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1. The formation pathways of compact elliptical galaxies

   https://doi.org/10.1093/mnras/stad2313  

   Deeley, Simon ; Drinkwater, Michael J. ; Sweet, Sarah M. ; Bekki, Kenji ; Couch, Warrick J. ; Forbes, Duncan A. ( August 2023 , Monthly Notices of the Royal Astronomical Society)

   Compact elliptical (cE) galaxies remain an elusively difficult galaxy class to study. Recent observations have suggested that isolated and host-associated cEs have different formation pathways, while simulation studies have also shown different pathways can lead to a cE galaxy. However, a solid link has not been established, and the relative contributions of each pathway in a cosmological context remains unknown. Here, we combine a spatially resolved observational sample of cEs taken from the Sydney-AAO Multi-object Integral field spectrograph Galaxy Survey with a matched sample of galaxies within the IllustrisTNG cosmological simulation to establish an overall picture of how these galaxies form. The observed cEs located near a host galaxy appear redder, smaller, and older than isolated cEs, supporting previous evidence for multiple formation pathways. Tracing the simulated cEs back through time, we find two main formation pathways; 32 ± 5 per cent formed via the stripping of a spiral galaxy by a larger host galaxy, while 68 ± 4 per cent formed through a gradual build-up of stellar mass in isolated environments. We confirm that cEs in different environments do indeed form via different pathways, with all isolated cEs in our sample having formed via in situ formation (i.e. none were ejected from a previous host), and 77 ± 6 per cent of host-associated cEs having formed via tidal stripping. Separating them by their formation pathway, we are able to reproduce the observed differences between isolated and host-associated cEs, showing that these differences can be fully explained by the different formation pathways dominating in each environment.

    

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2. The gas, metal, and dust evolution in low-metallicity local and high-redshift galaxies

   https://doi.org/10.1051/0004-6361/202037833  

   Nanni, A. ; Burgarella, D. ; Theulé, P. ; Côté, B. ; Hirashita, H. ( September 2020 , Astronomy & Astrophysics)

   null (Ed.)

   Context. The chemical enrichment in the interstellar medium (ISM) of galaxies is regulated by several physical processes: star birth and death, grain formation and destruction, and galactic inflows and outflows. Understanding such processes and their relative importance is essential to following galaxy evolution and the chemical enrichment through the cosmic epochs, and to interpreting current and future observations. Despite the importance of such topics, the contribution of different stellar sources to the chemical enrichment of galaxies, for example massive stars exploding as Type II supernovae (SNe) and low-mass stars, as well as the mechanisms driving the evolution of dust grains, such as for example grain growth in the ISM and destruction by SN shocks, remain controversial from both observational and theoretical viewpoints. Aims. In this work, we revise the current description of metal and dust evolution in the ISM of local low-metallicity dwarf galaxies and develop a new description of Lyman-break galaxies (LBGs) which are considered to be their high-redshift counterparts in terms of star formation, stellar mass, and metallicity. Our goal is to reproduce the observed properties of such galaxies, in particular (i) the peak in dust mass over total stellar mass (sMdust) observed within a few hundred million years; and (ii) the decrease in sMdust at a later time. Methods. We fitted spectral energy distribution of dwarf galaxies and LBGs with the “Code Investigating GALaxies Emission”, through which the total stellar mass, dust mass, and star formation rate are estimated. For some of the dwarf galaxies considered, the metal and gas content are available from the literature. We computed different prescriptions for metal and dust evolution in these systems (e.g. different initial mass functions for stars, dust condensation fractions, SN destruction, dust accretion in the ISM, and inflow and outflow efficiency), and we fitted the properties of the observed galaxies through the predictions of the models. Results. Only some combinations of models are able to reproduce the observed trend and simultaneously fit the observed properties of the galaxies considered. In particular, we show that (i) a top-heavy initial mass function that favours the formation of massive stars and a dust condensation fraction for Type II SNe of around 50% or more help to reproduce the peak of sMdust observed after ≈100 Myr from the beginning of the baryon cycle for both dwarf galaxies and LBGs; (ii) galactic outflows play a crucial role in reproducing the observed decline in sMdust with age and are more efficient than grain destruction from Type II SNe both in local galaxies and at high-redshift; (iii) a star formation efficiency (mass of gas converted into stars) of a few percent is required to explain the observed metallicity of local dwarf galaxies; and (iv) dust growth in the ISM is not necessary in order to reproduce the values of sMdust derived for the galaxies under study, and, if present, the effect of this process would be erased by galactic outflows. 

