OPTICAL SURVEY WITH KMTNET FOR DUSTY STAR-FORMING GALAXIES IN THE AKARI DEEP FIELD SOUTH

Abstract

We present an optical imaging survey of AKARI Deep Field South (ADF-S) using the Korea Microlensing Telescope Network (KMTNet), to find optical counterparts of dusty star-forming galaxies. The ADF-S is a deep far-infrared imaging survey region with AKARI covering around 12 deg², where the deep optical imaging data are not yet available. By utilizing the wide-field capability of the KMTNet telescopes (~4 deg²), we obtain optical images in B, R and I bands for three regions. The target depth of images in B, R and I bands is ~24 mag (AB) at 5σ, which enables us to detect most dusty star-forming galaxies discovered by AKARI in the ADF-S. Those optical datasets will be helpful to constrain optical spectral energy distributions as well as to identify rare types of dusty star-forming galaxies such as dust-obscured galaxy, sub-millimeter galaxy at high redshift.

1. INTRODUCTION

The light from the first luminous objects in the Universe has been absorbed by dust and then re-radiated the thermal energy in the far-infrared (far-IR) and submillimeter (submm) range. Approximately half of the energy is emitted in these wavelength range (Puget et al. 1996; Dwek et al. 1998). Although redshift surveys to map the large-scale structure of the Universe such as Sloan Digital Sky Survey (SDSS) and the 2dF Galaxy Redshift Survey (2dFGRS) have expanded our knowledge on the optical properties of the z<1 Universe, a significant portion of star formation is hidden by dust and difficult to study through ground-based observations alone. To overcome observational constraints, large and deep survey programs in various wavelength ranges are planned and conducted in ground and in space (e.g., Cosmological Evolution Survey (COSMOS); Scoville et al. 2007). These multi-wavelength data sets are essential to constrain physical properties of galaxies such as mass, star formation rate, age.

Over the last decade, dusty star-forming galaxies with high star-formation rate of 100 − 1,000 M⊙/yr (e.g., Ultraluminous Infrared Galaxies - ULIRGs) have been detected by Spitzer/AKARI /Herschel extragalactic surveys. The physical mechanisms underlying the formation of these rare galaxies will be key to understand the high-redshift Universe. The AKARI Deep Field South (hereafter ADF-S) toward the South Ecliptic Pole (SEP) is one of deep survey fields designed for the study of Cosmic Infrared Background (CIB). Since the ADF-S is located near the SEP region, the region is likely to be observed deeply in multiple exposures with future space missions like Euclid (Laureijs et al. 2012) and SPHEREx (Doré et al. 2014). The deep far-IR imaging survey in the ADF-S was initiated by AKARI with 3σ detection limit of ~20mJy at 90μm (Matsuura et al. 2011). Unlike other deep and wide survey fields, the ADF-S has 6 photometric data sets at the far-infrared bands (24, 65, 70, 90, 140 and 160μm) to characterize dust emissions from star-forming galaxies. In addition, interstellar and interplanetary dust extinction is minimal along the line of sight, which is advantageous for detecting faint objects. The existing submm data from Herschel observations enable us to push our understanding of star-forming galaxies to high redshifts. Using the fluctuation analysis of the ADFS, Matsuura et al. (2011) reported that the clustering feature of the CIB from unresolved galaxies shows a ULIRG-like spectrum at high redshift. IR observations of AKARI and Herschel reveal some of the sources that produce IR emission, but their relatively poor spatial resolution (20′′−50′′) make it difficult to do understand the nature of IR sources. To reveal the nature of sources contributing to the CIB, a multi-wavelength study is required for the properties of individual sources and possible link among different galaxy populations.

To complement the relatively low spatial resolution of IR data, optical surveys are commonly carried out for IR surveys. For example, optical imaging observations were performed in the AKARI deep field toward North Ecliptic Pole (Hwang et al. 2007; Jeon et al. 2010). For efficient optical survey of large areal region, the widefield telescope takes advantages of minimizing observation time and increasing depth of field. The Korea Microlensing Telescope Network (hereafter KMTNet) composed of three 1.6m optical telescopes with a wide field of view of 2 × 2 deg2 and a pixel scale of 0.4′′ (Kim et al. 2016) is suitable for this purpose.

