APOD: NGC 1566: A Spiral Galaxy from Webb... (2024 Feb 06)

[APOD Robot]

NGC 1566: A Spiral Galaxy from Webb and Hubble

Explanation: What's different about this galaxy? Very little, which makes the Spanish Dancer galaxy, NGC 1566, one of the most typical and photogenic spirals on the sky. There is something different about this galaxy image, though, because it is a diagonal combination of two images: one by the Hubble Space Telescope on the upper left, and the other by the James Webb Space Telescope on the lower right. The Hubble image was taken in ultraviolet light and highlights the locations of bright blue stars and dark dust along the galaxy's impressive spiral arms. In contrast, the Webb image was taken in infrared light and highlights where the same dust emits more light than it absorbed. In the rollover image, the other two sides of these images are revealed. Blinking between the two images shows which stars are particularly hot because they glow brighter in ultraviolet light, and the difference between seemingly empty space and infrared-glowing dust.

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[Ann]

Not all galaxies are like the cosmic beauty queen, NGC 1566!

NGC 1566: A Spiral Galaxy from Webb and Hubble Image Credit & Copyright: NASA, ESA, CSA, STScI, J. Lee (STScI), T. Williams (Oxford), R. Chandar (UToledo), D. Calzetti (UMass), PHANGS Team

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IC 5253 in visible and infrared light. Credit: ESA/Webb, NASA & CSA, J. Lee and the PHANGS-JWST and PHANGS-HST Teams

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Amazing, isn't it? In NGC 1566, the dust serves like the framework and the load-bearing beams of the entire galaxy. In IC 5253, the dust lanes look like a mess, or perhaps like a mesh, and the overall galactic spiral structure seems almost unrelated to the dust. The spiral shape appears to be created and upheld mostly by the stars, particularly by the old stellar population. (And by the dark matter, too?)

Not so with NGC 1566, where the dust and the stars seem to go "hand in hand". It is such a stunningly elegant galaxy! And its "dust framework" is incredibly elegant, too!

NGC 1566 in optical light. Credit: Mark Hanson.

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The dust structure of NGC 1566. Credit: JWST/NASA/Judy Schmidt.

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Not all of us can be the prettiest in our class. So, mirror, mirror on the wall, who is the fairest of them all?

Mirror, mirror on the wall, who is the fairest of them all?

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Well, it can only be Snow White, right? I mean, NGC 1566!

Ann

[Christian G.]

Not all galaxies are like the cosmic beauty queen, NGC 1566!

NGC 1566: A Spiral Galaxy from Webb and Hubble Image Credit & Copyright: NASA, ESA, CSA, STScI, J. Lee (STScI), T. Williams (Oxford), R. Chandar (UToledo), D. Calzetti (UMass), PHANGS Team

---

IC 5253 in visible and infrared light. Credit: ESA/Webb, NASA & CSA, J. Lee and the PHANGS-JWST and PHANGS-HST Teams

---

Amazing, isn't it? In NGC 1566, the dust serves like the framework and the load-bearing beams of the entire galaxy. In IC 5253, the dust lanes look like a mess, or perhaps like a mesh, and the overall galactic spiral structure seems almost unrelated to the dust. The spiral shape appears to be created and upheld mostly by the stars, particularly by the old stellar population. (And by the dark matter, too?)

Not so with NGC 1566, where the dust and the stars seem to go "hand in hand". It is such a stunningly elegant galaxy! And its "dust framework" is incredibly elegant, too!

NGC 1566 in optical light. Credit: Mark Hanson.

---

The dust structure of NGC 1566. Credit: JWST/NASA/Judy Schmidt.

---

Not all of us can be the prettiest in our class. So, mirror, mirror on the wall, who is the fairest of them all?

Mirror, mirror on the wall, who is the fairest of them all?

---

Well, it can only be Snow White, right? I mean, NGC 1566!

Ann

In my book the first thing one has to learn about the cosmos is how exhilarating it is! The second thing is all the knowledge, which can only be fun to acquire once you’ve learned the first thing. Ann, you are a great teacher at both!

