Next Contents Previous


With GALEX and HST Lyα observations now running to over hundred galaxies for each telescope, local samples are substantial. Nevertheless, several major questions still remain to be answered. This closing Section is dedicated to outlining a handful of the most pressing questions that, while perhaps challenging, can be addressed within current samples, or with possible extensions with present-day facilities.

8.1. What is the Atomic Gas Distribution in Star-Forming Galaxies?

The spatial distribution of emitted and absorbed Lyα must reflect some set of properties of the Hi gas. In very nearby galaxies, 21 cm observations have already revealed the structure of the cold atomic gas where, for example, the Hi Nearby Galaxy Survey (THINGS, Walter et al., 2008) and VLA-ANGST (Ott et al., 2012) surveys have found an atomic medium that is largely inhomogeneous and clumpy. It is clear that if Lyα photons were injected in these galaxies they would experience more scattering in some regions than others. Examining at least these 21 cm observations, we see that the Hi does not much resemble the shells and slabs in which many radiation transfer calculations are done. In order to empirically determine the effects that Hi and its distribution have on Lyα emission requires resolved observations of individual targets in both Hi and Lyα.

Currently we may contrast average Lyα surface brightness profiles (e.g. Hayes et al., 2014) with those of the intensity at 21 cm (e.g. Bigiel & Blitz, 2012). These samples show both Lyα and 21 cm intensity profiles that are best fit with Sérsic profiles that are close to exponential (n ∼ 1), while UV/optical wavelengths in the LARS sample show significantly higher Sérsic indices (n ≳ 3). Does the Lyα surface brightness trace the gas column density? While curious, this may be entirely coincidental as the sample selection is very different in the two cases. Such observations established in the same galaxies would be enormously instructive in interpreting the Lyα halos of both galaxies at low and high redshifts.

It is unfortunate that resolutions attainable in 21 cm and in the UV/optical are not well matched; sampling small physical scales at 21 cm requires very local galaxies, while the redshift requirements to observe Lyα imply targets must lie beyond several tens of Mpc. Progress has been made by Cannon et al., (2004) and Pardy et al., (2014), but with synthesized beam sizes of ≈ 15 arcsec in the best case (usually much coarser), such Hi observations can place only two resolution elements inside the linear size of the ACS/SBC camera. However VLA in configurations B and A can provide resolutions down to around 4 and 1.3 arcsec, respectively, albeit with a substantial increase in observing time. Observational programmes at higher resolution (VLA C and B configurations) are ongoing, but it is clear that large steps forward may be taken if Hi observations are pushed to the highest spatial resolutions, and such observations are now essential.

8.2. How is the Lyα Spectral Profile Built?

The total integrated Lyα profile of a galaxy is built by emission from different regions, likely with differing kinematics, orientation, and with different contributions to the total Lyα. The spectral profiles of Lyα measured in small apertures tend to be P Cygni, with radiation absorbed from the blue and re-radiated in the red. In these small apertures, Lyα escape fractions are also low. However imaging tells us that these photons are often not expunged from the system, but scattered back into the line of sight at different position, where large-aperture photometry measures significantly higher fescLyα. It is an unfortunately common feature of absorption spectroscopy that we rarely know the line-of-sight distance between the emitting sources and the absorbing gas.

Ideally we would like to know where the frequency redistribution of Lyα occurs, and whether the halo emission shares a similar profile to that of the more central regions where the starburst is located. For example if frequency redistribution occurs close to ionizing clusters and Lyα is singly scattered at large radii, the profile may indeed be similar over large distances. However if photons also get caught for many scatterings in halo gas then a new kinematic structure may be encoded in the Lyα, or double-peaked profiles may be seen.

To resolve this we may take high resolution spectra of the diffuse Lyα-emitting regions in galaxy halos. These observations would be best supported by aperture-matched optical spectroscopy of Hα or Hβ, to tightly constrain the rest velocity distribution of Lyα, and ideally also the highest possible resolution 21 cm observations (previous section). The best tool for this would be HST/STIS with narrow slits, which could provide resolutions of around 20 km s−1, but again at the cost of long integrations in faint regions. Mas-Hesse et al., (2003) have shown that in IRAS 08339+6517, the blue wing of the Lyα profile sets on at a similar wavelength over at least 10 kpc, suggesting the star forming regions are surrounded by a Hi medium that may be rather homogeneous in both space and velocity. However 21 cm observations find gas at much larger radii, where Lyα has not yet been spectroscopically detected; whether the profile bends smoothly and traces the edge of a bubble, and how this effect may vary in different galaxies are all currently unknown.

8.3. Ionization State of the Interstellar Medium

Section 6.2 discusses the effect of a large number of galaxy properties on the emission of Lyα. Many have been studied over time, but largely overlooked has been the effect of ionization state of the ISM. Specifically ionizing radiation from stars may not only produce recombination nebulae but also heat the diffuse warm medium. Thus as well as producing Lyα, the further propagation of LyC radiation may increase the ionization levels of the diffuse gas, lowering the optical depth of the Hii regions to Lyman radiation (e.g. Pellegrini et al., 2012). Given that the ISM of galaxies may be very inhomogeneous, and that Lyα may be absorbed close the nebulae in which it forms, the propagation of ionizing radiation could potentially have a large impact upon the first stages of Lyα transfer.

