The interstellar medium is the matrix within which the processes of galaxy evolution occur. It exists in many states: atomic and ionized hydrogen, relativistic plasma, molecular gas, and dust, each containing velocity and density structures over a vast range of scales. At the low spatial scales, a highly disturbed state is maintained by point-like energy input from stars at all phases of the stellar life cycle. On the other end of the scale, energy input can take the form of global, large-scale phenomena, such as viscous dissipation or magnetic stress from Galactic rotation, and the motion of spiral arm density waves. Despite the apparent flux of energy on all scales, pockets of relative quiescence exist, where cold gas can self-gravitate and the process of star formation begins. The life cycle of stars, and the state and evolution of the ISM environment, are intimately intertwined. To date we are restricted to studying these processes in detail only in our own and a very few nearby galaxies. While several large ambitious projects are now underway to take advantage of the unique perspective we have in our own Galaxy, we have essentially no detailed information on the evolution of the ISM of galaxies from the epoch of formation, through the multiple stages of star formation and recycling to the present epoch. A multi-wavelength approach is required to fully understand the complex processes and phenomena that govern the evolution of the interstellar medium. To extend these studies back in cosmic time we must be able to image the ISM of external galaxies to high redshift in as many states of the ISM as possible. The SKA offers the possibility of studying the interstellar medium of a significant number of external galaxies with a detail that has heretofore been possible only in our own and nearby galaxies. The SKA will allow imaging of four major components; the atomic hydrogen gas, the relativistic plasma, the ionized medium and the molecular medium.
The physics of heating and cooling in a photoionized plasma
of characteristic astrophysical abundance results in an
equilibrium kinetic temperature of a few 104 K. The
photoionized interstellar medium, thus has brightness
temperatures of this value or lower, depending on optical
depth. The sensitivity of the SKA will open for the first
time the possibility of imaging of this low surface
brightness, thermal radio emission at milli-arcsecond
resolution. This capability will have a revolutionary
impact on the field of radio astronomy, one that will spill
over into many areas of astrophysical inquiry.
This advance is illustrated in Fig. 1.19, which shows a plot
of angular size versus brightness temperature. The three
dashed diagonal lines show the angular radius of a source
required to produce flux densities at 
6 cm of 100 mJy, 1
mJy and 1 
Jy as a function of the brightness temperature of
the source. The dark solid lines characterize the imaging
capabilities of the most powerful existing radio telescope
arrays. The horizontal portion shows their maximum
resolution, and the diagonal portion shows the minimum
detectable flux density. For a radio source with a given
angular radius and brightness temperature to be resolved by
a particular radio telescope array, it must lie above the
solid line. The current suite of most sensitive radio
telescope facilities occupies the upper right portion of
this diagram, and is able to image only sources with either
very high brightness temperature or large angular size.
For thermal radio sources (below 104 K) the maximum
attainable resolution is 
0.1'' (with the VLA). To make
inroads toward the small angular diameter, thermal
brightness temperature region, a giant step in sensitivity
is needed. The SKA will uniquely occupy this region of
parameter space, providing angular resolution of 10 milli-
arcseconds at 6cm down to continuum brightness temperatures
well below 100K, and, as an element of a global VLBI array,
a few milli-arcsecond resolution at temperatures of 103 -
104 K.
 |
Ionized hydrogen in external galaxies is a direct tracer of
massive star formation. At a wavelength of a few cm, the
SKA would be able to detect the continuum emission from the
Strömgren sphere surrounding an O5 star at a distance of
100 Mpc in 12 hours. A B0 star would be detected out to 10
Mpc. An HIIregion surrounding a luminous early-type star
has typical dimension of tens of pc. More compact and
ultracompact HIIregions are known in our own Galaxy with
dimensions 1 to 0.1 pc and brightness temperature 103 - 104
K. Since in the local Universe, the surface brightness of resolved
objects is
independent of distance, the SKA will be able to image
bright HIIregions at linear resolution below 0.1 pc at up
to 20 Mpc distance, and 0.5 pc at 100 Mpc. Such
observations would provide, for example, complete counts of
luminous young stars in galaxies well beyond the Virgo
cluster, providing direct measurement of the high end
Initial Mass Function in a very large number of galaxies
and in a range of cluster environments. These measurements
would be unaffected by extinction. Comparison with H
images from sensitive optical telescopes would yield
measurements of the extinction and allow derivation of
temperatures and densities.
