Units and Apertures

Snapshot units

The analysis scripts use the GADGET conventions stored in each snapshot:

Quantity	Convention
Position	comoving \(\mathrm{kpc}/h\)
Physical position	coordinate \(\times a/h\)
Mass	\(10^{10}\,M_\odot/h\)
Grain radius	numerical GrainRadius values are in nm in the current implementation; do not convert them a second time
Scale factor	Header/Time
Redshift	Header/Redshift
Hubble parameter	Parameters/HubbleParam attribute (not Header)

Always use periodic coordinate differences before centering or measuring distances.

Halo and galaxy apertures

M200c and R200c are defined relative to 200 times the critical density. They describe the halo and should be measured consistently with the adopted spherical-overdensity routine.

Galaxy-integrated quantities such as stellar mass, gas mass, SFR, dust mass, D/G, and D/Z are aperture dependent. Every table and figure must state the aperture explicitly. Do not silently mix quantities measured within R200c, a fixed physical radius, and an ISM selection. The established single-halo diagnostics commonly use a 20 pkpc ISM aperture, while halo-wide budgets use R200c; the 12-halo paper must record its adopted galaxy aperture alongside every catalog value.

D/G and D/Z must also state whether gas and metals are total, gas phase, or restricted to an ISM phase selection.