Low-mass Clusters Research Articles

H α morphologies of star clusters in 16 LEGUS galaxies: Constraints on H ii region evolution time-scales

ABSTRACT The analysis of star cluster ages in tandem with the morphology of their H ii regions can provide insight into the processes that clear a cluster’s natal gas, as well as the accuracy of cluster ages and dust reddening derived from Spectral Energy Distribution (SED) fitting. We classify 3757 star clusters in 16 nearby galaxies according to their H α morphology (concentrated, partially exposed, no emission), using Hubble Space Telescope (HST) imaging from the Legacy ExtraGalactic Ultraviolet Survey (LEGUS). We find: (1) The mean SED ages of clusters with concentrated (1–2 Myr) and partially exposed H ii region morphologies (2–3 Myr) indicate a relatively early onset of gas clearing and a short (1–2 Myr) clearing time-scale. (2) The reddening of clusters can be overestimated due to the presence of red supergiants, which is a result of stochastic sampling of the IMF in low mass clusters. (3) The age-reddening degeneracy impacts the results of the SED fitting – out of 1408 clusters with M* ≥ 5000 M⊙, we find that at least 46 (3 per cent) have SED ages which appear significantly underestimated or overestimated based on H α and their environment, while the total percentage of poor age estimates is expected to be several times larger. (4) Lastly, we examine the dependence of the morphological classifications on spatial resolution. At HST resolution, our conclusions are robust to the distance range spanned by the sample (3–10 Mpc). However, analysis of ground-based H α images shows that compact and partially exposed morphologies frequently cannot be distinguished from each other.

Mass Accretion Rates of the HectoMAP Clusters of Galaxies

Abstract We estimate the mass accretion rate (MAR) of 321 clusters of galaxies in the HectoMAP Cluster Survey. The clusters span the redshift range 0.17–0.42 and the M 200 mass range ≈ (0.5–3.5) × 1014 M ⊙. The MAR estimate is based on the caustic technique along with a spherical infall model. Our analysis extends the measurement of MARs for 129 clusters at z < 0.3 from the Cluster Infall Regions in the Sloan Digital Sky Survey and the Hectospec Cluster Survey to redshift z ∼ 0.42. Averaging over redshift, low-mass clusters with masses near 0.7 × 1014 M ⊙ roughly accrete 3 × 104 M ⊙ yr−1; more massive clusters with masses near 2.8 × 1014 M ⊙ roughly accrete 1 × 105 M ⊙ yr−1. Low- and high-mass clusters increase their MAR by approximately 46% and 84%, respectively, as the redshift increases from z in the range 0.17–0.29 to z in the range 0.34–0.42. The MARs at fixed redshift increase with mass and MARs at fixed mass increase with redshift in agreement with the ΛCDM cosmological model for hierarchical structure formation. We consider the extension of MAR measurements to z ∼ 1.

A possible solution to the Milky Way’s binary-deficient retrograde stellar population

Context. The fraction of field binaries on retrograde orbits about the Milky Way is significantly lower compared to its prograde counterpart. Chemical and dynamical evidence suggests that the retrograde stellar population originates from ω Centauri, which is either the most massive globular cluster (GC) of the Milky Way or the putative core of a former dwarf galaxy. Aims. Star formation conditions required to produce the retrograde binary population are constrained assuming that the retrograde stellar population originates from ω Centauri’s progenitor. Methods. We match the observed low binary fraction with dynamical population synthesis models, including a universal initial binary population and dynamical processing in star clusters, making use of the publicly available binary population synthesis tool BIPOS1. Results. It is found that either the GC progenitor of ω Cen must have formed with a stellar density of ≈108 M⊙ pc−3 or the ω Centauri dwarf galaxy’s progenitor star cluster population must have formed in an extreme starburst with a star formation rate exceeding 1000 M⊙ yr−1 and probably a top-heavy embedded-cluster mass function with suppressed low-mass cluster formation. The separation and mass-ratio distribution for retrograde field binaries are predicted for comparison with future observations. Conclusions. A viable solution for the deficiency of binaries on retrograde orbits is presented, and star formation conditions for ω Centauri as well as orbital parameter distributions for the Milky Way’s retrograde binary population are predicted. The dwarf galaxy origin for ω Centauri is tentatively preferred within the present context.

