Longitude and Latitude velocity plots

Longitude-velocity and Latitude-velocity plots

Piyush R.Maharana

Observing velocity profiles of the HI lines at different longitudes, we can derive a diagram in which the intensity distribution is mapped in the position-velocity space, namely in the l-vr plane. This is called a longitude-velocity diagram. The LV diagram is used for studying the rotation characteristics of the galactic disk as well as to derive the spiral structure.[Y.Sofue, Galactic Radio Astronomy].

The HI spectrums that I observed give us the information needed to construct the l-v diagrams.

Data

I'll write more about the "observation" part in a later post. But for now, just take it that we get the velocities and the corresponding latitudes from this data.

Longitude-velocity plots

l-v plot at latitude b=0

l-v plot using data at b=0 latitude

In the longitude-velocity plot above you can see that the gas isn't spread everywhere i.e not randomly scattered across the velocity space but instead it forms these "structures".In the second l-v plot. I have used the complete gas spectrum instead of using the peak velocity points. This way we get a more detailed picture of these gas "structures". You might have guessed by now what these "structures" are. Yes, they are the spiral arms of the Milky Way galaxy.

The arm near the 0 km/s mark is the Local arm, while the -50 km/s is the Perseus and the one nearing the end around -100 km/s is the Outer arm. The upper triangle contains the Saggitarius-Carina and Scutum-Centaurus arm.

Gas with positive velocities represents those inside the solar circle (R < Ro), and negative velocities are those outside the solar circle R > Ro. The solar circle is the circle with the radius equalling the distance of the solar system to the center of the galaxy. (Ro=8.05 Kpc).

 Have a look at Koo et.al (2017) paper. They used different tracers to do the same. Unlike me they weren't limited to longitude 0-220 degrees, they had the whole sky to work on. Keep in mind they use the [-180:180] layout instead of the [0:360] layout for the longitude-velocity plots.[Link]

Latitude-velocity plots

In a similar way, we can obtain a latitude-velocity diagram by scanning in the direction of the latitude (perpendicular to the galactic plane). A  b-v diagram is used for obtaining information about the disk thickness and distribution of the gas in the z-direction. It is often used for studying the warping of the outer galactic disk.[Y.Sofue, Galactic Radio Astronomy].

For constructing the b-v plots  I observed at two different longitudes l=90 and l=120 degrees in the latitude range  -20<b<20 degrees.

l=120 degrees

Local, Perseus and the Outer arm are visible 

l=90 degrees

Local, Perseus and the Outer arm are visible 

The local arm is the bright intense line in these plots while the outermost one is the eponymous Outer arm, and between these two is the Perseus arm.

References

Koo, B., Park, G., Kim, W., Lee, M., Balser, D., & Wenger, T. (2017). Tracing the Spiral Structure of the Outer Milky Way with Dense Atomic Hydrogen Gas. arXiv: Astrophysics of Galaxies. Link

Sofue, Yoshiaki. “Galactic Radio Astronomy.” (2017).