APOD: Clouds Around Galaxy Andromeda (2022 Oct 24)

[APOD Robot]

Clouds Around Galaxy Andromeda

Explanation: What are those red clouds surrounding the Andromeda galaxy? This galaxy, M31, is often imaged by planet Earth-based astronomers. As the nearest large spiral galaxy, it is a familiar sight with dark dust lanes, bright yellowish core, and spiral arms traced by clouds of bright blue stars. A mosaic of well-exposed broad and narrow-band image data, this deep portrait of our neighboring island universe offers strikingly unfamiliar features though, faint reddish clouds of glowing ionized hydrogen gas in the same wide field of view. Most of the ionized hydrogen clouds surely lie in the foreground of the scene, well within our Milky Way Galaxy. They are likely associated with the pervasive, dusty interstellar cirrus clouds scattered hundreds of light-years above our own galactic plane. Some of the clouds, however, occur right in the Andromeda galaxy itself, and some in M110, the small galaxy just below.

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[Ann]

Wow, that's fascinating! We can clearly see the outline of Andromeda in hydrogen alpha light, and we can even see the beginnings of two outer spiral arms! And we can see that satellite galaxy NGC 205 (small appendix below Andromeda) is being stretched and glowing in Hα, too.

Clouds Around Galaxy Andromeda
Image Credit & Copyright: Andrew Fryhover

The halo of Andromeda. Photo: Giuseppe Donatiello.

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But what exactly are we seeing in the APOD? Is Andromeda actually immersed in a real, if faint, emission nebula? Or are we just seeing its halo, which is reddish in color due to the large number of small cool stars, which are intrinsically reddish in color, and because of the presence of dust, which is also intrinsically reddish?

In other words, would we have seen the same sort of halo if we had taken a very deep photo of Andromeda in another red but non-Hα wavelength, like, say, 649 nm?

Blackbody curves of stars of different temperatures. The cool red star at left emits a lot of red light of many wavelengths. Image: docs.kde.org

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The reddish color of the thickest dust in this APOD is a good illustration of the typical intrinsic color of dust. Photo: Howard Trottier.

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A cool red star emits red light of many wavelengths, and the specific Hα wavelength of 656.46 nm does not stand out among them. The Hα wavelength only stands out where hydrogen has been ionized.

So, in short: Ionized hydrogen emits red Hα light because it has been ionized either by hot stars or by shock waves of some sort. Small cool stars emit Hα light because Hα is just one wavelength out of millions in their spectra, nothing special, and dust is reddish because, well, because it is.

Ann

[Ann]

Speaking of Andromeda's halo: It is a fascinating thing!

At its largest scales Andromeda's halo is humongous and predominantly made of gas. Andromeda's inner halo, however, is rich in stars.

The full size of Andromeda's halo. The tiny little white thing in the middle
is the visible disk of Andromeda. Illustration: NASA.

Denizens and background objects of the inner halo of Andromeda. Photo: NASA, ESA and T.M. Brown (STScI)

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Read about the full size of Andromeda's halo here, and read about the inner halo here.

Note that the picture of details in Andromeda's inner halo were taken through a filter centered on 606 nm, which is this color, ███, and an infrared filter detecting light invisible to our eyes. This choice of filters will make the objects in the picture look bluer than they typically are. Therefore, a lot of small stars that look white in the Hubble image are probably small red dwarfs, which are reddish in color.

Ann

[sc02492]

A cool red star emits red light of many wavelengths, and the specific Hα wavelength of 656.46 nm does not stand out among them. The Hα wavelength only stands out where hydrogen has been ionized.

So, in short: Ionized hydrogen emits red Hα light because it has been ionized either by hot stars or by shock waves of some sort. Small cool stars emit Hα light because Hα is just one wavelength out of millions in their spectra, nothing special, and dust is reddish because, well, because it is.

Ann

Ann makes an excellent point. We are seeing two sources of Ha light here, one coming from the Milky Way itself, and the other coming from an envelope of Ha light surrounding M31 itself. However, the actual mechanism producing the Ha light in this image remains unclear to me. Conventionally we think of Ha light in the 656nm wavelength range as being emitted by hydrogen that has been excited (ionized HII) by UV radiation, and which can be captured by Ha narrowband filters. Whether that is actually the case for the Ha light seen in this image, or whether it (more likely) represents reflection of this particular wavelength by dust in the interstellar cirrus (or dust surrounding M31 and M110), is the question. The text in the APOD caption hints at this, but only in passing.

Steve Cannistra
www.starrywonders.com

[orin stepanek]

This is the kind of Andromeda photo that I like!

Halloween kitty?

[stefanz]

I agree to Ann.

It is often overseen that narrow band filters also capture continuum.

In the APOD the black levels for green and blue are chosen higher than black levels for red channel such that all faint regions become red.
Without that trick, some of the red structures would be white because they have a similar brightness in all channels (with white balance to star average), see there for example.

Another issue is that interstellar cirrus (or integrated flux nebulae) scatters continuum light. That region contains such nebulae. But the HII filaments, which are also there, are closer IMHO.

[Chris Peterson]

A cool red star emits red light of many wavelengths, and the specific Hα wavelength of 656.46 nm does not stand out among them. The Hα wavelength only stands out where hydrogen has been ionized.

