Cosmic Star Formation History Research Articles

Main sequence of star forming galaxies beyond the Herschel confusion limit

Context. Deep far-infrared (FIR) cosmological surveys are known to be affected by source confusion, causing issues when examining the main sequence (MS) of star forming galaxies. In the past this has typically been partially tackled by the use of stacking. However, stacking only provides the average properties of the objects in the stack. Aims. This work aims to trace the MS over 0.2 ≤ z < 6.0 using the latest de-blended Herschel photometry, which reaches ≈10 times deeper than the 5σ confusion limit in SPIRE. This provides more reliable star formation rates (SFRs), especially for the fainter galaxies, and hence a more reliable MS. Methods. We built a pipeline that uses the spectral energy distribution (SED) modelling and fitting tool CIGALE to generate flux density priors in the Herschel SPIRE bands. These priors were then fed into the de-blending tool XID+ to extract flux densities from the SPIRE maps. In the final step, multi-wavelength data were combined with the extracted SPIRE flux densities to constrain SEDs and provide stellar mass (M⋆) and SFRs. These M⋆ and SFRs were then used to populate the SFR-M⋆ plane over 0.2 ≤ z < 6.0. Results. No significant evidence of a high-mass turn-over was found; the best fit is thus a simple two-parameter power law of the form log(SFR) = α[log(M⋆) − 10.5] + β. The normalisation of the power law increases with redshift, rapidly at z ≲ 1.8, from 0.58 ± 0.09 at z ≈ 0.37 to 1.31 ± 0.08 at z ≈ 1.8. The slope is also found to increase with redshift, perhaps with an excess around 1.8 ≤ z < 2.9. Conclusions. The increasing slope indicates that galaxies become more self-similar as redshift increases. This implies that the specific SFR of high-mass galaxies increases with redshift, from 0.2 to 6.0, becoming closer to that of low-mass galaxies. The excess in the slope at 1.8 ≤ z < 2.9, if present, coincides with the peak of the cosmic star formation history.

Fast Radio Bursts’ Recipes for the Distributions of Dispersion Measures, Flux Densities, and Fluences

Abstract We investigate how the statistical properties of dispersion measure (DM) and apparent flux density/fluence of (nonrepeating) fast radio bursts (FRBs) are determined by unknown cosmic rate density history [ρ FRB(z)] and luminosity function (LF) of the transient events. We predict the distributions of DMs, flux densities, and fluences of FRBs taking account of the variation of the receiver efficiency within its beam, using analytical models of ρ FRB(z) and LF. Comparing the predictions with the observations, we show that the cumulative distribution of apparent fluences suggests that FRBs originate at cosmological distances and ρ FRB increases with redshift resembling the cosmic star formation history (CSFH). We also show that an LF model with a bright-end cutoff at log10 L ν (erg s−1 Hz−1) ∼ 34 are favored to reproduce the observed DM distribution if ρ FRB(z) ∝ CSFH, although the statistical significance of the constraints obtained with the current size of the observed sample is not high. Finally, we find that the correlation between DM and flux density of FRBs is potentially a powerful tool to distinguish whether FRBs are at cosmological distances or in the local universe more robustly with future observations.

GAMA/G10-COSMOS/3D-HST: the 0 < z < 5 cosmic star formation history, stellar-mass, and dust-mass densities