The optical survey with KMTNet is being performed in B, R and I bands. Here, we report the expected scientific outputs from our optical survey for the ADF-S. This paper is organized as follows. We begin with the explanations of the current data sets in the ADF-S and our optical survey with KMTNet in Section 2. We present our observations and data reduction in Section 3. In Section 4, we describe optically identified sources such as submm galaxy, dust-obscured galaxy and galaxy cluster. We summarize our results in Section 5. Throughout this work, we assume the following cosmological values: H0 = 73km s−1Mpc−1, Ωm = 0.27 and Ω∧ = 0.73. Unless explicitly noted otherwise, we use the Vega magnitude system.

 

2. THE ADF-S AND KMTNET SURVEY

The ADF-S is located at α = 4h44m00s and δ = - 53d20m00s covering an area of ~12 deg2 toward the South Ecliptic Pole (SEP). The ADF-S is a low background region with low levels of both Galactic cirrus and zodiacal emissions. The whole ADF-S was observed by the Far-Infrared Surveyor (FIS; Kawada et al. 2007) onboard AKARI (Murakami et al. 2007). In addition to the AKARI observations, other follow-up observations were performed for extragalactic studies with multi-wavelength data. The surveys covering most of the ADF-S were made from Spitzer/MIPS in midor far-IR (Scott et al. 2010; Clements et al. 2011) and Herschel Multi-tiered Extragalactic Survey (HerMES; Oliver et al. 2012) and Balloon-borne Large Aperture Submillimeter Telescope (BLAST; Valiante et al. 2010) in submm range. From HerMES and BLAST survey, tens of thousands of submm galaxies (hereafter SMGs) were detected in the ADF-S. The all-sky infrared survey mission, Wide-field Infrared Survey Explorer (WISE; Wright et al. 2010) gives us information on the midinfrared colors. Optical survey with KMTNet is currently ongoing. The optical survey regions with KMTNet and other surveys are shown in Figure 1. The information of available photometric data sets for the whole ADF-S is summarized in Table 1. The central part of the ADF-S (inside KMTNet-R2 region in Figure 1) was also observed by AKARI /IRC, APEX/LABOCA, ASTE/AzTEC and ATCA at 2-24μm, 870μm, 1.1mm and 20cm, respectively. Optical spectroscopic information is available only in this region (Sedgwick et al. 2011). Other follow-up multi-wavelength surveys for the central region of the ADF-S are described in Clements (2012).

Figure 1.Optical survey regions (left) by KMTNet (R1, R2 and R3 - large yellow box) together with other surveys in the ADF-S and BRI color composite image of the central region of galaxy cluster Abell S0463 by KMTNet (right). The overlaid map is the 100μm dust map by Schlegel, Finkbeiner & Davis (1998). The mean cirrus brightness in the ADF-S is ∼0.4MJy/sr at 100μm. Large cyan, red and magenta box show the AKARI /FIS, Spitzer/MIPS and Herschel/SPIRE (or BLAST) observations, respectively. We did not correct the saturated signals near the bright stars in this image.

Table 1λ denotes the wavelength. a FWHM (Full Width Half Maximum) of beam size b Skrutskie et al. (2006) c Wright et al. (2010) d Matsuura et al. (2011) e Clements et al. (2011) f Valiante et al. (2010) g Oliver et al. (2012)

To study the optical counterparts of far-infrared sources, we will achieve the target depth of 24 mag (AB) at 5σ in B, R and I optical bands, respectively (see Section 4 in detail). B-, R- and I -band filters are selected to construct the optical part of the spectral energy distribution (SED) of galaxies. From the test runs of observation for the ADF-S, we estimated total observation time of 5, 5 and 10 hours to reach the target sensitivity of 24 mag at B, R and I bands, respectively.

As seen in Figure 2, most of local star-forming galaxies detected at submm range can be identified in both AKARI and KMTNet observations. In addition, the deep data sets from 24μm, 90μm and submm bands enable us to expand those detections to high redshift range, although the detections at B band are expected to be poorer than those at R and I bands due to the red color of dusty star-forming galaxy.

Figure 2.Detection limits for local dusty star-forming galaxies at z=0.3. All SEDs are normalized to the detection limit at 500μm. The sensitivities listed in Tables 1 and 2 are shown as black squares.

