George's Random Astronomical Object

Object 69: North Ecliptic Pole

Podcast release date: 21 March 2022

Right ascension: 18:00:00.0

Declination: +66:33:39

Epoch: J2000

Constellation: Draco

Corresponding Earth location: The Arctic Circle near Pearce Lake in Nunavut, Canada

These coordinates don't actually point to an astronomical object. They actually point to a location that is perpendicular to the plane of the Earth's orbit around the Sun. This location is called the North Ecliptic Pole.

To understand what's so special about this place in the sky, we first need to understand what the ecliptic is in a little more detail, even though I would think that many of people listening to this podcast would have heard of it before. The ecliptic is the path that the Sun appears to take across the sky over the course of an entire year. On a celestial sphere, it looks like a circle tilted about 23.5 degrees relative to the celestial equator, while on a rectangular map of the night sky, it looks like a sine wave. It passes through the constellations of the zodiac that people are familiar with as well as a less familiar constellation called Ophiuchus which got left out of the zodiac because of a clerical error. It is where the plane of the Solar System intersects the night sky, so we also see the Moon, the other planets, most of the Solar System's asteroids, and many comets travelling within or close to the ecliptic.

The North Ecliptic Pole is located 90 degrees away from the ecliptic, or in other words, in a location that is perpendicular to the plane of the Solar System. It effectively points in the location of the axis of the Earth's orbit around the Sun as well as the axes of the other planets' orbits around the Sun. However, the characteristics of this location in the night sky make it a very special place for professional astronomical observations, especially in the infrared part of the electromagnetic spectrum.

First of all, if professional astronomers want to observe things outside our Solar System and want to avoid accidentally looking at asteroids and other Solar System objects, then the North Ecliptic Pole is an excellent place to look. I can tell you from my personal experience that one of the most annoying things to deal with when trying to make an image of a galaxy is having to deal with a bunch of photobombing asteroids passing in front of that galaxy. The weird faces and hand gestures that those asteroids make are not funny. The North Ecliptic Pole is a good place to avoid these jerks.

However, asteroids are not the only things that astronomers can avoid at the North Ecliptic Pole. The Solar System also contains a thin ring of interplanetary dust that orbits the Sun within the ecliptic at about the same distance as the Earth does. This dust is called zodiacal dust. The dust reflects light from the Sun, and while this reflected light is impossible to see during the day, it can sometimes be seen near the horizon after sunset or before sunrise when the night sky is sufficiently dark. This interplanetary dust also absorbs a little bit of energy from the Sun and re-radiates that energy as infrared light. When astronomers want to look at the sky in the infrared, they will also see the infrared light from the zodiacal dust if they are looking close to the ecliptic. For that reason, some infrared astronomers prefer to look away from the ecliptic, and the North Ecliptic Pole is as far as you can get.

Another thing worth saying is that this location also lies outside the plane of our galaxy [1]. This means that, if astronomers want to look at objects outside our galaxy as well as outside our Solar System, this is a really good place to look because the plane of our galaxy not only contains lots of stars but also lots of nebulae containing interstellar dust that will both absorb light in the visible part of the electromagnetic spectrum and re-radiate that energy in the infrared part of the electromagnetic spectrum, all of which makes it harder to see the galaxies behind the nebulae. The area around the North Ecliptic Pole doesn't have any nebulae, which means that astronomers can avoid the problems caused by interstellar dust in our galaxy by looking at this region.

Finally, it's also worth pointing out that many space telescopes end up spending lots of time observing the North Ecliptic Pole because those telescope are designed to point mainly in a direction that is perpendicular to the Sun. This is either because the telescopes have sunshields on their sides that protect the telescopes from the strong light of the Sun (which is particularly important for infrared telescopes) and/or because the telescopes have solar arrays mounted on their sides that need to be pointed towards the Sun at all times. Consequently, at any given time, the telescopes will only be able to observe circular regions of the sky. These circular regions rotate as the telescopes orbit the Sun (which includes situations where the telescopes are still in orbit around the Earth, but because the Earth orbits the Sun, the telescopes orbit the Sun, too). Even though the circular regions that the telescopes can see will change over time, the North Ecliptic Pole is always visible to these space telescopes because it is always at a location 90 degrees away from the Sun, so the telescopes may end up spending more time observing this region than other regions of the sky.