Compact object mergers in hierarchical triples from low-mass young star clusters

Abstract A binary star orbited by an outer companion constitutes a hierarchical triple system. The outer body may excite the eccentricity of the inner binary through the von Zeipel-Lidov-Kozai (ZLK) mechanism, triggering the gravitational wave (GW) coalescence of the inner binary when its members are compact objects. Here, we study a sample of hierarchical triples with an inner black hole (BH) – BH binary, BH – neutron star (NS) binary, and BH – white dwarf (WD) binary, formed via dynamical interactions in low-mass young star clusters. Our sample of triples was obtained self-consistently from direct N-body simulations of star clusters which included up-to-date stellar evolution. We find that the inner binaries in our triples cannot merge via GW radiation alone, and the ZLK mechanism is essential to trigger their coalescence. Contrary to binaries assembled dynamically in young star clusters, binary BHs merging in triples have preferentially low mass ratios (q ≃ 0.3) and higher primary masses ($m_{\rm p} \gtrsim 40 {\, \rm M_\odot }$). We derive a local merger rate density of 0.60, 0.11 and $0.5 {-1} \, \rm Gpc^{-3}$ for BH-BH, BH-NS and BH-WD binaries, respectively. Additionally, we find that merging binaries have high eccentricities across the GW spectrum, including the LIGO-Virgo-KAGRA (LVK), LISA, and DECIGO frequencies. About 7 per cent of BH-BH and 60 per cent of BH-NS binaries will have detectable eccentricities in the LVK band. Our results indicate that the eccentricity and the mass spectrum of merging binaries are the strongest features for the identification of GW mergers from triples.

The Unusually Weak and Exceptionally Steep Radio Relic in A2108

Abstract Mergers between galaxy clusters often drive shocks into the intracluster medium, the effects of which are sometimes visible via temperature and density jumps in the X-ray and via radio emission from relativistic particles energized by the shock’s passage. A2108 was selected as a likely merger system through comparing the X-ray luminosity to the Planck Sunyaev–Zeldovich signal, where this cluster appeared highly X-ray underluminous. Follow-up observations confirmed it to be a merging low-mass cluster featuring two distinct subclusters, both with a highly disturbed X-ray morphology. Giant Metrewave Radio Telescope (GMRT) data in bands 2, 3, & 4 (covering 120–750 MHz) show an extended radio feature resembling a radio relic near the location of a temperature discontinuity in the X-rays. We measure a Mach number from the X-ray temperature jump (  X = 1.6 ± 0.2 ). Several characteristics of radio relics (location and morphology of extended radio emission) are found in A2108, making this cluster one of the few low-mass mergers (M L−M = 1.8 ± 0.4 × 1014 M ⊙) likely hosting a radio relic. The radio spectrum is exceptionally steep (α = −2 at the lowest frequencies), and the radio power is very weak (P 1.4 GHz = 1 × 1022 W Hz−1). To account for the shock/relic offset, we propose a scenario in which the shock created the relic by re-accelerating a cloud of pre-existing relativistic electrons and then moved away, leaving behind a fading relic. The electron-aging timescale derived from the high-frequency steepening in the relic spectrum is consistent with the shock travel time to the observed X-ray discontinuity. However, the lower flux in GMRT band 4 data causing the steepening could be due to instrumental limitations and deeper radio data are needed to constrain the spectral slope of the relic at high frequencies. A background cluster, 4′ from the merger, may have contributed to the ROSAT and Planck signals, but SZ modeling shows that the merger system is still X-ray underluminous, supporting the use of this approach to identifying merger-disrupted clusters.

The challenge of simulating the star cluster population of dwarf galaxies with resolved interstellar medium

