Starship Asterisk*

Hubble images of galaxies are glorious. Yet looking at them, I can't help but notice something. In (nearly) all of the pictures, there is 1, or maybe 2, featured galaxies, with dozens of red, small background galaxies scattered everywhere. The red, distant galaxies are around 1/20 of the main one, diameter-wise. This led me to question: where are the "middle ground" galaxies? The ones that should appear 1/4 or 1/5 as big as the featured ones. In other words, if the large, beautiful galaxies captured by Hubble are ~100mly away and the little, red ones are ~10Bly away, then where are the galaxies ~1Bly away? Could you show me a lot of images where all 3 types of distance are present?

The Tadpole galaxy is a good place to start. Let's first look at an overview of the entire galaxy, plus a large number of background galaxies, and then let's look at a closeup.


You are going to have to click on the image at right to see my annotations. I have pointed out a mid-distant galaxy (which is interacting with a tiny satellite galaxy), a group of very red, probably dusty, not obviously starforming galaxies, a group of small blue starforming galaxies and an intrinsically tiny spheroidal galaxy which is at the same distance as the Tadpole galaxy, and it is interacting with the Tadpole.

Remember that galaxies are intrinsically large or small, and they are intrinsically red or blue dependent on whether or not they are forming a lot of stars, a moderate amount of stars or no stars. A very distant small blue galaxy that emits a huge amount of ultraviolet light can look bluer than a more nearby galaxy that contains no blue stars at all. But a starforming galaxy that produces a lot of dust (which is a common by-product of star formation) may look redder due to all its dust than a dust-free elliptical galaxy made up of old yellow stars only.

Also take a look at the Hubble Deep Field:


There are no nearby galaxies in the Hubble Deep Field, but a lot of mid-distant ones and huge numbers of very distant galaxies.

Please note, too, that galaxies are not randomly distributed in space. They aggregate in clusters or groups, which is why we so often see galaxies that really appear to be physically close to one another. But there are also "jumps" or gaps where few or no galaxies appear to reside.

It's like surveying a country and counting the cities, towns and villages. The houses and buildings clump together here in these concentrated areas of human dwellings. But in between there are stretches of "emptiness", where few people seem to live.

Read about cosmic filaments and voids here.

Ann

Thank you Ann. That annotated pic is great. I know that there are filaments and voids in the universe, and that fact is related to the issue, but couldn't formulate it into words in the OP questions. Now, let's try it. Using common logic deduction: if in any random pic taken by Hubble, we see hundreds of background galaxies and only 1-2 foreground ones, then the mid-ground number should be somewhere between those values, i.e. dozens. I presume that in the universe we know, the number of observed faraway galaxies is on orders of magnitude larger than that of mid-distant ones, which in turn is orders of magnitude bigger than that of close galaxies. That leads to some questions:
- Is there any catalog on the internet that has a function which allows us to filter galaxies by distant? For example, 100mly < d < 1Bly. I want to test if my prediction is correct.
- Is it true that the reason we don't see many mid-ground galaxies in Hubble images (compared to fore- & background) is that they're harder to do research on? Therefore the astronomers who ask for Hubble time prefer to look at near, big-angular galaxies. The fact that in your very example, the Tadpole is carefully studied, while PGC 212515 is mostly unknown to scientists despite looking pretty decent on the picture, is a testament for that.

longtry wrote: ↑Wed Apr 13, 2022 9:06 am Thank you Ann. That annotated pic is great. I know that there are filaments and voids in the universe, and that fact is related to the issue, but couldn't formulate it into words in the OP questions. Now, let's try it. Using common logic deduction: if in any random pic taken by Hubble, we see hundreds of background galaxies and only 1-2 foreground ones, then the mid-ground number should be somewhere between those values, i.e. dozens. I presume that in the universe we know, the number of observed faraway galaxies is on orders of magnitude larger than that of mid-distant ones, which in turn is orders of magnitude bigger than that of close galaxies. That leads to some questions:
- Is there any catalog on the internet that has a function which allows us to filter galaxies by distant? For example, 100mly < d < 1Bly. I want to test if my prediction is correct.
- Is it true that the reason we don't see many mid-ground galaxies in Hubble images (compared to fore- & background) is that they're harder to do research on? Therefore the astronomers who ask for Hubble time prefer to look at near, big-angular galaxies. The fact that in your very example, the Tadpole is carefully studied, while PGC 212515 is mostly unknown to scientists despite looking pretty decent on the picture, is a testament for that.

