The origin and fate of the most extended extragalactic neutral cloud known inthe local Universe, the Leo ring, is still debated 38 years after itsdiscovery. Its existence is alternatively attributed to leftover primordial gaswith some low level of metal pollution versus enriched gas stripped during agalaxy-galaxy encounter. Taking advantage of MUSE (Multi Unit SpectroscopicExplorer) operating at the VLT, we performed optical integral fieldspectroscopy of 3 HI clumps in the Leo ring where ultraviolet continuumemission has been found. We detected, for the first time, ionized hydrogen inthe ring and identify 4 nebular regions powered by massive stars. These nebulaeshow several metal lines ([OIII],[NII],[SII]) which allowed reliable measuresof metallicities, found to be close to or above the solar value. Given thefaintness of the diffuse stellar counterparts, less than 3 percent of theobserved heavy elements could have been produced locally in the main body ofthe ring and not much more than 15 percent in the HI clump towards M96. Thisinference, and the chemical homogeneity among the regions, convincinglydemonstrates that the gas in the ring is not primordial, but has beenpre-enriched in a galaxy disk, then later removed and shaped by tidal forcesand it is forming a sparse population of stars.

Protoclusters, the progenitors of the most massive structures in theUniverse, have been identified at redshifts of up to 6.6. Besides exploringearly structure formation, searching for protoclusters at even higher redshiftsis particularly useful to probe the reionization. Here we report the discoveryof the protocluster LAGER-z7OD1 at a redshift of 6.93, when the Universe wasonly 770 million years old and could be experiencing rapid evolution of theneutral hydrogen fraction in the intergalactic medium. The protocluster isidentified by an overdensity of 6 times the average galaxy density, and with 21narrowband selected Lyman-$\alpha$ galaxies, among which 16 have beenspectroscopically confirmed. At redshifts similar to or above this record,smaller protogroups with fewer members have been reported. LAGER-z7OD1 shows anelongated shape and consists of two subprotoclusters, which would have mergedinto one massive cluster with a present-day mass of $3.7 \times 10^{15}$ solarmasses. The total volume of the ionized bubbles generated by its membergalaxies is found to be comparable to the volume of the protocluster itself,indicating that we are witnessing the merging of the individual bubbles andthat the intergalactic medium within the protocluster is almost fully ionized.LAGER-z7OD1 thus provides a unique natural laboratory to investigate thereionization process.

The levels of heavy elements in stars are the product of enhancement byprevious stellar generations, and the distribution of this metallicity amongthe population contains clues to the process by which a galaxy formed. Mostfamously, the "G-dwarf problem" highlighted the small number of low-metallicityG-dwarf stars in the Milky Way, which is inconsistent with the simplest pictureof a galaxy formed from a "closed box" of gas. It can be resolved by treatingthe Galaxy as an open system that accretes gas throughout its life. Thisobservation has classically only been made in the Milky Way, but theavailability of high-quality spectral data from SDSS-IV MaNGA and thedevelopment of new analysis techniques mean that we can now make equivalentmeasurements for a large sample of spiral galaxies. Our analysis shows thathigh-mass spirals generically show a similar deficit of low-metallicity stars,implying that the Milky Way's history of gas accretion is common. By contrast,low-mass spirals show little sign of a G-dwarf problem, presenting themetallicity distribution that would be expected if such systems evolved aspretty much closed boxes. This distinction can be understood from the differingtimescales for star formation in galaxies of differing masses.

In this paper we present an attempt to estimate the redshift evolution of themolecular to neutral gas mass ratio within galaxies (at fixed stellar mass).For a sample of five nearby grand design spirals located on the Main Sequence(MS) of star forming galaxies, we exploit maps at 500 pc resolution of stellarmass and star formation rate ($M_{\star}$ and SFR). For the same cells, we alsohave estimates of the neutral ($M_{\rm HI}$) and molecular ($M_{\rm H_2}$) gasmasses. To compute the redshift evolution we exploit two relations: {\it i)}one between the molecular-to-neutral mass ratio and the total gas mass ($M_{\rmgas}$), whose scatter shows a strong dependence with the distance from thespatially resolved MS, and {\it ii)} the one between$\log(M_{\rm{H_2}}/M_{\star})$ and $\log(M_{\rm{HI}}/M_{\star})$. For bothmethods, we find that $M_{\rm H_2}$/$M_{\rm HI}$ within the optical radiusslightly decreases with redshift, contrary to common expectations of galaxiesbecoming progressively more dominated by molecular hydrogen at high redshifts.We discuss possible implications of this trend on our understanding of theinternal working of high redshift galaxies.