ABSTRACT We present results on the star cluster properties from a series of high resolution smoothed particles hydrodynamics (SPH) simulations of isolated dwarf galaxies as part of the griffin project. The simulations at sub-parsec spatial resolution and a minimum particle mass of 4 M⊙ incorporate non-equilibrium heating, cooling, and chemistry processes, and realize individual massive stars. The simulations follow feedback channels of massive stars that include the interstellar-radiation field variable in space and time, the radiation input by photo-ionization and supernova explosions. Varying the star formation efficiency per free-fall time in the range ϵff = 0.2–50${{\ \rm per\ cent}}$ neither changes the star formation rates nor the outflow rates. While the environmental densities at star formation change significantly with ϵff, the ambient densities of supernovae are independent of ϵff indicating a decoupling of the two processes. At low ϵff, gas is allowed to collapse more before star formation, resulting in more massive, and increasingly more bound star clusters are formed, which are typically not destroyed. With increasing ϵff, there is a trend for shallower cluster mass functions and the cluster formation efficiency Γ for young bound clusters decreases from $50 {{\ \rm per\ cent}}$ to $\sim 1 {{\ \rm per\ cent}}$ showing evidence for cluster disruption. However, none of our simulations form low mass (<103 M⊙) clusters with structural properties in perfect agreement with observations. Traditional star formation models used in galaxy formation simulations based on local free-fall times might therefore be unable to capture star cluster properties without significant fine tuning.

Too dense to go through: the role of low-mass clusters in the pre-processing of satellite galaxies

ABSTRACT We study the evolution of satellite galaxies in clusters of the c-eagle simulations, a suite of 30 high-resolution cosmological hydrodynamical zoom-in simulations based on the eagle code. We find that the majority of galaxies that are quenched at z = 0 (≳80${{\ \rm per\ cent}}$) reached this state in a dense environment (log10M200[M⊙] ≥13.5). At low redshift, regardless of the final cluster mass, galaxies appear to reach their quenching state in low-mass clusters. Moreover, galaxies quenched inside the cluster that they reside in at z = 0 are the dominant population in low-mass clusters, while galaxies quenched in a different halo dominate in the most massive clusters. When looking at clusters at z > 0.5, their in situ quenched population dominates at all cluster masses. This suggests that galaxies are quenched inside the first cluster they fall into. After galaxies cross the cluster’s r200 they rapidly become quenched (≲1 Gyr). Just a small fraction of galaxies ($\lesssim 15{{\ \rm per\ cent}}$) is capable of retaining their gas for a longer period of time, but after 4 Gyr, almost all galaxies are quenched. This phenomenon is related to ram pressure stripping and is produced when the density of the intracluster medium reaches a threshold of $\rho _{\rm ICM}\, \sim 3 \times 10 ^{-5}$ nH (cm−3). These results suggest that galaxies start a rapid-quenching phase shortly after their first infall inside r200 and that, by the time they reach r500, most of them are already quenched.

Close Stellar Flybys Common in Low-mass Clusters

Abstract Numerous protoplanetary disks show distinct spiral arms features. While possibly caused by a range of processes, detailed pattern analysis points at close stellar flybys as cause for some of them. Surprisingly, these disks reside in young low-mass clusters, where close stellar flybys are expected to be rare. This fact motivated us to take a fresh look at the frequency of close flybys in low-mass clusters. In the solar neighborhood, low-mass clusters have smaller half-mass radii than their more massive counterparts. We show that this observational fact results in the mean and central stellar density of low-mass clusters being approximately the same as in high-mass clusters, which is rarely reflected in theoretical studies. We perform N-body simulations of the stellar dynamics in young clusters obeying the observed mass–radius relation. Taking the mean disk truncation radius as a proxy for the degree of influence of the environment, we find that the influence of the environment on disks is more or less the same in low- and high-mass clusters. Even the fraction of small disks (<10 au) is nearly identical. Our main conclusion is that the frequency of close flybys seems to have been severely underestimated for low-mass clusters. A testable prediction of this hypothesis is that low-mass clusters should contain 10%–15% of disks smaller than 30 au truncated by flybys. These truncated disks should be distinguishable from primordially small disks by their steep outer edge.

Signature of a Massive Rotating Metal-poor Star Imprinted in the Phoenix Stellar Stream*