We use the energy-balance code MAGPHYS to determine stellar and dust masses, and dust corrected star-formation rates for over 200,000 GAMA galaxies, 170,000 G10-COSMOS galaxies and 200,000 3D-HST galaxies. Our values agree well with previously reported measurements and constitute a representative and homogeneous dataset spanning a broad range in stellar mass (10^8---10^12 Msol), dust mass (10^6---10^9 Msol), and star-formation rates (0.01---100 Msol per yr), and over a broad redshift range (0.0 < z < 5.0). We combine these data to measure the cosmic star-formation history (CSFH), the stellar-mass density (SMD), and the dust-mass density (DMD) over a 12 Gyr timeline. The data mostly agree with previous estimates, where they exist, and provide a quasi-homogeneous dataset using consistent mass and star-formation estimators with consistent underlying assumptions over the full time range. As a consequence our formal errors are significantly reduced when compared to the historic literature. Integrating our cosmic star-formation history we precisely reproduce the stellar-mass density with an ISM replenishment factor of 0.50 +/- 0.07, consistent with our choice of Chabrier IMF plus some modest amount of stripped stellar mass. Exploring the cosmic dust density evolution, we find a gradual increase in dust density with lookback time. We build a simple phenomenological model from the CSFH to account for the dust mass evolution, and infer two key conclusions: (1) For every unit of stellar mass which is formed 0.0065---0.004 units of dust mass is also formed; (2) Over the history of the Universe approximately 90 to 95 per cent of all dust formed has been destroyed and/or ejected.

Modelling the cosmic spectral energy distribution and extragalactic background light over all time

We present a phenomological model of the Cosmic Spectral Energy Distribution (CSED) and the integrated galactic light (IGL) over all cosmic time. This model, based on an earlier model by Driver et al. (2013), attributes the cosmic star formation history to two processes -- firstly, chaotic clump accretion and major mergers, resulting in the early-time formation of bulges and secondly, cold gas accretion, resulting in late-time disc formation. Under the assumption of a Universal Chabrier initial mass function, we combine the Bruzual & Charlot (2003) stellar libraries, the Charlot & Fall (2000) dust attenuation prescription and template spectra for emission by dust and active galactic nuclei to predict the CSED -- pre- and post-dust attenuation -- and the IGL throughout cosmic time. The phenomological model, as constructed, adopts a number of basic axioms and empirical results and has minimal free parameters. We compare the model output, as well as predictions from the semi-analytic model GALFORM to recent estimates of the CSED out to $z=1$. By construction, our empirical model reproduces the full energy output of the Universe from the ultraviolet to the far-infrared extremely well. We use the model to derive predictions of the stellar and dust mass densities, again finding good agreement. We find that GALFORM predicts the CSED for $z < 0.3$ in good agreement with the observations. This agreement becomes increasingly poor towards $z = 1$, when the model CSED is $\sim$50 per cent fainter. The latter is consistent with the model underpredicting the cosmic star formation history. As a consequence, GALFORM predicts a $\sim$30 per cent fainter IGL.

Star Formation, Supernovae, Iron, and α: Consistent Cosmic and Galactic Histories

Abstract Recent versions of the observed cosmic star formation history (SFH) have resolved an inconsistency with the stellar mass density history. We show that the revised SFH also scales up the delay-time distribution (DTD) of Type Ia supernovae (SNe Ia), as determined from the observed volumetric SN Ia rate history, aligning it with other field-galaxy SN Ia DTD measurements. The revised-SFH-based DTD has a form and a Hubble-time-integrated production efficiency of SNe Ia per of formed stellar mass. Using these revised histories and updated empirical iron yields of the various SN types, we re-derive the cosmic iron accumulation history. Core-collapse SNe and SNe Ia have contributed about equally to the total mass of iron in the universe today. We find the track of the average cosmic gas element in the [α/Fe] versus [Fe/H] abundance-ratio plane. The track is broadly similar to the observed main locus of Galactic stars in this plane, indicating a Milky Way (MW) SFH similar in form to the cosmic one. We easily find a simple MW SFH that makes the track closely match this stellar locus. Galaxy clusters appear to have a higher-normalization DTD. This cluster DTD, combined with a short-burst MW SFH peaked at z = 3, produces a track that matches remarkably well the observed “high-α” locus of MW stars, suggesting the halo/thick-disk population has had a galaxy-cluster-like formation mode. Thus, a simple two-component SFH, combined with empirical DTDs and SN iron yields, suffices to closely reproduce the MW’s stellar abundance patterns.

