PI: Greene, Jenny, Princeton University
Institutional PI: Wechsler, Risa, Stanford University
Institutional PI: Mao, Yao-Yuan, University of Utah, Salt Lake City
Wide-Field Science – Regular
Accurate distance measurements are fundamental to modern astronomy. It is critical for determining the physical properties of astronomical objects and mapping the local universe’s 3D structure (Valade et al. 2024). These measurements are crucial for testing cosmological models and understanding the nature of dark matter. The tip of the red giant branch (TRGB) method provides accurate distances for galaxies at 3-15 Mpc but requires deep space-based imaging to resolve individual giant stars, making it expensive and hard to apply to a large volume. Alternatively, the surface brightness fluctuation (SBF) method offers a reliable and cheaper alternative for semi-resolved galaxies (Tonry et al. 1988, Carlsten et al. 2022). Limited by atmospheric seeing, ground-based SBF only works within 20 Mpc (Greco et al. 2021). Roman’s Wide Field Instrument (WFI) presents a transformative opportunity: its unique combination of high spatial resolution and near-infrared (NIR) capability will likely extend SBF measurements to 50-100 Mpc. It is timely to develop the NIR SBF technique for Roman and investigate synergy with Rubin LSST to fully exploit Roman’s power from day one.
SBF arises from the Poisson scatter of bright stars per pixel, reflecting both galaxy distance and stellar population. Intriguingly, SBF is more significant in the NIR than in the optical because bright evolved stars enable SBF distances to beyond 50 Mpc in the NIR where TRGB distances are impractical. This is only achievable with Roman, as Euclid’s coarser resolution produces undersampled NIR images. At 20-100 Mpc, SBF distance complements redshifts, which are affected by peculiar velocities and challenging for low-mass quiescent galaxies. Therefore, SBF is the only viable method to measure direct distances to these galaxies, and Roman is the only facility that can deliver such distances efficiently.
We thus propose the following work:
(1) Deliver a new SBF-color calibration in NIR with a well-understood error budget. While optical SBF achieves ~10% distance precision, current NIR calibrations show a much higher scatter. Our preliminary work shows that combining optical colors (from Rubin LSST) with NIR data can reduce scatter to <0.2 dex, potentially exceeding optical SBF precision but applicable to a larger volume. We will validate this using HST, Euclid, and JWST datasets, alongside theoretical models (Choi et al. 2016, Nguyen et al. 2022).
(2) Develop an open-source SBF measurement software optimized for Roman. The package will serve multiple communities, from time-domain astronomers seeking distances to transient hosts, to galaxy evolution researchers studying dwarf galaxies. It will include robust background noise handling and uncertainty analysis, with documentation, tutorial Jupyter notebooks, and example datasets on GitHub and the Roman Science Platform. The software will be thoroughly tested on Euclid and simulated data before Roman’s launch, ensuring immediate applicability to Roman data.
(3) Create image simulation tools for semi-resolved galaxies for Roman WFI. We will use ArtPop (Greco & Danieli 2022) to generate mock galaxies and inject mock galaxies into Roman data of different reduction levels using Romanisim and STIPS. This framework will enable us to determine the SBF distance limit, quantify how systematics affect the SBF distance limit, and develop optimal strategies for Roman observations. These tools will be publicly available through GitHub and the Roman Science Platform.
(4) Investigate how redshift and SBF distance uncertainties affect large-scale structure mapping at 20-100 Mpc using numerical simulations. This will optimize future Roman observing strategies and provide a framework for interpreting the 3D distribution of galaxies where peculiar velocities affect redshift-distance measurements.