As for your first question, if there is a catalog on the internet that allows us to filter galaxies by distance, I don't know, but I wouldn't think so. The reason is that there is such a huge number of galaxies in the Universe, and when it comes to mid-distance galaxies, not enough is known about the more or less exact distance to too many of them to include them in a catalog that filters galaxies by their distance. That's my two cents. The person you should ask is probably starsurfer, because he has a great knowledge of all kinds of catalogs. You can contact him here. Click PM: Send private message.

As for your second question, yes, I really think that little is known about most mid-distance galaxies. The reason is that there are so many of them - yes, there really are - and there is so much astronomy to be done, and there are so few suitable big telescopes available, and there is just one Hubble Space Telescope. Brutally put, I don't think astronomers have much time for individual scrutiny of most mid-distance galaxies.

But as for examples of mid-distance galaxies, there is a beautiful one in today's APOD (April 14, 2022):



Let's have a look at the background galaxies:

1 is a perfect example of a mid-distant galaxy, an edge-on disk galaxy perhaps similar to NGC 4565. (By the way, when you look at the picture of NGC 4565, you may notice a distinctive elongated blue galaxy at upper left. Is that a mid-distance galaxy? No, because that galaxy is too detailed - you can see its irregular dust lanes very clearly - and at the same time galaxy's edges are fuzzy, and it is too faint and blue, and it lacks a bright yellow nucleus. A galaxy that looks like that is intrinsically faint and small. But there is a spiral galaxy at lower left that is a mid-distance one.)

2 is a mid-distance elliptical galaxy. It has got a very bright inner region and a very big elongated halo of old stars. It resembles M86 in the Virgo Cluster.

3 is a mid-distance galaxy too, and it is also an elliptical galaxy. To me it looks intrinsically smaller than the galaxy that I have labeled 2.

4 is not a mid-distance galaxy. It is very faint. It has an obvious but still faint blue nucleus. Not only is this object faint, but for a faint object it is relatively large. No intrinsically bright and distant object can look like that. I would call it a nucleated dwarf galaxy that is probably at the same distance as M96.


Let's have a look at another galaxy with mid-distance background galaxies:


1 is a stunning-looking barred galaxy. It could possibly be a satellite galaxy of NGC 1309. That's because the galaxy's blue arms, resolved blue star clusters, pale yellow bar and non-reddened colors make it look remarkably similar to NGC 1309 in stellar content (but obviously extremely different in shape). It could therefore be at more or less the same distance as NGC 1309, but I don't think so. This galaxy has a too elegant shape, with an extremely well-established bar and long elegant arms, to be puny in mass. And if you compare the sizes of the largest blue clusters in NGC 1309 and in the barred galaxy to the upper left of it, you can see that the clusters in NGC 1309 are larger in size. I think, however, that the largest clusters in these two galaxies are comparable in size, so if the clusters in the smaller galaxy look smaller, that is because the smaller-looking galaxy is farther away. Besides, NGC 1309 itself is small, only 30,000 light-years in diameter, and it is impossible that such an elegant galaxy as the one I have labeled "1" could be so small that it could be a true satellite of diminutive NGC 1309. So, in short, "1" is a mid-distant galaxy. However, for a mid-distance galaxy, it is a relatively nearby one.

2 is definitely a mid-distance elliptical galaxy. Note its reddened colors. We don't expect elliptical galaxies to be yellower in color than the yellow centers of spiral galaxies. It is certainly true that we see "2" partly through the disk of NGC 1309, but I don't think that this would redden the background galaxy so much. No, "2" is a redshift-reddened elliptical galaxy, both larger, a lot more massive and considerably more distant than "1".

3 is a mid-distant spiral galaxy. Note its reddened colors compared with "1". I'd say that "3" is a more massive galaxy than "1". Note that "3" is dominated by its bright yellow center, whereas "1" is dominated by its arms. Bright yellow centers are always massive. I think that "3" could be at more or less the same distance as "2".

4 is quite distant. It is not a mid-distant galaxy. It is very reddened, and even though it is seen through an arm of NGC 1309, I'd say that redshift-reddening contributes a lot more to its color than dust-reddening. The galaxy is faint, too. But it has an elegant shape, and I think that it is intrinsically a good-sized galaxy.

Ann

Ann, with answers like these you sure deserve the title of Color Commentator. My appreciation. I almost can't believe you managed to incorporate even the latest APOD into it! Have to agree that 2a & 1b are the perfect examples of mid-distant galaxies. One can't go wrong with your analysis. BTW, is the 4a galaxy right under number "4"?
(Of course) Aside from JWST, do NASA, ESA or somebody else have any plans to launch some workhorse telescope to space in the future? I pray for Hubble, but it seems like the end of the journey is within a decade or 2.

And thanks for the message tip! I'll contact him.