The study of dusty star-forming galaxies (DSFGs) at high redshifts has passed through major milestones over the last decade. At the time of the Blain et al. (2002) review, the principle DSFGs were identified at 850 µm as submillimeter galaxies (SMGs) with flux densities in excess of a few mJy scattered over a few square degrees. The rate of discovery of SMGs with the initial Scuba instrument on JCMT was at the level of one per 10 hours of observations on the sky. Since Scuba, new instruments on single-dish ground-based experiments, especially Laboca, AzTEC, Scuba-2, and SPT have expanded the submm surveys with an order of magnitude increase in the discovery rate. Moving from single-dish observations, interferometers such as VLA, SMA, CARMA and IRAM/PdBI allowed detailed follow-up studies on the nature of these galaxies at high resolution, especially on the molecular gas content and distribution.
Over the last five years, another order of magnitude improvement in our ability to discover DSFGs at high redshifts came from the space-based observations with the Herschel Space Observatory. The Spire instrument could image a square degree on the sky down below the source confusion noise of 6 to 8 mJy at 250, 350 and 500 µm, simultaneously, in less than two hours. A square degree imaged with Spire generally contains about 1000 sources individually detected, while the source confusion itself contains significant information on the counts and spatial distribution of the fainter sources below the individual detection threshold. With over 1200 square degrees mapped and existing in the data archive, Herschel has now left a lasting legacy for follow-up observations over the coming decades with new facilities.
Among the most prominent new facilities which will see countless advances in this area over the next decade is the Atacama Large Millimeter Array (ALMA). At its full capability, ALMA will be able to detect the ionized gas emission as traced by the 158 µm [CII] line from a Milky Way-like galaxy at z ~ 3 in less than 24 hours of observations. Reaching these depths will be critical in understanding fundamental differences between luminous DSFGs and normal galaxies at high redshift.
Moving forward, during the remainder of this decade and early next decade, we expect significant advances in our understanding of DSFGs at high redshifts from planned facilities like CCAT, GLT (Greenland 12 m Telescope), JWST, SPICA, and thirty meter-class telescopes with adaptive optics over arcminute areas on the sky in infrared wavelengths. We end our review with a list of scientific questions and goals for future science programs relating to DSFGs. Over the coming years we hope to:
Resolve 100% of the cosmic infrared background at submm wavelengths into individual galaxies. For reference we note that deep images with Herschel-Spire have only resolved 10% of the cosmic infrared background at 350 and 500 µm to individual galaxies, while indirect techniques like stacking on known galaxy populations or using gravitational lensing and high-resolution instruments like Scuba-2 can account for ~ 80% of the background. It will become necessary to resolve the background directly in submm wavelength imaging data to address if the galaxies that make up the remainder of the background are very high redshift star-forming galaxies or a fainter galaxy population at low redshifts, and to what extent the background is comprised of those two populations.
Build large, complete, luminosity limited samples of DSFGs which do not suffer from the selection biases plaguing present-day samples, which are defined by their selection at single-submillimeter bolometer band wavelengths. This will enable a proper accounting for the integrated contribution of dust-enshrouded star formation to the cosmic star-formation rate density (SFRD) and relative importance of DSFGs relative to the much more numerous and well-studied optically-selected galaxy populations.
Find statistically significant samples of z > 4 DSFGs and SMGs to be able to measure the LFs at z = 4 to the epoch of reionization at z > 6 to establish the proportion of the early cosmic SFRD which is enshrouded by dust. This will shed light on dust obscuration and dust production mechanisms shortly after the epoch of reionization. Currently only a handful of SMGs are known at z > 5 and most are gravitationally lensed with the accuracy of lensing models limiting our ability to use them for a cosmological measurement of the SFRD. Separately, measurements of the escape fraction of the Ly-α photons fesc of z > 6 SMGs (like HFLS3) will be necessary to establish if such sources with vigorous star-formation, are an important contributor to the UV ionizing photon background responsible for reionization at the highest redshifts of the Universe. If the SMGs are an important contributor they could easily dominate the UV photon budget and alleviate the current need for a reionization model dominated by UV photons from faint, small galaxies.
