Massive Hosts Research Articles

Environmental Quenching of Low-surface-brightness Galaxies Near Hosts from Large Magellanic Cloud to Milky Way Mass Scales

Low-surface-brightness galaxies (LSBGs) are excellent probes of quenching and other environmental processes near massive galaxies. We study an extensive sample of LSBGs near massive hosts in the local universe that are distributed across a diverse range of environments. The LSBGs with surface-brightness μeff,g>24.2magarcsec−2 are drawn from the Dark Energy Survey Year 3 catalog while the hosts with masses 9.0<log(M⋆/M⊙)<11.0 comparable to the Milky Way and the Large Magellanic Cloud are selected from the z0MGS sample. We study the projected radial density profiles of LSBGs as a function of their color and surface brightness around hosts in both the rich Fornax–Eridanus cluster environment and the low-density field. We detect an overdensity with respect to the background density, out to 2.5 times the virial radius for both hosts in the cluster environment and the isolated field galaxies. When the LSBG sample is split by g − i color or surface brightness μ eff, g , we find the LSBGs closer to their hosts are significantly redder and brighter, like their high-surface-brightness counterparts. The LSBGs form a clear “red sequence” in both the cluster and isolated environments that is visible beyond the virial radius of the hosts. This suggests preprocessing of infalling LSBGs and a quenched backsplash population around both host samples. More so, the relative prominence of the “blue cloud” feature implies that preprocessing is ongoing near the isolated hosts compared to the cluster environment where the LSBGs are already well processed.

Galaxy Quenching with Mass Growth History of Galaxy Groups and Clusters: The Importance of Post-processing

We investigate the fraction of quenched satellite galaxies in host galaxy groups and clusters using TNG300 in the IllustrisTNG cosmological magnetohydrodynamical simulations. The simulations show that most satellites are quenched after they fall into their final hosts, and that post-processing is a more dominant mechanism of galaxy quenching than pre-processing. We find that the fraction of quenched satellites at z = 0 increases with host mass, which implies that more massive hosts have higher quenching efficiency because they have more massive groups infalling. Furthermore, we find that hosts that have many early-infall satellites show a higher fraction of quenched satellites at z = 0 than those that have many late-infall satellites, which results in a scatter of the quenched fraction of satellites in a given mass range of hosts at z = 0. Our results highlight the significance of the mass of hosts and the different infall times of satellites in understanding galaxy quenching.

Jellyfish galaxies with the IllustrisTNG simulations – when, where, and for how long does ram pressure stripping of cold gas occur?

ABSTRACT Jellyfish galaxies are prototypical examples of satellite galaxies undergoing strong ram pressure stripping (RPS). We analyse the evolution of 512 unique, first-infalling jellyfish galaxies from the TNG50 cosmological simulation. These have been visually inspected to be undergoing RPS sometime in the past 5 Byr (since z = 0.5), have satellite stellar masses $M_\star ^{\rm sat}\sim 10^{8\!-\!10.5}\, {\rm M}_\odot$, and live in hosts with $M_{\rm 200c}\sim 10^{12\!-\!14.3}\, {\rm M}_\odot$ at z = 0. We quantify the cold gas (T ≤ 104.5 K) removal using the tracer particles, confirming that for these jellyfish, RPS is the dominant driver of cold gas loss after infall. Half of these jellyfish are completely gas-less by z = 0, and these galaxies have earlier infall times and smaller satellite-to-host mass ratios than their gaseous counterparts. RPS can act on jellyfish galaxies over long time-scales of ≈1.5–8 Gyr. Jellyfish in more massive hosts are impacted by RPS for a shorter time span and, at a fixed host mass, jellyfish with less cold gas at infall and lower stellar masses at z = 0 have shorter RPS time spans. While RPS may act for long periods of time, the peak RPS period – where at least 50 per cent of the total RPS occurs – begins within ≈1 Gyr of infall and lasts ≲2 Gyr. During this period, the jellyfish are at host-centric distances ∼0.2–2R200c, illustrating that much of RPS occurs at large distances from the host galaxy. Interestingly, jellyfish continue forming stars until they have lost ≈98 per cent of their cold gas. For groups and clusters in TNG50 $(M_{\rm 200c}^{\rm host}\sim 10^{13\!-\!14.3}\, {\rm M}_\odot)$, jellyfish galaxies deposit more cold gas ($\sim 10^{11\!-\!12}\, {\rm M}_\odot$) into haloes than what exists in them at z = 0, demonstrating that jellyfish, and in general satellite galaxies, are a significant source of cold gas accretion.

