Day 749

Tuesday.

1409.5792
Impact of star formation history on the measurement of star formation rates
Boquien, Buat, Perret

Calculate SED from hydrosims of 1<z<2 galaxies every 1 Myr.  Except for the Lyman continuum, classical SFR estimators calibrated over 100 Myr overestimate the SFR from ~25% in the FUV band to ~65% in the U band.  Such biases are due 1) to the contribution of stars living longer than 100 Myr, and 2) to variations of the SFR on timescales longer than a few tens of Myr.  Rapid variations of the SFR increase the uncertainty on the determination of the instantaneous SFR but have no long term effect.  The discrepancies between the true and estimated SFR may explain at least part of the tension between the integral of the SFR density and the stellar mass density at a given redshift.  To reduce possible biases, suggest to use SFR estimators calibrated over 1 Gyr rather than the usually adopted 100 Myr timescales.

1409.5905
SARCS strong-lensing galaxy groups: II - mass-concentration relation and strong-lensing bias
Foëx, Motta, Jullo, Limousin, Verdugo

Based on stacked WL analysis of 80 SL galaxy groups, find (i) the lensing signal does not allow a firm rejection of a simple SIS mass distribution compared to the expected NFW, (ii) obtain an average concentration c200=8.6pm2 that is much higher than the expected value from sims for the corresponding average mass M200=0.73e14 Msun, (iii) the combination of results with those at higher mass scales give a c(M) relation over nearly 2 decades in mass, with a slope in disagreement with predictions from numerical sims using unbiased populations of DM haloes, (iv) combined c(M) relation matches results from sims using only haloes with a large SL cross section, i.e., elongated with a major axis close to the LoS, (v) for the simplest case of prolate haloes, estimate with a toy model a lower limit on the minor:major axis ratio a/c=0.5 for the average SARCS galaxy group.  Analysis based on galaxy groups confirmed the results obtained at larger mass scales: SL present apparently too large concentrations, which can be explained by traixial haloes preferentially orientated with the LoS.  Because more massive systems already have large lensing cross section, they do not require a large elongation along the LoS, contrary to less massive galaxy groups.  Therefore, it is natural to observed large lensing (projected) concentrations for such systems, resulting in an overall mass-concentration relation steeper than that of non-lensing haloes.

1409.6273
Impact of atmospheric chromatic effects on weak lensing measurements
Meyers, Burchat

Use analytic and computational techniques to study the impact on shape measurements of two atmospheric chromatic effects for ground-based surveys such as the DES and LSST: (i) atmospheric differential chromatic refraction (DCR) and (ii) wavelength dependence of seeing. Investigate the effects of using the PSF measured with stars to determine the sale of a galaxy that has a different SED than the stars.  For (i), extend a study by Plazas & Bernstein based on analytic calculations that show that DCR leads to significant biases in galaxy shape measurements for future surveys, if not corrected.  For (ii), find that the wavelength dependence of seeing leads to significant biases for galaxy shape measurements - even for current ground-based surveys.  For both effects, investigate correction techniques based on multi-filter photometry.  Using simulated galaxy images, find a form of chromatic "model bias" that arises when fitting a galaxy image with a model that has been convolved with a stellar, instead of galactic, PSF.  Find that PSF-level corrections can reduce biases to levels that meet the requirements for the LSST survey.  Conclude that achieving the ultimate precision for WL from current and future ground-based imaging surveys requires a detailed understanding of the wavelength dependence of the PSF in the atmosphere, and from other sources such as optics and sensors.  The source code for analysis available on github.