Discovery of a Lensed Ultrabright Submillimeter Galaxy at z = 2.0439

We have obtained spectroscopy of the lensed galaxy with the optical imager and spectrograph OSIRIS at the 10.4 m Gran Telescopio de Canarias (GTC), on the night of 2017 April 18 in clear conditions and dark moon with $0\buildrel{\prime\prime}\over{.} 8$ seeing. Observations were made using the low-resolution, long-slit mode with a $0\buildrel{\prime\prime}\over{.} 8$ slit and the R500B grism. In this configuration, the resolution is $R\sim 540$, and we have 3.54 Å pix⁻¹ from ∼3700 to 7200 Å. The slit was oriented along the line of the arc structure of the lensed source (PA of 6422). We grouped observations in two observing blocks of two exposures of 1385 s each with a 60 s acquisition image per block in the g band (see Figure 2). The seeing limited image shows an arc-like galaxy in the expected position for which we measured ${g}_{\mathrm{AB}}=22.85\pm 0.03$ for region 1 and ${g}_{\mathrm{AB}}=23.43\pm 0.04$ for region 2. The spectroscopic observations, with a total integration time of 5540 s, were reduced using the noao/twodspec and noao/onedspec packages of IRAF to yield fluxed, wavelength-calibrated spectra. We extracted spectra for the full arc region along the slit and separately for encircled regions 1 and 2. Our GTC spectroscopy reveals that the two regions in the arc are the same source. In Figure 3, we show the individual spectra for each arc region and the total arc spectrum. The spectra of the two arc regions show strong absorption features. We derive the redshift from the fit of well-measured lines of Si ii $\lambda 1260.4$, OI $\lambda 1302$, C ii $\lambda 1334.5$, Si ii $\lambda 1526.7$, and Al ii $\lambda 1670.7$; for the full arc we find $z=2.0448\pm 0.0004$.

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Archival GEMINI NIR-spectroscopic observations were found in CADC for region 1 of the arc. The spectrum was obtained using the cross-dispersed (XD) mode of the Gemini Near-IR Spectrograph on the 8.1 m Gemini-North telescope on the night of 2014 May 10 (under PI: Jane Rigby program ID: XGN-2014A-C-3) with the "short blue" camera, 32 l/mm grating and $0\buildrel{\prime\prime}\over{.} 68$ slit. The slit used in this XD mode is $7^{\prime\prime}$ in length and the orientation was with a position angle (PA) of 897 in region 1. The telescope was nodded (typically $3\buildrel{\prime\prime}\over{.} 5$ distant) in an ABBA-type pattern. We took the raw and calibration data from the CADC and reduced them using the Gemini IRAF package version v1.13.1 and following Mason et al. (2015) and the instruction on the Reducing XD spectra webpage from GEMINI observatory.¹¹ The total integration time for the spectrum was 3600 s, and two telluric stars of spectral type A0V were observed immediately before and after the galaxy. Only in the K band have we found useful data with a clear detection of ${{\rm{H}}}_{\alpha }$ in emission. In the inset of Figure 3, we show the spectrum near the H_α emission line from which we obtain a redshift of $z=2.0439\pm 0.0006$ in agreement with the rest-frame UV-spectrum. This redshift is consistent with the value published in Oguri et al. (2012; $z\approx 2.04$) for a source coincident in position with the lensed galaxy.

We have used Lenstool¹² (Kneib et al. 1993; Jullo et al. 2007) to perform a mass reconstruction of the foreground cluster, assuming a parametric model for the distribution of dark matter. This model was constrained using the location of the multiple images identified in the cluster. We used a simple model with a single cluster-scale mass component, as well as individual galaxy-scale mass components centered on each cluster member. For each component, we used a dual pseudo-isothermal elliptical mass distribution (dPIE, also known as a truncated PIEMD; Limousin et al. 2005). Sextractor and visual inspection of the HST/WFPC3 images provided an identification of six families of multiply lensed background galaxies, arclets with two to six components each, based on proximity, colors, and shape. The redshift of the family of the Cosmic Eyebrow (A in Figure 1) was fixed at z = 2.044, and for the other families was set as a free parameter. When modeling the lensing, the redshift of three families (B, C, and E in Figure 1) was found very close to z = 2.044, so we fixed it to z = 2.044 and for only two families were the redshifts considered a free parameter. We checked the model with the reconstruction of the arclets, and found good agreement between the positions for image arcs and the reconstruction of each component of the families. The data are best fitted with a cluster-scale potential of ellipticity e = 0.226, PA = 4700, and velocity dispersion ${\sigma }_{\mathrm{PIEMD}}=830$ km s⁻¹. The enclosed mass within an aperture of 250 kpc is $M=1.8\pm 0.5\times {10}^{14}\,{M}_{\odot }$ with an Einstein radius of ${\theta }_{{\rm{e}}}=11.\,\pm \,0\buildrel{\prime\prime}\over{.} 4$ at z = 2.044. The amplification is obtained from the comparison between the main characteristic of the galaxies in the source plane and in the image plane; we find an amplification factor of 11 ± 2 for the two main members of the Cosmic Eyebrow family, which are our spectroscopy regions on the arc indicated in Figure 2. The mean amplification factors for each family with z = 2.044 are A $\sim \,11\pm 2$, B $\sim \,7\pm 2$, C $\sim \,14\pm 2$, and E $\sim \,7\pm 2$. The largest amplification is for the brightest member of family C $\sim \,20\pm 2$; for the other two families the mean amplification factors are D $\sim \,4\pm 2$ ($z\sim 2.9$) and F $\sim \,5\pm 2$ ($z\sim 1.0$).

