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Register a new userLyman-alpha Filter Prototype to Enable Astronomical Photometry in the Lyman Ultraviolet
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Observations of astronomical objects in the far ultraviolet (FUV wavelengths span 900-1800{AA}) from earths orbit has been impeded due to bright Lyman-{alpha} geocoronal emission. The Johns Hopkins Rocket Group is developing a hydrogen absorption cell that would act as a narrow band Lyman-{alpha} rejection filter to enable space-based photometric observation in bandpasses that span over the Lyman ultraviolet region shortward of the geocoronal line. While this technology has been applied to various planetary missions with single element photomultiplier detectors it has yet to be used on near earth orbiting satellites with a multi-element detector. We are working to develop a cell that could be easily incorporated into future Lyman ultraviolet missions. The prototype cell is a low-pressure (~ few torr) chamber sealed between a pair of MgF2 windows allowing transmission down to 1150 {AA}. It is filled with molecular hydrogen that is converted to its neutral atomic form in the presence of a hot tungsten filament, which allows for the absorption of the Lyman-{alpha} photons. Molecular hydrogen is stored in a fully saturated non-evaporable getter module (St707TM), which allows the cell pressure to be increased under a modest application of heat (a 20 degree rise from room temperature has produced a rise in pressure from 0.6 to 10 torr). Testing is now underway using a vacuum ultraviolet monochromator to characterize the cell optical depth to Lyman-{alpha} photons as functions of pressure and tungsten filament current. We will present these results, along with a discussion of enabled science in broadband photometric applications.
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There is unique and groundbreaking science to be done with a new generation of UV spectrographs that cover wavelengths in the Lyman Ultraviolet (LUV; 912 - 1216 Ang). There is no astrophysical basis for truncating spectroscopic wavelength coverage anywhere between the atmospheric cutoff (3100 Ang) and the Lyman limit (912 Ang); the usual reasons this happens are all technical. The unique science available in the LUV includes critical problems in astrophysics ranging from the habitability of exoplanets to the reionization of the IGM. Crucially, the local Universe (z <= 0.1) is entirely closed to many key physical diagnostics without access to the LUV. These compelling scientific problems require overcoming these technical barriers so that future UV spectrographs can extend coverage to the Lyman limit at 912 Ang.
We propose to infer ionising continuum leaking properties of galaxies by looking at their Lyman-alpha line profiles. We carry out Lyman-alpha radiation transfer calculations in two models of HII regions which are porous to ionising continuum escape: 1) the so-called density bounded media, in which massive stars produce enough ionising photons to keep the surrounding interstellar medium transparent to the ionising continuum, i.e almost totally ionised, and 2) riddled ionisation-bounded media, surrounded by neutral interstellar medium, but with holes, i.e. with a covering factor lower than unity. The Lyman-alpha spectra emergent from these configurations have distinctive features: 1) a classical asymmetric redshifted profile in the first case, but with a small shift of the maximum of the profile compare to the systemic redshift (Vpeak < 150 km/s); 2) a main peak at the systemic redshift in the second case (Vpeak = 0 km/s), with, as a consequence, a non-zero Lyman-alpha flux bluewards the systemic redshift. Assuming that in a galaxy leaking ionising photons, the Lyman-alpha component emerging from the leaking star cluster(s) dominates the total Lyman-alpha spectrum, the Lyman-alpha shape may be used as a pre-selection tool to detect Lyman continuum (LyC) leaking galaxies, in objects with well determined systemic redshift, and high spectral resolution Lyman-alpha spectra (R >= 4000). The examination of a sample of 10 local starbursts with high resolution HST-COS Lyman-alpha spectra and known in the literature as LyC leakers or leaking candidates, corroborates our predictions. Observations of Lyman-alpha profiles at high resolution should show definite signatures revealing the escape of Lyman continuum photons from star-forming galaxies.
