Day 1307

Wednesday.

1709.03992
HST Grism observations of a gravitationally lensed redshift 10 galaxy
Hoag, et al

Present deep spectroscopic observations of a Lyman-break galaxy candidate (MACS1149-JD) at z~10 with HST WFC3/IR prisms.  The grism observations were taken at 4 distinct position angles, totally 34 orbits with the G141 grism, although only 19 of the orbits are relatively uncontaminated along the trace of MACS1149-JD.  Fit a 3 parameter (z, F160W mag, and rest-frame Lyman-alpha equivalent width) Lyman-break galaxy template to the 3 least contaminated grism position angles using a MCMC approach.  he grism data alone are best fit with z = 9.53+0.39-0.60 (68% CL), in good agreement with the photometric estimate of z= 0.51+0.06-0.12.  The analysis rules out Lyman-alpha emission from MACS1149-JD above a 3 sigma rest-frame equivalent width of 21A, consistent with a highly neutral IGM.  Explore a scenario where the red Spitzer IRAC 3.6-4.5 color of the galaxy previously pointed out in the literature is due to strong rest-frame optical emission lines rather than a 4000 A break.  Find that this can provide an explanation for the observed IRAC photometry, but only with a probability of 0.05.  Instead, the grism data add credence to the scenario that the red IRAC color is best explained by a 4000 A break, characteristic of a relatively evolved stellar populations.  In this interpretation, the photometry indicates that a 340+29-35 Myr stellar population is already present in this galaxy only ~500 Myr after the Big Bang.


1709.04205
Photometric redshifts for the Kilo-Degree Survey.  Machine-learning analysis with artificial neural networks
Bilicki, et al

Present a machine-learning photometric redshift analysis of KiDS DR3 using 2 neural network based techniques: ANNz2 and MLPQNA.  Despite limited coverage of spectroscopic training sets, these ML codes provide photo-zs of quality comparable to, if not better than, those from the BPZ code, at least up to a-hot<0.9 and r<23.5.  At the bright end of r<20, where very complete spectroscopic data overlapping with KiDS are available, the performance of the ML photo-zs clearly surpasses that of BPZ, currently the primary photo-z method for KiDS.  Using the GAMA spectroscopic survey as calibration, further study how photo-zs improve for bright sources when photometric parameters additional to magnitudes are included in the photo-z derivation, as well as when VIKING and WISE infrared bands are added.  While the fiducial four-band ugri setup gives a photo-z bias delta z=-2e-4 and scatter sigma_z<0.022 at mean z=0.23, combining magnitudes, colors and galaxy sizes reduces the scatter by ~7% and the bias by an order of magnitude.  Once the ugri and IR magnitudes are joined into 12-band photometry spanning up to 12 mu, the scatter decreases by more than 10% for the fiducial case.  Finally, using the 12 bands together with optical colors and linear sizes gives delta z < 4e-5 and sigma_z<0.019.  This paper also serves as a reference for 2 public photo-z catalogues accompanying KiDS DR3, both obtained using the ANNz2 code.  The first one, of general purpose, includes all the 39 million KiDS sources with 4-band ugri measurements in DR3.  The second dataset, optimized for low-z studies such as galaxy-galaxy lensing, is limited to r<20, and provides photo-zs of much better quality than in the full-depth case thanks to incorporating optical magnitudes, colors, and sizes in the GAMA-calirated photo-z derivation.


1709.04444
Imitating intrinsic alignments: a bias to the CMB lensing-galaxy shape cross-correlation power spectrum induced by the large-scale structure bispectrum
Merkel, Schaefer

Cross-correlating the lensing signals of galaxies and CMB fluctuations is expected to provide valuable cosmological information.  In particular it may help tighten constraints on parameters describing the properties of intrinsically aligned galaxies at high z.  To access the information conveyed by the cross-correlation signal its accurate theoretical description is required.  Compute the bias to CMB lensing-galaxy shape cross-correlation measurements induced by nonlinear structure growth.  Using tree-level perturbation theory for the large-scale structure bispectrum find that the bias is negative on most angular scales, therefore mimicking the signal of IA.  Combining Euclid-like galaxy lensing data with CMB experiment comparable to the Planck satellite mission the bias becomes significant only on smallest scales (ell>~2500).  For improved CMB observations, however, the corrections amount to 10-15% of the CMB lensing-IA signal over a wide multipole range (10 <~ ell <~ 2000).  Accordingly the power spectrum bias, if uncorrected, translates to 2 sigma and 3 sigma errors in the determination of the IA amplitude in case of CMB stage III and stage IV experiments, respectively.