Friday.
1706.09507
Photometric redshifts with the LSST: evaluating survey observing strategies
Graham et al
Present and charaterize a nearest-neighbors color-matching photometric redshift estimator that features a direct relationship between the precision and accuracy of the input magnitudes and the output photometric redshifts. This aspect makes the estimator an ideal tool for evaluating the impact of changes to LSST survey parameters that affect the measurement errors of the photometry, which is the main motivation of this work (i.e., it is not intended to provide the "best" photometric z for LSST data). Show how the photometric redshifts will improve with time over the 10-yr LSST survey and confirm that the nominal distribution of visits per filter provides the most accurate photo-z results. Also demonstrate how deep LSST imaging of a spectroscopic galaxy sample can significantly improve photo-z quality, especially in the survey's early years. The LSST survey strategy naturally produces observations over a range of airmass, which offers the opportunity of using an SED- and z-dependent atmospheric affect on the observed photometry as a color-independent redshift indicator. Show that measuring this airmass effect and including it as a prior has the potential to improve the photometric z and can ameliorate extreme outliers, but also find that it will only be adequately measured for the brightest galaxies, which limits its overall impact on LSST photo-z. Ultimately intend for this to work to serve as a guide for the expectations and preparations of the LSST science community with regards to the minimum quality of photo-z as the survey progresses.
1706.09586
Dependency of photo-z accuracy on filter definition of photometric surveys
Cao et al
Accurate measurement of photo-z is crucial for photometric galaxy surveys, hence it is very important to study how the photo-z accuracy depends on the design of filter systems, and find the best filter parameters which can optimize photo-z fitting. Investigate this problem for the Chinese Space Station Optical Survey (CSS-OS) as an example, which covers wide wavelength range from near-UV to near-IR bands. Use COSMOS galaxy catalog to create mock flux data, whose galaxy redshift and magnitude distributions are similar to that of the CSS-OS. Compared different public photo-z SED template-fitting codes, and choose to use a modified LePhare code in the filter calibration process. The code exploits information in flux upper-limits, which has been proven to be effective in the photo-z measurement. Then investigate the effect of each filter passband and filter parameters on photo-z fitting. Find that the g, r, and i bands can significantly affect the photo-z accuracy and fraction of catastrophic redshift in the CSS-OS or any survey with similar magnitude limits, and the current CSS-OS filter design can optimize the photo-z fitting process.
1706.09723
Sample variance in the local measurements of the Hubble constant
Wu, Huterer
The current >3 sigma tension between the H0 measured from local distance indicators and from CMB is one of the most highly debated issues in cosmology, as it possibly indicates new physics or unknown systematics. In this work, explore whether this tension can be alleviated by the sample variance in the local measurements, which use a small fraction of the Hubble volume. Use a large-volume cosmo N-body simulation to model the local measurements and to quantify the variance due to local density fluctuations and sample selection. Explicitly take into account the inhomogeneous spatial distribution of SNe Ia. Despite the faithful modeling of the observations, the results confirm previous findings that sample variance in local Hubble constant measurements is small; find sigma(local H0)=0.31 km/s/Mpc, a nearly negligible fraction of the ~6 km/s/Mpc necessary t explain the difference between the local and the global H0 measurements. While the H0 tension could in principle be explained by the local neighborhood being a underdense region of radius ~150 Mpc, the extreme required under density of such a void (delta~=0.8) makes it very unlikely in a LCDM Universe, and it also violates existing observations constraints. Therefore, sample variance in a LCDM Universe cannot appreciably alleviates the tension in H0 measurements even after taking into account the inhomogeneous selection of SNe Ia.
1706.09853
Cosmological constraints from the redshift dependence of the volume effect using the galaxy 2-point correlation function across the line-of-sight
Li et al
Develop a methodology to use the z dependence of the galaxy 2PCF across the LoS, xi(r_p), as a probe of cosmo parameters. The positions of galaxies in comoving Cartesian space varies under different cosmo parameter choices, inducing a z-dependent scaling in the galaxy distribution. This geometrical distortion can be observed as a z-dependent rescaling in the measured xi(r_p). Test this methodology using a sample of 1.75 billion mock galaxies at z=0,0.5,1,1.5,2, drawn from the Horizon Runs 4 N-body sim. The shape of xi(r_p) can exhibit a significant z evolution when the galaxy sample is analyzed under a cosmology different from the true, simulated one. Other contributions, including the gravitational growth of structure, galaxy bias, and the z-space distortions, do not produce large z evolution in the shape. Show that one can make use of this geometrical distortion to constrain the values of cosmo parameters governing the expansion history of the universe. This method could be applicable to future large scale structure surveys, especially photometric surveys such as DES, LSST, ate derive tight cosmo constraints. This work is a continuation of previous works as a strategy to consorting cosmo parameters using z-invariang physical quantities.
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