Polarized extended Lyα emission from a z=2.3 radio galaxy,
We present spatially resolved spectropolarimetic measurements of the 100-kpc scale gaseous environment of the z=2.34 radio galaxy TXS 0211-122. The polarization level of the narrow Lyα emission is low centrally (P < 5 %), but rises to P=16.4±4.6 % in the Eastern part of the nebula, indicating that the nebula is at least partly powered by the scattering of Lyα photons by HI. Not only is this the first detection of polarized Lyα around a radio-loud active galaxy, it is also the second detection to date for any kind of Lyα nebula. We also detect a pair of diametrically opposed UV continuum sources along the slit, at the outer edges of the Lyα nebula, which we suggest may be the limb of a dusty shell, related to the large-scale HI absorbers often associated with high-z radio galaxies.
Luminous high-redshift Lyα nebulae have been used as an efficient way to select galaxies which present significant mass-assembly rates. However, is it still unclear which are the dominant physical mechanisms behind such emission, although several hypothesis already exist and there is consensus that no single mechanism is able to explain the emission from all these nebulae.
One of the proposed mechanisms is the scattering of Lyα photons by neutral hydrogen, which can cause a net observable polarization. Due to technical difficulties and to the large observation time required for polarimetric observations of high-redshift Lyα nebulae, polarimetric measurements of extended Lyα emission had been performed in only two nebulae, with conflicting results.
Many of these nebulae also allowed the detection of gaseous structures in front of them through HI absorption spectral features. Some of these structures are thought to be expanding shells surrounding the Lyα nebulae, and its detection in emission – instead of absorption – would allow a much better understanding of its physical properties.
Now, deep spectroscopy and spectropolarimetry data of TXS 0211-122 have been analyzed. TXS 0211-122 is a z=2.34 radio galaxy associated with a large scale Lyα nebula and a HI absorbing structure with a size of ≥100 kpc. The main observational data was obtained with the Low Resolution Imaging Spectrometer in polarimetry mode of the 10 m Keck II telescope, with data from the Infrared Spectrometer And Array Camera in long-slit mode at the 8 m Very Large Telescope also being used.
Several polarization measurements of the Lyα line were performed along the slit. It was possible to detect a region with a substantial Lyα polarization P=16.4±4.6%, although the core of the line only presents a modest polarization P<6.8% (Fig. 1). These results are consistent with theoretical predictions of Lyα scattering in HI halos around high redshift galaxies and adds to the growing body of evidence that Lyα scattering by HI may not be an uncommon powering mechanism for Lyα nebula found at high redshifts. This polarization detection is the first ever achieved in narrow Lyα emission from a nebula associated with a radio-loud active galaxy.
Within the spectroscopic slit, it was also possible to detect two faint and diametrically opposed continuum sources around the Lyα nebula (Fig. 2). Their characteristics are consistent with being the limb of a gas and dust shell, centered on TXS 0211-122, with a total gas mass of ≥4x108ʘ. Unlike previous detections of such shells, this is detected in emission, rather than absorption, due to being illuminated by the central AGN. Gas and dust swept up from the interstellar medium of the galaxy by a powerful starburst event, during which at least 105 supernovae explosions would have needed to occur, offers a plausible origin for this giant shell.
Andrew Humphrey was lead author and as such performed the majority of the data analysis and interpretation, with important contributions from the other authors.