (Z Sct, DD Cas, VX Per, SZ Cyg, SS CMa, XY Car, S Vul, X Pup and WZ Sgr). These data will improve the precision of the final parallaxes by identifying and eliminating binaries among ther eference stars, providing a longer baselien for proper motion determination, and providing direct overlap with Gaia observations.

GO 14709: HST/WFC3 Spectroscopy of < 400 AU Companions to Orion Young Stellar Objects

An image of the Orion Nebula superimposed on the 13CO map of Orion A (from this link ).

The Orion association is the largest nearby star-forming complex, providing a key laboratory for unlocking the secrets of star formation. As such, it has been subject to intense scrutiny at all wavelengths from both ground and space. Surveys at near-infrared and mid-infrared wavelenths, notably by Spitzer, have identified an extensive number of embedded sources, young stellar objects (YSOs) that are still accreting from the surrounding molecular gas. A follow-up HST proposal focused on more than 250 sources within the Orion A molecular cloud, the complex that includes the Orion Nebula Cluster. Initially,NICMOS was used to survey a subset of the protostars; following SM4, the WFC3-IR camera was applied to the task. The observations provided an excellent complement to Spitzer since, while HST cannot offer either the same areal coverage or sensitivity at mid-infrared wavelegths, HST gives a resolution close to 0.1 arcsecond, an order of magnitude higher than the Spitzer images. That program resulted in the detection of several very faint companions, with luminosities consistent with planetary mass (5 MJ) objects. The present program is using the G141 grism on WFC3-IR to obtain spectra and determine the true nature of these objects with the overall goal of constraining the mass function of sub-stellar-mass companions.