How Do Galaxies Trace a Large-scale Structure? A Case Study around a Massive Protocluster at Z = 3.13

In 2017 September, we obtain narrowband imaging of the protocluster candidate "D1UD01" and the surrounding region in the D1 field, one of the four CFHTLS deep fields. The pointing center is [α, δ] = [36316, −4493]. The data are taken with the Mosaic-3 Camera (Dey et al. 2016b) on the Mayall 4 m telescope of the Kitt Peak National Observatory (NOAO Program ID: 2017B-0087). The KPNO Mosaic [O iii] filter no. k1014 (o3 filter hereafter) is used, with a central wavelength of 5024.9 Å and an FWHM of 55.6 Å. The o3 filter samples redshifted Lyα lines in the range z = 3.132 ± 0.023, spanning a line-of-sight distance of 44 Mpc.

The individual exposure time of 1200 s is used with small-offset dithers (FILLGAP) optimized to fill in CCD chip gaps. We discard the frames taken with seeing >13. We identify and remove a handful of frames that appear to have been taken when the guide star was temporarily lost, resulting in the sources leaving visible trails in the image. The total exposure time of the new imaging is 14.0 hr. The mosaic image has a native pixel scale of 025.

We calibrate the astrometry with the IRAF task msccmatch using the stars identified in the CFHTLS deep survey catalog (Gwyn 2012) and re-project each image with a pixel scale of 0186 using the tangent point of the CFHTLS images. The relative intensity scale is determined using the IRAF task mscimatch. The reprojected frames are then combined into a final image stack using a weighted average, with the average weight inversely proportional to the variance of the sky noise measured in the reprojected frames. We trim the images, removing the area near the edges with less than 20% of the maximum exposure time, and mask areas near bright saturated stars. The final mosaic has an effective area of 0.32 deg² with a measured seeing of 12.

As most of our observations were taken in nonphotometric conditions, we calibrate the photometric zero-point using the CFHTLS broadband catalogs. The central wavelength of the g band is 4750 Å, reasonably close to that of the o3 filter at 5024.9 Å. We define a sample of galaxies that have g-band magnitudes of 21–25 mag with the blue g − r colors (g−r ≤ 0.2) and determine the o3-band zero-point such that the median o3−g color is zero. We further check our result by plotting the g − r colors versus o3−g colors for all photometric sources. We confirm that the intercept in the o3−g colors is zero.

In conjunction with the new o3 data, we use the deep ugriz images available from the CFHTLS Deep Survey (Gwyn 2012). The broadband images are trimmed to have an identical dimension to the o3-band data. The photometric depth (measured from the sky fluctuations by placing 2'' diameter apertures in random image positions) and native image quality of these bands are summarized in Table 1; their filter transmission curves are illustrated in Figure 1.

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We create a multiwavelength photometric catalog as follows. First, we homogenize the point-spread functions (PSFs) of the broadband data to match those of the worst-seeing data, i.e., the o3 image (FWHM = 12). The radial profile of the PSF in each image is approximated as a Moffat profile with the measured seeing FWHM, and a noiseless convolution kernel is derived using the IDL routine MAX_ENTROPY. The broadband data are then convolved with their respective kernels to create a PSF-matched image.

We create the narrowband catalog by running the SExtractor software (Bertin & Arnouts 1996) in the dual-image mode. The o3-band image is used for detection, while photometric measurements are performed in all the broadband images. The SExtractor parameter MAG_AUTO is used to estimate the total magnitude, while colors are computed from the fluxes within a fixed isophotal area (i.e., FLUX_ISO). As the images are PSF matched, aperture correction in all bands is assumed to be given by the difference between MAG_AUTO and MAG_ISO estimated in the detection band. A total of 43,940 sources are detected in the o3 image. We also use the broadband-only catalog released as part of the CFHTLS final data release¹⁰ (referred to as a "T0007" version hereafter); the T0007 catalog contains 249,771 sources where a gri-selected χ² is used as a detection image.

The primary goal of this paper is to investigate the possible presence of a large-scale structure in and around the five spectroscopic sources at z = 3.13 discovered by Toshikawa et al. (2016). The o3 filter is ideally suited for this task, as redshifted Lyα emission falls into it at z = 3.13 ± 0.02. The redshift selection function, converted from the filter transmission, is illustrated in the inset of Figure 1. The Lyα-based spectroscopic redshifts of the five galaxies confirmed by Toshikawa et al. (2016) are marked as vertical dashed lines.

We adopt the following criteria to select LAE candidates at z = 3.13:

$$\begin{eqnarray}&&\begin{array}{l}o3-g\lt -0.9\,\wedge {\rm{S}}/{\rm{N}}(o3)\geqslant 7\,\\ \wedge \,[u-g\gt 1.2\,\vee {\rm{S}}/{\rm{N}}(u)\lt 2],\end{array}\end{eqnarray} \tag{ 1 }$$

where the symbols ∨ and ∧ are the logical "OR" and "AND" operators, respectively, and S/N denotes the signal-to-noise ratio within the isophotal area. The u − g color criterion requires a strong continuum break falling between the two filters to ensure that the source lies at z ≳ 2.7.

To design the selection criteria, we synthesize the colors by generating model galaxies spanning a range of rest-frame UV continuum slope, Lyα emission-line equivalent width (EW), and Lyα luminosity. The galaxy's spectral energy distribution (SED) is constructed assuming a constant star formation history observed at the population age of 100 Myr, with a Salpeter (1955) initial mass function (IMF) and solar metallicity. We account for attenuation by intergalactic hydrogen using the H i opacity given by Madau (1995) and assume that the interstellar extinction obeys the Calzetti et al. (2000) reddening law.

To the reddened, redshifted galaxy SED, we add an Lyα emission with a Gaussian line profile centered at 1215.67 (1 + z) Å and an intrinsic line width of 3 Å. The redshift z = 3.13 is assumed. Given that the o3 filter is much wider than the line width, exact values assumed for the line width are not important as long as they reproduce the observed galaxy colors and line FWHM reasonably well. The Lyα limiting luminosity from the above criteria is ≈10^42.3 erg s⁻¹. No extinction is applied to the Lyα line, as it represents the observed luminosity.

In the left panel of Figure 2, we show the expected o3−g and g − r colors for different reddening values with different line luminosities. For the Lyα luminosities indicated in the same panel, we assume a continuum g-band magnitude of 25.5 mag, which is based on the median value of our LAE sample. Finally, we stress that our photometric criteria (Equation (1)) are sensitive to the line EW and redshifts of the source, but not to the choice of IMF and metallicity adopted to create the base galaxy SED. For example, if a subsolar metallicity (Z = 0.008) is assumed, the g − r colors would be bluer by 0.04 mag while the o3−g colors would remain unchanged.

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The middle and right panels of Figure 2 show the color–color and color–magnitude distributions of the o3-detected sources, respectively. The adopted selection criteria (Equation (1)) correspond to the rest-frame EWs ≳20 Å at the target redshift range and result in 94 LAE candidates. Their g − r colors suggest that the majority are consistent with being relatively dust-free with a few exceptions. The LAE candidates are distributed over a ≈4365 Mpc² (1156 arcmin²) field. With the exception of six (green triangles in Figure 2), all have robust continuum detections in the g or r band.

