# Infalling gas in a Lyman-α blob

Yiping Ao, Zheng Zheng, Christian Henkel, Shiyu Nie, Alexandre Beelen, Renyue Cen, Mark Dijkstra, Paul J. Francis, James E. Geach, Kotaro Kohno, et al.

► To cite this version:

Yiping Ao, Zheng Zheng, Christian Henkel, Shiyu Nie, Alexandre Beelen, et al. Infalling gas in a Lyman-α blob. Nature Astronomy, 2020, 4, pp.670-674. 10.1038/s41550-020-1033-3. insu-03744534

HAL Id: insu-03744534

https://insu.hal.science/insu-03744534v1

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# Infalling gas in a Lyman-α blob

Yiping Ao<sup>1,2,3✉</sup>, Zheng Zheng<sup>4</sup>, Christian Henkel<sup>5,6</sup>, Shiyu Nie<sup>4</sup>, Alexandre Beelen<sup>7</sup>, Renyue Cen<sup>8</sup>, Mark Dijkstra<sup>9</sup>, Paul J. Francis<sup>10</sup>, James E. Geach<sup>11</sup>, Kotaro Kohno<sup>12</sup>, Matthew D. Lehnert<sup>13</sup>, Karl M. Menten<sup>5</sup>, Junzhi Wang<sup>14</sup> and Axel Weiss<sup>5</sup>

Lyman-α blobs (LABs) are spatially extended nebulae of emission in the Lyman-α (Lyα) line of hydrogen, seen at high redshifts<sup>1,2</sup>, and most commonly found in the dense environment of star-forming galaxies<sup>3,4</sup>. A recent study showed that nearly 100% of the sky is covered by Lyα emission around high-redshift galaxies<sup>5-7</sup>. The origin of Lyα emission in the LABs is still unclear and under debate<sup>8</sup>. It may be powered by photo-ionization involving galactic superwinds/outflows, resonant scattering of Lyα photons from starbursts or active galactic nuclei<sup>9-16</sup>, or by cooling radiation from cold streams of gas accreting onto galaxies<sup>17,18</sup>, as demonstrated by recent simulations<sup>19</sup>. Here we analyse the gas kinematics within a LAB, providing rare observational evidence for infalling gas. This is consistent with the release of gravitational accretion energy as cold streams radiate Lyα photons. It also provides direct evidence for possible cold streams feeding the central galaxies. The mass of the infalling gas is not important in comparison to the gas mass consumed by star formation, and is also not the major powering source of Lyα emission, but it hints at another mechanism to explain the origin of the extended Lyα emission around young galaxies.

We used the Atacama Large Millimeter/Submillimeter Array (ALMA) to search for CO J=7–6 and C₁(³P₂ → ³P₁) line emission as well as for associated continuum emission towards Lyman-α blob 6 (LAB6) in the Francis cluster (J2143–4423)<sup>1,9,20</sup>. Its Lyman-α (Lyα) emission is distributed over a region of ~120 kpc, with a luminosity of ~5.2 × 10<sup>43</sup> ergs s<sup>-1</sup>. The field of view of ALMA at the central observed frequency of 246.6 GHz is about 24”, or 200 kpc (physical) projected on the sky at redshift z=2.37. LAB6 is notably detected in the 1.2 mm dust continuum with ALMA, as shown in Fig. 1a. The continuum source, LAB6.1, with a flux density of 1.57 ± 0.11 mJy is located at the centre of the LAB, with an apparent size of 0.82” × 0.70” (6.8 kpc × 5.8 kpc). It is marginally resolved with a deconvolved size of 0.52” × 0.46” (4.3 kpc × 3.8 kpc). To highlight the gas kinematics in the central galaxy, we show the line profiles of CO J=7–6 and C₁(³P₂ → ³P₁) towards LAB6.1 in Fig. 2. The CO J=7–6 line indicates a redshift of 2.3691 ± 0.0004. In Fig. 1a, there is another unresolved continuum source, LAB6.2, detected with a flux density of 1.60 ± 0.24 mJy. It is offset by ~16” (130 kpc) southwest from the centre, and may be located outside the LAB. Thus, LAB6.2 is unlikely to be physically associated with LAB6, which is

also supported by the evidence that no CO J=7–6 or C₁(³P₂ → ³P₁) line emission is detected from LAB6.2.