Identify and follow-up lensed DSFGs in resolved detail. While lensed DSFGs cause some issues with interpretation, they are also useful in many other ways in the pre-30 m class telescope era. Through spatial enhancement provided by extreme lensing magnification events, it will become necessary to study the internal structure of L = 1010 - 1013 L⊙ sub-LIRGs to HyLIRGs at z ~ 2 - 4 to study how their internal physical processes, on the scale of several 10-100 pc, might differ or be similar to star-forming molecular clouds in nearby galaxies.
Re-calibrate star formation rate indicators for dusty galaxies. The existing calibrations related to SFR and luminosity are limited to near-by galaxies, a handful of calibrators, or subsamples of distinctly optically-selected galaxy samples. In the future it will become necessary to re-calibrate extinction estimates at optical wavelengths, particularly relevant to studies during the epoch of reionization to establish the abundance of dust in the early Universe. In general, it is also crucial that we re-calibrate all of the star-formation indicators with new dust and gas information currently in hand at high redshifts as the existing relations may have significant evolutionary trends that are currently ignored.
Reveal the physical mechanisms driving the incredible luminosities in high-z DSFGs. While some evidence has pointed to their obvious merger-dominated histories, other work has argued strongly that the gas depletion timescales are long enough to be steady-state, fed through bombardment of gas from filaments in the early Universe. A clean computation of the fractional contribution of merger-driven activity towards cosmic star formation, particularly amongst DSFG populations, is needed. Likewise, we need to acquire an enhanced understanding of DSFGs' place in the context of the galaxy main sequence by disentangling uncertainties in stellar mass and SFR estimates. Future observations that will more precisely determine these quantities will be extremely valuable.
Gain a better understanding of the interplay and coevolution of star-forming galaxies and their active galactic nuclei (AGN). Probing the formation and evolution supermassive black holes at galaxy centers in tandem with their host galaxy's star formation history is critical to understanding how relevant different suggested evolutionary trajectories are to galaxy formation and evolution. Both observational and theoretical work in this area should improve drastically in the coming years with statistically larger data samples becoming available and enhanced models which will shed light on AGN feedback.
Map out [CII] and CO(1-0) gas in all z > 6 galaxies and combine these observations with low-frequency 21-cm radio interferometers studying the epoch of reionization. Measuring the ionizing bubble sizes of star-forming galaxies and the growth of ionized bubble size during reionization as a function of the star-formation rate and gas mass will shed light on the physical processes responsible for reionization. These studies are could be done in the context of intensity mapping where galaxies are not individually resolved with either [CII] or CO(1-0) line, but instead could be pursued with statisical analysis like intensity power spectra and cross power spectral analysis.
Understand the physical origins of [CII] emission, when it is a good SFR tracer, and where the [CII]-IR luminosity deficit originates. Also, we should aim to understand how [CII] line intensity varies with the metallicity. Similarly, we should aim at contrasting high and low-excitation tracers in order to observationally probe coeval AGN and starburst tracers. There are still large uncertainties on our understanding of molecular and ionized gas processes at submm and mm wavelengths.
Develop a comprehensive theoretical/simulated model that accounts for various DSFG observables: the submm number counts, DSFG clustering and environments, and the redshift distribution of various DSFG populations, while not violating other cosmic constraints gathered from other observation data (e.g. luminosity functions and stellar mass functions).
This list is by no means exhaustive, but is representative of some of the major goals of this burgeoning field in the coming years. Many of the future developments will depend on specific capabilities of forthcoming observatories, but the underlying theme is very clear: while we have made significant advances over the last ten years, in our ability to find and understand DSFGs, we are far from fully understanding the physical processes that govern high redshift dust-obscured star-formation and the assembly and evolution of the first galaxies.
