One thing that has become outstandingly clear in the course of this review is that we have only obtained cursory answers to the questions that IFS surveys have set out to investigate. Certainly, we can see some definite discs and mergers at high-redshift and make plausible physical models; however, the detailed abundance of these kinematics classes remains uncertain. The IFS surveys have been pioneering, however like all pioneers they have set off in different directions, explored limited areas and different terrain. When they compare notes they find they have all done things somewhat differently, they all agree on some of the major landmarks but the systematic detail is subject to a lot of difficult inter-comparisons.
6.1. Outstanding questions
The outstanding questions remaining follow the themes of the section of this review. I will highlight some of the most important in my mind:
discs at high-redshift? As I have discussed clearly many of the objects observed at high-redshift in IFS surveys are rotating with velocity fields that rise and turnover to a flat portion in a manner similar to local discs. It is not at all clear what fraction are discs at what redshift, the range is 30-100% of star-forming galaxies and different surveys probe to different depths, sample different redshifts, and select different mass ranges. It is clear though that pure consideration of imaging surveys is not enough to establish the epoch at which discs arise in the Universe, as is often still done (e.g. Mortlock et al. 2013). The scatter in the Tully-Fisher relationship may be increasing at high-redshift, or it may not. Deeper IFS surveys are needed to probe the turnover in galaxy rotation curves at z > 1. We also need greater overlap between broad-band HST surveys and kinematic AO surveys (only a few papers each with only a few objects). Some objects may be too small to resolve as discs in natural seeing, and some may even be too small (< 1 kpc) to resolve with AO data. The discs are certainly morphologically different to local ones: at a minimum they have much higher star-formation rates, have a high-velocity dispersion, and are physically thick. I note Law et al. (2012b) found they may be even more different: axial ratios provide some evidence that some may be triaxial ellipsoidal systems. Similar results have been found by Chevance et al. (2012) for compact red galaxies. Nor is it clear whether the discs we see at high redshift are evolving in to the thick discs of today's spirals, or S0 galaxies, or massive ellipticals (via major mergers). These questions could perhaps be tested in the future by considering space density (van Dokkum et al. 2010) and clustering (Adelberger et al. 2005) of such objects as a means of tracing from high to low-redshift. Parent populations have been studied (Hayashi et al. 2007, Lin et al. 2012) but current IFS sub-samples are not well-characterised in a mass-complete sense.
What are 'Dispersion dominated galaxies'? The existence of star-forming non-rotating galaxies is hard to explain. They start to appear at masses > 1010 M⊙ for z > 1. High-star formation implies large amounts of gas which naturally settles in to a disc on dynamical time. Unresolved (even at AO resolution) very compact discs would be one possibility as argued by Newman et al. (2013). Law et al. (2012b) found that the morphology is not truly disc-like and suggested that may in fact indeed be transient structures, not in equilibrium, perhaps merger driven. If transient events are common and enhance star-formation, they may naturally populate UV-selected samples. More detailed comparisons of the morphological axial ratios vs stellar mass and specific star-formation rate would be interesting especially given the wealth of new near-IR structural data coming from HST in the CANDELS survey (Grogin et al. 2011, Koekemoer et al. 2011). The final answer to the question of the structure of these compact galaxies may depend on 30m class telescope AO resolution, though it is possible that the sub-resolution kinematics could be tested with spectroastrometry.