For these various reasons, astronomers have spent lots of time observing the North Ecliptic Pole to search for very distant galaxies and other objects outside our own galaxy. I tried compiling a list of all of the telescopes that have observed this part of the sky, but it ended up just being a list of about half of all of the telescopes in existence. Having said that, it looks like people started paying close attention to this area in the 1990s. The X-ray telescope ROSAT was one of the first observatories to really observe this area very intensely, and it ended up identifying X-ray emission from very hot but diffuse gas within multiple clusters of galaxies [2, 3, 4, 5, 6, 7].

The next observatory to intensely observe the North Ecliptic Pole was the infrared telescope Akari in the early 2000s [8, 9, 10, 11, 12, 13, 14]. This telescope found literally thousands of infrared objects in this region, but the most notable objects many have been the very distant but very bright luminous and ultraluminous infrared galaxies scattered across the sky in this region. These are galaxies that are radiating huge amounts of infrared energy for one of two reasons. First, the galaxies could be forming abnormally large amounts of stars that are embedded within clouds of interstellar dust that absorb the visible light from the stars and re-radiate that energy as infrared light. Second, the galaxies could also contain central supermassive black holes millions or billions of times the mass of the Sun that are in the process of absorbing interstellar gas. However, that gas, as it falls inwards to those black holes, will get really hot and radiate lots of energy, a lot of which will again get absorbed by interstellar dust and re-radiated as infrared light. As a result of either or both of these phenomena, the galaxies will look really bright at infrared wavelengths, and astronomers will be able to find them from much further away that galaxies like the Milky Way. Since the North Ecliptic Pole avoids infrared emission from both the interplanetary dust in our Solar System and the interstellar dust in our galaxy, it has been a great place to look for these distant galaxies at infrared wavelengths.

References

[1] SIMBAD Astronomical Database, 2020

[2] Henry, J. P. et al., Groups of Galaxies in the ROSAT North Ecliptic Pole Survey, 1995, Astrophysical Journal, 449, 422

[3] Bower, R. G. et al., The ROSAT North Ecliptic Pole Deep Survey, 1996, Monthly Notices of the Royal Astronomical Society, 281, 59

[4] Voges, W. et al., The ROSAT North Ecliptic Pole Survey X-Ray Data, 2001, Astrophysical Journal Letters, 553, L119

[5] Henry, J. P. et al., Overview of the ROSAT North Ecliptic Pole Survey, 2001, Astrophysical Journal Letters, 553, L109

[6] Henry, J. Patrick et al., The ROSAT North Ecliptic Pole Survey: The X-Ray Catalog, 2006, Astrophysical Journal Supplement Series, 162, 304

[7] Hasinger, G. et al., The ROSAT Raster survey in the north ecliptic pole field. X-ray catalogue and optical identifications, 2021, Astronomy & Astrophysics, 645, A95

[8] Matsuhara, Hideo et al., Deep Extragalactic Surveys around the Ecliptic Poles with AKARI (ASTRO-F), 2006, Publications of the Astronomical Society of Japan, 58, 673

[9] Wada, Takehiko et al., AKARI/IRC Deep Survey in the North Ecliptic Pole Region, 2008, Publications of the Astronomical Society of Japan, 60, S517

[10] Lee, Hyung Mok et al., North Ecliptic Pole Wide Field Survey of AKARI: Survey Strategy and Data Characteristics, 2009, Publications of the Astronomical Society of Japan, 61, 375

[11] Pearson, C. P. et al., Source counts at 15 microns from the AKARI NEP survey, 2010, Astronomy & Astrophysics, 514, A8

[12] Kim, S. J. et al., The North Ecliptic Pole Wide survey of AKARI: a near- and mid-infrared source catalog, 2012, Astronomy & Astrophysics, 548, A29

[13] Takagi, T. et al., The AKARI NEP-Deep survey: a mid-infrared source catalogue, 2012, Astronomy & Astrophysics, 537, A24

[14] Murata, K. et al., AKARI North Ecliptic Pole Deep Survey. Revision of the catalogue via a new image analysis, 2013, Astronomy & Astrophysics, 559, A132

Podcast and Website: George J. Bendo

Music: Immersion by Sascha Ende

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

 

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

Last update: 25 April 2022