By the way can anyone spot tiny background galaxies in the APOD? I’m curious to see how those would compare between Hubble and Webb.

[JimB]

By the way can anyone spot tiny background galaxies in the APOD? I’m curious to see how those would compare between Hubble and Webb.

Looks like there may be one here, right on the left edge of the Webb image

APOD1.jpg

It's quite visible and blue-ish in the Webb view, but disappears in Hubble.

[Christian G.]

By the way can anyone spot tiny background galaxies in the APOD? I’m curious to see how those would compare between Hubble and Webb.

Looks like there may be one here, right on the left edge of the Webb image
APOD1.jpg

It's quite visible and blue-ish in the Webb view, but disappears in Hubble.

I see it, thanks!

[Ann]

In my book the first thing one has to learn about the cosmos is how exhilarating it is! The second thing is all the knowledge, which can only be fun to acquire once you’ve learned the first thing. Ann, you are a great teacher at both!

By the way can anyone spot tiny background galaxies in the APOD? I’m curious to see how those would compare between Hubble and Webb.

I'm very glad you appreciate my posts, Christian!


There is one galaxy that is seen right through the disk of NGC 1566 at upper left:

APOD 6 February 2024 detail annotated.png

APOD 6 February 2024 detail 2 annotated.png

In the Hubble version of this region, you can see that the orange object next to the arrow is quite fuzzy. That means that it isn't a star, so it must be a galaxy. (Okay, it might be a globular cluster, but that's not likely.) The orange color is due to the fact that this galaxy is quite distant, so it is redshift-reddened, but it is also seen through the disk of NGC 1566, so it is also quite heavily dust-reddened. The galaxy is most likely an elliptical galaxy with no spiral arms and no star formation, but we can't be sure about that. Galactic arms are almost always much fainter than the galactic core, even though there are a few exceptions (see here). The galaxy that we see through the disk of NGC 1566 could be a spiral like UGC 11105, which has a bright inner region and faint arms, or it could be an elliptical galaxy.

In any case, what Hubble detected from this galaxy was reddened light from a population of old red and yellow stars. What JWST detected from this galaxy was also light from red and yellow stars. Red stars are shown as blue in JWST images. Dust, by contrast, is shown as red or orange in JWST images. But we see no orange color from this galaxy in the JWST image. That means that JWST has not detected any appreciable amounts of dust in this galaxy. Again, this means that the galaxy is either a spiral galaxy with faint arms and little dust in its center, or else it is really an elliptical galaxy.

To see what I mean, consider two pictures of the iconic galaxy M104, one in visible light by ESO and one in infrared light by Spitzer:

M104 in visible light. Credit: ESO.

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M1904 in infrared light. Credit: Spitzer.

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In the Hubble image taken in visible light, the stars of M104 look yellowish and the dust looks brown. In the infrared image by Spitzer, the stars are cyan-colored and the dust is red, pink and brown.

If a galaxy looks "all blue" in an infrared image, we should definitely assume that it is dominated (and perhaps completely dominated) by old red and yellow stars.

Ann

[Ann]

We can also see at least two galaxies in the JWST image that appear to contain dust:

APOD 6 February 2024 detail 4 annotated.png

APOD 6 February 2024 detail 3 annotated.png

One galaxy looks quite pink in the JWST image. The pink color suggests to me that this galaxy is quite dusty. (Or possibly something went wrong during the processing.) In any case, this galaxy very obviously has a disk, so it is not an elliptical galaxy.

The other galaxy, which is almost invisible in the Hubble image, has a blue center in the JWST image and a purplish-blue disk. That suggests to me that there is dust in the disk.

Ann

[Christian G.]

In my book the first thing one has to learn about the cosmos is how exhilarating it is! The second thing is all the knowledge, which can only be fun to acquire once you’ve learned the first thing. Ann, you are a great teacher at both!

