[daip] Velocity - Frequency conversion in AIPS

[Toby Brown]

Hi,

I’m experiencing an issue converting between frequency and velocity with an AIPS produced cube I was wondering if you could help me with. I should caveat this with the fact I have zero experience with AIPS but plenty playing with radio cubes using python and casa etc.

We have a VLA 21 cm cube produced in AIPS with a frequency axis. When converted to velocity in python, CASA, CARTA, and by hand following either the optical or radio conventions, the cube is at a systematically higher velocity than the true system velocity of the galaxy. However, when the conversion from frequency to velocity is done using "ALTSWTCH" in AIPS, the result has the correct velocity.

The VLA cube can be downloaded here <https://www.dropbox.com/s/2yow86gqxoqjroc/NGC4383.cube.fits?dl=0>.

Here I show the VLA 21 cm spectrum (orange) and ALMA CO (grey) spectrum comparison. The ALMA spectrum matches published multi-wavelength velocities, while the VLA 21 cm cube is offset approximated +300 km/s.


Now I’m showing the spectrum from the VLA cube *after* converting to velocity with ALTSWITCH (orange) and the same ALMA CO (grey) spectrum. You can see that the offset goes away.



Do you know why AIPS is able to make the correct conversion between velocity and frequency, while the standard conventions do not work?

Cheers,
Toby


-- 
Research Officer
Herzberg Astronomy & Astrophysics Research Centre
National Research Council of Canada
drtobybrown.com <https://drtobybrown.com/>
@drtobybrown
 <https://twitter.com/DrTobyBrown>Pronouns: He/him/his

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