# Galactic longitude and radial velocities

1. Mar 23, 2016

### henrco

Hi,

I'm pretty happy with my answer to part i). But could I get guidance for my answers to part ii), iii) and iv). I think they are correct... However I'd like to understand that with the data provided is there a way of actually determining the distance for each component to the GC and therefore having a more accurate answer.

1. The problem statement, all variables and given/known data

From the two spectra below (brightness plotted against observed frequency) determine the
radial velocities for components 1 and 2 (galactic longitude l = 33º) and components 3 and 4
( l = 100º). Which of these four components originate further out from the Galactic Centre
than the Sun? Which of these four components originates closest to the Galactic Centre? Why
does the spectrum along l = 100º have no components at frequencies below 1420.406 MHz?

2. Relevant equations

3. The attempt at a solution

i) radial velocities for components 1 and 2 (galactic longitude l = 33º) and components 3 and 4
( l = 100º).

Using the above formulas, I calculated the following relative velocities.
Component 1 : 86 km/s receding
Component 2 : 12 km/s receding
Component 3 : -3 km/s towards
Component 4 : --58 km/s towards

ii) Which of these four components originate further out from the Galactic Centre
than the Sun?

Based on the galactic longitude system, component 4 which is on Galactic longitude l= 100 degrees is further out from the GC than the sun.

iii) Which of these four components originates closest to the Galactic Centre?

Based on the galactic longitude system, component 1 & 2 which is on Galactic longitude l=33 degrees, originates closer to the GC than the sun.

iv) Why does the spectrum along l = 100º have no components at frequencies below 1420.406 MHz?

This is because it’s longitude line from the Sun runs in between two spiral arms and only at
component 3 does it cut across a spiral arm.

2. Mar 28, 2016