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Register a new userFLaREON : a fast computation of Ly$alpha$ escape fractions and line profiles
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Added by
Siddhartha Gurung-L\\'opez
Publication date
2018
fields
Physics
and research's language is
English
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We present FLaREON (Fast Lyman-Alpha Radiative Escape from Outflowing Neutral gas), a public python package that delivers fast and accurate Lyman alpha escape fractions and line profiles over a wide range of outflow geometries and properties. The code incorporates different algorithms, such as interpolation and machine learning to predict Lyman alpha line properties from a pre-computed grid of outflow configurations based on the outputs of a Monte Carlo radiative transfer code. Here we describe the algorithm, discuss its performance and illustrate some of its many applications. Most notably, FLaREON can be used to infer the physical properties of the outflowing medium from an observed Lyman alpha line profile, including the escape fraction, or it can be run over millions of objects in a galaxy formation model to simulate the escape of Lyman alpha photons in a cosmological volume.
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We study the heating mechanisms and Ly{alpha} escape fractions of 35 Ly{alpha} blobs (LABs) at z = 3.1 in the SSA22 field. Dust continuum sources have been identified in 11 of the 35 LABs, all with star formation rates (SFRs) above 100 Msun/yr. Likely radio counterparts are detected in 9 out of 29 investigated LABs. The detection of submm dust emission is more linked to the physical size of the Ly{alpha} emission than to the Ly{alpha} luminosities of the LABs. A radio excess in the submm/radio detected LABs is common, hinting at the presence of active galactic nuclei. Most radio sources without X-ray counterparts are located at the centers of the LABs. However, all X-ray counterparts avoid the central regions. This may be explained by absorption due to exceptionally large column densities along the line-of-sight or by LAB morphologies, which are highly orientation dependent. The median Ly{alpha} escape fraction is about 3% among the submm-detected LABs, which is lower than a lower limit of 11% for the submm-undetected LABs. We suspect that the large difference is due to the high dust attenuation supported by the large SFRs, the dense large-scale environment as well as large uncertainties in the extinction corrections required to apply when interpreting optical data.
The Ly$alpha$ escape fraction is a key measure to constrain the neutral state of the intergalactic medium and then to understand how the universe was fully reionized. We combine deep narrowband imaging data from the custom-made filter NB393 and the $H_{2}S$1 filter centered at 2.14 $mu$m to examine the Ly$alpha$ emitters and H$alpha$ emitters at the same redshift $z=2.24$. The combination of these two populations allows us to determine the Ly$alpha$ escape fraction at $z=2.24$. Over an area of 383 arcmin$^{2}$ in the Extended Chandra Deep Field South (ECDFS), 124 Ly$alpha$ emitters are detected down to NB393 = 26.4 mag at the 5$sigma$ level, and 56 H$alpha$ emitters come from An14. Of these, four have both Ly$alpha$ and H$alpha$ emissions (LAHAEs). We measure the individual/volumetric Ly$alpha$ escape fraction by comparing the observed Ly$alpha$ luminosity/luminosity density to the extinction-corrected H$alpha$ luminosity/luminosity density. We revisit the extinction correction for H$alpha$ emitters using the Galactic extinction law with the color excess for nebular emission. We also adopt the Calzetti extinction law together with an identical color excess for stellar and nebular regions to explore how the uncertainties in extinction correction affect our results. In both cases, an anti-correlation between the Ly$alpha$ escape fraction and dust attenuation is found among the LAHAEs, suggesting that dust absorption is responsible for the suppression of the escaping Ly$alpha$ photons. However, the estimated Ly$alpha$ escape fraction of individual LAHAEs varies up to ~3 percentage points between the two methods of extinction correction. We find the global Ly$alpha$ escape fraction at $z=2.24$ to be ($3.7pm1.4$)% in the ECDFS. The variation in the color excess of the extinction causes a discrepancy of ~1 percentage point in the global Ly$alpha$ escape fraction.
We measure the Ly$alpha$ escape fraction of 935 [OIII]-emitting galaxies between $1.9 < z < 2.35$ by comparing stacked spectra from the Hubble Space Telescope/WFC3s near-IR grism to corresponding stacks from the Hobby Eberly Telescope Dark Energy Experiments Internal Data Release 2. By measuring the stacks H$beta$ to Ly$alpha$ ratios, we determine the Ly$alpha$ escape fraction as a function of stellar mass, star formation rate, internal reddening, size, and [OIII]/H$beta$ ratio. We show that the escape fraction of Ly$alpha$ correlates with a number of parameters, such as galaxy size, star formation rate, and nebular excitation. However, we also demonstrate that most of these relations are indirect, and the primary variables that control the escape of Ly$alpha$ are likely stellar mass and internal extinction. Overall, the escape of Ly$alpha$ declines from $gtrsim 18%$ in galaxies with $log M/M_{odot} lesssim 9$ to $lesssim 1%$ for systems with $log M/M_{odot} gtrsim 10$, with the samples mean escape fraction being $6.0^{+0.6%}_{-0.5%}$.
We analyze the spectra of $10$ Green Pea galaxies, previously studied by Henry et al. (2015), using a semi-analytical line transfer (SALT) model to interpret emission and absorption features observed in UV galactic spectra. We focus our analysis on various ionization states of silicon, associated with the cool ($sim 10^4$ K) and warm ($sim 10^5$ K) gas. By analyzing low-ionization lines, we study the relationships between the distribution and kinematics of the outflowing H I gas and the observed Ly$alpha$ escape fraction, $f_{esc}^{Lyalpha}$, as well as the Ly$alpha$ emission peak separation, $Delta_{peak}$. We find that outflow geometries which leave a portion of the source uncovered along the line of sight create the best conditions for Ly$alpha$ escape and have narrow peak separations, while geometries which block the observers view of the source create the worst conditions for Ly$alpha$ escape and have large peak separations. To isolate the effects of outflow kinematics, we restricted our testing set to galaxies with spherical outflows and found that $f_{esc}^{Lyalpha}$ and the Ly$alpha$ luminosity both increase with the extent of the galactic winds. A simple estimate suggests that the collisional excitation of neutral hydrogen by free electrons in the cool gas of the winds can account for the Ly$alpha$ luminosity observed in these objects. Finally, we speculate on the relationship between outflows and the escape of ionizing radiation from the CGM.
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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