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Unveiling the Nature of Submillimeter Galaxy SXDF850.6

We present an 880 micron Submillimeter Array (SMA) detection of the submillimeter galaxy SXDF850.6. SXDF850.6 is a bright source (S(850 micron) = 8 mJy) detected in the SCUBA Half Degree Extragalactic Survey (SHADES), and has multiple possible radio counterparts in its deep radio image obtained at the VLA. Our new SMA detection finds that the submm emission coincides with the brightest radio emission that is found ~8"north of the coordinates determined from SCUBA. Despite the lack of detectable counterparts in deep UV/optical images, we find a source at the SMA position in near-infrared and longer wavelength images. We perform SED model fits to UV-optical-IR photometry (u, B, V, R, i', z', J, H, K, 3.6 micron, 4.5 micron, 5.8 micron, and 8.0 micron) and to submm-radio photometry (850 micron, 880 micron, 1100 micron, and 21 cm) independently, and we find both are well described by starburst templates at a redshift of z ~= 2.2 (+/- 0.3). The best-fit parameters from the UV-optical-IR SED fit are a redshift of z = 1.87 (+0.15/-0.07), a stellar mass of M_star = 2.5 +2.2/-0.3 x 10^11 M_sun, an extinction of A_V = 3.0 (+0.3/-1.0) mag, and an age of 720 (+1880/-210) Myr. The submm-radio SED fit provides a consistent redshift of z ~ 1.8-2.5, an IR luminosity of L_IR = (7-26) x 10^12 L_sun, and a star formation rate of 1300-4500 M_sun/yr. These results suggest that SXDF850.6 is a mature system already having a massive amount of old stellar population constructed before its submm bright phase and is experiencing a dusty starburst, possibly induced by major mergers.

Millimeter/submillimeter surveys have revolutionized observational cosmology by uncovering a substantial new population of mm/submm-bright dusty starburst galaxies at high redshifts (SMGs) (e.g., Smail, Ivison & Blain 1997;Barger et al. 1998;Hughes et al. 1998;Scott et al. 2002;Coppin et al. 2006;Greve et al. 2004;Laurent et al. 2005;Scott et al. 2008;Perera et al. 2008;Austermann et al. 2009;Tamura et al. 2009, and see also Blain et al. 2002 for a review). The energy source of mm/submm emission is primarily from intense star formation activity, with star formation rates (SFRs) of 100 M ⊙ yr −1 to several 1000 M ⊙ yr −1 , and possibly partially from an active galactic nucleus (AGN) (e.g., Alexander et al. 2005). While spectroscopic observations of radio-identified SMGs find a median redshift of 2.2 , several SMGs have now been found up to and beyond z = 4 (e.g., Capak et al. 2008;Coppin et al. 2009;Daddi et al. 2009). There is a hypothesis that SMGs are progenitors of present-day massive ellipticals (e.g., Lilly et al. 1999;, however, little is known about their evolution process. While multi-wavelength analysis is essential in order to understand the nature of SMGs, the coarse angular resolution of single dish telescopes prevents a precise determination of the exact optical/NIR counterparts. One of the most successful ways to pinpoint the location of the submm emission is to obtain high resolution, deep radio imaging (e.g, Ivison et al. 1998Ivison et al. , 2000Ivison et al. , 2002Smail et al. 2000; Barger et al. 2000). Although this technique reveals robust radio counterparts of ∼50%-80% of submm sources (e.g., Ivison et al. 2005Ivison et al. , 2007Wagg et al. 2009), sometimes multiple radio counterpart candidates are found for a source.
The most accurate means of achieving high precision astrometry on the submillimeter emission is clearly to observe with high angular resolution at the wavelength of the original detection. In this respect, the Submillimeter Array (SMA; Ho et al. 2004) has proved to be a powerful instrument (Iono et al. 2006a,b;Wang et al. 2007;Younger et al. 2007Younger et al. , 2008Ivison et al. 2008).
Here we present the results from the SMA observations toward an 8 mJy submm source, SXDF 850.6, detected in the SCUBA Half Degree Extragalactic Survey (SHADES). SHADES has observed a large area of the sky (720 arcmin 2 ) with high sensitivity (1σ ∼ 2 mJy) with the purpose of obtaining a statistically significant unbiased sample of submillimeter sources (Mortier et al. 2005;Coppin et al. 2006). The regions covered by SHADES are divided between two fields, the Lockman Hole and the Subaru/XMM-Newton Deep Field (SXDF). SXDF 850.6 is a source in the SXDF with multiple optical, IR, and radio counterparts (Ivison et al. 2007;Clements et al. 2008), but no established submm source identification. The strongest radio emission has no confirmed optical counterpart, but the two secondary radio peaks both have apparent optical associations.
§ 2 outlines the observational and calibration details, and the results are presented in § 3. In § 4, multiwavelength data are described. The results of SED model fitting using the photometry are presented in § 5. In § 6, we discuss the nature of SXDF 850.6. A summary is presented in § 7. Throughout the paper, magnitudes are in the AB system, and we adopt a cosmology with H 0 = 70 km s −1 Mpc −1 , Ω M = 0.3, and Ω Λ = 0.7.

