IRMaGiC: Extending Luminous Red Galaxy Selection into the infrared with joint LSST and Roman HLIS Data

TL;DR

IRMaGiC addresses the challenge of selecting Luminous Red Galaxies (LRGs) and estimating their redshifts at $z$ up to 2 by integrating infrared data from the Roman HLIS with optical LSST data. Building on redMaGiC, it calibrates a red-sequence template across $1 \le z \le 2$ using seed red galaxies with spectroscopic redshifts from Roman HLSS, and it employs a color-color seed selection, Roman grism efficiency curves, and a redshift-afterburner to refine photometric redshifts. The paper demonstrates reduced scatter and bias in photo-$z$ for IRMaGiC compared with DC2 photo-$z$, and shows that combining LSST and Roman data extends LRG redshift coverage and improves cosmological leverage for future surveys. These results imply substantial gains for high-z cosmology, enabling more accurate redshift calibration and larger, higher-fidelity LRG catalogs in joint LSST-Roman analyses.

Abstract

We introduce IRMaGiC, an algorithm built based on RedMaGiC desgined to enhance the selection of Luminous Red Galaxies (LRGs) across the redshift range $1 \leq z \leq 2$. We show that this method extends the capabilities of the redMaGiC algorithm by applying it to simulated photometric data from the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) and the Nancy Grace Roman Space Telescope's High Latitude Wide Area Survey (HLWAS). By integrating infrared band coverage from Roman HLWAS with LSST's optical bands, IRMaGiC enables red-sequence calibration at higher redshifts. We demonstrate that IRMaGiC reduces scatter and bias in photometric redshift estimates for LRGs at higher redshift, providing more accurate redshift assessments compared to existing methods. Our findings suggest that incorporating infrared data can considerably improve the selection and redshift estimation of LRGs at higher redshift, offering substantial benefits for future cosmological surveys.

Figures (8)

• Figure 1: Throughput curves for the six LSST filters and four Roman filters. From left to right, the LSST u, g, r, i, z, y filters are represented by black dashed lines, while the Roman F106, F129, F158, F184 filters are shown with black solid lines. The red solid horizontal line marks the wavelength of the $4000$Å break as the galaxy spectra get redshifted. This illustration is intended to demonstrate the overall shape and wavelength coverage of the filters and does not represent the actual throughput values.
• Figure 2: The observed (Y - J) versus (g - Y) colors. Data are shown at different redshift bins, from z = 1 to z = 2. Galaxies are color coded depending on their sSFR and Red-sequence flag based on truth information. The red dots are quiescent galaxies and blue dots are star-forming galaxies. The black and red solid lines show the 68 and 95 percent contours of the number density of star-forming and quiescent, red galaxies respectively. The red and blue contours show the classification boundary from SVM. On the top left-hand side of each panel, we report the completeness (C) and False-positive (FP) fraction of the red, quiescent galaxy selection.
• Figure 3: Color evolution versus true redshift for all galaxies with $m_\mathrm{H} \leq 22\ \mathrm{mag}$ cross-matched from the 20 deg$^2$ region of the Roman simulation and LSST DC2. The blue points represent all galaxies included in this study, while the orange points indicate the seed galaxies selected for calibrating the red-sequence template. The five panels illustrate the redshift dependence of the $i - z$, $z - Y$, $Y - J$, $J - H$, and $H - F$ colors. Here, the $i$ and $z$ band magnitudes are sourced from LSST-DC2 measurements, whereas the $Y$, $J$, $H$, and $F$ magnitudes are derived from the Roman simulation.
• Figure 4: Roman grism efficiency curve for red, quiescent galaxies derived in Guo24.
• Figure 5: Color (i - z, z - Y, Y - J,J - H, H- F) as a function of redshift for the selected seed galaxies. The cyan points indicate the $a(z)$ values at the spline node positions, and the cyan, dashed lines are the spline interpolation. The red, dashed lines indicates the $3\sigma_{\mathrm{int}}$ range. Conversely, the larger number of outliers in the five colors above reflects the fact that the photometric errors are larger than the intrinsic width of the red sequence.