Lightcone Data

For several HACC simulations, the portal provides data in the lightcone frame: the contents of the past lightcone of an observer located at the simulation origin. Lightcone data resembles snapshot data, with one important difference — a single lightcone step spans a range of cosmic time, so scale factors are recorded per object rather than per step.

Methodology

The method used to compute lightcone crossings has evolved over the history of HACC simulations. For earlier simulations such as Last Journey, we follow the procedure described in The Last Journey. I. An Extreme-Scale Simulation on the Mira Supercomputer, with diagrams and further details in Lightcone Construction for HACC Cosmological Simulations with LANTERN. Particle crossings are computed at the sub-cycle level of the simulation and accumulated into high-resolution density maps at each step, while halo lightcone data is produced by interpolating halos between analysis steps using halo merger trees. Interpolation can introduce slight offsets between the density maps and the halo data when halos evolve significantly between analysis steps, but it provides a cost-effective way to place halos on the lightcone.

For the most recent simulations (Frontier-E), particle crossings are computed at the step level, and the FoF halo finder and its associated analysis codes — including, where applicable, galaxy finding — run directly on the lightcone particles. This approach guarantees consistency among all lightcone products, at the expense of more demanding in-situ analysis.

Differences from Snapshot Queries

Lightcone queries select objects by redshift and position on the sky rather than by simulation step:

• a redshift range, z_low < z < z_high
• angular bounds in co-latitude, theta_low < theta < theta_high, and longitude, phi_low < phi < phi_high

Lightcone outputs also contain additional columns beyond their snapshot counterparts. In the halo lightcone catalogs these include:

• fof_halo_center_a: the scale factor at which the object crosses the lightcone (for extended objects, that of the center particle), with the corresponding redshift also provided
• theta, phi: the co-latitude and longitude of the object
• chi: the comoving distance to the object
• replication: a tag identifying the replication of the simulation box used to tile the volume out to the lightcone depth

Other lightcone products carry analogous columns. Where a rotation is applied to the replicated boxes (not the case for Frontier-E), a rotation tag is included as well.

Map Data

Full particle data on the lightcone would be prohibitively large, so alongside the per-object catalogs we produce high-resolution full-sky maps. For Frontier-E, the maps are stored natively as HEALPix grids with nside 16384 in nested ordering. The native map products of gravity-only simulations include:

• density
• gravitational potential
• line-of-sight velocity

Hydrodynamic simulations additionally include:

• X-ray flux
• Compton-y parameter
• Doppler-b parameter
• X-ray-weighted temperature

A subset of these products is currently exposed through the portal's map query; see the dataset pages for the map types offered for each simulation. Maps can be matched spatially to objects in the lightcone catalogs through their co-latitude and longitude.