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What is the physical origin of strong Lya emission? II. Gas Kinematics and Distribution of Lya Emitters

We present a statistical study of velocities of Lya, interstellar (IS) absorption, and nebular lines and gas covering fraction for Lya emitters (LAEs) at z~2. We make a sample of 22 LAEs with a large Lya equivalent width (EW) of>50A based on our deep Keck/LRIS observations, in conjunction with spectroscopic data from the Subaru/FMOS program and the literature. We estimate the average velocity offset of Lya from a systemic redshift determined with nebular lines to be dv_Lya=234+-9 km s-1. Using a Kolmogorv-Smirnov test, we confirm the previous claim of Hashimoto et al. (2013) that the average dv_Lya of LAEs is smaller than that of LBGs. Our LRIS data successfully identify blue-shifted multiple IS absorption lines in the UV continua of four LAEs on an individual basis. The average velocity offset of IS absorption lines from a systemic redshift is dv_IS=204+-27 km s-1, indicating LAE's gas outflow with a velocity comparable to typical LBGs. Thus, the ratio, R^Lya_ IS = dv_Lya/dv_IS of LAEs, is around unity, suggestive of low impacts on Lya transmission by resonant scattering of neutral hydrogen in the IS medium. We find an anti-correlation between Lya EW and the covering fraction, f_c, estimated from the depth of absorption lines, where f_c is an indicator of average neutral hydrogen column density, N_HI. The results of our study support the idea that N_HI is a key quantity determining Lya emissivity.

1. INTRODUCTION Lyα Emitters (LAEs) are an important population of high-z star-forming galaxies in the context of galaxy formation. LAEs at z = 2 − 7 and beyond z = 7 are found by narrow-band (NB) imaging observations based on an NB excess resulting from their prominent Lyα emission (e.g., Cowie et al. 2010;Gronwall et al. 2007;Ciardullo et al. 2012;Ouchi et al. 2008;Ota et al. 2008;Ouchi et al. 2010;Hu et al. 2010;Finkelstein et al. 2007;Kashikawa et al. 2011Kashikawa et al. , 2006Shibuya et al. 2012). Observational studies on a morphology and spectral energy distribution (SED) of LAEs reveal that such a galaxy is typically young, compact, less-massive, lessdusty than other high-z galaxy populations, and a possible progenitor of Milky Way mass galaxies (e.g., Gronwall et al. 2011;Guaita et al. 2011;Ono et al. 2010;Gawiser et al. 2007;Dressler et al. 2011;Rauch et al. 2008;Dijkstra & Kramer 2012). Additionally, LAEs are used to measure the neutral hydrogen fraction at the reionizing epoch, because Lyα photons are absorbed by intergalactic medium (IGM).
Lyα emitting mechanism is not fully understood due to the highly-complex radiative transfer of Lyα in the interstellar medium (ISM). Many theoretical models have predicted that the neutral gas and/or dust distributions surrounding central ionizing sources are closely linked to the Lyα emissivity (e.g., Neufeld 1991;Finkelstein et al. 2008;Laursen et al. 2013Laursen et al. , 2009Laursen & Sommer-Larsen 2007;Duval et al. 2013;Zheng & Wallace 2013;Zheng et al. 2010;Yajima et al. 2012). Thus, resonant scattering in the neutral ISM can significantly attenuate the Lyα emission.
Lyα emissivity may not only depend on the spatial ISM distribution, but on the gas kinematics as well. The large-scale galactic outflows driven by starbursts or active galactic nuclei could allow Lyα photons to emerge at wavelengths where the Gunn-Peterson opacity is reduced, and consequently enhance the Lyα emissivity, particularly in the high-z Universe (e.g., Dijkstra & Wyithe 2010). The outflow may also blow out the Lyα absorbing ISM. The gas kinematics of LAEs has been evaluated from the Lyα velocity offset (∆v Lyα ) with respect to the systemic redshift (z sys ) traced by nebular emission lines (e.g, Hα, [O iii]) from their H ii regions. In the past few years, deep NIR spectroscopic studies have detected nebular emission lines from ∼ 10 LAEs at z = 2 − 3, and measured their ∆v Lyα (McLinden et al. 2011;Hashimoto et al. 2013;Guaita et al. 2013;Finkelstein et al. 2011;Chonis et al. 2013). The Lyα emission lines for these LAEs are redshifted from their z sys by a ∆v Lyα of 200 − 300 km s −1 . Hashimoto et al. (2013) find an anti-correlation between Lyα equivalent width (EW) and ∆v Lyα in a compilation of LAE and LBG samples. This result is in contrast to a simple picture where Lyα photons more easily escape in the presence of a galactic outflow.