The nature and driver of dispersion? The large resolved velocity dispersion of high-redshift galaxies was an unexpected observation. It is intrinsic and is not a beam-smearing effect. What is measured is the ionised gas dispersion as revealed by emission lines. One naturally then asks is it coming from HII regions bound to a disc, in which case it reflects the gas disc dispersion, or from outflowing ionised gas? The consensus seems to be the former, and in fact outflows are seen to be separately observed in even broader line wings of width 300-1000 km s-1 (Genzel et al. 2011, Wisnioski et al. 2012). A key test of this was measuring the dispersion of the cold molecular gas (Tacconi et al. 2010, 2013, Swinbank et al. 2011, Hodge et al. 2012), this will be improved to sub-galactic scales by the resolution and sensitivity of new telescopes such as ALMA. The resolved stellar kinematics of the young disc also ought to match the gas, however this has to be measured from absorption lines which will likely require 30m class telescopes. If we interpret the dispersion as a turbulent gas disc then the energy source powering it is not known. The picture of a Q=1 marginally stable discs requires but does not specify this. Cosmic accretion, star-formation feedback, clump formation and stirring are all interesting candidates and progress will require a difficult quantitative estimate of these across large samples. Structure seems apparent in numerous dispersion maps of galaxies (see discussion in Section 5.1) but is never commented on. Is it real and if so what does is correlate with? This may provide additional physical insight.
The physics of clumps? We have seen a picture of the large clumps seen in high-redshift galaxies as Jeans mass objects embedded in galaxy discs. This explains the important observation of irregular morphology but smooth velocity fields without significant perturbations at the clump locations. Their masses, luminosities, sizes and velocity dispersion seem to scale with star-formation rates, however it is not clear if there are single scaling relations connecting them with galactic HII regions today or two sequences. Part of this is that size measurements are particularly difficult to define even locally, HII regions clump together in complexes in spiral arms and the apparent size depends on the spatial resolution and image depth (Rozas et al. 1996). More uniform and systematic approaches to comparing local and high-redshift galaxies (including quantitative artificial redshifting) are needed.
We do know that the clumps we see in high-redshift galaxies are not just a resolution effect, i.e. the morphology is fundamentally different from artificially redshifted local galaxies (Elmegreen et al. 2009b), however we do not know if these clumps will break up in to sub-clumps when viewed at even higher angular resolution. If they do, then which clump scale is important for scaling relations? Are the clumps (or sub-clumps) single bound structures with the velocity dispersion providing virial support? Do they also have rotational support (Ceverino et al. 2010)? How does metallicity, ionisation, stellar population age, and dust extinction compare for young clumps vs the surrounding disc and radius within a galaxy? These could all be uncovered by future IFS observations. One would presume that clumps also contain large amounts of cold molecular gas fuelling the star-formation. Do they contain super-Giant Molecular Clouds and what are their structure? One particularly clear and notable example of molecular clumps has been seen at z = 4.05 in a sub-mm galaxy (albeit one of the most luminous) by Hodge et al. (2012). Future observations from ALMA and other facilities will produce many more such observations of the general galaxy population extending to lower redshift. They are likely to test the picture that clumps form in regions of Q < 1 by measuring Q properly using direct gas surface density measurements. Finally, one must ask what is the fate of giant clumps, do they last a long time and gradually spiral in to the centre of a galaxy and form a bulge or are they quickly destroyed by intense feedback? Simulations support both short (Genel et al. 2012) and long lifetime (Ceverino et al. 2012) scenarios depending on assumptions, observations (Genzel et al. 2011, Wuyts et al. 2012, Guo et al. 2012) have yet to settle the question.
The Star Formation Law? We have seen that stars form in high-redshift galaxies in very different conditions than they do locally. The discs are more gas rich implying much greater pressure, the star-formation feedback is more intense especially within clumps and the dispersion of the disc is much greater. Given the star-formation history of the Universe (Hopkins & Beacom 2006) most stars formed under these conditions. One must therefore ask basic questions, for example is the star-formation law the same? Does a Kennicutt-Schmidt-like law apply or something different? The classical Kennicutt-Schmidt relationship simply relates projected surface densities of gas and star-formation via a power law. The thickness of high-redshift discs would imply quite different results from laws that depend on projected surface densities vs volumetric densities (Krumholz et al. 2012). There are many other proposed variations on this theme. For example, there may be 'thresholds' to star-formation (e.g. above some critical density, Lada et al. 2010, Heiderman et al. 2010). At high-redshift, it has been suggested that there are in fact two relations — a 'sequence of starbursts' and a 'sequence of discs' but which may be unified by introducing a dynamical time in to the formulation (Daddi et al. 2010b). Alternatively, it may simply reflect issues with CO conversion factors (Kennicutt & Evans 2012). Direct resolved tests of star-formation laws in high-redshift galaxies (see Freundlich et al. 2013) for a first step towards this) are critical and will improve with the advent of high-resolution ALMA data in the next few years. Another related topic is the Initial Mass Function (IMF) for star-formation. The possibility of IMF variations is important (see review of Bastian et al. 2010) and evidence for variations in galaxies has attracted considerable recent interest and some tantalising results (e.g. Hoversten & Glazebrook 2008, Meurer et al. 2009, van Dokkum & Conroy 2010, Cappellari et al. 2012). It seems plausible that the IMF could be different in high-redshift discs and/or in clumps (e.g. Narayanan & Davé 2012) and perhaps could be investigated by comparing colours and spectra as are done at low-redshift.
