Object 121: NGC 34

Podcast release date: 01 April 2024

Right ascension: 00:11:06.6


Epoch: ICRS

Constellation: Cetus

Corresponding Earth location: About 1150 km west of Benguela, Angola, in the Atlantic Ocean

This episode's coordinates point to the galaxy NGC 34 located at a distance of approximately 28 million light years (85 Mpc) [1, 2] in the constellation Cetus, a stupid constellation that is supposed to look like a whale or sea monster but only looks like that if you take psychoactive substances. Anyway, because of a bookkeeping error in the New General Catalogue, which is where the abbreviation NGC comes from, the galaxy NGC 34 is also identified as NGC 17. Although most professional astronomy papers refer to this object as NGC 34, quite a few websites, including Wikipedia [3], call it NGC 17 instead. Very weirdly, the Hubble Space Telescope uses different names for the object on different webpages [4, 5].

Anyhow, NGC 34 is a very chaotic-looking galaxy. It has a kind-of distorted, lopsided disk of stars in the center, a weird hazy patch of stars on one side of that disk, a really long wisp of stars coming out of a different side of the disk, and a stubby arc of stars coming out of the other side [6]. This galaxy, in fact, formed from the merger of two other spiral galaxies, one of which was bigger than the other [6]. However, even though the nuclei of the galaxies appear to have merged together to form a single larger nucleus at the center of that distorted disk thing, the merger process is still ongoing, which is why this object looks so chaotic. That long wisp of stars and the stubby arc that I mentioned earlier are what are commonly referred to as tidal tails, and they are basically the distorted outer remnants of the disks of the spiral galaxies that got flung outwards into empty space during the merger process.

NGC 34 is also classified as a luminous infrared galaxy, which is a term that I have used a couple of times previously in this podcast series. Luminous infrared galaxies are defined as galaxies that produce an amount of infrared emission that is equivalent to over 100 billion times the total energy emitted by the Sun. This emission comes from interstellar dust in these galaxies that absorbs light at ultraviolet and visible wavelengths and then re-radiates that energy at infrared wavelengths.

To produce so much infrared radiation, luminous infrared galaxies need a very strong power source. Generally, this power source could be either a starburst or an active galactic nucleus (AGN).

A starburst is basically a situation where a lot of stars form all at once, and merging galaxies tend to produce starbursts. The gas clouds in the merging galaxies will either collide with each other and collapse or will be induced to collapse through the actions of various tidal forces on those clouds, and the collapse of these clouds triggers the process of converting the gas in these clouds into new stars. Starbursts are usually surrounded by gas clouds containing a lot of interstellar dust, so it's basically a situation with a lot of stars producing a lot of ultraviolet and visible light that gets aborbed by a lot of dust and re-emitted as an abnormally huge amount of infrared light.

NGC 34 very clearly contains such a starburst. Multiple observations of the galaxy have revealed the presence of many blue stars with relatively short lifespans that must have formed relatively recently [6, 7, 8]. The rates at which the galaxy is forming stars has been measured to be in the range of 50 to 90 solar masses per year [9, 10]. This could be thought of as approximately equivalent to 50 to 90 stars forming every year, although keep in mind that stars take hundreds of thousands or millions of years to form, so it might be more accurate to think of this as 50 million to 90 million stars forming every million years.

However, it is still possible that NGC 34 also contains an AGN. As a brief review, an AGN contains a supermassive black hole millions or billions of times the mass of the Sun, a disk of gas falling into the black hole, and jets of gas that emerge from the poles of the AGN that are produced by infalling material that gets very hot and expands and then gets deflected away from the black hole by magnetic fields within the system. Because the infalling gas is very hot, it produces very large amounts of multiwavelength electromagnetic radiation, including infrared emission.

