15 replies to this topic

[akmal89]

Hi,
Would like to have info from the experts here. May I know, what the horizontal light line represent in shown spectroscopy spectrum?
Is it light pollution? If so, why there is no light pollution emission line represented in spectrum profile (expect some spike rather than profile onset).
Or the green circle represent Na1 emission line from HPS light?

 

 

Attached Thumbnails

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[twoc]

That spectrum will have had the sky background / light pollution subtracted. I reckon the horizontal line is continuum mostly from all the bright OB stars near the core of NGC 604.

Re the green circle, it looks about where the HeI 5876 line is to me. I eyeball it at around 10x less intense than Hb, which after a quick search is roughly in line with measured HeI/Hb ratios in NGC 604.

https://articles.ads...MNRAS.226...19D

https://iopscience.i...1086/344104/pdf

Edited by twoc, 18 January 2025 - 12:26 PM.

[robin_astro]

An earlier spectrum, also by Christian Buil can be found in his library of nebula spectra here

https://buil.astrosu...ula/nebula.html

and yes the sky background will have been subtracted as normal and the continuum comes from the hot stars exciting this HII region

 

The digitised spectrum file can be downloaded from there and plotted with the lines identified using Plotspectra for example. Not sure what the line at 3722A is though as that is outside the range I normally cover

 

 

Cheers

Robin

Edited by robin_astro, 18 January 2025 - 03:52 PM.

[robin_astro]

 Not sure what the line at 3722A is though 

Probably the wavelength calibration is probably a bit off at that end and it is  [OIII] at 3726/3729. Here is a line list of commonly seen galaxy emission lines

http://astronomy.nms...ssionlines.html

 

Cheers

Robin

[robin_astro]

Probably the wavelength calibration is probably a bit off at that end and it is  [OIII] at 3726/3729. 

Actually looking more closely, all the lines in Christian Buil's spectrum are slightly blue shifted by a few Angstrom.  This would be expected for an object in M33 which is moving towards us at   ~180km/s  giving  a doppler shift to the blue of 3 Angstrom at 5000A 

 

Cheers

Robin

[JonL]

The horizontal line is the nebular continuum. A emission nebula produce its own continuum.

Here the text form Kaler book “stars and their spectra”:

​10.6 The nebular continuum

Note also in Figure 10.10 the presence of a faint continuum, which can be better seen in the spectra of planetary nebulae (Figure 11.4), which we will examine in the next chapter. Some of it in the Orion Nebula's spectrum may be due to faint background stars, but a component is produced by the atomic processes of the nebula itself. Part of the nebular continuum is caused simply by the free-bound (recombination) and free-free processes that we introduced in Section 2.7. The free-free continuum is easily observed in the radio spectrum. Most of it, however, is produced by the two-quantum mechanism. Usually, an electron in hydrogen's level 2 will simply jump downward to level 1 with the production of the Lyman a line at 1216Á. However it is also possible that two photons can be emitted simultaneously. The sum of their energies must equal that of Lyman a, but otherwise there are no restrictions; they can be of equal energy, or one may be high, the other low. The result is a continuum that pervades the spectrum with a limit at 1216A. Since a Lyman a photon, once created, will suffer numerous successive re-absorp-tions and re-emissions as it tries to work its way out of the nebula, the odds of it breaking down into two photons become reasonably good, and a strong continuum is produced A similar process in ionized helium will produce additional continuous radiation in the spectra of planetary nebulae the exciting stars of H II regions are too cool to produce He-*) down to He Il Lyman a at 304 A.

 

Edited by JonL, 18 January 2025 - 11:14 PM.

[akmal89]

I found a paper about NGC 1514 spectroscopy. The author stated that recorded spectroscopy shows the light pollution line was presented in the spectrum profile - please refer to figure 10 of such paper.

https://www.research...C_1514_-_NGC_40

 

From author spectrum image, I tried to come out with profile using RSpec. By keeping the light pollution and UNchecked the substract background feature, here what I found.

(Calibrated Oiii at 5000nm and Halpha at 6560nm).

 

1. Claimed light pollution emission line at 5575nm didnt match to Hg and Na element emission line. Then what it is?

2. When substract background unchecked, Hg and Na spike can be seen. Is that the light pollution should be?

 

Please correct me.

