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ABSTRACT We present the first statistical analysis of kinematically resolved, spatially extended $$\rm Ly\alpha$$ emission around z = 2–3 galaxies in the Keck Baryonic Structure Survey (KBSS) using the Keck Cosmic Web Imager (KCWI). Our sample of 59 star-forming galaxies (zmed = 2.29) comprises the subset with typical KCWI integration times of ∼5 h and with existing imaging data from the Hubble Space Telescope and/or adaptive optics-assisted integral field spectroscopy. The high-resolution images were used to evaluate the azimuthal dependence of the diffuse $$\rm Ly\alpha$$ emission with respect to the stellar continuum within projected galactocentric distances of ≲30 proper kpc. We introduce cylindrically projected 2D spectra (CP2D) that map the averaged $$\rm Ly\alpha$$ spectral profile over a specified range of azimuthal angle, as a function of impact parameter around galaxies. The averaged CP2D spectrum of all galaxies shows clear signatures of $$\rm Ly\alpha$$ resonant scattering by outflowing gas. We stacked the CP2D spectra of individual galaxies over ranges of azimuthal angle with respect to their major axes. The extended $$\rm Ly\alpha$$ emission along the galaxy principal axes is statistically indistinguishable, with residual asymmetry of ≤2 per cent (∼2σ) of the integrated $$\rm Ly\alpha$$ emission. The symmetry implies that the $$\rm Ly\alpha$$ scattering medium is dominated by outflows in all directions within 30 kpc. Meanwhile, we find that the blueshifted component of $$\rm Ly\alpha$$ emission is marginally stronger along galaxy minor axes for galaxies with relatively weak $$\rm Ly\alpha$$ emission. We speculate that this weak directional dependence of $$\rm Ly\alpha$$ emission becomes discernible only when the $$\rm Ly\alpha$$ escape fraction is low. These discoveries highlight the need for similar analyses in simulations with $$\rm Ly\alpha$$ radiative transfer modelling.
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The Keck Baryonic Structure Survey: using foreground/background galaxy pairs to trace the structure and kinematics of circumgalactic neutral hydrogen at z ∼ 2
ABSTRACT We present new measurements of the spatial distribution and kinematics of neutral hydrogen in the circumgalactic and intergalactic medium surrounding star-forming galaxies at z ∼ 2. Using the spectra of ≃3000 galaxies with redshifts 〈z〉 = 2.3 ± 0.4 from the Keck Baryonic Structure Survey, we assemble a sample of more than 200 000 distinct foreground-background pairs with projected angular separations of 3–500 arcsec and spectroscopic redshifts, with 〈zfg〉 = 2.23 and 〈zbg〉 = 2.57 (foreground, background redshifts, respectively.) The ensemble of sightlines and foreground galaxies is used to construct a 2D map of the mean excess $$\rm{H\,{\small I}}$$\rm Ly\,\alpha$$ optical depth relative to the intergalactic mean as a function of projected galactocentric distance (20 ≲ Dtran/pkpc ≲ 4000) and line-of-sight velocity. We obtain accurate galaxy systemic redshifts, providing significant information on the line-of-sight kinematics of $$\rm{H\,{\small I}}$$ gas as a function of projected distance Dtran. We compare the map with cosmological zoom-in simulation, finding qualitative agreement between them. A simple two-component (accretion, outflow) analytical model generally reproduces the observed line-of-sight kinematics and projected spatial distribution of $$\rm{H\,{\small I}}$$. The best-fitting model suggests that galaxy-scale outflows with initial velocity vout ≃ 600 km s$$^{-1}\,$$ dominate the kinematics of circumgalactic $$\rm{H\,{\small I}}$$ out to Dtran ≃ 50 kpc, while $$\rm{H\,{\small I}}$$ at Dtran ≳ 100 kpc is dominated by infall with characteristic vin ≲ circular velocity. Over the impact parameter range 80 ≲ Dtran/pkpc ≲ 200, the $$\rm{H\,{\small I}}$$ line-of-sight velocity range reaches a minimum, with a corresponding flattening in the rest-frame $$\rm Ly\,\alpha$$ equivalent width. These observations can be naturally explained as the transition between outflow-dominated and accretion-dominated flows. Beyond Dtran ≃ 300 pkpc (∼1 cMpc), the line-of-sight kinematics are dominated by Hubble expansion.
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- PAR ID:
- 10208465
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 499
- Issue:
- 2
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- 1721 to 1746
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/mnras/staa2808
