Object 30: NGC 3621

Podcast release date: 21 September 2020

Right ascension: 11:18:16.3


Epoch: ICRS

Constellation: Hydra

Corresponding Earth location: 339 km northwest of the North Island of New Zealand and 444 km south-southeast of Norfolk Island

NGC 3621 is actually a galaxy that I have done a lot of scientific research on. The galaxy, which is located in the constellation Hydra, is classified as a spiral galaxy. A typical spiral galaxy has a spherical bulge of stars in its center and a disk of stars that contains spiral structures. However, NGC 3621 does not have a bulge. It only has a disk [1]. Moreover, the disk does not contain very sharply-defined spiral arms but instead has a series of wispy filamentary structures [2]. Astronomers use the technical term flocculent to describe galaxies with this type of spiral structure.

NGC 3621 is actually bright enough to be seen with an amateur astronomy telescope. Don't ask me how to find it. The stars in this part of the sky don't seem to form any distinct shapes, and I've never looked at it through a telescope myself. One of my reference described NGC 3621 as looking like a "bright oval" [3], which sounds like what I would expect.

As a professional astronomer, I have been working with this galaxy for about 18 years. A lot of my research has focused on interstellar dust, which absorbs starlight in the ultraviolet and visible parts of the spectrum and re-radiates the energy as infrared light. Since interstellar dust tends to be found in dense nebulae where stars are forming, a lot of infrared radiation comes from dust absorbing the light from very young stars. However, I found that, in NGC 3621 and a few other spiral galaxies with no bulges and wispy spiral filaments, a significant fraction of the far-infrared radiation appears to come from dust absorbing starlight from old stars, although my results for NGC 3621 are not quite as clear as for other galaxies [4]. I haven't quite figured out exactly what's happening in these galaxies, but at the moment, I think my results might have something to do with the overall structures of the galaxies or the amount of interstellar dust within the galaxies.

As a side note, NGC 3621 has been a little difficult for me to work with because two nearby stars within our own galaxy fall between us and the southern half of NGC 3621. In the mid-infrared, these stars are very bright and make it difficult to actually analyze the infrared emission from the galaxy itself, so a lot of my analysis has been restricted to the northern and central parts of the galaxy.

In any case, this galaxy may be more interesting for reasons that don't necessarily involve the interstellar dust. In 2007, a group of astronomers working with infrared data from the Spitzer Space Telescope identified the presence of highly ionized neon in the center of NGC 3621 [5]. (Yes, that's the same type of neon found in neon lights here on Earth.) That highly ionized neon was the first hint that NGC 3621 contains a supermassive black hole. The black hole itself does not produce light, but gas falling into the black hole gets highly compressed by gravity, and that makes it really hot and causes it to radiate a huge amount of electromagnetic radiation. The ionized neon in the centre of NGC 3621 is associated with this really hot gas.

Observations with the Chandra X-ray Observatory found a point-like X-ray source at the centre of NGC 3621 that could only come from the environment around a supermassive black hole, thus confirming that it was there [6]. In some Hubble Space Telscope images previously used for completely different purposes (one of which I will describe in a couple of minutes), astronomers found a small cluster of stars orbiting the potential black hole and used that to estimate its mass, which is about 3 million times the mass of the Sun [7].

This discovery is particularly important because it is a supermassive black hole found at the center of a galaxy that has no bulge. Typically, astronomers find supermassive black holes in the centers of galaxies with stellar bulges, and the mass of the black hole is known to be directly proportional to the mass of the bulge [8]. This relation indicates that the growth of galaxies' bulges and the growth of galaxies' central supermassive black holes are somehow interconnected, although it's not quite clear how. However, NGC 3621 has no stellar bulge, so it does not quite fit into this relation very well. It's also not the only galaxy with a supermassive black hole but without a bulge, although it's a really good example of one. Astronomers are still trying to figure out exactly what's going on with these specific black holes, which means that they will be spending much more time observing NGC 3621 in the future.

Now for a completely different topic. NGC 3621 is close enough to Earth that it is possible to identify the individual stars within the galaxy. This is important for being able to measure distances to NGC 3621. In our own galaxy, we can use a geometrical technique called parallax (which I am not going to describe in detail right now) to measure the distances to individual stars. If we know the distances to specific types of stars in our galaxy and if we measure the apparent brightnesses of these types of stars as seen from Earth, then we can determine how much power is emitted by these stars. Astronomers use the term luminosity to refer to the total power emitted by any astronomical object. If we see these types of stars in other galaxies and compare the apparent brightnesses of these stars to the luminosities that we expect these stars to have, we can calculate the distances to these galaxies.

