Imaging Spectropolarimetry – A New Observing Mode on the HST/ACS Instrument

The polarization state of light observed from astrophysical sources can provide vital information that is otherwise unobtainable from intensity alone. Examples of such information include (but are not limited to): The origin of emission (intrinsically luminous vs. scattered); Nature of emitting or scattering particles (e.g., relativistic electrons, small dust particles); Geometry, orientation and detailed structure of an object; Magnetic field strengths and directions; and Scattering media properties such as sizes, morphologies, and complex refractive indices.

Measurement of the polarization state of the light as a function of wavelength, known as Spectropolarimetry, can provide new and valuable constraints on the nature of the light source. While Spectropolarimetry is a technique that is accessible from ground-based observatories, the superb spatial resolution afforded by the HST/ACS combined with the slitless nature of HST/ACS grism spectroscopy opens up the possibility to study polarized extended emission in a way that is not possible even with AO facilities on the ground.

This new observing mode, which has been commissioned in HST Cycle 30 and is being offered to HST observers in Cycles 31+, could help to study QSOs, AGN & Radio Galaxies (central illuminating sources & synchrotron emission), pre-planetary Nebulae, proto-planetary & Debris Disks, Supernovae & Supernova Remnants, Solar System Targets (e.g., comets, asteroids, moons, minor planets), ISM Dust Properties, Alignment, Photo-Dissociation Regions, Young O&B Stars & Associations, AGB Stars, Cataclysmic Variables, and Galactic Magnetic Fields.

In this presentation, we will discuss the results from the calibration programs used to commission and calibrate a specific mode of Imaging Spectropolarimetry on ACS, which will provide a useful capability for the science community. For details and updates refer to the latest version of the ACS Instrument Handbook and ACS Data Handbook.