The ionization parameter (the number of hydrogen-ionizing photons per atom) governs the excitation of the gas, which is usually quantified observationally by the excitation parameter (an emission line ratio that contrasts high and low ionization species). This is done most effectively by taking ratios of p2 and p3 ions: Zastrow et al. (2011, 2013) performed ‘ionization parameter mapping' of several local starbursts using the [Siii]λ 9069 / [Sii]λ 6716 ratio, to identify highly ionized cones that could signpost LyC emission. Similar integrated measurements of ‘Green Pea' galaxies found high [Oiii]λ 5007 / [Oii]λ 3727 ratios (Jaskot & Oey, 2013) that implies high ionization states, and indeed these galaxies have been found to be high-EW Lyα emitters (Jaskot & Oey, 2014; Henry et al., in preparation).

Currently we do not know how these highly ionized regions affect Lyα on small scales in the ISM. Resolved Lyα imaging has now been obtained for over 50 galaxies with ACS, but as yet the ionization structure has only been mapped in one of them. Bik et al., (2015) used VLT/MUSE observations of [Siii] and [Oiii] to map the ionization parameter and gas kinematics in ESO 338-IG04, and compare with the Lyα imaging from Hayes et al., (2005). Two outflowing and highly-ionized conical regions are revealed, that approximately align with the brightest Lyα regions. This would be consistent with a scenario in which Lyα may propagate with less scattering through these highly ionized regions, but the observations of one system cannot establish a causal relation, and a large sample of similar observations needs to be obtained. Pertinent observational questions include whether Lyα emission is systematically enhanced in regions of high [Siii] / [Sii], or whether chimneys through the ISM may feed brighter regions of Lyα emission in extended halos.

8.4. Can we Predict Lyα Observables from Other Information?

Given the importance of Lyα observations at both low and high redshift (see Section 1.1), a vital question becomes whether we can predict the Lyα escape fraction, equivalent width, or line profile from a given set of quantities. I.e. if we are given a dust content, a characteristic velocity for outflowing atomic gas, etc, do our predictions for Lyα emission/absorption match reality?

Given the absence of strong correlations involving Lyα (for example in Figure 12), one may be tempted to conclude that our understanding is not this sophisticated. However if, on the other hand, the Lyα that we observe is governed by geometrical considerations such as viewing angle, then averaged over many galaxies the answer may be more encouraging. For example it has been shown that transfer models inside homogeneous shells can reproduce very wide ranges of line profiles (Schaerer et al., 2011), similar to those that are observed globally at high-z; furthermore when coupled with semi-analytical models of galaxy formation, such models are able to reproduce the broad features of the Lyα LF over a wide range of redshift (Garel et al., 2012). This may imply that our more general picture of Lyα transport is correct.

It is not clear whether current samples are sufficiently large, or span a high enough dynamic range in luminosity/SFR for such an exploration. However in the coming years, larger databases of global properties will become available; GALEX LAEs will remain at around 100 objects; HST spectroscopic samples already exceed this while imaging observations remain somewhat smaller. This should permit statistical studies studies using linear discriminant analyses, to determine how combinations of properties produce the observed Lyα characteristics. Many such possibilities may be envisaged if the signal is strong enough and the samples are sufficiently large.

More empirically motivated simulations can be performed using the wealth of data available for low-z galaxies as input. In such an approach, nature sets up the ISM instead of computers. For example, transport calculations have been run in synthetic galaxies output by hydrodynamical simulations (Laursen et al., 2009, Verhamme et al., 2012, Yajima et al., 2014), although as yet there has been no attempt to construct realistic input conditions of a galaxy based upon observation. Such an experiment is not easy, especially without detailed knowledge of the Hi, but as that becomes available the more the study becomes a possibility. With maps of the ionizing stellar population, nebular gas and ionization structure, Hi distribution including large and small scale kinematics, it will become possible to generate sets of model galaxies that are based upon real systems for which Lyα observations have been obtained. Transfer simulations in such models will then recover the Lyα morphology and spectral profile, that can be tested against observation.


I acknowledge the support of the Swedish Research Council, Vetenskapsrådet and the Swedish National Space Board (SNSB). This research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. I would like to extend warm thanks to my friends and collaborators for comments and feedback on the manuscript: John Cannon, Mark Dijkstra, Daniel Kunth, Peter Laursen, J. Miguel Mas-Hesse, Jens Melinder, Héctor Otí Floranes, Ivana Orlitová, Daniel Schaerer and Anne Verhamme. Lucia Guaita is thanked for making equivalent width tables available for Figure 11. Further I would like to thank Göran Östlin, Claudia Scarlata, Sebastiano Cantalupo, Len Cowie, and many others, especially those from the 2013 NORDITA Lyα workshops, for valuable and stimulating discussions. I thank the anonymous referee for several careful readings of the manuscript, and providing many thoughtful comments that have greatly improved the content.

Next Contents Previous