M31 “Radio Map” using data from Green Bank 20m Radio Telescope 13/03/25
My first “map” of M31 – results of my first observing programme using the 20m dish at Green Bank Observatory. The spectra show a clear change in Doppler shift with increasing RA, and with increasing declination, reflecting the rotation of the galaxy. Also, the side that the peak is shifted from 1420MHz demonstrates that M31 as a whole is moving towards us.
It is possible that my analysis above and below is incorrect:
From: Stephen Tzikas <Tzikas@alum.rpi.edu>
Sent: Saturday, March 15, 2025 8:57:36 AM
Subject: Re: Mapping M31 using 20m Green Bank RT
Hi Andy,
Your observations are not of the Andromeda Galaxy. You are picking up the Doppler shift of the Milky Way. In fact, with the parameters you have set, you weren’t even detecting any hydrogen from Andromeda.
Even if you did know the best set of parameters, it would still be difficult to do what you proposed. The 20m isn’t that sensitive, though you could detect Andromeda’s core with varying degrees of success or failure.
Welcome to Radio Astronomy. It is nothing like optical astronomy. I encourage you to discover why and why you can’t observe certain targets. It will keep you busy for years.
The radio beam of the 20m will always detect something no matter where you point it. That beam is detecting all radio emitting matter between you and the target, as well as beyond the target. The target itself might not be emitting anything in radio, or it is too weak to detect. You will need to learn what is in the beam of the target when you point and the limitations of the radio telescope itself. These two items introduce many observational and mechanical variables. FYI: the Milky Way’s hydrogen is everywhere in the sky at different intensities.
The GBO 20m site has a search box for all radio observations, not just SARA’s. It is open to the public. It is a rather “dumb” search box, so it takes time to learn how to manipulate it.
https://www.gb.nrao.edu/20m/find20mproj.html
You will find hundreds of observations on M31 or Andromeda from amateurs to professional radio astronomers. Frank Ghigho and Dan Reichart are the best. I often follow them on their observations and can guess what they are trying to do and if they were successful or not.
The GBO site also has some very simple instructions, and how to detect hydrogen in nearby galaxies.
See my latest presentation on Andromeda and Hydrogen observations from the 2024 SARA conference (Hydrogen off the beaten path):
https://radio-astronomy.org/node/200
Also see Jason’s recent presentation,Hydrogen Line Observing in Nearby Galaxies Using Drift Scanning With a Small Dish and Also Using the GBO 20m Dish:
https://radio-astronomy.org/node/198
Also see the GBO instructions/observations at:
https://www.gb.nraoedu/20m/obsadvice.html
https://www.gb.nrao.edu/20m/peak/log.htm#bottom
Happy Exploring. Thank you for you enthusiasm. One of the best ways to learn is reading professional papers. There are a few on the 20m by Dr. Dan Reichart. Search Google for them. You can also look at volunteer opportunities at SARA. That will help you meet others and get a deeper feel on what is happening at SARA. There are the many observation videos on the SARA You Tube channel as well.
Good Luck.
Steve
However, another point of view I have been given is more positive, from another expert:
I disagree with Steve on this. The doppler shift ranges for the Milky Way have an absolute magnitude of less than 100 km/s. Your peak for m31 observation is clearly above that range and right in the proper blueshift range for m31. We should talk more about this. I believe what you are proposing is reasonably doable with the 20 meter dish.