Our ALMA observations show the distribution and gas kinematics from the central galaxy. Figure 1 shows that the Lyα emission is clearly peaked around the location of the galaxy and is featured with extended morphology in the southeast. Does the central submillimetre galaxy power the physically extended Lyα emission? To answer this question, we used the X-shooter on Unit Telescope 2 of the Very Large Telescope (VLT) at the European Southern Observatory (ESO) to conduct follow-up spectroscopic observations to measure the line profile of Lyα. A slit was placed along LAB6 as shown in Fig. 1a and the results are presented in Fig. 1c,d. The Lyα line emission is clearly detected around LAB6.1 with its peak approximately –150 km s<sup>-1</sup> off the systemic velocity obtained from CO J=7–6.

Given the resonant nature of the scattering cross-section, high near the line centre and low in the line wings, Lyα photons usually escape from a static cloud with a double-peaked profile<sup>21</sup>. With outflowing (inflowing) neutral gas, the Doppler effect would lead to an enhanced red (blue) peak<sup>21–23</sup>. In Fig. 2, we overlay the line profile of Lyα on those of CO J=7–6 and C₁(³P₂ → ³P₁), showing an offset of roughly –150 km s<sup>-1</sup>. It is very rare to observe a blue-skewed Lyα line profile from ionized gas compared with the systemic velocity of molecular or atomic gas in a LAB. Such line profiles can be well reproduced in cold-stream models<sup>21–23</sup>, while star-forming-based models with outflows driven by mechanical feedback generally give rise to characteristic red-skewed asymmetric profiles. However, star-forming-based models with infalling gas may also produce the observed line profile. When a contracting shell model (see details in Methods) is used to fit the observed Lyα spectrum (the results are shown in Supplementary Fig. 5), the inferred velocity of about –34 km s<sup>-1</sup> of the shell suggests that the infalling gas may become stalled near the galaxy. For the 1.2 mm continuum source detected by ALMA, the inferred far-infrared (FIR) luminosity of LAB6.1 is (6.5 ± 0.13) × 10<sup>12</sup> L<sub>☉</sub> and its corresponding star-formation rate (SFR) is approximately 1,100 M<sub>☉</sub> yr<sup>-1</sup>. Such a high degree of star formation can easily provide enough ionizing ultraviolet (UV) photons to produce extended Lyα emission if they escape. Therefore, the observed Lyα emission with a blue-skewed line profile can originate either from cooling radiation or from a central photoionizing source, where the photons are scattered by the infalling gas.

<footnote n="1">¹Purple Mountain Observatory and Key Laboratory for Radio Astronomy, Chinese Academy of Sciences, Nanjing, China. ²School of Astronomy and Space Science, University of Science and Technology of China, Hefei, China. ³National Astronomical Observatory of Japan, Mitaka, Japan. ⁴Department of Physics and Astronomy, University of Utah, Salt Lake City, UT, USA. ⁵Max-Planck-Institut für Radioastronomie, Bonn, Germany. ⁶Astronomy Department, King Abdulaziz University, Jeddah, Saudi Arabia. ⁷Institut d'Astrophysique Spatiale, CNRS UMR 8617, Université Paris-Sud, Université Paris-Saclay, Orsay, France. ⁸Department of Astrophysical Sciences, Princeton University, Princeton, NJ, USA. ⁹Institute of Theoretical Astrophysics, University of Oslo, Oslo, Norway. ¹⁰Physics Education Centre and Research School of Astronomy and Astrophysics, The Australian National University, Canberra, Australian Capital Territory, Australia. ¹¹Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, Hatfield, UK.</footnote>

<footnote n="12">¹²Institute of Astronomy, The University of Tokyo, Mitaka, Japan. ¹³Sorbonne Université, CNRS, UMR 7095, Institut d'Astrophysique de Paris, Paris, France.</footnote>

