--- title: 'Phantom Run: MOND Galaxy Simulations' url: https://www.emergentmind.com/topics/phantom-run type: topic --- # Phantom Run: MOND Galaxy Simulations Searching arXiv for papers relevant to “Phantom Run,” especially Phantom of RAMSES and its simulation workflow. “Phantom Run” denotes the practical workflow for setting up, executing, and analysing galaxy simulations with **Phantom of RAMSES (PoR)**, a MOND-capable patch to the grid-based **RAMSES** code. PoR is not a standalone code; it is a customized version of RAMSES that replaces RAMSES’s standard gravity treatment with a MOND Poisson solver when desired, while retaining RAMSES’s strengths in **adaptive mesh refinement (AMR)**, **particles**, **hydrodynamics**, and **star formation** [2101.11011]. In this usage, a phantom run can be performed in **Milgromian dynamics (MOND/QUMOND)** or in ordinary **Newtonian gravity**, and is especially suited to disc-galaxy simulations with baryons only, including isolated and interacting systems [2101.11011]. ## 1. Definition and computational scope Within the PoR manual, the operative meaning of a phantom run is a simulation campaign carried out by combining a patched RAMSES executable, MOND-aware disc-galaxy initial conditions, a problem-specific namelist, and post-processing tools [2101.11011]. The code implements **QUMOND**, and in MOND the modification becomes relevant below the acceleration scale \[ a_0 = 1.2\times 10^{-10}\,\mathrm{m\,s^{-2}}, \] with the deep-MOND limit summarized by \[ g \approx \sqrt{g_\mathrm{N} a_0}\qquad \text{for } g_\mathrm{N}\ll a_0. \] In PoR, the MOND potential \(\Phi\) satisfies \[ \nabla^2 \Phi \equiv -\nabla\cdot \mathbf{g} = -\nabla\cdot(\nu\,\mathbf{g}_\mathrm{N}), \] while the Newtonian field is obtained from \[ \nabla\cdot \mathbf{g}_\mathrm{N} = -4\pi G\rho_b, \] using the “simple” interpolating function \[ \nu(y)=\frac{1}{2}+\sqrt{\frac{1}{4}+\frac{1}{y}}. \] For isolated systems, the far-field MOND boundary condition is the logarithmic potential \[ \Phi \sim \sqrt{GMa_0\ln R}, \] rather than a Newtonian \(1/r\) asymptotic form [2101.11011]. This computational framing matters because PoR is designed to evolve stars and gas self-consistently without dark matter haloes. A plausible implication is that the phrase “phantom run” refers less to a single executable mode than to a numerically consistent end-to-end procedure for MOND or Newtonian disc-galaxy evolution inside the RAMSES ecosystem. ## 2. Code architecture and relation to earlier MOND solvers PoR is presented as part of a longer development line of MOND simulation software. The manual notes the first multigrid Milgromian \(N\)-body code by **Brada & Milgrom**, used for disc stability and for warps via the **external field effect (EFE)**; the **Tiret et al.** AQUAL solver extended to gas via sticky particles; **n-mody**, which solved the MOND equation in spherical coordinates and was used for dynamical friction, orbit instabilities, and stellar kinematics; cosmological MOND solvers by **Llinares, Angus, and collaborators**; and **raymond**, which implemented both AQUAL and QUMOND for cosmology and galaxy-cluster problems [2101.11011]. The manual’s stated limitation of many earlier codes is that they are specialized and may not handle generic mixtures of particles, gas, and star formation. PoR’s distinctive role is precisely that it patches RAMSES so those components can be evolved in one framework [2101.11011]. Because RAMSES already provides AMR, particles, hydrodynamics, and star-formation modules, PoR changes the gravity sector while inheriting the rest of the baryonic machinery. This suggests that the operational difficulty of a phantom run lies mainly in keeping the gravity patch, the initial-condition generator, and the runtime namelist mutually consistent. ## 3. Installation and compilation workflow The recommended setup is **RAMSES 2015**, because later RAMSES versions are not compatible with the PoR patch [2101.11011]. RAMSES is compiled from its `bin` directory by editing the makefile, with settings such as: ```text NDIM = 3 NVAR = 6 SOLVER = hydro ``` and a patch chosen according to the target problem. For particle-only MOND or Newtonian simulations, the relevant patch is ```text PATCH = ../patch/phantom_staticparts ``` whereas for hydrodynamical MOND simulations the patch is ```text PATCH = ../patch/hydro/phantom_extfield ``` The manual stresses that **only one patch should be active at a time** [2101.11011]. A Newtonian run still uses the phantom patch but sets `mond = .false.