Defining Frequency and Velocity

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3.3.1.9 Defining Frequency and Velocity

If the user plans to plot Frequency, the reference frame must be defined. By default, the plotted frequency is simply that observed at the telescope. However, transformations can be made by choosing a Frame from the drop-down menu in the Plots > Trans tab. Frequency reference frames can be chosen to be:

LSRK — local standard of rest (kinematic)
LSRD — local standard of rest (dynamic)
BARY — barycentric
GEO — geocentric
TOPO — topocentric
GALACTO — galactocentric
LGROUP — Local Group
CMB — cosmic microwave background dipole

Velocity is affected by the user’s choice of Frame, but it is also impacted by the choice of velocity definition and spectral line rest frequency. The velocity definition is chosen from the Velocity Defn drop-down menu in the Plots > Trans tab, offering selections of Radio, True, or Optical.

For more information on frequency frames and spectral coordinate systems, see the paper by Greisen et al. (A&A, 446, 747, 2006) ¹.

Finally, the spectral line’s rest frequency in units of MHz should be typed into the blank box next to Rest Freq in the Plots > Trans tab. You can use the me.spectralline tool method to turn transition names into frequencies

CASA <16>: me.spectralline(’HI’)
Out[17]:
{’m0’: {’unit’: ’Hz’, ’value’: 1420405751.786},
’refer’: ’REST’,
’type’: ’frequency’}

For a list of known lines in the CASA measures system, use the toolkit command me.linelist(). For example:

CASA <21>: me.linelist()
Out[21]: ’HI H186A H185A H184A H183A H182A H181A H180A H179A H178A H177A H176A H175A
H174A H173A H172A H171A H170A H169A H168A H167A H166A H165A H164A H163A H162A H161A H160A...
He182A He181A He180A He179A He178A He177A He176A He175A He174A He173A He172A He171A He170A
He169A He168A He167A He166A He165A He164A He163A He162A He161A He160A He159A He158A He157A...
C186A C185A C184A C183A C182A C181A C180A C179A C178A C177A C176A C175A C174A C173A C172A
C171A C170A C169A C168A C167A C166A C165A C164A C163A C162A C161A C160A C159A C158A C157A...
NH3_11 NH3_22 NH3_33 NH3_44 NH3_55 NH3_66 NH3_77 NH3_88 NH3_99 NH3_1010 NH3_1111 NH3_1212
OH1612 OH1665 OH1667 OH1720 OH4660 OH4750 OH4765 OH5523 OH6016 OH6030 OH6035 OH6049 OH13433
OH13434 OH13441 OH13442 OH23817 OH23826 CH3OH6.7 CH3OH44 H2O22 H2CO4.8 CO_1_0 CO_2_1 CO_3_2
CO_4_3 CO_5_4 CO_6_5 CO_7_6 CO_8_7 13CO_1_0 13CO_2_1 13CO_3_2 13CO_4_3 13CO_5_4 13CO_6_5
13CO_7_6 13CO_8_7 13CO_9_8 C18O_1_0 C18O_2_1 C18O_3_2 C18O_4_3 C18O_5_4 C18O_6_5 C18O_7_6
C18O_8_7 C18O_9_8 CS_1_0 CS_2_1 CS_3_2 CS_4_3 CS_5_4 CS_6_5 CS_7_6 CS_8_7 CS_9_8 CS_10_9
CS_11_10 CS_12_11 CS_13_12 CS_14_13 CS_15_14 CS_16_15 CS_17_16 CS_18_17 CS_19_18 CS_12_19
SiO_1_0 SiO_2_1 SiO_3_2 SiO_4_3 SiO_5_4 SiO_6_5 SiO_7_6 SiO_8_7 SiO_9_8 SiO_10_9 SiO_11_10
SiO_12_11 SiO_13_12 SiO_14_13 SiO_15_14 SiO_16_15 SiO_17_16 SiO_18_17 SiO_19_18 SiO_20_19
SiO_21_20 SiO_22_21 SiO_23_22’

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