The Cosmic Wheel and the Legacy of the AKARI archive: from galaxies and stars to planets and life197
Multiwavelength study of the North Ecliptic Pole Field andIdentiﬁcation of candidate high-redshift galaxies
F. Mazyed,1, 2 D. Burgarella,1 F.-T. Yuan,3, 1 N. Oi,4 T. Goto,5 and the AKARI-NEP team6
1Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, 13388, Marseille, France2University of Wasit, 52001, Wasit, Iraq3Shanghai Astronomical Observatory, China4Tokyo University of Science, 1-3, Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan5Institute of Astronomy, National Tsing Hua University, No. 101, Section 2, Kuang-Fu Road, Hsinchu, Taiwan 30013, R.O.C.6coordinated by H. Matsuhara, ISAS, JAXA, Sagamihara, Kanagawa 229-8510, Japan
ABSTRACTMulti-wavelength data from the far-UV to the far-IR have been collected in the North Ecliptic Pole (NEP). These data arecrucial to understand the galaxy populations detected in this study. More speciﬁcally, this work made use of GALEX datain far- and near-UV, SUBARU HSC data in the optical, AKARI and WISE data in the near- and mid-IR and Herschel data inthe far-IR to try and identify a sample of candidate high-redshift, possibly lensed galaxies. From the list of candidates, wehave obtained follow-up observations at IRAM/30m, NOEMA, Gemini North and SMA to better understand our galaxies.This work is the beginning of a promising project dedicated to the study of high-redshift galaxies in the NEP.
1. INTRODUCTION
Although studying individual galaxies can be quite informative, understanding a global phenomenon like the galaxyformation and evolution means that we need large galaxy samples. This theme needs statistics. This is one of the reasonswhy we build wide ﬁeld surveys to collect large samples. Another one is because wide ﬁelds are necessary to detect andidentify rare objects. The color-color selection method (aka dropout) has been used to build large samples of star-forminggalaxies at 0.7 < z < 10 (see e.g., Burgarella et al. 2007; Bouwens et al. 2016). Far-IR and sub-mm telescopes measuredtheir dust properties up to z ∼ 4 (Gruppioni et al. 2013; Burgarella et al. 2013). In this work, we have generated multi-bandpoint source catalogs from observations of the NEP ﬁeld with GALEX, and Herschel. Finally, to model and interpret thismulti-wavelength dataset, we need an eﬃcient and versatile tool able to physically model all the various components of agalaxy and the underlying physics. Here, we use CIGALE (Burgarella et al. 2005; Noll et al. 2009) which is available tothe community (http://cigale.lam.fr) and supported by the authors.
2. MULTI-WAVELENGTH DATA: THE NEED FOR THE FAR-UV AND THE FAR-IR
Galaxies are complex systems that contain gas, dust, stars and sometimes an active galactic nuclei in their core. Physicallymodeling the emission of a galaxy implies that we are able to model all the physical processes occurring in a galaxy.These processes emit (or can emit) in diﬀerent wavelength ranges covering the entire spectral energy distribution (SED)of galaxies. In this work, we perform an analysis from the far-UV to the far-IR, including AKARI and deep new SUBARUdata (Figure 1) to identify and study high-redshift candidates that would show a red color in the far-IR. The negativeK-correction leads to high-redshift galaxies being easier to detect at sub-millimeter wavelengths as compared with theirlow-redshift counterparts. We make use of the red colors as a selection and develop color criteria.
3. THE GRAVITATIONAL LENSING EFFECT
Gravitational lensing is the bending of light caused by a massive mass located between the source and the observer. Inaddition to providing an information on the lensed galaxy, gravitational lensing probes the total mass of the foregroundlens, including the relative content of dark and luminous mass. One of the by-products of this work is to select outpossible high-redshift candidates via their red far-IR colors. The magniﬁcation helps alleviating the photon starvation,and, facilitates follow-up observations of galaxies obscured by dust and in particular the determination of their redshift.