` in the namelist [2101.11011]. Compilation uses MPI-capable Fortran settings such as ```text F90 = mpif90 -frecord-marker=4 -O3 -ffree-line-length-none -g -fbacktrace FFLAGS = -x f95-cpp-input $(DEFINES)$ ``` followed by `make`. If needed, one can clean and recompile with: ```bash make clean && make ``` The manual also cautions that some parameter-file details may differ from the modern code base, so unfamiliar parameters should not be edited blindly [2101.11011]. ## 4. Initial conditions and problem classes PoR relies on a modified version of **DICE** (Disk Initial Conditions Environment) for galaxy initial conditions [2101.11011]. Two versions are distinguished: - **p-dice** in `dice_particle` for particle-only disc models - **h-dice** in `dice_gas` for gas + stars Both are adapted to MOND and compiled with the usual CMake workflow after installing **CMake, GSL, and FFTW** [2101.11011]. The example package contains configurations for MW, M31, and M33, but those are the cases the authors state are “rated to work” [2101.11011]. A Milky Way-like example uses a configuration such as: ```text Galaxy = ../../example/params_files/testMilkyWay.params Filename = dice_highz ICformat = Gadget2 Nthreads = 32 ``` with the `redshift` entry unused in the MONDified DICE [2101.11011]. Running DICE is then ```bash ./dice ../../example/test_mw.config ``` which produces, for particle-only runs, a file such as `Milky_Way_output_p2_k0.txt`; for hydrodynamics it also outputs `Milky_Way_rotation_curve.txt` [2101.11011]. For a **particle-only isolated disc**, the workflow is explicitly: 1. compile RAMSES with `phantom_staticparts`, 2. generate the DICE particle IC file, 3. feed that file into PoR using a namelist such as `PoR-static.nml` [2101.11011]. A representative namelist includes the mandatory blocks: - `RUN_PARAMS` - `AMR_PARAMS` - `OUTPUT_PARAMS` - `INIT_PARAMS` - `POISSON_PARAMS` - `BOUNDARY_PARAMS` with MOND-specific switches ```text poisson = .true. pic = .true. mond = .true. ``` and Poisson controls ```text a0_ms2 = 1.2e-10 m_threshold = 1.e+30 gravity_type = 0 cg_levelmin = 999 ``` The particle-only patch integrates particles below a threshold mass, while particles above `m_threshold` remain static but still contribute to the Newtonian field \(\mathbf{g}_\mathrm{N}\) used by the MOND solver. If all stars are intended to evolve, the manual suggests choosing an extremely large threshold like \(10^{30}M_\odot\) [2101.11011]. For a **hydrodynamical isolated disc** without star formation, PoR uses **h-dice** and a more elaborate setup. The Milky Way parameter file is modified with entries such as ```text v200 = 200.0 m200 = 9.15 Gas_fraction = 0.2 Gas_T = 50000.0 ``` The manual emphasizes that this gas temperature is not literally a thermodynamic temperature but a velocity-dispersion-like parameter controlling stability, and it should match the PoR namelist parameter `T2_ISM` [2101.11011]. It also states that the gas fraction must exceed the outer-disc mass fraction used by DICE, about 17.64% in the default setup, because of how h-dice distributes gas between components [2101.11011]. ## 5. Runtime control, AMR parameters, and numerical caveats The AMR controls given in the manual are central to a successful phantom run: ```text levelmin = 7 levelmax = 12 ngridmax = 2000000 npartmax = 2000000 boxlen = 1024.0 ``` with the note that `ngridmax` and `npartmax` should be of order \(10^6\) to avoid memory errors [2101.11011]. The output schedule may be set through entries such as ```text foutput = 8000 noutput = 100 delta_tout = 100. # Myr tend = 10000. # Myr = 10 Gyr ``` A run can be launched with MPI, for example ```bash mpiexec -n 32 ../ramses3d ../filename.nml ``` or without MPI: ```bash ../bin/ramses3d ../filename.nml ``` The manual warns that **SEGSEV / invalid memory reference** errors usually indicate `npartmax` or `ngridmax` that are too small; suggested fixes are to increase them, sometimes up to `npartmax ~ 10^7` and `ngridmax ~ 8\times 10^6`, although these are stated as upper practical limits [2101.11011]. Restart support is built in: if a run stops at output 45, one sets `nrestart = 45` and resumes [2101.11011]. These details define the most practical meaning of phantom run: it is a RAMSES-based production run under the PoR patch, where resource limits, patch selection, and physics switches directly determine whether the evolution is numerically meaningful. ## 6. Hydrodynamics, star formation, Newtonian mode, and the external field effect For hydrodynamical runs, PoR often uses the merger-template `condinit` from RAMSES’s hydro/merger patch [2101.11011]. Although this template normally sets up two disc galaxies, an isolated-galaxy configuration can be obtained by turning off the second system through parameter choices such as ```text gal_center2 = 2000,0,0 # kpc Vgal2 = 0,0,0 ``` while placing the primary galaxy at the box center and using ```text Vgal1 = 0,0,0 Vgal2 = 0,0,0 ``` for isolated runs [2101.11011]. A