Software: CRIME

The code

CRIME (Cosmological Realizations for Intensity Mapping Experiments) is a set of computational tools designed to generate fast simulations of the main components of intensity mapping observations:

• The cosmological signal is generated by a C-code called GetHI. GetHI uses the lognormal transformation (Coles & Jones, 1991) to generate a realization of the cosmological matter density and peculiar velocity fields. These are then transformed into 21cm brightness temperature fluctuations and interpolated into temperature maps in different frequencies.
• Galactic and extra-galactic foregrounds are one of the main challenges facing intensity mapping observations. ForGet, another program included in CRIME, can generate simulated temperature maps for different foregrounds, including unpolarized and polarized galactic synchrotron, galactic and extra-galactic free-free emission and extra-galactic point sources. ForGet can also be used to simulate a custom foreground by varying the input parameters.
• We have also included a small C program called JoinT that adds the different components, including polarization leakage, applies a Gaussian beam and adds uncorrelated noise to generate the observed maps. This tool uses overly simplistic models for each of these steps and is only included for convenience. This post-processing is instrument-dependent, and therefore we strongly discourage the use of JoinT for any realistic analysis.
• NEWS! CRIME has been fully rewritten in C.

The models and approximations used by CRIME are best described in its accompanying paper. For a description of its functionality, input and output format, compilation, etc. we refer the user to CRIME's README files.

Features in the current version

• Cosmological signal includes light-cone evolution, redshift-space distortions and time-dependent linear bias.
• Unpolarized galactic synchrotron emission is implemented from the Haslam template, using a spatially-varying spectral index and including slight frequency decorrelations
• The model for polarized synchrotron is able to reproduce the most relevant features of this emission: depolarization towards the galactic centre, emission from a number of independent Faraday screens and large frequency decorrelation due to Faraday rotation.
• Point sources and free-free emission are modelled as Gaussian random fields including frequency decorrelation.

Limitations

We have tried to reach a compromise between computational speed and precision. Thus, while CRIME is able to simulate many of the most important characteristics of 21cm intensity mapping observations, it is not all-powerful. We advise the user to take the following caveats into account:

• CRIME uses the lognormal transformation to generate the cosmological signal. While this is an extremely fast method, it cannot be expected to yield satisfactory results in the non-linear regime (conservatively it shouldn't be trusted below scales of ~10 Mpc/h), or for very large values of the linear bias (b(z) D(z) ~ 2).
• CRIME relies on two external functions in order to generate the cosmological signal, namely the evolution of the neutral hydrogen fraction and the linear bias of HI. The simulations generated by CRIME can be trusted inasmuch as one trusts the models for these two quantities.
• The models used to generate the foreground maps were thought to yield amplitudes and frequency dependences that were similar to the real ones, so that they can be used, for instance, to asess de validity of different foreground-subtraction techniques. This does not mean that the models themselves describe the underlying astrophysical sources realistically in any other respect.
• The previous point must be particularly stressed in the case of the polarized synchrotron emission: the lack of observational data in different frequencies makes it impossible to develop a model purely based on observations. Thus, the model used by CRIME in its current version has been compared with the results from Hammurabi, which is itself based on a number of analytical models. CRIME's polarization can then be trusted as much as these models.

Wishlist for the future

As almost any scientific code, CRIME is in a perpetual phase of developement. Here are the changes we intend to implement in future versions in order of likelihood/urgency:

• NEWS!!!: GetHI is now MPI-parallelized.
• Use a more realistic modelling for the spatial distribution of galactic free-free emission.
• Create a flat-sky version and a cut-sky version, with the aim of accelerating the simulations as much as possible.
• Integrate a fully-blown 3D line-of-sight integrator for the polarized and unpolarized synchrotron foregrounds (similar to Hammurabi).
• 2LPT code to generate the dark-matter field, as was done in Manera et al. 2012.

Download

Dowload version 1.3
Version 1.3 may (and probably will) contain errors and bugs. It may also undergo substantial changes in future versions. If you experience any trouble running or compiling it, please do not hesitate to contact the author (details below).

CRIME is publicly available under the GPL lincense (see COPYING in the root directory). We kindly ask you to cite the program's website and companion paper when using it for published scientific results.

Simulated data

Here you can find one of the "Fast" simulations used in Alonso et al. 2014. Please, refer to CRIME's README files for a description of the file formats and units.

Contact

Regarding bugs, suggestions, questions or petitions, feel free to contact the author: david.alonso@astro.ox.ac.uk