Based on the photometric data, we derive the physical properties of our LAE candidates, including the rest-frame Lyα EW (W₀), Lyα luminosity (L_Lyα), UV continuum luminosity at the rest-frame 1700 Å (L₁₇₀₀), and UV spectral slope (β: defined as f_λ ∝ λ^β). The Lyα luminosity and EW are derived following the prescription given in Xue et al. (2017), which fully takes into account the Lyα forest attenuation in the relevant filters. The UV slope is computed from a linear regression fitting of the riz photometric data; the continuum luminosity L₁₇₀₀ is then extrapolated from the i-band flux density assuming the slope β. These quantities are listed in Table 2.

Four galaxies in our LAE sample are significantly redder (g−r > 1.0) than the majority. We check them in the image to verify that these sources are real and robust detections. One is likely an AGN with an extremely high UV luminosity (r = 21.7 mag) and a point-like morphology. The other three may be more dust reddened than the other 90 LAE candidates. Dusty LAEs are rare but have been reported in the literature (Oteo et al. 2012; Bridge et al. 2013), some of which are IR-luminous galaxies detected in mid-infrared surveys. Assuming the Calzetti et al. (2000) dust law, their UV slope β values correspond to the color excess of the stellar continuum E(B − V) of 0.20, 0.16, and 0.23, respectively, compared to the median value of 0.10 for the full LAE sample. These values are comparable to those measured for dusty LAEs with Herschel/PACS detection studied by Oteo et al. (2012).

Three of the five spectroscopic sources in the "D1UD01" structure satisfy our LAE selection; their IDs in the Toshikawa et al. (2016) study are D1UD01-8, D1UD01-9, and D1UD01-6. Their Lyα EWs estimated from spectroscopy are 7.8, 21.0, and 81.5 Å, respectively. The remaining two, Toshikawa source ID D1UD01-7 and D1UD01-10, do not meet our LAE selection because they are too faint in the o3 band (S/N in the range of 4–5); however, their o3−g colors, −1.61 ± 0.10 and −1.33 ± 0.08, are consistent with the Lyα EWs, 36.2 and 34.3 Å, measured from spectroscopy.

Sample Contamination. At the central wavelength of the o3 filter (5024.9 Å), the only plausible contaminants of our photometric LAE sample are [O ii] emitters at z ∼ 0.35, since our survey samples an inadequately small volume for [O iii] emitters that would lie at z ∼ 0.01. The adopted o3−g color cut corresponds to the observed line EW of 83 Å, much larger than the values measured for [O ii] emitters, which are mostly ≲50 Å (Hogg et al. 1998; Ciardullo et al. 2013) at z = 0.35. The requirement that the galaxies have red u − g colors provides an additional assurance that the Lyman break falls in the u band (i.e., the sources lie at z > 2.7).

Low-luminosity AGNs with a broad Lyα emission line at z > 2.7 can potentially contaminate our LAE sample, although the contamination is expected to be generally low (at ∼1%: Gawiser et al. 2006; Ouchi et al. 2008; Zheng et al. 2010; Sobral et al. 2018). We cross-correlate the source positions with the X-ray sources listed in the XMM survey in the field (Chiappetti et al. 2005) and find no match. However, the brightest source in our sample (QSO30046, o3 = 21.06, r = 21.69 mag) is detected in the Spitzer MIPS 24 μm data. QSO30046 is also observed by the VIMOS VLT Deep Survey (VVDS; Le Fèvre et al. 2013) and classified as an AGN at z_spec = 3.86. Given its redshift, the blue o3−g color is due to broad emission from Lyβ and O vi (Vanden Berk et al. 2001). While we list its properties in Table 2, we remove this source from our LAE catalog.

We also cross-match our LAEs with spectroscopic redshift sources published by Toshikawa et al. (2016) and those in the VVDS and VIMOS Ultra Deep Survey (VUDS; Le Fèvre et al. 2015). Four matches are found; three are part of the LBG overdensity reported by Toshikawa et al. (2016), and the fourth lies at z = 3.133, but well outside it spatially. The relatively low number of matches is not surprising given that all these spectroscopic surveys are limited to sampling only relatively bright sources (i.e., i < 25). In comparison, the mean i-band magnitude of our LAE sample is ∼26. Furthermore, the "D1UD01" region is excluded from the VUDS survey coverage.

We also identify a sample of UV-luminous star-forming galaxies at z ∼ 3 by applying the Lyman break color selection technique to the ugr data from the CFHTLS T0007 catalog. The technique can identify star-forming galaxies with a modest amount of dust by detecting spectral features produced by the Lyman limit at λ_rest = 912 Å and absorption by the intervening Lyα forest at λ_rest = 912–1216 Å. At 2.7 < z < 3.4, both of these features fall between the u and g bands.

In the right panel of Figure 3, we show the expected redshift evolution of broadband colors from z = 2.7 in steps of Δz = 0.1. Four reddening parameters are assumed, E(B − V) = 0.0, 0.1, 0.2, and 0.3 (from left to right). The synthetic colors of lower-redshift galaxies are also computed using the Coleman et al. (1980) template of S0 galaxies redshifted out to z = 2 (black lines). As can be seen in the figure, most of the z ∼ 3.1 sources are located at u−g > 1.0 while safely avoiding the locus of z < 2 galaxies. Based on these considerations, we adopt the following criteria to select LBG candidates:

$$\begin{eqnarray}&&\begin{array}{l}u-g\gt 1.0\,\wedge -\,1.0\lt (g-r)\lt 1.2\\ \wedge \,(u-g)\gt 1.5(g-r)+0.75.\end{array}\end{eqnarray} \tag{ 2 }$$

These are identical to those used by Toshikawa et al. (2016).

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In the left panel of Figure 3, we show the locations of all the sources in the two-color diagram. For the sources undetected in the u band, we show lower limits by adopting the 2σ limiting magnitude (28.5 mag). We also require that the candidates be detected with more than 3σ (7σ) significance in the g (r) bands to ensure that their detection and color measurements are robust. A total of 6,913 galaxies are selected as our LBG candidates. A total of 80 (86%) of the LAEs satisfy the adopted LBG criteria, with most of the remaining LAEs lying close to the selection criteria, confirming the similarity of the two populations. Our LBG catalog recovers 24 LBGs spectroscopically confirmed by Toshikawa et al. (2016), including all five "D1UD01" sources. Of 6913 galaxies, 210 have spectroscopic redshifts measured from the VVDS and VUDS surveys and by Toshikawa et al. (2016). Of those, 27 lie at z < 2.7, yielding a contamination rate of 13%.

The majority of these 27 galaxies have redshifts close to z = 2.7, suggesting that they are simply scattered into the LBG window. To quantify the role of photometric scatter, we carry out realistic galaxy simulations similar to those described in Lee et al. (2012). First, we create SEDs spanning a wide range of physical parameters (age, reddening, and redshift) and compute input photometry of these SEDs in the observed passbands. Mock galaxies are inserted into the images, and detection and photometric measurements are performed using an identical manner to the real data. The galaxies that satisfy our LBG criteria are collated into the master list. The redshift distribution of LBG-selected mock galaxies in the magnitude range r = 22–28 (matching the optical brightness of our LBG sample) peaks at z ∼ 3.1 with an FWHM of ∼0.7. Of those, 12% lie at z < 2.7, nearly identical to the contamination rate of 13% estimated from spectroscopy.

