Simulating the Radio Sky

One of the main purposes of cora is for generating simulations of the radio sky with a particular focus on cosmological 21cm observations (frequencies in the range 100 MHz up to 1.4 GHz). It focuses on three different sources of radio emission:

  • 21cm radiation from neutral hydrogen at cosmological distances. Currently the support for reionisation is poor.

  • Synchrotron from our galaxy, including the effect of emission from a broad range of Faraday depths within our galaxy.

  • Radio emission from point sources. This incluces multiple components:

    • Real point sources derived from NVSS and VLSS.

    • A synthetic population of dimmer point sources.

    • A gaussian random field to model an unresolved background of sources.

These models are described in more detail in arXiv:1302.0327 and arXiv:1401.2095

Command Line Tools

The primary way of generating simulations is to use the command line tool cora-makesky, which is able to generate maps of each of these components. If you installed cora with easy_install, pip or used the setup.py this command should be in your path.

A typical call takes the form of:

$ cora-makesky COMPONENT NSIDE FREQ_LOWER FREQ_UPPER NFREQ outputmap.hdf5

where the required options are:

  • COMPONENT is the type of sky to simulate. Must be one of:

    • galaxy - synchrotron from our galaxy.

    • pointsource - emission from radio point sources.

    • foreground - both of the above.

    • 21cm - cosmological 21cm emission.

  • NSIDE is the Healpix resolution. Must be a power of two. 256 is a good value to start with.

  • FREQ_LOWER the lowest frequency in MHz.

  • FREQ_UPPER the highest frequency in MHz.

  • NFREQ gives the number of evenly spaced frequency channels with in the band.

  • outmap.hdf5 replace with your desired output filename.

Map Format

Maps are stored as hdf5 files which can be easily read in Python using healpy. Currently there is minimal structure to the written files. There is a single dataset called map which contains Healpix maps of the whole sky for each frequency and polarisation. Within this the files are packed by frequency, polarisation and Healpix pixel.