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Exploiting IRAM 30 m CO spectroscopy, we find that SDSS post-merger galaxies display gas fractions and depletion times enhanced by 25-50%, a mildly higher CO excitation, and standard molecular-to-atomic gas ratios, compared to non-interacting galaxies with similar redshift, stellar mass ($M_{\star}$) and star-formation rate (SFR). To place these results in context, we compile further samples of interacting or starbursting galaxies, from pre-coalescence kinematic pairs to post-starbursts, carefully homogenising gas mass, $M_{\star}$ and SFR measurements in the process. We explore systematics by duplicating our analysis for different SFR and $M_{\star}$ estimators, finding good qualitative agreement in general. Molecular gas fractions and depletion times are enhanced in interacting pairs, albeit less than for post-mergers. Among all samples studied, gas fraction and depletion time enhancements appear largest in young (a few 100 Myr) post-starbursts. While there is only partial overlap between post-mergers and post-starbursts, this suggests that molecular gas reservoirs are boosted throughout most stages of galaxy interactions, plausibly due to torque-driven inflows of halo gas and gas compression. The gas fraction and depletion time offsets of mergers and post-starbursts anti-correlate with their distance from the galaxy main sequence $\Delta({\rm MS})$, evidencing the role of SFE in driving the high SFRs of the strongest starbursts. Post-starbursts display the steepest dependency of gas fraction and SFE-offsets on $\Delta({\rm MS})$, with an evolving normalisation that reflects gas reservoir depletion over time. Our multi-sample analysis paints a coherent picture of the starburst-merger throughout the low-z merger sequence. It reconciles contradictory literature findings by highlighting that gas fraction enhancements and SFE variations both play their part in merger-driven star formation., Comment: 46 pages, 21 figures. Submitted to the Open Journal of Astrophysics

High-redshift, luminous, dusty star forming galaxies (DSFGs) constrain the extremity of galaxy formation theories. The most extreme are discovered through follow-up on candidates in large area surveys. Here we present 850 $\mu$m SCUBA-2 follow-up observations of 188 red DSFG candidates from the \textit{Herschel} Multi-tiered Extragalactic Survey (HerMES) Large Mode Survey, covering 274 deg$^2$. We detected 87 per cent with a signal-to-noise ratio $>$ 3 at 850~$\mu$m. We introduce a new method for incorporating the confusion noise in our spectral energy distribution fitting by sampling correlated flux density fluctuations from a confusion limited map. The new 850~$\mu$m data provide a better constraint on the photometric redshifts of the candidates, with photometric redshift errors decreasing from $\sigma_z/(1+z)\approx0.21$ to $0.15$. Comparison spectroscopic redshifts also found little bias ($\langle (z-z_{\rm spec})/(1+z_{\rm spec})\rangle = 0.08 $). The mean photometric redshift is found to be 3.6 with a dispersion of $0.4$ and we identify 21 DSFGs with a high probability of lying at $z > 4$. After simulating our selection effects we find number counts are consistent with phenomenological galaxy evolution models. There is a statistically significant excess of WISE-1 and SDSS sources near our red galaxies, giving a strong indication that lensing may explain some of the apparently extreme objects. Nevertheless, our sample should include examples of galaxies with the highest star formation rates in the Universe ($\gg10^3$ M$_\odot$yr$^{-1}$)., Comment: 22 pages, 14 figures and 3 tables

We have developed a new prior-based source extraction tool, XID+, to carry out photometry in the Herschel SPIRE maps at the positions of known sources. XID+ is developed using a probabilistic Bayesian framework which provides a natural framework in which to include prior information, and uses the Bayesian inference tool, Stan, to obtain the full posterior probability distribution on flux estimates. In this paper, we discuss the details of XID+ and demonstrate the basic capabilities and performance by running it on simulated SPIRE maps resembling the COSMOS field, and comparing to the current prior-based source extraction tool DESPHOT. We show that not only does XID+ perform better on metrics such as flux accuracy and flux uncertainty accuracy, we illustrate how obtaining the posterior probability distribution can help overcome some of the issues inherent with maximum likelihood based source extraction routines. We run XID+ on the COSMOS SPIRE maps from HerMES, using a 24 $\mathrm{\mu m}$ catalogue as a prior and show the marginalised SPIRE colour-colour plot and marginalised contribution to the cosmic infrared background at the SPIRE wavelengths. XID+ is a core tool arising from the Herschel Extragalactic Legacy Project (HELP) and we discuss how additional work within HELP providing prior information on fluxes can and will be utilised. The software is available at https://github.com/H-E-L-P/XID_plus. We also provide the data product for COSMOS. We believe this is the first time that the full posterior probability of galaxy photometry has been provided as a data product., Comment: 14 pages, 10 Figures

The Herschel Extragalactic Legacy Project (HELP) brings together a vast range of data from many astronomical observatories. Its main focus is on the Herschel data, which maps dust obscured star formation over 1300 deg$^2$. With this unprecedented combination of data sets, it is possible to investigate how the star formation vs stellar mass relation (main-sequence) of star-forming galaxies depends on environment. In this pilot study we explore this question between 0.1 < z < 3.2 using data in the COSMOS field. We estimate the local environment from a smoothed galaxy density field using the full photometric redshift probability distribution. We estimate star formation rates by stacking the SPIRE data from the Herschel Multi-tiered Extragalactic Survey (HerMES). Our analysis rules out the hypothesis that the main-sequence for star-forming systems is independent of environment at 1.5 < z < 2, while a simple model in which the mean specific star formation rate declines with increasing environmental density gives a better description. However, we cannot exclude a simple hypothesis in which the main-sequence for star-forming systems is independent of environment at z < 1.5 and z > 2. We also estimate the evolution of the star formation rate density in the COSMOS field and our results are consistent with previous measurements at z < 1.5 and z > 2 but we find a $1.4^{+0.3}_{-0.2}$ times higher peak value of the star formation rate density at $z \sim 1.9$., Comment: 14 pages, 7 figures. MNRAS accepted