Satellites of Milky Way- and M31-like galaxies with TNG50: quenched fractions, gas content, and star formation histories

ABSTRACT We analyse the quenched fractions, gas content, and star formation histories of ∼1200 satellite galaxies with M* ≥ 5 × 106 M⊙ around 198 Milky Way- (MW) and Andromeda-like (M31) hosts in TNG50, the highest-resolution simulation of IllustrisTNG. Satellite quenched fractions are larger for smaller masses, for smaller distances to their host galaxy, and in the more massive M31-like compared to MW-like hosts. As satellites cross their host’s virial radius, their gas content drops: Most satellites within 300 kpc lack detectable gas reservoirs at z = 0, unless they are massive like the Magellanic Clouds and M32. Nevertheless, their stellar assembly exhibits a large degree of diversity. On average, the cumulative star formation histories are more extended for brighter, more massive satellites with a later infall, and for those in less massive hosts. Based on these relationships, we can even infer infall periods for observed MW and M31 dwarfs, e.g. 0–4 Gyr ago for the Magellanic Clouds and Leo I, and 4–8 and 0–2 Gyr ago for M32 and IC 10, respectively. Ram pressure stripping (in combination with tidal stripping) deprives TNG50 satellites of their gas reservoirs and ultimately quenches their star formation, even though only a few per cent of the present-day satellites around the 198 TNG50 MW/M31-like hosts appear as jellyfish. The typical time since quenching for currently quenched TNG50 satellites is $6.9\substack{+2.5\\-3.3}$ Gyr ago. The TNG50 results are consistent with the quenched fractions and stellar assembly of observed MW and M31 satellites, however, satellites of the SAGA survey with M* ∼ 108–109 M⊙ exhibit lower quenched fractions than TNG50 and other, observed analogues.

Photometric and structural parameters of newly discovered nuclear star clusters in Local Volume galaxies

ABSTRACTWe use high-resolution Hubble Space Telescope imaging data of dwarf galaxies in the Local Volume ($\lesssim {11}\, \mathrm{Mpc}$) to parameterize 19 newly discovered nuclear star clusters (NSCs). Most of the clusters have stellar masses of $M_{\star }^{\mathrm{nsc}} \lesssim 10^{6}{\, {\rm M}_{\odot }}$ and compare to Galactic globular clusters in terms of ellipticity, effective radius, stellar mass, and surface density. The clusters are modelled with a Sérsic profile and their surface brightness evaluated at the effective radius reveals a tight positive correlation to the host galaxy stellar mass. Our data also indicate an increase in slope of the density profiles with increasing mass, perhaps indicating an increasing role for in situ star formation in more massive hosts. We evaluate the scaling relation between the clusters and their host galaxy stellar mass to find an environmental dependence: for NSCs in field galaxies, the slope of the relation is $\alpha = 0.82^{+0.08}_{-0.08}$ whereas $\alpha = 0.55^{+0.06}_{-0.05}$ for dwarfs in the core of the Virgo cluster. Restricting the fit for the cluster to $M_{\star }^{\mathrm{gal}} \ge 10^{6.5}{\, {\rm M}_{\odot }}$ yields $\alpha = 0.70^{+0.08}_{-0.07}$, in agreement with the field environment within the 1σ interval. The environmental dependence is due to the lowest-mass nucleated galaxies and we speculate that this is either due to an increased number of progenitor globular clusters merging to become an NSC, or due to the formation of more massive globular clusters in dense environments, depending on the initial globular cluster mass function. Our results clearly corroborate recent results in that there exists a tight connection between NSCs and globular clusters in dwarf galaxies.