<footnote n="14">¹⁴Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai, China. ✉e-mail: ypao@pmo.ac.cn</footnote>
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**Fig. 1** | ALMA and X-shooter data in LAB6. **a**, The ALMA 1.2 mm continuum image in blue contours (contour levels are 5, 10, 15 and 20σ and 1σ = 40 µJy beam⁻¹) overlaid onto the Lyα image²⁰ in grey (also shown as red contours with contour levels of 30%, 50%, 70% and 90% of the peak intensity). The two continuum sources detected at >5σ levels are labelled. The big circle shows the primary beam of the 12 m ALMA array. The synthesized beam is shown at the bottom left. The rectangle shows the slit position for the X-shooter observations, with a size of 1.2" × 11". The image is centred at the ALMA continuum peak, right ascension α(J2000) = 21h 42m 42.631s and declination δ(J2000) = −44° 30′ 09.44". Primary beam correction is not applied to the 1.2 mm continuum image for clarity. North is up and east to the left. **b**, Zoom-in on the central region of interest. **c**, X-shooter two-dimensional Lyα spectra along the slit as indicated in **a**. To improve the signal-to-noise ratio in the spectrum, four contiguous pixels (4 × 0.16") were smoothed along the slit. The blue and red horizontal dashed lines denote the positions of the ALMA 1.2 mm continuum and the Lyα peak, respectively. **d**, X-shooter extracted Lyα spectra at the position of the ALMA 1.2 mm continuum in blue and at the Lyα peak in red. Note that the velocity is relative to z = 2.3691.

In both scenarios, infalling gas is needed to produce the observed line profiles presented in this paper. As a whole, our observational result represents direct kinematic evidence that gas is falling into the central region where the submillimetre galaxy is located, but it does not necessarily imply cooling radiation as the powering mechanism of the LAB. We note that the gas infall rate is approximately 1.7 $M_{\odot}$ yr⁻¹ (see details in Methods), which is negligible compared with the consumed material of approximately 1,100 $M_{\odot}$ yr⁻¹ by star formation. The spatial offset (~5.4 kpc) between the Lyα peak and the dust continuum peak of LAB6 (Fig. 1b) may be a result of anisotropic gas and dust distributions and may be orientation dependent, as seen in simulations¹⁵.

Recently proposed star-forming-based models¹² predict that LABs at high redshift may correspond to protoclusters containing the most massive galaxies and cluster halos in the early Universe as well as ubiquitous strong infrared sources undergoing extreme starbursts. These models also predict that the Lyα emission from photons that escape from a galaxy is expected to be significantly polarized. This has been confirmed for one LAB in the SSA22 field²⁴ and the detection of polarized radiation is inconsistent with in situ production of Lyα photons, indicating that they have been produced in the galaxies hosted within the nebula, and scattered by neutral hydrogen. Of course, the galaxies already formed in the LABs will likely contribute to the extended Lyα emission with photoionization by young stars and/or active galactic nuclei. Stacking of faint Lyα emitters (LAEs) in the SSA22 field⁴ shows extended and faint Lyα emission surrounding the bright sources at the centre, indicating a possible connection of the extended Lyα emission with the central powering source. In our case, star formation or an active galactic nucleus instead of cooling radiation are expected to be the major powering source of LAB6 because the inferred gas infall rate is much lower than the SFR and the cooling radiation is much less than the observed Lyα luminosity (see details in Methods).

For the scenario of cold streams, the radiatively cooled gas can continuously fall onto the centre of the LAB to form a new galaxy or to feed the existing central galaxy. The gas inflow rate may be high enough to feed the central starburst and notably increase the galaxy's mass, as shown in simulations²⁵⁻²⁷. This is in apparent

contrast with our inferred low accretion rate from the contracting shell model. Note that the morphology and the three-dimensional distribution of cold streams²⁶,²⁷ do not resemble a spherical shell structure. Theoretical models suggest that the accretion mode changes from cold to hot as halo mass increases and in massive halos cold streams are suppressed at $z < 2$ and a complete shutdown of star

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**Fig. 2** | Line profiles from molecular and ionized gas in LAB6. Observed CO J=7-6 and C(I(³P₂ →³P₁)) line profiles obtained with ALMA towards LAB6.1 are presented by filled histograms. X-shooter Lyα spectra at the position of the ALMA 1.2 mm continuum and at the peak of Lyα emission are shown in blue and red, respectively. The velocity scale refers to that of the CO J=7-6 line at z = 2.3691. Note that CO J=7-6 and C(I(³P₂ →³P₁)), in units of mJy, are shown in the same panel but the latter with an offset along the y axis. Lyα flux is on an arbitrary scale.
formation is assumed to follow²⁸. Cold streams in massive halos are assumed to supply cold gas for further disk growth and star formation at $z>2$. However, it is not clear how much gas accretes onto massive galaxies as cold streams²⁹. While the shell model result supports the cold-stream scenario, details need to be scrutinized to provide more realistic models. It is likely that the cold accretion streams are connected to large-scale filaments. Observational evidence includes the kinematics of the gas distribution in the cosmic web near a quasar³⁰, a 12 Mpc (comoving) filamentary structure traced by LABs in an overdense region³¹ and intense star formation occurring within the galaxies embedded in multiple filaments extending more than 4 Mpc (comoving) in the SSA22 field³². Given such a picture, LABs could represent a class of objects that host a diverse sample of galaxies, or in the extreme case, are devoid of galaxies. In the latter case, LABs are part of the cosmic web in which galaxies have not yet formed or are still in a very early stage of evolution. Our recent deep observations¹⁰ reveal that central heating sources are still missing in about two-thirds of the LABs in the SSA22 field. This may be caused by insufficient observational sensitivities or by the fact that the extended Lyα emission is dominated by cooling radiation in some LABs.