Insights on star-formation histories and physical properties of 1.2 ≤z ≲ 4 Herschel-detected galaxies

We test the impact of using variable star forming histories (SFHs) and the use of the IR luminosity (LIR) as a constrain on the physical parameters of high redshift dusty star-forming galaxies. We explore in particular the stellar properties of galaxies in relation with their location on the SFR-M* diagram. We perform SED fitting of the UV-NIR and FIR emissions of a large sample of GOODS-Herschel galaxies, for which rich multi-wavelength observations are available. We test different SFHs and imposing energy conservation in the SED fitting process, to face issues like the age-extinction degeneracy and produce SEDs consistent with observations. Our models work well for the majority of the sample, with the notable exception of the high LIR end, for which we have indications that our simple energy conservation approach cannot hold true. We find trends in the SFHs fitting our sources depending on stellar mass M* and z. Trends also emerge in the characteristic timescales of the SED models depending on the location on the SFR-M* diagram. We show that whilst using the same available observational data, we can produce galaxies less star-forming than usually inferred, if we allow declining SFHs, while properly reproducing their observables. These sources can be post-starbursts undergoing quenching, and their SFRs are potentially overestimated if inferred from their LIR. Fitting without the IR constrain leads to a strong preference for declining SFHs, while its inclusion increases the preference of rising SFHs, more so at high z, in tentative agreement with the cosmic star formation history. Keeping in mind that the sample is biased towards high LIR, the evolution shaped by our model appears as both bursty (initially) and steady-lasting (later on). The global SFH of the sample follows the cosmic SFH with a small scatter, and is compatible with the "downsizing" scenario of galaxy evolution.

Constraining the contribution of galaxies and active galactic nuclei to cosmic reionization

Understanding the detailed process of cosmic reionization is one of the remaining problems in astrophysics and cosmology. Here we construct a model of cosmic reionization that includes contribution from high-$z$ galaxies and active galactic nuclei (AGNs), and calculate the reionization and thermal histories with the model. To keep the model to be general and realistic, we vary the escape fraction of ionizing photons, $f_{\rm esc}$, and the faint-end slope of the AGN luminosity function at high redshifts, $\alpha_{\rm hz}$, within the constraints from the observed cosmic star formation history and the observed bright-end UV luminosity functions at $z \leq 6$. Additionally, we model the spectral energy distribution (SED) of AGNs which depends on the Eddington ratio and the black-hole mass. By comparing the computed reionization histories with the observed \ion{H}{i} fractions and the optical depth for Thomson scattering from Planck, we find that $\alpha_{\rm hz} > -1.5$ and $f_{\rm esc} < 0.15$ are favored when we employ the bright-end luminosity function obtained by Giallongo et al. Our result suggests that an AGN-dominated model with the abundance of faint-AGNs being as large as the estimate by Giallongo et al. is allowed only if the contribution from high-z galaxies is almost negligible, while a galaxy-dominant model is also allowed. We also find that the shape of SED has a significant impact on the thermal history. Therefore it is expected that measurements of the thermal state of the IGM provide useful information on properties of ionizing sources.

Galaxy Protoclusters as Drivers of Cosmic Star Formation History in the First 2 Gyr

Abstract Present-day clusters are massive halos containing mostly quiescent galaxies, while distant protoclusters are extended structures containing numerous star-forming galaxies. We investigate the implications of this fundamental change in a cosmological context using a set of N-body simulations and semi-analytic models. We find that the fraction of the cosmic volume occupied by all (proto)clusters increases by nearly three orders of magnitude from z = 0 to z = 7. We show that (proto)cluster galaxies are an important and even dominant population at high redshift, as their expected contribution to the cosmic star formation rate density rises (from 1% at z = 0) to 20% at z = 2 and 50% at z = 10. Protoclusters thus provide a significant fraction of the cosmic ionizing photons, and may have been crucial in driving the timing and topology of cosmic reionization. Internally, the average history of cluster formation can be described by three distinct phases: at z ∼ 10–5, galaxy growth in protoclusters proceeded in an inside-out manner, with centrally dominant halos that are among the most active regions in the universe; at z ∼ 5–1.5, rapid star formation occurred within the entire 10–20 Mpc structures, forming most of their present-day stellar mass; at z ≲ 1.5, violent gravitational collapse drove these stellar contents into single cluster halos, largely erasing the details of cluster galaxy formation due to relaxation and virialization. Our results motivate observations of distant protoclusters in order to understand the rapid, extended stellar growth during cosmic noon, and their connection to reionization during cosmic dawn.