During the preparation of this manuscript, many colleagues and collaborators in the community contributed data to this work and/or provided helpful feedback on its content. We would like to thank the COSMOS collaboration for permitting the public use of their data, which was used to make Figure 10. We also thank Justin Spilker for sharing the composite millimeter spectrum of SPT DSFGs from his upcoming paper (plotted here in Figure 16), and Scott Chapman for sharing the composite rest-frame ultraviolet spectrum of 850 µm-selected SMGs from his upcoming paper (plotted here in Figure 26). We are extremely grateful to the many other members of the community that willingly shared their published data or simulation results for the purposes of plots in this review, including Andrew Baker, Manda Banerji, Carlton Baugh, Andrew Benson, Matt Bothwell, Chris Carilli, Anna Danielson, Romeel Davé, Tanio Diaz-Santos, Duncan Farrah, Fabio Fontanot, Hai Fu, Jian Fu, Javier Gracia-Carpio, Steve Hailey-Dunsheath, Chris Hayward, Jacqueline Hodge, Mark Krumholz, Cedric Lacey, Claudia Lagos, Jen Donovan Meyer, Gergo Popping, Dominik Riechers, Dimitra Rigopoulou, Karin Sandstrom, Kim Scott, Chelsea Sharon, Ikko Shimizu, Rachel Somerville, Mark Swinbank, and Fabian Walter. We also wish to thank the many colleagues who gave us permission to reprint figures from their previous published papers here, including Phil Hopkins (Figure 2), Sam Kim (Figure 9), Laura Mocanu (Figure 15), David Alexander (Figure 27), Karin Menéndez-Delmestre (Figure 28), Susannah Alaghband-Zadeh (Figure 29), Scott Chapman and Jeyhan Kartaltepe (Figure 30), Mark Swinbank (Figure 31), Jacqueline Hodge (Figure 32), Mattia Negrello (Figure 33), Shane Bussmann (Figure 34), Darren Dowell and Alex Conley (Figure 35), Yashar Hezaveh (Figure 36), Manuela Magliocchetti, Ryan Hickox and Christina Williams (Figure 38), Cameron Thacker (Figures 40 and 41), and Gil Holder (Figure 42).
Furthermore, we would like to thank the many expert members of our community who took time to read and offer comments on a preliminary draft of this review; the manuscript was significantly improved with their feedback: Susannah Alaghband-Zadeh, Andrew Benson, Matthieu Béthermin, R. Shane Bussmann, Alexander Conley, Edward Chapin, Romeel Davé, Aaron Evans, Neal Evans, Duncan Farrah, Chris Hayward, Jacqueline Hodge, Rob Ivison, Claudia Lagos, Dan Marrone, Alexandra Pope, David Sanders, Ian Smail, Joaquin Vieira, and Marco Viero. We would also like to thank our anonymous reviewer who provided many excellent and insightful suggestions for improving the manuscript. Also, we would like to acknowledge the contributions of many collaborators, especially, members of the Herschel/SPIRE Instrument Science Team, HerMES, H-ATLAS, and COSMOS.
In addition, we would like to thank the participants of the Aspen Center for Physics Summer 2013 Workshop “The Obscured Universe: Dust and Gas in Distant Starburst Galaxies” for spurring the discussions which motivated the writing of this review. We thank Marc Kamionkowski for inviting us to submit a review article on the dusty star-forming galaxies and for his help during the writing and editorial process. CMC would like to acknowledge generous support from a McCue Fellowship through the University of California, Irvine's Center for Cosmology and support from a Hubble Fellowship, grant HST-HF-51268.01-A from Space Telescope Science Institute for support during the preparation of this review. DN is supported by the NSF via grant AST-1009452. AC is supported by a combination of NSF AST-1313319 and NASA/JPL funding for US guaranteed time and open time programs with the Herschel Space Observatory.