For the origins of the infalling gas, besides the long-sought-after cold accretion streams, it is possible that the gas was ejected earlier as part of a galactic wind from the central region and is falling back to the galaxy from where it originated, forming a so-called galactic fountain. Compared with the cold accretion streams, gas in a galactic fountain is expected to have a larger covering factor. Model fitting results including the effect of dust show that the metallicity of the infalling gas is at the level of 0.03–0.2 $Z_\odot$, suggesting that the gas may come from cold streams, instead of a galactic fountain (see details in Methods). If cold streams or infall of recycled gas are common, blue-skewed Lyα line profiles should not be so rare³³. However, the gas distribution can become more complex with feedback from central sources and the effect on Lyα emission can depend on the viewing direction³⁴, which may help explain the rarity of the blue-skewed profiles in the observed Lyα spectra. In addition, recent simulations³⁵ show high-redshift galaxies experiencing cycles of inflow and outflow phases due to supernova feedback, which also explain the rarity. To constrain the origin of the infalling gas, it would be useful to study LABs without strong feedback from central sources with the help of deep ALMA observations. Observations that provide constraints on the properties of the gas (for example, metallicity and column density) and Lyα radiative transfer modelling with realistic gas distributions from galaxy formation simulations could further discriminate different scenarios of its origin, substantially improving our understanding of galaxy formation in the early Universe.

## Methods

**ALMA observations.** We observed LAB6 in the protocluster J2143–4423³⁹. The observations were carried out on 7 May 2016 with 36 12 m antennas and baselines between 16.5 and 629 m. The total on-source observing time was 25 min. We used the correlators in the frequency division mode at the central frequency of 238.76 GHz in the lower sideband and in the time division mode at the central frequency of 254.51 GHz in the upper sideband. The data were reduced with the Common Astronomy Software Application (CASA) package in a standard manner. Originally, Pallas was adopted as the flux calibrator. The Pallas-based flux calibration resulted in a flux density of 1.17 Jy at 253.5 GHz for the bandpass calibrator J2056–4714. This is 1.28 times the flux density based on the ALMA calibrator catalogue, estimated by interpolation between data entries in band 3 (97.5 GHz) and band 7 (343.5 GHz). In the ALMA calibrator catalogue, there is no signature of high variability for J2056–4714 around the observation date. The flux discrepancy may be attributed to the Pallas-based flux calibration. Therefore, the flux density of the phase/gain calibrator J2139–4235 has been re-scaled using the flux of the bandpass calibrator interpolated with the measurements in the ALMA catalogue. The primary beam size is about 24" at the central frequency of 246.6 GHz. The full-width at half-maximum of the achieved synthesized beam size is 0.64" × 0.51" with a position angle of 9° east of north. The continuum maps have a root-mean-square noise level of 40 µJy beam⁻¹. The flux densities of the detected sources were measured with the CASA task IMFIT.

Two lines are detected towards LAB6.1 (Fig. 2). Accounting for the known offset between the frequencies of the CO $J=7-6$ and $C1(3P_2 \rightarrow 3P_1)$ lines, one can estimate an accurate redshift of 2.3796 ± 0.0004 for LAB6.1. This is within the uncertainties consistent with the redshift of the Lyα emission of LAB6. Supplementary Fig. 1 shows an 1.2 mm continuum map, integrated intensity maps of CO $J=7-6$ and $C1(3P_2 \rightarrow 3P_1)$, as well as the velocity field traced by CO $J=7-6$ towards LAB6.1. In Supplementary Fig. 1d, the projected velocity varies by about 200 km s⁻¹ across ~0.7" (~6 kpc). It can be a hint of rotation of the molecular gas in the central galaxy. The other possibility is gas inflow or outflow. We note that the gradient of the velocity field approximately coincides with the direction towards the diffuse Lyα emission to the southeast (Fig. 1).