Abstract The Phoenix stellar stream has a low intrinsic dispersion in velocity and metallicity that implies the progenitor was probably a low-mass globular cluster. In this work we use Magellan/Magellan Inamori Kyocera Echelle (MIKE) high-dispersion spectroscopy of eight Phoenix stream red giants to confirm this scenario. In particular, we find negligible intrinsic scatter in metallicity ( σ ( [ Fe II / H ] ) = 0.04 − 0.03 + 0.11 ) and a large peak-to-peak range in [Na/Fe] and [Al/Fe] abundance ratios, consistent with the light element abundance patterns seen in the most metal-poor globular clusters. However, unlike any other globular cluster, we also find an intrinsic spread in [Sr ii/Fe] spanning ∼1 dex, while [Ba ii/Fe] shows nearly no intrinsic spread ( σ ( [ Ba II / H ] ) = 0.03 − 0.02 + 0.10 ). This abundance signature is best interpreted as slow-neutron-capture element production from a massive fast-rotating metal-poor star (15–20 M ⊙, v ini/v crit = 0.4, [Fe/H] = −3.8). The low inferred cluster mass suggests the system would have been unable to retain supernovae ejecta, implying that any massive fast-rotating metal-poor star that enriched the interstellar medium must have formed and evolved before the globular cluster formed. Neutron-capture element production from asymptotic giant branch stars or magneto-rotational instabilities in core-collapse supernovae provide poor fits to the observations. We also report one Phoenix stream star to be a lithium-rich giant (A(Li) = 3.1 ± 0.1). At [Fe/H ] = −2.93; it is among the most metal-poor lithium-rich giants known.

Synthetic photometry of OB star clusters with stochastically sampled IMFs: analysis of models and HST observations

ABSTRACT We present a pilot library of synthetic NUV, U, B, V, and I photometry of star clusters with stochastically sampled IMFs and ionized gas for initial masses, Mi = 103, 104, and 105 M⊙; t = 1, 3, 4, and 8 Myr; Z = 0.014 and Z = 0.002; and log(US) = −2 and −3. We compare the library with predictions from deterministic models and observations of isolated low-mass (<104 M⊙) star clusters with co-spatial compact H ii regions. The clusters are located in NGC 7793, one of the nearest galaxies observed as part of the HST LEGUS and Hα-LEGUS surveys. (1) For model magnitudes that only account for the stars: (a) the residual |deterministic mag - median stochastic mag| can be ≥0.5 mag, even for Mi = 105 M⊙; and (b) the largest spread in stochastic magnitudes occurs when Wolf–Rayet stars are present. (2) For Mi = 105 M⊙: (a) the median stochastic mag with gas can be >1.0 mag more luminous than the median stochastic magnitude without gas; and (b) nebular emission lines can contribute with $\gt 50{{\ \rm per\ cent}}$ and $\gt 30{{\ \rm per\ cent}}$ to the total emission in the V and I bands, respectively. (3) Age-dating OB-star clusters via deterministic tracks in the U-B versus V-I plane is highly uncertain at Z = 0.014 for Mi ∼ 103 M⊙ and Z = 0.002 for Mi ∼ 103–105 M⊙. (4) For low-mass clusters, the V-band extinction derived with stochastic models significantly depends on the value of log(US). (5) The youngest clusters tend to have higher extinction. (6) The majority of clusters have multi-peaked age PDFs. (7) Finally, we discuss the importance of characterizing the true variance in the number of stars per mass bin in nature.

Two striking head–tail galaxies in the galaxy cluster IIZW108: insights into transition to turbulence, magnetic fields, and particle re-acceleration

ABSTRACT We present deep Jansky Very Large Array observations at 1.4 and 2.7 GHz (full polarization), as well as optical OmegaWINGS/WINGS and X-ray observations of two extended radio galaxies in the IIZW108 galaxy cluster at z = 0.04889. They show a bent tail morphology in agreement with a radio lobed galaxy falling into the cluster potential. Both galaxies are found to possess properties comparable with narrow-angle tail galaxies in the literature even though they are part of a low mass cluster. We find a spectral index steepening and an increase in fractional polarization through the galaxy jets and an ordered magnetic field component mostly aligned with the jet direction. This is likely caused by either shear due to the velocity difference of the intracluster medium and the jet fluid and/or magnetic draping of the intracluster medium across the galaxy jets. We find clear evidence that one source is showing two active galactic nuclei (AGN) outbursts from which we expect the AGN has never turned off completely. We show that pure standard electron cooling cannot explain the jet length. We demonstrate therefore that these galaxies can be used as a laboratory to study gentle re-acceleration of relativistic electrons in galaxy jets via transition from laminar to turbulent motion.