However, the Lyα velocity offset is thought to increase with both resonant scattering in H i gas clouds as well as galactic outflow velocity (e.g., Verhamme et al. 2006Verhamme et al. , 2008. The anti-correlation could result from a difference in H i column density (N HI ) rather than outflowing velocity. The gas kinematics can be investigated more directly from the velocity offset between interstellar (IS) absorption lines of the rest-frame UV continuum and z sys (IS velocity offset; ∆v IS ). The IS velocity offset traces the speed of outflowing gas clouds, and may help to distinguish the two effects on ∆v Lyα .
For UV-continuum selected galaxies, the ∆v IS has been measured for > 100 objects (e.g. Pettini et al. 2001;Christensen et al. 2012;Kulas et al. 2012;Schenker et al. 2013;Steidel et al. 2010). Steidel et al. (2010) find that LBGs have an average of ∆v IS = −164 km s −1 in their sample of 89 LBGs at z ∼ 3. This statistical study indicates ubiquitousness of galactic outflow in LBGs. However, there have been no NB-selected galaxies with a ∆v IS measurement to date except for a stacked UV spectrum in Hashimoto et al. (2013). This is because it is difficult to estimate ∆v IS for individual LAEs, especially for galaxies with a large Lyα EW of 50Å due to their faint UV-continuum emission, while ∆v IS are measured for some UV-selected galaxies with EW(Lyα) ∼ 50Å (e.g., Erb et al. 2010). A statistical investigation of Lyα kinematics for LAEs could shed light on the physical origin of the anti-correlation and the underlying Lyα emitting mechanism. This is the second paper in the series exploring the Lyα emitting mechanisms 9 . In this paper, we present the results of our optical and NIR spectroscopy for a large sample of z = 2.2 LAEs with Keck/LRIS and Subaru/FMOS to verify possible differences of ∆v Lyα and ∆v IS between LAEs and LBGs. These spectroscopic observations are in an extension of the project of Hashimoto et al. (2013) aiming to confirm the anti-correlation between Lyα EW and ∆v Lyα . The organization of this paper is as follows. In Section 2, we describe the details of the LAEs targeted for our spectroscopy. Next, we show our optical and NIR spectroscopic observations in Section 3. We present methods to reduce the spectra, and to measure kinematic quantities such as ∆v Lyα and ∆v IS in 4. We perform SED fitting to derive physical properties in Section 5. We compare kinematic properties between LAEs and LBGs in Section 6, and discuss physical origins of possible differences in these quantities in Section 7. In the last section Section 8, we summarize our findings.
2. TARGETS FOR SPECTROSCOPY Our targets for optical and NIR spectroscopy are z = 2.2 LAEs selected by observations of the 9 The first paper presents a study on LAE structures (Shibuya et al. 2014).
Subaru/Suprime-Cam (Miyazaki et al. 2002) equipped with the narrow-band (NB) filter, NB387 (λ c = 3870Å and FWHM = 94Å) (Nakajima et al. 2012. The details of observations and selection for LAEs are given in these papers, but we provide a brief description as follows. The Suprime-Cam observations have been carried out for LAEs at z = 2.2 with NB387 in a total area of ∼ 1.5 square degrees. Based on the color selection of B − N B387 and u * − N B387, the Suprime-Cam observations have located 619, 919, 747, 950, and 168 LAEs in the Cosmic Evolution Survey (COSMOS) (Scoville et al. 2007), the Subaru/XMM-Newton Deep Survey (SXDS) (Furusawa et al. 2008), the Chandra Deep Field South (CDFS) (Giacconi et al. 2001), the Hubble Deep Field North (HDFN) (Giavalisco et al. 2004), and the SSA22 (e.g., Steidel et al. 2000) fields, respectively. In the above five fields, a total of ∼ 3400 LAEs have been selected down to a Lyα EW of 20 − 30Å in rest-frame (Nakajima et al. in prep.). This large sample size enables us to study statistically various properties of high-z LAEs, such as structural properties (Shibuya et al. 2014) and the statistics of Lyα halos (Momose et al. 2014 Oke et al. 1995;Steidel et al. 2004) on the Keck I telescope in order to detect their redshifted Lyα emission lines. We used six multi-object slit (MOS) masks for LAEs selected in the NB387 imaging observations in the COSMOS, HDFN, HUDF, SSA22, and SXDS fields. The mask for the objects in the HUDF includes two LAEs whose nebular emission lines were detected in the 3D-HST survey (H. Atek et al. in preparation). The total number of LAEs observed with these LRIS masks is 83. The observations were conducted on March 19-21 and November 14-15, 2012 (UST) with seeing sizes of 0. ′′ 7-1. ′′ 6. Spectrophotometric