We make a qualitative comparison of the T0007 catalog with the Toshikawa et al. (2016) catalog (hereafter T16). The major difference is a detection image that is a gri-based χ² image for the T0007 catalog and an i-band image for the T16 catalog. The detection setting (including the threshold) is also different. Overall, we find that the T16 catalog is more inclusive of fainter objects with a median r-band magnitude of 26.5 mag, compared to 26.0 mag for the T0007 catalog. The two catalogs have 4219 sources in common, which accounts for 61% and 54% of the T0007 and T16 catalogs, respectively.

We search for sources that are significantly extended in their Lyα emission; such sources are often referred to as a giant Lyα nebula or "Lyα blob" (LAB hereafter). The largest LABs reported to date can be as large as ≳100 kpc across (e.g., Dey et al. 2005). Multiple discoveries of luminous LABs in and around galaxy overdensities (e.g., Steidel et al. 2000; Matsuda et al. 2004; Palunas et al. 2004; Dey et al. 2005; Prescott et al. 2008; Yang et al. 2010; Mawatari et al. 2012; Bădescu et al. 2017) have led to a claim that they may be a signpost for massive large-scale structures.

To enable a sensitive search, we first create an Lyα line image by estimating and subtracting out the continuum emission from the o3 image. Following the procedure described in Xue et al. (2017, see their Equation (11)), the line flux is expressed as F_Lyα = af_AB,o3 − bf_AB,g, where f is the monochromatic flux density in the respective bands and a and b are coefficients that depend on the corresponding bandwidth and optical depth of the intergalactic medium, as well as the UV continuum slope. For example, at z = 3.1, with a UV slope β of −2.0, a ∼ 7.3 × 10¹² and b ∼ 7.7 × 10¹².

We run the SExtractor software (Bertin & Arnouts 1996) on the Lyα image as a detection band and perform photometry on the o3- and g-band data. For detection, we require a minimum area of 16 pixels above the threshold 1.5σ, which corresponds to 27.81 mag arcsec⁻² or 1.80 × 10⁻¹⁸ ergs s⁻¹ cm⁻² arcsec⁻². Our LAB search is slightly different from our LAE selection in that source detection is made on the Lyα image and is tuned to be more sensitive to extended low surface brightness sources. The same o3−g color cut (Equation (1)) as our LAE selection is applied. Our search yields a single Lyα blob candidate in the entire field, which we name LAB17139. It is also identified as an LAE. At the Lyα luminosity of ≈10^43.3 erg s⁻¹, it has the highest luminosity in our LAE sample. We estimate the isophotal area to be 31.2 arcsec² (1920 kpc² assuming z = 3.13). The postage-stamp images of LAB17139 are shown in Figure 4, and its properties are listed in Table 2.

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At the centroid of its Lyα emission, no apparent counterpart exists in any of the broadband data (gri). If its Lyα emission originates from a single galaxy, its continuum luminosity is fainter than r = 28.6 mag (2σ). We do not find any plausible galaxy candidate in its vicinity that may lie at the same redshift. There are two UV-bright sources just outside the isophote (one directly north and the other at the southwestern end); having the u − g color of 0.57 ± 0.19 and 0.90 ± 0.12, neither of them satisfies our LBG selection. Therefore, it is unlikely that they lie at the same redshift as LAB17139.

We search for its possible infrared counterpart utilizing two publicly available Spitzer observations in the D1 field, namely, the Spitzer Wide-area InfraRed Extragalactic survey (SWIRE; Lonsdale et al. 2003) and the Spitzer Extragalactic Representative Volume Survey (SERVS; Mauduit et al. 2012). The former includes all IRAC and MIPS bands, while the latter was taken as part of post-cryogenic IRAC observations (3.6 and 4.5 μm bands only, which are deeper than the SWIRE counterpart). In Figure 4, we show postage-stamp images of these data centered on LAB17139.

A single IR-bright source is identified within the LAB isophote that lies ≈12 away from the center of LAB17139; the source is securely detected in the 3.6 and 4.5 μm bands and marginally detected in the MIPS 24 μm band but not in the 70 μm band. In the optical (gri) images, the source appears very diffuse and spans at least 2''. If it is a single source, it is likely an interloper, as it is too large to lie at z ≳ 3. Given its clear positional offset from the centroid of the LAB, it is unlikely that the source is solely responsible for the Lyα emission. Thus, the physical association of this diffuse source and LAB17139 remains unclear.

Intrinsic Size of LAB17139. We investigate the intrinsic size of LAB17139 by carrying out extensive image simulations. First, we insert artificial point sources with a range of luminosities into the Lyα image after convolving them with the image PSF, and we recover them using the same detection setting as our LAB search. On the top left panel of Figure 5, we show how measured isophotal size correlates with luminosity for point sources (gray symbols). It is evident that the majority of our LAEs follow the same sequence except for a few highest-luminosity LAEs. On the other hand, LAB17139 lies well above the point-source locus, i.e., its high luminosity is insufficient to explain its large size.

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Having established that the source is extended, we repeat the simulation, but this time assuming that the radial profile of the source declines exponentially: S(r) ∝ exp[−1.6783 (r/r_s)]. In Figure 5, we show the luminosity−isophotal area scaling relation for the sources with half-light radii of 3'' (r_s = 18), 5'' (r_s = 30), and 8'' (r_s = 48); at z = 3.13, these values correspond to 24, 39, and 63 kpc, respectively. At a fixed line luminosity, the scatter in the recovered isophotal area increases with sizes as expected owing to lower surface brightness. Nevertheless, Figure 5 shows that a unique scaling relation exists at a fixed intrinsic size.

Utilizing this trend, with luminosities fixed in the simulation, we estimate that the half-light radius of LAB17139 must lie in the range of 39–55 kpc, provided that its surface brightness falls exponentially. Based on the average stack of 11 Lyα blobs at z = 2.65, Steidel et al. (2011) reported the exponential scale length of r_s = 27.6 kpc, which corresponds to a half-light radius of 46.4 kpc. Thus, we conclude that LAB17139 has a similar size to z ∼ 2.6 LABs.

In Figure 5 (right panel), we also show the line luminosity distribution of known Lyα blobs at z = 2–4 (Matsuda et al. 2004; Dey et al. 2005; Yang et al. 2010; Erb et al. 2011; Bădescu et al. 2017). LAB17139 lies at a relatively high luminosity regime. The size distribution of LABs is more difficult to characterize because the measured isophotal size of an LAB is determined by the combination of intrinsic source brightness, redshift, and imaging sensitivity. For example, with everything else being equal, the same source can have larger isophotal size as the imaging depth increases. In order to construct the intrinsic size distribution of Lyα nebulae, image simulations such as the one adopted here need to be run on each of the relevant data sets.

The LAE distribution in the sky appears to be highly inhomogeneous, suggesting that there may be overdense structures. To quantify their spatial distribution, we start by estimating the mean LAE density. After removing the regions near saturated stars (hatched circular regions in Figure 6), the effective area is 1156 arcmin², over which 93 LAEs are distributed. Thus, the LAE surface density is $\bar{{\rm{\Sigma }}}$ = 0.08 ±0.01 arcmin⁻², where the error reflects the Poisson noise.