The Merger Rate? As we have seen there seems to be some tension between some IFS results and those of other techniques. In particular, at z ~ 0.5 deeper and higher resolution IFS observations are needed to determine if nearly half of all star-forming galaxies have major kinematic disturbances or whether this is just an artefact of low angular resolution or not being sensitive to galaxy outskirts. There is a clear tension with the estimates of the merger rate by close pairs, and at these redshifts this technique is quite sensitive. At high-redshift (z > 2) some estimates for Lyman Break Galaxies put the merger fraction close to 100%, this may be compatible with the existence of massive discs and the lower rates found in IFS surveys as the UV-selected objects are at the lower mass end. Can we define a consistent merger rate across the various merger phases from close approach through to coalescence as a function of stellar mass, merger ratio, and redshift? This would be a powerful constraint on galaxy formation models. Deeper and more numerous IFS observations will help, but in my opinion it is equally important to find new techniques to extract time scales and mass ratios from IFS maps (which would obviously have to be calibrated on simulations).
In a sense, every galaxy at high-redshift is being subject to a continual accretion of matter of some degree of lumpiness, it is a question of degree and how often. Every disc at high-redshift has probably had some sort of kinematic disturbance in it's recent past, likewise every major merger remnant is probably busily regrowing a disc from new infall. Being able to quantify these effects continuously would be more helpful in comparison with models than the current somewhat artificial distinction between 'disc' and 'merger' which is predicated on the modern Universe where mergers are infrequent.
6.2. New surveys
Clearly, one next and critical step at high-redshift is large scale IFS surveys of thousands of objects with uniform, homogenous selection functions. Current surveys suffer from diversity — selection is done using UV flux, near-IR flux, sub-mm flux, emission line flux, or some difficult to evaluate combination of all these. Even then selection from the parent sample is not necessarily homogenous. Of course the limiting factor in survey size to date has been the necessity to observe one object at a time with IFS (with the notable exception of the IMAGES survey using the optical FLAMES-GIRAFFE instrument). The instrument that is most likely to transform this is KMOS, recently commissioned at the end of 2012, on the VLT (Sharples et al. 2012) which will work in the near-infrared and offer a 24-IFU multiplex (in natural seeing). Multiplexed observations facilitate an improvement in numbers of course, but they also permit an improvement in depth as well as the telescope time is less expensive per object. A number of groups are proposing IFS surveys of this scale with KMOS. It is remains desirable to select galaxies for IFS from spectroscopic redshift surveys with prior information of the strength of the emission lines and their proximity to night sky lines. New redshift surveys using slitless spectroscopy in space will allow this (Brammer et al. 2012); as will new near-IR redshift surveys using new multi-slit instruments such as MOSFIRE on Keck (McLean et al. 2012). They will also provide well-defined environments for the IFS kinematic observations at high-redshift; a topic that so far has been completely unaddressed. If turbulent discs are fuelled by ongoing cosmic accretion, one might speculate on seeing strong environmental trends in their incidence and star-formation rates.