standard stars were observed on each night for flux calibrations. The spectral resolution is R ∼ 1000. The number of observed LAEs, grisms, central wavelength and observing time in each slit-masks are summarized in Table 1  Our LRIS spectra in each MOS mask are reduced with the public Low-Redux (XIDL) pipeline 10 , for longslit and multi-slit data from the spectrographs on the Keck, Gemini, MMT, and Lick telescopes. We reduce the spectra of LAEs with this software in the following manner. First, we create flats, calibrate wavelengths with the arc data, and reject sources illuminated by the cosmic ray injections for 2-D spectra in the MOS masks. Next, we automatically identify emission lines and continua, and trace them in each slit in individual one-frame masks. After the source identification, we subtract the sky background, and correct for the distortion of the 2-D MOS mask images using sky lines. According to the information on the source identifications, we extract 1-D spectra from each slit in individual mask images. Finally, we stack the extracted 1-D spectra.
The public XIDL software extracts 1-D spectra from each one-exposure frame before combining these 2-D mask images. This process makes it difficult to detect faint emission lines and continua that are undetectable in individual one-exposure images. Then, we additionally search for faint emission lines from stacked 2-D images by visual inspection after combining one-exposure frames.
In total, the Lyα emission lines are detected from 26 objects in the LRIS spectroscopy. Figure 1 shows the spectroscopic success rate in the detection of Lyα emission. The success rate is ∼ 70% for bright objects with NB387 24.5. However, low detection and/or selection completeness at NB387 24.5 reduces largely the success rate (∼ 20%). The photometric and spectroscopic properties of these Lyα-detected objects are listed in Table 2. Among these LRIS spectra, we identify eight LAEs with detections of Lyα and nebular emission lines excluding AGN-like objects.

Measurement of Lyα Velocity Offset
We measure the Lyα velocity offset for the eight LAEs with detections of Lyα and nebular lines: where c, z Lyα , and z sys , are the speed of light, and Lyα and systemic redshifts, respectively. The systemic redshift is determined from nebular emission lines obtained with FMOS.  Prior to the measurement of ∆v Lyα , we measure the wavelength of Lyα in the following line-fitting procedures. We use the peak wavelength of the best-fit asymmetric Gaussian profile for measurements of the Lyα wavelength. We first search automatically for an emission line in a wavelength range of 3500 − 4000Å in each spectrum. This range includes the wavelength range of the NB387 filter. Next, we fit an asymmetric Gaussian profile to the detected lines. The asymmetric Gaussian profile is expressed as where A, λ 0 , and f 0 are the amplitude, peak wavelength of the emission line, and continuum level, respectively. The asymmetric dispersion, σ asym , is represented by σ asym = a asym (λ − λ asym 0 ) + d, where a asym and d are the asymmetric parameter and typical width of the line, respectively. An object with a positive (negative) a asym value has a skewed line profile with a red (blue) -Observed Lyα emission line (black) for an example LAE, 10600, and its best-fit profiles. The curves are the best-fit symmetric Gaussian profiles in the wavelength range where the flux drops to 70 % (green) and 10 % (blue) of its peak, and the asymmetric Gaussian profile (red). The vertical bold lines denote the corresponding peak wavelengths of the best-fit profiles. The peak wavelengths are 3903.77, 3904.02, and 3903.62Å, respectively, with the central, symmetric Gaussian, and asymmetric Gaussian profiles. The vertical dashed lines indicate the wavelengths where the flux drops to 70 % (gray) and 10 % (black) of its peak. See details in §4.2.
wing. The fitting with the asymmetric Gaussian profile is efficient for Lyα line from high-z galaxies affected by complex kinematic structure of infalling and/or outflowing gas and IGM absorption. For fitting, we use data points over the wavelength range where the flux drops to 10% of its peak value at the redder and bluer sides of the emission line. We use the peak flux, wavelength of the line peak, 0.4, 1.0 × 10 −17 , and 2.0 as the initial parameters of A, λ asym 0 , a asym , f 0 , and d for the line-fitting. The last two are typical values of our spectra. If profile fitting does not converge to the minimum in χ 2 , we search for the best-fit by changing the initial value of a asym .
