IAA-CSIC: I Zw 18 Reveals the Universe's History

[bystander]

I Zw 18: The Galaxy that Reveals the Universe's History
Institute of Astrophysics of Andalusia | 2015 Mar 23

[url=http://hubblesite.org/newscenter/archive/releases/2004/35/image/a/][b][i]I Zwicky 18: Possibly the Youngest Galaxy Ever Seen[/i][/b][/url] [b][i]Credit: NASA, ESA, Y. Izotov (MAO, Kyiv, UA) and T. Thuan (UVA)[/i][/b]

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A map of ionized helium in the galaxy has just been published which indicates the presence of peculiar stars similar to the first that ever shone in the universe

The first galaxies were formed some 13.3 billion years ago, mainly composed of hydrogen and helium, the primary elements that emerged from the Big Bang. Their study to date has been technically very challenging due to their great distance from us, but the observation of analogous galaxies in our vicinity has turned out to be an excellent shortcut.

"Dwarf galaxy IZw18 is the least abundant in metals (in astrophysics, elements heavier than hydrogen and helium) in the nearby universe, and one of the most akin to the primeval galaxies. Its study therefore allows us to catch glimpses of the conditions that prevailed in the primordial universe”, says Carolina Kehrig, researcher at the Institute of Astrophysics of Andalusia in charge of the study that analyses the properties of IZw18.

The study has found a very large region in this small galaxy of ionized helium, which tends to be more frequent in very distant galaxies with low presence of metals. The ionization of helium implies the presence of objects emitting a radiation intense enough to knock electrons off the helium atoms. "In this study we propose a new interpretation of the origin of this radiation in galaxy IZw18, a subject which is still enigmatic”, Kehrig says. ...

The Extended He II λ4686-emitting Region in IZw 18 Unveiled: Clues for Peculiar Ionizing Sources - Carolina Kehrig et al

• Astrophysical Journal Letters 801(2) L28 (10 Mar 2015) DOI: 10.1088/2041-8205/801/2/L28
  arXiv.org > astro-ph > arXiv:1502.00522 > 02 Feb 2015

[Ann]

I've been meaning to write a comment on this before, but I've only now come around to it.

http://www.iaa.es/content/izw18-galaxy- ... y-universe wrote:

The study has found a very large region in this small galaxy of ionized helium, which tends to be more frequent in very distant galaxies with low presence of metals. The ionization of helium implies the presence of objects emitting a radiation intense enough to knock electrons off the helium atoms.
...
Conventional sources of ionization, such as Wolf-Rayet stars – very massive and with very violent stellar winds – or shocks generated by remnants of supernovae, cannot provide the energy necessary to explain the halo of ionized helium present on IZw18, so researchers considered other possibilities.

"Our data point to the fact that extremely hot stars, such as supermassive stars with low metal content or massive stars practically devoid of metals may hold the key to the enigma of the excitement of helium on IZw18, even though the existence of these stars has not yet been confirmed by observations on any galaxy”, says Carolina Kehrig (IAA-CSIC).

We would be talking about very hot stars analogous to first generation stars (known as Population III stars) which, according to theoretical models, would be composed only of hydrogen and helium and could be hundreds of times more massive than the Sun. These stars are believed to have played a decisive role in the “reionization” of the universe, during which period the first stars and galaxies became visible and which is still little known.

Wow. A stellar population in I Zwicky 18 analogous to Population III stars, the first generation of stars in the universe, supermassive absolutely searingly hot stars, responsible for reionizing the universe. That's absolutely amazing. If this holds up, it would be like having a little piece of the babyhood of the universe being somehow transported almost right into our midst by some kind of incredible time machine.

(And those blue stars inside I Zwicky 18 - you can imagine me drooling at the thought of them, although even I am grateful that they are not right on my doorstep!)

I've been scratching my head over the weird colors of the image of I Zwicky 18. I found a Hubble source which says, or so I think, that the picture has been made from exposures through different infrared filters. Do you agree, Geck, that this is an infrared image?

Ann

[geckzilla]

That particular image is a two-filter, ACS/WFC wideband green (555nm peak) and near-infrared (814nm peak) image. There are infrared images by NICMOS in the Hubble archive but they weren't used for making this image. NICMOS is hard to work with for making 'pretty' pictures like this one. I've done it a few times and it always takes quit a bit of fiddling. You almost never see NICMOS imagery in press releases.

[Ann]

Thanks. It doesn't look like an infrared image, because it is too sharply detailed. After all, infrared images of galaxies are typically fuzzy.

But it doesn't look quite like the typical two-filter 606 and 814 nm, either. Perhaps the 555 instead of 606 nm makes a difference.

I still don't understand the colors. What emission wavelengths are passing through by the 555 nm filter to create that brilliant blue-looking nebula that surrounds almost all of I Zwicky 18? Is the 555 nm filter broad enough to detect both Ha and OIII, for example? And is the 814 nm filter broad enough to detect Ha and SIII? Would that explain the greenish color of the outer filaments of the nebula?