 

3. OBSERVATIONS AND DATA REDUCTION

3.1. Observations

To fill out the gaps of ~3′ or ~10′ between mosaic CCDs (Kim et al. 2016), dithering observations composed of 7 adjacent positions were performed. We took image frames with 2-minute exposure which is optimal to maximize single exposure and to minimize saturations from bright stars. We also acquired short 30-second exposures to correct saturated signals. The KMTNet observations in the ADF-S are summarized in Table 2. Current observational data were obtained at the Cerro-Tololo Inter-American Observatory (CTIO) in Chile. During the observations, the median seeings were below 1′′. Considering seeing conditions, it is expected to achieve a spatial resolution of ~1′′. Precise positional information gives us increased chances to access the large facilities in the southern hemisphere, e.g., Atacama Large Millimeter/submillimeter Array and Giant Magellan Telescope.

Table 2a Estimated from initial data sets considering the underestimated photometric error from SExtractor

3.2. Data Reduction

For the feasibility study, we have reduced 2-night observational data of KMTNet-R3 region in this paper. Total exposure accumulated during two nights in February, 2015 is 53, 53 and 92 minutes at B, R and I bands, respectively. The raw data was preprocessed by the standard KMTNet pipeline (see Kim et al. 2016, in detail). The telescope optics with the wide field of view exhibits distortion within ±2′′. We have used SCAMP for the correction of astrometry (Bertin 2006) with the USNO CCD Astrograph Catalog version 3 (Zacharias et al. 2010) and SWarp for combining dithered images (Bertin et al. 2002). Figure 3 shows the astrometric errors in KMTNet-R3 region after the correction. We confirmed that the 1σ astrometric error is below 0.2′′ corresponding to a half pixel of CCD.

Figure 3.Astrometric errors after the correction. The 1σ errors in α and δ are 0.09′′ and 0.13′′ , respectively.

To generate the source catalog, we have derived the ‘AUTO’ magnitudes by the SExtractor (Bertin & Arnouts 1996). For reliable detection, we set the optimal parameters in SExtractor of 2.5σ for DETECT THRESH, 3 connected pixels for DETECT MINAREA, 256 for BACK SIZE and 3 for BACK FILTERSIZE. First, we have reduced the R-band catalog and then cross-matched sources to B and I bands. As shown in Figure 4, the mean magnitude limits with photometric error below 0.1 are 23.5, 23.0 and 22.3 at B, R and I bands, respectively. The photometric error from SExtractor does not include correlations in the background noise when stacking sub-pixel dithered images. Due to this effect, the photometric errors from SExtractor are in general underestimated (see Jeon et al. (2010)). We performed photometric calibrations with the standard star catalog near the SEP region from Clem & Landolt (2013). This calibration converts from the instrumental magnitude to the standard Johnson UBV and Kron-Cousins RI systems. Figure 5 shows the calibration result with color corrections. The 1σ deviation of magnitude difference after the calibration is below 0.05 mag in all bands. Since the photometric errors of stars with < 19 mag do not decrease below 0.01 mag rms, similar calibration uncertainties in all magnitude ranges may originate from the telescope system.

Figure 4.Photometric errors in B, R and I bands. The dashed line shows mean photometric error. The magnitude limit with mean photometric error below 0.1 mag (dotted line) is 23.5, 23.0 and 22.3 at B, R and I bands, respectively. These errors are formal errors returned by SExtractor, and could be underestimated by a factor of two or so.

Figure 5.The magnitude difference after the photometric calibration with standard star catalog from Clem & Landolt (2013).

To estimate completeness, we have compared with other galaxy counts at R band from Jeon et al. (2010) and Capak et al. (2004). After the statistical removal of stars using the TRILEGAL star count model (Girardi et al. 2005), the estimated magnitude of completeness 50% for the stacked data set is ~23 mag at R band (see Figure 6). Note that the contribution from galaxies fainter than 21 mag to source counts is dominant compared to that from stars (Jeon et al. 2010). As we mentioned, the fact that 50% completeness of R-band count is R magnitude of ~23 suggests that the photometric errors in Figure 4 are likely to be underestimated by about a factor of two.

Figure 6.The galaxy number counts at R band. The completeness is corrected in other source counts. Due to the limiting magnitude, the galaxy count drops below 50% at ∼23 mag.