We show the best-fit asymmetric Gaussian profile for an example spectrum in Figure 2. We also fit a symmetric Gaussian profile to the emission lines in addition to asymmetric one. For the symmetric Gaussian fitting, we adopt two wavelength ranges where the flux drops to 70% and 10% of its peak value, and denote the corresponding peak wavelengths by λ gauss 0 and λ cent 0 , respectively. The fitting procedure in the former narrow range is similar as in Hashimoto et al. (2013) in terms of avoiding systematic effects due to asymmetric line profile. As shown in Fig. 2, the best-fit λ asym 0 is broadly equal to λ cent 0 for the example line. The wavelength difference is ∼ +0.1Å(∼ +10 km s −1 at z = 2.2). In contrast, λ gauss 0 differs from λ asym 0 by ∼ +0.4Å which corresponds to a velocity difference of ∼ +30 km s −1 at z = 2.2. This is likely to be caused by the sharp drop on the blue side and the extended red tail which cannot be fit well with symmetric profiles. This trend is more clearly shown in Figure Asymmetric parameter a asym λ cent − λ asym λ gauss − λ asym λ asym 0 as a function of the asymmetric parameter, a asym . The wavelengths of individual profiles are in good agreement for almost symmetric lines with a asym ∼ 0. Even for moderately-asymmetric profile with |a asym | 0.2, λ cent 0 tends to correct for systematic effects of skewed lines compared to λ gauss 0 . However, both of λ gauss 0 and λ cent 0 are redshifted (blueshifted) from λ asym 0 by ∼ 0.5 − 1.0Å for highly-asymmetric lines with a asym ∼ +0.4 (−0.4).
After correcting for the heliocentric motion of Earth for the redshifts of Lyα and nebular lines 11 , we calculate ∆v Lyα following Equation 1. Table 2 lists the z Lyα , z sys , and ∆v Lyα for the 26 Lyα-detected objects observed with LRIS. Figure 4 present Lyα spectra as a function of velocity for LAEs with detections of nebular emission lines. In Table 3, we also list these quantities of the four LAEs with detections of Lyα and nebular lines obtained by previous Magellan/IMACS observations (Nakajima et al. 2012). Almost all objects observed with LRIS have a ∆v Lyα of ∼ 200 km s −1 which is consistent with values in previous studies (e.g., Hashimoto et al. 2013). The values of ∆v Lyα in the IMACS sample are calculated to be smaller than the LRIS results. This could be caused by large uncertainties due to the IMACS spectroscopy with a lower spectral resolution than LRIS.

Measurement of IS Velocity Offset
We measure the IS velocity offset of IS absorption lines for our LAEs. Due to the faintness of their UV continuum emission, it is difficult to detect IS absorption lines from high Lyα EW galaxies with EW 50Å in individual spectra. However, owing to the high sensitivity of Keck/LRIS, the rest-frame UV continuum emission is clearly detected from four individual LAEs, LAE 12805, 13636, and 14212 in COSMOS, and LAE 10600 in SXDS, among the 26 Lyα-detected objects.
We first fit a power-law curve to the UV continuum emission in four individual objects in order to normalize the continuum level, and derive the properties of IS absorption lines. The normalized continuum emission in the rest-frame is shown in Figure 6. Next, we fit the symmetric Gaussian profile to each IS absorption line in a wavelength range of ±5Å around the expected line center. We summarize the best-fit peak wavelength, line depth, width, and equivalent width in Table 4. The noise in each line is estimated from spectra at 1250 − 1700Å avoiding regions close to the absorption lines. Most absorption lines are found to be detected at the > 5σ levels except for several LIS lines.
We calculate ∆v IS in a similar manner as for Lyα in Section 6.1. Several pairs of absorption lines such as O i λ1302-S i λ1304, and C iv λ1548-C iv λ1550 are likely to be blended at the resolution of our spectroscopy. For this reason, we define the wavelengths of the line pairs as central values between the pairs. We also derive the properties of fine-structure emission lines such as Si ii * as summarized in Table 5. We find that the velocity offsets of these ion lines from z sys are almost zero, indicating that the fine-structure emission lines also trace the systemic redshift of galaxies. This is because these emission lines come from nebular regions photoionized by radiation from massive stars (e.g., Shapley et al. 2003).

