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A giant protogalactic disk linked to the cosmic web


The specifics of how galaxies form from, and are fuelled by, gas from the intergalactic medium remain uncertain. Hydrodynamic simulations suggest that ‘cold accretion flows’—relatively cool (temperatures of the order of 104 kelvin), unshocked gas streaming along filaments of the cosmic web into dark-matter halos1,2,3—are important. These flows are thought to deposit gas and angular momentum into the circumgalactic medium, creating disk- or ring-like structures that eventually coalesce into galaxies that form at filamentary intersections4,5. Recently, a large and luminous filament, consistent with such a cold accretion flow, was discovered near the quasi-stellar object QSO UM287 at redshift 2.279 using narrow-band imaging6. Unfortunately, imaging is not sufficient to constrain the physical characteristics of the filament, to determine its kinematics, to explain how it is linked to nearby sources, or to account for its unusual brightness, more than a factor of ten above what is expected for a filament. Here we report a two-dimensional spectroscopic investigation of the emitting structure. We find that the brightest emission region is an extended rotating hydrogen disk with a velocity profile that is characteristic of gas in a dark-matter halo with a mass of 1013 solar masses. This giant protogalactic disk appears to be connected to a quiescent filament that may extend beyond the virial radius of the halo. The geometry is strongly suggestive of a cold accretion flow.

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Figure 1: Spectral image and pseudo-slit spectrum of the QSO UM287 field.
Figure 2: Spectral image and narrow-band image created from a sheared-velocity window.
Figure 3: Physical properties of the extended disk.
Figure 4: Sketch of the geometry of the QSO–disk system.


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We thank T. Tombrello and S. Kulkarni for their support of PCWI. This work was supported by the National Science Foundation and the California Institute of Technology.

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Authors and Affiliations



D.C.M. is the principal investigator of PCWI, led the observations and analysis of UM287, and was principal author on the paper. M.M., D.C., and P.M. designed, constructed, and operated PCWI. A.M. was the project and technical manager (2006–2010). J.D.N., M.M., and D.C.M. developed the PCWI/KCWI data pipeline and produced the final data cubes. M.M., P.M., S.C., and J.X.P. contributed Keck data and helped edit the paper.

Corresponding author

Correspondence to D. Christopher Martin.

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Extended data figures and tables

Extended Data Figure 1 Illustration of raw and conventionally smoothed data.

a, Raw-data spectral image of pseudo-slit obtained in the slit shown in b. 1σ error is about 3 kLU or approximately one colour scale step. b, Raw-data narrow-band image obtained in the 3,970–4,000 Å band. 1σ error is 50 kLU or about 0.5 colour scale steps. c, Raw spectral image shown in a, conventionally boxcar smoothed by 10 pixels. d, Raw narrow-band image shown in b, conventionally boxcar smoothed by 10 pixels. e, Spectral image obtained by 3D adaptive smoothing as discussed in the Methods. f, Narrow-band image (3,970–4,000 Å) obtained by summing over the 3D adaptively smoothed data cube, as discussed in the Methods.

Extended Data Figure 2 Channel maps of the UM287 data cube.

al, Panels show individual velocity channels that are 150 km s−1 wide, corresponding to a 2 Å width. Sources A–D near QSO UM287 are plotted. Velocities are with respect to the UM287 systemic velocity. QSO A has been subtracted by calculating an average PSF for the QSO over the 3,970–4,000 Å band, and then subtracting this slice by slice, within an elliptical radius of 6 arcsec in the x direction and 7.2 arcsec in the y direction. The residual flux in certain channels is due to three effects. (1) At the centre of QSO A, the residual flux is due to emission lines in the QSO around Lyα. (2) At the elliptical boundary surrounding the QSO, a small subtraction residual can be seen outside the ellipse within which the subtraction is performed. This is typically 3–5 kLU, and can be seen clearly without additional sources in c and d. (3) Emission sources are present near the QSO in certain channels. Source C shows line emission primarily in j and k (430 km s−1 < v < 731 km s−1), and its continuum emission is subtracted along with the QSO. Any emission near the subtraction boundary above 5 kLU is not a subtraction residual due to the QSO. The disk emission appears bright in b southeast of QSO A, and moves north, approaching the QSO as the velocity moves redward. In the two central velocity channels, the emission appears almost ring-like and continues to move north, roughly centred on source D. The emission continues to move north in hj. The emission is 15–25 kLU and therefore cannot be QSO subtraction residuals. The emission is also not partially subtracted emission from source C. The emission in i and j are several arcseconds east of source C. Further evidence for this point is given in Extended Data Figs 4 and 5. To indicate the approximate disk location, we show an elliptical contour with a major axis radius of 8.5 arcsec, position angle of 15.5°, and an ellipticity corresponding to an inclination of 70°.

Extended Data Figure 3 Comparison of PCWI data and Keck narrow-band and continuum images.

a, PCWI narrow-band image created by summing 3,970–4,000 Å data-cube slices from the adaptively smoothed data cube. Sources A, B, C, and D are shown. PCWI image is not continuum subtracted. b, Keck continuum-subtracted narrow-band image on the same intensity scale as the PCWI image in a. c, Keck V-band image. Sources A, C, and D are shown; continuum magnitudes in the V band are approximately (±2 mag) 16.6 AB, 22.2 AB, and 23.8 AB respectively. d, Keck continuum-subtracted narrow-band image on an expanded intensity scale.

Extended Data Figure 4 Channel maps of the UM287 data cube.

Panels show individual velocity channels that are 150 km s−1 wide, corresponding to a 2 Å width, as in Extended Data Fig. 2. In this case, no source subtraction has been performed. Sources near the QSO are plotted. Ellipses are drawn as in Extended Data Fig. 2. Velocities are with respect to the UM287 systemic velocity.

Extended Data Figure 5 Channel maps of the UM287 data cube.

al, Individual velocity channels that are 150 km s−1 wide, corresponding to a 2 Å width, as in Extended Data Fig. 2. In this case, the wavelength-averaged cube has been subtracted from the cube over the full field of view. Sources near the QSO are plotted. Ellipses are drawn as in Extended Data Fig. 2. Velocities are with respect to the UM287 systemic velocity. This subtraction removes the average continuum from all sources, including source C. The emission centred on source C is present in j and k from line emission (presumably Lyα). The progression of the disk emission can be seen as in Extended Data Fig. 2 from v = −700 km s−1 to v = +600 km s−1.

Extended Data Figure 6 Spectral image and rectangular pseudo-slit slices of the UM287 data cube.

ac, The spectral image shown at left is in the same format as Fig. 1, and the narrow-band image that is obtained using a −600 km s−1 < v < 600 km s−1 velocity cut is shown on the right with the corresponding slit location from which the spectral image is obtained. The vertical slits are 2.5 arcsec wide, and are positioned at 0 arcsec, 3.75 arcsec, and 5.0 arcsec in the positive x direction (northeast) with respect to the 0 reference position. The average QSO A spectrum has been subtracted in the bright regions. In each spectral image, strong emission appears with a large, quasi-linear velocity shear centred on the QSO velocity. Also, the narrow-band image in this velocity range illustrates that, as the emission approaches the QSO, there is an offset to the northeast that is not consistent with a direct entry into the QSO. df, The same plots with the minimum intensity range set to a negative value to show negative residuals from the QSO subtraction.

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Martin, D., Matuszewski, M., Morrissey, P. et al. A giant protogalactic disk linked to the cosmic web. Nature 524, 192–195 (2015).

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