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.
Acknowledgements
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.