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L'astronomia extragalattica si sviluppò all'Osservatorio di Arcetri verso la fine degli anni '70, con l'arrivo del nuovo direttore Franco Pacini. La prima linea di ricerca dedicata a questo nuovo settore emerse da una collaborazione con la Cornell University sullo studio dell'idrogeno atomico. Successivamente il gruppo crebbe rapidamente durante gli anni '80 e '90 grazie all'inserimento di giovani ricercatori di talento. Oggi i temi scientifici includono lo studio dei nuclei galattici attivi, l'evoluzione chimica delle galassie, gli ammassi di galassie, la dinamica delle galassie e lo studio del mezzo interstellare. Il futuro degli studi extragalattici ad Arcetri trarrà vantaggio da strutture di osservazione all'avanguardia come l'Extremely Large Telescope, che entrerà in funzione nei prossimi anni. Parole chiave. Astronomia extragalattica, nuclei galattici attivi, spettroscopia. Extragalactic science was introduced at the Arcetri Observatory towards the end of the 1970's, upon the arrival of a new director, Franco Pacini. The first dedicated line of research emerged in collaboration with Cornell University and the study of neutral hydrogen. The group grew rapidly during the 80's and the 90's, with the inclusion of talented young researchers. The science themes studied today include active galactic nuclei, the chemical evolution of galaxies, galaxy clusters, galaxy dynamics and the study of the interstellar medium. The future of extragalactic work at Arcetri will profit from state-of-the-art observational facilities such as the Extremely Large Telescope which will become operational in the next few years. Keywords. Extragalactic astronomy, active galactic nuclei, spectroscopy.

Extragalactic science was introduced at the Arcetri Observatory towards the end of the 1970’s, upon the arrival of a new director, Franco Pacini. The first dedicated line of research emerged in collaboration with Cornell University and the study of neutral hydrogen. The group grew rapidly during the 80’s and the 90’s, with the inclusion of talented young researchers. The science themes studied today include active galactic nuclei, the chemical evolution of galaxies, galaxy clusters, galaxy dynamics and the study of the interstellar medium. The future of extragalactic work at Arcetri will profit from state-of-the-art observational facilities such as the Extremely Large Telescope which will become operational in the next few years.

Francesco Palla, Director of the Arcetri Astrophysical Observatory up to 2011, died suddenly last January. He was also well known in Florence for his commitment to spreading knowledge of science and was one of the founders of this journal. Here we should like to remember Francesco in his role of eclectic scientist and passionate communicator. Keywords. Arcetri Astrophysical Observatory, Francesco Palla. Lo scorso gennaio e improvvisamente mancato Francesco Palla, direttore dell'Osservatorio Astrofisico di Arcetri dal 2005 al 2011, molto conosciuto a Firenze anche per le sua attivita di comunicazione della scienza, e tra i fondatori di questa rivista. Ne ricordiamo la figura di scienziato e di divulgatore appassionato. Parole chiave. Osservatorio Astrofisico di Arcetri, Francesco Palla.

Francesco Palla, Director of the Arcetri Astrophysical Observatory up to 2011, died suddenly last January. He was also well known in Florence for his commitment to spreading knowledge of science and was one of the founders of this journal. Here we should like to remember Francesco in his role of eclectic scientist and passionate communicator.

L'astronomia extragalattica si sviluppò all'Osservatorio di Arcetri verso la fine degli anni '70, con l'arrivo del nuovo direttore Franco Pacini. La prima linea di ricerca dedicata a questo nuovo settore emerse da una collaborazione con la Cornell University sullo studio dell'idrogeno atomico. Successivamente il gruppo crebbe rapidamente durante gli anni '80 e '90 grazie all'inserimento di giovani ricercatori di talento. Oggi i temi scientifici includono lo studio dei nuclei galattici attivi, l'evoluzione chimica delle galassie, gli ammassi di galassie, la dinamica delle galassie e lo studio del mezzo interstellare. Il futuro degli studi extragalattici ad Arcetri trarrà vantaggio da strutture di osservazione all'avanguardia come l'Extremely Large Telescope, che entrerà in funzione nei prossimi anni.

We present recent results from our MAGNUM survey of nearby active galactic nuclei (AGN), which exploits observations from the optical/near-IR integral field spectrograph MUSE at VLT. We detect strongly enhanced line widths in emission line maps of four galaxies perpendicularly to their low-power jets and AGN ionisation cones, indicative of turbulent/outflowing material. The observation of a similar phenomenon in other works suggests that it originates from an interaction mechanism between the jet and the galaxy disc through which it propagates.