**VLT/X-shooter observations.** We obtained a UV to near-infrared (NIR) spectrum (programme ID: 297.A-5059) at the VLT with the VLT/X-shooter instrument²⁶ from 20 to 26 October 2016. The X-shooter instrument consists of three spectroscopic arms: UVB, VIS and NIR, ranging from 293.6 nm to 2,480 nm. The observations were taken with 1.6" × 11", 1.5" × 11" and 1.2" × 11" slits for the three arms, respectively. The total on-source integration time is 86 min. The data reduction was performed using the reflex X-shooter pipeline 2.9.1 (ref. ³⁷).

Note that only the UVB arm covering the Lyα spectroscopic data is presented in this paper. To improve the signal-to-noise ratio of the spectrum, four contiguous pixels (4 × 1.6") were smoothed along the slit. To check the Lyα line emission in the southeastern part (see Fig. 1a for this extended emission), we present a typical spectrum from this region in Supplementary Fig. 2, where line emission is not detected. Apparently, this region is too far for our X-shooter observations. For the spectrum at the Lyα peak, the blue peak is obvious and the signal-to-noise ratios in seven channels are higher than 4. For the red peak, there are nine continuous channels with signal-to-noise ratios around 2, and the signal-to-noise ratio will become 6 by smoothing all channels. The Lyα luminosity around LAB6.1 recovered by X-shooter is ~5.0 × 10²⁴ erg s⁻¹, which is about 10% of the total luminosity in the whole LAB²⁰. In combination with the data at other bands discussed in the next section, the Lyα escape fraction is estimated to be only ~5%.

Except for the Lyα emission, there is no notable interstellar absorption line detected among the three arms of the VLT/X-shooter instrument.

**Near- to mid-infrared data and constraints on the central source.** To constrain the physical properties of the central galaxies in the LABs of the Francis cluster J2143–4423, Herschel PACS and SPIRE data have been used for data analysis²⁹. However, the coarse angular resolution may induce large uncertainties as shown in Fig. 1a where two sources with a separation of only 16" are revealed by ALMA. Higher angular resolution is therefore required.

NIR data were taken with the High Aćuity Wide field K-band Imager (HAWK-I) on VLT during October and November 2008. Broadband filters, J band at 1.258 µm and Ks band at 2.146 µm, were used for the observations. Mid-infrared data were obtained using the Spitzer Infrared Array Camera (IRAC) at 3.6, 4.5, 5.8 and 8.0 µm (ref. ³⁸). Details of the data reduction can be found in the literature³⁸. Data are listed in Supplementary Table 1 and images are presented in Supplementary Fig. 3.

Using Bayesian Analysis of Galaxies for Physical Inference and Parameter Estimation (Bagpipes)¹⁹⁻⁴¹, we constrained the physical properties of the host galaxy from spectral energy distribution (SED) modelling with near- to mid-infrared data. If the redshift is left as a free parameter, the photometric redshift derived from Bagpipes is 2.53±0.33, consistent with that determined from the submillimetre lines. To constrain the physical properties, we fixed the redshift to 2.3691 and found that the SED can be well described by a recent burst with an SFR of 1.18±0.39 × 10³ $M_\odot$ yr⁻¹, a stellar age of 62±22 Myr and a logarithmic stellar mass $\log(M_{\text{stellar}}/M_\odot) = 11.00±0.08$ as shown in Supplementary Fig. 4.

**Molecular gas mass, dust mass and SFR.** Here we derive the physical properties of LAB6.1 using CO $J=7-6$ and dust emission. For LAB6.1, the measured flux density at 1.2 mm is 1.57 ± 0.11 mJy, which is significantly lower than the predicted value of 2.43 mJy by the SED model of LAB6¹. This latter higher value may be due to the contribution from other spatial components. As shown in Fig. 1, there is a continuum source located 16" southwest from LAB6.1. In the Herschel SPIRE bands used for the SED model, this source is blended with LAB6.1, which may explain the overprediction of the 1.2 mm flux density from the previous SED model. Assuming that the total observed FIR luminosity² originates from both sources and the fractional contribution from LAB6.1 follows that of the 1.2 mm continuum, we derive the FIR luminosity of LAB6.1 to be (6.5 ± 0.13) × 10¹² $L_\odot$, which translates to an SFR of 960 $M_\odot$ yr⁻¹ based on the SFR–$L_{\text{FIR}}$ relation¹², where $L_{\text{FIR}}$ is integrated from 40 µm to 200 µm in the rest frame.