OBSERVATIONS AND DATA REDUCTION
SXDF 850.6 was observed on September 21 and October 7, 2004, and on October 9 and 14, 2005, using a compact configuration with 7 -8 antennas of the SMA. The phase center was positioned at the submm source centroid which is α(J2000) = 02 h 17 m 29.80 s and δ(J2000) = −05 • 03 ′ 26 ′′ .00. The unprojected baseline length ranged from 23 m to 139m. The SMA correlator was equipped with 2 GHz total bandwidth in each sideband, yielding a total of 4 GHz bandwidth for continuum observations. A continuum channel was generated by vector averaging all of the channels after calibration. The SIS receivers were tuned to 345 GHz for the USB, yielding 335 GHz for the LSB. Interferometric pointing was checked at the beginning of the track and the pointing offsets were usually within ∼ ±5 ′′ (15% of the primary beam) for all antennas. We used an integration time of 30 seconds.
The raw SMA data was calibrated using the MIR package (Scoville et al. 1993). Passband calibration was done using bright QSOs and planets observed during the track. Antenna based phase calibration was done using J0238+166 (1.02 Jy), J0423-013 (1.67 Jy) and J0132-169 (0.84 Jy). The flux levels of all sources were normalized using the the quasar flux estimates derived from the primary flux standard Uranus. Imaging was carried out in MIRIAD (Sault et al. 1995). Maximum sensitivity was achieved by adopting natural weighting, which gave a final synthesized beam size of 2. ′′ 32 × 2. ′′ 19 (P.A. = 79.1 • ) and an RMS noise of 1.2 mJy. Because the source Signal-to-Noise Ratio is close to zero in declination, the uv coverage is undersampled in the north-south direction and the resultant synthesized beam has multiple sidelobes at the 30% level along the north-south direction about 14 ′′ away from the synthesized beam. The final map was made after adding the data from all four days and two sidebands, and corrected for the attenuation by the 35 ′′ primary beam of the SMA. The astrometric accuracy is likely dominated by the low S/N of the image, and we assess ∼0. ′′ 4.

RESULTS
The synthesized map is shown in Figure 1. A source was detected at about 6 sigma significance 8. ′′ 2 north of the SCUBA coordinates. The source appears slightly elongated in the northeast-southwest direction but this is likely caused by the low S/N, and the source is entirely unresolved with the SMA beam. Based on a point source fit to the visibilities, the derived flux is 6.9 ± 1.2 mJy, and the coordinates are α(J2000) = 02 h 17 m 29.79 s and δ(J2000) = −05 • 03 ′ 18. ′′ 65. The derived flux is consistent with the SCUBA 850 µm flux of 8.15 ± 2.2 mJy (Coppin et al. 2006) within the uncertainties of both measurements.

