Object 42: NGC 891

Podcast release date: 08 March 2021

Right ascension: 02:22:32.9


Epoch: ICRS

Constellation: Andromeda

Corresponding Earth location: Slightly more than 50 km northeast of Sinop, Turkey, in the Black Sea

NGC 891 is a spiral galaxy located at a distance of about 31.3 million light years (9.6 Mpc) in the constellation Andromeda [1, 2, 3]. If you are unfamiliar with the constellation Andromeda, it looks like a warped V, with the vertex of the V attached to the constellation Pegasus, which looks like a square. NGC 891 is located east of Gamma Andromedae, which is at the tip of the east part of the V shape. Also, it's worth mentioning that the most famous galaxy in the constellation Andromeda is the Andromeda Galaxy, but that galaxy has nothing to do with NGC 891 and is a subject for another podcast.

In some sense, NGC 891 is a relatively ordinary-looking nearby spiral galaxy. It's actually similar to the Milky Way, but then a lot of other galaxies are like the Milky Way. It actually looks very nice in astronomical images in the visible part of the electromagnetic spectrum. The stellar disk is viewed edge-on, and the symmetric disk with the small bulge in the center is bisected by a dark disk of interstellar gas and dust. However, NGC 891 is more than just a pretty nearby galaxy. Because NGC 891 is relatively nearby, because it is seen edge-on, and because it is symmetric, many types of sophisticated scientific analyses can be done relatively easily with this galaxy.

One of the important features of this galaxy is that disk of interstellar gas and dust that I mentioned earlier. Because we are looking through the edge of this thin disk of dust, it's easy to make measurements of how that interstellar dust either absorbs or scatters the light from the stars in the galaxy. When that interstellar dust absorbs light in the ultraviolet and visible parts of the electromagnetic spectrum, it warms up a little and then re-radiates that energy as infrared radiation. All of that infrared radiation, as seen from Earth, comes from a very narrow region on the sky, so the infrared radiation ends up looking very bright. Because of these things, astronomers often go look at NGC 891 first if they want to understand interstellar dust [4, 5, 6, 7, 8, 9, 10, 11, 12]. In fact, many astronomers have used NGC 891 very specifically to develop rather sophisticated computer models that describe how the interstellar dust absorbs starlight and re-radiates that energy in the infrared [13, 14, 15, 16, 17].

Now, my research is primarily focused on interstellar dust and star formation in nearby galaxies, and while I have published some images of the infrared light from the interstellar dust in NGC 891 (which you can download from the web), I haven't personally done much research specifically on NGC 891 itself. However, I have attended a few conferences on interstellar dust, and what is rather eerie is that, even though I normally show up with something like science results on M81 or the Sombrero Galaxy, everyone else at the meeting, all of who are working independently from each other, will just happen to show up with various research results involving NGC 891. These meetings last a few days, so imagine three or four days worth of talks specifically about interstellar dust in NGC 891. It often reaches the point where, by the third day, someone could just show an image of NGC 891, and the room would break into laughter. It would be like if I added a laugh track to this podcast every time I mention NGC 891. Maybe this is too much of an inside joke, but it does illustrate how important NGC 891 is to infrared astronomy.

The other important thing about NGC 891 is that, because it is a nearby spiral galaxy that is viewed almost exactly edge-on, it is an excellent place for astronomers to measure the vertical structure within a typical spiral galaxy. (This naturally means that it is useless in terms of studying spiral structure in the disk, but that doesn't matter.) As I indicated before, NGC 891 has not only the thin disk of gas and dust but also a thicker disk of stars. Those stars would have initially formed inside the thinner disk of gas and dust, but over time, their orbits get altered, and they end up orbiting the galaxy outside of the thinner disk. This is actually something that happens in our galaxy and other spiral galaxies as well, but it's much easier to see this in NGC 891 [4, 13, 18, 19, 20, 21, 22]. Also, the gas and dust does not always stay inside the thinner disk. Sometimes, when supernovae explode in the disk, this causes gas and dust to get blown out of the plane of the galaxy, and many astronomers have spent a lot of time looking at this phenomenon specifically in NGC 891 [23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35]. I myself am even a co-author on a paper discussing interstellar dust that made its way outside the thinner dust disk [34].

Other people have specifically observed NGC 891 because it is possible to study the galaxy's halo, which is a relatively dark, roughly spherical structure that contains very few stars and very little interstellar gas but that encompasses the whole of the disk of the galaxy. When looking at a spiral galaxy that is oriented face-on so that we can see the spiral structures in its disk, it is impossible to distinguish anything in the galaxy's halo above the disk because the disk is so bright in comparison. In NGC 891, however, it's much easier to see things in the halo because we can just look on either side of the edge-on disk. Therefore, if astronomers want to understand what the interstellar gas or stars or globular clusters are like in a typical spiral galaxy's halo, they will go look at NGC 891 [3, 23, 24, 25, 26, 27, 28, 29, 32, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58].

While most astronomers are interested in looking at NGC 891 because they can spend time looking at its disk of dust or looking at how the galaxy is structured vertically, I can also mention a couple of other minor scientific results regarding the galaxy. First, NGC 891 might contain a bar-shaped structure in its center. This is very difficult to actually see in an edge-on galaxy because the bar lies within the plane of the galaxy, but the bulge as seen from the side looks kind of box-shaped, which implies that a bar is actually there [59]. The second interesting fact is that NGC 891 is the second brightest galaxy in a group of galaxies called the NGC 1023 Group [60], which is named after the group's brightest galaxy, the lenticular galaxy NGC 1023. The group also contains seven other spiral galaxies and numerous other dwarf elliptical and irregular galaxies as well. In fact, because dwarf galxies are often found in the halos of spiral galaxies, a lot of people have spent time searching for dwarf galaxies in the halo of NGC 891 [61].

Finally, if you enjoy stargazing, NGC 891 is a galaxy that can be seen in an amateur astronomy telescope. As I indicated before, the galaxy is located to the east of the star named Gamma Andromedae at the east tip of the V-shape in the constellation Andromeda. NGC 891 can actually be located by looking halfway between that star and the bright open cluster M34. Now, this is not a galaxy that I have seen in an amateur telescope myself, so I am relying on what other people have said about observing it. Based on my references, the galaxy will not look like much in a small telescope with a diameter of about 10 cm (4 inches) or less, but in a 20 cm (8 inch) telescope, it is possible to begin to see the disk with the bulge in its center bisected by the dust disk [62, 63]. However, the galaxy is sufficiently faint that you should really try to view it in a larger telescope if you can.


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[45] Howk, J. Christopher and Savage, Blair D., The Multiphase Halo of NGC 891: WIYN Hα and BVI Imaging, 2000, Astronomical Journal, 119, 644

[46] Rossa, Jörn et al., A Hubble Space Telescope WFPC2 Investigation of the Disk-Halo Interface in NGC 891, 2004, Astronomical Journal, 128, 674

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