4. HIGH-REDSHIFT SUB-MILLIMETER GALAXIES
Corresponding author: Denis Burgarelladenis.burgarella@lam.fr
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Figure 1. The area coverage of the NEP ﬁeld. The backgroundimage is the Herschel/SPIRE 250 µm, the same area is coveredby Herschel/SPIRE 350 and 500 µm, (OT2-sserje01-2, PI: S. Ser-jeant), and this work. Overlaid areas are: violet circle is the GALEX(GI4-057001-AKARI-NEP, PI: M. Malkan), cyan dashed and solidsquares are the CFHT optical surveys from Oi et al. (2014), blue rect-angle for CFHT NIR Y, J, and Ks, red solid and dashed circles areAKARI (NEP-Wide: Kim et al. 2012, and NEP-Deep: Takagi et al.2012, respectively), black solid squares are the PACS (OT1-sserj01-1, PI: S. Serjeant), yellow solid circle is the Sub-mm of SCUBA-2(Geach et al. 2017), green solid circle is WSRT radio 21 cm (Whiteet al. 2010).
Figure 2. A color-ﬂux diagram of Herschel/SPIRE photometry ofall galaxies detected in the NEP-ﬁeld with SNR > 3 at 500 µm.The vertical dashed lines are the selection limits at S500 > 100 mJy,and S500 > 70 mJy of our primary and supplementary lensed can-didates respectively. The sources that satisfy the selection criteriaare highlighted and classiﬁed as (red) gravitationally lensed sourcecandidates at high-redshift, (blue) local galaxies, and (green) radiogalaxies. The black square is the planetary nebula NGC 6543.
High-z sub-mm galaxies (SMGs) constitute an important population of massive systems in the early universe that haveprobably formed most of their stellar mass and heavy elements in intense, dust-enshrouded starbursts with high starformation rates, in the most massive dark matter halos. They are thought to be driven by major mergers of gas-richgalaxies at early epochs (e.g., Tacconi et al. 2008). But, it remains unknown how soon after the Big Bang such massivestarburst progenitors appeared. So, by identifying these sources, we can learn more about the nature and properties ofthese populations in terms of their mass and redshift distributions; number densities; and environments that may presenta challenge to current models of early cosmic structure formation (e.g., Negrello et al. 2010). According to the theoreticalpredictions of the evolutionary models such as those at 500 µm from, e.g., Negrello et al. (2007), the main populationsof sources observed with ﬂux densities above 100 mJy, consists of three populations: late-type galaxies at z > 0.1;ﬂat-spectrum radio AGNs; and a signiﬁcant fraction of bright strongly lensed SMGs at high redshift. Also, about 50% ofgalaxies with ﬂux densities above 80 mJy are expected to be lensed SMGs, and the rest can be easily identiﬁed as localgalaxies or as radio-loud AGNs from shallow optical imaging and radio surveys respectively. This method has been shownto be eﬃcient at making a robust list of gravitationally lensed candidates high redshift galaxies.