The evolution of CNO isotopes: a new window on cosmic star formation history and the stellar IMF in the age of ALMA

We use state-of-the-art chemical models to track the cosmic evolution of the CNO isotopes in the interstellar medium (ISM) of galaxies, yielding powerful constraints on their stellar initial mass function (IMF). We re-assess the relative roles of massive stars, asymptotic giant branch (AGB) stars and novae in the production of rare isotopes such as 13C, 15N, 17O and 18O, along with 12C, 14N and 16O. The CNO isotope yields of super-AGB stars, novae and fast-rotating massive stars are included. Having reproduced the available isotope enrichment data in the solar neighbourhood, and across the Galaxy, and having assessed the sensitivity of our models to the remaining uncertainties, e.g. nova yields and star-formation history, we show that we can meaningfully constrain the stellar IMF in galaxies using C, O and N isotope abundance ratios. In starburst galaxies, where data for multiple isotopologue lines are available, we find compelling new evidence for a top-heavy stellar IMF, with profound implications for their star-formation rates and efficiencies, perhaps also their stellar masses. Neither chemical fractionation nor selective photodissociation can significantly perturb globally-averaged isotopologue abundance ratios away from the corresponding isotope ones, as both these processes will typically affect only small mass fractions of molecular clouds in galaxies. Thus the Atacama Large Millimetre Array now stands ready to probe the stellar IMF, and even the ages of specific starburst events in star-forming galaxies across cosmic time unaffected by the dust obscuration effects that plague optical/near-infrared studies.

Radiation Backgrounds at Cosmic Dawn: X-Rays from Compact Binaries

We compute the expected X-ray diffuse background and radiative feedback on the intergalactic medium (IGM) from X-ray binaries prior and during the epoch of reionization. The cosmic evolution of compact binaries is followed using a population synthesis technique that treats separately neutron stars and black hole binaries in different spectral states and is calibrated to reproduce the observed X-ray properties of galaxies at z<4. Together with an updated empirical determination of the cosmic history of star formation, recent modeling of the stellar mass-metallicity relation, and a scheme for absorption by the IGM that accounts for the presence of ionized HII bubbles during the epoch of reionization, our detailed calculations provide refined predictions of the X-ray volume emissivity and filtered radiation background from "normal" galaxies at z>6. Radiative transfer effects modulate the background spectrum, which shows a characteristic peak between 1 and 2 keV. While the filtering of X-ray radiation through the IGM slightly increases the mean excess energy per photoionization, it also weakens the radiation intensity below 1 keV, lowering the mean photoionization and heating rates. Numerical integration of the rate and energy equations shows that the contribution of X-ray binaries to the ionization of the bulk IGM is negligible, with the electron fraction never exceeding 1%. Direct HeI photoionizations are the main source of IGM heating, and the temperature of the largely neutral medium in between HII cavities increases above the temperature of the cosmic microwave background (CMB) only at z<10, when the volume filling factor of HII bubbles is already >0.1. Therefore, in this scenario, it is only at relatively late epochs that the bulk of neutral intergalactic hydrogen may be observable in 21-cm emission against the CMB.

Does the evolution of the radio luminosity function of star-forming galaxies match that of the star formation rate function?

