George's Random Astronomical Object

Object 88: NGC 6052

Podcast release date: 26 December 2022

Right ascension: 16:05:12.9

Declination: +20:32:33

Epoch: J2000

Constellation: Hercules

Corresponding Earth location: About 930 km west-southwest of the southern tip of Baja California

As seen from Earth, NGC 6052 looks like a spiral galaxy that has smashed into a wall of stars. This is rather close to what is actually happening, as NGC 6052 is a pair of spiral galaxies that are in the process of merging. One of those galaxies is seen face-on, while the other galaxy, which looks like the "wall of stars", is seen edge-on. These days, we have a decent understanding of the basics of what happens when two spiral galaxies merge together, but the scientific literature for this object extends back several decades to a time when galaxies and mergers were not as well understood, and it's kind of interesting to look at what those scientific papers were saying. So, in this episode, we are going on an adventure into scientific history with NGC 6052. Cue some sort of sound effect.

To begin with, the "NGC" in the name "NGC 6052" (as well as in the names of a lot of other objects featured in this podcast series) refers to the New General Catalogue. This is a catalog of "nebulae and clusters of stars" which was created by John Dreyer in 1888 [1]. This was an update of the older General Catalogue of Nebulae created by John Herschel in 1864 [2], which is why the New General Catalogue is called "new", even though it is over 130 years old at this point. The New General Catalog or NGC designations have stuck over time, so many people who work with these objects typically refer to them by their NGC numbers rather than anything else. Also, I just want to point out that the New General Catalogue listed things that basically looked fuzzy in nineteenth century telescopes, and while it advertises itself as a catalog of "nebulae and clusters of stars", many of the objects listed in the catalog are entire galaxies. When the New General Catalogue was created, astronomers did not know that these galaxies were outside our own galaxy; they thought that they were star clusters or nebulae within the Milky Way.

NGC 6052 was, as you would expect, the 6052nd object listed in the New General Catalogue. As best as I can tell, it looks like it was originally discovered by Albert Marth in the mid-1800s before it was included in the New General Catalogue [3]. The entry for NGC 6052 lists its description as a series of unintelligible abbreviations that basically indicate that the object looked faint, pretty large, and irregularly round (whatever that means) [1].

As far as I can tell, people largely ignored NGC 6052 for the next few decades. Then came Halton Arp in the 1960s. Arp was trying to come up with come up with evidence for an alternative to the Bing Bang theory for why we observe the expansion of the universe, which I'm not going to discuss here but which I will say does not make much sense if you think too hard about it, and most people now ignore his alternative descriptions of cosmology. However, in 1966, Arp created the Atlas of Peculiar Galaxies to try to support his hypotheses [4]. This atlas was a collection of astronomical photographs of interacting and otherwise strange-looking galaxies that astronomers found to be a very useful reference even if they didn't buy into Arp's alternative version of cosmology. Some galaxies in this atlas are actually quite famous and known by the designations given in this atlas. NGC 6052 was listed in this atlas as an example of a galaxy with "irregularities, absorption, and resolution" (whatever that means), and it was given the designation Arp 209 [4].

Soon after this, Benjamin Markarian detected NGC 6052 in his survey of the sky at ultraviolet wavelengths and included the object in his catalogs of galaxies with strong ultraviolet emission. The object was given the alternative designation of Markarian 297 (Mrk 297) [5]. Because the excessive ultraviolet emission from the galaxies in Markarian's catalogs looked so unusual, a lot of people spent a lot of time studying those galaxies in detail, although it took people several years to identify how the ultraviolet emission was produced.

Following the publications of both Arp's and Markarian's catalogs, people knew that NGC 6052 was some sort of weird, ultraviolet freak of nature, and they began to look at NGC 6052 much more frequently. However, it wasn't until 1979 that anyone (or, more specifically, Danielle Alloin and Renee Duflot) published a paper suggesting that NGC 6052 was actually two galaxies merging together [5], thus explaining why the object looked so abnormal that it made it into the Atlas of Peculiar Galaxies. Additionally, they came to the conclusion that this merger event had caused the interstellar gas clouds in these two galaxies to collide with each other, and this caused the gas clouds to collapse and form lots of new stars, including really big, really hot blue stars with very short lifespans that produce lots of ultraviolet emission [5]. This explained why NGC 6052 appeared in Markarian's survey of ultraviolet sources. While, by today's standards, this seems like a simple, straightforward explanation for what is happening in NGC 6052, this new theory was a major breakthrough in 1979.

