North Ecliptic Pole (NEP) Survey

The North Ecliptic Pole (NEP) is one of the most frequently visited areas on the sky because of easy accessibility to the space missions. We now have extensive data sets covering almost all wavelengths from UV to radio for a region centered at the NEP, however with some missing bands conspicuous by their absence. In addition, eROSITA, to be launched early 2018, is expected to conduct ultra deep 2-10 keV imaging toward NEP, and we expect unprecedentedly numerous Compton-thick AGN candidates when combined with the multi-wavelength data in NEP region.

The addition of the 850μm SCUBA-2 data to the comprehensive multi-wavelength data over the NEP region will dramatically reduce the uncertainties in SED fitting for infrared sources. The uncertainty of the infrared SED is mainly due to the dust temperature and the dust composition, and such uncertainty propagates to the uncertainties in the derived physical parameters including total infrared luminosity, dust attenuation, star formation rates, and accurate AGN vs. star formation decomposition. Long wavelengths data in the Rayleigh-Jeans side of the infrared SED peak are crucial for reliable infrared SED fitting. The following figure shows the ensemble SEDs of the z < 1 galaxies, which ranges from red, passive galaxies to violently star-forming galaxies. The complete mid-infrared coverage of AKARI/IRC provides a profound opportunity to characterize PAH for galaxies at z < 1, by filling the previous gap of Spitzer between 8-24μm. However, without a data point that reflects cold dust emission at the longer submilimeter wavelengths, it is impossible to construct a full-wavelength model for dusty galaxies and the resulting parameters such as star formation rates, total infrared luminosity, dust attenuation, dust mass, and dust temperatures are highly uncertain.

Ensemble SEDs of the galaxies with spectroscopic redshifts of z < 0.3 (blue) and 0.3 < z < 0.8 (red) that are detected in Herschel/SPIRE 250μm band. Gray points are SEDs of the galaxies without 250μm detection at z < 0.8. Dotted, dashed, and dot-dashed line shows how the 850μm data point (covers rest-frame wavelength range for gray shaded region) in addition to Herschel/SPIRE photometry can constrain the dust temperature in these galaxies. Overplotted solid lines are empirical templates of Arp 220 and Sd galaxies (Polletta et al. 2007).

The NEP survey field and its multi-wavelength dataset is uniquely equipped to face the challenge of both identifying galaxies that harbour an actively accreting SMBH and reliably decomposing its emission from that of its host galaxy. There are at least four ways that we can currently employ to identify AGN in the AKARI-NEP field: (1) mid-IR emission (2) radio emission (3) optical spectroscopy and (4) X-ray emission that however is confined in the Deep part of the survey. A coverage map, showing the radio and optical spectroscopic observed sources in the NEP, together with the Herschel, GALEX, and CFHT coverage of the field is shown in Figure below.

Multi-wavelength coverage of NEP Region. Green squares denote radio-detected sources at 1.4 GHz with WSRT. Yellow stars denote spectroscopically confirmed Type 1 AGNs, while all sources with optical spectra are shown with gray circles.

We plan to observe the entire large of around 4 square degrees centered on NEP with SCUBA-2 850μm to fill the gap between the Herschel 500μm and ground-based radio data at 21cm. By adding the new data from our survey, we will be able to address the following outstanding questions regarding the evolution of the galaxies:

  • What is the origin of the Cosmic Infrared Background? Is it due to the very first population of stars, the intracluster halo stars or something else?
  • What is the nature of very red objects and the history of dust obscured star formation?
  • What is the role of the AGN in the evolution of galaxies? Under what circumstances do they inhibit or promote star formation?

Tiling map of the proposed observation. The entire NEP-wide field excluding the previously covered NEP-Deep area, is decomposed into 21 Pong1800 mappings. Total expected survey time is about 400 hours.


Coordinators:  Hyung Mok Lee (South Korea), Tomotsugu Goto (Taiwan), Stephen Serjeant (UK), Hideo Matsuhara (Japan), Linhua Jiang (China)

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