Object 8: II Zw 96

Podcast release date: 18 November 2019

Right ascension: 20:57:23.6


Epoch: ICRS

Constellation: Delphinus

Corresponding Earth location: The Mid-Atlantic Ridge between the Caribbean Sea and Mauritania

II Zw 96 is one of many objects outside our galaxy that was initially identified in surveys by Fritz Zwicky in the mid-twentieth century. As a side note, Fritz Zwicky is one of those people who could be described as an angry scientist as well as a mad scientist [1,2]. The object is a pair of galaxies at a distance of about 520 million light years (160 Mpc) [3] that are in the process of merging together to form one larger galaxy.

II Zw 96 is one of many galaxies classified as a luminous infrared galaxy (LIRG) [4]. These are defined as galaxies where the total amount of infrared light from the galaxies is equivalent to somewhere between 100 billion and one trillion times the amount of light produced by the Sun. In space, infrared light is primarily produced by interstellar dust. The dust absorbs ultraviolet and visible light from stars and other astronomical objects and then re-emits that energy at infrared wavelengths. This happens in our galaxy and in many other galaxies, but the amount of infrared light that is produced is weak compared to LIRGs like II Zw 96.

The huge amount of energy radiated as infrared light by interstellar dust in LIRGs needs to originate from a very strong energy source. One possibility is that the energy comes from large numbers of very bright, very hot stars. Since these stars have very short lifespans, they are usually indicative of where star formation has taken place very recently, and if an abnormally large number of stars appear to be forming in any one location, it is usually referred to as a starburst, and these starbursts can provide all the energy that is absorbed by the interstellar dust and re-radiated as infrared light. The other possibility is that the energy comes from an active galactic nucleus (AGN), which consists of a supermassive black hole and a large disk of gas and dust that gets very hot as it slowly falls into the black hole. The disk would produce a lot of infrared light itself, and if the active galactic nucleus is surrounded by more dust, this will absorb extra energy from the disk and re-radiate it as infrared light.

In LIRGs powered by starbursts, the stars typically form in the center of the galaxy or galaxies in the system, making these location the brightest infrared sources in the galaxies. In LIRGs powered by AGN, the AGN is by definition found in the centers of the galaxies, and so is most of the infrared light. However, II Zw 96, like its namesake, behaves differently from other LIRGs just to aggravate astronomers. Most of the infrared emission in II Zw 96 comes from a location in the outskirts of one of the two galaxies involved [5,6]. The centers of the galaxies do not produce much infrared light in comparison.

The discovery of this off-center infrared-bright region in II Zw 96 prompted astronomers to investigate whether it could be powered by a starburst or an AGN (even though the AGN would not be in the nucleus of either galaxy, which would seem to contradict the definition of an AGN). With the use of mid-infrared spectroscopy, it is possible to investigate what is happening in places where interstellar dust obscures all of the visible light, and when a mid-infrared spectrum was made of the infrared-bright thing in II Zw 96, astronomers saw no sign of an AGN being present [6]. They also made estimates of the rate at which stars are forming. Astronomers like to describe this rate in terms of the mass of gas (in units of the mass of the Sun) that is converted into new stars per unit time. If all of the infrared emission comes from a starburst, it looks like up to 120 solar masses of gas per year could be forming into stars in this off-center region [6]. For comparison, the star formation rate for the entire Milky Way is only 1.65 solar masses per year [7].

However, astronomers can also see through interstellar dust using radio telescopes, and a group of astronomers used the MERLIN telescope (now called eMERLIN) to observe regions called masers within II Zw 96. Masers are like radiowave lasers in space. By observing the motions of these masers, MERLIN was able to show that this off-center region in this galaxy might contain a very compact object about one billion times the mass of the Sun [8], which would mean that a supermassive black hole could be present and that people may need to find a name other than AGN to describe things that contain supermassive black holes.

As far as I can tell, people are still trying to figure out exactly what is happening in II Zw 96, and I think some astronomers would be interested in trying to model the system to figure out if the infrared bright thing might actually be the center of a third hidden galaxy behind the first two. In any case, II Zw 96 shows that sometimes the brightest infrared sources in luminous infrared galaxies are not their centers but strange regions in the outskirts. A lot of astronomers debate exactly what is happening inside LIRGs with a particular focus on how much star formation and AGN are involved in producing the large amounts of infrared emission from these galaxies, but things like II Zw 96 make those debates even more confusing.


[1] Johnson, John Jr., Zwicky: The Outcast Genius Who Unmasked the Universe, 2019, Harvard University Press

[2] de Swart, Jaco, Deciphering dark matter: the remarkable life of Fritz Zwicky, 2019, Nature, 573, 32

[3] NASA/IPAC Extragalactic Database, 2019

[4] Armus, L. et al., GOALS: The Great Observatories All-Sky LIRG Survey, 2009, Publications of the Astronomical Society of the Pacific, 121, 559

[5] Goldader, Jeffrey D. et al., Heavily Obscured Star Formation in the II ZW 96 Galaxy Merger, 1997, Astronomical Journal, 113, 1569

[6] Inami, H. et al., The Buried Starburst in the Interacting Galaxy II Zw 096 as Revealed by the Spitzer Space Telescope, 2010, Astronomical Journal, 140, 63

[7] Licquia, Timothy C. and Newman, Jeffrey A., Improved Estimates of the Milky Way's Stellar Mass and Star Formation Rate from Hierarchical Bayesian Meta-Analysis, 2015, Astrophysical Journal, 806, 96

[8] Migenes, V. et al., Optical and OH megamaser observations of the starburst galaxy IIZw 096, 2011, Monthly Notices of the Royal Astronomical Society, 416, 1267


Podcast and Website: George J. Bendo

Special Guest Contribution: Adam Avison

Music: Immersion by Sascha Ende

Sound Effects: art6nik, dashatron, dronemachine, ivolipa, jameswrowles, Samulis, SkibkaMusic, and Xulie at The Freesound Project

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