Context. V838 Mon erupted in 2002, quickly becoming the prototype of a new type of stellar eruption known today as the (luminous) red nova. Red nova outbursts are thought to be caused by stellar mergers. The merger in V838 Mon took place in a triple or higher system involving two B-type stars. Aims. We wish to characterize the merger remnant ~17 yr after the eruption to learn more about the remaining system, the progenitor, and the merger physics. Methods. We mapped the merger site with ALMA at a resolution of ~25 mas, or 148 au for a distance of 5.9 kpc, in continuum dust emission and in rotational lines of simple molecules, including CO, SiO, SO, SO2, AlOH, and H2S. We use radiative transfer calculations to reproduce the architecture of the remnant at the epoch of the ALMA observations. We also make use of optical spectroscopy data obtained mainly with VLT/X-shooter and SALT/HRS. Results. For the first time, we identify the position of the B-type companion relative to the outbursting component of V838 Mon. The stellar remnant is surrounded by a clumpy wind with characteristics similar to those of the winds of red supergiants. The merger product is also associated with an elongated structure of 17.6 × 7.6 mas (104 × 45 au) seen in continuum emission, and which we interpret as a disk seen at a moderate inclination. Maps of continuum and molecular emission also show a complex region of interaction between the B-type star (its gravity, radiation, and wind) and the flow of matter ejected in 2002. The remnant’s molecular mass is about 0.1 M⊙ and the dust mass is 8.3 × 10−3 M⊙. The mass of the atomic component remains unconstrained. Conclusions. The most interesting region for understanding the merger of V838 Mon lies in its direct vicinity and appears elongated, but details of its substructure remain unknown. To study it further and in more detail will require even higher angular resolutions. ALMA maps show us an extreme form of interaction between the merger ejecta and the distant (~250 au) companion. This interaction is similar to that known from the Anteres AB system but at a much higher mass loss rate. The B-type star not only deflects the merger ejecta but also changes its molecular composition with an involvement of circumstellar shocks. The ALMA view of V838 Mon offers the best images of a merger site so far.

Red ultra-compact massive galaxies, called red nuggets were formed at high redshifts (z ∼ 2 − 3). Survivors of red nuggets, known as relics, observed at lower redshifts (z < 2) are believed to remain almost unchanged since their formation. For the first time, we verify the environmental properties of red nuggets at intermediate redshift (0.5 < z < 0.9 ) using 42 red, massive (log(Mstar/M⊙) ≥ 10.9) and ultra-compact (Re < 1.5 kpc) from the VIMOS Public Extragalactic Redshift Survey (VIPERS). We found that the increasing fraction of red galaxies, when moving to denser environments, is driven by the red massive normal-size galaxies. Red nuggets, similarly to red intermediate-mass (10.4 ≲ log (Mstar/M⊙) < 10.9) ultra-compact galaxies, are found in various types of environments, with consistent (within 1σ) fractions across all local densities. Analysis of red nugget stellar ages suggests that relics are preferably found in high-density regions while quiescent red nuggets are overabundant in low-density environments. We speculate that red nuggets have survived to lower redshifts via two channels: i) in low-density environments where the fraction of red nuggets decreases as time passes due to (very) limited merger activity, ii) in high-density environments, where the number of red nuggets drops at higher redshift due to merger activity and is preserved at lower redshift as the high velocities of clusters prevent them from being cannibalised. Even more, the fraction of red nuggets in clusters may increase due to the addition of red massive normal-size galaxies deprived of their envelopes with cosmic time.

Context. Red nuggets are a rare population of passive compact massive galaxies thought to be the first massive galaxies that formed in the Universe. First found at z ∼ 3, they are even less abundant at lower redshifts, and it is believed that with time they mostly transformed through mergers into today's giant ellipticals. The red nuggets that managed to escape this fate can serve as unique laboratories to study the early evolution of massive galaxies. Aims. In this paper we aim to make use of the unprecedented statistical power of the VIMOS Public Extragalactic Redshift Survey to build the largest up-to-date catalogue of spectroscopically confirmed red nuggets at the intermediate redshift 0.5 < z < 1.0. Methods. Starting from a catalogue of nearly 90 000 VIPERS galaxies we selected sources with stellar masses M star > 8×10 10 M ⊙ and effective radii R e < 1.5 kpc. From these sources we selected red passive galaxies with old stellar populations based on colour-colour NUVrK diagram, star formation rate values, and verification of their optical spectra. Results. Verifying the influence of the limit of the source compactness on the selection, we found that the sample size can vary by up to two orders of magnitude, depending on the chosen criterion. Using one of the most restrictive criteria with additional checks on their spectra and passiveness, we spectroscopically identified only 77 previously unknown red nuggets. The resultant catalogue of 77 red nuggets is the largest such catalogue built based on the uniform set of selection criteria above the local Universe. The number density calculated on the final sample of 77 VIPERS passive red nuggets per comoving Mpc 3 increases from 4.7×10 −6 at z ∼ 0.61 to 9.8 × 10 −6 at z ∼ 0.95, which is higher than values estimated in the local Universe, and lower than the values found at z > 2. It fills the gap at intermediate redshift.Conclusions. A catalogue of red nuggets presented in this paper is a golden sample for future studies of this rare population of objects at intermediate redshift. In addition to covering a unique redshift range and careful selection of galaxies, the catalogue is spectroscopically identified.