The assessment of the relationship between radio continuum luminosity and star formation rate (SFR) is of crucial importance to make reliable predictions for the forthcoming ultra-deep radio surveys and to allow a full exploitation of their results to measure the cosmic star formation history. We have addressed this issue by matching recent accurate determinations of the SFR function up to high redshifts with literature estimates of the 1.4 GHz luminosity functions of star forming galaxies (SFGs). This was done considering two options, proposed in the literature, for the relationship between the synchrotron emission ($L_{\rm synch}$), that dominates at 1.4 GHz, and the SFR: a linear relation with a decline of the $L_{\rm synch}$/SFR ratio at low luminosities or a mildly non-linear relation at all luminosities. In both cases we get good agreement with the observed radio luminosity functions but, in the non-linear case, the deviation from linearity must be small. The luminosity function data are consistent with a moderate increase of the $L_{\rm synch}$/SFR ratio with increasing redshift, indicated by other data sets, although a constant ratio cannot be ruled out. A stronger indication of such increase is provided by recent deep 1.4 GHz counts, down to $\mu$Jy levels. This is in contradiction with models predicting a decrease of that ratio due to inverse Compton cooling of relativistic electrons at high redshifts. Synchrotron losses appear to dominate up to $z\simeq 5$. We have also updated the Massardi et al. (2010) evolutionary model for radio loud AGNs.

High Mass X-ray Binaries: Beacons in a Stormy Universe.

The discovery of gravity waves from the mergers of black hole binaries has focused the astronomical community on the high mass X-ray binaries (HMXBs) as the potential progenitors of close pairs of compact stars. This symposium gathered experts in observational and theoretical work for a very timely review of our understanding of the processes that drive the X-ray luminosity of the diverse kinds of binaries and what evolutionary stages are revealed in the observed cases. Here I offer a condensed summary of some of the results about massive star properties, the observational categories of HMXBs, their accretion processes, their numbers in the Milky Way and other galaxies, and how they may be related to the compact binaries that merge in a burst of gravity waves.

ALMA observations of cold molecular gas in AGN hosts at z ∼ 1.5 – evidence of AGN feedback?

Similarly to the cosmic star formation history, the black hole accretion rate density of the Universe peaked at 1 < z < 3. This cosmic epoch is hence best suited for investigating the effects of radiative feedback from active galactic nucleus (AGN). Observational efforts are under way to quantify the impact of the AGN feedback, if any, on their host galaxies. Here, we present a study of the molecular gas content of AGN hosts at z ∼ 1.5 using CO[2−1] line emission observed with Atacama Large Millimeter/sub-mm Array (ALMA) for a sample of 10 AGNs. We compare this with a sample of galaxies without an AGN matched in redshift, stellar mass and star formation rate. We detect CO in three AGNs with LCO ∼ 6.3–25.1 × 109 L⊙, which translates to a molecular hydrogen gas mass of 2.5–10 × 1010 M⊙ assuming conventional conversion factor of αCO ∼ 3.6. Our results indicate a >99 per cent probability of lower depletion time-scales and lower molecular gas fractions in AGN hosts with respect to the non-AGN comparison sample. We discuss the implications of these observations on the impact that AGN feedback may have on star formation efficiency of z >1 galaxies.

HYPER SUPRIME-CAMERA SURVEY OF THE AKARI NEP WIDE FIELD

The extragalactic background suggests half the energy generated by stars was reprocessed into the infrared (IR) by dust. At z ∼1.3, 90% of star formation is obscured by dust. To fully understand the cosmic star formation history, it is critical to investigate infrared emission. AKARI has made deep mid-IR observation using its continuous 9-band filters in the NEP field (5.4 deg2), using ∼10% of the entire pointed observations available throughout its lifetime. However, there remain 11,000 AKARI infrared sources undetected with the previous CFHT/Megacam imaging (r ∼25.9ABmag). Redshift and IR luminosity of these sources are unknown. These sources may contribute significantly to the cosmic star-formation rate density (CSFRD). For example, if they all lie at 1 < z < 2, the CSFRD will be twice as high at the epoch. We are carrying out deep imaging of the NEP field in 5 broad bands (g, r, i, z, and y) using Hyper Suprime-Camera (HSC), which has 1.5 deg field of view in diameter on Subaru 8m telescope. This will provide photometric redshift information, and thereby IR luminosity for the previously-undetected 11,000 faint AKARI IR sources. Combined with AKARI's mid-IR AGN/SF diagnosis, and accurate mid-IR luminosity measurement, this will allow a complete census of cosmic star-formation/AGN accretion history obscured by dust.