Planets orbiting close to hot stars experience intense extreme-ultraviolet radiation, potentially leading to atmosphere evaporation and to thermal dissociation of molecules. However, this extreme regime remains mainly unexplored due to observational challenges. Only a single known ultra-hot giant planet, KELT-9b, receives enough ultraviolet radiation for molecular dissociation, with a day-side temperature of ~4,600 K. An alternative approach uses irradiated brown dwarfs as hot-Jupiter analogues. With atmospheres and radii similar to those of giant planets, brown dwarfs orbiting close to hot Earth-sized white dwarf stars can be directly detected above the glare of the star. Here we report observations revealing an extremely irradiated low-mass companion to the hot white dwarf WD 0032–317. Our analysis indicates a day-side temperature of ~8,000 K, and a day-to-night temperature difference of ~6,000 K. The amount of extreme-ultraviolet radiation (with wavelengths 100–912 Å) received by WD 0032–317B is equivalent to that received by planets orbiting close to stars as hot as late B-type stars, and about 5,600 times higher than that of KELT-9b. With a mass of ~75–88 Jupiter masses, this near-hydrogen-burning-limit object is potentially one of the most massive brown dwarfs known.WD 0032–317B is a 75–88-Jupiter mass companion orbiting a hot white dwarf with a period of 2.3 h. It has a day-side temperature of about 8,000 K and a day–night difference of ~6,000 K. WD 0032–317B is amenable to detailed characterization and can be used as a proxy for strongly irradiated ultra-hot giant planets.

We carry out sensitive searches for the CO J=1-0 and J=2-1 lines in the giant extragalactic HI ring in Leo to investigate the star formation process within environments where gas metallicities are close to solar but physical conditions are different than those typical of bright galaxy disks. Our aim is to check the range of validity of known scaling relations. We use the IRAM-30m telescope to observe eleven regions close to HI gas peaks or where sparse young massive stars have been found. For all pointed observations we reached a spectral noise between 1 and 5~mK for at least one observed frequencies at 2~km/s spectral resolution. We marginally detect two CO J=1-0 lines in the star forming region Clump~1 of the Leo ring, whose radial velocities are consistent with those of Halpha lines but line widths are much smaller than observed for virialized molecular clouds of similar mass in galaxies. The low signal-to-noise ratio, the small line widths and the extremely low number densities suggest that a more standard population of molecular clouds, still undetected, might be in place. Using upper limits to the CO lines, the most sensitive pointed observations show that the molecular gas mass surface density is lower than expected from the extrapolation of the molecular Kennicutt-Schmidt relation established in the disk of galaxies. The sparse stellar population in the ring, possibly forming ultra diffuse dwarf galaxies, might then be the result of a short molecular gas depletion time in this extreme environment.}

Chemical abundances in the Leo ring, the largest HI cloud in the local Universe, have recently been determined to be close or above solar, incompatible with a previously claimed primordial origin of the ring. The gas, pre-enriched in a galactic disk and tidally stripped, did not manage to form stars very efficiently in intergalactic space. We map nebular lines in 3 dense HI clumps of the Leo ring and complement these data with archival stellar continuum observations to investigate the slow building up of a sparse population of stars in localized areas of the ring. Individual young stars as massive as O7-types are powering some HII regions. The average star formation rate density is of order of 10^{-5} Msun/yr/kpc^2 and proceeds with local bursts a few hundred parsecs in size, where loose stellar associations of 500-1000 Msun occasionally host massive outliers. The far ultraviolet-to-Halpha emission ratio in nebular regions implies recent stellar bursts, from 2 to 7 Myr ago. The relation between the local HI gas density and the star formation rate in the ring is similar to what is found in dwarfs and outer disks with gas depletion times as long as 100~Gyrs. We find a candidate planetary nebula in a compact and faint Halpha region with [OIII]/Halpha line enhancement, consistent with the estimated mean stellar surface brightness of the ring. The presence of 1 kpc partial ring emitting weak Halpha lines around the brightest and youngest HII region suggests that local shocks might be the triggers of new star forming events.

The metallicity of local galaxies is tightly related not only to stellar mass, i.e. the mass-metallicity relation, but also to the star formation rate (SFR) through the so-called fundamental metallicity relation (FMR); more active galaxies show lower metallicities at fixed mass. Interestingly, high-z galaxies up to z~2.5 follow the same relation defined by SDSS locally. However, different shapes have been proposed for local galaxies, and the existence of a FMR and the role of the SFR has been recently questioned by some authors. In this paper we first discuss the various parametrizations of this mass-metallicity-SFR relation that has appeared in the literature to understand the origin of their different shapes. We then reanalysed data from CALIFA and SDSS-IV MaNGA surveys, which were used to suggest no dependency of metallicity on the SFR in local galaxies. Contrary to those claims, we find that those datasets are instead fully consistent with the predictions, showing the expected dependency on the SFR at fixed mass. Finally, we analysed those high-z data whose consistency with the local relation was questioned. While an internal dependency on the SFR among the subsamples is difficult to detect at high-z because of the limited dynamic range sampled in the three parameters and the intrinsic scatter and uncertainties of such small samples, all these datasets are compatible with the relation defined locally by SDSS galaxies. This confirms the lack of evolution of the FMR in these data up to z~2.3.