MULTI-WAVELENGTH DATA
Multi-wavelength images around SXDF 850.6 are shown in Figure 2. While an obvious optical counterpart is not seen in the images from u to z ′ bands, the NIR images represented in JHK and longer wavelength images show a presence of a counterpart. The photometry of the counterpart and upper limits at the SMA position are presented in Table 1. map based on the probability analysis method of Downes et al. (1986). All three radio sources have corresponding MIPS 24 µm sources (Clements et al. 2008), as shown in Figure 1. The SMA observation reveals that the correct radio counterpart is the most distant source from the SCUBA centroid, and the brightest in 24 µm emission. This demonstrates the effectiveness of the SMA in identifying SMG counterparts, especially in situations with multiple counterpart candidates.
The radio coordinates derived from reanalysis of the VLA image (Arumugam et al.

Mid-IR
Spitzer/IRAC and MIPS data are taken from the SpUDS archive. The 3σ detection limits are 0.58, 0.89, 5.7, and 5.3 µJy in IRAC bands (3.6, 4.5, 5.8, and 8.0 µm), and 36 µJy in MIPS 24 µm band. The photometry was performed using the SExtractor package (Bertin & Arnouts 1996). The photometry in IRAC bands is conducted with 3. ′′ 8 diameter aperture, and the photometric zero-points of the images are determined using colors of early-type stars between K-band and IRAC bands following Lacy et al. (2005). In order to derive total flux, the aperture photometry are corrected by assuming IRAC PSFs by a factor of 1.36, 1.40, 1.65, and 1.84, respectively, The photometry in the 24 µm band is conducted using 12 ′′ diameter aperture. The aperture magnitude is corrected by a factor of 1.70.
We find emission at the SMA position in all IRAC bands and MIPS 24 µm band. Note that the 24 µm flux in Table 1 should be used as an upper limit since sources around the SMA position are blended in the 24 µm image ( Figure 2).  (Laurent et al. 2005). The 3σ detection limits are 24.28, 24.08, and 24.18 mag, respectively.
A faint source is detected at the SMA position in all bands. We perform photometry using the SExtractor. Aperture photometry with 1. ′′ 8 diameter aperture in the J, H, and K bands are corrected by difference between the aperture and the total (MAG AUTO) magnitudes in the K-band.
4.4. uBV Ri ′ z ′ We present a u-band image of MOSAIC II on CTIO 4-m telescope (Fujishiro et al. in prep.), B, V, R, i ′ , and z ′ images of SuprimeCam on Subaru telescope in Figure  2. B, V , R, i ′ , and z ′ band images are obtained from Subaru/XMM-Newton Deep Survey database (SXDS; Furusawa et al. 2008). No galaxy is detected at the SMA position in these bands. The 3σ detection limits are 26. 00, 27.94, 27.68, 27.43, 27.45, and 26.43 mag, respectively. Near the SMA position, three galaxies are seen; ∼2 ′′ south-east, ∼0. ′′ 8 south-east, and ∼1 ′′ south-west to the SMA position. We derive photometric redshifts for the sources and find that they are likely to be at z < 1, and it is unlikely that they are related to the SMA source (see § 5).

SED FITTING
We perform SED model fitting to the photometry data from UV to radio to estimate photometric redshifts and  other physical properties (stellar mass, V -band extinction, star formation timescale, and metallicity). The photometric data are divided into two wavelength ranges: (1) from UV to 8.0 µm, where stellar emission is dominant, (2) from submm to radio, where thermal dust and synchrotron emission is dominant.