With this prospect, I also think it is critical to see a move to a uniformity of application of kinematic techniques; a good example is disc fitting where every group has developed their own bespoke code. Large surveys need to develop a best practice with common codes and whole papers need to be devoted to describing and evaluating codes with full treatments of errors, fit qualities, and degeneracies. This is the same transformation as the photometric redshift community has gone through in the last decade as deep high-redshift imaging surveys have become industrialised.
Another analogy is with the first deep imaging and spectroscopic surveys done with CCDs in the 1980's and 1990's, it was immediately apparent that the local comparison surveys done with photography were inadequate and this spawned the Sloan Digital Sky Survey (York et al. 2000). We seem to be in a similar position today with IFS surveys, they have been fruitful at handling objects with the complex morphological structures common at high-redshift, however the majority of the local comparison to date is with traditional work done with long-slit spectroscopy. This is changing rapidly as hundreds of local galaxies have been observed with wide field IFS instruments, notably the CALIFA (Sánchez et al. 2012), ATLAS3D (Cappellari et al. 2011a) and DISKMASS surveys (Bershady et al. 2010). Low-redshift surveys are beginning with multi-IFU instruments; the MANGA survey in the U.S. and the SAMI survey in Australia (Croom et al. 2012) will both observe several thousand galaxies in the next few years with well-defined selection and environments. They will provide a 'kinematic SDSS' and allow the statistical comparison of rotation, velocity dispersion, and angular momentum vs galaxy properties across a range of environments from the field to rich clusters allowing fundamental tests of galaxy formation models. I predict we will always move from simple scaling relations such as some measure of mass vs rotation towards distribution functions, for example the space density vs mass and angular momentum compared to theoretical models. These local surveys will also be extremely important for comparison with high-redshift; in particular the application of uniform techniques and the provision of large samples for artificial redshifting tests. This well-defined approach is necessary to settle the question of the evolution in the Tully-Fisher relationship — for example it is critical to test for selection biases to uncover the small amounts of evolution if any. Particularly important is these will provide high-quality baseline samples of galaxy mergers where kinematic features as well as low surface brightness photometric features (such as tidal tails) are available, confirming the merger nature but also providing approximate mass ratio estimates by comparison with simulations. We will also likely see an increasing number of other rare objects discovered that are similar to high-redshift galaxies (see Section 3.9) and whose close proximity will facilitate detailed astrophysical observation, in particular multi-wavelength observations to measure gas content and it's role in shaping galaxy kinematics.
Future AO surveys will also be critical. It has been surprising how much progress has been made using natural seeing surveys given how under-sampled the galaxies are are. AO surveys can deliver the kpc resolution required to resolve detailed internal structure and to make fundamental kinematic classifications of compact galaxies. Detailed study of individual galaxies will remain an important complement to the large surveys of thousands of galaxies with lower resolution. The main difficulty is that AO surveys remain small and it is difficult to see how substantial progress will be made in increasing sample size in the near-future given AO systems generally correct a small field-of view, hence no multiplexing of targets. Another difficulty of the current situation is the lack of significant samples which have had AO and non-AO observations of the same galaxies for comparison. Even groups who have done AO and non-AO observations have not done so for the same objects (a notable exception being (Newman et al. 2013) however only limited comparisons have so far been made). Part of the reason for the limited size of AO overlap samples is the requirement for bright guide stars — even with laser guide star AO it is currently necessary to have a R ≲ 17 mag tip-tilt correction star and this has severely limited sample selection to only 10-20% of possible targets. The other issue is of course sensitivity — at higher spatial resolution one has less photons per spaxel but also light is lost in the AO optical system and through the imperfect correction (i.e Strehl ratios well less than unity). Thus, more compact sources or those with highly clumped high surface-brightness emission tend to be favoured and AO surveys have only had moderate completeness rates except when very long integration times have been attempted. Yet another restriction is the redshift coverage — strong emission lines need to be used and AO works best at the redder near-IR wavelengths. We are currently subject to an 'AO redshift desert' at 0.3 < z < 1.2 where we can not attain kpc resolution. The reddest strong emission line is Hα which only achieves good Strehl in the H-band for z > 1.2. The next reddest strong star-formation line is Pa α but that redshifts in to the thermal infrared for z > 0.3.
