Object 60: UM 425

Podcast release date: 15 November 2021

Right ascension: 11:23:20.7


Epoch: ICRS

Constellation: Leo

Corresponding Earth location: About 230 km west of Tarawa Atoll in the Gilbert Islands

The history of UM 425 is rather interesting. The object was originally discovered in 1981 in a survey led by Gordon M. MacAlpine at the University of Michigan [1], which I am fairly certain is why the name of this astronomical object begins with UM. MacAlpine had been searching for quasars in the visible part of the electromagnetic spectrum. A quasar is one of many different types of galaxies containing an active galactic nucleus, which itself consists of a supermassive black hole millions or billions of times more massive than the Sun, a disk of really hot gas falling into that black hole, and jets of gas above the poles of the black hole that originate from gas that got too hot as it was falling into the black hole and that ended up getting blown away instead. Many different galaxies have supermassive black holes, but in quasars, the jets of gas end up aimed roughly towards Earth, so the quasars tend to look bright and point-like. Quasars also are generally found at distances of billions of light years from our galaxy. UM 425 itself is at a distance of about 9.3 billion light years from Earth [2].

While it is interesting that UM 425 was identified as a quasar by Gordon MacAlpine and his crew of trusty astronomy assistants and part-time Michigan Wolverines sports fans, some later observations of UM 425 by G. Meylan and S. Djorgovski in 1987 revealed that this was more than just a single ordinary quasar. Their more detailed images revealed the presence of a second bright spot just above the center of the quasar [3]. This second bright spot looked like a second quasar at the same distance as the brighter quasar. It seemed a little too unusual to have two very similar quasars right next to each other in the sky at the same distance from Earth, and this led Meylan and Djorgovski to propose that they were not looking at two separate quasars but two gravitationally-lensed images of one quasar [3].

Gravitational lenses are really exciting in astronomy. They are basically situations where, on Earth, we see a very distant object outside our galaxy, such as a very distant galaxy, behind another object that is still outside our galaxy but not quite as far away, such as another galaxy or maybe a cluster of galaxies. The gravitational forces from the closer object can actually bend the light from the more distant galaxy, and this can produce all sorts of really cool effects, including distorting the light from the more distant object into weird circular shapes, magnifying the light from the more distant object, and producing multiple images (and this last point is why people identified UM 425 as a potential gravitational lens). Gravitational lenses can be used for all sorts of different things in astronomy. First of all, astronomers can model how the closer objects distorts the more distant galaxies to determine how mass is distributed within the closer objects. This in turn allows astronomers to search for dark matter, which is stuff that neither absorbs nor emits light or any other form of electromagnetic radiation but which exerts gravitational forces that help to hold galaxies together [4]. Astronomers can also look at the distorted light from the more distant galaxies, which otherwise be too faint to see if they were not lensed, to understand them better, and astronomers can even use models to reconstruct what these distorted-looking galaxies actually looked like [5,6]. Additionally, statistical information on the numbers of gravitational lenses in the sky could also be used to test models of how the universe formed and how it has been expanding over time [7,8].

When UM 425 was identified as a potential gravitational lens in 1987, it was particularly exciting because, up to that point, astronomers had been having severe difficulty finding gravitational lenses even though they were expected to exist theoretically. Follow up observations of UM 425 in the 1990s and early 2000s sought to confirm that the object is indeed a gravitational lens and not just a pair of quasars sitting next to each other, and astronomers concluded that they could not reach any conclusions regarding the actual nature of UM 425.

Astronomers did find a cluster of galaxies located at a distance of about 6.8 billion light years in the same direction as UM 425 [9]. This distance places the cluster in between the Earth and the quasar (or quasars), and the cluster of galaxies as a whole appears to have the mass to gravitationally lens the light from the more distant quasar. However, astronomers did not find any of the distorted arc-like things that people have seen when looking at other gravitational lenses, and they could not exactly figure out how the closer cluster could lens the more distant quasar unless the cluster had a really massive but very faint galaxy with enough gravity to do the lensing [9]. This would imply that UM 425 could actually be a pair of quasars. However, if UM 425 was actually a pair of quasars, it would be peculiar that the quasars are not only located at exactly the same distance but also have very similar spectra. In fact, the probability of selecting two quasars from anywhere in the sky with the same type of spectra as seen in the two quasars in UM 425 is 1 in 20 at best [9].

Unfortunately, since astronomers couldn't figure out what was happening with UM 425, they moved on to working with other things where they could definitely tell that the objects were either pairs of quasars or gravitationally lensed individual quasars. Maybe someday, though, astronomers will take another look at UM 425 and will be able to figure out exactly what's happening.


[1] MacAlpine, G. M. and Williams, G. A., Curtis Schmidt-thin prism survey for extragalactic emission-line objects : University of Michigan, List V, 1981, Astrophysical Journal Supplement Series, 45, 113

[2] Abazajian, Kevork N. et al., The Seventh Data Release of the Sloan Digital Sky Survey, 2009, Astrophysical Journal Supplement Series, 182, 543

[3] Meylan, G. and Djorgovski, S., UM 425: A New Gravitational Lens Candidate, 1989, Astrophysical Journal Letters, 338, L1

[4] Treu, Tommaso, Strong Lensing by Galaxies, 2010, Annual Reviews of Astronomy and Astrophysics, 48, 87

[5] Swinbank, A. M. et al., Intense star formation within resolved compact regions in a galaxy at z = 2.3, 2010, Nature, 464, 733

[6] Dye, S. et al., Revealing the complex nature of the strong gravitationally lensed system H-ATLAS J090311.6+003906 using ALMA, 2015, Monthly Notices of the Royal Astronomical Society, 452, 2258

[7] Grillo, C. et al., Cosmological parameters from strong gravitational lensing and stellar dynamics in elliptical galaxies, 2008, Astronomy & Astrophysics, 477, 397

[8] Eales, S. A., Practical cosmology with lenses, 2015, Monthly Notices of the Royal Astronomical Society, 446, 3224

[9] Green, Paul J. et al., Discovery of a Galaxy Cluster in the Foreground of the Wide-Separation Quasar Pair UM 425, 2005, Astrophysical Journal, 630, 142


Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

Sound Effects: dronemachine, eldiariosonoro_, ivolipa, jameswrowles, josefpres, LudwigMueller, newagesoup, Reitanna Seishin, Rudmer_Rotteveel, spectral9, and Veiler at The Freesound Project

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