George's Random Astronomical Object

Object 47: NGC 3504

Podcast release date: 17 May 2021

Right ascension: 11:03:11.2

Declination: +27:58:21

Epoch: J2000

Constellation: Leo Minor

Corresponding Earth location: Slightly less than 1000 km north of Wake Island in the Pacific Ocean

NGC 3504 is a galaxy located in the constellation Leo Minor at a distance of about 106 million light years (32.4 Mpc) [1], although this distance measurement isn't very well constrained. You may have already guessed that the name of the constellation Leo Minor means "little lion", but if you haven't, then I can tell you now that the name of the constellation means "little lion" [2]. The constellation is actually located just north of the constellation Leo in the Zodiac, and it was made up in 1687 by a Polish astronomer named Johannes Hevelius to occupy what was otherwise a relatively blank part of the sky [2,3]. The stars are so faint that only about six of them can be seen without a telescope (and notice that I did not say that the stars could be "easily seen without a telescope").

Anyway, back to NGC 3504. This could be described as just a spiral galaxy, but that would be a little oversimplistic. All spiral galaxies have disks of stars and gas that contains the spiral patterns that we associate with spiral galaxies, and many of these galaxies also have spherical bulges of stars in their centers. However, the size of the bulge relative to the disk varies from galaxy to galaxy, and different spiral galaxies contain different types of spiral structures. In the case of NGC 3504, a lot of different things have been found in this galaxy's disk.

To begin with, the disk contains what is known as a bar [4]. Some people, when they hear the word "bar", might think of a long cylindrical object, and indeed the galaxy contains such a cylindrical structure running through its center. Other people, when they hear the word "bar", might think of a business establishment that sells alcohol, but it is unclear whether NGC 3504 contains such a bar in its center as astronomers have not yet detected any alien civilization in this galaxy able to issue alcohol sales licenses.

Back to the bar that is the long cylindrical structure. This bar is roughly 40 thousand light years long. To put that into context, the distance from the Earth to the center of the Milky Way is somewhere between 25000 and 30000 light years, and NGC 3504 itself is a little over 80 thousand light years in diameter. The bar is basically a structure made out of stars that exert gravitational forces on themselves to stay in the bar shape. As I mentioned before, the structure runs through the center of the galaxy, and the spiral arms are attached to the ends of the bar. The stars in the bar still rotate around the center of the galaxy even though they stay inside the bar, and the bar itself also spins around the center.

Bars like the bar in NGC 3504 exert strong gravtational forces on interstellar gas. In a typical unbarred spiral galaxy, both stars and interstellar gas clouds would basically travel in circles around the center of the galaxy, but when interstellar gas clouds encounter a bar in a typical barred galaxy, the bar can potentially divert the gas clouds either inwards towards the center of the galaxy or outwards to the ends of the bar. The bar in NGC 3504 has actually done both of these things. The gas that was driven outwards formed a patchy ring-like structure attached to the ends of the bar. This gas then formed stars that trace the same ring structure.

The gas that fell inwards, however, did something a bit more complicated. It actually formed a small, disk-like structure that looks like a miniature spiral galaxy with miniature spiral arms [5]. It even contains a mini-bar (or, technically, an inner bar) about 3000 light years in size and a small ring-like structure that encompasses the mini-bar [5]. (The mini-bar, however, does not contain any overpriced drinks, like a smaller-than-average can of Coke or tiny bottles of wine or vodka.) This type of overall miniature spiral structure in the center of this galaxy is called a pseudobulge. In older astronomy images, especially photographs from the previous century, structures like the one in the center of NGC 3504 would have appeared overexposed and therefore would have looked like a three-dimensional spherical bulge of stars like we see, for example, in the Andromeda or Sombrero Galaxies. It was only when astronomers started using digital detectors about 30 years ago that they began to discover that some of these "bulges" (in air quotes) in the centers of some galaxies were not smooth spherical objects but actually had complex spiral structures. Hence, they are called pseudobulges, or "fake bulges".

The mini-bar in the psuedobulge functions very similarly to the larger bar in NGC 3504, and it's also funneling interstellar gas from the disk of the psuedobulge inwards to the nucleus. The nucleus does not contain a third even smaller spiral structure or a third even smaller bar (at least as far as I know), but it does contain a supermassive black hole. A group of astronomers led by Dieu Nguyen have measured the rotation of the gas in the very inner part of the mini-bar to determine that the mass of this supermassive black hole is 16 million times the mass of the Sun [1]. That sounds large, but actually, it's kind of an average size as far as supermassive black holes go.

So, that covers the inner parts of NGC 3504, but I still have one thing to mention about the outer disk. As I said before, the galaxy's spiral arms are attached to the ends of the larger bar. When these arms reach the edge of the galaxy's disk, they wrap around in such a way that they look like they form a ring [3]. A lot of galaxies are encircled by rings of stars and/or gas, but the ring in NGC 3504 isn't quite the same, so it gets called a psuedoring (which actually has nothing to do with the pseudobulge at the center of the galaxy).

So, to summarize, NGC 3504 is a spiral galaxy with three ring-like structures, two bars, a pseudobulge, a supermassive black hole, and no licensed drinking establishments (at least as far as we know).

References

[1] Nguyen, Dieu D. et al., The MBHBM* Project. I. Measurement of the Central Black Hole Mass in Spiral Galaxy NGC 3504 Using Molecular Gas Kinematics, 2020, Astrophysical Journal, 892, 68

[2] Ridpath, Ian, Star Tales: Revised and Expanded Edition, 2018

[3] Ford, Dominic, The Constellation Leo Minor, 2021, In-The-Sky.org

[4] Buta, Ronald J. et al., The de Vaucouleurs Atlas of Galaxies, 2007

[5] Wu, Yu-Ting et al., Morphological and kinematical analysis of the double-barred galaxy NGC 3504 using ALMA CO (2-1) data, 2021, Monthly Notices of the Royal Astronomical Society, 504, 3111

Podcast and Website: George J. Bendo

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© George Bendo 2021. See the acknowledgments page for additional information.

Last update: 16 May 2021