[Paper Review] Infalling gas in a Lyman-alpha blob

Yiping Ao, Zheng Zheng+12 more|arXiv (Cornell University)|Jan 1, 2020

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy64 references27 citations

TL;DR

This study presents direct kinematic evidence of infalling gas in Lyman-alpha blob LAB6, detected via ALMA observations of CO J=7−6 and C I(3P2→3P1) lines and X-shooter spectroscopy of Lyα emission. The blue-skewed Lyα line profile—unusual in star-forming systems—best explained by gas infall onto the central submillimeter galaxy, supporting cooling radiation from accreting cold streams as a viable mechanism for extended Lyα emission in some LABs.

ABSTRACT

Lyman-α blobs (LABs) are spatially extended nebulae of emission in the Lyman-α (Lyα) line of hydrogen, seen at high redshifts1,2, and most commonly found in the dense environment of star-forming galaxies3,4. A recent study showed that nearly 100% of the sky is covered by Lyα emission around high-redshift galaxies5,6,7. The origin of Lyα emission in the LABs is still unclear and under debate8. It may be powered by photoionization involving galactic superwinds/outflows, resonant scattering of Lyα photons from starbursts or active galactic nuclei9,10,11,12,13,14,15,16, or by cooling radiation from cold streams of gas accreting onto galaxies17,18, as demonstrated by recent simulations19. 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.

Motivation & Objective

• To determine the origin of extended Lyα emission in Lyman-alpha blobs (LABs), which remains debated between photoionization and cooling radiation from cold streams.
• To investigate the kinematics of gas in the central region of LAB6, a massive high-redshift LAB in the Francis cluster.
• To test whether infall or outflow motions can explain the observed blue-skewed Lyα line profile, which is rare in star-forming systems.
• To assess the role of cold accretion streams in powering extended Lyα emission, particularly in the absence of a strong central ionizing source.
• To constrain the gas inflow rate and its connection to the high star formation rate in the central submillimeter galaxy (SMG) of LAB6.

Proposed method

• Conducted deep ALMA observations at 246.6 GHz to detect CO J=7−6 and C I(3P2→3P1) emission lines and 1.2 mm dust continuum toward LAB6.
• Used X-shooter spectroscopy on the VLT to obtain high-resolution Lyα line profiles along a slit aligned with the LAB’s extended emission.
• Measured the systemic redshift of molecular gas from the CO J=7−6 line at z = 2.3691 ± 0.0004 to anchor the velocity frame.
• Fitted the observed Lyα line profile with a contracting shell model to test whether infall can reproduce the blue skewness.
• Calculated the far-infrared luminosity and star formation rate (SFR) from the 1.2 mm continuum flux, assuming dust emission from a starburst.
• Compared the spatial offset between Lyα peak and dust continuum peak to assess anisotropy and orientation effects in gas distribution.

Experimental results

Research questions

• RQ1 Can the observed blue-skewed Lyα line profile in LAB6 be explained by infalling gas rather than outflows?
• RQ2 Is there direct kinematic evidence linking gas inflow to the central submillimeter galaxy in LAB6?
• RQ3 Does the observed Lyα emission profile support the cold stream accretion model over photoionization by a central source?
• RQ4 What is the mass inflow rate of gas into the central galaxy, and how does it compare to the star formation rate?
• RQ5 How do the spatial and kinematic properties of the Lyα emission relate to the distribution of molecular gas and dust?

Key findings

• The Lyα line profile in LAB6 is blue-skewed by approximately −150 km s⁻¹ relative to the systemic velocity of molecular gas, indicating infall rather than outflow.
• ALMA detected a 1.2 mm dust continuum source (LAB6.1) at the center of the LAB with a flux density of 1.57 ± 0.11 mJy, corresponding to a far-infrared luminosity of (6.5 ± 0.13) × 10¹² L⊙.
• The inferred star formation rate in LAB6.1 is approximately 1100 M⊙ yr⁻¹, indicating a powerful central source capable of ionizing extended gas.
• The gas inflow rate is estimated at 1.7 M⊙ yr⁻¹, which is negligible compared to the star formation rate, suggesting infall is not a major mass supply mechanism but may be a feedback or recycling process.
• The Lyα peak is offset by ∼5.4 kpc from the dust continuum peak, likely due to anisotropic gas and dust distribution or orientation effects.
• The contracting shell model provides a good fit to the Lyα profile, with an inferred shell velocity of ∼−34 km s⁻¹, supporting the infall scenario as the cause of the blue skew.