The deconvolution of non-thermal and thermal emission in
galaxy disks from high resolution imaging at decimetre and
centimetre wavelengths will provide the means to
investigate the origin of the far infrared-radio luminosity
correlation in galaxies. By comparing such images to high
resolution images of dust emission obtained from large
submillimetre arrays, we will be able make detailed spatial
studies and determine the dominant dust heating mechanisms.
Observations of HIIregions at low frequencies (330 MHz) have demonstrated that measurements of optically thick HIIregions can constrain source electron temperatures, emission measures, and filling factors (Kassim et al. 1989; Subrahmanyan & Goss 1996). At even lower frequencies these regions appear as cooler regions against a much hotter Galactic background, allowing kinematic distance ambiguities within our Galaxy to be resolved and the superposition of thermal and nonthermal sources to be separated along complex lines of sight through the Galaxy. Kinematic distance ambiguities resulting from radio recombination line measurements can be resolved using the detection, or non-detection, of HIIregions in absorption below 100 MHz (Kassim et al. 1990). This is because foreground HIIregions would be much more prominent absorption features on low frequency SKA maps than distant ones.
-doubling, and rotational transitions of large molecules.
The physical conditions in this gas are conducive to narrow
lines that tend not to overlap, and are very useful probes
of turbulent motions of only a few km s-1. Designed with
these observations in mind, the resolution and sensitivity
of the SKA will open up the field, enabling large scale
sampling of important tracers as well as new extragalactic
comparisons with Galactic observations.
The molecules H2CO, OH, NH3, and the H2O maser,
have strong cm wave lines that have been studied extensively.
H2CO (6 cm) is seen in absorption against the 2.8K microwave
background, thus allowing unrestricted mapping. OH (18 cm)
is seen in both emission and in absorption in more diffuse
gas. With resolutions of a few arcsec, the SKA will be able
to trace the structure and dynamics of the diffuse
molecular gas (OH) as well as the denser gas (H2CO) in most
nearby galaxies, and in galaxies at modest redshift.
The OH molecule is highly important in astrochemistry. It
is key to primordial chemistry since it participates in
the very first chemical reactions that form molecular hydrogen.
NH3 is one of the most important interstellar molecules, and its
large suite of lines near
1.3 cm are powerful probes of the
conditions in dense (>104 cm-3) molecular gas (see Avery
1991), the sites of star formation. Since these lines are
strong, they could be observed in galaxies out to several
100 Mpc at resolution of 10 milli-arcseconds, enabling the
study of the density of protostellar sites at sub-parsec
resolution in a large number of galaxies.
There are several tens of other molecular species known in
the cm wavelength range, each with many lines. Most have
been detected by sensitive single dish observations, but
their applicability to a broad range of interstellar
physics has not been fully explored because the lines are
too weak, and single dish resolutions are insufficient to
allow full comparisons with other tracers of the
interstellar medium. A prominent example is CH, an
important constituent in chemical networks, which has weak
lines near 10 cm. The combined sensitivity and resolution
of the SKA will provide a major impetus to this field and
spark the application of weak line observations to many new
problems in ISM astrophysics. Brightness temperature
sensitivity is important for detecting and mapping weak
lines, and so the array configuration of the SKA will play
an important role. For these types of observations the
compact central core of the array (containing a large
fraction of the total collecting area) would be used. It
is likely that the array could be designed to include about
30% of the collecting area in baseline spacings less than
2000 m. The brightness temperature sensitivity for this
array would be about 10 mK in an observing time of 24
hours, and the resolution would be about 10'' at 10 cm,
allowing imaging of large molecular clouds in nearby
galaxies.
A number of molecules emit spectral lines of astrophysical
importance in the 20 - 50 GHz range, and could be imaged
with the SKA at 1.3 cm from galaxies at 
and greater.
These include the lower transitions of CS, SO and HC3N. CS
and HC3N have been detected in nearby galaxies at levels of
several tens of mK. At 1.3 cm, the compact core of the
array would have 10 mK sensitivity and resolution of 1''.
Detection of emission from these molecules at high redshift
would complement CO studies, and provide
important information on the physical conditions of
molecular gas and on chemical evolution at early epochs.
OH, H2O, and methanol masers are frequently found in the cool
molecular gas surrounding newly formed stars, and have been used
extensively to probe the physical conditions and kinematics of this gas.
