BAN #407: A gorgeous but weird galactic mystery

07 March 2022 Issue #407

March 07, 2022

Blog Jam

[Detail of the DKIST image of AR12822 at full resolution. From Wednesday’s article. Photo: NSO/AURA/NSF]

Pic o’ the Letter

A cool or lovely or mind-bending astronomical image/video with a short description so you can grok it

It’s been a while since I’ve posted a gorgeous deep sky shot by my friend Adam Block, so let’s fix that now, shall we?

[The NGC 128 Group. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona in collaboration with the Chart32 team for providing additional color data.]

And very, very interesting.

This is called the NGC 128 Group, a small clump of galaxies that are more or less at the same distance. Or are they…?

The bigger galaxy to the upper left is NGC 128, a nearly edge-on disk galaxy (here’s the same image with the galaxies labeled, which is helpful). Spiral galaxies are a subset of disk galaxies, and they all have a central bulge of stars, which is usually shaped like a flattened sphere. But things can get complicated.

In our solar system, where the Sun dominates the mass and sits in the center, planetary orbits are relatively simple. They can be circles, or ellipses, and some comets can have parabolic or hyperbolic orbits. But always conic sections, if you remember those from high school geometry.

But when you have a lot of mass that’s spread out, like a few billion stars in the center of the galaxy, orbits can be weirder. They can change, both in shape and orientation over time. I wrote about this a while back, but in the end stars orbiting near the center of a disk galaxy can wind up making the bulge elongated, like a lozenge or a Tic Tac.

Even weirder, you can get orbits at an angle to each other, and from some angles that can look like the bulge is peanut shaped or even a big X. Our Milky Way has one of those! It’s not common, but it happens.

Clearly, NGC 128 has one too. I’ve never seen one so obvious before! Very cool.

But when you look outside NGC 128, things get really weird.

You can see two smaller galaxies near it; NGC 130 above it and NGC 127 to the lower right. The latter is a mess, and it’s clearly undergoing an interaction with NGC 128. The gravity of the much bigger galaxy has distorted it and torn out a streamer of gas, dust, and stars; you can see the dust silhouetted in front of NGC 128. That’s a smoking gun.

[An inverted (negative) image of the NGC 128 group, showing much fainter detail. Credit: Adam Block/Mount Lemmon SkyCenter/University of Arizona]

Adam links to a negative version of this as well (at a slightly smaller scale) which is a much deeper exposure, and you can see streamers all over the place. Clearly NGC 128 is busy disrupting other galaxies. The gravitational interactions with those smaller galaxies may be why it has a peanut bulge, too, from them poking and prodding at its stars.

But now look to the lower right in the original color image. That more face-on spiral is NGC 125. It presents a problem. It looks like it’s part of the group, but its redshift is significantly higher than the others’.

The Universe is expanding, and sweeping galaxies along with it. As they move away from us, their light is shifted to longer wavelengths, what we call a redshift. The faster it moves away, the more the wavelengths are stretched. We can measure the velocities of the galaxies by how much the wavelengths are redshifted, and for the four galaxies (including small spiral NGC 130 there toward the left) you get a velocity of around 4000 – 4400 kilometers per second.

But NGC 125 sticks out; its velocity is 5300 km/sec. That’s a big difference.

Normally we’d use that to calculate a distance, and if you do the four galaxies are about 190 – 200 million light years away. But NGC 125 is 250 million light years away. That’s a big difference, and if true it’s not part of the group.

But I wonder. Look at the inverted image again. NGC 125 also has what looks like streamers pulled out of it, and you can just barely see them in Adam’s original image as well. That makes me wonder if it interacted with NGC 128 as well, and maybe got flung away from us. Now, that extra 1,000 km/sec is a lot, and way more than you’d expect from a galactic collision. But if it’s not from a collision it’s pretty weird that it appears so close to those others, is about the same size, but is 50 million light years farther away. That’s way too far to be a part of the same group (the Milky Way is part of a small clutch of galaxies called the Local Group, and that whole thing is less than 10 million light years across).

It doesn’t make sense to me. It’s close to the others but not that close, but farther yet not that much farther.

It could be coincidence, or it could mean something peculiar happened. I have no idea. There’s a technical paper about NGC 128’s bulge that mentions 125 as well, but they don’t say much if anything about it except the redshift is weird.

I don’t have anything more to add here scientifically. Some things are just mysteries until someone figures things out, and so hopefully some astronomer somewhere will see something about this weird group and come up with observations that will help unravel this. Until then, ¯\_(ツ)_/¯.

Et alia

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