Measurement of HI Covering Fraction
We estimate the covering fraction, f c , of surrounding H i gas from the depth of low ionization IS absorption lines for our four continuum-detected LAEs. If the H i gas is distributed in a spherical shell, the depth of the lines may be related to f c . The covering fraction of any ion is estimated from where τ , I, and I 0 are optical depth of an absorption line, its residual intensity, and the continuum level, respectively. The optical depth is liked to the column density as where f , λ, and N are the ion oscillator strength, wavelength of the absorption line inÅ, and the column density of the ion in cm −2 (km s −1 ) −1 , respectively. Jones et al. (2013) use Si ii λ1260, 1304, and 1526 lines in order to solve the above two equations, and estimate f c for gravitationally-lensed LBGs at z ∼ 4. They find bestfit values of N and f c by fitting observed the Si ii line profiles with the intensity as a function of N and f c , I(N, f c ), derived from the above equations. In addition to the fitting to Si ii lines, they use several strong absorption lines, Si ii λ1260, O i λ1302, Si ii λ1304, C ii λ1334, and Si ii λ1526 to put a lower limit on f c via which is a simplified case of Equation 3 when τ ≫ 1. For our LAEs, we estimate f c in the latter method for the following reasons: (1) it is relatively difficult to fit our Si ii line profiles with a low S/N due to the faintness of UV-continuum emission and low resolution of our spectroscopy; and (2) Jones et al. (2013) use mainly the f c value derived in the latter method in their discussion. We would like to compare f c for LAEs with that for LBGs in the same manner.
We derive the average absorption line profile of these  strong transitions as a function of velocity. In the calculation, we do not use O i λ1302 and Si ii λ1304 transitions, since they could be heavily blended owing to the low spectral resolution. The derived average line profiles are shown in Figure 7. The covering fractions are estimated to be ∼ 0.7 for LAE 12805, ∼ 0.3 for 13636, ∼ 0.9 for 14212, and ∼ 0.4 for 10600 from the residual intensity in the core of the absorption line profiles. We additionally calculate the average depth of each best-fit Gaussian profile derived in Section 4.3. This alternative is helpful to estimate adequately the depth of a profile with a low S/N. The values of f c are comparable to those derived from the average line profiles, with the exception of LAE 14212. The difference for LAE 14212 is because the f c of the average line profile is affected by a singular count of C ii λ1334 line profile.
The spectral resolution of our LRIS spectroscopy is ∼ 4 times lower than that of Jones et al. (2013), preventing us from making a fair comparison between our LAEs and LBGs. We alternatively estimate EW of strong LIS absorption lines, EW(LIS), for our UV-continuum detected LAEs, and compare with results of composite LBG spec-tra in Section 6.3.

SED FITTING
In order to derive physical properties from stellar components, we perform SED fitting to the eight LAEs with known z sys . These LAEs have been imaged in several filters in the COSMOS or SXDS surveys. We use B, V , r, i ′ , and z ′ data taken with Subaru/Suprime-Cam, J data obtained with UKIRT/WFCAM, K s data from CFHT/WIRCAM (McCracken et al. 2010), and Spitzer/IRAC 3.6, 4.5, 5.8, and 8.0 µm photometry from the Spitzer legacy survey of the UDS field.