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3. Dwarfs in Void Environments (DIVE): The Stellar Kinematics of Void Dwarf Galaxies Using the Keck Cosmic Web Imager

   https://doi.org/10.3847/1538-4357/acd189  

   de los Reyes, Mithi A. C. ; Kirby, Evan N. ; Zhuang, Zhuyun ; Steidel, Charles C. ; Chen, Yuguang ; Wheeler, Coral ( July 2023 , The Astrophysical Journal)

   Dwarf galaxies located in extremely underdense cosmic voids are excellent test beds for disentangling the effects of large-scale environments on galaxy formation and evolution. We present the first results of the Dwarfs in Void Environments Survey, which has obtained integral field spectroscopy for low-mass galaxies (M_⋆= 10⁷–10⁹M_⊙) located inside (N= 21) and outside (N= 9) cosmic voids using the Keck Cosmic Web Imager. Using measurements of stellar line-of-sight rotational velocityv_rotand velocity dispersionσ_⋆, we test the tidal stirring hypothesis, which posits that dwarf spheroidal galaxies are formed through tidal interactions with more massive host galaxies. We measure low values ofv_rot/σ_⋆≲ 2 for our sample of isolated dwarf galaxies, and we find no trend betweenv_rot/σ_⋆and the distance from a massive galaxy$d_{L^{\star }}$out to$d_{L^{\star }}\sim 10$Mpc. These suggest that dwarf galaxies can become dispersion-supported, “puffy” systems even in the absence of environmental effects like tidal interactions. We also find indications of an upward trend betweenv_rot/σ_⋆and galaxy stellar mass, perhaps implying that stellar disk formation depends on mass rather than environment. Although some of our conclusions may be slightly modified by systematic effects, our main result still holds: that isolated low-mass galaxies may form and remain as puffy systems rather than the dynamically cold disks predicted by classical galaxy formation theory.

    

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4. SCATTER: A New Common Envelope Formalism

   https://doi.org/10.3847/1538-4357/acae9b  

   Di Stefano, Rosanne ; Kruckow, Matthias U. ; Gao, Yan ; Neunteufel, Patrick G. ; Kobayashi, Chiaki ( February 2023 , The Astrophysical Journal)

   One of the most mysterious astrophysical states is the common envelope (CE) phase of binary evolution, in which two stars are enshrouded by the envelope shed by one of them. Interactions between the stars and the envelope shrinks the orbit. The CE can lead to mergers or to a subsequent phase of interactions. Mergers may involve any combination of two compact objects and/or stars. Some involving white dwarfs may produce Type Ia supernovae, while merging neutron stars may yield gamma-ray bursts, and merging compact objects of all kinds produce gravitational radiation. Since CEs can arise from a variety of different initial conditions, and due to the complexity of the processes involved, it is difficult to predict their end states. When many systems are being considered, as in population synthesis calculations, conservation principles are generally employed. Here we use angular momentum in a new way to derive a simple expression for the final orbital separation. This method provides advantages for the study of binaries and is particularly well suited to higher-order multiples, now considered to be important in the genesis of potential mergers. Here we focus on CEs in binaries, and the follow-up paper extends our formalism to multiple-star systems within which a CE occurs.

    

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5. Young, Blue, and Isolated Stellar Systems in the Virgo Cluster. II. A New Class of Stellar System

   https://doi.org/10.3847/1538-4357/ac7c6c  

   Jones, Michael G. ; Sand, David J. ; Bellazzini, Michele ; Spekkens, Kristine ; Karunakaran, Ananthan ; Adams, Elizabeth A. K. ; Battaglia, Giuseppina ; Beccari, Giacomo ; Bennet, Paul ; Cannon, John M. ; et al ( August 2022 , The Astrophysical Journal)

   We discuss five blue stellar systems in the direction of the Virgo cluster, analogous to the enigmatic object SECCO 1 (AGC 226067). These objects were identified based on their optical and UV morphology and followed up with Hiobservations with the Very Large Array (and Green Bank Telescope), Multi Unit Spectroscopic Explorer (on the Very Large Telescope) optical spectroscopy, and Hubble Space Telescope imaging. These new data indicate that one system is a distant group of galaxies. The remaining four are extremely low mass (M_*∼ 10⁵M_⊙), are dominated by young blue stars, have highly irregular and clumpy morphologies, are only a few kiloparsecs across, yet host an abundance of metal-rich,$12+\log (\mathrm{O}/\mathrm{H})>8.2$, Hiiregions. These high metallicities indicate that these stellar systems formed from gas stripped from much more massive galaxies. Despite the young age of their stellar populations, only one system is detected in Hi, while the remaining three have minimal (if any) gas reservoirs. Furthermore, two systems are surprisingly isolated and have no plausible parent galaxy within ∼30′ (∼140 kpc). Although tidal stripping cannot be conclusively excluded as the formation mechanism of these objects, ram pressure stripping more naturally explains their properties, in particular their isolation, owing to the higher velocities, relative to the parent system, that can be achieved. Therefore, we posit that most of these systems formed from ram-pressure-stripped gas removed from new infalling cluster members and survived in the intracluster medium long enough to become separated from their parent galaxies by hundreds of kiloparsecs and that they thus represent a new type of stellar system.

    

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