Context. Over the last decade a large number of dusty star forming galaxies has been discovered up to redshift z = 2 − 3 and recent studies have attempted to push the highly-confused Herschel SPIRE surveys beyond that distance. To search for z ≥ 4 galaxies they often consider the sources with fluxes rising from 250 µm to 500 µm (so-called "500 µm-risers"). Herschel surveys offer a unique opportunity to efficiently select a large number of these rare objects, and thus gain insight into the prodigious star-forming activity that takes place in the very distant Universe. Aims. We aim to implement a novel method to obtain a statistical sample of "500 µm-risers" and fully evaluate our selection inspecting different models of galaxy evolution. Methods. We consider one of the largest and deepest Herschel surveys, the Herschel Virgo Cluster Survey. We develop a novel selection algorithm which links the source extraction and spectral energy distribution fitting. To fully quantify selection biases we make end-to-end simulations including clustering and lensing. Results. We select 133 "500 µm-risers" over 55 deg 2 , imposing the criteria: S 500 > S 350 > S 250 , S 250 > 13.2 mJy and S 500 > 30 mJy. Differential number counts are in a fairly good agreement with models, displaying better match than other existing samples. The estimated fraction of strongly lensed sources is 24 +6 −5 % based on models. Conclusions. We present the faintest sample of "500 µm-risers" down to S 250 = 13.2 mJy. We show that noise and strong lensing have an important impact on measured counts and redshift distribution of selected sources. We estimate the flux-corrected star formation rate density at 4 < z < 5 with the "500 µm-risers" and found it close to the total value measured in far-infrared. It indicates that colour selection is not a limiting effect to search for the most massive, dusty z > 4 sources.

We present an ALMA high-resolution (0.18″ × 0.21″) observation of the 840 μm continuum and [CII] λ157.74 μm line emission in the WISE-SDSS selected hyper-luminous (WISSH) quasi-stellar object (QSO) J1015+0020, at z ∼ 4.4. Our analysis reveals an exceptional overdensity of [CII]-emitting companions with a very small (<150 km s−1) velocity shift with respect to the QSO redshift. We report the discovery of the closest companion observed so far in submillimetre observations of high-z QSOs. It is only 2.2 kpc distant and merging with J1015+0020, while two other [CII] emitters are found at 8 and 17 kpc. Two strong continuum emitters are also detected at <3.5 arcsec from the QSO. They are likely associated with the same overdense structure of J1015+0020, as they exceed by a factor of 100 the number of expected sources, considering the log(N)–log(S) at 850 μm. The host galaxy of J1015+0020 shows a star formation rate (SFR) of about 100 M⊙ yr−1, while the total SFR of the QSO and its companion galaxies is a factor of ∼10 higher, indicating that substantial stellar mass assembly at early epochs may have taken place in the QSO satellites. For J1015+0020 we computed a black hole mass MBH ∼ 6 × 109 M⊙. As we resolve the [CII] emission of the QSO, we can compute a dynamical mass of Mdyn ∼ 4 × 1010 M⊙. This translates into an extreme ratio Mdyn/MBH ∼ 7, i.e. two orders of magnitude smaller than what is typically observed in local galaxies. The total stellar mass of the QSO host galaxy plus the [CII] emitters in the ALMA field of view already exceeds 1011 M⊙ at z ∼ 4.4. These sources will likely merge and develop into a giant galaxy of ∼1.3 × 1012 M⊙. Under the assumption of constant Ṁacc or λEdd equal to the observed values, we find that the growth timescale of the host galaxy of J1015+0020 is comparable or even shorter than that inferred for the SMBH.

Context. Spectral synthesis is a powerful tool for interpreting the physical properties of galaxies by decomposing their spectral energy distributions (SEDs) into the main luminosity contributors (e.g. stellar populations of distinct age and metallicity or ionised gas). However, the impact nebular emission has on the inferred properties of star-forming (SF) galaxies has been largely overlooked over the years, with unknown ramifications to the current understanding of galaxy evolution. Aims. The objective of this work is to estimate the relations between stellar properties (e.g. total mass, mean age, and mean metallicity) of SF galaxies by simultaneously fitting the stellar and nebular continua and comparing them to the results derived through the more common purely stellar spectral synthesis approach. Methods. The main galaxy sample from SDSS DR7 was analysed with two distinct population synthesis codes: Fado, which estimates self-consistently both the stellar and nebular contributions to the SED, and the original version of Starlight, as representative of purely stellar population synthesis codes. Results. Differences between codes regarding average mass, mean age and mean metallicity values can go as high as ∼0.06 dex for the overall population of galaxies and ∼0.12 dex for SF galaxies (galaxies with EW(Hα)>3 Å), with the most prominent difference between both codes in the two populations being in the light-weighted mean stellar age. Fado presents a broader range of mean stellar ages and metallicities for SF galaxies than Starlight, with the latter code preferring metallicity solutions around the solar value (Z = 0.02). A closer look into the average light-and mass-weighted star formation histories of intensively SF galaxies (EW(Hα)>75 Å) reveals that the light contributions of simple stellar populations (SSPs) younger than ≤ 10 7 (10 9 ) years in Starlight are higher by ∼5.41% (9.11%) compared to Fado. Moreover, Fado presents higher light contributions from SSPs with metallicity ≤ Z /200 (Z /50) of around 8.05% (13.51%) when compared with Starlight. This suggests that Starlight is underestimating the average light-weighted age of intensively SF galaxies by up to ∼0.17 dex and overestimating the light-weighted metallicity by up to ∼0.13 dex compared to Fado (or vice versa). The comparison between the average stellar properties of passive, SF and intensively SF galaxy samples also reveals that differences between codes increase with increasing EW(Hα) and decreasing total stellar mass. Moreover, comparing SF results from Fado in a purely stellar mode with the previous results qualitatively suggests that differences between codes are primarily due to mathematical and statistical differences and secondarily due to the impact of the nebular continuum modelling approach (or lack thereof). However, it is challenging to adequately quantify the relative role of each factor since they are likely interconnected. Conclusions. This work finds indirect evidence that a purely stellar population synthesis approach negatively i...