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To create an LAE density map, we place point sources in the masked regions whose numbers are commensurate with that expected at random locations to avoid producing artificial underdensities. On the positional map containing 93 LAEs and point sources, we apply a Gaussian kernel of an FWHM of 10 Mpc (51; σ = 4.25 Mpc). A similar smoothing scale has been used to identify LAE overdensities in the literature (e.g., Lee et al. 2014; Bădescu et al. 2017). The resultant map is shown in the top left panel of Figure 6 as contour lines and gray shades. The contour line values represent the local surface density relative to the mean value. The positions of individual LAEs are also shown.

The highest LAE overdensity is located ∼5' west of the field center. Twenty-one galaxies are enclosed within the purple contour (2.4 $\bar{{\rm{\Sigma }}}$ isodensity line), within which the effective area is 72.8 arcmin² (275 Mpc²). We choose this region as the LAE overdensity. Scaling from the mean LAE surface density (0.08 arcmin²), the expected number of galaxies within this region is 5.8 ± 2.4. Thus, the region contains 3.6 times more galaxies than expected (${\delta }_{{\rm{\Sigma }}}\equiv ({\rm{\Sigma }}-\bar{{\rm{\Sigma }}})/\bar{{\rm{\Sigma }}}=2.6\pm 0.8$).

We recompute the mean density after excluding those in the LAE overdensity and obtain $\bar{{\rm{\Sigma }}}$ = 0.067 ± 0.008 arcmin⁻²; this estimate is insensitive to inclusion or exclusion of the "D1UD01" region, which contains only a few LAEs. The revised overdensity is δ_Σ = 3.28 ± 0.94. Interestingly, LAB17139 is located at the outskirts of the LAE overdensity (see Section 5.2 for more discussion).

We test the robustness of our overdensity estimate by computing the number of LAEs expected in our survey assuming the field Lyα luminosity functions at z ∼ 3.1 (Gronwall et al. 2007; Ouchi et al. 2008). The expected number of LAEs in a magnitude bin [m_k, m_k + Δm] and redshift bin [z_j, z_j + Δz] is

$$\begin{eqnarray}&&{N}_{\mathrm{LAE}}({z}_{j},{m}_{k})={V}_{j}p({m}_{k})S({z}_{j}){\int }_{{L}_{k}}\phi (L){dL},\end{eqnarray} \tag{ 3 }$$

where S(z) is the normalized redshift selection function, defined from the effective filter transmission T(λ) of the o3 filter expressed as S(z_j) ≡ T(1215.67 × (1 + z_j))/max(T), V_j is the effective comoving volume, and p(m_k) is the completeness limit of the o3 image in the magnitude bin, which is derived from our image simulations of point sources. For our calculation, we use Δz = 0.003 and Δm = 0.1 mag. The total number of LAEs is ${N}_{\mathrm{LAE}}={\displaystyle \sum }_{j}{\displaystyle \sum }_{k}{N}_{\mathrm{LAE}}({z}_{j},{m}_{k})$.

The expected number of LAEs in our field is 71 ± 8 using the Gronwall et al. (2007) best-fit parameters and 102 ± 10 using the Ouchi et al. (2008) values. As for the errors, we assume Poisson statistics, which are underestimated, as they do not include cosmic variance. The observed number of LAEs in our survey field is consistent with that expected in an average field. Using these values as the field LAE density, the overdensity outlined by the purple contour in Figure 6 is δ_Σ = 2.2–3.6, consistent with our previous estimate.

The significance of the newly discovered LAE overdensity is comparable to those found in known structures in the literature. Kurk et al. (2000) reported an LAE overdensity of δ_Σ = 3 around a radio galaxy at z = 2.16 (Venemans et al. 2007). Another radio galaxy at z = 4.1 is associated with an LAE overdensity of δ_Σ = 3.7 (Venemans et al. 2002, 2007). The line-of-sight distances probed by these surveys are similar to this study (Δz ≈ 0.04–0.05). Lee et al. (2014) reported two structures with similar LAE overdensities, which were later confirmed spectroscopically as protoclusters (Dey et al. 2016a).

If the LAE overdensity we discovered at z ∼ 3.13 represents a genuine protocluster, the same region is expected to be traced by non-LAEs at the same redshift. Existing observations suggest that LAEs represent a subset of star-forming galaxies likely observed through sight lines with the lowest optical depths (Shapley et al. 2003) and otherwise obey similar scaling relations to UV color-selected star-forming galaxies (e.g., Lee et al. 2014; Shi et al. 2019). However, some LAEs appear to have lower metallicities, have higher ionization parameters (Finkelstein et al. 2011; Nakajima et al. 2013; Song et al. 2014), and be less massive with younger ages (e.g., Gawiser et al. 2007; Guaita et al. 2011; Hathi et al. 2016) than non-LAEs.

Isolating non-LAEs at the same redshift is a formidable task. In principle, similar to the LAE selection, one can look for narrowband "deficit" sources to find galaxies with strong Lyα absorption (e.g., Steidel et al. 2000). However, the depth of our imaging data is inadequate for this method to be effective. Alternatively, one can use the surface density of LBGs as a proxy to search for high overdensity regions. Using the Millennium simulations, Chiang et al. (2013) demonstrated that the progenitors of the most massive galaxy clusters reside in regions of elevated densities even at the redshift smoothing scale of Δz ≲ 0.2–0.3. Observationally, several confirmed protoclusters are discovered initially as LBG overdense regions (e.g., Lee et al. 2014; Toshikawa et al. 2016).

To investigate the possibility of LBG overdensities, we use two LBG samples, namely, our fiducial LBG sample selected from the T0007 catalog, and one from the T16 catalog constructed by Toshikawa et al. (2016). The numbers of LBGs in these catalogs are slightly different: 6913 and 7793, respectively, which mainly reflects the differences in their source detection setting as discussed in Section 3.2.

Similar to the LAE density map, we smooth the positions of each LBG using a Gaussian kernel. In determining the size of a smoothing kernel, two factors need to be taken into consideration: the source surface density and the volume within which a galaxy overdensity is enclosed. For instance, using a kernel size smaller than the typical distance between two nearest neighbors is undesired, as most "overdensities" will consist of a single galaxy. On the other hand, using too large a kernel size effectively averages out cosmic volumes that are much greater than a typical size of a galaxy overdensity, thereby washing away the very signal one is searching for.

In the case of LBGs, the source density is sufficiently high (and the distance to the nearest neighbor small) that the cosmic volume consideration becomes the main determinant of the kernel size. Toshikawa et al. (2016) used a top-hat filter with a diameter 1.5 Mpc (physical) in their search of z ∼ 3–6 LBG overdensities. The size was justified as a typical angular size enclosing protoclusters in cosmological simulations (Chiang et al. 2013). At z = 3.13, this corresponds to 6.2 Mpc.

In the bottom left and right panels of Figure 6, we show the resultant LBG maps using a smoothing FWHM of 6 Mpc. Gray scales and contour lines indicate the density fluctuations, together with the positions of the LAEs (cyan circles) and five spectroscopic sources at z = 3.13 (green squares).