 

4. OPTICAL IDENTIFICATIONS

The detection of star-forming galaxies in optical bands is inefficient due to their strong extinction by dust near star forming regions. Since the fraction of optical emission is quite modest, follow-up observations of this faint population in the optical region require deep optical imaging. Since the beam size of far-IR and submm images by AKARI and Herschel is 30′′−50′′ and 20′′−30′′, respectively, optical identifications can provide precise locations for other follow-up observations at other wavelengths. It will also constrain the optical SED. From mid- to far-infrared range, 90μm and 24μm data sets are optimal to detect dusty star-forming galaxies (see the information on sensitivity in Table 1). Regarding optical identifications of local star-forming galaxies, we have used sources cross-matched in both 24μm and 90μm (or 70μm) data. In addition, the cross-identified sources in both 24μm and submm data can push us to detect sources at higher redshift range. Since the positional uncertainty of 24μm data is less than 6′′, this matching strategy will increase reliable detections for dusty star-forming galaxies in a variety of redshift range. Figure 7 shows one sample identified from visible to submm range. It shows the difficulties in optical identifications using only far-IR and submm data.

Figure 7.Postage-stamp images from visible to submm range with 2.5′ × 2.5′ . The 3-band color composites (blue, green and red) with inverted color map were created from the images of KMTNet (B, R and I ), 2MASS (J, H and K), WISE (3.4, 4.6 and 12μm bands) and HerMES (250, 350 and 500μm bands). 24 - 70μm and 90 - 140μm images are from Spitzer and AKARI, respectively. The high spatial resolution in the optical range enables us to identify the morphology of the detected star-forming galaxy.

4.1. Dusty Star-Forming Galaxies

Due to intense star formation, the spectral energy distribution of star-forming galaxies peaks in the far-IR range. AKARI far-IR observations detected actively star-forming galaxies such as starburst (M82) and ULIRG (Arp220). From our optical survey, properties such as optical color, morphological type and angular size can be determined for local star-forming galaxies, which give us an detailed view of cosmic star formation in the local Universe.

To understand the dusty star formation history, recent observational efforts were made to search the high-redshift populations. Traditionally, to characterize the physics in high-z galaxies, bright emission lines, e.g., Lyα, Hα were used. The detection of emission lines at high redshifts enable us to broaden our knowledge on the star formation related to dust production after reionization. As simple selection criterion for high-redshift objects, imaging at submm range is usually used, since star-forming galaxies at high redshift with dust temperature of 20 ~ 70K (Casey et al. 2012) can be detected in submm ranges due to a negative K-correction. We have used the third Data Release (DR3) HerMES submm point source catalog crossmatched with 24μm sources (Wang et al. 2014). The cross-identified 24μm sources help us reduce false detection owing to higher spatial resolution of 6′′. Most of AKARI far-IR sources (>90%) observed by Herschel in the KMTNet-R3 region are detected in Spitzer 24μm or the HerMES-DR3 catalog. The fraction of optical identifications with HerMES-DR3 catalog is about 65% at R band in KMTNet-R3 region and ~8% of sources among the optically identified sources have multiple detections within 4′′ radius. Regarding multiple detections, we have estimated the likelihood ratio (Sutherland & Saunders 1992) for each association to find the true counterparts. Figure 8 shows the color-color diagram for KMTNet-R3 region from 3 submm bands (> 5σ detection in all bands) and the redshift track for SED templates of starburst and ULIRG. Although we could not determine the optical counterparts for the high redshift (z>3) candidates with certainty, forthcoming stacked imaging data may be expected to give us more reliable detections. These optical identifications will supply us with accurate positional information for spectroscopic or high-resolution imaging follow-up observations to study individual galaxies in detail.

Figure 8.High-redshift candidates from submm color-color diagram for KMTNet-R3 region. After matching with R-, I -band and 24μm sources, several optical counterparts (star symbol) are detected even at redshifts z>2. In addition, DOGs with submm counterparts are shown in asterisk symbol.