By the way can anyone spot tiny background galaxies in the APOD? I’m curious to see how those would compare between Hubble and Webb.

I'm very glad you appreciate my posts, Christian!


There is one galaxy that is seen right through the disk of NGC 1566 at upper left:

APOD 6 February 2024 detail annotated.png

APOD 6 February 2024 detail 2 annotated.png

In the Hubble version of this region, you can see that the orange object next to the arrow is quite fuzzy. That means that it isn't a star, so it must be a galaxy. (Okay, it might be a globular cluster, but that's not likely.) The orange color is due to the fact that this galaxy is quite distant, so it is redshift-reddened, but it is also seen through the disk of NGC 1566, so it is also quite heavily dust-reddened. The galaxy is most likely an elliptical galaxy with no spiral arms and no star formation, but we can't be sure about that. Galactic arms are almost always much fainter than the galactic core, even though there are a few exceptions (see here). The galaxy that we see through the disk of NGC 1566 could be a spiral like UGC 11105, which has a bright inner region and faint arms, or it could be an elliptical galaxy.

In any case, what Hubble detected from this galaxy was reddened light from a population of old red and yellow stars. What JWST detected from this galaxy was also light from red and yellow stars. Red stars are shown as blue in JWST images. Dust, by contrast, is shown as red or orange in JWST images. But we see no orange color from this galaxy in the JWST image. That means that JWST has not detected any appreciable amounts of dust in this galaxy. Again, this means that the galaxy is either a spiral galaxy with faint arms and little dust in its center, or else it is really an elliptical galaxy.

To see what I mean, consider two pictures of the iconic galaxy M104, one in visible light by ESO and one in infrared light by Spitzer:

M104 in visible light. Credit: ESO.

---

M1904 in infrared light. Credit: Spitzer.

---

In the Hubble image taken in visible light, the stars of M104 look yellowish and the dust looks brown. In the infrared image by Spitzer, the stars are cyan-colored and the dust is red, pink and brown.

If a galaxy looks "all blue" in an infrared image, we should definitely assume that it is dominated (and perhaps completely dominated) by old red and yellow stars.

Ann

Another much appreciated post!

[johnnydeep]

What's with all the bright blue stars in the JWST image, that appear as either white or red in the Hubble image?

EDIT: ok, above, I seen Ann said "Red stars are shown as blue in JWST images.", but that doesn't explain it for me. Why are some white stars shown as blue as well?

[Ann]

What's with all the bright blue stars in the JWST image, that appear as either white or red in the Hubble image?

EDIT: ok, above, I seen Ann said "Red stars are shown as blue in JWST images.", but that doesn't explain it for me. Why are some white stars shown as blue as well?

All stars that are not heavily, heavily reddened by dust (such as stars that have not yet made it out of their birth cocoons) are shown as blue in infrared images. That's because almost all stars (except heavily dust-reddened stars and very cool brown dwarfs) emit the kind of longwave light (some 700-1,000 nm) that is the most shortwave and therefore the "bluest" light that infrared detectors can detect.

Cool stars peak - that is, reach the top of their blackbody curve - at infrared wavelengths. Stars like the Sun peak at blue-green wavelengths, and stars of spectral class A and hotter peak at ultraviolet wavelengths.

Blackbody curves of stars of different temperatures. The emission of cool red stars peaks in the infrared part of the spectrum. The yellow (probably Sunlike) star peaks in the blue-green part of the spectrum. And the hot blue star peaks inte the ultraviolet part of the spectrum.

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But even though stars of different temperatures peak in different parts of the spectrum, they all emit the kind of longwave light that is detected by infrared detectors:

Blackbody curves of different stars infrared annotated.png

I found the illustration of the blackbody curves of stars of different temperatures here. The dotted red line, which I added, is approximately the shortest and "bluest" wavelength that an infrared camera would detect. It is (approximately) this wavelength that is shown as blue in infrared images. Almost all stars emit this wavelength, and almost all (reasonably nearby) stars are detected by infrared cameras. However, hot stars, which emit large and in some cases absolutely copious amounts of ultraviolet light, tend to look comparatively very faint in infrared images (because they emit comparatively little infrared light).


