Object 12: ELAIS N1

Podcast release date: 13 January 2020

Right ascension: 16:10:01.0


Epoch: ICRS

Constellation: Draco

Corresponding Earth location: Partly-forested land in West Central Alberta, Canada

ELAIS N1 is one of many extragalactic deep fields that astronomers have observed very intensely in the past 25 years.

The most famous of these deep fields is the Hubble Deep Field. Back in 1995, the Hubble Space Telescope pointed at what seemed like a relatively blank part of sky for about 10 days [1]. This blank part of the sky is located outside of the plane of the Milky Way, so it contains almost no bright stars or other objects from our galaxy, and it also contains no nearby galaxies that people already knew about. The observations revealed a population of faint galaxies at distances of billions of light years that changed astronomers' perception of what galaxies were like when the universe was younger and how those galaxies have evolved over time [1]. The successfulness of using the Hubble Deep Field observations to find distant galaxies led other astronomers to try to identify other relatively blank parts of the sky so that they too could point other telescopes at those blank areas for insanely long periods of time. This sound like the astronomical equivalent of watching paint dry, but the results are usually much more scientifically important.

ELAIS N1 is one of the deep fields that was initially defined for such deep field observations with the Infrared Space Observatory (ISO), which was an infrared space telescope that operated from 1995 to 1998. The name ELAIS is an acronym that stands for European Large Area ISO Survey; the people who developed this survey won an award for placing an acronym within an acronym from the International Astronomical Union in 2000.

The focus of this survey was to observe mid-infrared and far-infrared emission from very distant galaxies. Most of this emission would come from interstellar dust within these galaxies that absorbs ultraviolet and visible light from stars and that re-radiates the energy as infrared light. One of the major complicating issues with observing infrared emission from interstellar dust in distant galaxies is that the Milky Way Galaxy also contains interstellar dust that produces the same type of infrared emission. The astronomers using ISO wanted to avoid our galaxy's dust emission, so they chose a place in the sky where data from earlier low resolution infrared all-sky surveys showed that there was very little foreground dust. This meant that the survey would point to locations outside the plane of the Milky Way [2]. The Solar System also contains a ring of dust called zodiacal dust that orbits the Sun at about the same distance as the Earth and that produces a lot of mid-infrared emission. The astronomers using ISO wanted to avoid this dust, so they chose locations outside the plane of the Solar System as well [2]. The observers also avoided any really bright infrared sources that were already well-known, as these sources could saturate ISO's detectors, and they made some adjustments to their field selection based on some concerns about the telescope's observing schedule [2].

The resulting survey consisted of 11 separate fields [2]. ELAIS N1 is one of the three large fields located in the northern half of the sky, which is why it has the designation N1. The original size of the field was 2.0 degrees by 1.3 degrees in size. For reference, the Sun and the Moon as seen from Earth are both half a degree in diameter, so ELAIS N1 is about 4 times as wide as the Moon and slightly less than three times as tall.

The ISO observations found thousands of galaxies in the infrared images of all of the ELAIS fields [3]. The data from ISO were compared to models of how the universe evolves, and one of the key results is that more stars were forming in galaxies when the universe was half the age that it is today [4,5,6].

Since the observations by ISO, other infrared space telescopes, including the Spitzer Space Telescope, AKARI, and the Herschel Space Observatory, have since observed ELAIS N1 [7,8,9,10,11]. The field has also been observed in X-rays, visible light, and radio waves [12,13,14,15]. Some of these surveys covered larger areas than the original ELAIS N1 field, and some of these follow-up observations have identified tens of thousands of galaxies in ELAIS N1.

The data from both ISO and these other telescopes have led to the publication of a lot of papers, and I'm not going to try to summarize them all. However, one of results stand out to me. The earliest Spitzer Space Telescope observations of ELAIS N1 indicated that more than have of the galaxies seen in this field at mid-infrared wavelengths are luminous infrared galaxies, which are galaxies where the amount of light emitted at infrared wavelengths is between 100 billion and 1 trillion times the total amount of energy emitted at all wavelengths by the Sun [7]. These are typically either galaxies where a lot of stars are forming all at once, galaxies that contain an active galactic nucleus where gas is falling into a supermassive black hole and getting very hot, or some combination of these things. If you were to go outside our galaxy and away from it as well as away from any other nearby galaxies and then look around, most of the galaxies that you would see at infrared wavelengths would be these luminous infrared galaxies that we see in ELAIS N1.


[1] Williams, Robert E. et al., The Hubble Deep Field: Observations, Data Reduction, and Galaxy Photometry, 1996, Astronomical Journal, 112, 1335

[2] Oliver, Seb et al., The European Large Area ISO Survey - I. Goals, definition and observations, 2000, Monthly Notices of the Royal Astronomical Society, 316, 749

[3] Vaccari, M. et al., Final analysis of ELAIS 15-μm observations: method, reduction and catalogue, 2005, Monthly Notices of the Royal Astronomical Society, 358, 397

[4] Efstathiou, Andreas et al., The European Large Area ISO Survey - III. 90-μm extragalactic source counts, 2000, Monthly Notices of the Royal Astronomical Society, 319, 1169

[5] Héraudeau, Ph. et al., The European Large Area ISO Survey - VIII. 90-μm final analysis and source counts, 2004, Monthly Notices of the Royal Astronomical Society, 354, 924

[6] Rowan-Robinson, M. et al., The European Large-Area ISO Survey (ELAIS): the final band-merged catalogue, 2004, Monthly Notices of the Royal Astronomical Society, 351, 1290

[7] Chary, R. et al., The Nature of Faint 24 Micron Sources Seen in Spitzer Space Telescope Observations of ELAIS-N1, 2004, Astrophysical Journal Supplement Series, 154, 80

[8] Negrello, M. et al., Photometric redshift accuracy in AKARI deep surveys, 2009, Monthly Notices of the Royal Astronomical Society, 394, 375

[9] Davidge, H. et al., AKARI/IRC source catalogues and source counts for the IRAC Dark Field, ELAIS North and the AKARI Deep Field South, 2017, Monthly Notices of the Royal Astronomical Society, 472, 4259

[10] Kovács, Tímea O. et al., Star formation and polycyclic aromatic hydrocarbons in ELAIS N1 galaxies as seen by AKARI, 2019, Publications of the Astronomical Society of Japan, 71, 27

[11] Oliver, S. J. et al., The Herschel Multi-tiered Extragalactic Survey: HerMES, 2012, Monthly Notices of the Royal Astronomical Society, 424, 1614

[12] Manners, J. C. et al., The ELAIS deep X-ray survey - I. Chandra source catalogue and first results, 2003, Monthly Notices of the Royal Astronomical Society, 343, 293

[13] Kim, Jae-Woo et al., Clustering of extremely red objects in Elais-N1 from the UKIDSS DXS with optical photometry from Pan-STARRS 1 and Subaru, 2014, Monthly Notices of the Royal Astronomical Society, 438, 825

[14] Taylor, A. R. et al., Radio Polarimetry of the ELAIS N1 Field: Polarized Compact Sources, 2007, Astrophysical Journal, 666, 201

[15] Garn, Timothy et al., A 610-MHz survey of the ELAIS-N1 field with the Giant Metrewave Radio Telescope - observations, data analysis and source catalogue, 2008, Monthly Notices of the Royal Astronomical Society, 383, 75


Podcast and Website: George J. Bendo

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