ΛCDM halo substructure properties revealed with high-resolution and large-volume cosmological simulations

ABSTRACT In this work, we investigate the structural properties, distribution and abundance of ΛCDM dark matter subhaloes using the Phi-4096 and Uchuu suite of N-body cosmological simulations. Thanks to the combination of their large volume, high mass resolution and superb statistics, we are able to quantify – for the first time consistently over more than seven decades in ratio of subhalo-to-host-halo mass – dependencies of subhalo properties on mass, maximum circular velocity, Vmax, host halo mass, and distance to host halo centre. We also dissect the evolution of these dependencies over cosmic time. We provide accurate fits for the subhalo mass and velocity functions, both exhibiting decreasing power-law slopes and with no significant dependence on redshift. We also find subhalo abundance to depend weakly on host halo mass. Subhalo structural properties are codified via a concentration parameter, cV, that does not depend on any pre-defined density profile and relies only on Vmax. We derive the cV−Vmax relation and find an important dependence on distance of the subhalo to the host halo centre. Interestingly, we also find subhaloes of the same mass to be significantly more concentrated when they reside inside more massive hosts. Finally, we investigate the redshift evolution of cV, and provide accurate fits. Our results offer an unprecedented detailed characterization of the subhalo population, consistent over a wide range of subhalo and host halo masses, as well as cosmic times. Thus, we expect our work to be particularly useful for any future research involving dark matter halo substructure.

The present-day gas content of simulated field dwarf galaxies

ABSTRACT We examine the gas content of field dwarf galaxies in a high-resolution cosmological simulation. In agreement with previous work, we find that galaxies inhabiting dark matter haloes with mass below a critical value, M200 ≲ Mcrit ≈ 5 × 109 M⊙, are quiescent at the present day. The gas content of these galaxies is thus insensitive to feedback from evolving stars. Almost half of these quiescent systems today have gas masses much smaller than that expected for their mass. We find that gas-deficient galaxies originate from (1) past interactions with massive hosts, in which a dwarf loses gas and dark matter via tidal and ram-pressure forces; and (2) from hydrodynamic interactions with the gaseous filaments and sheets of the cosmic web, in which a dwarf loses gas via ram pressure. We refer to these systems as ‘flybys’ and ‘COSWEBs’. Flybys locate in high-density regions, tracing the location of the most massive galaxies in the simulation. In contrast, COSWEBs are dispersed throughout the volume and trace the cosmic web. For sub-critical systems, M200 &amp;lt; Mcrit, the fraction of COSWEB galaxies can be as high as $35 {{\ \rm per\ cent}}$, and much higher for flybys, which make up 100 per cent of the galaxies with $M_{200}\lt 3\times 10^8 \ \rm M_{\odot }$. The deficit of gas caused by these mechanisms may preclude the detection of a large fraction of field dwarfs in future H i surveys. For galaxies inhabiting haloes with mass M200 &amp;gt; Mcrit, we find that cosmic web stripping, on average, shuts down star formation in more than $70{{\ \rm per\ cent}}$ of the affected systems.

The Missing Satellite Problem outside of the Local Group. II. Statistical Properties of Satellites of Milky Way–like Galaxies

We present a new observation of satellite galaxies around seven Milky Way (MW)–like galaxies located outside of the Local Group (LG) using Subaru/Hyper Suprime-Cam imaging data to statistically address the missing satellite problem. We select satellite galaxy candidates using magnitude, surface brightness, Sérsic index, axial ratio, FWHM, and surface brightness fluctuation cuts, followed by visual screening of false positives such as optical ghosts of bright stars. We identify 51 secure dwarf satellite galaxies within the virial radius of nine host galaxies, two of which are drawn from the pilot observation presented in Paper I. We find that the average luminosity function of the satellite galaxies is consistent with that of the MW satellites, although the luminosity function of each host galaxy varies significantly. We observe an indication that more massive hosts tend to have a larger number of satellites. Physical properties of the satellites such as the size–luminosity relation are also consistent with the MW satellites. However, the spatial distribution is different; we find that the satellite galaxies outside of the LG show no sign of concentration or alignment, while that of the MW satellites is more concentrated around the host and exhibits a significant alignment. As we focus on relatively massive satellites with M V < −10, we do not expect that the observational incompleteness can be responsible here. This trend might represent a peculiarity of the MW satellites, and further work is needed to understand its origin.