With ALMA, the measured CO $J=7-6$ integrated flux density is 1.14 ± 0.12 Jy km s⁻¹ and the corresponding line luminosity is (1.10 ± 0.12) × 10⁸ $L_\odot$ or (6.50 ± 0.71) × 10⁹ K km s⁻¹ pc⁻². Following the SFR calibration using the CO $J=7-6$ line luminosity¹³, the derived SFR is 1,240 $M_\odot$ yr⁻¹. From the mean value of the SFRs from the above two methods, the derived SFR of LAB6.1 is approximately 1,100 $M_\odot$ yr⁻¹, which is consistent with the value derived with the near- and mid-infrared SED fitting.
Assuming a line ratio of 0.24 for $L_{CO(7-6)}/L_{CO(1-0)}$ (refs. $^{44,45}$) and the standard factor $X_{CO}=0.8 M_{\odot}$ (K km s⁻¹ pc²)⁻¹ for ultraluminous infrared galaxies⁶⁶, we find a molecular gas mass of (2.2 ± 1.1) × 10¹⁰ M$_{\odot}$. The dust mass from the SED model⁹ is (2.1 ± 0.5) × 10¹⁰ M$_{\odot}$ after rescaling the result according to the continuum flux density measured by ALMA. This implies a gas-to-dust-mass ratio of 105 ± 60.

**Shell model fit to the Lyα spectrum.** The Lyα line emergent from Lyα-emitting galaxies and LABs is usually characterized by a profile skewed towards the red side of the systemic velocity⁷, which can be understood as Lyα emission escaping from outflowing gas after experiencing resonant scatterings off neutral hydrogen atoms. The resonant scattering cross-section is high near the line centre and low in the wings, and Lyα emission from a static cloud (for example, with no inflow or outflow) would have frequencies shifted away from the line centre to escape, leading to a double-peaked line profile²¹. For an expanding cloud (with outflowing gas), the Doppler effect makes Lyα photons backscattered from the far side of the cloud easier to escape than from the near side. This results in a red-skewed Lyα line profile²¹,²², producing a Lyα line with an enhanced red peak and a suppressed blue peak. However, the Lyα spectrum of LAB6.1 shows a blue-skewed line profile, with an enhanced blue peak and a suppressed red peak. Such spectral features indicate gas infall.

For LAB6.1, there is a slight offset between the peak of the Lyα emission and the central galaxy associated with the 1.2 mm dust continuum (Fig. 1), which is about 0.64" (5.4 kpc). This raises the question about the nature of the gas probed by the Lyα emission. If we take the velocity (roughly −50 km s⁻¹) near the trough between the red and blue peaks in the Lyα spectrum as an indication of the bulk velocity of the gas (with respect to the continuum source), the gas can be either on the near side outflowing from the galaxy or on the far side inflowing towards the galaxy. However, the blue-skewed Lyα spectrum suggests that the gas itself is collapsing. Furthermore, the Lyα spectrum centred on the galaxy has a similar shape to that near the Lyα peak. The shape of the spectrum at the peak Lyα emission and that at the position of the central galaxy provide strong support that the gas is falling into the central galaxy. The small spatial offset of the Lyα emission from the galaxy may be caused by the non-uniform distribution of the gas, for example, accretion along filaments, or by notable absorption near the continuum source, leaving the dominant contribution to the surviving Lyα emission biased towards the low-density accretion flows.

