George's Random Astronomical Object

Object 92: NGC 4125

Podcast release date: 20 February 2023

Right ascension: 12:08:06.0

Declination: +65:10:27

Epoch: J2000

Constellation: Draco

Corresponding Earth location: Within the Gulf of Anadyr slightly more than 30 km off the southern shore of the Chukchi Peninsula at the eastern end of Russia

For the most part, NGC 4125 is a seemingly ordinary-looking elliptical galaxy located at a distance of 63.6 million light years (19.5 Mpc) [1] in the constellation Draco. As suggested by the galaxy's classification, it's a giant ellipsoidal collection of stars. At a magnitude of about 9.7 [2] and with a diameter of 3.3 arcminutes (or slightly over one tenth the diameter of the Moon) [3], it's actually something that can be seen in a medium-sized amateur telescope. If you want to go find it, it kind of forms the vertex of a right angle with the two stars in the top of the cup part of the Big Dipper. If you don't want to find it, then I recommend moving to Tierra del Fuego, where you will never be able to see it. I didn't find much information on what it should look like in an amateur telescope, maybe because it's a seemingly ordinary-looking elliptical galaxy, but I basically expect it to look like a fuzzy oval thing.

Anyhow, desipte NGC 4125's seemingly-ordinary appearance, it is actually a vampire ready to prey on other mortal galaxies and... no, wait, I've been reading too many horror-themed webcomics following the everyday lives of vampires and other members of the undead. Let's start over.

Anyhow, despite NGC 4125's seemingly-ordinary appearance, it actually contains a lot of interstellar dust for an elliptical galaxy [4]. It also appears to be an extremely rare example of a galaxy that contains cold interstellar dust without the accompanying cold interstellar gas, which is weird [5].

The classical view of elliptical galaxies is that the interstellar medium in these galaxies is mostly hot, ionized, X-ray emitting gas with no interstellar dust whatsoever, and I can say based on my experience with making infrared images of interstellar dust in other galaxies that I have seen quite a few elliptical galaxies with no interstellar dust. However, a substantial amount of research in the past 20 years or so has demonstrated that a subset of elliptical galaxies contain really small disks of cold molecular gas in their centers [6]. It's quite common to find interstellar dust intermixed with cold interstellar gas within many different types of galaxies, including elliptical galaxies with molecular gas discs [6]. However, interstellar dust tends to disintegrate in environments that are really hot, like the X-ray gas found in elliptical galaxies without central molecular gas disks.

NGC 4125 stands out in that it contains a lot of interstellar dust but no cold interstellar gas to go with that dust, which is a little weird [5]. This leads to the question of why. Well, to begin with, it looks like another galaxy merged with NGC 4125 about 6 to 8 billion years ago, producing a lot of stars in the center of the galaxy [7]. Today, those stars would have evolved and expanded into large red stars, and while some of these could be called "red giants" by both lay people and professional astronomers, the professionals use the technical terms "horizontal branch star" and "asymptotic giant branch star" to describe some of these large red stars. (I'm not going to get into what these names mean in this episode, but if you want detailed information on these types of stars, I recommend listening to episodes 72 and 84.)

In any case, the asymptotic giant branch stars are stars near the ends of their lives with inert carbon cores, with helium fusing into carbon in shells around those cores, and with hydrogen fusing into helium in shells around all of that. Dust and molecular gas gets formed in the outer layers of these stars in NGC 4125 and then gets blown away into the galaxy's interstellar medium. So, the elliptical galaxy can currently generate its own interstellar molecular gas and dust. However, it looks like the cold interstellar dust can last longer than the cold interstellar molecular gas when both of those things are injected into the hot X-ray emitting gas in NGC 4125 [5]. In other words, the cold molecular gas gets heated up into hot ionized interstellar gas before the interstellar dust gets destroyed by the hot ionzed gas in the galaxy, so the galaxy will appear to have an abnormally large amount of dust and relatively little cold molecular gas to go with that dust.

Astronomers are still working out the details about what happens to interstellar dust in elliptical galaxies in general, as it should eventually all be destroyed. NGC 4125 appears to represent a very rare example of a galaxy with substantial amount of cold interstellar dust but no accompanying cold interstellar gas, and it will be key in understanding the processes that destroy interstellar dust.

I also wanted to mention that I actually played a rather unique role in the observations that produced these science results. In my first position after getting my PhD, I worked with the Spitzer Infrared Nearby Galaxies Survey, which used the Spitzer Space Telescope to survey the infrared emission from warm and hot interstellar dust in a set of 75 nearby galaxies that included NGC 4125. After this, I worked with several different groups using the Herschel Space Observatory to survey the infrared emission from colder interstellar dust in many different samples of nearby galaxies, including one survey called the Very Nearby Galaxies Survey led by Christine Wilson. This survey covered only 12 galaxies, including some galaxies that are very well known to both amateur and professional astronomers, such as the Whirlpool Galaxy, Messier 81, and Messier 83; some galaxies that are very famous among professional astronomers but not as well known by the general public, including Arp 220 and NGC 4151; and a few other galaxies to round out the sample, including NGC 891 (a galaxy that is an inside joke among astronomers and that I discussed in episode 42).

Chris wanted to add an elliptical galaxy from the Spitzer Infrared Nearby Galaxies Survey to the Very Nearby Galaxies Survey and originally chose the elliptical galaxy NGC 1404, but I persuaded the team to not look at that galaxy with the Herschel Space Observatory. The images of NGC 1404 from the Spitzer Space Telescope that I had made showed that most of the far-infrared emission came from an off-center infrared source, and I was worried that the infrared source was not something in NGC 1404 itself but actually a more distant galaxy behind NGC 1404. If we had pointed the Herschel Space Observatory at NGC 1404 and if the infrared source was indeed a galaxy in the background, we would not have learned anything about dust in that elliptical galaxy. When I explained this to everyone in the collaboration, Chris selected NGC 4125 in place of NGC 1404. I personally felt like observing NGC 4125 was a safer choice because, in the images that I had made from the Spitzer Space Telescope, the infrared emission came from the center of the galaxy and therefore seemed more likely to be associated with the galaxy itself.

NGC 4125 turned out to be a very good choice, as the combination of observations from the Spitzer Space Telescope and the Herschel Space Observatory led to the interesting results about the interstellar dust in the center of NGC 4125 that were published by Chris herself and that I just described in this episode.


[1] Tully, R. Brent et al., Cosmicflows-2: The Data, 2013, Astronomical Journal, 146, 86

[2] Gil de Paz, Armando et al., The GALEX Ultraviolet Atlas of Nearby Galaxies, 2007, Astrophysical Journal Supplement Series, 173, 185

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

[4] Smith, M. W. L. et al., The Herschel Reference Survey: Dust in Early-type Galaxies and across the Hubble Sequence, 2012, Astrophysical Journal, 748, 123

[5] Wilson, C. D. et al., Cold Dust but Warm Gas in the Unusual Elliptical Galaxy NGC 4125, 2013, Astrophysical Journal Letters, 776, L30

[6] Young, Lisa M. et al., The ATLAS3D project - IV. The molecular gas content of early-type galaxies, 2011, Monthly Notices of the Royal Astronomical Society, 414, 940

[7] Schweizer, Francois and Seitzer, Patrick, Correlation Between UBV Colors and Fine Structure in E and SO, 1992, Astronomical Journal, 104, 1039

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: afleetingspeck, Cbeeching, ehohnke, ivolipa, jameswrowles, metrostock99, NachtmahrTV, newagesoup, and strangehorizon at The Freesound Project

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


© George Bendo 2023. See the acknowledgments page for additional information.

Last update: 1 March 2023