So, as I said, NGC 3621 is close enough that we can identify some specific types of individual stars in the galaxy. Back in 1997, the Hubble Space Telescope was used to identify stars called Cepheids within NGC 3621. A Cepheid is a variable star where the period of the variation in the star's brightness is directly related to the star's luminosity, which means that a comparison of a Cepheid's variation in brightness to how its average brightness looks from Earth can be used to estimate the distance to the star. A total of 69 Cepheids were initially found with the Hubble Space Telescope in NGC 3621, and the distance to the galaxy based on those stars is 21.7 million light years (6.64 Mpc) [9,10], although a lot of people have spent a lot of time re-analyzing these measurements and often get slightly different answers.

However, astronomers have also been able to identify both the brightest red giant stars and blue supergiant stars within NGC 3621. These types of stars are a little more tricky to identify, in part because they don't blink like Cepheids do, and it requires a little more work to figure out accurate luminosities for these stars, but they can still be used to make accurate distance measurements to other galaxies. The brightest red giant stars give a distance of 23.2 million light years (7.11 Mpc) [11], while the blue supergiants give a distance of 21.3 million light years (6.52 Mpc) [12]. If we take the average of these measurements and the one from Cepheids, we get 22.0 million light years (6.76 Mpc), and we could probably say that the average has an accuracy of about a few percent.

These measurements of highly accurate distances to other galaxies may sound like some sort of esoteric technical competition that astronomers participate in so that they can win free trips to Hawaii or South America, complete with a hotel room and three free meals a day. However, these types of distance measurements are extremely important scientifically for calibrating the various other techniques used to measure distances to galaxies where we can't see the individual stars, and the measurements are also important for studying the expansion of the Universe. This expansion is actually seen by looking at the Doppler shifting of light from other galaxies, which, including NGC 3621, all seem to be moving away from our own galaxy. However, we know that more distant galaxies are moving away faster than nearby ones. We expect, at least for galaxies within a couple hundred million light years of Earth, that the ratio of the velocity at which the galaxy is moving from Earth to its distance should be constant, so if we can measure that ratio for nearby galaxies like NGC 3621, we can accurately estimate the distances to more distant galaxies using measurements of their velocities. This is why all of these highly accurate distance measurements are so important.


[1] de Vaucouleurs, Gerard et al., Third Reference Catalogue of Bright Galaxies, 1991

[2] Elmegreen, Debra Meloy and Elmegreen, Bruce G., Arm Classifications for Spiral Galaxies, 1987, Astrophysical Journal, 314, 3

[3] Eicher, David J., The Universe from Your Backyard, 1988

[4] Bendo, G. J. et al., The identification of dust heating mechanisms in nearby galaxies using Herschel 160/250 and 250/350 μm surface brightness ratios, 2015, Monthly Notices of the Royal Astronomical Society, 448, 135

[5] Satyapal, S. et al., The Discovery of an Active Galactic Nucleus in the Late-Type Galaxy NGC 3621: Spitzer Spectroscopic Observations, 2007, Astrophysical Journal Letters, 663, L9

[6] Gliozzi, Mario et al., A Chandra View of NGC 3621: A Bulgeless Galaxy Hosting an AGN in Its Early Phase?, 2009, Astrophysical Journal, 700, 1759

[7] Barth, Aaron J. et al., Dynamical Constraints on the Masses of the Nuclear Star Cluster and Black Hole in the Late-Type Spiral Galaxy NGC 3621, 2009, Astrophysical Journal, 690, 1031

[8] Kormendy, John and Ho, Luis C., Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies, 2013, Annual Reviews of Astronomy and Astrophysics, 51, 511

[9] Rawson, Daya M. et al., The Extragalactic Distance Scale Key Project. VIII. The Discovery of Cepheids and a New Distance to NGC 3621 Using the Hubble Space Telescope, 1997, Astrophysical Journal, 490, 517

[10] Freedman, Wendy L. et al., Final Results from the Hubble Space Telescope Key Project to Measure the Hubble Constant, 2001, Astrophysical Journal, 553, 47

[11] Rizzi, Luca et al., Tip of the Red Giant Branch Distances. II. Zero-Point Calibration, 2007, Astrophysical Journal, 661, 815

[12] Kudritzki, Rolf-Peter et al., Stellar Metallicity of the Extended Disk and Distance of the Spiral Galaxy NGC 3621, 2014, Astrophysical Journal, 788, 56


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: felix.blume, Infernus2, ivolipa, jameswrowles, metrostock99, newagesoup, slumbermonkey, and speedyrce at The Freesound Project

Image Viewer: Aladin Sky Atlas (developed at CDS, Strasbourg Observatory, France)