OVERVIEW OF NORTH ECLIPTIC POLE DEEP MULTI-WAVELENGTH SURVEY (NEP-DEEP)

The recent updates of the North Ecliptic Pole deep (0.5~deg$^2$, NEP-Deep) multi-wavelength survey covering from X-ray to radio-wave is presented. The NEP-Deep provides us with several thousands of 15~$\mu$m or 18~$\mu$m selected sample of galaxies, which is the largest sample ever made at this wavelengths. A continuous filter coverage in the mid-infrared wavelength (7, 9, 11, 15, 18, and 24~$\mu$m) is unique and vital to diagnose the contributions from starbursts and AGNs in the galaxies out to $z$=2.The new goal of the project is to resolve the nature of the cosmic star formation history at the violent epoch (e.g. $z$=1--2), and to find a clue to understand its decline from $z$=1 to present universe by utilizing the unique power of the multi-wavelength survey. The progress in this context is briefly mentioned.

Local Group ultra-faint dwarf galaxies in the reionization era

Abstract Motivated by the stellar fossil record of Local Group (LG) dwarf galaxies, we show that the star-forming ancestors of the faintest ultra-faint dwarf galaxies (UFDs; MV ∼ −2 or M⋆ ∼ 102 at z = 0) had ultraviolet (UV) luminosities of MUV ∼ −3 to −6 during reionization (z ∼ 6–10). The existence of such faint galaxies has substantial implications for early epochs of galaxy formation and reionization. If the faint-end slopes of the UV luminosity functions (UVLFs) during reionization are steep (α ≲ −2) to MUV ∼ −3, then (i) the ancestors of UFDs produced &amp;gt;50 per cent of UV flux from galaxies; (ii) galaxies can maintain reionization with escape fractions that are more than two times lower than currently adopted values; (iii) direct Hubble Space Telescope and James Webb Space Telescope observations may detect only ∼10–50 per cent of the UV light from galaxies; and (iv) the cosmic star formation history increases by ≳ 4–6 at z ≳ 6. Significant flux from UFDs, and resultant tensions with LG dwarf galaxy counts, is reduced if the high-redshift UVLF turns over. Independent of the UVLF shape, the existence of a large population of UFDs requires a non-zero luminosity function to MUV ∼ −3 during reionization.

Advanced LIGO Constraints on Neutron Star Mergers and r-process Sites

Abstract The role of compact binary mergers as the main production site of r-process elements is investigated by combining stellar abundances of Eu observed in the Milky Way, galactic chemical evolution (GCE) simulations, and binary population synthesis models, and gravitational wave measurements from Advanced LIGO. We compiled and reviewed seven recent GCE studies to extract the frequency of neutron star–neutron star (NS–NS) mergers that is needed in order to reproduce the observed [Eu/Fe] versus [Fe/H] relationship. We used our simple chemical evolution code to explore the impact of different analytical delay-time distribution functions for NS–NS mergers. We then combined our metallicity-dependent population synthesis models with our chemical evolution code to bring their predictions, for both NS–NS mergers and black hole–neutron star mergers, into a GCE context. Finally, we convolved our results with the cosmic star formation history to provide a direct comparison with current and upcoming Advanced LIGO measurements. When assuming that NS–NS mergers are the exclusive r-process sites, and that the ejected r-process mass per merger event is 0.01 M , the number of NS–NS mergers needed in GCE studies is about 10 times larger than what is predicted by standard population synthesis models. These two distinct fields can only be consistent with each other when assuming optimistic rates, massive NS–NS merger ejecta, and low Fe yields for massive stars. For now, population synthesis models and GCE simulations are in agreement with the current upper limit (O1) established by Advanced LIGO during their first run of observations. Upcoming measurements will provide an important constraint on the actual local NS–NS merger rate, will provide valuable insights on the plausibility of the GCE requirement, and will help to define whether or not compact binary mergers can be the dominant source of r-process elements in the universe.

