ALMA Obs Calc

ALMA_obs_calc.py is a small Python utility for estimating line fluxes and observing properties for ALMA targets. It converts galaxy properties such as star formation rate, redshift, line width, and source classification into predicted quantities for CO, [CII] 158 um, and [OIII] 88 um.

The code is based on a script from Gareth Jones and was modified by Jan Scholtz.

What it does

The main ALMA_calc class can:

• Estimate molecular gas mass from star formation rate for main-sequence or starburst galaxies.
• Convert molecular gas mass into CO line luminosity and flux density.
• Estimate [CII] 158 um and [OIII] 88 um flux densities from star formation rate.
• Compute a simple Gaussian line profile and sample it into spectral channels.
• Report representative frequency, channel width, peak amplitude, and spectral window placement.

Requirements

• Python 3
• numpy
• astropy

Install the dependencies with:

pip install numpy astropy

Input data

The calculator expects a galaxy dictionary with fields such as:

• NAME # Name of the object
• SFR # SFR - used for calculation of [CII], [OIII] or CO (for CO only if MH2 is not provided)
• ZRED # Redshift
• MSorSB #
• alpha # Alpha CO
• FWHM # FWHM of the line
• numchan # number of channel over the FWHM
• flipper # flip the spectral windows if you are close to the band edge
• ZP #
• Cosmo # Cosmology - if None, code uses: FlatLambdaCDM(H0=70, Om0=0.3, Tcmb0=2.725)
• PSF_n # Number of beams to cover the object- 1 for unresolved
• mu # magnification - for field just use 1.
• CO_ladder # CO ladder to use: 'SF' or 'AGN'

Optional fields include:

• MH2 # supplying MH2 overrides SFR for CO.

Example usage

from astropy.cosmology import FlatLambdaCDM
from ALMA_obs_calc import ALMA_calc

gal = {
    'NAME': 'Example Galaxy',
    'SFR': 50,
    'ZRED': 2.5,
    'MSorSB': 'MS',
    'alpha': 4.3,
    'FWHM': 300,
    'numchan': 9,
    'flipper': [False, False],
    'ZP': 1.0,
    'Cosmo': FlatLambdaCDM(H0=70, Om0=0.3, Tcmb0=2.725),
    'PSF_n': 1,
    'mu': 1,
    'CO_ladder': 'SF',
}

calc = ALMA_calc(gal)

calc.CO_calc([1, 2, 3])
calc.CII_calc()
calc.OIII_calc()

You can overide the [CII] and [OIII] estimate based on SFR by supplying LCII or LOIII variable in calling of the fuction:

calc.CII_calc(LCII_Lsol = 1e9)
calc.OIII_calc(LOIII_Lsol = 1e9)

Notes

• The script prints results directly to stdout and does not currently write files or expose a command-line interface.
• Several calculations depend on empirical relations from the literature, so the outputs are intended as estimates rather than exact predictions.
• The CO ladder can be switched between star-forming and AGN-like assumptions with the CO_ladder field.

Output

Each calculation prints values such as:

• Line luminosity or integrated flux
• Representative observing frequency
• Gaussian peak amplitude
• Per-channel flux estimates
• Suggested spectral window centers

Reference

The script includes relations drawn from the following sources mentioned in the code comments:

• Sargent et al. 2014
• Narayanan et al. 2012
• Carilli & Walter 2013
• De Looze et al. 2014
• Herrera-Camus et al. 2021