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The radial acceleration relation (RAR) connects the total gravitational acceleration of a galaxy at a given radius, atot(r), with that accounted for by baryons at the same radius, abar(r). The shape and tightness of the RAR for rotationally-supported galaxies have characteristics in line with MOdified Newtonian Dynamics (MOND) and can also arise within the cosmological constant + cold dark matter (ΛCDM) paradigm. We use zoom simulations of 20 galaxies with stellar masses of M⋆ ≃ 107–11 M⊙ to study the RAR in the FIRE-2 simulations. We highlight the existence of simulated galaxies with non-monotonic RAR tracks that ‘hook’ down from the average relation. These hooks are challenging to explain in Modified Inertia theories of MOND, but naturally arise in all of our ΛCDM-simulated galaxies that are dark-matter dominated at small radii and have feedback-induced cores in their dark matter haloes. We show, analytically and numerically, that downward hooks are expected in such cored haloes because they have non-monotonic acceleration profiles. We also extend the relation to accelerations below those traced by disc galaxy rotation curves. In this regime, our simulations exhibit ‘bends’ off of the MOND-inspired extrapolation of the RAR, which, at large radii, approach atot ≈ abar/fb, where fb is the cosmic baryon fraction. Future efforts to search for these hooks and bends in real galaxies will provide interesting tests for MOND and ΛCDM.

 

Ecdysteroid molting hormone synthesis is directed by a pair of molting glands or Y-organs (YOs), and this synthesis is inhibited by molt-inhibiting hormone (MIH). MIH is a member of the crustacean hyperglycemic hormone (CHH) neuropeptide superfamily, which includes CHH and insect ion transport peptide (ITP). It is hypothesized that the MIH receptor is a Class A (Rhodopsin-like) G protein-coupled receptor (GPCR). The YO of the blackback land crab,Gecarcinus lateralis, expresses 49 Class A GPCRs, three of which (Gl-CHHR-A9, -A10, and -A12) were provisionally assigned as CHH-like receptors. CrusTome, a transcriptome database assembled from 189 crustaceans and 12 ecdysozoan outgroups, was used to deorphanize candidate MIH/CHH GPCRs, relying on sequence homology to three functionally characterized ITP receptors (BNGR-A2, BNGR-A24, and BNGR-A34) in the silk moth,Bombyx mori. Phylogenetic analysis and multiple sequence alignments across major taxonomic groups revealed extensive expansion and diversification of crustacean A2, A24, and A34 receptors, designatedCHHFamilyReceptorCandidates (CFRCs). The A2 clade was divided into three subclades; A24 clade was divided into five subclades; and A34 was divided into six subclades. The subclades were distinguished by conserved motifs in extracellular loop (ECL) 2 and ECL3 in the ligand-binding region. Eleven of the 14 subclades occurred in decapod crustaceans. InG. lateralis, seven CFRC sequences, designated Gl-CFRC-A2α1, -A24α, -A24β1, -A24β2, -A34α2, -A34β1, and -A34β2, were identified; the three A34 sequences corresponded to Gl-GPCR-A12, -A9, and A10, respectively. ECL2 in all the CFRC sequences had a two-stranded β-sheet structure similar to human Class A GPCRs, whereas the ECL2 of decapod CFRC-A34β1/β2 had an additional two-stranded β-sheet. We hypothesize that this second β-sheet on ECL2 plays a role in MIH/CHH binding and activation, which will be investigated further with functional assays.

 

Understanding the evolution of satellite galaxies of the Milky Way (MW) and M31 requires modelling their orbital histories across cosmic time. Many works that model satellite orbits incorrectly assume or approximate that the host halo gravitational potential is fixed in time and is spherically symmetric or axisymmetric. We rigorously benchmark the accuracy of such models against the FIRE-2 cosmological baryonic simulations of MW/M31-mass haloes. When a typical surviving satellite fell in ($3.4\!-\!9.7\, \rm {Gyr}$ ago), the host halo mass and radius were typically 26–86 per cent of their values today, respectively. Most of this mass growth of the host occurred at small distances, $r\lesssim 50\, \rm {kpc}$, opposite to dark matter only simulations, which experience almost no growth at small radii. We fit a near-exact axisymmetric gravitational potential to each host at z = 0 and backward integrate the orbits of satellites in this static potential, comparing against the true orbit histories in the simulations. Orbital energy and angular momentum are not well conserved throughout an orbital history, varying by 25 per cent from their current values already $1.6\!-\!4.7\, \rm {Gyr}$ ago. Most orbital properties are minimally biased, ≲10 per cent, when averaged across the satellite population as a whole. However, for a single satellite, the uncertainties are large: recent orbital properties, like the most recent pericentre distance, typically are ≈20 per cent uncertain, while earlier events, like the minimum pericentre or the infall time, are ≈40–80 per cent uncertain. Furthermore, these biases and uncertainties are lower limits, given that we use near-exact host mass profiles at z = 0.

