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Register a new userHigh Accuracy Imaging Polarimetry with NICMOS
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Authors
D. Batcheldor
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The ability of NICMOS to perform high accuracy polarimetry is currently hampered by an uncalibrated residual instrumental polarization at a level of 1.2-1.5%. To better quantify and characterize this residual we obtained observations of three polarimetric standard stars at three separate space-craft roll angles. Combined with archival data, these observations were used to characterize the residual instrumental polarization to enable NICMOS to reach its full polarimetric potential. Using these data, we calculate values of the parallel transmission coefficients that reproduce the ground-based results for the polarimetric standards. The uncertainties associated with the parallel transmission coefficients, a result of the photometric repeatability of the observations, dominate the accuracy of p and theta. However, the new coefficients now enable imaging polarimetry of targets with p~1.0% at an accuracy of +/-0.6% and +/-15 degrees.
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The findings of a nine orbit calibration plan carried out during HST Cycle 15, to fully determine the NICMOS camera 2 (2.0 micron) polarization calibration to high accuracy, are reported. Recently Ueta et al. and Batcheldor et al. have suggested that NICMOS possesses a residual instrumental polarization at a level of 1.2-1.5%. This would completely inhibit the data reduction in a number of GO programs, and hamper the ability of the instrument to perform high accuracy polarimetry. We obtained polarimetric calibration observations of three polarimetric standards at three spacecraft roll angles separated by ~60deg. Combined with archival data, these observations were used to characterize the residual instrumental polarization in order for NICMOS to reach its full potential of accurate imaging polarimetry at p~1%. Using these data, we place an 0.6% upper limit on the instrumental polarization and calculate values of the parallel transmission coefficients that reproduce the ground-based results for the polarimetric standards. The uncertainties associated with the parallel transmission coefficients, a result of the photometric repeatability of the observations, are seen to dominate the accuracy of p and theta. However, the updated coefficients do allow imaging polarimetry of targets with p~1.0% at an accuracy of +/-0.6% and +/-15deg. This work enables a new caliber of science with HST.
NICMOS cameras 1 and 2 each carry a set of three polarizing elements to provide high sensitivity observations of linearly polarized light. The polarizers are bandpass limited and provide diffraction-limited imaging in camera 1 at 0.8 - 1.3um, and in camera 2 at 1.9-2.1um. The NICMOS design specified the intra-camera primary axis angles of the polarizers to be differentially offset by 120 degree, and with identical polarizing efficiency and transmittance. While this ideal concept was not strictly achieved, accurate polarimetry in both cameras, over their full (11 and ~19.2 square) fields of view was enabled through ground and on-orbit calibration of the as-built and HST-integrated systems. The Cycle 7 & 7N calibration program enabled and demonstrated excellent imaging polarimetric performance with uncertainties in measured polarization fractions <=1%. After the installation of the NICMOS Cooling System (NCS), the polarimetric calibration was re-established in Cycle 11, resulting in systemic performance comparable to (or better than) Cycle 7 & 7N. The NCS era NICMOS performance inspired the development of an earlier conceived, but non-implemented, observing mode combining high contrast coronagraphic imaging and polarimetry in camera 2. We successfully executed a program to calibrate and commission the Coronagraphic Polarimetry mode in NICMOS in Cycle 13, and the mode was made available for GO use in Cycle 14. We discuss the data reduction and calibration of direct and coronagraphic NICMOS polarimetry. Importantly, NICMOS coronagraphic polarimetry provides unique access to polarized light near bright targets over a range of spatial scales intermediate between direct polarimetry and ground-based (coronagraphic) polarimetry using adaptive optics.
We have obtained near-infrared (1.6 micron) images of 11 powerful 3CR radio galaxies at redshifts 0.8 < z < 1.8 using NICMOS on board HST. The high angular resolution permits a detailed study of galaxy morphology in these systems at rest-frame optical wavelengths, where starlight dominates over the extended, aligned UV continuum. The NICMOS morphologies are mostly symmetric and are consistent with dynamically relaxed, elliptical host galaxies dominated by a red, mature stellar population. The aligned structures are sometimes faintly visible, and nuclear point sources may be present in a few cases which manifest the ``unveiled AGN that is obscured from view at optical wavelengths. Our observations are consistent with the hypothesis that the host galaxies of z ~ 1-2 radio galaxies are similar to modern-day gE galaxies. Their sizes are typical of gE galaxies but smaller than present-day cD and brightest cluster galaxies, and their surface brightnesses are higher, as expected given simple luminosity evolution.
The nuclei of Seyfert 1 galaxies exhibit a range of optical polarization characteristics that can be understood in terms of two scattering regions producing orthogonal polarizations: an extended polar scattering region (PSR) and a compact equatorial scattering region (ESR), located within the circum-nuclear torus. Here we present NICMOS 2.0 micron imaging polarimetry of 6 polar scattered Seyfert 1 (S1) galaxies, in which the PSR dominates the optical polarization. The unresolved nucleus (<0.58 arcsec) is significantly polarized in only three objects, but 5 of the 6 exhibit polarization in a 0.58 to 1.5 arcsec circum-nuclear annulus. In Fairall 51 and ESO 323-G077, the polarization position angle at 2 microns (theta2m) is consistent with the average for the optical spectrum (thetav), implying that the nuclear polarization is dominated by polar scattering at both wavelengths. The same is probably true for NGC 3227. In both NGC 4593 and Mrk 766, there is a large difference between theta2m and thetav off nucleus, where polar scattering is expected to dominate. This may be due to contamination by interstellar polarization in NGC 4593, but there is no clear explanation in the case of the strongly polarized Mrk 766. Lastly, in Mrk 1239, a large change (~ 60deg) in theta2m between the nucleus and the annulus indicates that the unresolved nucleus and its immediate surroundings have different polarization states at 2 microns, which we attribute to the ESR and PSR, respectively. A further implication is that the source of the scattered 2 micron emission in the unresolved nucleus is the accretion disk, rather than torus hot dust emission.
We present the first results from a major Hubble Space Telescope program designed to investigate the cosmological evolution of quasar host galaxies from z~2 to the present day. Here we describe J and H-band NICMOS imaging of two quasar samples at redshifts of 0.9 and 1.9 respectively. Each sample contains equal numbers of radio-loud and radio-quiet quasars, selected to lie within the same narrow range of optical absolute magnitude (-24 > M_V > -25). Filter and target selection were designed to ensure that at each redshift the images sample the same part of the objects rest-frame spectrum, avoiding potential contamination by [OIII]lambda5007 and H-alpha emission lines. At z=1 the hosts of both radio-loud and radio-quiet quasars lie on the same Kormendy relation described by 3CR radio galaxies at comparable redshift. There is some evidence for a gap of ~1 mag between the host luminosities of RLQs and RQQs, a difference that cannot be due to emission-line contamination given the design of our study. However, within current uncertainties, simple passive stellar evolution is sufficient to link these galaxies with the elliptical hosts of low-redshift quasars of comparable nuclear output, implying that the hosts are virtually fully assembled by z=1. At z=2 the luminosity gap appears to have widened further to ~1.5 mag. Thus while the hosts of radio-loud quasars remain consistent with a formation epoch of z>3, allowing for passive evolution implies that the hosts of radio-quiet quasars are ~2-4 times less massive at z=2 than at low z.
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