Context. The mechanism of dust formation in galaxies at high redshift is still unknown. Asymptotic giant branch (AGB) stars and explosions of supernovae (SNe) are possible dust producers, and non-stellar processes may substantially contribute to dust production, for example grain growth in the interstellar medium (ISM). Aims. Our aim is to determine the contribution to dust production of AGB stars and SNe in nine galaxies at z ∼ 6-8.3, for which observations of dust have been recently attempted. Methods. In order to determine the origin of the observed dust we have determined dust yields per AGB star and SN required to explain the total amounts of dust in these galaxies.Results. We find that AGB stars were not able to produce the amounts of dust observed in the galaxies in our sample. In order to explain these dust masses, SNe would have to have maximum efficiency and not destroy the dust which they formed. Conclusions. Therefore, the observed amounts of dust in the galaxies in the early universe were formed either by efficient supernovae or by a non-stellar mechanism, for instance the grain growth in the interstellar medium.

Context. An important aspect of quenching star formation is the removal of the cold interstellar medium (ISM; non-ionised gas and dust) from a galaxy. In addition, dust grains can be destroyed in a hot or turbulent medium. The adopted timescale of dust removal usually relies on uncertain theoretical estimates. It is tricky to track dust removal because the dust is constantly being replenished by consecutive generations of stars. Aims. Our objective is to carry out an observational measurement of the timescale of dust removal. Methods. We explored an approach to select galaxies that demonstrate detectable amounts of dust and cold ISM coupled with a low current dust production rate. Any decrease of the dust and gas content as a function of the age of such galaxies must, therefore, be attributed to processes governing ISM removal. We used a sample of the galaxies detected by Herschel in the far-infrared with visually assigned early-type morphology or spirals with red colours. We also obtained JCMT/SCUBA-2 observations for five of these galaxies. Results. We discovered an exponential decline of the dust-to-stellar mass ratio with age, which we interpret as an evolutionary trend for the dust removal of these galaxies. For the first time, we have directly measured the dust removal timescale for such galaxies, with a result of τ = (2.5 ± 0.4) Gyr (the corresponding half-life time is (1.75 ± 0.25) Gyr). This quantity may be applied to models in which it must be assumed a priori and cannot be derived. Conclusions. Any process which removes dust in these galaxies, such as dust grain destruction, cannot happen on shorter timescales. The timescale is comparable to the quenching timescales found in simulations for galaxies with similar stellar masses. The dust is likely of internal, not external origin. It was either formed in the past directly by supernovae (SNe) or from seeds produced by SNe, and with grain growth in the ISM contributing substantially to the dust mass accumulation.

Context. Unusual stellar explosions represent an opportunity to learn about both stellar and galaxy evolution. Mapping the atomic gas in host galaxies of such transients can lead to an understanding of the conditions triggering them. Aims. We provide resolved atomic gas observations of the host galaxy, CGCG137-068, of the unusual, poorly-understood transient AT 2018cow searching for clues to understand its nature. We test whether it is consistent with a recent inflow of atomic gas from the intergalactic medium, as suggested for host galaxies of gamma-ray bursts (GRBs) and some supernovae (SNe). Methods. We observed the Hi hyperfine structure line of the AT 2018cow host with the Giant Metrewave Radio Telescope.Results. There is no unusual atomic gas concentration near the position of AT 2018cow. The gas distribution is much more regular than those of GRB/SN hosts. The AT 2018cow host has an atomic gas mass lower by 0.24 dex than predicted from its star formation rate (SFR) and is at the lower edge of the galaxy main sequence. In the continuum we detected the emission of AT 2018cow and of a star-forming region in the north-eastern part of the bar (away from AT 2018cow). This region hosts a third of the galaxy's SFR.Conclusions. The absence of atomic gas concentration close to AT 2018cow, along with a normal SFR and regular Hi velocity field, sets CGCG137-068 apart from GRB/SN hosts studied in Hi. The environment of AT 2018cow therefore suggests that its progenitor may not have been a massive star. Our findings are consistent with an origin of the transient that does not require a connection between its progenitor and gas concentration or inflow: an exploding low-mass star, a tidal disruption event, a merger of white dwarfs, or a merger between a neutron star and a giant star. We interpret the recently reported atomic gas ring in CGCG 137-068 as a result of internal processes connected with gravitational resonances caused by the bar.

We investigate the distribution of different classes of spectroscopically identified sources and theoretical models in the color-color diagrams (CCDs) combining the near-infrared (NIR) and mid-infrared (MIR) data to develop a method to classify Outer Galaxy sources detected with the Spitzer Space Telescope (hereafter Spitzer ) SMOG survey in the IRAC 3.6-8.0 µm and MIPS 24 µm bands. We supplement the Spitzer data with the data from other satellite and ground-based surveys. The main goal of our study is to discover and characterize the population of intermediate-and low-mass young stellar objects (YSOs) in the Outer Galaxy and use it to study star formation in a significantly different environment than the Galaxy inside the solar circle. Since the YSOs can be confused with evolved stars in the MIR, these classes of objects need to be carefully separated. Here we present the initial results of our analysis using the Ks-[8.0] vs. Ks-[24] CCD as an example. The evolved stars separated from YSOs in the YSO selection process will be investigated in detail in the follow-up study.

Long (>2 s) gamma-ray bursts (GRBs) are associated with explosions of massive stars, although in three instances, supernovae (SNe) have not been detected, despite deep observations. With new H i line and archival optical integral-field spectroscopy data, we characterize the interstellar medium (ISM) of the host galaxy of one of these events, GRB 111005A, in order to shed light on the unclear nature of these peculiar objects. We found that the atomic gas, radio continuum, and rotational patterns are in general very smooth throughout the galaxy, which does not indicate a recent gas inflow or outflow. There is also no gas concentration around the GRB position. The ISM in this galaxy differs from that in hosts of other GRBs and SNe, which may suggest that the progenitor of GRB 111005A was not an explosion of a very massive star (e.g., a compact object merger). However, there are subtle irregularities of the GRB 111005A host (most at a 2σ level), which may point to a weak gas inflow or interaction. Because in the SE part of the host there is 15% more atomic gas and half the molecular gas than in the NW part, the molecular gas fraction is low. In the SE part there is also a region with a very high Hα equivalent width. There is more continuum 1.4 GHz emission to the SE and an S-shaped warp in the UV. Finally, there is also a low-metallicity region 3.″5 (1 kpc) from the GRB position. Two galaxies within 300 kpc or a past merger can be responsible for these irregularities.