2011.10553
Mapping Gaia parallax systematic errors over the sky with faint Milky Way stars
Fardal, et al
Parallaxes measured by the Gaia mission have huge significance for astronomy, but parallaxes in Gaia DR2 are known to have systematic errors that depend on the source position and other quantities. We use the abundant information in faint Milky Way stars, along with the GOG simulation of the Gaia catalog, to probe the spatial dependence of Gaia DR2 parallax systematic errors in an empirical way. The parallax signal, concentrated in thick disk turnoff stars with magnitude G ~ 17, is sufficient to construct maps of the parallax systematic error over the majority of the sky. These maps show a locally regular "waffle pattern" on ~1 degree scales following Gaia scan directions, stronger linear "scar" features, and coherent variations on larger scales. The parallax bias maps also retain traces of astrophysical effects such as dust clouds. The waffle pattern, known from earlier maps of the Magellanic Clouds, extends over the entire sky; its local rms amplitude averages 15 microarcsec and varies by about a factor of two. The strength of this pattern increases by a factor ~6 from magnitude G = 13 to G = 20. Correlations with parallaxes of quasars and of stars with independent distance estimates support our bias estimates. Using similar methods, we map systematic errors in the proper motion and examine the relationship with the parallax systematics. We provide a code package to access and query our bias maps. Similar tests on the general stellar population should be useful in quantifying systematic errors in future Gaia releases.
2011.10578
Completeness of the Gaia-verse III: using hidden states to infer gaps, detection efficiencies and the scanning law from the DR2 light curves
Boubert, et al
The completeness of the Gaia catalogues heavily depends on the status of that space telescope through time. Stars are only published with each of the astrometric, photometric and spectroscopic data products if they are detected a minimum number of times. If there is a gap in scientific operations, a drop in the detection efficiency or Gaia deviates from the commanded scanning law, then stars will miss out on potential detections and thus be less likely to make it into the Gaia catalogues. We lay the groundwork to retrospectively ascertain the status of Gaia throughout the mission from the tens of individual measurements of the billions of stars, by developing novel methodologies to infer both the orientation and angular velocity of Gaia through time and gaps and efficiency drops in the detections. We have applied these methodologies to the Gaia DR2 variable star epoch photometry -- which are the only publicly available Gaia time-series at the present time -- and make the results publicly available. We accompany these results with a new Python package scanninglaw (https://github.com/gaiaverse/scanninglaw) that you can use to easily predict Gaia observation times and detection probabilities for arbitrary locations on the sky.
2011.11604
HSTCosmicrays: A Python package for analyzing cosmic rays in HST calibration data
Miles, et al
HSTCosmicrays is a python-based pipeline designed to find and characterize cosmic rays found in dark frames (exposures taken with the shutter closed). Dark exposures are obtained routinely by all the Hubble Space Telescope (HST) instruments for calibration. The main processing pipeline runs locally or in the cloud on AWS. To date, we have characterized more than 1.2 billion cosmic rays in ~76,000 dark frames obtained with CCDs from the four active instruments ACS/HRC, ACS/WFC, STIS, WFC3/UVIS, and the legacy instrument WFPC2.
2011.11613
Strong detection of the CMB lensingxgalaxy weak lensing cross-correlation from ACT-DR4, Planck Legacy and KiDS-1000
Robertson, et al
We measure the cross-correlation between galaxy weak lensing data from the Kilo Degree Survey (KiDS-1000, DR4) and cosmic microwave background (CMB) lensing data from the Atacama Cosmology Telescope (ACT, DR4) and the Planck Legacy survey. We use two samples of source galaxies, selected with photometric redshifts, $(0.1<z_{\rm B}<1.2)$ and $(1.2<z_{\rm B}<2)$, which produce a combined detection significance of the CMB lensing/weak galaxy lensing cross-spectrum of $7.7\sigma$. With the lower redshift galaxy sample, for which the cross-correlation is detected at a significance of $5.3\sigma$, we present joint cosmological constraints on the matter density parameter, $\Omega_{\rm m}$, and the matter fluctuation amplitude parameter, $\sigma_8$, marginalising over three nuisance parameters that model our uncertainty in the redshift and shear calibration, and the intrinsic alignment of galaxies. We find our measurement to be consistent with the best-fitting flat $\Lambda$CDM cosmological models from both Planck and KiDS-1000. We demonstrate the capacity of CMB-weak lensing cross-correlations to set constraints on either the redshift or shear calibration, by analysing a previously unused high-redshift KiDS galaxy sample $(1.2<z_{\rm B}<2)$, with the cross-correlation detected at a significance of $7\sigma$. This analysis provides an independent assessment for the accuracy of redshift measurements in a regime that is challenging to calibrate directly owing to known incompleteness in spectroscopic surveys.
2011.11648
Unraveling the origin of magnetic fields in galaxies
Martin-Alvarez, et al
Despite their ubiquity, there are many open questions regarding galactic and cosmic magnetic fields. Specifically, current observational constraints cannot rule out if magnetic fields observed in galaxies were generated in the Early Universe or are of astrophysical nature. Motivated by this we use our magnetic tracers algorithm to investigate whether the signatures of primordial magnetic fields persist in galaxies throughout cosmic time. We simulate a Milky Way-like galaxy in four scenarios: magnetised solely by primordial magnetic fields, magnetised exclusively by SN-injected magnetic fields, and two combined primordial + SN magnetisation cases. We find that once primordial magnetic fields with a comoving strength $B_0 >10^{-12}$ G are considered, they remain the primary source of galaxy magnetisation. Our magnetic tracers show that, even combined with galactic sources of magnetisation, when primordial magnetic fields are strong, they source the large-scale fields in the warm metal-poor phase of the simulated galaxy. In this case, the circumgalactic and intergalactic medium can be used to probe $B_0$ without risk of pollution by magnetic fields originated in the galaxy. Furthermore, whether magnetic fields are primordial or astrophysically-sourced can be inferred by studying local gas metallicity. As a result, we predict that future state-of-the-art observational facilities of magnetic fields in galaxies will have the potential to unravel astrophysical and primordial magnetic components of our Universe.
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