Black Hole TON618 - Wavelength Spectrum

[nanjo]

Several websites quote the Lyman-alpha wavelength as 121.567nm for black hole TON618. Is this value the observed wavelength or the source wavelength? As both values are required to determine z, can anyone tell me what the value of the missing wavelength is?

 

[Orodruin]

The Lyman-alpha emitted wavelength is 121.567 nm.

You can easily compute the observed wavelength from the redshift value z = 2.219.

 

[nanjo]

Thanks Orodruin for your prompt reply and confirming that the 121.567nm is the emitted wavelength at TON618. The value z = 2.219 is stated as a fact in the data provided for TON618. It is calculated from knowledge of the source and emitted wavelengths. What I need to know is the spectrum value initially observed, from which the value of z was originally derived.

 

[Ibix]

Observed wavelength is $(1+z)$ times the emitted wavelength, so you can calculate the observed wavelength from the two numbers you have.

 

[nanjo]

Thank you Ibix. But this is not a chicken and egg controversy. The wavelengths came first and z was calculated later. An astronomer would record an observed wavelength and with knowledge of a baseline spectrum would locate source wavelength then calculate z. Where can I find information that includes the ACTUAL observed value of wavelength? Please excuse the capitalised emphasis. This information is missing from all black hole web sites that I have come across.

 

[Ibix]

Sure somebody observed a wavelength and divided it by the emitted wavelength to get the redshift. Then they can either publish the observed wavelength (and leave everybody to recalculate the redshift if they need it) or the redshift (and leave everybody to calculate the observed wavelength) or both. Given that you don't seem to be able to find the observed wavelength and that the redshift is by far the more interesting number, they may well have gone with the second option. You can get the observed wavelength by the trivial calculation in my last post - what value would publishing the observed wavelength add? A check that they did a division correctly?

If you really want to find raw data you'd have to track down the original publications around the discovery, and quite probably have to end up delving into the relevant datasets to see what's there. That seems like a lot of work to avoid doing one multiplication.

Fundamentally, I don't understand why you want to see a number written down when you can easily reverse engineer it from available data. Perhaps if you explain that we might be better able to help.

 

[phyzguy]

While I agree with @Ibix , if you really want the spectra, you can go to NED, enter TON618 and hit Go. Then if you click "Spectra", you can see the spectra, with references to where they came from. This may leave you with more questions than answers,

 

[nanjo]

Hi Ibix
Thanks again for the prompt response to which I respond on some points:

<Given that you don't seem to be able to find the observed wavelength (and the redshift is by far the more interesting number) …..>

I disagree - the observed wavelength shows the redshift and is the more interesting presentation, while z is just a ratio.

< You can get the observed wavelength by the trivial calculation in my last post - what value would publishing the observed wavelength add? A check that they did a division correctly?>

That they did the division correctly (and more) - Yes. Many years ago, I was told that education at university level and beyond was supposed to teach one to think and to question. However, experience has taught me otherwise. During further education one was expected to listen, learn, take it all in and pass one’s exams. If one dared to question a lecturer the response was usually “That’s the way it is – just accept what you are being told, and you will understand when you know more about the subject. Then you will understand”. Well, that time has long passed, and my understanding is that there is much controversy that needs exploring further.

< If you really want to find raw data you'd have to track down the original publications around the discovery, and quite probably have to end up delving into the relevant datasets to see what's there.>

Yes, that’s exactly what I want. In fact this forum’s member “phyzguy” has already provided some useful information which I shall follow up in due course.

 

[nanjo]

Hi phyzguy,
Thanks for giving me that lead to some very useful information. I shall follow that up with the hope that I may ask you for a pointer or two at some later date.

 

[nanjo]

Many thanks for you help phyzguy. I've seen the spectrum data and have confirmed the value of z for myself thus satisfying my curiosity.

 

[Ibix]

I disagree - the observed wavelength shows the redshift and is the more interesting presentation, while z is just a ratio.

The point about a redshift, though, is that it relates the emitted and observed wavelengths for all wavelengths emitted by that source. It isn't specific to one line. And it's related to the distance to the object via the cosmological scale factor. And it relates the apparent rate of processes in the gas cloud to the rate you would see if you were there. Similarly observed temperatures. On the other hand, the observed wavelength is nothing more than the observed wavelength. If I observe some other line in another object, I can't meaningfully compare my observed wavelength to TON618's H-alpha line, but the redshifts are directly comparable.

Dimensionless quantities are often (not always, but often) far more useful that unitful ones.

 

[nanjo]

Raw data can be extremely useful, especially when not wanting to be tied to the very controversial Doppler effect and its popular inference.

 

[phyzguy]

The interpretation of redshift for distant objects is not controversial. Virtually all physicists and astronomers accept it. Of course you can find people who disagree, just like you can find flat-earthers. Would you say that the statement, "the Earth is a sphere" is controversial?

 

[Orodruin]

Raw data can be extremely useful, especially when not wanting to be tied to the very controversial Doppler effect and its popular inference.

Even if we take the Doppler effect being controversial seriously (it really isn't controversial at all, it is an experimentally well-established effect), this does not matter for what you are trying to find out at all. The astronomer(s) that made the inference of $z$ used the definition of $1+z = \lambda/\lambda_0$ with $\lambda_0$ being the wavelength of the Lyman-alpha emission of hydrogen at rest. It is therefore obvious what the value of $\lambda$ was given this and your objection here holds no water whatsoever.

 

[berkeman]

tied to the very controversial Doppler effect and its popular inference.

