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Register a new userThe Line Polarization Within a Giant Lyman-alpha Nebula
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Recent theoretical work has suggested that Lyman-alpha nebulae could be substantially polarized in the Lyman-alpha emission line, depending on the geometry, kinematics, and powering mechanism at work. Polarization observations can therefore provide a useful constraint on the source of ionization in these systems. In this Letter, we present the first Lyman-alpha polarization measurements for a giant Lyman-alpha nebula at z~2.656. We do not detect any significant linear polarization of the Lyman-alpha emission: P_{Lyman-alpha}=2.6+/-2.8% (corrected for statistical bias) within a single large aperture. The current data also do not show evidence for the radial polarization gradient predicted by some theoretical models. These results rule out singly scattered Lyman-alpha (e.g., from the nearby AGN) and may be inconsistent with some models of backscattering in a spherical outflow. However, the effects of seeing, diminished signal-to-noise ratio, and angle averaging within radial bins make it difficult to put strong constraints on the radial polarization profile. The current constraints may be consistent with higher density outflow models, spherically symmetric infall models, photoionization by star formation within the nebula or the nearby AGN, resonant scattering, or non-spherically symmetric cold accretion (i.e., along filaments). Higher signal-to-noise ratio data probing to higher spatial resolution will allow us to harness the full diagnostic power of polarization observations in distinguishing between theoretical models of giant Lyman-alpha nebulae.
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We present Spitzer observations of Lya Blobs (LAB) at z=2.38-3.09. The mid-infrared ratios (4.5/8um and 8/24um) indicate that ~60% of LAB infrared counterparts are cool, consistent with their infrared output being dominated by star formation and not active galactic nuclei (AGN). The rest have a substantial hot dust component that one would expect from an AGN or an extreme starburst. Comparing the mid-infrared to submillimeter fluxes (~850um or rest frame far infrared) also indicates a large percentage (~2/3) of the LAB counterparts have total bolometric energy output dominated by star formation, although the number of sources with sub-mm detections or meaningful upper limits remains small (~10). We obtained Infrared Spectrograph (IRS) spectra of 6 infrared-bright sources associated with LABs. Four of these sources have measurable polycyclic aromatic hydrocarbon (PAH) emission features, indicative of significant star formation, while the remaining two show a featureless continuum, indicative of infrared energy output completely dominated by an AGN. Two of the counterparts with PAHs are mixed sources, with PAH line-to-continuum ratios and PAH equivalent widths indicative of large energy contributions from both star formation and AGN. Most of the LAB infrared counterparts have large stellar masses, around 10^11 Mo. There is a weak trend of mass upper limit with the Lya luminosity of the host blob, particularly after the most likely AGN contaminants are removed. The range in likely energy sources for the LABs found in this and previous studies suggests that there is no single source of power that is producing all the known LABs.
Detailed analysis of the substructure of Lya nebulae can put important constraints on the physical mechanisms at work and the properties of galaxies forming within them. Using high resolution HST imaging of a Lya nebula at z~2.656, we have taken a census of the compact galaxies in the vicinity, used optical/near-infrared colors to select system members, and put constraints on the morphology of the spatially-extended emission. The system is characterized by (a) a population of compact, low luminosity (~0.1 L*) sources --- 17 primarily young, small (Re~1-2 kpc), disky galaxies including an obscured AGN --- that are all substantially offset (>20 kpc) from the line-emitting nebula; (b) the lack of a central galaxy at or near the peak of the Lya emission; and (c) several nearly coincident, spatially extended emission components --- Lya, HeII, and UV continuum --- that are extremely smooth. These morphological findings are difficult to reconcile with theoretical models that invoke outflows, cold flows, or resonant scattering, suggesting that while all of these physical phenomena may be occurring, they are not sufficient to explain the powering and large extent of Lya nebulae. In addition, although the compact galaxies within the system are irrelevant as power sources, the region is significantly overdense relative to the field galaxy population (by at least a factor of 4). These observations provide the first estimate of the luminosity function of galaxies within an individual Lya nebula system, and suggest that large Lya nebulae may be the seeds of galaxy groups or low-mass clusters.