Several overdensities are present, but each with a much lower significance than our LAE overdensity. This is not surprising considering the line-of-sight distances sampled by them. Our image simulation suggests that the FWHM in the redshift selection function, at r ≈ 24.5, is Δz = 0.7, corresponding to 666 Mpc. Using the o3 filter FWHM, the LAE redshift range is z = 3.109–3.155, spanning just 44 Mpc in the line-of-sight distance, more than an order of magnitude smaller than that of the LBGs. The large Δz of the LBGs can also result in artificial overdensities because of chance alignments along the line of sight. It is easy to understand that, even in a sight line of a massive protocluster, LBGs with no physical association with the structure will outnumber those in it.

In both LBG density maps (T0007 and T16 LBG samples), smaller overdensities and underdense regions spanning ≳20 Mpc are found in identical locations, and the largest and most significant overdensity structures are found at the western end of the field. In the T0007 map, the region consists of three adjacent overdensities labeled as "A," "B," and "C" in Figure 6. In the T16 map, the overdensity A is noticeably more pronounced, while the B and C overdensities, which are merged into a single overdensity, appear less significant.

The LAE overdensity largely coincides with the "B+C" region and stretches toward the "A" region, where a concentration of four LAEs lies. It is intriguing that in the "A" region, which Toshikawa et al. (2016) found to be the most significant LBG overdensity, relatively few LAEs are found. Assuming that all LAE candidates lie at z = 3.13, there are a total of just six galaxies in the "A" region (including the spectroscopic sources that barely escape the LAE selection). In comparison, the LAE overdensity contains 21 LAEs. Extending the overdensity region slightly would include two additional LAEs and one Lyα nebula (Section 3.3).

Comparing the LAE and LBG maps, it is evident that their sky distributions are disparate. Other than the main LAE overdensity, none of the LAE density peaks coincide with the LBG overdensities. This likely suggests that there is only a single large-scale structure that exists at z = 3.13, and smaller LAE overdensities are a product of Poisson fluctuations or, alternatively, belong to much less significant cosmic structures than the one that the main LAE overdensity inhabits.

Next, we make a quantitative comparison of the LBG and LAE distributions in the general LAE overdensity region. To this end, we perform a series of 2D Kolmogorov–Smirnov (K-S) tests (Peacock 1983; Fasano & Franceschini 1987). We define a rectangular region enclosing the LAE overdensity as outlined in the left panel of Figure 6 whose area is 115 arcmin². Running the 2D K-S test in the LBG and LAE distributions yields the p value of 0.28 (0.20) using the fiducial (T16) LBG samples. The large p value indicates the similarity of the two distributions.

As the 2D K-S test is less reliable than the 1D test, we perform a control test to interpret the p values. First, we create two random samples that are uniformly distributed in the rectangular region, each matching the number of LAEs and LBGs in our samples, and calculate the corresponding p value. The process is repeated 1000 times, and the p value is recorded each time. We obtain a median (mean) p value of 0.23 (0.27). Second, we assume that the underlying distribution is a 2D Gaussian function with σ = 5' centered at the middle of the rectangle, and we repeat the test, obtaining similarly large p values (median and mean value of 0.27 and 0.31).

Finally, we test the similarity of the two galaxy samples in the entire field by moving the rectangle to random locations. Whenever a masked region falls within the subfield, we randomly populate the area with the expected number of point sources therein before performing the test. The median (mean) p value is 4.5 × 10⁻¹⁵ (9 × 10⁻⁴). These tests give strong support to the possibility that the cosmic structure traced by the LAE overdensity is also well populated by LBGs at a level not observed in other parts of the survey field.

All in all, our analyses strongly suggest the presence of a significantly overdense cosmic structure, which includes 21 LAEs, 5 spectroscopically confirmed z = 3.13 LBGs, and 1 luminous Lyα nebula. A large number of LBGs exist in the general region, although without spectroscopy it is difficult to know how many of them truly belong to the structure. A segregation of the highest LAE and LBG overdensity is also curious. We discuss possible implications of our results in Section 5.2.

5.1.1. The Present-day Mass of the LAE Overdensity

Given that the o3 filter samples ≈44 Mpc in the line-of-sight direction, a surface overdensity computed based on the angular distribution of galaxies should scale closely with a given intrinsic galaxy overdensity with a minimal contamination from foreground and background interlopers. In this section, we estimate the true galaxy overdensities and infer their descendant (present-day) masses.

Based on the Millennium runs (Springel et al. 2005), Chiang et al. (2013) calibrated the relationship between galaxy overdensity (δ_g) and present-day mass M_z=0 at a given redshift. Galaxy overdensity is measured in a (15 Mpc)³ volume (δ_g,15 hereafter) using the galaxies whose host halos have the bias value of b ≈ 2. This value is comparable to that typically measured for LAEs (Gawiser et al. 2007; Guaita et al. 2010; Lee et al. 2014), which lie at similar redshift and are of comparable line luminosities to those in our sample. Thus, it is safe for us to apply the Chiang et al. calibration without further corrections.

The transverse area enclosing the LAE overdensity is 275 Mpc², reasonably close to that of (15 Mpc)² used by Chiang et al. (2013). However, the line-of-sight distance sampled by the o3 filter is ∼3 times larger than their sampled volume. Generally, averaging over a larger volume reduces the significance of the overdensity. We correct this effect using their Figure 13, where they show how, for a fixed δ_g,15, measured (surface) overdensity drops with increasing redshift uncertainty Δz. Correcting the measured overdensity (Section 4.1) accordingly results in δ_g,15 = 5.5 ± 1.6. Inferred from Figure 10 of Chiang et al. (2013), the corresponding descendant mass at z = 0 is M_tot ≈ (0.6–1.3) × 10¹⁵ M_⊙. The estimated overdensity well exceeds the value δ_g,15 = 3.14, above which there is >80% confidence that it will evolve into a galaxy cluster by z = 0. These considerations lend confidence that the newly identified LAE overdensity is a genuine massive protocluster.

Alternatively, a more empirical method may be employed similar to that taken by Steidel et al. (1998). If all mass enclosed within the overdensity will be gravitationally bound and virialized by z = 0, the total mass can be expressed as

$$\begin{eqnarray}&&{M}_{z=0}=(1+{\delta }_{m})\langle \rho \rangle {V}_{\mathrm{true}},\end{eqnarray} \tag{ 4 }$$

where $\langle \rho \rangle$ is the mean density of the universe and V_true is the true volume of the overdensity. The matter overdensity δ_m is related to the galaxy overdensity through a bias parameter. The bias parameter b can be described as 1 + bδ_m = C(1 + δ_g), where C represents the correction factor for the effect of redshift-space distortion due to peculiar velocities. The true volume V_true is underestimated by the same factor as V_true = V_obs/C. In the simplest case of spherical collapse, it is expressed as

$$\begin{eqnarray}&&C({\delta }_{m},z)=1+{{\rm{\Omega }}}_{m}^{4/7}(z)[1-{\left(1+{\delta }_{m}\right)}^{1/3}].\end{eqnarray} \tag{ 5 }$$

Equation (5) and the equation relating the matter and galaxy overdensity can be evaluated iteratively to determine C and δ_m. The observed overdensity is δ_g = 3.3 ± 0.9, and the estimated survey volume is V_obs = 1.21 × 10⁴ Mpc³. The bias value is assumed to be b ≈ 2 (Gawiser et al. 2007; Guaita et al. 2010; Lee et al. 2014). We obtain the matter overdensity in the range of δ_m = 0.8–1.3; thus, the total mass enclosed in this overdensity is M_z=0 ≈ (1.0–1.5) × 10¹⁵ M_⊙, in good agreement with the simulation-based estimate. We conclude that the LAE overdensity will evolve into a Coma-like cluster by the present-day epoch (see, e.g., Kubo et al. 2007).