As an attempt to model the Lyα spectrum, we adopt a simple model that infalling gas is uniformly located in a thin shell of neutral gas and that the Lyα radiation from the central source is scattered by the hydrogen atoms in the shell. This is the reverse version of the expanding shell model²³ commonly used to model the Lyα spectra from star-forming galaxies. We characterize the model with three parameters, the infalling velocity V of the shell and the temperature T and column density $N_{\text{H}}$ of the neutral hydrogen in the shell. A Markov chain Monte Carlo calculation is performed to constrain the parameters, based on interpolating a compilation of Lyα spectra¹⁸ from the shell model. We find that the best fit (model 1) has $V = -33.8 \times 10^{-3.1}$ km s⁻¹, $\log[T(\text{K})] = 3.60 \pm 0.27$ and $\log[N_{\text{H}}(\text{cm}^{-2})] = 19.80 \pm 0.05$ with $\chi^2 = 53.5$ for 74 degrees of freedom. The $\chi^2$ value is acceptable for 74 degrees of freedom (which has a $1\sigma$ range of $\chi^2$ values of 74 ± 12), and it does not imply over-fitting. The other acceptable but much less favoured solution (model 2) with $\chi^2=68.9$ has $V = -93.9 \times 10^{-20.47}$ km s⁻¹, $\log[T(\text{K})] = 4.19 \pm 0.22_{-0.19}^{+0.18}$ and $\log[N_{\text{H}}(\text{cm}^{-2})] = 19.62 \pm 0.05$²⁰. The two model fit spectra are shown in Supplementary Fig. 5. While the first model clearly works better, it misses the Lyα flux near the velocity of 0 in the red peak of the spectrum, which implies that a uniform shell model is over-simplified. The presence of Lyα flux between the two peaks suggests possible contributions to the Lyα emission of extended origin (that is, cooling, photoionization and so on). The crude picture from the model is that the infalling gas (with velocity of ~34 km s⁻¹) has a temperature of ~4,000 K and a neutral hydrogen column density of $10^{19.8}$ cm⁻² (in the regime of the Lyman-limit system). Given the spatial extent of the major Lyα emission in the two-dimensional spectrum, which is about 1" in radius (~8 kpc on a physical scale) and the offset from the central galaxy (~5.4 kpc), the gas can be regarded as part of the circumgalactic medium (CGM).

There could be two possible origins of the infalling gas. First, it may come from the long-sought-after cold streams. The infalling gas may be getting stalled near the central galaxy (hence the low velocity). The other scenario may be that the gas was ejected earlier as part of the galactic wind from the central region and is falling back (the so-called galactic fountain). If we take $r \sim 8$ kpc as the radius of the shell, the total amount of hydrogen mass is then $4\pi r^2 N_{\text{H}} m_{\text{H}} \sim 4.0 \times 10^8 M_{\odot}$, where $m_{\text{H}}$ is the mass of atomic hydrogen. The dynamical timescale is $R/|V| \approx 230$ Myr for $|V| \approx 34$ km s⁻¹. The gas inflow rate is then about $1.7 M_{\odot} \text{yr}^{-1}$. This suggests that while the infalling gas is important in shaping the Lyα emission, it is not the main component responsible for the growth of the central galaxy. The shell model is overly simplified, and the gas distribution can be more complex. For example, the inferred SFR in the power source should produce powerful outflows, which in turn alter the gas kinematics even in the CGM. The photoionization of the CGM gas by ionizing photons from the central source can also change the distribution of the neutral gas as well as produce Lyα emission. The above modelling result should be taken as a rough estimate. Lyα radiative transfer calculation with galaxies

and gas distribution in hydrodynamic galaxy formation simulations might lead to complementary insights into systems like LAB6.1.

We further fit the observed Lyα line profile by including in the contracting shell model the effect of dust¹⁹. We find that the observed profile does not favour models with a large amount of dust. The dust optical depth across the shell is constrained to be at the level of (1–6) × 10⁻³, with neutral hydrogen column density of ~2 × 10¹⁹ cm⁻² and shell velocity ~78 km s⁻¹. The best fit is shown as the dashed curve in Supplementary Fig. 5. With the dust optical depth and the neutral hydrogen column density, we estimate the metallicity of the gas in the shell is at the level of 0.03–0.2 Z$_{\odot}$. The results suggest that the infalling gas comes from cold streams, instead of galactic fountain. Following the above calculation of gas inflow rate $\dot{M}$ from the shell, the cooling radiation luminosity is estimated to be $GM\dot{M}/R \approx 3.4 \times 10^{41} [M (10^{12} M_{\odot})[\dot{M} (1.0 M_{\odot} \text{yr}^{-1})] \text{erg s}^{-1}$, where M is the gravitational mass inside the shell radius and G is the gravitational constant. This is far short of the observed Lyα luminosity. The likely scenario is that star formation contributes the majority of Lyα photons, which are scattered in the shell of cold streams.