Discovery of a Bimodal Environmental Distribution of Compact Ellipticals in the Local Universe

Low-mass compact stellar systems (CSSs; M ⋆ < 1010 M ⊙) are thought to be a mixed bag of objects with various formation mechanisms. Previous surveys of CSSs were biased to relatively high-density environments and cannot provide a complete view of the environmental dependence of the formation of CSSs. We conduct the first-ever unbiased flux-limited census of nearby quiescent CSSs over a total sky area of ∼200 deg2 observed by the GAMA spectroscopic survey. The complete sample includes 82 quiescent CSSs, of which 85% fall within the stellar mass range of classical compact ellipticals (cEs). By quantifying the local environment with the normalized projected distance D/R vir to the nearest luminous neighboring galaxy, we find that these CSSs have a bimodal D/R vir distribution, with one group peaking near ∼0.1 × R vir (satellite) and the other peaking near ∼10 × R vir (field). In contrast to the CSSs, ordinary quiescent galaxies of similar masses have a unimodal D/R vir distribution. Satellite CSSs are older and more metal-rich than field CSSs on average. The bimodal D/R vir distribution of quiescent CSSs reinforces the existence of two distinct formation channels (tidal stripping and born-to-be) for cEs and may be understood in two mutually inclusive perspectives, i.e., substantial tidal stripping happens only when satellite galaxies travel sufficiently close to their massive hosts, and there exists an excess of high-density cE-bearing subhalos close to massive halos.

A scaled-up planetary system around a supernova progenitor

Context. Virtually all known exoplanets reside around stars with M &lt; 2.3 M⊙ either due to the rapid evaporation of the protostellar disks or to selection effects impeding detections around more massive stellar hosts. Aims. To clarify if this dearth of planets is real or a selection effect, we launched the planet-hunting B-star Exoplanet Abundance STudy (BEAST) survey targeting B stars (M &gt; 2.4 M⊙) in the young (5−20 Myr) Scorpius-Centaurus association by means of the high-contrast spectro-imager SPHERE at the Very Large Telescope. Methods. In this paper we present the analysis of high-contrast images of the massive (M ~ 9 M⊙) star μ2 Sco obtained within BEAST. We carefully examined the properties of this star, combining data from Gaia and from the literature, and used state-of-the-art algorithms for the reduction and analysis of our observations. Results. Based on kinematic information, we found that μ2 Sco is a member of a small group which we label Eastern Lower Scorpius within the Scorpius-Centaurus association. We were thus able to constrain its distance, refining in turn the precision on stellar parameters. Around this star we identify a robustly detected substellar companion (14.4 ± 0.8 MJ)at a projected separation of 290 ± 10 au, and a probable second similar object (18.5 ± 1.5 MJ) at 21 ± 1 au. The planet-to-star mass ratios of these objects are similar to that of Jupiter to the Sun, and the flux they receive from the star is similar to those of Jupiter and Mercury, respectively. Conclusions. The robust and the probable companions of μ2 Sco are naturally added to the giant 10.9 MJ planet recently discovered by BEAST around the binary b Cen system. While these objects are slightly more massive than the deuterium burning limit, their properties are similar to those of giant planets around less massive stars and they are better reproduced by assuming that they formed under a planet-like, rather than a star-like scenario. Irrespective of the (needed) confirmation of the inner companion, μ2 Sco is the first star that would end its life as a supernova that hosts such a system. The tentative high frequency of BEAST discoveries is unexpected, and it shows that systems with giant planets or small-mass brown dwarfs can form around B stars. When putting this finding in the context of core accretion and gravitational instability formation scenarios, we conclude that the current modeling of both mechanisms is not able to produce this kind of companion. The completion of BEAST will pave the way for the first time to an extension of these models to intermediate and massive stars.

Comparison of the star formation in X-ray-selected AGN in eFEDS with that of star-forming galaxies