Thread is closed for Moderation...

 

[berkeman]

After a Mentor review, the thread is reopened provisionally. The Doppler effect is not controversial in this context.

 

[nanjo]

The interpretation of redshift for distant objects is not controversial. Virtually all physicists and astronomers accept it. Of course you can find people who disagree, just like you can find flat-earthers. Would you say that the statement, "the Earth is a sphere" is controversial?

I disagree. I would say that many (not virtually all) physicists and astronomers accept it. Those that are not among the "many" are unlikely to have gained their promotions nor kept their employment as such. They would have become unknowns or even dissidents.

As regards flat-earthers (pre 6th century BC), the phrase "the Earth is a sphere" must have been controversial at some time, but in this case it turned out to be proven as correct. The interpretation of cosmological redshift still remains controversial. The fact that many (or virtually all) physicists and astronomers accept it does not make it correct.

 

[Bandersnatch]

Much like with the shape of the Earth, the cosmological interpretation of the redshift is accepted for a reason, not because it's en vogue. Unlike with Eratosthenes' experiments, the substantiation is a bit more involved. Luckily, it is accessible from cosmology textbooks, which one is encouraged to study. Probably a more purposeful use of one's time than checking for arithmetic errors in random findings.

 

[weirdoguy]

Those that are not among the "many" are unlikely to have gained their promotions nor kept their employment as such. They would have become unknowns or even dissidents.

Prove it. Or stop making up some nonsensical conspiracy theories.

If one dared to question a lecturer the response was usually “That’s the way it is – just accept what you are being told, and you will understand when you know more about the subject. Then you will understand”.

Well that is the problem of your university. I am a theoretical physicist and I have never encountered such behaviour. To the contrary, there were a lot of discussions, and lecturers were there to explain everything. My alma mater is Warsaw University

 

[Orodruin]

I disagree - the observed wavelength shows the redshift and is the more interesting presentation, while z is just a ratio.

You are missing the entire point. If you know the emitted wavelength and the redshift then you know the observed frequency simply by the fact that redshift z by definition is a particular function of emitted and observed wavelength. You can question whether or not that change in wavelength is due to the expansion of the universe or not, but questioning the relationship itself is equivalent to claim that astronomers (many of them) are unable to use a calculator correctly to compute a very simple function of two numbers.

 

[nanjo]

Hi Bandersnatch,
<.... not because it's en vogue ....>
There are those that believe it's en vogue with the general public.

<Probably a more purposeful use of one's time than checking for arithmetic errors in random findings.>
Of course - but why jump directly into the Doppler route. Starting with raw data allows one to proceed in another direction. Using z puts a limit on the next step.

 

[nanjo]

Hi Weirdoguy,
<Prove it. Or stop making up some nonsensical conspiracy theories.>
There are many examples on the internet. Research it for yourself. I only asked for one piece of basic but not readily accessible information which was kindly provided by the very helpful Physguy.

 

[nanjo]

<It is therefore obvious what the value of λ was given this and your objection here holds no water whatsoever.>
Granted but, as in any discipline it is reassuring to see the raw data including any sidebands and/or shape of the 'discrete'.

 

[nanjo]

Hi berkeman,
<The Doppler effect is not controversial in this context.>
But it is disputed by many. For just one example see 'The Static Universe 'by Hilton Ratcliffe.

 

[Orodruin]

<It is therefore obvious what the value of λ was given this and your objection here holds no water whatsoever.>
Granted but, as in any discipline it is reassuring to see the raw data including any sidebands and/or shape of the 'discrete'.

The raw data is not a wavelength. The raw data is going to be some counts in a detector that needs to be appropriately calibrated and set up. Getting to an actual wavelength is going to require some amount of mathematically transforming that data. Do you want to check that too? Converting into a redshift is just a single extra mathematical transformation in order to present it as a parameter that is more useful in the current theory. (Not thereby saying that the data would necessarily conform with said theory.) If I give you the number 2 and tell you that a second number is 4 times larger, then that is exactly the same thing as telling you that the second number is 8. This is no different.

 

[nanjo]

Hi Orodruin,
<You are missing the entire point. If you know the emitted wavelength .....>
Sorry - You are correct, I did miss this implication. All I actually wanted was the raw spectrum data.

 

[nanjo]

Hi Orodruin,
<The raw data is not a wavelength. The raw data is going to be some counts in a detector that needs to be appropriately calibrated and set up. >
The information provided by phyzguy was a wavelength spectrum and is exactly what I was looking for. As I am more familiar with spectrum analysers, then perhaps raw data was the wrong term for me to use if spectrum analysis is not a first stage in data collection. I don't question how spectrum analysers work, so I don't need to question how data collection works. Thanks for the data collection information.

 

[Orodruin]

I don't question how spectrum analysers work, so I don't need to question how data collection works.

But you question how basic division and addition works?
(Because that is all the difference between being told the observed wavelength and the redshift.)

 

[nanjo]

No!

 

[Orodruin]

Then the answer to your original question is:

"The Lyman-alpha emitted wavelength is 121.567 nm.

You can easily compute the observed wavelength from the redshift value z = 2.219."

If you want the actual number, then $1 + z = \lambda/\lambda_0$ implies that
$$
\lambda = \lambda_0(1+z) = (121.567\ {\rm nm}) (1+2.219) \simeq 391.3\ {\rm nm}
$$

This is irrespective of whether you believe that the observed wavelength is 391.3 nm because the Universe expanded by a factor of 3.319 or not.