The fitting procedure is the same as in Ono et al. (2010). We create a spectral energy distribution (SED) of a starburst galaxy using a stellar population synthesis model, GALAXEV (Bruzual & Charlot 2003) including nebular emission (Schaerer & de Barros 2009), with a Salpeter initial mass function with lower and upper mass cutoffs of m L = 0.1M ⊙ and m U = 100M ⊙ . We assume a constant star formation history with a metallicity of Z/Z ⊙ = 0.2. We use Calzetti's law (Calzetti et al. 2000) for the stellar continuum extinction E(B − V ). These   parameters are selected to be the same as those used in Hashimoto et al. (2013) for consistency. The IGM absorption is applied to the spectra using the model of Madau (1995). The best-fit parameters and model spectra are shown in Table 6 and Figure 8, respectively. The best-fit stellar mass of our LAEs ranges from log M * ∼ 9 to ∼ 10 which is broadly comparable to that of LBGs. This is because we choose bright objects from our LAE sample for the spectroscopic observations. Thus, the small ∆v Lyα of LAEs does not appear to be caused by a difference in stellar mass between LAEs and LBGs. 6. RESULTS 6.1. Difference in ∆v Lyα between LAEs and LBGs In this section, we investigate statistically the difference in ∆v Lyα between LAEs and LBGs in a compilation of LAEs with a ∆v Lyα measurement in the previous studies including our 12 LAEs. The Lyα velocity offsets have previously been estimated for two objects in McLinden et al. (2011), three in the HETDEX survey (Finkelstein et al. 2011;Chonis et al. 2013), four from , and two LAEs in the MUSYC project (Guaita et al. 2013) at z ∼ 2 − 3. Among  the objects in previous studies, COSMOS 13636 from  is included in our sample of the 12 LAEs. We combine these 11 LAEs with our new 11, and construct a large sample consisting of 22 objects, which doubles the number of LAEs with ∆v Lyα measurements. Figure 9 shows the histogram of ∆v Lyα using the newly-enlarged sample. This is the updated version of Figure 6 in Hashimoto et al. (2013). Similar to Hashimoto et al. (2013), we confirm that ∆v Lyα of LAEs is systematically smaller than the values of LBGs. We carry out the non-parametric Kolmogorv-Smirnov test for the two populations. The K-S probability is calculated to be ∼ 10 −7 , indicating that ∆v Lyα is definitively different between LBGs and LAEs. The weighted mean of the 22 objects is ∆v Lyα = 234 ± 9 km s −1 .
We plot EW(Lyα) and ∆v Lyα of our new sample in the right panel of Figure 10. We confirm the anticorrelation between EW(Lyα) and ∆v Lyα in high-z starforming galaxies including objects with a high Lyα EW (Hashimoto relation) suggested by Hashimoto et al. (2013). The larger sample clarifies that ∆v Lyα decreases with increasing EW(Lyα). The similar trend has been found for UV-continuum selected galaxies at z ∼ 2 − 3. Shapley et al. (2003) have calculated velocity offsets between Lyα emission and IS absorption (∆v em−abs ) for composite spectra of LBGs, and have investigated relation between Lyα EW and ∆v em−abs . Four LBG sub- samples divided according to their Lyα EW reveal a trend that Lyα EW increases with decreasing ∆v em−abs in the EW range of −15 − +50Å. The ∆v em−abs of our UV-continuum detected LAEs is very consistent with the trend of Shapley et al. (2003), as shown in the top panel of Figure 11. Nevertheless, Lyα and IS velocity offsets from z sys would be capable of distinguishing effects of Lyα resonant scattering and galactic outflow on ∆v em−abs .

Difference in ∆v IS between LAEs and LBGs
We additionally examine a possible difference in ∆v IS between LAEs and LBGs. The weighted means of ∆v IS of the absorption lines are calculated to be −134 ± 67, −261 ± 48, −216 ± 56, and −169 ± 52 km s −1 for LAE 13636, 10600, 14212, and 12805, respectively. In the calculation of average ∆v IS for each object, we exclude several line-pairs with a large ∆v IS of −500 km s −1 which are not reliably determined due to a line blending. As shown in Table 4, we find that almost all IS absorption lines are blueshifted with respect to z sys by −200 km s −1 , which indicates that gaseous outflows are present in the continuum-detected LAEs.
The left panel in Fig. 10 represents the relation between EW(Lyα) and ∆v IS . The average of the four is ∆v IS = −204 ± 27 km s −1 , which is comparable to that of LBGs (e.g., Erb et al. 2006b;Steidel et al. 2010) in contrast to ∆v Lyα , although the current small sample of LAEs with a ∆v IS is insufficient to provide a definitive conclusion on ∆v IS of LAEs and LBGs.