5. THE SELECTION WITH HERSCHEL-SPIRE
Taking advantage of this method, we used our Herschel SPIRE data to build a list of gravitationally lensed sourcecandidates. The identiﬁcation of gravitationally lensed galaxies is performed by searching for bright and red sources via aselection in the far-IR color-ﬂux diagram. A similar procedure is used by, e.g., Wardlow et al. (2013). We start by usingthe SPIRE 500 µm to select the lensed SMG candidates with a cut S500µm > 100 mJy (Figure 2). For further studies,we make also a supplementary candidate with ﬂux (70 ≥ S500µm [mJy] ≥ 100), where 50% of galaxies are expected tobe lensed SMGs (Negrello et al. 2010). A relatively low contamination from unlensed galaxies can be present in thecandidate list. The primary contamination are late-type galaxies at z > 0.1 (Serjeant et al. 2005), and ﬂat-spectrum radioquasars (or blazars) at higher redshifts (z ≳ 1.0) (Gonzalez et al. 2010). Both types of galaxies can have similar colorto the high-redshift galaxies, but they can easily be identiﬁed from shallow optical and radio surveys. Local galaxies arediscarded from the initial list by searching the spatially resolved galaxies in the optical catalog from Oi et al. (2014), and
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The Cosmic Wheel and the Legacy of the AKARI archive: from galaxies and stars to planets and life199
Multiwavelength study of the NEP Field and identification of candidate high-redshift galaxiesO32 - 3
Figure 3. Upper panel: Images of the 1.3 mm continuum of our lensed candidates from NOEMA. The sources (from top left to bottomright) are NEP-05, NEP-09, NEP-12, NEP-23, NEP-25, and NEP-52. The images show evidence of multiple features in 3 sources(NEP-09, NEP-12, and NEP-25). Two others, (NEP-23, and NEP-52), are extended and display distinct lensed morphologies. Lowerpanel: The best available optical or near-IR images at the position of the candidate lensed SMGs. The overlaid red contours are fromNOEMA 1.3 mm images. The sources (from top left to bottom right) are NEP-05, NEP-09, NEP-12, NEP-23, NEP-25, and NEP-52.
NASA/IPAC Extragalactic Database1 at the position of the selected candidates. We found in total 18 bright low-redshiftgalaxies (z < 0.1), with various populations from spiral to starburst galaxies. Radio sources are discarded by searchingfor radio emission in the radio catalogs from VLA-NEP (Kollgaard et al. 1994) and WSRT (White et al. 2010).The main list contains 6 sources with (S500µm > 100 mJy), and 55 sources in the supplementary list with (70 ≥S500µm [mJy] ≥ 100). Assuming all of them are trustworthy high-redshift lensed galaxies would result in a surfacedensity on the sky of ∼ 0.44 deg−2, and ∼ 6.1 deg−2, for the main and the supplementary candidate lists respectively. Thenumber density of the main list is the highest among the values of previous studies (e.g., ∼ 0.23 deg−2, ∼ 0.14 deg−2, and∼ 0.26 deg−2 from Negrello et al. 2010, Wardlow et al. 2013 and Bussmann et al. 2013), respectively. The diﬀerence inthe surface densities is not very signiﬁcant from the statistical point of view, and it could be caused by cosmic variance.
6. IRAM-NOEMA INTERFEROMETRY
NOEMA (previously PdBI) is the most powerful millimeter radio telescope in the Northerm hemisphere that includedeight 15-m diameter antennas, (at the time of the observing in 2016). The NOEMA conﬁgurations provide high angularresolutions up to 1.′′6, 1.′′1, 0.′′4 at 230 GHz with point source sensitivities of 0.08–0.14 mJy. We obtained NOEMAcontinuum observations at 1.3 mm band, for 6 sources in our list of candidates. The observations were executed, (proposalW16DD, P.I.: F. Mazyed), during the Winter cycle 2016–2017, in conditions of good to excellent atmospheric phasestability (seeing ∼ 0.′′1 to ∼ 0.