How it works
A satellite galaxy's present-day projected phase-space coordinates — its line-of-sight velocity offset from its host cluster and its projected radius — encode statistical information about its orbital history: when it fell into the cluster, how many pericentric passages it has had, and whether it is virialized, infalling, or a backsplash galaxy. The mapping from orbit to present-day phase space is many-to-one and projected, so any one galaxy's history can only be estimated probabilistically.
The orbit-library method addresses this by drawing a large sample of satellite orbits from cosmological N-body simulations, building the probability distribution of orbital parameters conditioned on observable projected phase-space coordinates. The library is then convolved with a simple, parametrised model for environmental processing — star formation quenching, gas stripping — and fit to observed galaxy properties (colours, star-formation rates, HI fluxes, stellar ages) as a function of phase-space position. The result is a set of best-fit timescales (delay times, quenching durations, stripping efficiencies) that would be invisible without folding the projection back through the orbit distribution.
The approach trades the realism of full hydrodynamic simulations for a clean, controllable parametrisation of environmental physics — useful precisely because the bottleneck for satellite quenching is no longer the dynamics but the small-scale gas physics that governs cooling and removal.
Building and validating the library
In Oman, Hudson & Behroozi 2013 we built the original orbit library from merger trees in the MultiDark Run 1 N-body simulation and showed that infalling, backsplash, and virialized populations occupy distinct (though partially overlapping) regions of projected phase space. With all available phase-space information we recover the time since infall to ±2.58 Gyr in 68% of cases — substantially better than a blind estimate (±3.10 Gyr). This calibration paper sets the precision floor for every science analysis built on the library.
Applications
The orbit library has been applied to study how, when, and where environmental processing acts on satellite galaxies — the quenching of star formation, the stripping of cold gas, the spatial signature of ram-pressure stripping in IFU spectroscopy, and the differential effects of cluster vs group environments. See the Quenching of star formation topic page for the science results and the references behind them (Oman & Hudson 2016; Owers, Hudson & Oman et al. 2019; Oman, Bahé, Healy, Hess, Hudson & Verheijen 2021; Reeves, Hudson & Oman 2023).
Inputs
MultiDark / Hydrangea
N-body and hydrodynamical simulations
MultiDark Run 1 supplies the original satellite-orbit catalogues; the Hydrangea cosmological hydrodynamical suite validates the stripping & quenching framework on realistic cluster zooms.
SDSS
Photometry, spectroscopy and cluster catalogues
Sloan Digital Sky Survey provides the bulk of our cluster membership, projected phase-space coordinates, and integrated galaxy properties.
ALFALFA
21-cm HI fluxes
The Arecibo Legacy Fast ALFA survey lets us track when neutral gas is removed, separately from when star formation ceases.
SAMI
Integral-field spectroscopy
The Sydney–AAO Multi-object Integral-field Galaxy Survey spatially resolves where in each galaxy quenching is happening, testing outside-in stripping predictions.