Difference in f c between LAEs and LBGs
We compare the H i covering fraction f c of LAEs derived in Section 4.4 with that of z ∼ 2 − 3 LBGs in Jones et al. (2013). Note that we here place lower limits on f c when τ ≫ 1. Figure 12 displays the relation between f c and Lyα EW, indicating a tenta-tive trend that f c decreases with Lyα EW. This trend has already been found in Jones et al. (2013) using an LBG sample. We find that the trend continues into objects with a higher Lyα EW. Our slope of the trend is slightly steeper than that in Jones et al. (2013), which would result from the wider dynamic range in Lyα EW. However, this trend could arise from the difference in the spectral resolution, although this tendency may marginally be found for LAEs alone. In addition to f c , we compare EW(LIS) between LAEs and z = 3 − 4 LBGs in the bottom panel of Figure 11. Shapley et al. (2003) have found that EW(LIS) decreases with increasing EW(Lyα) with composite spectra of LBGs. Our LAEs with EW(Lyα) = 30 − 90Å follow the trend between EW(Lyα) and EW(LIS), which might be indicative of a low velocity dispersion and/or low f c , as suggested by Shapley et al. (2003). These results related to the low f c imply the need for modeling Lyα line profiles emitted from a non-spherical shell of neutral gas (e.g., Zheng & Wallace 2013;Behrens et al. 2014).
7. DISCUSSION 7.1. Origin of Small ∆v Lyα in LAEs As described in the previous sections, we definitely confirm the anti-correlation between ∆v Lyα and Lyα EW by using a larger LAE sample than previously available. In this section, we explore the physical origin of the small ∆v Lyα in high Lyα EW galaxies.
Models predict that the redshift of the Lya emission line should increase with either outflow velocity or neutral hydrogen column density (N HI ) (Verhamme et al. 2006(Verhamme et al. , 2008. We have shown that the outflow velocities of LAE are comparable to those of LBGs, so the smaller ∆v Lyα for LAE is likely to be due to lower column densities in these objects.
In order to address the origin of the small ∆v Lyα in LAEs, we introduce the velocity offset ratio, The value of R Lyα IS could trace purely physical properties such as N HI and the dust amount by excluding the kinematic effect of a bulk outflow, since the quantity is normalized by the outflowing velocity, as suggested in (Verhamme et al. 2006). Hashimoto et al. (2013) infer the average value of ∆v IS for LAEs from a stacked spectrum of four LAEs with a z sys , and compare R Lyα IS between LAEs and LBGs. In the stacking analysis, R Lyα IS is found to be ∼ 1 for LAEs which is slightly-smaller than that of LBGs, but the uncertainties are large.
Here, we estimate R Lyα IS for the four continuumdetected LAEs, and compare the quantities with those of z = 2 − 3 LBGs in Erb et al. (2006a,b). In the comparison, we use LBGs with a negative ∆v IS value that indicates the outflow is present. The LAEs have a R Lyα IS of ∼ 0.6 − 1.4, while LBGs have a wide variety of the quantity from 0 to ∼ 10. Nevertheless, the average R Lyα IS for the LAEs is systematically smaller than that of LBGs. This indicates that LAEs tend to have a small N HI compared to LBGs based on the expanding gas shell model of Verhamme et al. (2006)  ) and the EW distribution (Reddy et al. 2008).
be indicative of a small N HI in LAEs.
Next, we examine possible correlations of ∆v Lyα and R Lyα IS with physical properties inferred from the SED fitting (Section 5). In correlation tests, ∆v Lyα and R Lyα IS correlate most strongly with mass-related quantities, and SFR, respectively. Figures 13 and 14 show the correlations of these quantities, respectively, including LAEs and LBGs with a z sys in the literatures. The SFR value of several LAEs is based on a Hα flux through the relation of Kennicutt (1998). The SFR based on a Hα flux is found to be comparable to the value inferred from SED fitting . We conduct Spearman rank correlation tests in order to find the most related physical quantities to ∆v Lyα and R Lyα IS in the same manner as Steidel et al. (2010). Table 8 summarizes the results of the Spearman rank correlation tests.
For Lyα velocity offsets, we find that the ∆v Lyα strongly correlates with SFR and stellar mass, which has not been observed previously in an LBG sample . These correlations may have merged because our sample covers larger dynamic ranges of SFR and M * . The correlation between ∆v Lyα and sSFR may arise from the stellar mass.
As far as the velocity offset ratio is concerned, we do not find a notable correlation between R Lyα IS and the physical properties. Nonetheless, the correlation tests indicate that R Lyα IS most correlates with SFR among the four physical quantities. The correlation may reflect the connection between star formation and N HI , if R Lyα IS is sensitive to N HI . A larger sample of LAEs with a R Lyα IS measurement might reveal its physical connections with galactic properties.