Take a look at this O9.5V (very hot) runaway star Zeta Ophiuchi in visible and infrared light:

Hot star Zeta Ophiuchi. The arrow shows the direction in which the star is travelling. No other star is easily detected in the direction in which Zeta Ophiuchi is travelling. Credit: Apollo (Scott) of Cloudy NIghts.

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Zeta Ophiuchi in infrared and X-ray light. The star itself is probably only seen in infrared, not in X-rays. Zeta Ophiuchi creates a powerful bow shock or shock wave, probably in the direction in which it is travelling. Another reasonably bright star is seen on the other side of the shock wave. Credit: Image credit: X-ray: NASA/CXC/Dublin Inst. Advanced Studies/S. Green et al.; Infrared: NASA/JPL/Spitzer

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In the optical image by Apollo of Cloudy Nights, no other star is seen in the direction in which Zeta Ophiuchi is travelling. But in the infrared/X-ray image, another reasonably bright star is seen on the other side of the shock wave. In my opinion, we have every reason to believe that this other star peaks in infrared light and is really quite faint in visible light, and we see it so clearly in the infrared image because the infrared camera is so good at detecting its longwave light. At the same time, Zeta Ophiuchi itself is made to look many times fainter than it would if it was photographed in optical light, since only its faint longwave light is detected.

Note that all the stars in the infrared/X-ray image look blue except one star, which looks red. My guess is that this is either a runaway unborn star wrapped in a dusty cocoon, or else it is light from a very distant and reddened light source like, perhaps, a quasar? Or else it is an artifact.

Actually there are two red dots, and that makes it more likely that these red objects are in fact extremely heavily dust-reddened (and dust-enshrouded) stars. Unless the red color is used to show X-rays in this image, in which case the red dots could be distant sources of X-rays.

Ann

[johnnydeep]

What's with all the bright blue stars in the JWST image, that appear as either white or red in the Hubble image?

EDIT: ok, above, I seen Ann said "Red stars are shown as blue in JWST images.", but that doesn't explain it for me. Why are some white stars shown as blue as well?

...

Take a look at this O9.5V (very hot) runaway star Zeta Ophiuchi in visible and infrared light:

Hot star Zeta Ophiuchi. The arrow shows the direction in which the star is travelling. No other star is easily detected in the direction in which Zeta Ophiuchi is travelling. Credit: Apollo (Scott) of Cloudy NIghts.

---

Zeta Ophiuchi in infrared and X-ray light. The star itself is probably only seen in infrared, not in X-rays. Zeta Ophiuchi creates a powerful bow shock or shock wave, probably in the direction in which it is travelling. Another reasonably bright star is seen on the other side of the shock wave. Credit: Image credit: X-ray: NASA/CXC/Dublin Inst. Advanced Studies/S. Green et al.; Infrared: NASA/JPL/Spitzer

---

In the optical image by Apollo of Cloudy Nights, no other star is seen in the direction in which Zeta Ophiuchi is travelling. But in the infrared/X-ray image, another reasonably bright star is seen on the other side of the shock wave. In my opinion, we have every reason to believe that this other star peaks in infrared light and is really quite faint in visible light, and we see it so clearly in the infrared image because the infrared camera is so good at detecting its longwave light. At the same time, Zeta Ophiuchi itself is made to look many times fainter than it would if it was photographed in optical light, since only its faint longwave light is detected.

Note that all the stars in the infrared/X-ray image look blue except one star, which looks red. My guess is that this is either a runaway unborn star wrapped in a dusty cocoon, or else it is light from a very distant and reddened light source like, perhaps, a quasar? Or else it is an artifact.

Actually there are two red dots, and that makes it more likely that these red objects are in fact extremely heavily dust-reddened (and dust-enshrouded) stars. Unless the red color is used to show X-rays in this image, in which case the red dots could be distant sources of X-rays.