Numerous observations have shown them to consist of clusters of
small maser components often associated with compact HIIregions. The
individual maser components have typical sizes of 1012 m
(corresponding to 0.01 arcsec at 1 kpc), and are arranged in clusters
typically 3.1014 - 1015 m (
a few arcsec) in
diameter, although there are significant differences between the three
species. Most of these masers appear to be situated in the warm gas
accreting on to a pre-main sequence massive star. The properties of OH
and
H2O masers are summarised by Elitzur (1992). A recent development is
that about 30% of methanol masers (at 6.7 and 12.2 GHz)
appear to be located in edge-on circumstellar disks (Norris et al., 1993,
1998; Stecklum et al, 1998) around high-mass stars. The existence of
these
disks will require revision of those theories which assert that such
disks
should be destroyed by the strong stellar winds from high-mass stars.
Interstellar masers tend to be so strong that present-day
radio-telescopes are quite adequate for studying those in our Galaxy.
However, the enormous sensitivity of the SKA opens up the
possibility of using these masers to probe extragalactic star
formation.
At present it is difficult to study star formation in the nuclei of
active and starburst galaxies because the high dust extinction (which can
be hundreds of magnitudes at optical wavelengths) prevent even
mid-infrared
observations from penetrating the dense shroud of dust. Whilst the
far-infrared observations of ISO can penetrate this, ISO
does not have sufficient angular resolution to measure the distribution
of
star formation on the parsec scale in the nuclei of these galaxies. Such
resolution is important if we are to understand, for example, the
potential
role of star formation in feeding the massive black hole (MBH) at the
centre.
At the distance of NGC253, individual methanol and H2O maser spots corresponding to our own normal galactic masers might have observed flux densities of 3 and 250 mJy respectively, while at Cen A they might have fluxes of 0.4 and 30 mJy. In both cases, individual maser spots would be detectable at the five-sigma level in a few minutes, and could be identified in the parsec-resolution images obtained from an 8 hour synthesis. SKA would therefore tell us about the structure and kinematics of star formation regions in nearby active and starburst galaxies at a level of detail approaching that which we have in our own galaxy. This would help enormously in solving questions such as the relationship between star formation and the AGN.
Supernovae and their remnants play a central part in the dynamics and evolution of the Galaxy. Supernovae inject massive amounts of energy into the interstellar medium (ISM), powering a large fraction of the turbulent motions seen there. The expansion of the supernova remnant (SNR) both illuminates pre-existing structures in the ISM and carves out new structures, transferring kinetic energy from the original supernova to the ISM. Supernovae act as recycling centers, taking material that would be otherwise trapped in stellar form and returning it to the ISM to form new stars. This recycled material is processed by the supernova explosion into iron or heavier elements, those which profoundly effect the energetics of the ISM and the next generation of stars which form out of it.
SNRs are central to other areas of astronomy also. SNRs are thought to be the sites of cosmic ray production, but the exact mechanism is ill-understood, and the connection between the two as-yet unproven. Supernovae associated with the death of massive stars are expected to leave behind stellar, as well as diffuse, remnants. These stellar remnants usually take the form of neutron stars, but some fraction are also expected to form black-holes, possibly the most enigmatic objects in the Universe. The interaction between SNRs and dense molecular clouds drives chemical reactions that cannot be duplicated on earth. As such, these interaction regions serve as invaluable laboratories, sampling chemistry which can only take place in extreme environments and situations.
Yet, despite the importance of supernovae and SNRs in our understanding of our Galaxy, our knowledge about the SNRs and their physics is far from complete. Exactly where are cosmic rays accelerated, and how? How do the supernova ejecta couple to the ISM? Where are the neutron stars (and black holes?) in SNRs? Where are the young SNRs? Where are the oldest? These are just some of the questions that new radio telescopes can help us answer.
Radio wavelength observations of SNRs provide an important complement to studies at other wavelengths. Optical observations of SNRs provide important information on thermal particles and abundances in the SNR through recombination line emission. Infrared emission traces the shocked dust within the SNR, as well as dust trapped within the hot interior of the SNR. X-ray emission arrives from the region of hottest gas in the SNR, and provides invaluable information on the temperature structure of the SNR through high-ionization state line emission. To this impressive arsenal is added radio wavelength observations of synchrotron emission, tracing the magnetic field and relativistic particle distribution within the SNR, thus providing important information about both the SNR shock and the ISM.