In the next 15 years after Alloin and Duflot's publication, NGC 6052 would continue to attract a lot of attention as what was at the time seen as a unique case of two galaxies merging together. However, it seemed like some people were a little hesitant to refer to this object as a merger. I found a few papers that called it a "clumpy irregular galaxy" [6, 7, 8, 9, 10], which is an accurate description but which seems overly vague, much in the same way that the term "car" could be used to refer to a Formula One racecar. Anyway, two sets of science results from the 1980s and early 1990s stood out to me. First, observations of NGC 6052 at radio wavelengths demonstrated that the object was not only producing stars at a very high rate but was also producing lots of supernovae as well [11, 12]. This is because, as I mentioned earlier, the object contains an excessive amount of really big blue stars. These stars tend to live for relatively short periods of time before they explode as supernovae, and when they do explode as supernovae, they tend to produce a lot of excess radio emission. The second science results for NGC 6052 that I noticed were early computer simulations published in 1991 that tried to provide an explanation for what was happening in the collision [10]. Just a few years after this paper was published, I worked on an analysis of computer simulations of merging galaxies for a project for my master's degree [13], and at the time, performing these computer simulations was really difficult because they needed a lot of computational power. I therefore personally found it quite interesting to see this simulations paper from 1991 focused specifically on NGC 6052.

These days, NGC 6052 does not attract that much attention, and I think that is because everyone now understands how galaxies merge and trigger the formation of stars and because everyone has identified many more examples of where this is happening in both nearby galaxies and more distant galaxies. Nonetheless, people still occasionally target NGC 6052 to study specific aspects of the galaxy merging process, and it will continue to be a potential future science target for people who want to understand more about merging galaxies.

References

[1] Dreyer, J. L. E., A New General Catalogue of Nebulae and Clusters of Stars, being the Catalogue of the late Sir John F. W. Herschel, Bart, revised, corrected, and enlarged, 1888, Memoirs of the Royal Astronomical Society, 49, 1

[2] Herschel, John Frederick William, A General Catalogue of Nebulae and Clusters of Stars, 1864, Philosophical Transactions of the Royal Society of London Series I, 154, 1

[3] Dreyer, J. L. E., A Supplement to Sir John Herschel's "General Catalogue of Nebulae and Clusters of Stars'', 1878, Transactions of the Royal Irish Academy, 26, 381

[4] Arp, Halton, Atlas of peculiar galaxies, 1966

[5] Markarian, B. E., Galaxies with Ultraviolet Continuum II, 1969, Astrofizika, 5, 581

[6] Alloin, D. and Duflot, R., NGC 6052: a collision of two late spirals?, 1979, Astronomy & Astrophysics, 78, L5

[7] Heeschen, D. S. et al., A variable radio source in the clumpy irregular galaxy Markarian 297., 1983, Astrophysical Journal Letters, 267, L73

[8] Sofue, Y. et al., A search for the CO line emission toward two clumpy irregular galaxies, Markarian 8 and 297., 1986, Publications of the Astronomical Society of Japan, 38, 161

[9] Sofue, Yoshiaki et al., CO-Line Emission from the Clumpy Irregular Galaxy Markarian 297, 1990, Publications of the Astronomical Society of Japan, 42, L45

[10] Taniguchi, Yoshiaki and Noguchi, Masafumi, Wing Galaxies: A Formation Mechanism of the Clumpy Irregular Galaxy Markarian 297, 1991, Astronomical Journal, 101, 1601

[11] Yin, Q. F. and Heeschen, D. S., Two radio supernovae in the unusual galaxy Markarian 297, 1991, Nature, 354, 130

[12] Yin, Q. F., The Clumpy Galaxy Markarian 297 and Its Supernova Activity, 1994, Astrophysical Journal, 420, 152

[13] Bendo, George J. and Barnes, Joshua E., The line-of-sight velocity distributions of simulated merger remnants, 2000, Monthly Notices of the Royal Astronomical Society, 316, 315

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: betterchinese, dronemachine, ivolipa, jameswrowles, Meutecee, newagesoup, sagetyrtle, smokinghotdog, strangehorizon, and Xulie at The Freesound Project

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

 

© George Bendo 2022. See the acknowledgments page for additional information.

Last update: 26 December 2022