I am a student studying HI HVC clouds, and I need to compare results from several HI surveys.
I ran into a problem of “How to convert HIPASS barycentric velocities to LSRK velocities”.
HIPASS uses ‘VELO-HEL’ /Barycentric velocity; radio convention, while more recent surveys like HI4PI and CRAFTS use LSRK velocities.
These two systems have a slight difference of about 17 km/s but I do not know how to convert them.
I found Working with Velocities in Astropy Coordinates: Radial Velocity Corrections, but it does not give me good methods.
CARTA can do velocity conventions, but I don’t know how they calculate it
This table is what I found on CARTA:
# V_bary ~ V_lsrk | Delta_V (m/s)
# 600101.679 ~ 583017.524 | 17084.155
# 309892.611 ~ 292791.884 | 17100.727
# 6492.221 ~ -10625.830 | 17118.051
# -6699.100 ~ -23817.905 | 17118.905
# -283716.847 ~ -300851.470 | 17134.623
# -600308.558 ~ -617461.258 | 17152.700
I found that Delta_V is not a constant, so I cannot simply subtract 17 km/s from V_bary.
It does depend on RA-Dec, when I checked for several different images.
Any help is welcome! If you could provide a simple Python code snippet, I would be very grateful.