NED (NASA/IPAC) Spectral Energy Distributions - Query

[Norris Adams]

I have attached a sample SED plot for M83 which was downloaded from the above database.

It purports to show emissions across the full EM spectrum (from radio to X ray).

My question is this. Why are there such big gaps in the data? There are 240 data points in total (according to the supporting table - not attached) but nothing between  frequencies 1e+10 to 1e+11.5Hz (i.e. millimetre range) , and again nothing between 1e+16 to 1e+17.5 (soft X rays).

With the data presented as it is, it would be very difficult to draw a best fit curve.

Logically this means that either the galaxy emits no radiation at these wavelengths or no-one has bothered to collect the data. Neither of these conclusions makes much sense! Obviously I am missing something, but what?

Other galaxies exhibit similar discontinuities (e.g. M31, M33).

If anyone can advise I would be delighted to know.

Norris (the engineer)!

M83 - SED Plot.docx

[Astronomist]

The earth's atmosphere is quite opaque to both x rays and most microwave wavelengths, and thus these signals are observed by space telescopes. I assume whoever made the graph just didn't have any space telescope data available.

[robin_astro]

The data in NED is a (very incomplete) collection of some of the observations which have been made public, compiled from a wide range of sources.  Making measurements from radio to gamma rays involves a wide range of differing instruments, some of which need to be space based and some regions of the spectrum are particularly technically challenging. There is  a table of the sources they included. 

The SED will also depend on where in M83 you measure it as just like our milky way, it contains a wide range of different types of object with very different spectral characteristics so most of the time with an object like M83 astronomers would be looking at specific objects within it rather than measuring the total spectral energy density. 

Even if you were to draw a curve through the points it would not tell the whole story as there is much more detail in the spectrum to be found as you go to higher resolution.

Cheers

Robin

[robin_astro]

21 minutes ago, robin_astro said:

The SED will also depend on where in M83 you measure it

If you look at the images in NED it will give  an idea of how inhomogeneous and different it appears at various wavelengths

https://ned.ipac.caltech.edu/byname?objname=M83

Cheers

Robin

[Norris Adams]

18 hours ago, Astronomist said:

The earth's atmosphere is quite opaque to both x rays and most microwave wavelengths, and thus these signals are observed by space telescopes. I assume whoever made the graph just didn't have any space telescope data available.

Thanks Harry - that makes sense. I hadn't twigged regarding the opacity in the atmosphere to certain wavelengths.

 

[Norris Adams]

11 hours ago, robin_astro said:

If you look at the images in NED it will give  an idea of how inhomogeneous and different it appears at various wavelengths

https://ned.ipac.caltech.edu/byname?objname=M83

Cheers

Robin

Thanks Robin - I think I have been looking for an oversimplified classification system.

I guess when something is tens of kiloparsecs wide and emitting in multiple wavelengths a single plot capturing its 'essence' is going to be a tall order! A bit like trying to fully describe a person using only their shoe size (for example).

Some of those images are striking though. It underlines just how incomplete relying on optical morphology is!

Norris

[robin_astro]

2 hours ago, Norris Adams said:

It underlines just how incomplete relying on optical morphology is!

Yes according to this  mass budget diagram from ESA, even discounting dark matter, the stuff that is hot enough to be visible (stars) accounts for only about half the ordinary matter in a typical galaxy and only 7% of the ordinary matter in the Universe.

https://www.esa.int/ESA_Multimedia/Images/2018/06/The_cosmic_budget_of_ordinary_matter

[Norris Adams]

Thanks Robin,

So now I'm trying to guess what the bloke's wife and family looks like based on his shoe size!!!!

Norris