′′3). Band 3 (201–267 GHz) has been used with the WideX correlator, allowing to detect thecontinuum as well as any other emission lines, if present (but we had no line detections).A ﬁrst interesting result is that the Herschel sources can be divided into several components. This result is similar towhat Karim et al. (2013) found using the Atacama Large Millimeter Array (ALMA). Faint SMGs in the 870-µm bandderived from arcsecond-resolution observations with ALMA, like our NOEMA observations, allowed to get rid from theeﬀects of confusion that we undergo with Herschel. However, their and our observations show that most of the brightestsources in our NEP sample are formed from the emission of multiple sources. Karim et al. (2013) suggested that the brightSMGs can be composed of multiple fainter SMGS. In all cases, NOEMA images have clearly identiﬁed the backgroundlensed sources, but there is no indication for continuum emission from the foreground lenses. The identiﬁcation of thelenses requires high spatial resolution (probably adaptive optics) observations at optical or NIR wavelengths. We usedthe most recent optical imaging of the SUBARU Hyper Suprime-Camera (HSC, Miyazaki et al. 2012; Goto et al. 2015).All the sources have optical imaging with HSC, except NEP9 which is out of HSC ﬁeld-of-views (we used Spitzer IRAC4.5 µm image). Figure 3 shows the best available optical or near-IR images at the position of the candidate lensed SMGs.The overlaid red contours from NOEMA 1.3 mm images are also shown. For all candidates, the background lensed sourceappears to be undetected in the optical images. Also for NEP9, it is hard to ﬁnd any morphological structure that could beassociated with the background source since the source is likely to be hidden behind the foreground source. This resultconﬁrms the high dust obscuration of these kind of galaxies, and that they would have been totally missed by standardoptical/NIR imaging. On the other hand, the optical/NIR images support the idea of a gravitational lensing hypothesis.According to Figure 3, the image separations suggest the presence of a gravitational potential wells, for the lenses, typicalof isolated galaxies (for NEP5 and NEP9), or small numbers of galaxies (for the other sources).The ﬁnal interpretation of the results is still uncertain. Given that we do not have enough information, yet, it is stillimpossible to apply a simple lensing model to estimate the basic parameters (e.g., deﬂection, distortion, ampliﬁcation andtime delay). Indeed, we would at least the ratio of the distances of the lens and lensed galaxies.
1 https://ned.ipac.caltech.edu/
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Figure 4. Left: Mosaic from the far-UV to the far-IR showing NEP-L. The upper panel shows detections of an object in the near-IRfrom 2 to 12 µm that is now conﬁrmed to be a massive galaxy at zspec = 0.856 (right panel). And in the Far-IR from 250 to 500 µm thatcan be a high-redshift lensed galaxy. Lower panel : The deep HSC/SUBARU observations for this object (Goto et al. 2015), conﬁrmthat the central bright source is the lens, a massive (maybe) elliptical galaxy. In the r-band image are superimposed the photometricredshifts with the two possible optical counterparts at z ∼ 5.1. Right: Gemini GMOS spectrum of the lens.
7. NEP-LEFT
NEP-Left (NEP-L) is our best candidate gravitational lensed galaxy at high redshift. Figure 4 shows the SED overdiﬀerent bands for NEP-L region. At the location of NEP-L, we have detections in the optical, NIR and MIR. However, nostrong emission is detected, in the other band in X-rays, ultraviolet (UV) and radio. This source is not compatible with alocal galaxy, or with a blazar that could produce such a rising far-infrared SED. Therefore, the best alternate explanation isthat NEP-L is a high-redshift gravitationally-lensed galaxy. The lensing nature is supported by the fact that the MIR SEDis not compatible with the FIR SED. We have not been able to ﬁnd a satisfactory complete SED ﬁtting using CIGALE.We deﬁnitely need to have two diﬀerent objects at diﬀerent redshifts. This anomalous is in agreement with the hypothesismentioned in Rowan-robinson et al. (2014): 11 per cent of 500 µm selected sources are good lensing candidates.Finally, from additional data, we can safely conclude that, it is very likely that we have a galaxy-galaxy gravitationallensing, and that the very bright source detected at the optical to mid-IR emission is the lens at zphot ≃ 0.856 (Figure 4)and that the source producing the FIR emission is the lensed one at zphot ≃ 5.1. But still more information are neededto understand NEP-L. Some very recent SMA observations of NEP-L have been secured and are now under processing.NOEMA observations will also be carried out by May 2018.
ACKNOWLEDGMENTS
We thank Johan Richard for his help in the interpretation of the Gemini GMOS spectra.
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