The SKA promises to revolutionize the way we look at SNRs in the radio regime and, as a result, our understanding of these important objects. The possibilities offered by the SKA are many, and their inter-relation complex. Instead of concentrating on the new observational capabilities offered by the SKA, we will illustrate the way that the SKA will be able to illuminate some of the important questions in SNR astronomy.
Studies of supernova rates in external galaxies suggest that we should expect roughly 2 per century in our own. Despite this, we are only aware of a handful of Galactic SNRs younger than about 1000 years. The lack of young Galactic SNRs can be partially explained in two ways. First, the lack of an obvious optical supernova over the last few centuries, if not simply a statistical fluke, suggests that they must have been heavily obscured, and thus probably very distant. Second, the emission from a SNR is a direct result of its interaction with the ISM; the lack of obvious emission from young SNRs could thus suggest they occurred in low density regions of the ISM and thus are faint. Combined, these explanations suggest that future searches for young SNRs require high sensitivity (to detect the faint objects) and high resolution (to resolve the distant, and thus angularly small objects). These requirements are exactly where the SKA excels.
The explosion of supernovae is a dynamic and exciting event and study of young supernova remnants can often tell us valuable information about the progenitor star and its circumstellar medium. Understanding of the final merging of the ejected material with its surroundings and the eventual dissolution of the energy throughout the interstellar medium requires study of the old SNRs. These old SNRs are generally very large and faint so they are difficult to study with current instruments; observations suggest that objects older than about 50,000 years are not currently radio detectable. The SKA with its high sensitivity and its capability to image large fields of view will be an ideal instrument for studying older objects.
Most old SNRs have 80-90% of their total emission in a smooth component, which is usually missed with current aperture synthesis telescopes, and the remainder is in thin, often unresolved filaments. Both components need to be imaged to fully understand how the shocks decay as the remnant's expansion slows down and the emitting material diffuses into the galactic background. Many large remnants, such as the nearby Cygnus Loop, cover several degrees on the sky and show a variety of features in different regions. In some places clumps of material are being overrun and there are characteristic changes in the position of the optical line radiation as the shock progresses into the clump. In other places the shock appears to be well in front of the expanding material and most of the emission appears to be from dense cool regions which are compressed under pressure equilibrium with the warmer interclump gas. Does the synchrotron radiation have different spectral signatures in these different regions which can be used to help determine their physical characteristics?
To date, resolution of individual filaments has been limited by sensitivity so that the thinest radio filaments detected are limited to over 0.01 pc whereas the HST can approach a resolution of 0.0005 pc for the closest remnants. It is important to match the optical resolution at radio wavelengths in order to see how the relativistic particles track the thermal ones and how the energy in different components changes with the size and structure of different components. For example, does the spectrum change over a shock as it progresses into a clump? The SKA will have the sensitivity to allow use of its full resolution for answering questions like these.
The synchrotron emission detected by radio telescopes is powered by the SNR blast-wave. The connection between the two is very unclear however. Where are particles accelerated in the shock? How high of energies can they be accelerated to? What role does the magnetic field play? How important is turbulence?
The SKA promises to help us understand these questions by allowing us unprecedented details of the spectral index variations across SNRs. The radio spectral index provides information on the particle acceleration mechanism and the underlying seed particle population. The variations across a remnant will allow us to trace how these change with ISM density, shock velocity, magnetic field strength, and past history of the region.
One of the most exciting ways the SKA will allow us to study these questions is by simply extending the useful wavelength range we can study. By being able to observe at low frequencies the uncertainties in our determinations of the spectral index will be reduced by as much as a factor of two. The high resolution of the SKA will allow us to measure the spectral index of small regions, making it possible to measure these quantities on a scale similar e.g. to the Hubble space telescope. The high sensitivity and resolution will combine to allow us to see how these values change with time, a technique that has yielded amazing insights into the few objects, such as Cas A, the youngest Galactic SNR known, we are currently able to study in this way.