Hidden in the haystack: low-luminosity globular clusters towards the Milky Way bulge

ABSTRACT Recent wide-area surveys have enabled us to study the Milky Way with unprecedented detail. Its inner regions, hidden behind dust and gas, have been partially unveiled with the arrival of near-infrared (IR) photometric and spectroscopic data sets. Among recent discoveries, there is a population of low-mass globular clusters, known to be missing, especially towards the Galactic bulge. In this work, five new low-luminosity globular clusters located towards the bulge area are presented. They were discovered by searching for groups in the multidimensional space of coordinates, colours, and proper motions from the Gaia EDR3 catalogue and later confirmed with deeper VVV survey near-IR photometry. The clusters show well-defined red giant branches and, in some cases, horizontal branches with their members forming a dynamically coherent structure in proper motion space. Four of them were confirmed by spectroscopic follow-up with the MUSE instrument on the ESO VLT. Photometric parameters were derived, and when available, metallicities, radial velocities, and orbits were determined. The new clusters Gran 1 and 5 are bulge globular clusters, while Gran 2, 3 and 4 present halo-like properties. Preliminary orbits indicate that Gran 1 might be related to the Main Progenitor, or the so-called ‘low-energy’ group, while Gran 2, 3 and 5 appears to follow the Gaia-Enceladus/Sausage structure. This study demonstrates that the Gaia proper motions, combined with the spectroscopic follow-up and colour–magnitude diagrams, are required to confirm the nature of cluster candidates towards the inner Galaxy. High stellar crowding and differential extinction may hide other low-luminosity clusters.

Stability of Cool Cores during Galaxy Cluster Growth: A Joint Chandra/SPT Analysis of 67 Galaxy Clusters along a Common Evolutionary Track Spanning 9 Gyr

Abstract We present the results of a joint analysis of Chandra X-ray and South Pole Telescope (SPT) Sunyaev–Zel’dovich (SZ) observations targeting the first sample of galaxy clusters at 0.3 < z < 1.3, selected to be the progenitors of well-studied nearby clusters based on their expected accretion rate. We develop a new procedure in order to tackle the analysis challenge that is estimating the intracluster medium (ICM) properties of low-mass and high-redshift clusters with ∼150 X-ray counts. One of the dominant sources of uncertainty on the ICM density profile estimated with a standard X-ray analysis with such shallow X-ray data is due to the systematic uncertainty associated with the ICM temperature obtained through the analysis of the background-dominated X-ray spectrum. We show that we can decrease the uncertainty on the density profile by a factor varying between 2 and 8 with a joint deprojection of the X-ray surface brightness profile measured by Chandra and the SZ-integrated Compton parameter available in the SPT cluster catalog. We apply this technique to the whole sample of 67 clusters in order to track the evolution of the ICM core density during cluster growth. We confirm that the evolution of the gas density profile is well modeled by the combination of a fixed core and a self-similarly evolving non-cool-core profile. We show that the fraction of cool cores in this sample is remarkably stable with redshift although clusters have gained a factor of ∼4 in total mass over the past ∼9 Gyr.

Dynamics of binary black holes in low-mass young star clusters

ABSTRACT Young star clusters are dynamically active stellar systems and are a common birthplace for massive stars. Low-mass star clusters (∼300–103 M⊙) are more numerous than massive systems and are characterized by a two-body relaxation time-scale of a few Myr: the most massive stars sink to the cluster core and dynamically interact with each other even before they give birth to compact objects. Here, we explore the properties of black holes (BHs) and binary black holes (BBHs) formed in low-mass young star clusters, by means of a suite of 105 direct N-body simulations with a high original binary fraction (100 per cent for stars with mass >5 M⊙). Most BHs are ejected in the first ∼20 Myr by dynamical interactions. Dynamical exchanges are the main formation channel of BBHs, accounting for ∼40–80 per cent of all the systems. Most BBH mergers in low-mass young star clusters involve primary BHs with mass <40 M⊙ and low-mass ratios are extremely more common than in the field. Comparing our data with those of more massive star clusters (103 − 3 × 104 M⊙), we find a strong dependence of the percentage of exchanged BBHs on the mass of the host star cluster. In contrast, our results show just a mild correlation between the mass of the host star cluster and the efficiency of BBH mergers.