We present the discovery of a candidate of giant radio-quiet Lyman-alpha (Lya) blob (RQLAB) in a large-scale structure around a high-redshift radio galaxy (HzRG) lying in a giant Lya halo, B3 J2330+3927 at redshift z=3.087. We obtained Lya imaging around B3 J2330+3927 with Subaru/Suprime-Cam to search for Lya emitters (LAEs) and absorbers (LAAs) at redshift z=3.09+-0.03. We detected candidate 127 LAEs and 26 LAAs in the field of view of 31 x 24. We found that B3 J2330+3927 is surrounded by a 130 kpc Lya halo and a large-scale (60 x 20 comoving Mpc) filamentary structure. The large-scale structure contains one prominent local density peak with an overdensity of greater than 5, which is 8 (15 comoving Mpc) away from B3 J2330+3927. In this peak, we discovered a candidate 100 kpc RQLAB. The existence of both types of Lya nebulae in the same large-scale structure suggests that giant Lya nebulae need special large-scale environments to form. On smaller scales, however, the location of B3 J2330+3927 is not a significant local density peak in this structure, in contrast to the RQLAB. There are two possible interpretations of the difference of the local environments of these two Lya nebulae. Firstly, RQLAB may need a prominent (delta ~ 5) density peak of galaxies to form through intense star-bursts due to frequent galaxy interactions/mergers and/or continuous gas accretion in an overdense environment. On the other hand, Lya halo around HzRG may not always need a prominent density peak to form if the surrounding Lya halo is mainly powered by its radio and AGN activities. Alternatively, both RQLAB and Lya halo around HzRG may need prominent density peaks to form but we could not completely trace the density of galaxies because we missed evolved and dusty galaxies in this survey.
High-redshift Lyman-alpha blobs are extended, luminous, but rare structures that appear to be associated with the highest peaks in the matter density of the Universe. Their energy output and morphology are similar to powerful radio galaxies, but the source of the luminosity is unclear. Some blobs are associated with ultraviolet or infrared bright galaxies, suggesting an extreme starburst event or accretion onto a central black hole. Another possibility is gas that is shock excited by supernovae. However some blobs are not associated with galaxies, and may instead be heated by gas falling into a dark matter halo. The polarization of the Ly-alpha emission can in principle distinguish between these options, but a previous attempt to detect this signature returned a null detection. Here we report on the detection of polarized Ly-alpha from the blob LAB1. Although the central region shows no measurable polarization, the polarized fraction (P) increases to ~20 per cent at a radius of 45 kpc, forming an almost complete polarized ring. The detection of polarized radiation is inconsistent with the in situ production of Ly-alpha photons, and we conclude that they must have been produced in the galaxies hosted within the nebula, and re-scattered by neutral hydrogen.
There is a thin transition region (TR) in the solar atmosphere where the temperature rises from 10,000 K in the chromosphere to millions of degrees in the corona. Little is known about the mechanisms that dominate this enigmatic region other than the magnetic field plays a key role. The magnetism of the TR can only be detected by polarimetric measurements of a few ultraviolet (UV) spectral lines, the Lyman-$alpha$ line of neutral hydrogen at 121.6 nm (the strongest line of the solar UV spectrum) being of particular interest given its sensitivity to the Hanle effect (the magnetic-field-induced modification of the scattering line polarization). We report the discovery of linear polarization produced by scattering processes in the Lyman-$alpha$ line, obtained with the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) rocket experiment. The Stokes profiles observed by CLASP in quiet regions of the solar disk show that the $Q/I$ and $U/I$ linear polarization signals are of the order of 0.1 % in the line core and up to a few percent in the nearby wings, and that both have conspicuous spatial variations with scales of $sim 10$ arcsec. These observations help constrain theoretical models of the chromosphere-corona TR and extrapolations of the magnetic field from photospheric magnetograms. In fact, the observed spatial variation from disk to limb of polarization at the line core and wings already challenge the predictions from three-dimensional magnetohydrodynamical models of the upper solar chromosphere.
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