5.1.2. The Structure and Descendant Mass of the LBG Overdensity

The observational data presented in this work paint an incomplete picture, leaving several unanswered questions. First, the spatial segregation between the LAE- and LBG-traced structure is puzzling because the spectroscopically confirmed sources in the latter lie at the same redshift as the former. If the galaxies in the LBG overdensity trace a single structure, the implication would be that structure "A" genuinely lacks Lyα-emitting galaxies whereas structure "B+C" is populated by both LAEs and LBGs (and one large Lyα nebula).

The expected comoving size of galaxy clusters observed at z ∼ 3 is in the range of 15–20 Mpc (Chiang et al. 2013), in agreement with recent estimates of several confirmed protoclusters (Dey et al. 2016a; Bădescu et al. 2017). In comparison, the projected end-to-end size of the combined structure, at ≈30 Mpc, is simply too large, suggesting that the two are two separate structures. If they lie at the same redshift (i.e., the projected distance is close to the true separation), the dynamical timescale ($\tau \sim \sqrt{{R}^{3}/{GM}}$) is on the order of the Hubble time.

If "A" and "B+C" represent two separate systems, we speculate on several physical scenarios consistent with the current observational constraints. First, we may be witnessing galaxy assembly bias: a baryonic response to the well-known halo assembly bias. The latter generally refers to the fact that the spatial distribution of dark matter halos depends not only on mass but also on other properties such as concentration parameter, spin, large-scale environment, and halo formation time (e.g., Gao et al. 2005; Wechsler et al. 2006; Li et al. 2008; Zentner et al. 2014). In the present case, a given halo's environment and formation time are particularly relevant considering that clusters are expected to be the sites of the earliest star formation in the densest environment.

Zehavi et al. (2018) examined the importance of the halo formation time and its large-scale environment in determining the relations of halo mass (M_h) to stellar mass (M_star) and of halo mass to clustering strength scaling. They found that, while controlling for M_h, halos with an earlier formation time tend to host galaxies with larger M_star, have fewer satellites, and are more strongly clustered in space compared to those that formed at a later time. Similar dependence was found for the halos' large-scale environment (measured as dark matter density smoothed in a 5h⁻¹ Mpc scale, where h = 0.7), where a higher density and earlier formation time have similar effects on galaxies' properties.

In this context, we speculate that the "B+C" structure may have formed more recently than the "A" structure and thus is traced by numerous young and low-mass galaxies, many of which are observed as LAEs (see, e.g., Guaita et al. 2010, 2011). In comparison, as an older and more settled system, the "A" protocluster has had more time to accrete surrounding matter and to merge with lower-mass satellite halos, and thus it is expected to have more evolved (i.e., more stars and dust) star-forming galaxies that are observed as LBGs, while having fewer low-mass systems such as LAEs. Zehavi et al. (2018) also found a strong variation in their clustering amplitude at scales 5–10 Mpc (see their Figure 10). One implication may be that it is not appropriate to use a galaxy bias representative of the field galaxy population of the same type (as we have done in Section 5.1) in estimating the total descendant mass of a structure that likely formed the earliest.

A slight variant of the above hypothesis is that while the two have similar present-day masses, "A" is simply a more massive and relaxed halo at the time of observation than "B+C," which is an aggregate of two or more smaller halos. Similar environment-dependent processes are expected to those in the assembly bias scenario.

In these conjectures, it follows that both LAE- and LBG-based protocluster searches would be sensitive to different evolutionary stages (or ages) of cluster formation in which the former (latter) method favors younger (older) structures. Similarly, the presence of galaxies with old stellar populations should be predominantly found in the LBG-selected structures but not in LAE-traced ones. While several known structures support this picture (Steidel et al. 2005; Wang et al. 2016; Shi et al. 2019), only a handful of protocluster systems have been characterized using multiple galaxy tracers (including LAEs; e.g., Steidel et al. 2000; Kurk et al. 2000, 2004) in a similar manner to the present work, making it difficult to evaluate the validity of such a hypothesis. A rigorous test would require a well-controlled statistical approach in which large samples of LBG- and LAE-selected overdensities are identified independently and compared for the level of their cohabitation. The same test can also inform us about how baryonic physics can impact the manner in which galaxies trace the underlying large-scale structure in dense environments (e.g., Orsi et al. 2016).

The final and most innocuous scenario to explain the curious configuration of "A" and "B+C" is as follows: the spectroscopically confirmed sources embedded in the "A" region may be spatially disjoint from the majority of LBGs therein and are part of a small group falling in toward the "B+C" structure. The "A" region then could represent just another protocluster with no physical association with the LAE overdensity. It is unlikely, however, because within the "A" region Toshikawa et al. (2016) confirmed 30 galaxies between z = 2.73 and z = 3.56, and there was only one significant redshift overdensity at z = 3.13. More extensive spectroscopy in all of the "A+B+C" regions can elucidate the true configurations of these structures.

Finally, we contemplate the significance of the Lyα nebula in the context of protocluster formation. As described in Section 3.3, our search of the entire field resulted in a single LAB. The fact that it is located at the southwestern end of the LAE overdensity ("B+C") is significant.

There is mounting evidence that luminous Lyα nebulae are preferentially found in dense environments. Matsuda et al. (2004) identified 35 Lyα nebulae candidates in an LAE- and LBG-rich protocluster at z = 3.09 and reported that the LAEs and LABs trace one another. Yang et al. (2010) conducted a systematic search for LABs in four separate fields, each comparable in size to our survey field. The number of LABs they identified in each field ranged from 1 to 16; they argued that high cosmic variance implies a very large galaxy bias expected for group-sized halos. Small groups of galaxies are observed to be embedded in several luminous blobs (e.g., Dey et al. 2005; Yang et al. 2011, 2014; Prescott et al. 2012) in agreement with the assessment by Yang et al. (2010).

It is notable that LAB17139 lies at the periphery of the "B+C" overdensity traced by LAEs. Recently, Bădescu et al. (2017) compiled the LAE/LAB data for five protoclusters at z = 2.3 and z = 3.1 and showed that LABs are preferentially found in the outskirts of each of the LAE overdensities. They speculated that these blobs may be signposts for group-sized halos (harboring galaxy "proto-groups") falling in toward the cluster-sized parent halo traced by LAEs, where Lyα-lit gas traces the stripped gas from galaxy–galaxy interactions.

A significant variation of their numbers implies a relatively short timescale for the LAB phenomenon; that, combined with their preferred locations at the outskirts, requires a physical explanation involving the protocluster environment. The kinematics of protocluster galaxies showing relatively low velocity dispersions¹¹ and in multiple groupings (≲400 km s⁻¹: Dey et al. 2016a) indicate that the structure is far from virialization.