representative namelist excerpt includes ```text mond = .true. Activate_g_ext = .false. rad_profile = 'double_exp' z_profile = 'sech_sq' Mgas_disc1 = 45.75 Mgas_disc2 = 0 IG_density_factor = 1.0e-2 T2_ISM = 40.d3 scale_a2 = 1. Vcirc_dat_file1 = 'Milky_Way_rotation_curve.txt' Vcirc_dat_file2 = 'Milky_Way_rotation_curve.txt' ic_part_file_gal1 = 'Milky_Way_output_p2_k0.txt' ic_part_file_gal2 = 'Milky_Way_output_p2_k0.txt' ``` The gas component is not taken directly from DICE’s output; PoR reads the rotation-curve file and uses namelist parameters such as gas mass and temperature to construct the gas distribution internally. The manual therefore stresses that **DICE and PoR settings must be made mutually consistent** [2101.11011]. The **external field effect** is available in the hydro patch, and `Activate_g_ext = .false.` turns it off [2101.11011]. The same numerical machinery can also be operated in **Newtonian mode** by setting `mond = .false.`, which allows direct MOND-versus-Newtonian comparisons within essentially the same framework [2101.11011]. For hydrodynamical runs with star formation, PoR inherits RAMSES’s baryonic treatment and adds the `PHYSICS_PARAMS` block with standard RAMSES controls, for example: ```text cooling = .true. g_star = 1.6666D0 n_star = 0.1D0 eps_star = 0.0D0 t_star = 3.0d0 T2_star = 4.0d4 ``` The crucial switch is `t_star`: if finite and non-zero, star formation is active [2101.11011]. Since PoR only alters the gravitational solver, the hydrodynamic and star-formation modules remain RAMSES’s own. ## 7. Analysis pipeline and scientific applications Post-processing is performed with **extract_por** or **extract_por_sfr**, utilities that read RAMSES outputs and produce particle data products and optional plots [2101.11011]. Installation is a simple ```bash make xpordata ``` The main configuration file is `fmtRAMSES.par`, which includes the path to the output directory, the output number to read, the number of CPU threads, optional center-of-mass reset, radial binning settings, and image or plot settings [2101.11011]. A typical extraction setup is ```text path/to/your/outputfiles 38 # output number 32 # number of threads 0 # COM reset 10. 500 # binning radius, nbins ``` The resulting output files include: | File | Content | Notes | |---|---|---| | `part.asc` | particle data in ASCII | columns 1–3 position, 4–6 velocity, 7 mass, 8 particle ID | | `sfr.dat` | star-formation rate data | column 1 time interval in Myr, column 2 SFR in \(M_\odot/\mathrm{Myr}\) | | `image.dat` | optional image data | may be plotted with gnuplot | The SFR is computed by comparing stellar mass between snapshots, so higher temporal resolution can be obtained by reducing `delta_tout` during the run or by extracting particle birth times [2101.11011]. For visualization, the manual recommends grayscale (`1:gray`) for ordinary cases rather than `rgb` modes, and suggests tuning the binning radius and image field of view for better resolution [2101.11011]. A corresponding plotting command is ```bash plot "image.dat" with image ``` The manual also describes a random-turbulence option based on **square-square subdivision**, conceptually similar to diamond-square terrain generation but intended to give higher-quality 3D randomness with fewer artifacts [2101.11011]. It is controlled by extra `MERGER_PARAMS` entries such as ```text flg_qqm3d = -1 devflat_dens = 1.0 devscal_dens = 0.1 devflat_vel = 1.0 devscal_vel = 0.1 scale_objsize = 1.0 ``` with `flg_qqm3d = -1` meaning off, while the accompanying file `qqm3d.par` specifies recursion details and the power spectrum [2101.11011]. The manual notes that only certain `hr_mode` values should be used for science: `hr_mode = 4` uses the power-spectrum weights, and `hr_mode = 10` uses a flat spectrum [2101.11011]. PoR has been used for polar ring galaxies, Antennae-like encounters, Sagittarius and Palomar 5 tidal streams, satellite-plane formation in the Local Group, spontaneous disc formation in collapsing gas clouds, and M33’s long-term evolution [2101.11011]. In that sense, a phantom run is not a narrow technical mode but a reusable MOND simulation workflow with broad applicability to baryonic galaxy dynamics. A common misconception is that PoR is a separate simulation code or a MOND-only package. The manual states instead that it is a patch to RAMSES, and that the same infrastructure can be run in Newtonian mode by setting `mond = .false.` [2101.11011]. Another misconception is that DICE alone defines the gas disc; in the hydro workflow, PoR constructs the gas distribution internally from the rotation-curve file and namelist parameters, so consistency between the two stages is essential [2101.11011]. Source: https://www.emergentmind.com/topics/phantom-run