Many of these issues are gradually being improved. Next generation AO systems will deliver higher throughput and higher Strehl at shorter wavelengths enabling AO observations of z < 1 galaxies. Signal:noise is also improved by new near-IR detectors with lower readout noise (which is an issue due to the high spectral resolution of kinematic observations). Guide star availability is being improved through more efficient wavefront sensors, near-IR wavefront sensors, which helps because so many faint stars are red M-stars (Max et al. 2008), the development of compromise 'no tip tilt' laser AO modes (Davies et al. 2008) and the development of 'Adaptive Optics Deep Fields' with low galactic extinction and high stellar density (Damjanov et al. 2011). Multiple object integral field AO observations (denoted 'MOAO') may also become possible due to the development of compact deployable wavefront sensors (Andersen et al. 2006b) allowing greater number of objects and longer integration times. AO work will extend down in to the optical as technology improves, for example the next generation MUSE instrument on VLT (Arsenault et al. 2008) will offer a diffraction limited visible imaging mode with a 7.5 arcsec IFU (as well as a contiguous 1 arcmin wide field mode). Finally, the advent of 20-40m class telescopes in the 2020's will increase both AO resolution (from the diffraction limit) and light gathering power. Ultimately in my view large and complete AO IFS surveys will have greater impact on our physical understanding, but will take longer to arrive, than large seeing-limited surveys.
Even AO surveys can be under-sampled when some galaxy sizes approach a kpc at high-redshift (van Dokkum et al. 2008, Damjanov et al. 2009, Wuyts et al. 2011). The future prospects are also very bright for taking advantage of the extra spatial resolution boost from gravitational strong-lensing which coupled with AO has allowed us to probe sub-kpc scales (Stark et al. 2008, Jones et al. 2010, Livermore et al. 2012). New sky surveys such as the Dark Energy Survey (Flaugher 2005), the Hyper Suprime-Cam survey (Takada 2010), and the Large Synoptic Sky Telescope (Tyson 2002) survey will produce thousands to tens of thousands of new strong-lens candidates allowing a greater diversity of objects to be studied and statistics to be assembled. As such targets only have a sky density of order one per deg2 they do not suffer a relative disadvantage from the single-object nature of AO and there ought to be ample with suitable tip-tilt stars.
I predict the most important developments in the immediate future (the next five years) will not be at optical wavelengths. The Atacama Large Millimeter / sub-millimeter Array (ALMA) (Hills & Beasley 2008) is being commissioned in Chile and is being officially inaugurated this year and is likely to dominate the near-future of high-redshift galaxy kinematics. Why do I make this statement? Today, high-redshift is dominated by optical and near-IR observations which are mainly sensitive to stars and hot ionised gas (e.g. from star-formation or AGN). However, it is important to consider 'the fuel as well as the fire'. We have seen from existing sub-mm observations that high-redshift galaxies are rich in molecular gas (Daddi et al. 2010b, Tacconi et al. 2010). Current sub-mm telescopes barely resolve high-redshift galaxies with their beams of 0.5-1 arcsec and require many hours of integration per galaxy. However, integration time performance of radio telescopes scales much faster with increased area (∝ A2) than do background-limited optical telescopes (∝ A). ALMA will have three times more collecting area and baselines up to 16 km and hence will improve resolution and integration times by factors of ten. In the northern hemisphere, upgrades to the Plateau de Bure Interferometer (the 'NOEMA' project) will double the number of dishes (increasing the collecting area to 40% of full ALMA) and maximum baselines (allowing sub-arcsec resolution) by 2018. Upgrades to lower frequency radio interferometers may enable such studies to be extended to even higher redshifts. These new facilities will enable kpc-resolution morphology and kinematics of molecular gas and dust in normal star-forming galaxies to be routinely made. The 'turbulent clumpy disc model' predicts galaxies to be gas rich and thick. Will we see thick cold molecular gas discs co-rotating and aligned with the young stars seen by the near-IR IFS observations? Will we see super-giant molecular clouds associated with the bright giant star-forming regions see in the UV? I predict we will! It should be noted though that the observations are likely to be even more time-consuming than optical/near-IR. Even with the full ALMA of 50 dishes I calculate that 0.3 arcsec/50 km s-1 resolution CO(3-2) observations of a z = 2.0 galaxy with 1011 M⊙ of molecular hydrogen would take 20 h. 22 On the other hand the ~ one arcmin field-of-view and wide bandwidth of ALMA could allow multiple targets at similar redshifts to be observed simultaneously, somewhat offsetting this.