 

Thanks.

 

 

 

 

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[robin_astro]

The horizontal line is the nebular continuum. A emission nebula produce its own continuum.

 

There may be some contribution  nebular continuum but I suspect in this case may be from the central stars, unresolved at this distance as it is confined to the central region while the emission extends some significant distance either side. A flux calibrated spectrum of the central region might solve the issue as if it is due to the hot stars it would show the continuum of a hot star.

 

Cheers

Robin

[robin_astro]

I found a paper about NGC 1514 spectroscopy. The author stated that recorded spectroscopy shows the light pollution line was presented in the spectrum profile -

Note this a student paper so should not be relied on for accuracy.

 

Pollution from the sky background is easily distinguishable from the source being measured as the sky lines will extend at approximately the same intensity over the full length of the slit. If done accurately, the sky background subtraction (a fundamental step in processing spectra) removes these. If you specifically want to produce the spectrum of the sky background from this spectrum, set a wide binning zone above or below the target, excluding the target, with sky background subtraction unchecked.

 

Sky spectra in light polluted areas can be very complex and there are also natural sky lines from air glow. The line at 5577 A is one of these (from oxygen in our own atmosphere)

 

Cheers

Robin

[robin_astro]

 

Sky spectra in light polluted areas can be very complex and there are also natural sky lines from air glow. The line at 5577 A is one of these (from oxygen in our own atmosphere)

 

 

You can see it here in my spectrum before sky background subtraction for example

https://britastro.or...5ef1da74745623d

 

Cheers

Robin

[robin_astro]

With planetary nebulae 

 

 I suspect in this case may be from the central stars, unresolved at this distance as it is confined to the central region while the emission extends some significant distance either side. A flux calibrated spectrum of the central region might solve the issue as if it is due to the hot stars it would show the continuum of a hot star.

 

 

With planetary nebulae (A different excitation source compared with this HII region), the spectrum of  the central star can sometimes be isolated from the nebula spectrum as in ngc 7026 here for example

https://britastro.or...fb7b9c36516d29a

 

Cheers

Robin

[robin_astro]

 A flux calibrated spectrum of the central region might solve the issue as if it is due to the hot stars it would show the continuum of a hot star.

 

 

I had a closer look at the continuum in Buil's spectrum of NGC604 I posted above.

 

 

Although weak and noisy, it is effectively flat and certainly not the thermal continuum of a hot star so it does appear that the continuum is indeed produced by non thermal processes within the nebula. I wonder why is appears confined to the inner part of the nebula ? (Distance from the ionising source, temperature, density?)

 

Cheers

Robin

[Organic Astrochemist]

The horizontal line is the nebular continuum. A emission nebula produce its own continuum.

Here the text form Kaler book “stars and their spectra”:

​10.6 The nebular continuum

Note also in Figure 10.10 the presence of a faint continuum, which can be better seen in the spectra of planetary nebulae (Figure 11.4), which we will examine in the next chapter. Some of it in the Orion Nebula's spectrum may be due to faint background stars, but a component is produced by the atomic processes of the nebula itself. Part of the nebular continuum is caused simply by the free-bound (recombination) and free-free processes that we introduced in Section 2.7. The free-free continuum is easily observed in the radio spectrum. Most of it, however, is produced by the two-quantum mechanism. Usually, an electron in hydrogen's level 2 will simply jump downward to level 1 with the production of the Lyman a line at 1216Á. However it is also possible that two photons can be emitted simultaneously. The sum of their energies must equal that of Lyman a, but otherwise there are no restrictions; they can be of equal energy, or one may be high, the other low. The result is a continuum that pervades the spectrum with a limit at 1216A. Since a Lyman a photon, once created, will suffer numerous successive re-absorp-tions and re-emissions as it tries to work its way out of the nebula, the odds of it breaking down into two photons become reasonably good, and a strong continuum is produced A similar process in ionized helium will produce additional continuous radiation in the spectra of planetary nebulae the exciting stars of H II regions are too cool to produce He-*) down to He Il Lyman a at 304 A.

