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Lightcone data

For a number of HACC simulations we provide data in the lightcone frame, or in other words the data that an observer located at the simulation origin would have in their past lightcone. The data itself is typically similar to snapshot data, with the exception that one step in lightcone data now encompasses a range of time, with scale factors provided per object rather than per step.

Methodology

The methodology for computing lightcone crossing times has changed over the history of HACC simulations. For earlier data (Last Journey), we follow the procedures listed in https://arxiv.org/pdf/2006.01697 (see also https://arxiv.org/pdf/1906.08355 for diagrams and further details). This involves computing particle crossing at the sub-cycle level of the simulation and using them to create high resolution density maps at each step. We then compute halo lightcone data by interpolating halo objects between analysis timesteps using halo merger trees. This can create slight offsets between the density maps and the halo data in the case of significant evolution between analysis steps, but gives a cost-effective way of moving halos to the lightcone.

In the most recent simulations (Frontier-E), we compute particle crossings at the step level and then directly run an FOF halo finder and associated analysis codes on these lightcone particles. When applicable we also run galaxy finding on the lightcone particles directly. This approach allows perfect consistency between lightcone products, at the expense of more expensive in-situ analysis.

Lightcone differences

The main difference in lightcone products is the spatial selections required

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

The output will then contain extra columns, in particular

  • a or object_center_a, the scale factor (for an extended object the scale factor of the center particle)
  • replication or object_replication, the replication tag describing the tiling of the box to create the lightcone
  • theta, phi the co-latitude and longitude of the object
  • chi, the comoving distance to the object
  • rotation, a tag describing the rotation of the replication box, where applicable (not applicable for Frontier-E)

Map data

We do not typically make full-particle data available on the lightcone due to the large volumes at play, instead in addition to per-object data we provide full-sky maps at high resolution. For Frontier-E these are natively in Healpix grids of nside 16384 and nested format, and include for gravity-only simulations:

  • density
  • gravitational potential
  • line of sight velocity and for hydrodynamic simulations we additionally have
  • X-ray flux
  • Compton-y parameter
  • Doppler-b parameter
  • X-ray-weighted temperature When available, these can be accessed using the map query, and matched spatially using the co-latitude and longitude to the objects in the lightcone catalogs.