Ann

Thanks, Ann, but I don't think I understand these images. Where's the bow shock in the optical image on the left? I guess it must be perpendicular to your arrow? I was first thinking that the two most easily seen stars on a horizontal line dead center in each image showed the same pair, and the two images were at about the same scale. But that can't be right based on your description.

[Ann]

What's with all the bright blue stars in the JWST image, that appear as either white or red in the Hubble image?

EDIT: ok, above, I seen Ann said "Red stars are shown as blue in JWST images.", but that doesn't explain it for me. Why are some white stars shown as blue as well?

...

Take a look at this O9.5V (very hot) runaway star Zeta Ophiuchi in visible and infrared light:

Hot star Zeta Ophiuchi. The arrow shows the direction in which the star is travelling. No other star is easily detected in the direction in which Zeta Ophiuchi is travelling. Credit: Apollo (Scott) of Cloudy NIghts.

---

Zeta Ophiuchi in infrared and X-ray light. The star itself is probably only seen in infrared, not in X-rays. Zeta Ophiuchi creates a powerful bow shock or shock wave, probably in the direction in which it is travelling. Another reasonably bright star is seen on the other side of the shock wave. Credit: Image credit: X-ray: NASA/CXC/Dublin Inst. Advanced Studies/S. Green et al.; Infrared: NASA/JPL/Spitzer

---

In the optical image by Apollo of Cloudy Nights, no other star is seen in the direction in which Zeta Ophiuchi is travelling. But in the infrared/X-ray image, another reasonably bright star is seen on the other side of the shock wave. In my opinion, we have every reason to believe that this other star peaks in infrared light and is really quite faint in visible light, and we see it so clearly in the infrared image because the infrared camera is so good at detecting its longwave light. At the same time, Zeta Ophiuchi itself is made to look many times fainter than it would if it was photographed in optical light, since only its faint longwave light is detected.

Note that all the stars in the infrared/X-ray image look blue except one star, which looks red. My guess is that this is either a runaway unborn star wrapped in a dusty cocoon, or else it is light from a very distant and reddened light source like, perhaps, a quasar? Or else it is an artifact.

Actually there are two red dots, and that makes it more likely that these red objects are in fact extremely heavily dust-reddened (and dust-enshrouded) stars. Unless the red color is used to show X-rays in this image, in which case the red dots could be distant sources of X-rays.

Ann

Thanks, Ann, but I don't think I understand these images. Where's the bow shock in the optical image on the left? I guess it must be perpendicular to your arrow? I was first thinking that the two most easily seen stars on a horizontal line dead center in each image showed the same pair, and the two images were at about the same scale. But that can't be right based on your description.

The bow shock isn't seen in visual light. I agree with you that it should be perpendicular to the arrow (which I didn't put there, by the way).

I also don't think that the pictures are at the same scale, and I doubt that north is up in the image with the bow shock.

The second brightest star (after Zeta Oph itself) that is seen in the bow shock image might - mind you, might - be HD 150068. It is located in the right direction away from Zeta Ophiuchi according to the "arrow image" (that is, it is to the northeast of Zeta), and it is an 8th magnitude star compared with Zeta's bright 3rd magnitude. It is spectral class K0III, and it is definitely yellower than the Sun, but it is not deeply orange or very very infrared.

I found this spectrum of Pollux, which is similar to HD 150068:

Spectrum of Pollux Wolfcreek Space.png

Spectrum of Pollux by Wolfcreek Space

As you can see, the spectrum of Pollux is definitely more red than blue, and the spectrum of HD 150068 would be similar.

For comparison, the spectrum of Alnitak, which is reasonably similar at least in temperature to Zeta Ophiuchi, looks like this:

Spectrum of Alnitak Wolfcreek Space.png

Spectrum of Alnitak. Credit: Wolfcreek Space.

But I don't know if HD 150068 is the star that is seen next to Zeta Ophiuchi. I'm just guessing.

Ann

[johnnydeep]

Ok - thanks again, Ann!