Submm -Radio
Photometry at 850 µm, 880 µm, 1100 µm, and 21 cm is used for submm-radio SED fits. The MIPS photometry is also used to check for consistency. We use the following starburst SED models: Arp 220 (Silva et al. 1998), the average SED of 76 SMGs with spectroscopic redshifts (Micha lowski et al. 2009), and 105 SED models of Chary & Elbaz (2001). We perform minimized χ 2 fits with free parameters of redshifts and flux scaling factors. We find the best-fit redshifts of z = 2.1 +0.5 −0.4 for Arp 220 (χ 2 = 0.3), z = 1.8 +0.5 −0.3 for the average SMG (χ 2 = 0.4), and z = 2.5 +0.6 −0.4 for an SED of Chary & Elbaz (2001) (χ 2 = 0.09). The errors are 99% confidence intervals. Note that there are other templates in the library of Chary & Elbaz (2001) that fit the photometry data with similar χ 2 values in the redshift range of z ∼ 1-3. The derived redshifts are consistent with the result in the UV-optical-IR SED fit in the previous section. The IR luminosities are estimated to be L IR = (7-26) × 10 12 L ⊙ from the intrinsic IR luminosities of SED templates multiplied by scaling factors for the best-fit SEDs. The bestfit SED models are shown in Figure 4 with photometry data from UV to radio. All of the SEDs overestimate the UV-NIR photometry, suggesting that stellar emission is heavily attenuated by dust. This is consistent with the large extinction (A V = 3.0) derived from the UV-optical-IR SED fit. Finally, we note that the SMG sample in Micha lowski et al. (2009) is biased against optically faint objects. It is clearly important to take into account a heavily obscured SMG like SXDF 850.6 (or e.g., GOODS 850-5 in Wang et al. 2009, AzTEC1 in Tamura et al. in prep.) to properly understand the overall picture of SMGs.

AGN Contribution
The rest-frame 1.6 µm bump is clearly seen at 5.8 µm (Figure 3), suggesting that the NIR emission detected in the IRAC bands is star-formation dominated (e.g., Weedman et al. 2006;Farrah et al. 2008). The flatter Table 2. Best-fit Results in UV-Optical-IR SED Fit  (Silva et al. 1998) at z = 2.1, and a starburst template of Chary & Elbaz (2001) at z = 2.5. Photometry data from UV to radio are overplotted. spectral slope in the IRAC emission compared to AGN populations favours a star formation as a dominant heating source (Yun et al. 2008;Ivison et al. 2004). This is also supported by the fact that SXDF 850.6 appears close to the starburst model tracks in the S 8.0 /S 4.5 vs. S 24 /S 8.0 color-color diagram of Ivison et al. (2004). These facts suggest that SXDF 850.6 favours star formation as a dominant heating source and the physical quantities derived in the SED fit are less affected by the AGN.
The 3σ upper limit on the rest-frame X-ray luminosity derived from 0.2-12 keV flux is 1.3×10 44 erg s −1 (assuming z = 1.87 and an effective photon index of Γ = 1.8). Since this value is not properly corrected for hydrogen attenuation, we do not constrain the AGN contribution from the X-ray luminosity.
By adopting a gas-to-dust mass ratio of 54, which is an average value for SMGs in Kovács et al. (2006), the molecular gas mass is M gas = (2-5) ×10 10 M ⊙ .

