Day 1372

Thursday.

1802.06914
A binary offset effect in CCD readout and its impact on astronomical data
Boone, Aldering, ... Perlmutter, et al

A discovery of an anomalous behavior of CCD readout electronics that affects their use in many astronomical applications.  An offset in the digitization of the CCD output voltage that depends on the binary encoding of one pixel is added to pixels that are read out one, two and/or three pixels later.  One result of this effect is the introduction of a differential offset in the background when comparing regions with and without flux from science targets.  Conventional data reduction methods do not correct for this offset.  Find this effect in 16 of 22 instruments investigated, covering a variety of telescope and many different front-end electronics systems.  The affected instruments include LRIS and DEIMOS on the Keck telescopes, WFC3-UVIS and STIS on HST, MegaCam on CFHT, SNIFS on the UH88 telescope, GMOS on the Gemini telescopes, HSC on Subaru, and FORS on VLT.  The amplitude of the introduced offset is up to 4.5 ADU per pixel, and it is not directly proportional to the measured ADU level.  Developed a model that can be used to detect this "binary offset effect" in data and correct for it.  Understanding how data are affected and applying a correction for the effect is essential for precise astronomical measurements.


1802.07708
A radial measurement of the galaxy tidal alignment magnitude with BOSS data
Martens, Hirata, Ross, Fang

The anisotropy of galaxy clustering in redshift space has long been used to probe the rate of growth of cosmological perturbations.  However, if galaxies are aligned by large-scale tidal fields, then a sample with an orientation-dependent selection effect has an additional anisotropy imprinted onto its correlation function.  Use the LOWZ and CMASS catalogs of SDSS-III BOSS DR12 to divide galaxies into two sub-samples based on their offset from the Fundamental Plane, which should be correlated with orientation.   These sub-samples must trace the same underlying cosmology, but have opposite orientation-dependent selection effects.  Measure the clustering parameters of each sub-sample and compare them in order to calculate the dimensionless parameter B, a measure of how strongly galaxies are aligned by gravitational tidal fields.  Found that for CMASS (LOWZ), the measured B was -0.024±0.015 (-0.030±0.016).  This result can be compared to the theoretical predictions of Hirata 2009, who argued that since galaxy formation physics does not depend on the direction of the observer, the same intrinsic alignment parameters that describe galaxy-ellipticity correlations should also describe intrinsic alignments in the radial direction.  Find that the ratio of observed to theoretical values is 0.15±0.32 (0.77±0.41) for CMASS (LOWZ).  Combine the results to obtain a total Obs/Theory = 0.61±0.26.  This measurement constitutes evidence (between 2 and 3 sigma) for radial intrinsic alignments, and is consistent with theoretical expectations (<2 sigma difference).