We use approximately 1800 X-ray active galactic nuclei (AGN) in the eROSITA Final Equatorial-Depth Survey (eFEDS) that span over two orders of magnitude in X-ray luminosity, LX, 2−10 keV ≈ 1043−45 erg s−1, and compare their star-formation rate (SFR) to that of non-AGN star-forming systems, at 0.5 &lt; z &lt; 1.5. To this end, we compiled a reference galaxy catalogue of about 17 000 sources. Both samples have the same photometric coverage, from optical to far-infrared. We construct the spectral energy distributions (SEDs) of all sources and fit them using the CIGALE code, using the same templates and parametric grid for both samples, and accounting for the mass incompleteness and exclude quiescent systems from both datasets. This allows us to compare the SFR of the two populations in a uniform manner, minimising systematic effects. Based on our analysis, AGN at low and moderate LX (LX, 2−10 keV &lt; 1044 erg s−1) have SFR that is lower than, or at most equal to, that of star-forming galaxies, in agreement with previous studies. The large number of luminous X-ray AGN available in the eFEDS dataset enable us to expand our investigations at higher LX to test previous, tentative results. At LX, 2−10 keV &gt; 1044.2 erg s−1, the SFR of AGN appears enhanced by ∼30% compared to that of star-forming sources, for systems with stellar mass in the range of 10.5 &lt; log [M*(M⊙)] &lt; 11.5, confirming indications found in previous studies. The most massive sources, log [M*(M⊙)] &gt; 11.5, present a flat SFRnorm–LX relation up to LX, 2−10 keV ∼ 1044.5 erg s−1, with SFR similar to that of star-forming galaxies. However, at higher LX (LX, 2−10 keV ∼ 1045 erg s−1), we find indications that the SFR of these massive AGN hosts may be enhanced compared to that of non-AGN systems.

A New Method to Constrain the Appearance and Disappearance of Observed Jellyfish Galaxy Tails

We present a new approach for observationally constraining where the tails of Jellyfish (JF) galaxies in groups and clusters first appear and how long they remain visible for with respect to the moment of their orbital pericenter. This is accomplished by measuring the distribution of their tail directions, with respect to their host centers, and their distribution in a projected velocity–radius phase-space diagram. We then model these observed distributions using a fast and flexible approach, where JF tails are painted onto dark matter halos, according to a simple parameterized prescription, before a Bayesian analysis is performed to estimate the parameters. We demonstrate the effectiveness of our approach using observational mocks, then apply it to a known observational sample of 106 JF galaxies, with radio-continuum tails located inside 68 hosts such as groups and clusters. We find that, typically, the radio-continuum tails become visible on first infall, when the galaxy reaches roughly three-quarters of r 200, and the tails remain visible for a few hundred Myr after pericenter passage. Lower-mass galaxies in more massive hosts tend to form visible tails further out and their tails disappear more quickly after pericenter. We argue that this indicates that they are more sensitive to ram pressure stripping. With upcoming large-area surveys of JF galaxies in progress, this is a promising new method for constraining the environmental conditions in which visible JF tails exist.

Being KLEVER at cosmic noon: Ionized gas outflows are inconspicuous in low-mass star-forming galaxies but prominent in massive AGN hosts

ABSTRACTWe investigate the presence of ionized gas outflows in a sample of 141 main-sequence star-forming galaxies at 1.2 &amp;lt; z &amp;lt; 2.6 from the KLEVER (KMOS Lensed Emission Lines and VElocity Review) survey. Our sample covers an exceptionally wide range of stellar masses, 8.1 &amp;lt; log (M⋆/M⊙) &amp;lt; 11.3, pushing outflow studies into the dwarf regime thanks to gravitationally lensed objects. We stack optical rest-frame emission lines (H β, [O iii], H α, and [N ii]) in different mass bins and seek for tracers of gas outflows by using a novel, physically motivated method that improves over the widely used, simplistic double Gaussian fitting. We compare the observed emission lines with the expectations from a rotating disc (disc + bulge for the most massive galaxies) model, whereby significant deviations are interpreted as a signature of outflows. We find clear evidence for outflows in the most massive, log (M⋆/M⊙) &amp;gt; 10.8, AGN-dominated galaxies, suggesting that AGNs may be the primary drivers of these gas flows. Surprisingly, at log (M⋆/M⊙) ≤ 9.6, the observed line profiles are fully consistent with a rotating disc model, indicating that ionized gas outflows in dwarf galaxies might play a negligible role even during the peak of cosmic star-formation activity. Finally, we find that the observed mass loading factor scales with stellar mass as expected from the TNG50 cosmological simulation, but the ionized gas mass accounts for less than 2 ${{\ \rm per\ cent}}$ of the predicted value. This suggests that either the bulk of the outflowing mass is in other gaseous phases or the current feedback models implemented in cosmological simulations need to be revised.