Star Formation Activity and Nature of SXDF 850.6
The UV-IR SED exhibits quiescent star forming activity dominated by old stellar components. The current SFR estimated from the SED fit is approximately zero. This is because the large part of the stellar mass was formed at the early phase of star formation and SFR decreased with time following exponential decay (τ = 20 Myr for the best-fit SED). On the other hand, the IR luminosity of the best-fit submm-radio SED provides SFR IR = 1300-4500 M ⊙ yr −1 (Kennicutt 1998). The SFR derived from 1.4 GHz radio emission is 1400 M ⊙ yr −1 following the equation of Bell (2003), and 1100 M ⊙ yr −1 following Yun & Carilli (2002) (with a spectral index of −0.8 and a radio-FIR normalization factor of 1).
To see whether the observational data allow the coexistence of the old stellar SED with quiescent star formation and the dusty starburst SED, we create synthetic SEDs at UV-IR wavelength using the GALAXEV library. The synthetic SEDs are composites of an SED with the same parameters as the best-fit SED which is dominated by old stellar population and starburst SEDs with different parameters. We find that the composite SEDs with plausible parameters for starburst SEDs (e.g., SFR ∼ 2000 M ⊙ yr −1 , A V = 3.0, and age = 1 Myr) are consistent with the photometry data ( Figure 5). Note that combining the SEDs increases the total stellar mass by only ∼1%.
Combining these results allows us to infer the nature of SXDF 850.6: it is a mature system with a large fraction of old stellar components, and currently experiencing a vigorous dusty starburst. The coexistence of old stars and a current starburst in an SMG is suggested by Wang et al. (2009) from a detailed SED analysis. SXDF 850.6 has enough molecular gas mass as estimated in § 6.2 to maintain intense star formation. Such significant star formation is likely caused by major mergers (e.g., Greve et al. 2005;Tacconi et al. 2006Tacconi et al. , 2008Narayanan et al. 2009). If the star formation continues with SFR ∼ a few 10 3 M ⊙ yr −1 , the gas consumption time is ∼ a few 10 Myr. This scenario is well described by hydrodynamic simulations of Narayanan et al. (2009) in which a major merger with a ∼10 13 M ⊙ dark matter halo produces a peak flux of ∼7-8 mJy at 850 µm for a short duration (<50 Myr). We are observing the shortlived glow of SXDF 850.6 as a bright SMG. Even if all the molecular gas converts into stars, the total stellar mass will increase by at most 30%. This supports an idea that the large fraction of stellar mass in SMGs is constructed before the current submm bright phase (e.g., Borys et al. 2005;Dye et al. 2008;Micha lowski et al. 2009).
The specific star formation rate (SSFR; SFR per unit stellar mass, an estimator of current star-forming activity) of ∼5-20 Gyr −1 places SXDF 850.6 on the high mass end of the correlation between stellar mass and SFR for z ∼ 2 star-forming galaxies or above the correlation (e.g., Erb et al. 2006b;Reddy et al. 2006;Daddi et al. 2007). If the large population of stars were constructed before the current starburst phase, the enhancement of star formation could be due to merging of massive star forming galaxies (e.g., merging BzKs; Takagi et al. 2008), and it is possible that this galaxy evolves into a massive present-day elliptical through major merger events (e.g., Lilly et al. 1999;.

SUMMARY
We conducted SMA 880 µm observations of a submillimeter galaxy, SXDF 850.6, detected with SCUBA in the Subaru/XMM-Newton Deep Field, which has multiple radio and IR counterpart candidates. The precise astrometry with the SMA shows that the correct counterpart is the most distant radio source from the SCUBA coordinates. Although there is a lack of a corresponding source at the SMA position in deep images of u, B, V , R, i ′ , z ′ bands, we find a counterpart at J, H, K, 3.6 µm, 4.5 µm, 5.8 µm, 8.0 µm, 24 µm, 1100 µm, and 21 cm. A detailed analysis of fitting a library of synthetic SEDs to the photometry from the u-band to 8.0 µm gives the bestfit parameters of z = 1.87 +0.15 −0.07 , M ⋆ = 2.5 +2.2 −0.3 × 10 11 M ⊙ , A V = 3.0 +0.3 −1.0 mag, and age = 720 +1880 −210 Myr. The SED fit to the submm-radio photometry using the three different starburst SED models provides the consistent redshift of z ∼ 1.8-2.5 and L IR = (7-26) × 10 12 L ⊙ . The IR color diagnostic suggests that the AGN contribution to the SED is small. The SFR derived from the IR luminosity is 1300-4500 M ⊙ yr −1 , and the SSFR is ∼10 Gyr −1 . If the intense starburst continues, the estimated molecular gas mass of (2-5) ×10 10 M ⊙ will last for a few Myr. Given these facts, it is suggested that SXDF 850.6 is a mature system that already contains a dominant fraction of its expected final stellar mass in an old stellar population, and which is also experiencing a significant starburst. The enhancement of star formation could be due to major mergers.