What is the Physical Origin of Strong Lyα
Emission? With our larger sample of LAEs with a z sys , we confirm conclusively that LAEs typically have a smaller ∆v Lyα than LBGs with a lower Lyα EW, while their outflowing velocities are similar in the two populations. These results yield a small R Lyα IS in LAEs, which indicates a small N HI in galaxies with a high Lyα EW. The anticorrelations of f c and EW(LIS) with Lyα EW in Figures  12 and 11 are consistent with the small N HI in LAEs. The patchy H i gas clouds surrounding the central source would lead to a small flux-averaged N HI corresponding to a small R Lyα IS . In this condition, Lyα photons could easily escape less affected by resonant scattering in the clouds. The results of our kinematic analyses support the idea that the H i column density is a key quantity determining Lyα emissivity.
Moreover, recent NIR spectroscopy by     Rhoads et al. (2014). b Based on Hα flux. c The ∆vLyα of this object is calculated to be 197 ± 102 km s −1 in our FMOS observation with higher spectral resolution than that of the Keck-II/NIRSPEC spectrosocpy in Hashimoto et al. (2013) (see Table 2).  Jones et al. 2012).
In our first paper of the series investigating LAE structures, we find that LAEs with a high Lyα EW tend to be a non-merger, to show a small Lyα spatial offset between Lyα and stellar continuum emission δ Lyα , and to have a small ellipticity by using a large sample of 426 LAEs (Shibuya et al. 2014). On the basis of these results on the gas distribution, the difference in H i column density explains the Lyα-EW dependences of the merger fraction, the Lyα spatial offset, and the galaxy inclination. For objects with density-bounded H ii regions, Lyα photons would directly escape from central ionizing sources, which produce a small δ Lyα . The low H i abundance along the line of sight also induces the preferential escape of Lyα to the face-on direction.
All of the above results suggest that ionized regions with small amounts of H i gas dominate in galaxies with a high Lyα.

SUMMARY AND CONCLUSION
We carry out deep optical spectroscopy for our large sample of LAEs at z = 2.2 in order to detect their Lyα lines with Keck/LRIS. We compare redshifts of the Lyα and nebular emission lines detected with Subaru/FMOS, and calculate ∆v Lyα for new 11 LAEs. This observation doubles the sample size of LAEs with a ∆v Lyα measurement in literatures.
The conclusions of this study are summarized below.
• Almost all of our new LAEs have a ∆v Lyα of ∼ 200 km s −1 which is systematically-smaller than that of LBGs. Using 22 LAEs with ∆v Lyα measurements taken from our new observations and the literature, we definitively confirm the anti-correlation between Lyα EW and ∆v Lyα suggested by previous work.
• Long exposure times and the high sensitivity of LRIS at blue wavelengths enabled us to successfully detect IS absorption lines against faint UV continua from four individual LAEs. These IS absorption lines are found to be blueshifted from the systemic redshift by 200 − 300 km s −1 , indicating strong gaseous outflows are present even in LAEs.
• We estimate R Lyα IS (≡ ∆v Lyα /∆v IS ) that would be a quantity sensitive to N HI for the four UV continuum-detected LAEs. We find the value of R Lyα IS in LAEs to be smaller than that of LBGs, indicating a lower N HI in LAEs. We performed a test for correlations between R Lyα IS and physical properties inferred from SED fitting. As a result, we tentatively conclude that SFR may be most closely related to R Lyα IS . The correlation may suggest that the star formation preferentially occurs in systems with large amounts of neutral hydrogen gas, which would have a larger value of R Lyα IS . • We estimate the covering fraction, f c , of surrounding H i gas from the depth of LIS absorption lines the four LAEs. We identify a tentative trend for f c to decrease with increasing Lyα EW, as suggested by a study for LBGs in Jones et al. (2013). A central source being covered by patchy H i gas clouds would lead to a small flux-averaged N HI corresponding to a small R Lyα IS . In this condition, Lyα photons could easily escape less affected by resonant scattering in the clouds.
• The results of our kinematic analyses support the idea that the H i column density is a key quantity  Table 4) and average LIS absorption line profiles (filled squares; Fig. 7). The gray circles indicate LBGs at z = 2 − 3 in Jones et al. (2013). The dashed line denotes a linear fit to the data.
In this kinematic study, we obtain ∆v IS , R Lyα IS , and f c only for objects with a moderate Lyα EW of 20 − 100Å which overlaps with the Lyα EW range of LBG samples in e.g., Shapley et al. (2003). We need to estimate these quantities for objects with a higher Lyα EW in order to check whether such objects follow the kinematic trends found in this study.