If a galaxy overdensity is a superposition of multiple overdensities in physical proximity, LABs' preferred location at their outskirts may signify their first group–group interactions, enabling a host of galaxy–galaxy interactions, which in turn bring about starbursts, AGNs, and stripped gas lighting up an extended region surrounding these galaxies.

We investigate whether local environment influences the properties of the LAEs. To this end, we define two LAE subsamples according to their measured galaxy surface density. The "overdensity" sample includes 21 LAEs within the purple contour shown in Figure 6, as well as three of the Toshikawa et al. (2016) galaxies that we recover as LAEs. The remaining 69 LAEs belong to the "field" sample.

Apart from the line luminosities and EWs (Section 2.2), we also convert the measured UV continuum slope, β, to the extinction parameter E(B − V) assuming the dust reddening law of local starburst galaxies (Calzetti et al. 2000). For the sources with relatively robust β measurements (Δβ < 0.9), we also derive dust-corrected SFRs by correcting the continuum luminosity accordingly using the Kennicutt (1998) calibration. In the overdensity and field sample, 21 (88%) and 42 (61%) LAEs, respectively, have the SFR estimates. The difference stems from the fact that the former sample is on average more UV luminous (see later). However, our SFR estimates are only approximate given a relatively large uncertainty in the measured UV slopes; increasing (decreasing) β value by Δβ = 0.4 (which is well within a typical uncertainty) would lead to a 41% increase (58% decrease) in the SFR estimate.

The mean properties of each subsample are listed in Table 3, with the errors corresponding to the standard deviation of the mean, and the overall distributions of these parameters are illustrated in Figure 7. In both, we show our results for the full sample containing 93 LAEs (top) and for the 63 LAEs with reliable SFR estimates (bottom). We find that our conclusions do not change depending on which sample we consider.

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In terms of both line and continuum luminosities, we find a possible enhancement for the LAEs in the overdense regions compared to those in the field. The enhancement in UV luminosity is 74% ± 32% if we compare all LAEs in both samples, and 5%8 ± 22% if only the LAEs with robust β measurements are considered. As for Lyα line luminosity, the enhancement relative to the field is 32% ± 15% and 55% ± 18% for all LAEs and those with β measurements, respectively. The median EW and E(B − V) values are comparable in both samples.

To assess the similarity of the overall distribution of the physical quantities between the two samples, we perform the 1D K-S test. The p values obtained for each distribution are indicated in Figure 7. The values obtained for the Lyα and UV luminosity distributions lie around p ∼ 0.05, corresponding to 2σ in the confidence level. As for the EWs and UV slopes β, the distributions are statistically indistinguishable for the two environmental bins.

The same trend is visualized in Figure 8. In the left panel, we show the LAE positions overlaid with a 2D Voronoi tessellated map of the whole field (Marinoni et al. 2002; Cooper et al. 2005). Each LAE is embedded in a Voronoi polygon with an area A_V, and its 2D density scales inversely with the radius of the equivalent circular region defined as ${r}_{V}\equiv \sqrt{{A}_{V}/\pi }$. The map is color-coded by the 2D density, with the size of each star increasing with increasing density. The LAE overdensity clearly stands out as a region with the highest concentration of blue stars.

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In the other two panels of Figure 8, we show the same tessellated LAE map, but the LAEs are color-coded by Lyα (middle) and UV luminosity (right). A large fraction of blue stars representing a top third populate the combined region of the LBG and LAE overdensities. The trend is particularly evident for the case of continuum luminosity (right panel). Of the 30 blue stars, 17 (57%) reside within the LAE overdensity region. No radial dependence is found for the luminosity enhancement within the group, although our sample may be too small to discern any trend.

The overall correlation between the LAE density and its luminosity and the level of enhancement are consistent with the similar trends we reported in Dey et al. (2016a) for the constituents of another protocluster at z = 3.78. The present work takes a step further by examining the UV and line luminosity of the same galaxies, which was not possible previously owing to the relatively shallow depth of the broadband data.

The higher UV and Lyα mean luminosities observed for protocluster LAEs are curious and cannot be fully explained by the level of overdensity. If the protocluster LAEs obey the same UV or Lyα luminosity function as measured in the field but are simply scaled up by a factor of (1 + δ_g), the expected mean or median values would be identical to those in the field. Observationally, the trend can be due to either the lack of low-luminosity (and low-mass) galaxies or the excess of high-luminosity LAEs in overdense environments relative to the field.

The lack of low-luminosity, low-mass galaxies near cluster-sized halos may be caused by a variety of astrophysical processes. Orsi et al. (2016) showed that AGN feedback (from a quasar or radio galaxy hosted by the central halo) can alter the clustering and abundance of galaxies inhabiting the satellite halos. They also noted that the spatial distribution of LAEs may be more affected than other galaxy tracers—such as Hα emitters—due to complex radiative transfer effects. Cooke et al. (2014) studied radio galaxy MRC 2104-242 at z = 2.5 and reported the lack of low-mass galaxies (M_star ≲ 10¹⁰ M_⊙) in their 7 arcmin² survey field. It is certainly conceivable that there was a radio galaxy or quasar in the past (which has since turned off) that had influenced the formation histories of these galaxies. While these trends warrant further exploration with upcoming surveys such as DESI, it is far from conclusive at this time given the small sample size and areal coverage. Further, the most likely candidate hosting a powerful AGN is not at the center but at the outskirts of this structure. Future availability of deep near-infrared imaging would be helpful in closer examination of the stellar mass distribution and its radial dependence, which can be compared with those in galaxy simulations.

Alternatively, our results can be interpreted as a mild but widespread enhancement of star formation in the protocluster LAEs. One possible explanation for the higher-luminosity value may be that the luminosity function (and SFR function) is more "top-heavy" in protocluster environments, producing a larger fraction of UV-luminous galaxies. This may be brought on by faster-growing halos as suggested by Chiang et al. (2017), or by a different star formation efficiency in clusters whereby a galaxy is more luminous at a fixed halo mass (Y.-K. Chiang 2019, private communication). Alternatively, it is also possible that protocluster LAEs simply have different ages and/or metallicities than elsewhere; however, the overall similarities in observed colors and EWs in the two environmental bins studied here argue against this possibility.

Our result is seemingly at odds with some of the existing studies that found that galaxies in dense environments largely grow at a similar rate to those in average fields (Lemaux et al. 2018a; Shi et al. 2019), perhaps with an exception at the massive end (e.g., Lemaux et al. 2014). However, it is worth noting that these studies focused on more UV-luminous, LBG-like galaxies that are, on average, a factor of ≳5–10 more massive than the LAE population studied in this work. To discern a clearer trend and to study how it depends on the galaxy's luminosity and stellar mass and on galaxy types (LBGs, LAEs, etc.), a more comprehensive study is needed.