We plot the SED (Figure 4) assuming the lensed source in the HST and GTC images is the counterpart of the SCUBA-2 and WISE detections (all positions coincident within 1 arcsec) and list the photometry data in Table 1. We calculate the upper limit for the K_s and H bands from the UKIDSS catalog¹³ and for 60 and 100 μm from the IRAS Sky Survey Atlas.¹⁴ This SED is consistent with a source at redshift $z=2-2.5$, and as expected, it is very well fitted by the SED of the Cosmic Eyelash in that redshift range. At all frequencies from optical to radio, the Cosmic Eyebrow is brighter than the Cosmic Eyelash.

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We calculate the rest-frame 8–1000 μm luminosity, ${L}_{\mathrm{IR}}$, from direct integration of the data fit, and obtain an intrinsic luminosity for our galaxy $L(\mathrm{Eyebrow})=(1.3\pm 0.1)\times {10}^{13}\,{L}_{\odot }$, indicating an SFR of $\sim 2000\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$ (Kennicutt 1998), which assumes a Salpeter IMF, assuming a Chabrier IMF would possibly be a factor of ∼1.8 lower (Casey et al. 2014).

We also use the MAGPHYS code that allows us to fit simultaneously the ultraviolet-to-radio SED, so we can constrain physical parameters (Da Cunha et al. 2015). The fit is shown in Figure 4; we would benefit from measurements in photometric bands between 22 and 350 μm in order to establish which SED better describes the Cosmic Eyebrow. From this fit we give the median-likelihood estimates (and confidence ranges) of several physical parameters corrected for lensing amplification, stellar mass $\mathrm{log}({M}_{* }/{M}_{\odot })={11.49}_{-0.05}^{+0.06}$, star formation rate $\mathrm{log}(\mathrm{SFR}/{M}_{\odot }\,{\mathrm{yr}}^{-1})={2.73}_{-0.01}^{+0.02}$, mass-weighted age $\mathrm{log}({\mathrm{age}}_{M}\,{\mathrm{yr}}^{-1})={9.28}_{-0.01}^{+0.01}$, average V-band dust attenuation ${A}_{V}={3.30}_{-0.02}^{+0.02}$, H-band mass-to-light ratio $\mathrm{log}({M}_{* }/{L}_{H})\,={0.14}_{-0.01}^{+0.01}$, total dust luminosity $\mathrm{log}({L}_{\mathrm{dust}}/{L}_{\odot })={13.27}_{-0.02}^{+0.01}$, luminosity-averaged dust temperature ${T}_{\mathrm{dust}}/K={40}_{-2}^{+7}$, and total dust mass $\mathrm{log}({M}_{\mathrm{dust}}/{M}_{\odot })={9.19}_{-0.08}^{+0.15}$. These physical parameters compare well with the average properties of the ALESS SMGs in Da Cunha et al. (2015). The SFR obtained with MAGPHYS is lower than estimated with the Kennicutt (1998) relation. This is partially due to the use of the Chabrier IMF and to the additional dust heating caused by the relatively old stellar populations of our galaxy that increases the dust luminosity at fixed SFRs (Da Cunha et al. 2015).

The UV rest-frame spectrum of the Cosmic Eyebrow resembles that of Lyman break galaxies (LBGs; Shapley et al. 2003) and displays the strong interstellar absorption features typical of the spectra of starburst galaxies (Casey et al. 2013). We identify in the observed spectrum low-ionization resonance interstellar metal lines such as Si ii $\lambda 1260.4$, OI 1302 Si ii $\lambda 1304$, C ii $\lambda 1334$, Si ii $\lambda 1526$, and Al ii $\lambda 1670$ that are associated with the neutral interstellar medium. We also detect high-ionization metal lines of Si iv $\lambda \lambda 1393,1402$ and C iv $\lambda \lambda 1548,1550$ that are associated with ionized interstellar gas and PCygni stellar wind features. The C iv feature exhibits a strong interstellar absorption component plus a weaker blueshifted broad absorption and marginal evidence of redshifted emission associated with stellar winds that likely originate in main-sequence, giant, and supergiant O stars (Walborn & Panek 1984). Our data do not show evidence for the He ii $\lambda 1640$, which could be an indication of a low ratio of Wolf–Rayet to O stars (Schaerer & Vacca 1998).

In the spectrum of the region 1 of the arc, we find H_α at $19982\pm 4\,\mathring{\rm A}$ and the [N ii] $\lambda 6583$ at $20041\pm 5\,\mathring{\rm A}$. Full width at half maximum (FWHM) line widths are 386 ± 20 km s⁻¹ and 331 ± 20 km s⁻¹; after correction for the instrumental profile, we estimate that the intrinsic width of these lines is 150 ± 25 km s⁻¹. The low [N ii]$\lambda 6583$/H_α ratio of 0.33 ± 0.04 suggests a star-forming region (Kewley et al. 2013). The EW_rest (H_α) = $111\pm 10\,\mathring{\rm A}$ compares well with values reported in other well-known SMGs of a similar luminosity (Olivares et al. 2016).