 

We study the kinematics of stars both at their formation and today within 14 Milky Way (MW)-mass galaxies from the FIRE-2 cosmological zoom-in simulations. We quantify the relative importance of cosmological disc settling and post-formation dynamical heating. We identify three eras: a Pre-Disc Era (typically ≳ 8 Gyr ago), when stars formed on dispersion-dominated orbits; an Early-Disc Era (≈8–4 Gyr ago), when stars started to form on rotation-dominated orbits but with high velocity dispersion, σform; and a Late-Disc Era (≲ 4 Gyr ago), when stars formed with low σform. σform increased with time during the Pre-Disc Era, peaking ≈8 Gyr ago, then decreased throughout the Early-Disc Era as the disc settled and remained low throughout the Late-Disc Era. By contrast, the dispersion measured today, σnow, increases monotonically with age because of stronger post-formation heating for Pre-Disc stars. Importantly, most of σnow was in place at formation, not added post-formation, for stars younger than ≈10 Gyr. We compare the evolution of the three velocity components: at all times, σR, form > σϕ, form > σZ, form. Post-formation heating primarily increased σR at ages ≲ 4 Gyr but acted nearly isotropically for older stars. The kinematics of young stars in FIRE-2 broadly agree with the range observed across the MW, M31, M33, and PHANGS-MUSE galaxies. The lookback time that the disc began to settle correlates with its dynamical state today: earlier-settling galaxies currently form colder discs. Including stellar cosmic-ray feedback does not significantly change disc rotational support at fixed stellar mass.

 

We present an in-depth analysis of gas morphologies for a sample of 25 Milky Way–like galaxies from the IllustrisTNG TNG50 simulation. We constrain the morphology of cold, warm, hot gas, and gas particles as a whole using a local shell iterative method and explore its observational implications by computing the hard-to-soft X-ray ratio, which ranges between 10⁻³and 10⁻²in the inner ∼50 kpc of the distribution and 10⁻⁵–10⁻⁴at the outer portion of the hot gas distribution. We group galaxies into three main categories: simple, stretched, and twisted. These categories are based on the radial reorientation of the principal axes of the reduced inertia tensor. We find that a vast majority (77%) of the galaxies in our sample exhibit twisting patterns in their radial profiles. Additionally, we present detailed comparisons between (i) the gaseous distributions belonging to individual temperature regimes, (ii) the cold gas distributions and stellar distributions, and (iii) the gaseous distributions and dark matter (DM) halos. We find a strong correlation between the morphological properties of the cold gas and stellar distributions. Furthermore, we find a correlation between gaseous distributions with a DM halo that increases with gas temperature, implying that we may use the warm–hot gaseous morphology as a tracer to probe the DM morphology. Finally, we show gaseous distributions exhibit significantly more prolate morphologies than the stellar distributions and DM halos, which we hypothesize is due to stellar and active galactic nucleus feedback.

 

Low-mass galaxies are highly susceptible to environmental effects that can efficiently quench star formation. We explore the role of ram pressure in quenching low-mass galaxies ($M_{*}\sim 10^{5}{-}10^{9}\, \rm {M}_{\odot }$) within 2 Mpc of Milky Way (MW) hosts using the FIRE-2 simulations. Ram pressure is highly variable across different environments, within individual MW haloes, and for individual low-mass galaxies over time. The impulsiveness of ram pressure – the maximum ram pressure scaled to the integrated ram pressure prior to quenching – correlates with whether a galaxy is quiescent or star forming. The time-scale between maximum ram pressure and quenching is anticorrelated with impulsiveness, such that high impulsiveness corresponds to quenching time-scales <1 Gyr. Galaxies in low-mass groups ($M_\mathrm{*,host}10^{7}{-}10^{9}\, \rm {M}_{\odot }$) outside of MW haloes experience typical ram pressure only slightly lower than ram pressure on MW satellites, helping to explain effective quenching via group preprocessing. Ram pressure on MW satellites rises sharply with decreasing distance to the host, and, at a fixed physical distance, more recent pericentre passages are typically associated with higher ram pressure because of greater gas density in the inner host halo at late times. Furthermore, the ram pressure and gas density in the inner regions of Local Group-like paired host haloes are higher at small angles off the host galaxy disc compared to isolated hosts. The quiescent fraction of satellites within these low-latitude regions is also elevated in the simulations and observations, signaling possible anisotropic quenching via ram pressure around MW-mass hosts.

 

Transcriptomes from nontraditional model organisms often harbor a wealth of unexplored data. Examining these data sets can lead to clarity and novel insights in traditional systems, as well as to discoveries across a multitude of fields. Despite significant advances in DNA sequencing technologies and in their adoption, access to genomic and transcriptomic resources for nontraditional model organisms remains limited. Crustaceans, for example, being among the most numerous, diverse, and widely distributed taxa on the planet, often serve as excellent systems to address ecological, evolutionary, and organismal questions. While they are ubiquitously present across environments, and of economic and food security importance, they remain severely underrepresented in publicly available sequence databases. Here, we present CrusTome, a multispecies, multitissue, transcriptome database of 201 assembled mRNA transcriptomes (189 crustaceans, 30 of which were previously unpublished, and 12 ecdysozoans for phylogenetic context) as an evolving and publicly available resource. This database is suitable for evolutionary, ecological, and functional studies that employ genomic/transcriptomic techniques and data sets. CrusTome is presented in BLAST and DIAMOND formats, providing robust data sets for sequence similarity searches, orthology assignments, phylogenetic inference, etc. and thus allowing for straightforward incorporation into existing custom pipelines for high-throughput analyses. In addition, to illustrate the use and potential of CrusTome, we conducted phylogenetic analyses elucidating the identity and evolution of the cryptochrome/photolyase family of proteins across crustaceans.