I think the statement that "Usually, an electron in hydrogen's level 2 will simply jump downward to level 1 with the production of the Lyman a line at 1216Á" is a little misleading. At first approximation, the energy levels of a hydrogen electron are like steps and electrons can jump from level 2 to level 1. But if the interaction between the electron's spin and its orbital motion around the nucleus is considered (spin-orbit coupling), then the transition from 2s to 1s (l= 0 for both) is strongly forbidden (not impossilble) and the 2s state has a relative long lifetime -- long enough to allow the inefficient two-photon decay. Tennyson suggests that this mechanism is responsible for roughly half the continuum radiation of a nebula ( Astronomical Spectroscopy, p. 55).

I do like the explanation that if a Lyman photon is produced, it will likely be re-absorbed - whereas the many continuum photons will evade absorption (their wavelengths don't match any transitions).

I suspect that the higher the column density of hydrogen, the greater the continuum emission, all things being equal, although proximity to some ionizing source is required to excite the hydrogen. We can only see Balmer emission if those photons are not subsequently reabsorbed by (excited) hydrogen. Whereas most continuum emission is unlikely to ever be re-absorbed by hydrogen.

[Organic Astrochemist]

I had a closer look at the continuum in Buil's spectrum of NGC604 I posted above.

 

ngc604_20130810_098_cbuil_continuum.png

 

Although weak and noisy, it is effectively flat and certainly not the thermal continuum of a hot star so it does appear that the continuum is indeed produced by non thermal processes within the nebula. I wonder why is appears confined to the inner part of the nebula ? (Distance from the ionising source, temperature, density?)

 

Cheers

Robin

That's a very interesting spectrum. I assume it has been corrected for instrumental response, so the fact that the continuum looks even more weak and noisy in the extreme blue and extreme red truly reflects some underlying wavelength dependence.

 

I have a hunch that I would love if anyone could comment on:

 

A) more absorption in the blue might be due to the very general trend of a transition probability being inversely related to the wavelength. Shorter wavelength transitions tend to have stronger absorptions.

 

B) more absorption in the red might be due to transitions from very high quantum states. The classic example of this are radio recombination lines, which have very high transition probabilities but I suspect that some of the reasons for these high probabilities (see chatGPT for good explanation) might generally apply even when the transitions begin at high quantum states. These high quantum states are not normal in stars but could be found in ionizing environments like or near HII regions. I tried to ask ChatGPT to calculate the Einstein A coefficients for the n=200 to n=3 transition. I got a range of values, but when I asked to carefully consider the large dipole moments of the higher quantum states, I generally got relatively higher Einstein A values compared to transitions from lower quantum states.

 

C) based on A, when I asked ChatGPT to calculate and compare the Einstein A coefficients for n=200 to n=3 vs n=200 to n=2 the latter had higher values. So perhaps the higher quantum states explains greater absorption in the blue and red (near the Balmer and Paschen limits respectively) 

 

The continuum that we see is a result of photons that evade the increased chance of absorption by the above two mechanisms.

Edited by Organic Astrochemist, 20 January 2025 - 04:43 PM.

[robin_astro]

That's a very interesting spectrum. I assume it has been corrected for instrumental response, so the fact that the continuum looks even more weak and noisy in the extreme blue and extreme red truly reflects some underlying wavelength dependence.

 

The response corrected continuum looks ~flat to me (The blue line is a linear fit)  The lower signal  at the blue and red ends due the underlying response of the instrument will naturally increase the noise in these regions.  It is a very weak signal though relative to the emission line intensity and the noise will likely also be high due to the application of a wide binning zone to include the region with emission.. Binning just the region with the continuum would be better for measuring the continuum. 

 

Interstellar extinction is another factor we should take into consideration which is probably significant local to the nebula and will redden the spectrum. Perhaps the decrement in the Balmer emission lines could be used to estimate this.

 

Cheers

Robin 

Edited by robin_astro, 21 January 2025 - 08:32 AM.

[robin_astro]

The response corrected continuum looks ~flat to me (The blue line is a linear fit)  

Note that this refers to the spectrum measured by Christian Buil from this reference

https://buil.astrosu...ula/nebula.html

 

The spectrum posted at the top of this thread, also measured by Christian Buil but a later date using a different instrument however shows the continuum clearly rising from red to blue

https://www.cloudyni...ent/?p=13917328

so we should be cautious about drawing any conclusions about the continuum shape from these spectra.

 

Cheers

Robin