The history of star formation in the universe

Our primary means of studying how galaxies form and evolve over cosmic time is through measurements of the rate at which massive stars are born in galaxies per unit comoving volume of the universe. Only recently have the most distant, most massively star-forming galaxies in the universe even been discovered, and these discoveries are challenging the standard cosmological scenario in which galaxies form hierarchically, with low-mass objects collapsing first and then merging to form larger and larger systems over cosmic time. We now know that these incredibly luminous galaxies, which have all but vanished in the local universe, are cocooned in dust, hiding them from optical view. They can only be detected at long wavelengths where the dust reradiates the starlight that it has absorbed. In this review, I trace the spectacular progress that has been made over the last decade towards understanding the cosmic history of star formation, with a particular emphasis on the important role of dusty sources in that history.

The Horizon-AGN simulation: evolution of galaxy properties over cosmic time

We compare the predictions of Horizon-AGN, a hydro-dynamical cosmological simulation that uses an adaptive mesh refinement code, to observational data in the redshift range 0<z<6. We study the reproduction, by the simulation, of quantities that trace the aggregate stellar-mass growth of galaxies over cosmic time: luminosity and stellar-mass functions, the star formation main sequence, rest-frame UV-optical-near infrared colours and the cosmic star-formation history. We show that Horizon-AGN, which is not tuned to reproduce the local Universe, produces good overall agreement with these quantities, from the present day to the epoch when the Universe was 5% of its current age. By comparison to Horizon-noAGN, a twin simulation without AGN feedback, we quantify how feedback from black holes is likely to help shape galaxy stellar-mass growth in the redshift range 0<z<6, particularly in the most massive galaxies. Our results demonstrate that Horizon-AGN successfully captures the evolutionary trends of observed galaxies over the lifetime of the Universe, making it an excellent tool for studying the processes that drive galaxy evolution and making predictions for the next generation of galaxy surveys.

Evolution of cosmic star formation in the SCUBA-2 Cosmology Legacy Survey

We present a new exploration of the cosmic star-formation history and dust obscuration in massive galaxies at redshifts $0.5< z<6$. We utilize the deepest 450 and 850$\mu$m imaging from SCUBA-2 CLS, covering 230arcmin$^2$ in the AEGIS, COSMOS and UDS fields, together with 100-250$\mu$m imaging from Herschel. We demonstrate the capability of the T-PHOT deconfusion code to reach below the confusion limit, using multi-wavelength prior catalogues from CANDELS/3D-HST. By combining IR and UV data, we measure the relationship between total star-formation rate (SFR) and stellar mass up to $z\sim5$, indicating that UV-derived dust corrections underestimate the SFR in massive galaxies. We investigate the relationship between obscuration and the UV slope (the IRX-$\beta$ relation) in our sample, which is similar to that of low-redshift starburst galaxies, although it deviates at high stellar masses. Our data provide new measurements of the total SFR density (SFRD) in $M_\ast>10^{10}M_\odot$ galaxies at $0.5<z<6$. This is dominated by obscured star formation by a factor of $>10$. One third of this is accounted for by 450$\mu$m-detected sources, while one fifth is attributed to UV-luminous sources (brighter than $L^\ast_{UV}$), although even these are largely obscured. By extrapolating our results to include all stellar masses, we estimate a total SFRD that is in good agreement with previous results from IR and UV data at $z\lesssim3$, and from UV-only data at $z\sim5$. The cosmic star-formation history undergoes a transition at $z\sim3-4$, as predominantly unobscured growth in the early Universe is overtaken by obscured star formation, driven by the build-up of the most massive galaxies during the peak of cosmic assembly.