Deciphering the Lyman α blob 1 with deep MUSE observations

Abstract

Context. Lyman α blobs (LABs) are large-scale radio-quiet Lyman α (Lyα) nebula at high-z that occur predominantly in overdense proto-cluster regions. In particular, there is the prototypical SSA22a-LAB1 at z = 3.1, which has become an observational reference for LABs across the electromagnetic spectrum. Aims. We want to understand the powering mechanisms that drive the LAB so that we may gain empirical insights into the galaxy-formation processes within a rare dense environment at high-z. Thus, we need to infer the distribution, the dynamics, and the ionisation state of LAB 1’s Lyα emitting gas. Methods. LAB 1 was observed for 17.2 h with the VLT/MUSE integral-field spectrograph. We produced optimally extracted narrow band images, in Lyαλ1216, He IIλ1640, and we tried to detect C IVλ1549 emission. By utilising a moment-based analysis, we mapped the kinematics and the line profile characteristics of the blob. We also linked the inferences from the line profile analysis to previous results from imaging polarimetry. Results. We map Lyα emission from the blob down to surface-brightness limits of ≈6 × 10−19 erg s−1 cm−2 arcsec−2. At this depth, we reveal a bridge between LAB 1 and its northern neighbour LAB 8, as well as a shell-like filament towards the south of LAB 1. The complexity and morphology of the Lyα profile vary strongly throughout the blob. Despite the complexity, we find a coherent large-scale east-west velocity gradient of ∼1000 km s−1 that is aligned perpendicular to the major axis of the blob. Moreover, we observe a negative correlation of Lyα polarisation fraction with Lyα line width and a positive correlation with absolute line-of-sight velocity. Finally, we reveal He II emission in three distinct regions within the blob, however, we can only provide upper limits for C IV. Conclusions. Various gas excitation mechanisms are at play in LAB 1: ionising radiation and feedback effects dominate near the embedded galaxies, while Lyα scattering contributes at larger distances. However, He II/Lyα ratios combined with upper limits on C IV/Lyα are not able to discriminate between active galactic nucleus ionisation and feedback- driven shocks. The alignment of the angular momentum vector parallel to the morphological principal axis appears to be at odds with the predicted norm for high-mass halos, but this most likely reflects that LAB 1 resides at a node of multiple intersecting filaments of the cosmic web. LAB 1 can thus be thought of as a progenitor of a present-day massive elliptical within a galaxy cluster.

The MUSE Extremely Deep Field: The cosmic web in emission at high redshift

“…While these observations have revealed some filamentary structures of ionized gas in very massive structures, they are biased to specific environments. For example, the SSA22 protocluster with its large overdensity of active galactic nuclei (AGNs), submillimeter galaxies and Lyα blobs (Lehmer et al 2009;Umehata et al 2018Umehata et al , 2019Herenz et al 2020) is an extreme environment. Furthermore, the environment of a QSO with anisotropic UV radiation and a possible excess of tidal debris from past interactions (Canalizo & Stockton 2001) may not be representative of the generic IGM.…”

ALMA Observations of Lyα Blob 1: Multiple Major Mergers and Widely Distributed Interstellar Media

“…Two LABs were individually identified and labeled as LAB1 and LAB8, originally (Steidel et al 2000;Matsuda et al 2004). A deeper Lyα map has uncovered that the two LABs are connected to each other (Geach et al 2016;Herenz et al 2020;Li et al 2021). As shown in Figure 1, observations in ALMA Band 7 and Band 8 cover LAB1 almost entirely, while ALMA Band 3 observations cover both LAB1 and LAB8 within the field of view.…”

“…LAB1 has an extent of ≈200 kpc and a luminosity L Lyα = 1.1 × 10 44 erg s −1 (Matsuda et al 2004), making LAB1 one of the most spectacular LABs known to date. Together with its environment, a remarkable protocluster, LAB1 has been intensively investigated by a number of works (e.g., Chapman et al 2001Chapman et al , 2004Bower et al 2004;Geach et al 2005Geach et al , 2009Geach et al , 2014Geach et al , 2016Matsuda et al 2007;Weijmans et al 2010;Hayes et al 2011;Uchimoto et al 2012;Tamura et al 2013;Hine et al 2016;Kubo et al 2016;Ao et al 2017;Umehata et al 2017a;Herenz et al 2020;Li et al 2021).…”

The Origin and Evolution of Lyα Blobs in Cosmological Galaxy Formation Simulations

“…Nilsson et al (2006) also argued against the presence of AGNs or superwinds based on the lack of a continuum counterpart detection in a z=3.16 blob (though this is debated, see, e.g., Prescott et al 2015). Other recent studies have supported arguments of a lack of AGNs with measurements of the kinematics of circumgalactic gas in order to argue for gas accretion as the dominant power source (e.g., Daddi et al 2020;Herenz et al 2020). At the same time, other authors have argued against gravitational cooling dominating the power source owing to a lack of kinematic inflow signatures (e.g., Yang et al 2011Yang et al , 2014.…”

“…Indeed, some authors think that LABs may be powered by a variety of mechanisms (Nilsson et al 2006;Prescott et al 2009;Scarlata et al 2009;Webb et al 2009;Ao et al 2015). There is increasing acknowledgement of the kinematic complexities of these objects (Herenz et al 2020), and recent evidence that the presence of infalling gas does not necessarily predict that cooling dominates Lyα emission (Ao et al 2020;Smith 2020). Additionally, the physics of Lyα escape from high-redshift galaxies remains an open problem in connection with LABs, though it is deeply coupled to the emission thereof (Smith et al 2019).…”