 

We employ the Feedback In Realistic Environments (FIRE-2) physics model to study how the properties of giant molecular clouds (GMCs) evolve during galaxy mergers. We conduct a pixel-by-pixel analysis of molecular gas properties in both the simulated control galaxies and galaxy major mergers. The simulated GMC pixels in the control galaxies follow a similar trend in a diagram of velocity dispersion (σ_v) versus gas surface density (Σ_mol) to the one observed in local spiral galaxies in the Physics at High Angular resolution in Nearby GalaxieS (PHANGS) survey. For GMC pixels in simulated mergers, we see a significant increase of a factor of 5–10 in both Σ_molandσ_v, which puts these pixels above the trend of PHANGS galaxies in theσ_vversus Σ_moldiagram. This deviation may indicate that GMCs in the simulated mergers are much less gravitationally bound compared with simulated control galaxies with virial parameters (α_vir) reaching 10–100. Furthermore, we find that the increase inα_virhappens at the same time as the increase in global star formation rate, which suggests that stellar feedback is responsible for dispersing the gas. We also find that the gas depletion time is significantly lower for high-α_virGMCs during a starburst event. This is in contrast to the simple physical picture that low-α_virGMCs are easier to collapse and form stars on shorter depletion times. This might suggest that some other physical mechanisms besides self-gravity are helping the GMCs in starbursting mergers collapse and form stars.

 

ABSTRACT We introduce a suite of cosmological volume simulations to study the evolution of galaxies as part of the Feedback in Realistic Environments project. FIREbox, the principal simulation of the present suite, provides a representative sample of galaxies (∼1000 galaxies with $M_{\rm star}\gt 10^8\, M_\odot$ at z  = 0) at a resolution ($\Delta {}x\sim {}20\, {\rm pc}$ , $m_{\rm b}\sim {}6\times {}10^4\, M_\odot$ ) comparable to state-of-the-art galaxy zoom-in simulations. FIREbox captures the multiphase nature of the interstellar medium in a fully cosmological setting (L = 22.1 Mpc) thanks to its exceptionally high dynamic range (≳106) and the inclusion of multichannel stellar feedback. Here, we focus on validating the simulation predictions by comparing to observational data. We find that star formation rates, gas masses, and metallicities of simulated galaxies with $M_{\rm star}\lt 10^{10.5-11}\, M_\odot$ broadly agree with observations. These galaxy scaling relations extend to low masses ($M_{\rm star}\sim {}10^7\, M_\odot$ ) and follow a (broken) power-law relationship. Also reproduced are the evolution of the cosmic HI density and the HI column density distribution at z ∼ 0–5. At low z , FIREbox predicts a peak in the stellar-mass–halo-mass relation but also a higher abundance of massive galaxies and a higher cosmic star formation rate density than observed, showing that stellar feedback alone is insufficient to reproduce the properties of massive galaxies at late times. Given its high resolution and sample size, FIREbox offers a baseline prediction of galaxy formation theory in a ΛCDM Universe while also highlighting modelling challenges to be addressed in next-generation galaxy simulations. 

We investigate the formation of Milky Way–mass galaxies using FIRE-2 ΛCDM cosmological zoom-in simulations by studying the orbital evolution of stars formed in the main progenitor of the galaxy, from birth to the present day. We classify in situ stars as isotropic spheroid, thick-disc, and thin-disc according to their orbital circularities and show that these components are assembled in a time-ordered sequence from early to late times, respectively. All simulated galaxies experience an early phase of bursty star formation that transitions to a late-time steady phase. This transition coincides with the time that the inner CGM virializes. During the early bursty phase, galaxies have irregular morphologies and new stars are born on radial orbits; these stars evolve into an isotropic spheroidal population today. The bulk of thick-disc stars form at intermediate times, during a clumpy-disc ‘spin-up’ phase, slightly later than the peak of spheroid formation. At late times, once the CGM virializes and star formation ‘cools down,’ stars are born on circular orbits within a narrow plane. Those stars mostly inhabit thin discs today. Broadly speaking, stars with disc-like or spheroid-like orbits today were born that way. Mergers on to discs and secular processes do affect kinematics in our simulations, but play only secondary roles in populating thick-disc and in situ spheroid populations at z = 0. The age distributions of spheroid, thick disc, and thin disc populations scale self-similarly with the steady-phase transition time, which suggests that morphological age dating can be linked to the CGM virialization time in galaxies.