Another extremely important question for these high-resolution sub-mm facilities is the nature of the star-formation law relating gas density to star-formation rate, a critical theoretical ingredient of numerical galaxy formation simulations (this is often referred to as the 'sub-grid physics'). Around 80% of the stars in the Universe formed at z > 1 but we have seen throughout this review that galaxies in the the high-redshift Universe are physically very different from today's galaxies. Will the star-formation law be the same as in today's galaxies or quite different? Future facilities will bring a highly superior set of data to bear on this important problem and I will predict some surprises! Finally one interesting new prediction that could perhaps be tested by ALMA is the possible existence of dark turbulent discs (Elmegreen & Burkert 2010). The prediction is that turbulence in gas discs starts initially in a dark accretion-driven phase lasting for ~ 180 Myr before star-formation turns on and renders the galaxy optically visible. The gas would be cold and molecular — the actual visibility of such objects to ALMA has not yet been calculated, but would make for an interesting paper.
6.3. Final words
I am fortunate in the timing of this review as I sense that in 2013 we are now at the end of the first major phase of high-redshift IFS kinematic studies which started around 2005. My impression of the topic is that in the next few years, we are going to see a phase change in the field and an avalanche of new data from large surveys with instruments such as KMOS and the first sub-mm wavelength kinematic studies at high angular resolution. Large surveys with consistent selection will allow us to firmly address the statistical questions about the incidence of kinematic structures that have been identified at high-redshift and longer wavelength observations will allow us to view the cold molecular gas, both before and after forming stars, directly. The combination of improved AO instruments and sub-mm telescopes will allow us to test the detailed physics of internal star-formation and probe galactic structure at high-redshift. I will look forward to seeing some of the outstanding physical questions raised by the first generation of surveys answered.
I would like to thank Bridget Glazebrook for her love and support in the long weekends required to write this review and the various little Glazebrooks for not creating too much disruption. I would also like to thank Bryan Gaensler for inviting me to write the very first Dawes review and continual reminders about my progress! Special thanks for providing useful useful references along the way go to Jeff Cooke, Andy Green, Luc Simard and Emily Wisnioski. I would like to thank Roberto Abraham for a careful and critical review of the draft manuscript. Thanks also go to Reinhard Genzel, Richard Ellis, Sarah Miller, Susan Kassin, Joss Hawthorn, Karín Menéndez-Delmestre, Enrica Bellocchi, Tucker Jones and Francois Hammer for providing useful comments and suggestions on the submitted manuscript and I have tried to reflect most of them to the best of my ability. I would like to specially thank Cara Faulkner of Ivanhoe Girls' Grammar School for her valuable assistance in correcting the final arXiv version of this review. The final responsibility and/or blame for the content and opinions of this review remain with me alone. Finally I would like to extend great thanks to the referee, Natascha Förster Schreiber, for many useful comments and suggestions which have greatly added to this review.
22 I use equation 1 of Tacconi et al. (2013), which represented normal z ~ 2 star-forming galaxies, to relate H2 masses to total CO fluxes and the ALMA Sensitivity Calculator at http://almascience.eso.org/proposing/sensitivity-calculator Back.