So, in short: Ionized hydrogen emits red Hα light because it has been ionized either by hot stars or by shock waves of some sort. Small cool stars emit Hα light because Hα is just one wavelength out of millions in their spectra, nothing special, and dust is reddish because, well, because it is.

When we view stars through an Hα filter, almost all the light we see is from ionized hydrogen, not from the (thermal) continuum. That is, most of the light emitted at 656 nm by stars (including cool ones) is from ionized hydrogen. But that represents only a tiny fraction of the total energy produced by stars, which is why it only stands out if we use a narrow band filter.

[Ann]

A cool red star emits red light of many wavelengths, and the specific Hα wavelength of 656.46 nm does not stand out among them. The Hα wavelength only stands out where hydrogen has been ionized.

So, in short: Ionized hydrogen emits red Hα light because it has been ionized either by hot stars or by shock waves of some sort. Small cool stars emit Hα light because Hα is just one wavelength out of millions in their spectra, nothing special, and dust is reddish because, well, because it is.

When we view stars through an Hα filter, almost all the light we see is from ionized hydrogen, not from the (thermal) continuum. That is, most of the light emitted at 656 nm by stars (including cool ones) is from ionized hydrogen. But that represents only a tiny fraction of the total energy produced by stars, which is why it only stands out if we use a narrow band filter.

Absolutely, Chris. My question was whether the red-glowing halo of Andromeda is really narrowband Hα emission, or if it just one of innumerable wavelengths from the thermal continuum brought out by the Hα filter.

Ann

[Chris Peterson]

A cool red star emits red light of many wavelengths, and the specific Hα wavelength of 656.46 nm does not stand out among them. The Hα wavelength only stands out where hydrogen has been ionized.

So, in short: Ionized hydrogen emits red Hα light because it has been ionized either by hot stars or by shock waves of some sort. Small cool stars emit Hα light because Hα is just one wavelength out of millions in their spectra, nothing special, and dust is reddish because, well, because it is.

When we view stars through an Hα filter, almost all the light we see is from ionized hydrogen, not from the (thermal) continuum. That is, most of the light emitted at 656 nm by stars (including cool ones) is from ionized hydrogen. But that represents only a tiny fraction of the total energy produced by stars, which is why it only stands out if we use a narrow band filter.

Absolutely, Chris. My question was whether the red-glowing halo of Andromeda is really narrowband Hα emission, or if it just one of innumerable wavelengths from the thermal continuum brought out by the Hα filter.

Yeah, stefanz is on the right track with that. We could be seeing reflected Hα, we could be seeing reflected continuum, we could be seeing emission (not likely, but not impossible). To bring up that very low level signal, an extreme transfer function was applied, and we'd need to know how the different channels were balanced. Push the red signal and anything will look red, even if it isn't.

[VictorBorun]

if we talking narrow-band Hα why not show Doppler's shift?
If Andromeda's outskirts have line-of-sight velocities that differ by 300 km/s, then z differs by 1/1000, 656.46 nm ± 0.66 nm

[Chris Peterson]

if we talking narrow-band Hα why not show Doppler's shift?
If Andromeda's outskirts have line-of-sight velocities that differ by 300 km/s, then z differs by 1/1000, 656.46 nm ± 0.66 nm

High quality Hα filters intended for solar viewing have widths of less than 0.1 nm, but the best interference filters used for deep sky imaging have widths of 3 nm (with 5-10 nm quite common). So not useful for detecting Doppler shift at those kinds of speeds.

[stefanz]

Then we view stars through an Hα filter, almost all the light we see is from ionized hydrogen, not from the (thermal) continuum. That is, most of the light emitted at 656 nm by stars (including cool ones) is from ionized hydrogen.

Depends on the width of the filter pass band. If you observe the sun using a 0.5Å filter, most of the details are from H-alpha. If you use an 5nm (=50Å) filter, you won't see any difference compared to a wideband red filter. The narrowest filters used for deepsky are 3nm wide.

The red colored stuff we see in the APOD consists in:

1. Irregular halo of M31, correcttly depicted and nicely annotated in the picture from Giuseppe Donatiello posted by Ann. (AFAIK the shape comes from previous accreted galaxies.) Only a very small fraction of that light comes from H-alpha emissions of stars. (Unless the photographer used a custom made <1Å wide filter which also takes the 6.5Å blueshift into account ...)
2. HII filaments, i.e. their H-alpha emissions
3. A little bit light from the milky way (mostly from stars) which was scattered at interstellar cirrus

Color of 1. and 3. should be approximately equal (white with natural white balance). Only 2. should be red.

I could not resist, I ionized the fat cat (using the tonal curve tool of GIMP):

ik.jpg (31.69 KiB) Viewed 4410 times

[VictorBorun]

if we talking narrow-band Hα why not show Doppler's shift?
If Andromeda's outskirts have line-of-sight velocities that differ by 300 km/s, then z differs by 1/1000, 656.46 nm ± 0.66 nm

High quality Hα filters intended for solar viewing have widths of less than 0.1 nm, but the best interference filters used for deep sky imaging have widths of 3 nm (with 5-10 nm quite common). So not useful for detecting Doppler shift at those kinds of speeds.

too bad
it's all about the manufacturing difficulties, is not it
Because a deep sky exposure in Hα gains no additional photons of interest using filters wider than the width of the main of the two Hα lines