Quantifying the Impact of LSST $u$-band Survey Strategy on Photometric Redshift Estimation and the Detection of Lyman-break Galaxies

TL;DR

This study evaluates how LSST’s $u$-band depth—shaped by the mirror coatings and survey strategy—affects photometric redshift accuracy and Lyman-break galaxy detection. Using OpSim-based survey realizations and analytic LBG population models, it shows that deeper $u$-band imaging yields meaningful photo-$z$ gains for $1.5\lesssim z\lesssim 2.5$ while incurring modest losses at $z<1$, and that $u$-band depth can dramatically boost $z\sim3$ LBG detections. The authors quantify LBG densities under baseline and modified strategies, finding substantial increases (e.g., $n_u$ rising from $69$ to $2113\ \mathrm{deg^{-2}}$ between year 1 and year 10 for the baseline) and endorsing strategies such as $[1.1\text{x},38\text{s }u]$ with Ag-Ag-Ag coatings. They discuss remaining systematics, notably interloper contamination, and emphasize the role of deep, multiwavelength data and synergy with spectroscopic surveys like DESI-II in enabling robust high-redshift cosmology with LSST LBGs.

Abstract

The Vera C. Rubin Observatory will conduct the Legacy Survey of Space and Time (LSST), promising to discover billions of galaxies out to redshift 7, using six photometric bands ($ugrizy$) spanning the near-ultraviolet to the near-infrared. The exact number of and quality of information about these galaxies will depend on survey depth in these six bands, which in turn depends on the LSST survey strategy: i.e., how often and how long to expose in each band. $u$-band depth is especially important for photometric redshift (photo-$z$) estimation and for detection of high-redshift Lyman-break galaxies (LBGs). In this paper we use a simulated galaxy catalog and an analytic model for the LBG population to study how recent updates and proposed changes to Rubin's $u$-band throughput and LSST survey strategy impact photo-$z$ accuracy and LBG detection. We find that proposed variations in $u$-band strategy have a small impact on photo-$z$ accuracy for $z < 1.5$ galaxies, but the outlier fraction, scatter, and bias for higher redshift galaxies varies by up to 50%, depending on the survey strategy considered. The number of $u$-band dropout LBGs at $z \sim 3$ is also highly sensitive to the $u$-band depth, varying by up to 500%, while the number of $griz$-band dropouts is only modestly affected. Under the new $u$-band strategy recommended by the Rubin Survey Cadence Optimization Committee, we predict $u$-band dropout number densities of $110$ deg$^{-2}$ (3200 deg$^{-2}$) in year 1 (10) of LSST. We discuss the implications of these results for LSST cosmology.

Figures (12)

• Figure 1: Comparison of Rubin Observatory $ugrizy$ throughput curves, assuming original Al-Ag-Al and new Ag-Ag-Ag mirror coatings. The transition to all-silver coatings decreased throughput in the $u$ band, but increased throughput in all of the $grizy$ bands. These curves include contributions from the atmosphere (assuming airmass 1.2), mirror reflectivities, lens and filter throughputs, and detector sensitivity.
• Figure 2: Maps of extinction-corrected coadded $5\sigma$ point-source depth in the LSST $u$ band, assuming survey strategy baseline v4.0. The left panel displays the depth map for LSST year 1, while the right displays year 10. By year 10 the $u$-band depth is much deeper (notice the change in color bar limits) as well as significantly more uniform.
• Figure 3: Transmission of the inoue2014 IGM model in black, plotted for several different source redshifts. The transmission of the LSST photometric bandpasses (with Ag-Ag-Ag coating) are plotted in color to help visualize how much IGM extinction impacts each band at different redshifts.
• Figure 4: IGM magnitude increments in the Rubin $u$ and $g$ bands for $z > 1.5$ galaxies in the simulated catalog. The scatter is due to scatter in UV slope, $\beta_\text{UV}$.
• Figure 5: photo-$z$ metrics for simulations of several different $u$-band survey strategies. Each metric is averaged within 19 overlapping redshift bins of width 0.3, spanning the range $0 \leq z \leq 3$. The redshift value for each bin is the mean redshift of galaxies in that bin. Uncertainties are estimated by bootstrapping 1000 times. For clarity, we plot only a small subset of the $u$-band strategies considered. Each panel displays the corresponding requirement for LSST science as a horizontal gray line lsstSRD. These requirements provide a sense of scale for each metric, but whether or not this photo-$z$ estimator achieves each limit in different redshift ranges is not predictive of the photo-$z$ performance of DESC cosmology, due to the considerations discussed in Section \ref{['sec:cmnn']}.