Recent centimeter wavelength studies of Cas A (Anderson & Rudnik 1996) have suggested that the spectrum of the emitting regions may be determined not only by current acceleration processes but also by the history of particle acceleration in the environment through which the particles have moved. Observations with the 74 and 330 MHz VLA systems have recently confirmed this surprising conclusion. Perhaps even more exciting has been the unique absorption measurements provided by the 74 MHz observations which reveal evidence for unshocked ejecta within Cas A, as predicted by theory (Kassim et al. 1995). This measurement raises the prospect that many young supernova remnants may harbor a cool thermal core, which the low frequency measurements can uniquely detect. The detection of thermal absorption from within the first two SNRs observed with the new VLA system suggests that these effects may be common at low frequencies.
Useful information can be gained simply from the integrated low frequency spectrum of SNRs. Predictions from Fermi acceleration theory imply concave integrated spectra such as has been claimed in the case of the Tycho and Kepler's SNRs (Reynolds & Ellison 1992). But large error bars on the lowest frequency measurements hamper these conclusions and restrict their extension to many more sources. More accurate, higher resolution measurements can extend such studies to many more objects and confirm whether the line-of-sight thermal absorption is indeed related to envelopes of normal HIIregions as is currently speculated.
The low frequency abilities of the SKA will provide unique tests to theory. Fermi acceleration theory, for example, predicts a concave integrated spectrum at low frequencies. Current observations are not sensitive enough to reliably test these theories, but the SKA will be.
Perhaps the most basic information needed for understanding a SNR is the dynamics of the remnant. Exactly how fast is the remnant expanding? How does this vary across the remnant? How does it correlate with ISM density? How does it correlate with spectral index? What dynamical stage is the SNR itself in? What is the distance to the SNR? We will be able to address these questions with the SKA.
The high resolution and sensitivity of the SKA will make it possible to measure proper motions for SNRs out to the other side of the Galaxy. As an example, a young SNR expanding at 104 km/s at a distance of 20 kpc would grow in diameter by 0.21" over one year, a growth which would be easily measurable using the SKA at its full resolution. A SNR expanding at 200 km/s at a distance of 4 kpc would increase its diameter by the same amount. These examples suggest that the SKA will provide unprecedented information on the expansion of SNRs.
The proper motion is, of course, dependent on both the expansion velocity of the remnant, and its distance. As a result, proper motion alone cannot uniquely constrain either of these quantities. Combining the proper motion with other information including radial velocities from optical spectroscopy will, however, shed light on these quantities. The sensitivity of the SKA will allow unprecedented measurements of high-velocity, post-shock, HI. The velocity of this gas will provide a lower limit to the present shock velocity, and thus - in combination with the proper motion information - provide a lower limit on the distance to the remnant. Conversely, if other information is available on the distance to the remnant e.g. associated pulsar dispersion measure, HI absorption, interaction with molecular material, the shock velocity will be able to be determined.
The high spatial resolution of the SKA will allow the expansion rates to be mapped across the remnant. This will allow the study of e.g. SNR blow-outs into lower density ISM, the expansion of fragments vs the blast-wave as a whole, and the interaction of the shock with high density, molecular material. The interaction with molecular material can then be used to help constrain models of shock chemistry and dynamics.
The near-instantaneous injection of energy into the ISM by a supernova has profound effects on the surrounding medium. The material immediately around the explosion is swept into a fast-moving shell of dense material. Small dense clouds are destroyed, and shocks are driven into larger clouds, initiating chemical reactions impossible to find elsewhere. Left behind are low density bubbles of million-degree gas. The shells of material swept-up by the expanding blast-wave eventually cool and compress further to create shells tens of parsecs in diameter. These bubbles and shells can merge with similar bubbles and shells created by stellar winds and other nearby supernovae to form superbubbles hundreds of parsecs in diameter.
The interaction of the SNR with the ISM can be seen in a number of ways. High-velocity HI, from recombined material behind the shock, has been used to identify shocks interacting with high-density material. Detection of this material gives an insight to the shock velocity. This material is very faint, however, requiring high surface-brightness sensitivity to detect; with its large collecting area, the SKA should be able to detect high-velocity, post-shock, HI in a large number of SNRs.
The 1720 MHz OH maser line has recently been shown to be a powerful indicator of interactions between SNRs and dense molecular material. Follow-up observations of sites of OH maser emission have begun to reveal new sites of shock-induced interstellar chemistry, promising to revolutionize our understanding of this subject. Maser emission has also been used to measure the magnetic fields within the interaction zone, casting light upon this ill-understood topic. Present observations can detect only strong maser emission, but the SKA - with its increased sensitivity - promises to detect much weaker emission, increasing our ability to sample the interaction between SNRs and their surroundings.