Unfortunately, it has been very difficult to determine whether NGC 34 contains an AGN. Some spectroscopic observations measured a spectrum that looks like the spectrum of an AGN [11, 12, 13], while others measured spectra that looked like what was expected from just a starburst without any AGN present [14, 15]. Having said that, quite a few people have found an X-ray source in the center of NGC 34 [12, 16, 17], and while starbursts can also produce X-ray emission, the X-rays from the center of NGC 34 are so high in energy that they look like they have to be associated with an AGN [17].

So it looks like NGC 34 contains both a starburst and an AGN. However, I don't think this research on the energy sources in NGC 34 has reached a final conclusion. One of the ways I like to irritate other astronomers is, when they find both an AGN and a starburst in a luminous infrared galaxy, to ask whether the starburst or the AGN could be responsible for most of the galaxy's infrared emission. After all, just because both are present does not mean that they split the total amount of infrared emission 50/50. I will be interested to read what other people find in future observations of this galaxy.


[1] Rothberg, B. and Joseph, R. D., A Survey of Merger Remnants. II. The Emerging Kinematic and Photometric Correlations, 2006, Astronomical Journal, 131, 185

[2] Samsonyan, Anahit et al., Neon and [C II] 158 μm Emission Line Profiles in Dusty Starbursts and Active Galactic Nuclei, 2016, Astrophysical Journal Supplement Series, 226, 11

[3] NGC 34, 2023, Wikipedia

[4] NASA et al., Hubble Interacting Galaxy NGC 17, 2008, Hubblesite

[5] ESA/Hubble et al., Beauty From Chaos, 2020, ESA/Hubble

[6] Schweizer, François and Seitzer, Patrick, Remnant of a "Wet'' Merger: NGC 34 and Its Young Massive Clusters, Young Stellar Disk, and Strong Gaseous Outflow, 2007, Astronomical Journal, 133, 2132

[7] Riffel, R. et al., A 0.8-2.4 μm spectral atlas of active galactic nuclei, 2006, Astronomy & Astrophysics, 457, 61

[8] Dametto, N. Z. et al., Probing the circumnuclear stellar populations of starburst galaxies in the near-infrared, 2014, Monthly Notices of the Royal Astronomical Society, 443, 1754

[9] Prouton, O. R. et al., Modelling the Spectral Energy Distribution of compact luminous infrared galaxies: Constraints from high frequency radio data, 2004, Astronomy & Astrophysics, 421, 115

[10] Valdés, J. R. et al., NIR spectroscopy of luminous infrared galaxies and the hydrogen recombination photon deficit, 2005, Astronomy & Astrophysics, 434, 149

[11] Yuan, T. -T. et al., The Role of Starburst-Active Galactic Nucleus Composites in Luminous Infrared Galaxy Mergers: Insights from the New Optical Classification Scheme, 2010, Astrophysical Journal, 709, 884

[12] Brightman, Murray et al., An XMM-Newton spectral survey of 12 μm selected galaxies - II. Implications for AGN selection and unification, 2011, Monthly Notices of the Royal Astronomical Society, 414, 3084

[13] Motter, J. C. et al., A Gemini-NIFS view of the merger remnant NGC 34, 2021, Monthly Notices of the Royal Astronomical Society, 506, 4354

[14] Mulchaey, John S. et al., An Emission-Line Imaging Survey of Early-Type Seyfert Galaxies. I. The Observations, 1996, Astrophysical Journal Supplement Series, 102, 309

[15] Riffel, Rogério et al., The stellar spectral features of nearby galaxies in the near infrared: tracers of thermally pulsing asymptotic giant branch stars?, 2015, Monthly Notices of the Royal Astronomical Society, 450, 3069

[16] Guainazzi, M. et al., X-ray obscuration and obscured AGN in the local universe, 2005, Astronomy & Astrophysics, 444, 119

[17] Esquej, P. et al., The starburst-active galactic nucleus connection in the merger galaxy Mrk 938: an infrared and X-ray view, 2012, Monthly Notices of the Royal Astronomical Society, 423, 185


Podcast and Website: George J. Bendo

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