Ann

[geckzilla]

They (the infrared images) are not at all fuzzy, at least as far as Hubble goes. They are not as high resolution so they tend to be scaled up in order to match the resolution of visible light imagery. You could say that they are fuzzy after interpolation but in their raw form they're not fuzzy. Anyway, nitpicking.

I've processed I Zw 18 before. The nebula appears green and blue in mine, if you'd like to look. Of course, it's not an RGB image and the three filters used were F814W (R), F606W (G), and F555W (B) nm. It doesn't look red because it barely visible at 814nm (yes, it is still visible in infrared wavelengths!).
https://www.flickr.com/photos/geckzilla/15605557588/

I forgot if I've said it before here. I think I have, but anyway, I don't think emission nebulae are really red. I'm not convinced that if I could see one with my eyes it would even be pink. Maybe a very pale pink? I see them all the time in the broadband green and red filters so often used by Hubble. They show up pretty well in F555W. The thing about narrowband imagery and the H-alpha filter is that the stars do not glow so brightly in H-alpha, which allows the nebula to be seen so much more clearly. It's a way of isolating just those photons emitted by the nebula. The nebula surely glows relatively brightly at 656 nm but that does not mean it is only glowing at that wavelength. It is glowing across the spectrum.

[Ann]

Thanks, Geck, very interesting! I really appreciate your large size I Zwicky 18 image. Yes, the nebulosity in your image looks a bit greener than in the I Zwicky 18 picture that we usually see, but it is clearly greenish-blue, not reddish at all. In other words, the 814 nm light makes a minimal contribution here.

You wrote:

They (the infrared images) are not at all fuzzy, at least as far as Hubble goes. They are not as high resolution so they tend to be scaled up in order to match the resolution of visible light imagery. You could say that they are fuzzy after interpolation but in their raw form they're not fuzzy. Anyway, nitpicking.

I agree, the Hubble 814 nm images don't look fuzzy. I was thinking of the Spitzer images of galaxies. But surely the Spitzer images are typically not 814 nm images?

You wrote:

I forgot if I've said it before here. I think I have, but anyway, I don't think emission nebulae are really red. I'm not convinced that if I could see one with my eyes it would even be pink. Maybe a very pale pink? I see them all the time in the broadband green and red filters so often used by Hubble. They show up pretty well in F555W.

The way I understand it, the light from nebulas really is composed of different narrowband emission lines. We are talking about Ha, Hβ, OIII, SIII and other lines. But if there are many emission lines, you will get a mix of colors that are unlikely to look red to the human eye, if only because our eyes are not very sensitive to wavelengths so far into the red part of the spectrum. We are much better at seeing green and yellow light, for example.

I wonder, though, what emission lines will be picked up by the 555W filter. Perhaps the red 656 nm Ha light and the green 502 nm OIII light will combine and produce a yellowish light in the brightest parts of emission nebulas?

Ann

[geckzilla]

Spitzer's imagery isn't fuzzy either as far as I can remember. You'd have to show me an example. It's very likely that whatever fuzziness you saw was due to the image being scaled up and interpolated. I don't know if there's anything inherently fuzzy about any kind of imagery from any part of the electromagnetic spectrum. Something not imaged with high resolution will always look fuzzy when enlarged and interpolated.

Regarding the mixing of colors... I don't know. I know the emission lines are not always in the same position. OIII is brighter near the center, closer to the stars, usually. So if you want to imagine the nebula with enhanced human eyes as I am guessing you want to, then maybe imagine it something like a flame with a brighter yellow where the OIII is and overlaps with Ha and then redder trailing away where the Ha is more dominant. But it's still probably a kind of pastel palette rather than the rich colors you (indeed, most humans) enjoy so much.

[Ann]

Geck wrote:
So if you want to imagine the nebula with enhanced human eyes as I am guessing you want to, then maybe imagine it something like a flame with a brighter yellow where the OIII is and overlaps with Ha and then redder trailing away where the Ha is more dominant. But it's still probably a kind of pastel palette rather than the rich colors you (indeed, most humans) enjoy so much.

Indeed, I love rich colors. Check out this page and set the wavelength to 440 or 437 nm - the beauty of the intense color displayed is enough to, almost, make me gasp.

But I can appreciate pale colors, too. In the right kind of setting, pastels are fine.

I actually love color pictures characterized by a rich palette of pastels. Check out Sebastian Colombo's lovely image of NGC 3576 (and NGC 3603). Note the pale pink color of the moderately nearby emission nebula NGC 3576, and the more salmon-colored, dust-reddened distant emission nebula NGC 3603. Note that the blue field stars are still blue, although they are a very pale shade of blue. Note the subtle hues of the starry Milky Way background.

I love it!

Ann