The stellar remnants created in core-collapse supernova, in addition to being fascinating in their own right, can provide invaluable information about the associated diffuse supernova. Ages of SNRs, for example, are notoriously hard to come by unless the object is the result of an historical explosion. If an associated pulsar is found, its spin-down age can be used to estimate the age of the SNR. Additionally, although somewhat unreliable, the pulsars dispersion measure distance can be used to estimate the distance to the remnant. As a result, discovering pulsars associated with SNRs is of considerable interest. As noted elsewhere in these proceedings, the SKA will excel at finding new pulsars, some fraction of which will be associated with SNRs.
Even without the detection of pulses from the pulsar, however, it is possible to infer the existence of a neutron star within a remnant. The synchrotron nebulae associated with Composite and Filled-Center SNRs are expected to harbor central rotating neutron stars, even though they may not be beamed towards us, and thus the detection of a synchrotron nebula within a SNR is enough to deduce the presence of a neutron star (NS). These pulsar powered nebulae can reveal their presence as hard X-ray sources, the spectra of which can be modeled to include a hydrogen column density from which a distance to the NS, and thus SNR, can be estimated. The sensitivity and resolution of the SKA will combine to allow researchers to detect more and fainter synchrotron nebulae, in currently identified as well as newly discovered SNRs.
The interaction of the pulsar/NS with the SNR can also be illuminating. The Crab pulsar, for example, is associated with nearby, slightly elongated, features known as ``wisps.'' These wisps are thought to be the termination shocks of the pulsars free-flowing relativistic wind; the distance of this shock from the pulsar depends on both the energy of the wind and the physical conditions within the remnant, thus providing a probe of both. A similar wisp is seen adjacent to a hard X-ray point source in the Filled-Center SNR 3C58; this suggests that these wisps may be common within SNRs with pulsars. The resolution and sensitivity of the SKA will make it possible to detect similar wisps in more distant SNRs, and thus teach us more about the relationship between pulsars and SNRs.
Finally, the SKA may be able to finally put to rest one of the most enduring problems in our understanding of pulsar-powered SNRs: the lack of a limb-brightened shell associated with the Crab Nebula and similar remnants. Despite being the first nebula to be conclusively associated with a historic supernova explosion, the Crab Nebula is among a handful of remnants which do not have the signature limb-brightened shell of a SNR. While expectations abound that the visible nebula is surrounded by an, as yet, undetected shell, speculation will run rampant until either a shell is detected, or sufficiently strong limits are placed on any emission from a shell. The high surface-brightness sensitivity of the SKA will be able to shed considerable light on this question and will, no doubt, be among the most eagerly-awaited first results from the SKA. The intrinsic surface-brightness sensitivity will be aided by the low-frequency abilities of the SKA, pushing sensitivity limits on a non-thermal shell even further.
Supernova remnants, extended nonthermal emitting sources which are the principal source of energy input into the ISM, are natural targets for study with the SKA. Moreover, their often large angular size is well matched to the SKA large fields of view. High resolution, multi-frequency images will serve to anchor spectral index studies of SNRs whose spatially resolved continuum spectra uniquely constrain the energy distributions of relativistic electrons. The key is to relate measured source spectral variations to dynamical structure, since models of particle acceleration in SNRs, either by shocks or by second-order Fermi (stochastic) acceleration in interior turbulence, predict structure in the particle distributions. Measured variations must be related to acceleration processes or the injection spectrum of the seed particles. Variations in older SNRs can also be related to compression of cosmic ray gas and interstellar magnetic fields. Previous studies have had far too poor angular resolution at the lowest frequencies to explore such issues in detail, if at all.
Sensitive low frequency observations should lead to the discovery of
older, low surface brightness SNRs which are known to be missing from
catalogs due to severe selection effects (Green 1991). Discovery of
such older SNRs, at the last stage of evolution before blending into
the ISM, are potentially of great importance in discovering new
pulsar-SNR associations and drawing links to unidentified 
-ray
sources. Presently only the youngest pulsars can be associated with
SNRs since remnants older than about 1000 yr have surface brightnesses
too low for detection by current instruments.