We speculate on a potential implication of our result in the cosmological context. By following the structures identified as cluster-sized dark matter halos at z = 0 in the Millennium simulations, Chiang et al. (2017) estimated that the fractional contribution to the total star formation rate density (SFRD) from galaxies that will end up in clusters increases dramatically with redshift, from only a few percent at z = 0.5–1.0, to ≈20%–30% at z = 2–4, and to nearly 50% at z > 8. This change is mainly driven by large cosmic volumes occupied by protoclusters well before their final coalescence (see their Figure 1), as well as high galaxy overdensities and the top-heavy halo mass function therein (Chiang et al. 2017).

If the observed higher luminosity of protocluster LAEs has an astrophysical origin (e.g., a higher efficiency in converting gas into stars) rather than a cosmological one, it would follow that the total contribution to the cosmic SFRD from protoclusters would be even greater than the Chiang et al. (2017) estimate. Separating out these effects will be challenging, however, and will require a much larger sample of protoclusters and a better characterization of halo statistics in different environments.

BCGs are the most massive galaxies in galaxy clusters. In the local universe, they are typically elliptical galaxies residing near the cluster center defined by the X-ray emission peak (e.g., Lin & Mohr 2004). Identification and characterization of their progenitors ("proto-BCGs") at high redshift would illuminate the early stages of their formation.

At z ∼ 3, the rest-frame optical/near-IR luminosity (0.5–1.6μm) tracing the total stellar content is redshifted into the K_S band and beyond. Thus, the most effective search should be based on the photometric properties at infrared wavelengths. Although the D1 field was imaged in the near-IR JHK_S bands by the WIRCam Deep Survey (Bielby et al. 2012), the newly discovered galaxy overdensities unfortunately lie near the edge of its coverage (see their Figure 2 for the coverage map). Approximately 20% of the area enclosing the "A" and 'B+C' structures has no K_S-band coverage, while an additional 10% of the area has only partial coverage (<50% of the full exposure of 4.7 hr).

Given this limitation, we caution that any search based on the existing data would be severely limited by the depth and areal coverage in obtaining a complete census of massive galaxies in this structure. While a more comprehensive search of massive galaxies in this region based on the Spitzer IRAC 3.6 μm detection will be presented in the future (J. Toshikawa et al. 2019, in preparation; K. Shi et al. 2019, in preparation), in this work we base our proto-BCG search on our existing LBG catalog instead, focusing on UV-luminous galaxies that already have a large stellar content. We require that a given galaxy must have r-band magnitude r ≤ 24 (roughly corresponding to ≳1.6 ${L}_{\mathrm{UV},z\sim 3}^{* }$; Reddy & Steidel 2009). In addition, to further constrain its stellar mass, it should also be well detected in the K_S-band catalog. A total of 80 galaxies satisfy these criteria, corresponding to a surface density of 0.06 ± 0.01 arcmin⁻².

In the left panel of Figure 9, we show the J − K_S colors versus K_S-band magnitudes for all selected sources. The majority have K_S > 22.5 and relatively blue J − K_S colors. Using the EZGal software (Mancone & Gonzalez 2012), we also compute the expected color and luminosity evolution assuming several different star formation histories. The Bruzual & Charlot (2003) stellar population synthesis models and local starburst-like dust reddening curve (Calzetti et al. 2000) are adopted for the calculation. The model magnitudes are normalized to a lower-redshift (z = 1.8) M_⋆ cluster galaxy in the 3.6 μm band (Mancone et al. 2010), assuming passive evolution from z ∼ 3.1 to z ∼ 1.8. The model tracks represent the time evolution of the galaxy.

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Of the 80 galaxies, only 4 reside in the combined LBG and LAE overdensity region. The area covered by the WIRDS data is 93.7 arcmin², and thus the expected number therein is 6 ± 2. The K_S-brightest galaxy (K_S = 21.05), which we dub G411155, is shown as a red circle in the left panel of Figure 9. Its location is also marked in Figure 6 (red diamond). G411155 is the reddest LBG in the entire field (J − K_S = 1.92) and would easily meet a typical color selection for distant red galaxies (DRGs) at high redshift (J − K_S > 1.4; Franx et al. 2003; van Dokkum et al. 2003). G411155 is very bright in the IRAC 8 μm and MIPS 24 μm bands, having flux densities of 0.13 and 1.43 mJy, respectively. The remaining three galaxies have relatively modest K_S-band brightness (K_S = 23–24) and are bluer (J − K_S < 1.4). None of the five galaxies have an X-ray or radio counterpart (Bondi et al. 2003; Chiappetti et al. 2005).

Using the Bielby et al. (2012) and Lonsdale et al. (2003) catalogs, we extract the multiwavelength photometry (ugrizJHK_S[3.6][4.5][5.8][8.0][24][70][160]) of G411155 using the Kron-like total fluxes. We perform the SED fitting with the CIGALE software (Noll et al. 2009; Boquien et al. 2019) using both galaxy and AGN templates. Star formation histories are modeled as an exponentially declining function with characteristic timescale τ values of 100 Myr to 1 Gyr. AGN models from Fritz et al. (2006) are used as templates.

In the right panel of Figure 9, we show the best-fit SED model together with the photometric measurements. In the inset, we also show the photometric redshift probability density, which peaks at z ∼ 3.1. The best-fit physical parameters suggest that G411155 has a short star-forming timescale with a luminosity-weighted age of ≈200 Myr. The model fit also suggests a dust-obscured AGN component that dominates the infrared energy budget: 70% of the total IR luminosity originates from the AGN. The galaxy is already ultramassive at M_star ≈ 2 × 10¹¹ M_⊙, and it continues to form stars at a rate of SFR ∼ 500 M_⊙ yr⁻¹!

We compare our proto-BCG candidate with those found in the literature. Lemaux et al. (2014) identified a proto-BCG candidate in a z ∼ 3.3 protocluster. It contains a powerful Type I AGN (relatively unobscured by dust with broad lines) with a K_S-band magnitude of 20.67 (z − K_S = 0.1) with an estimated stellar mass and age of ∼8 × 10¹⁰ M_⊙ and ∼300 Myr, respectively. The SFR inferred from the total IR luminosity is ∼750 M_⊙ yr⁻¹. Although we do not have a spectrum for G411155, these two proto-BCG candidates have comparable physical properties.

Near the center of a protocluster at z = 3.09, Kubo et al. (2015, 2016) discovered a dense group of massive galaxies consisting of seven K_S bright (K_S ∼ 22–24) and red galaxies (J − K_S > 1.1) with a combined stellar mass of ≈6 × 10¹¹ M_⊙. They argued that the group is likely in the merger phase, which will evolve into a BCG observed in the local universe. Wang et al. (2016) reported an overdensity of 11 massive (≳10¹¹ M_⊙) DRGs within a compact core (80 kpc) in another z = 2.51 structure and speculated that their findings may signify a rapid buildup of a cluster core. Identifying and studying similar systems in a larger sample of protoclusters will elucidate evolutionary stages of cluster BCGs.

Finally, G411155 lies very close to the spectroscopic sources at z = 3.13, in particular four sources, two of which are also LAEs, as illustrated in Section 4.1. Given its photometric redshift (see inset of Figure 9), it is possible that these galaxies are members of the same group, which is falling toward the center of its parent halo located at a projected distance ≈1 Mpc (physical) away from it. Given its optical brightness, it should be relatively easy to measure its redshift and thereby unambiguously determine its physical association with these sources.