The Type II AGN-host galaxy connection

We present a study of optically selected Type II active galactic nuclei (AGN) at 0.5 &lt; z &lt; 0.9 from the VIPERS and VVDS surveys, to investigate the connection between AGN activity and the physical properties of their host galaxies. The host stellar mass is estimated through spectral energy distribution fitting with the CIGALE code, and star formation rates are derived from the [OII]λ3727 Å line luminosity. We find that 49% of the AGN host galaxies are on or above the main sequence (MS), 40% lie in the sub-MS locus, and 11% in the quiescent locus. Using the [OIII]λ5007 Å line luminosity as a proxy of the AGN power, we find that at fixed AGN power Type II AGN host galaxies show a bimodal behaviour: systems with host galaxy stellar mass &lt; 1010 M⊙ reside along the MS or in the starbursts locus (high-SF Type II AGN), while systems residing in massive host galaxies (&gt; 1010 M⊙) show a lower level of star formation (low-SF Type II AGN). At all stellar masses the offset from the MS is positively correlated with the AGN power. We interpret this correlation as evidence of co-evolution between the AGN and the host, possibly due to the availability of cold gas. In the most powerful AGN with host galaxies below the MS we find a hint, though weak, of asymmetry in the [OIII] line profile, likely due to outflowing gas, consistent with a scenario in which AGN feedback removes the available gas and halts the star formation in the most massive hosts.

The Dependence of the Type Ia Supernova Host Bias on Observation or Fitting Technique

Abstract More luminous Type Ia supernovae prefer less massive hosts and regions of higher star formation. This correlation is inverted during width–color–luminosity light-curve standardization resulting in step-like biases of distance measurements with respect to host properties. Using the PMAS/PPak Integral-field Supernovahosts COmpilation (PISCO) supernova host sample and Sloan Digital Sky Survey, Galaxy Evolution Explorer, and Two Micron All Sky Survey photometry, we compare host stellar mass and specific star-formation rate (sSFR) from different observation methods, including local versus global, and fitting techniques to measure their impact on the host step biases. Mass-step measurements for all our mass samples are consistent within a 1σ significance from −0.03 ± 0.02 mag to −0.04 ± 0.02 mag. Including or excluding UV information had no effect on measured mass-step size or location. sSFR step sizes are more significant than mass-step measurements and varied from 0.05 ± 0.03 mag (Hα) and 0.06 ± 0.02 mag (UV) for a 51 host sample. The sSFR step location is influenced by the mass sample used to normalize star formation and by sSFR tracer choice. The step size is reduced to 0.04 ± 0.03 mag when using all available 73 hosts with Hα measurements. This 73 PISCO host subsample overall lacked a clear step signal, but here we are searching for whether different choices of mass or sSFR estimation can create a step signal. We find no evidence that different observation or fitting techniques choices can create a distance measurement step in either mass or sSFR.

The Active Fraction of Massive Black Holes in Dwarf Galaxies

Abstract The population of massive black holes (MBHs) in dwarf galaxies is elusive, but fundamentally important to understand the coevolution of black holes with their hosts and the formation of the first collapsed objects in the universe. While some progress was made in determining the X-ray detected fraction of MBHs in dwarfs, with typical values ranging from 0%–6%, their overall active fraction,  , is still largely unconstrained. Here, we develop a theoretical model to predict the multiwavelength active fraction of MBHs in dwarf galaxies starting from first principles and based on the physical properties of the host, namely, its stellar mass and angular momentum content. We find multiwavelength active fractions for MBHs, accreting at typically low rates, ranging from 5%–22%, and increasing with the stellar mass of the host as  ∼ log 10 M ⋆ 4.5 . If dwarfs are characterized by low-metallicity environments, the active fraction may reach ∼30% for the most massive hosts. For galaxies with stellar mass in the range of 107 &lt; M ⋆[ M ⊙] &lt; 1010, our predictions are in agreement with occupation fractions derived from simulations and semi-analytical models. Additionally, we provide a fitting formula to predict the probability of finding an active MBH in a dwarf galaxy from observationally derived data. This model will be instrumental to guide future observational efforts to find MBHs in dwarfs. The James Webb Space Telescope, in particular, will play a crucial role in detecting MBHs in dwarfs, possibly uncovering active fractions ∼3 times larger than current X-ray surveys.