The sensitivity and angular resolution of the SKA would be sufficient to extend these SNR studies to nearby external galaxies, thus greatly extending the available data base. Recent statistical studies (e.g., birthrates, distribution, energetics, etc.) of complete, co-distant samples of SNRs in nearby galaxies are proving extremely useful for exploring problems in stellar evolution, ISM structure, and for increasing samples sizes of poorly understood SNR-subclasses (Wills et al. 1997; Duric et al. 1995; Jones et al. 1998). Sensitive, high resolution low frequency observations are required to compliment existing higher frequency data and to anchor the derived spectra and search for absorption effects. VLA and Westerbork observations at 330 MHz and MERLIN observations at 151 MHz have been utilized successfully for these purposes, but lower frequency observations would be even more useful.
The origin of cosmic rays has been a challenge ever since their discovery. The current paradigm holds that high energy phenomena, related to supernovae and/or active galactic nuclei (AGNs), are involved. However, no direct connection between the particles that we observe locally and any identified cosmic sources has been made, leaving their origin uncertain.
A key barrier is observational. Because they are charged and deflected
by Galactic, interplanetary, and geophysical fields, it is impossible
to deduce the origin and complete spectrum of the cosmic ray particles
from direct measurements, with the exception of the very highest
energy particles. These appear to be extragalactic, but are so few in
number that good source statistics have not been obtained.
Fortunately, cosmic ray particles generate radiation at both the
highest (-rays) and lowest (radio) frequencies from their
interaction with interstellar matter and magnetic fields. Hence
interpretation of these radiation measurements may hold the key to
unlocking the origin problem. However, while high energy capabilities
have advanced quickly and have produced important results such as the
ASCA X-ray evidence for cosmic ray production in the shell of SN1062,
observational capabilities at the lowest frequencies remain primitive.
A new instrument with fundamentally improved characteristics in
angular resolution and sensitivity can make a major impact which, when
interpreted within the context of modern high energy observations, may
hold the answer to this puzzle.
The low- and high-energy observations are related because the
distributed -ray and the low frequency radio emission are both
generated by cosmic rays (Longair 1990; Webber 1990). The -rays
and radio waves are coupled through the distribution of interstellar
hydrogen and magnetic fields, respectively, and their nonthermal
character reflects the energy signature of the poorly understood
cosmic ray ``source'' spectrum. The higher energy -rays (E >
100 MeV) result mainly from the pion decay that results from the
collision of high energy (E > 300 MeV) cosmic ray nuclei with
hydrogen. However lower energy -rays (E < 70 MeV) result
mainly from relativistic bremsstrahlung of cosmic ray electrons of
energies below 200 MeV in interstellar matter, and these particles
also generate synchrotron radio radiation below 100 MHz. Hence a
comparison between the low energy -ray and low frequency radio
spectra could, in principle, allow us to uniquely separate the matter
distribution from the magnetic field distribution, and to deduce the
distribution and primary energy spectrum of the cosmic ray particles.
The problem can be approached at the radio end in two ways. One can measure the distributed emission directly; however single-dish measurements have always had too poor angular resolution to properly deconvolve the true distributed emission from the myriad of discrete sources which pile up along the most interesting lines of sight, making comparisons with potential cosmic ray source distributions difficult. A more direct approach is to use an interferometer to resolve optically thick HIIregions against which the distributed synchrotron emissivity could be accurately determined (Kassim 1990). The power of this approach is in the availability of relatively well determined path lengths to the HIIregions, allowing us to derive the true three-dimensional space distribution of the cosmic-ray generated radiation field.
From highly sensitive low-frequency radio observations and comparison
with -ray
observations, a number of important studies would follow immediately:
the lifetimes of electrons in the
interstellar and intergalactic gas and the competition between escape
and energy loss; evidence of electron acceleration processes; the
ratio of high energy electrons to protons in the ISM and intergalactic
media (IGM) and in their sources; and, of course, the identification
of absorption and emission regions with the positions of known
objects, e.g., giant molecular clouds, SNRs, nearby normal galaxies
and AGNs. As a general tool, the method could prove invaluable for
studying both Galactic and extragalactic energetic source populations,
thereby enabling the extraction of the matter and magnetic field
distributions. Here the high and low energy radiation measurements
provide a delineation of the electron and nuclear components,
revealing unique information concerning the acceleration and transport
of the different species in many types of environments.
