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Register a new userUnlocking the Full Potential of Extragalactic Ly$alpha$ through Its Polarization Properties
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Marius B. Eide
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Lyman-$alpha$ (Ly$alpha$) is a powerful astrophysical probe. Not only is it ubiquitous at high redshifts, it is also a resonant line, making Ly$alpha$ photons scatter. This scattering process depends on the physical conditions of the gas through which Ly$alpha$ propagates, and these conditions are imprinted on observables such as the Ly$alpha$ spectrum and its surface brightness profile. In this work, we focus on a less-used observable capable of probing any scattering process: polarization. We implement the density matrix formalism of polarization into the Monte Carlo radiative transfer code tlac. This allows us to treat it as a quantum mechanical process where single photons develop and lose polarization from scatterings in arbitrary gas geometries. We explore static and expanding ellipsoids, biconical outflows, and clumpy multiphase media. We find that photons become increasingly polarized as they scatter and diffuse into the wings of the line profiles, making scattered Ly$alpha$ polarized in general. The degree and orientation of Ly$alpha$ polarization depends on the kinematics and distribution of the scattering HI gas. We find that it generally probes spatial or velocity space asymmetries and aligns itself tangentially to the emission source. We show that the mentioned observables, when studied separately, can leave similar signatures for different source models. We conclude by revealing how a joint analysis of the Ly$alpha$ spectra, surface brightness profiles, and polarization can break these degeneracies and help us extract unique physical information on galaxies and their environments from their strongest, most prominent emission line.
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Ly$alpha$ nebulae are giant ($sim$100 kpc), glowing gas clouds in the distant universe. The origin of their extended Ly$alpha$ emission remains a mystery. Some models posit that Ly$alpha$ emission is produced when the cloud is photoionized by UV emission from embedded or nearby sources, while others suggest that the Ly$alpha$ photons originate from an embedded galaxy or AGN and are then resonantly scattered by the cloud. At least in the latter scenario, the observed Ly$alpha$ emission will be polarized. To test these possibilities, we are conducting imaging polarimetric observations of seven Ly$alpha$ nebulae. Here we present our results for LABd05, a cloud at $z$ = 2.656 with an obscured, embedded AGN to the northeast of the peak of Ly$alpha$ emission. We detect significant polarization. The highest polarization fractions $P$ are $sim$10-20% at $sim$20-40 kpc southeast of the Ly$alpha$ peak, away from the AGN. The lowest $P$, including upper-limits, are $sim$5% and lie between the Ly$alpha$ peak and AGN. In other words, the polarization map is lopsided, with $P$ increasing from the Ly$alpha$ peak to the southeast. The measured polarization angles $theta$ are oriented northeast, roughly perpendicular to the $P$ gradient. This unique polarization pattern suggests that 1) the spatially-offset AGN is photoionizing nearby gas and 2) escaping Ly$alpha$ photons are scattered by the nebula at larger radii and into our sightline, producing tangentially-oriented, radially-increasing polarization away from the photoionized region. Finally we conclude that the interplay between the gas density and ionization profiles produces the observed central peak in the Ly$alpha$ emission. This also implies that the structure of LABd05 is more complex than assumed by current theoretical spherical or cylindrical models.
The unification scheme of active galactic nuclei (AGNs) invokes an optically thick molecular torus component hiding the broad emission line region. Assuming the presence of a thick neutral component in the molecular torus characterized by a ion{H}{I} column density > $10^{22}{rm cm^{-2}}$, we propose that far UV radiation around Ly$alpha$ can be significantly polarized through Rayleigh scattering. Adopting a Monte Carlo technique we compute polarization of Rayleigh scattered radiation near Ly$alpha$ in a thick neutral region in the shape of a slab and a cylindrical shell. It is found that radiation near Ly$alpha$ Rayleigh reflected from a very thick slab can be significantly polarized in a fairly large range of wavelength $Deltalambdasim 50$ AA exhibiting a flux profile similar to the incident one. Rayleigh transmitted radiation in a slab is characterized by the central dip with a complicated polarization behavior. The optically thick part near Ly$alpha$ center is polarized in the direction perpendicular to the slab normal, which is in contrast to weakly polarized wing parts in the direction parallel to the slab normal. A similar polarization flip phenomenon is also found in the case of a tall cylindrical shell, in which the spatial diffusion along the vertical direction near the inner cylinder wall for core photons leads to a tendency of the electric field aligned to the direction perpendicular to the vertical axis. Observational implications are briefly discussed including spectropolarimetry of the quasar PG~1630+377 by Koratkar et al. in 1990 where Ly$alpha$ is strongly polarized with no other emission lines polarized.