UGMRT detection of cluster radio emission in low-mass Planck Sunyaev–Zel’dovich clusters

ABSTRACT Low-mass (M500 < 5 × 1014 M⊙) galaxy clusters have been largely unexplored in radio observations, because of the inadequate sensitivity of existing telescopes. However, the upgraded Giant Metrewave Radio Telescope (uGMRT) and the Low Frequency ARray (LoFAR), with unprecedented sensitivity at low frequencies, have paved the way to study less massive clusters more closely than before. We have started the first large-scale programme to systematically search for diffuse radio emission from low-mass galaxy clusters, chosen from the Planck Sunyaev–Zel’dovich cluster catalogue. We report here the detection of diffuse radio emission from four of the 12 objects in our sample, shortlisted from the inspection of the LoFAR Two-Meter Sky Survey data release 1 (LoTSS-I), followed up by uGMRT Band 3 deep observations. The clusters PSZ2 G089 (Abell 1904) and PSZ2 G111 (Abell 1697) are detected with relic-like emission, while PSZ2 G106 is found to have an intermediate radio halo and PSZ2 G080 (Abell 2018) seems to be a halo-relic system. PSZ2 G089 and PSZ2 G080 are among the lowest-mass clusters discovered with a radio-relic and a halo-relic system, respectively. A high ($\sim \! 30{{\ \rm per\ cent}}$) detection rate, with powerful radio emission (P1.4 GHz ∼ 1023 W Hz−1) found in most of these objects, opens up prospects of studying radio emission in galaxy clusters over a wider mass range, to much lower-mass systems.

Revealing the Relation between Star Formation Activity of Jellyfish Galaxies and Ram Pressure Stripping

AbstractJellyfish galaxies are starburst galaxies with ram-pressure-stripped tails and blue star-forming knots. These galaxies show a snapshot of star formation enhancement triggered by ram pressure stripping (RPS), being important targets for studying the RPS-induced star formation in gas-rich galaxies. Here we investigate the star formation activity of five jellyfish galaxies in massive clusters, using Gemini GMOS/IFU observations. From the Hα-derived star formation rates (SFRs), we find that our sample shows higher SFR excess to the star formation main sequence than the jellyfish galaxies in low-mass clusters. From the compiled sample of jellyfish galaxies in low-mass to high-mass host clusters, we suggest that the star formation activity of jellyfish galaxies has positive correlations with host cluster mass and degree of RPS. These relationships imply that higher ram pressure environments tend to trigger stronger starbursts in jellyfish galaxies in the early stage of RPS.

Discovery of a Radio Halo (and Relic) in a M 500 < 2 × 1014 M ⊙ Cluster

Abstract Radio halos are diffuse synchrotron sources observed in dynamically unrelaxed galaxy clusters. Current observations and models suggest that halos trace turbulent regions in the intracluster medium where mildly relativistic particles are reaccelerated during cluster mergers. Due to the higher luminosities and detection rates with increasing cluster mass, radio halos have been mainly observed in massive systems ( M 500 ≳ 5 × 10 14 M ⊙ ). Here, we report the discovery of a radio halo with a largest linear scale of ≃750 kpc in PSZ2G145.92-12.53 (z = 0.03) using LOw Frequency ARray (LOFAR) observations at 120–168 MHz. With a mass of M 500 = ( 1.9 ± 0.2 ) × 10 14 M ⊙ and a radio power at 150 MHz of P 150 = ( 3.5 ± 0.7 ) × 10 23 W Hz−1, this is the least powerful radio halo in the least massive cluster discovered to date. Additionally, we discover a radio relic with a mildly convex morphology at ∼1.7 Mpc from the cluster center. Our results demonstrate that LOFAR has the potential to detect radio halos even in low-mass clusters, where the expectation to form them is very low (∼5%) based on turbulent reacceleration models. Together with the observation of large samples of clusters, this opens the possibility to constrain the low end of the power–mass relation of radio halos.