4.2. Dust-Obscured Galaxies

The selection of massive galaxies as rare population using the submm colors is described in Section 4.1. Another method is the selection of 24μm-bright, optically faint objects detected from Spitzer/MIPS data. Dey et al. (2008) selected sources from 8.14 deg2 region of the Boötes field with (R-[24]) > 14 corresponding to F24μm/FR ≥ 982, that are redder than the color of a typical ULIRG. 24 μm-bright Dust-Obscured Galaxies (hereafter DOGs) are expected to be very faint in visible range due to the large amount of obscuring dust, while being bright in the infrared range. We also selected DOGs with the same criteria from our optical survey data. As seen in Figure 9, we found 12 DOGs per 4 deg2 in the KMTNet-R3 region, which is consistent with the expected number density of DOG with F24μm > 1 mJy of ~6 per deg2 (Dey et al. 2008), considering the completeness of our survey region. We confirmed that our DOGs have red color in the optical range. Half the DOGs have far-IR or submm counterparts and the submm colors of DOGs with flux data at all submm bands are quite different from those of starburst or ULIRG in submm range (see Figure 8). From our optical survey of the whole ADF-S, more than 100 DOGs could be expected at our target depth of optical survey. Other follow-up observations like high-redshift SMGs will help to understand the detailed properties of DOGs.

Figure 9.The identified dust-obscured galaxies (DOGs) in our optical survey of KMTNet-R3 region.

The galaxies selected at sub-millimeter (SMGs) are a unique population to understand the formation and evolution of massive galaxies. The infrared-bright DOGs are identified as pure starbursts, obscured AGN-dominated objects, obscured quasars or high-redshift ULIRGs at the redshift z>2 (Riguccini et al. 2015). The DOGs are also thought to be plausible progenitors of massive galaxies (Dey et al. 2008). The relationship between SMGs and DOGs is one of the key issues in the evolution of massive galaxies.

4.3. Galaxy Cluster Abell S0463

Galaxy cluster Abell S0463 (z=0.0399) is known as a typical regular cluster (Dressler 1980) with type I-II in the Bautz-Morgan classification and 0 in Abell richness (Quintana & Ramirez 1995). As seen in Figure 10, the galaxy clusters like Coma and Virgo show a clear red sequence in the B-R versus R color-magnitude diagram at B-R color of ~1.6 and R magnitudes between 12.5 and 18. Recent X-ray observations with the Chandra showed that Abell S0463 has a large separation of 3.240′ between emission peak and emission-weighted centers which corresponds to ~152 kpc (Eckmiller, Hudson & Reiprich 2011). In addition, there are two X-ray peaks of roughly the same size and luminosity, implying that this cluster system is dynamically disturbed. Interestingly, some red-sequence members of the cluster are detected in the far-IR, which may reflect ongoing star formation activity (Małek et al. 2010). We have compiled red-sequence galaxies with a wide area of ~1 deg diameter, together with spectroscopically confirmed member galaxies. We expect to trace specific properties of candidates of cluster members related to dynamical state of the cluster.

Figure 10.B-R versus R color-magnitude diagram for galaxy cluster Abell S0463 (black) at z∼0.04 in the ADF-S by KMTNet, together with Coma (green; data from Godwin, Metcalfe & Peach 1983) and Virgo (blue; data from Kim et al. 2014) clusters.

 

5. SUMMARY

The optical survey in B, R and I bands is being undertaken with KMTNet for optical identifications of dusty star-forming galaxies detected by AKARI in ADF-S. The final optical catalog will be reduced with a target depth of 24 mag (AB) at 5σ. Optical data were used for identifying the optical counterparts for accurate positions of dusty star-forming galaxies. It will be also useful to design other spectroscopic or high-resolution imaging follow-up observations in various wavelength ranges. For the initial study, we have reduced ~15% of the KMTNet survey data in the KMTNet-R3 region among the entire expected survey. We found optical counterparts for most of local dusty star-forming galaxies resolved by AKARI as well as rare objects such as 24μm-bright DOGs. Half the DOGs have far-IR or submm counterparts, which enables us to understand the nature of DOGs. The complete deep optical imaging data may identify SMGs or DOGs even at high redshifts. In addition to the study of dusty star-forming galaxies, we obtained the complete optical catalog for galaxy cluster Abell S0463 in the ADF-S. We expect to derive the fundamental properties of the cluster such as evolutionary stage, cluster environment from identified cluster members. The cluster members with far-IR counterparts will be helpful to reveal the star-formation activity in the cluster.

The optical survey with KMTNet is still underway. We expect that more stacked data will improve the detections of fainter objects and accomplish our scientific objectives.