We report on a search for ultraluminous Lyman alpha emitting galaxies (LAEs) at z=6.6 using the NB921 filter on Hyper Suprime-Cam on the Subaru telescope. We searched a 30 degree squared area around the North Ecliptic Pole, which we observed in broadband g, r, i, z, and y and narrowband NB816 and NB921, for sources with NB921 < 23.5 and z - NB921 > 1.3. This corresponds to a selection of log L(Ly-alpha) > 43.5 erg/s. We followed up seven candidate LAEs (out of thirteen) with the Keck DEIMOS spectrograph and confirmed five z=6.6 LAEs, one z=6.6 AGN with a broad Ly-alpha line and a strong red continuum, and one low-redshift ([OIII]5007) galaxy. The five ultraluminous LAEs have wider line profiles than lower luminosity LAEs, and one source, NEPLA4, has a complex line profile similar to that of COLA1. In combination with previous results, we show that the line profiles of the z=6.6 ultraluminous LAEs are systematically different than those of lower luminosity LAEs at this redshift. This result suggests that ultraluminous LAEs generate highly ionized regions of the intergalactic medium in their vicinity that allow the full Lyman alpha profile of the galaxy---including any blue wings---to be visible. If this interpretation is correct, then ultraluminous LAEs offer a unique opportunity to determine the properties of the ionized zones around them, which will help in understanding the ionization of the z ~ 7 intergalactic medium. A simple calculation gives a very rough estimate of 0.015 for the escape fraction of ionizing photons, but more sophisticated calculations are needed to fully characterize the uncertainties.
Ly$alpha$ photons scattered by neutral hydrogen atoms in the circumgalactic media or produced in the halos of star-forming galaxies are expected to lead to extended Ly$alpha$ emission around galaxies. Such low surface brightness Ly$alpha$ halos (LAHs) have been detected by stacking Ly$alpha$ images of high-redshift star-forming galaxies. We study the origin of LAHs by performing radiative transfer modeling of nine $z=3.1$ Lyman-Alpha Emitters (LAEs) in a high resolution hydrodynamic cosmological galaxy formation simulation. We develop a method of computing the mean Ly$alpha$ surface brightness profile of each LAE by effectively integrating over many different observing directions. Without adjusting any parameters, our model yields an average Ly$alpha$ surface brightness profile in remarkable agreement with observations. We find that observed LAHs cannot be accounted for solely by photons originating from the central LAE and scattered to large radii by hydrogen atoms in the circumgalactic gas. Instead, Ly$alpha$ emission from regions in the outer halo is primarily responsible for producing the extended LAHs seen in observations, which potentially includes both star-forming and cooling radiation. With the limit on the star formation contribution set by the ultra-violet (UV) halo measurement, we find that cooling radiation can play an important role in forming the extended LAHs. We discuss the implications and caveats of such a picture.
We examine the dust geometry and Ly{alpha} scattering in the galaxies of the Lyman Alpha Reference Sample (LARS), a set of 14 nearby (0.02 < $z$ < 0.2) Ly{alpha} emitting and starbursting systems with Hubble Space Telescope Ly{alpha}, H{alpha}, and H{beta} imaging. We find that the global dust properties determined by line ratios are consistent with other studies, with some of the LARS galaxies exhibiting clumpy dust media while others of them show significantly lower Ly{alpha} emission compared to their Balmer decrement. With the LARS imaging, we present Ly{alpha}/H{alpha} and H{alpha}/H{beta} maps with spatial resolutions as low as $sim$ 40 pc, and use these data to show that in most galaxies, the dust geometry is best modeled by three distinct regions: a central core where dust acts as a screen, an annulus where dust is distributed in clumps, and an outer envelope where Ly{alpha} photons only scatter. We show that the dust that affects the escape of Ly{alpha} is more restricted to the galaxies central regions, while the larger Ly{alpha} halos are generated by scattering at large radii. We present an empirical modeling technique to quantify how much Ly{alpha} scatters in the halo, and find that this characteristic scattering distance correlates with the measured size of the Ly{alpha} halo. We note that there exists a slight anti-correlation between the scattering distance of Ly{alpha} and global dust properties.
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