STARFORGE: Towards a comprehensive numerical model of star cluster formation and feedback

ABSTRACT We present STARFORGE (STAR FORmation in Gaseous Environments): a new numerical framework for 3D radiation magnetohydrodynamic (MHD) simulations of star formation that simultaneously follow the formation, accretion, evolution, and dynamics of individual stars in massive giant molecular clouds (GMCs), while accounting for stellar feedback, including jets, radiative heating and momentum, stellar winds, and supernovae. We use the gizmo code with the MFM mesh-free Lagrangian MHD method, augmented with new algorithms for gravity, time-stepping, sink particle formation and accretion, stellar dynamics, and feedback coupling. We survey a wide range of numerical parameters/prescriptions for sink formation and accretion and find very small variations in star formation history and the IMF (except for intentionally unphysical variations). Modules for mass-injecting feedback (winds, SNe, and jets) inject new gas elements on the fly, eliminating the lack of resolution in diffuse feedback cavities otherwise inherent in Lagrangian methods. The treatment of radiation uses GIZMO’s radiative transfer solver to track five frequency bands (IR, optical, NUV, FUV, ionizing), coupling direct stellar emission and dust emission with gas heating and radiation pressure terms. We demonstrate accurate solutions for SNe, winds, and radiation in problems with known similarity solutions, and show that our jet module is robust to resolution and numerical details, and agrees well with previous AMR simulations. STARFORGE can scale up to massive (>105 M⊙) GMCs on current supercomputers while predicting the stellar (≳0.1 M⊙) range of the IMF, permitting simulations of both high- and low-mass cluster formation in a wide range of conditions.

Hierarchical black hole mergers in young, globular and nuclear star clusters: the effect of metallicity, spin and cluster properties

ABSTRACTWe explore hierarchical black hole (BH) mergers in nuclear star clusters (NSCs), globular clusters (GCs) and young star clusters (YSCs), accounting for both original and dynamically assembled binary BHs (BBHs). We find that the median mass of both first- and nth-generation dynamical mergers is larger in GCs and YSCs with respect to NSCs because the lighter BHs are ejected by supernova kicks from the lower mass clusters. Also, first- and nth-generation BH masses are strongly affected by the metallicity of the progenitor stars: the median mass of the primary BH of a nth-generation merger is ∼24–38 M⊙ (∼9–15 M⊙) in metal-poor (metal-rich) NSCs. The maximum BH mass mainly depends on the escape velocity: BHs with mass up to several thousand M⊙ form in NSCs, while YSCs and GCs host BHs with mass up to several hundred M⊙. Furthermore, we calculate the fraction of mergers with at least one component in the pair-instability mass gap (fPI) and in the intermediate-mass BH regime (fIMBH). In the fiducial model for dynamical BBHs with metallicity Z = 0.002, we find fPI ≈ 0.05, 0.02 and 0.007 (fIMBH ≈ 0.01, 0.002 and 0.001) in NSCs, GCs and YSCs, respectively. Both fPI and fIMBH drop by at least one order of magnitude at solar metallicity. Finally, we investigate the formation of GW190521 by assuming that it is either a nearly equal-mass BBH or an intermediate-mass ratio inspiral.

WISDOM project – VIII. Multiscale feedback cycles in the brightest cluster galaxy NGC 0708

ABSTRACT We present high-resolution (synthesized beam size 0${_{.}^{\prime\prime}}$088 × 0${_{.}^{\prime\prime}}$083 or 25 × 23 pc2) Atacama Large Millimetre/submillimetre Array 12CO(2–1) line and 236 GHz continuum observations, as well as 5 GHz enhanced Multi-Element Radio Linked Interferometer Network (e-MERLIN) continuum observations, of NGC 0708; the brightest galaxy in the low-mass galaxy cluster Abell 262. The line observations reveal a turbulent, rotating disc of molecular gas in the core of the galaxy, and a high-velocity, blueshifted feature ≈0${_{.}^{\prime\prime}}$4 (≈113 pc) from its centre. The submillimetre continuum emission peaks at the nucleus, but extends towards this anomalous CO emission feature. No corresponding elongation is found on the same spatial scales at 5 GHz with e-MERLIN. We discuss potential causes for the anomalous blueshifted emission detected in this source, and conclude that it is most likely to be a low-mass in-falling filament of material condensing from the hot intracluster medium via chaotic cold accretion, but it is also possible that it is a jet-driven molecular outflow. We estimate the physical properties this structure has in these two scenarios, and show that either explanation is viable. We suggest future observations with integral field spectrographs will be able to determine the true cause of this anomalous emission, and provide further evidence for interaction between quenched cooling flows and mechanical feedback on both small and large scales in this source.