Day 1276

Friday.



1706.02739

Cosmology at at Crossroads: Tension with the Hubble Constant 

Freedman

The accuracy in measurement of the Hubble constant has vastly improved, but a recent tension has arisen that is either signaling new physics or as-yet unrecognized uncertainties.

1706.07054
Predicting the locations of possible long-lived low-mass first stars: importance of satellite dwarf galaxies
Magg, et al

The search for metal-free stars has so far been unsuccessful, proving that if there are surviving stars from the first generation, they are rare, they have been polluted, or we have been looking in the wrong place.  To predict the likely location of Pop III survivors, model semi-analytically early SF in progenitors of MW-like galaxies and their environments.  Base the model on merger trees from the high-resolution DM only simulation suite Caterpillar.  Radiative and chemical feedback are taken into account self-consistently, based on the spatial distribution of the haloes.  The results are consistent with the non-detection of Pop III survivors in the MW today.  Find that possible surviving Pop III stars are more common in MW satellites than in the main Galaxy.  In particular, low mass MW satellites contain a much larger fraction of Pop III stars than the MW.  Such nearby, low mass MW satellite are promising targets for future attempts to find Pop III survivors, especially for high-resolution, high signal-to-noise spectroscopic observations.  Provide the probabilities to find a Pop III survivor in the red giant branch phase for all known MW satellites to guide future observations.


1706.07400
Is flat fielding safe for precision CCD astronomy?
Baumer et al

The ambitious goals of precision cosmology with wide-field optical surveys such as DES and LSST demand, at their foundation, precision CCD astronomy.  This in turn requires an understanding of previously uncharacterized sources of systematic error in CCD sensors, many of which manifest themselves as static effective variations in pixel area.  Such variation renders a critical assumption behind the traditional procedure of flat fielding--that a sensor's pixels comprise a uniform grid--invalid.  In this work, present a method to infer a curl-free model of a sensor's underlying pixel grid from flat field images, incorporating the superposition of all electrostatic sensor effects--both known and unknown--present in flat field data.  Use these pixel grid models to estimate the overall impact of sensor systematics on photometry, astrometry, and PSF shape measurements in a representative sensor effects for which corrections are currently being developed within DES.  For an LSST prototype CCD with pixel-response non-uniformity (PRNU) of 0.4%, find the impact of "improper" flat-fielding on these observables is negligible in nominal .7" seeing conditions.  These errors scale linearly with the PRNU, so for future LSST production sensors, which may have larger PRNU, the method provides a way to assess whether pixel-level calibration beyond flat fielding will be required.