We present rest-frame optical spectra of 60 faint ($R_{AB}sim 27$; $Lsim0.1 L_*$) Ly$alpha$-selected galaxies (LAEs) at $zapprox2.56$. The average LAE is consistent with the extreme low-metallicity end of the continuum-selected galaxy distribution at $zapprox2-3$. In particular, the LAEs have extremely high [OIII] $lambda$5008/H$beta$ ratios (log([OIII]/H$beta$) $sim$ 0.8) and low [NII] $lambda$6585/H$alpha$ ratios (log([NII]/H$alpha$) $<-1.15$). Using the [OIII] $lambda$4364 auroral line, we find that the star-forming regions in faint LAEs are characterized by high electron temperatures ($T_eapprox1.8times10^4$K), low oxygen abundances (12 + log(O/H) $approx$ 8.04, $Z_{neb}approx0.22Z_odot$), and high excitations with respect to more luminous galaxies. Our faintest LAEs have line ratios consistent with even lower metallicities, including six with 12 + log(O/H) $approx$ 6.9$-$7.4 ($Z_{neb}approx0.02-0.05Z_odot$). We interpret these observations in light of new models of stellar evolution (including binary interactions). We find that strong, hard ionizing continua are required to reproduce our observed line ratios, suggesting that faint galaxies are efficient producers of ionizing photons and important analogs of reionization-era galaxies. Furthermore, we investigate physical trends accompanying Ly$alpha$ emission across the largest current sample of combined Ly$alpha$ and rest-optical galaxy spectroscopy, including 60 faint LAEs and 368 more luminous galaxies at similar redshifts. We find that Ly$alpha$ emission is strongly correlated with nebular excitation and ionization and weakly correlated with dust attenuation, suggesting that metallicity plays a strong role in determining the observed properties of these galaxies by modulating their stellar spectra, nebular excitation, and dust content.
Virtually all of deep learning literature relies on the assumption of large amounts of available training data. Indeed, even the majority of few-shot learning methods rely on a large set of base classes for pretraining. This assumption, however, does not always hold. For some tasks, annotating a large number of classes can be infeasible, and even collecting the images themselves can be a challenge in some scenarios. In this paper, we study this problem and call it Small Data setting, in contrast to Big Data. To unlock the full potential of small data, we propose to augment the models with annotations for other related tasks, thus increasing their generalization abilities. In particular, we use the richly annotated scene parsing dataset ADE20K to construct our realistic Long-tail Recognition with Diverse Supervision (LRDS) benchmark by splitting the object categories into head and tail based on their distribution. Following the standard few-shot learning protocol, we use the head classes for representation learning and the tail classes for evaluation. Moreover, we further subsample the head categories and images to generate two novel settings which we call Scarce-Class and Scarce-Image, respectively corresponding to the shortage of samples for rare classes and training images. Finally, we analyze the effect of applying various additional supervision sources under the proposed settings. Our experiments demonstrate that densely labeling a small set of images can indeed largely remedy the small data constraints.
In this work we model the observed evolution in comoving number density of Lyman-alpha blobs (LABs) as a function of redshift, and try to find which mechanism of emission is dominant in LAB. Our model calculates LAB emission both from cooling radiation from the intergalactic gas accreting onto galaxies and from star formation (SF). We have used dark matter (DM) cosmological simulation to which we applied empirical recipes for Ly$alpha$ emission produced by cooling radiation and SF in every halo. In difference to the previous work, the simulated volume in the DM simulation is large enough to produce an average LABs number density. At a range of redshifts $zsim 1-7$ we compare our results with the observed luminosity functions of LABs and LAEs. Our cooling radiation luminosities appeared to be too small to explain LAB luminosities at all redshifts. In contrast, for SF we obtained a good agreement with observed LFs at all redshifts studied. We also discuss uncertainties which could influence the obtained results, and how LAB LFs could be related to each other in fields with different density.
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