In a previous study, one $z \approx 2.3$ LAB system, CDFS-LAB10⁴⁷, was found to have a Lyα component that is blueshifted (about −500 km s⁻¹) with respect to a reference galaxy. However, this Lyα component itself is not associated with any galaxy, and is about 2" (16.4 kpc) away from the reference galaxy. Compared with LAB6.1, this component in CDFS-LAB10 is much farther away from the galaxy, both in projected distance (16.4 kpc versus 5.4 kpc) and in velocity (about −500 km s⁻¹ versus about −50 km s⁻¹). In addition, the Lyα spectrum of the component in CDFS-LAB10 appears to be double-peaked with roughly symmetric red and blue peaks, indicating that the gas itself is not contracting or expanding. This is in direct contrast to the blue-skewed Lyα profile of LAB6.1. Therefore, given the large spatial and velocity separation of the Lyα component in CDFS-LAB10 from the reference galaxy and the roughly symmetric double peaks, its relation to the galaxy is not clear, with infalling, outflowing or photoionized gas at large distances all remaining as possibilities. Lyα emission with prominent blue peaks is also detected in two $z \approx 3.3$ LABs⁵⁰, and both show multiple emission components in the images. Also in these LABs, the Lyα line profiles are double-peaked, more symmetric than that of LAB6.1, making it hard to establish a clear association with inflowing gas. If we extend the case to Lyα-emitting galaxies (rather than LABs), it is found that in a sample of 237 MUSE Lyα spectra about 5% are well-fitted through an inflowing shell²¹, suggesting the existence of infalling gas in a small fraction of systems.

Compared with previous detections of Lyα spectra with blue peaks, the LAB6.1 case exhibits a more blue-skewed Lyα line profile. It also has molecular/atomic gas and dust detections. The peculiar Lyα line profile and the rich data make this object worth further investigations. For LAB6.1, the close association of the Lyα emission in position with the central galaxy probed by the 1.2 mm continuum (Fig. 1) and the more extended morphology suggest that gas probed by the Lyα emission surrounds the galaxy. The blue-skewed Lyα line can be produced by gas infall. The reasonably good fit to the line profile with a contracting shell model lends further support to such a scenario. In summary, the Lyα observation in combination with the ALMA observation of LAB6.1 provides a clear case of blue-skewed Lyα line in a LAB system, which indicates gas infall.

## Data availability

All the raw data used in this study are available at the ALMA Science Archive (https://almascience.nao.ac.jp/aq/) under programme ID 2015.1.00952.S, at the ESO Science Archive Facility (http://archive.eso.org/eso/eso_archive_main.html) under programme IDs 082.A-0846 and 297.A-5059, and at the NASA/IPAC Infrared Science Archive (https://irsa.ipac.caltech.edu/applications/Spitzer/SHA) under programme ID GO-3699. The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

## Code availability

To fit the Lyα line profile, we developed our own code by adopting a simple contracting model that inflating gas is uniformly located in a thin shell of neutral gas and that Lyα radiation from the central source is scattered by the hydrogen atom in the shell. Codes used in this study are not publicly released yet but are available from the corresponding author on reasonable request.

Received: 22 May 2019; Accepted: 27 January 2020;
Published online: 9 March 2020

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## Acknowledgements

Y.A. acknowledges financial support by NSFC grants 11988101, 11933011 and 11373007. J.E.G. is supported by a Royal Society University Research Fellowship. This paper makes use of the following ALMA data: ADS/JAO.ALMA#2015.1.00952.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. This research was based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme IDs 297.A-5059(A) and 082.A-0846(B). This research has made use of the SVO Filter Profile Service (http://svo2.cab.inta-csic.es/theory/fps/) with support from the Spanish MINECO through grant AyA2014-55216.

## Author contributions

Y.A. is the principal investigator of the ALMA and VLT/X-shooter observing proposals. Y.A. reduced the data and wrote the initial manuscript. Z.Z. conducted the data analysis with the SED modelling, drafted the interpretation and discussion of the Lyα emission and polished the manuscript. S.N. and Z.Z. performed the Lyα spectrum fitting with an infalling shell model. C.H. helped to polish the manuscript. All authors discussed and commented on the manuscript.

## Competing interests

The authors declare no competing interests.

## Additional information

Supplementary information is available for this paper at https://doi.org/10.1038/s41550-020-1033-3.

Correspondence and requests for materials should be addressed to Y.A.

Peer review information Nature Astronomy thanks Aaron Smith and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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