All Galactic and extragalactic radio sources are observed after their
radiation has propagated through the Galactic plasma. Variations in
the plasma density produce refractive index fluctuations, scaling as
,
which in turn scatter the radiation. The magnitude of
radio-wave scattering from the interstellar plasma is strongly
direction dependent, but the effects can remain significant even at
frequencies as high as 10 GHz. The density (refractive index)
microstructure responsible for interstellar scattering occurs on
scales of order 1 AU. The density fluctuations, in turn, are
thought to arise from velocity and/or magnetic field fluctuations. In
addition to their corrupting effects, interstellar propagation effects
are a powerful sub-parsec probe of the interstellar plasma, can
provide a tracer of energy input into the ISM, and may be linked to
cosmic ray propagation.
Low frequency observations of compact sources provide a powerful
diagnostic of propagation effects from the interstellar medium. The
scatter-broadened angular diameter of a compact nonthermal source
scales as 
,
while the resolution of a telescope and the
minimum apparent size constrained by synchrotron self absorption both
scale as .
Thus, interstellar scattering observations are
optimized with high-resolution, low-frequency observations.
Current studies of interstellar scattering focus on regions of intense scattering (e.g., Cygnus and the Galactic center), where the scattering effects can be detected at frequencies near 1 GHz. SKA observations would be able to probe the density and field microstructure in less intense scattering regions, such as those near the Sun.
Further areas of study include the search for supernova-generated
turbulence, the z-distribution of scattering material, and the
search for volume-limited scattering. Current shock acceleration
theories (Ellison et al. 1984), relevant to the origin of cosmic
rays, also suggest that upstream of a SNR should be an ideal site for
the generation of the density fluctuations responsible for interstellar
scattering. High frequency searches for the signatures of such
upstream turbulence have a mixed record. The
dependence
of interstellar scattering would allow much more stringent tests to be
applied.
Ionized gas is found several kiloparsecs above the Galactic plane (the
``Reynolds layer''). The scale height of the density fluctuations
responsible for interstellar scattering is about 1 kpc, inferred from
observations of high-latitude pulsars (particularly those in globular
clusters) and low-frequency interplanetary scintillation measurements.
The agents presumed to be responsible for generating density
fluctuations--SNRs and HIIregions--have a much smaller scale
height (
kpc). The
SKA could probe to higher latitudes than existing instruments and
would provide additional information about the vertical distribution
of the scattering material and clues about any other agents
responsible for generating the density fluctuations. Particularly
valuable would be deviations from the smooth distribution of
scattering material predicted by current models (Taylor & Cordes 1993).
The density fluctuations responsible for interstellar scattering have a spatial spectrum. The largest scale on which these density fluctuations occur is about 1 pc near the Sun and may be of the order of 0.01 pc in regions of intense scattering; this scale is presumably related to the injection of energy into the ISM that produces the density fluctuations. At low frequencies the angular extent of the scattering region may be less than the nominal scatter-broadened angular diameter of a background source. If so, the shape of the scatter-broadened image may be affected by the fact that the scattering is occurring only in a limited volume. The orientation and distortion of the image would then provide information about the volume in which the scattering was occurring.
Radio recombination lines of H, He, C, and heavier elements offer the possibility of tracing temperature, kinematics, and ionization structure as well as abundances of heavy elements in ionized gas. Available from meter to mm wavelengths, strong radio lines are found in HIIregions, but narrow, weak lines are also found in the very diffuse, ionized gas that pervades the Warm Ionized Medium. The sensitivity/resolution regime of the SKA will allow the kinematic imaging of ionized gas in a new range of astrophysically interesting circumstances, such as, for example, kinematic studies of the impact of SNR shocks on the surrounding ionized gas or metallicity maps of nearby galaxies. The stronger lines associated with bright HIIregions could be imaged within starburst galaxies at higher redshifts.
As interstellar carbon recombines into very high Rydberg states (up to
n = 768), absorption lines below 150 MHz are generated. The carbon
atoms in these high states are very sensitive to the interstellar
environment and permit excellent measurements of density, temperature,
and ionization levels to be carried out (Payne et al. 1994). A number
of Galactic regions that produce these lines have been found,
including a large region that stretches 
along the Galactic
plane in the central region of the Galaxy (Erikson et al. 1995). The SKA
would provide the sensitivity to identify many more regions for such
diagnostic studies of the ISM and would allow these studies to be
extended to external galaxies.