Day 1791

Monday, Tuesday, Wednesday.

2101.03699

Precise photometric measurements from a 1903 photographic plate using a commercial scanner

Cenry, et al

We demonstrate the feasibility of determining magnitudes of stars on archival photographic plates using a commercially available scanner. We describe one photometric approach that could serve as a useful example for other studies. In particular, we measure and calibrate stellar magnitudes from a 1903 photographic plate from the Yerkes Observatory collection, and demonstrate that the overall precision from our methods is better than 0.10 mag. Notably, these measurements are dominated by intrinsic plate noise, rather than noise introduced through the scanning/digitization process. The low expense of this approach expands the scientific potential to study variable stars in the archives of observatory plate collections. We use the serendipitous discovery of a candidate transient at photographic magnitude pg = 16.60 in the spiral galaxy NGC 7331 to illustrate our photometric methods. If this unknown source is a supernova, it would represent the fourth known supernova in NGC 7331.

2101.03723

The PAU survey: narrowband photometric redshifts using Gaussian processes

Soo, et al

We study the performance of the hybrid template-machine-learning photometric redshift (photo-z) algorithm Delight on a subset of the early data release of the Physics of the Accelerating Universe Survey (PAUS). We calibrate the fluxes of the 40 PAUS narrowbands with 6 broadband fluxes (uBVriz) in the COSMOS field using three different methods, including a new method which utilises the correlation between the apparent size and overall flux of the galaxy. We use a rich set of empirically derived galaxy spectral templates as guides to train the Gaussian process, and we show that our results are competitive with other standard photometric redshift algorithms, including the bespoke template-based BCNz2 previously applied to PAUS, lowering the photo-z 68th percentile error to σ68≈0.009(1+z) without any quality cut for galaxies with iauto<22.5. We use the combined Delight and BCNz2 results to identify a small number of tentative catastrophic failures among the secure spectroscopic redshift measurements in zCOSMOS. In the process, we introduce performance metrics derived from the results of both algorithms which improves the photo-z quality such that it achieves σ68<0.0035(1+z) at a magnitude of iauto<22.5 while keeping 50 per cent objects of the galaxy sample.

2101.04675

How not to obtain the redshift distribution from probabilistic redshift estimates: under what conditions is it no inappropriate to estimate the redshift distribution N(z) by stacking photo-z PDFs?

Malz

The scientific impact of current and upcoming photometric galaxy surveys is contingent on our ability to obtain redshift estimates for large numbers of faint galaxies. In the absence of spectroscopically confirmed redshifts, broad-band photometric redshift point estimates (photo-zs) have been superseded by photo-z probability density functions (PDFs) that encapsulate their nontrivial uncertainties. Initial applications of photo-z PDFs in weak gravitational lensing studies of cosmology have obtained the redshift distribution function N(z) by employing computationally straightforward stacking methodologies that violate the laws of probability. In response, mathematically self-consistent models of varying complexity have been proposed in an effort to answer the question, "What is the right way to obtain the redshift distribution function N(z) from a catalog of photo-z PDFs?" This letter aims to motivate adoption of such principled methods by addressing the contrapositive of the more common presentation of such models, answering the question, "Under what conditions do traditional stacking methods successfully recover the true redshift distribution function N(z)?" By placing stacking in a rigorous mathematical environment, we identify two such conditions: those of perfectly informative data and perfectly informative prior information. Stacking has maintained its foothold in the astronomical community for so long because the conditions in question were only weakly violated in the past. These conditions, however, will be strongly violated by future galaxy surveys. We therefore conclude that stacking must be abandoned in favor of mathematically supported methods in order to advance observational cosmology.