Extremely efficient mergers of intermediate-mass black hole binaries in nucleated dwarf galaxies

ABSTRACT Gravitational waves emitted by merging black holes between ∼104 and 107 M⊙ will be detectable by the Laser Interferometer Space Antenna (LISA) with signal-to-noise ratios of several hundred out to redshift 20. Supermassive black hole (107–1010 M⊙) binary formation, coalescence, and merger within massive galaxies is well-studied. However, intermediate-mass black holes (IMBHs) between ∼104 and 106 M⊙ are hosted by low mass and dwarf galaxies; it is not trivial to extrapolate black hole merger time-scales to this IMBH binary regime, due to the starkly different host galaxy structure, kinematics, and morphology compared to massive galaxy hosts. We perform ultrahigh resolution N-body simulations to study IMBH dynamics in nucleated dwarf galaxies whose structural parameters are obtained from observations of nearby dwarf galaxies. Starting from 50 pc, an IMBH quickly forms a binary. Thereafter, the binary orbit shrinks rapidly due to the high central stellar densities furnished by nuclear star clusters (NSCs). We find high eccentricities (e ∼ 0.4–0.99) in our suite of IMBH binaries, and residual eccentricity may persist to the LISA regime. IMBH merger times are typically a few hundred million years, with a few exceptionally short merger times for high eccentricities. We find that IMBH-stellar encounters originate pre-dominantly from NSCs, if the NSC-to-IMBH binary mass ratio is greater than 10; otherwise, bulge stars contribute significantly. As the IMBH binary ejects stars, however, the NSCs are disrupted. We conclude that comparable-mass IMBHs merge very efficiently in nucleated dwarf galaxies, making them promising LISA sources, as well as a channel for IMBH growth.

The cumulative star formation histories of dwarf galaxies with TNG50. I: environment-driven diversity and connection to quenching

ABSTRACT We present the cumulative star formation histories (SFHs) of &amp;gt;15 000 dwarf galaxies ($M_{\rm *}=10^{7-10}\, {\rm M}_{\odot }$) simulated with the TNG50 run of the IllustrisTNG suite across a vast range of environments. The key factors that determine the dwarfs’ SFHs are their central/satellite status and stellar mass, with centrals and more massive dwarfs assembling their stellar mass at later times, on average, compared to satellites and lower mass dwarfs. Satellites (in hosts of mass $M_{\rm 200c, host}=10^{12-14.3}\, {\rm M}_{\odot }$) assembled 90 per cent of their stellar mass ${\sim}7.0_{-5.5}^{+3.3}$ Gyr ago, on average and within the 10th to 90th percentiles, while the centrals did so only ${\sim}1.0_{-0.5}^{+4.0}$ Gyr ago. TNG50 predicts a large diversity in SFHs, so that individual dwarfs can have significantly different cumulative SFHs compared to the stacked median SFHs. Satellite dwarfs with the highest stellar mass to host cluster mass ratios have the latest stellar mass assembly. Conversely, satellites at fixed stellar and host halo mass found closer to the cluster centre or accreted at earlier times show significantly earlier stellar mass assembly. These trends and the shapes of the SFHs themselves are a manifestation of the varying proportions within a given subsample of quenched versus star-forming galaxies, which exhibit markedly distinct SFH shapes. Finally, satellite dwarfs in the most massive hosts have higher SFRs at early times, well before accretion into their z = 0 host, compared to a control sample of centrals mass-matched at the time of accretion. This is the result of the satellites being preprocessed in smaller hosts prior to accretion. Our findings are useful theoretical predictions for comparison to future resolved stellar population observations.

Dynamics of a superdense cluster of black holes and the formation of the Galactic supermassive black hole

ABSTRACT The centre of our Galaxy is known to host a massive compact object, Sgr A*, which is commonly considered as a supermassive black hole of ${\sim} 4\times 10^6 \, \text{${\rm M}_{\odot } \, $}$. It is surrounded by a dense and massive nuclear star cluster, with a half-mass radius of about 5 pc and a mass larger than $10^{7} \, \text{${\rm M}_{\odot } \, $}$. In this paper, we studied the evolutionary fate of a very dense cluster of intermediate-mass black holes, possible remnants of the dissipative orbital evolution of massive globular cluster hosts. We performed a set of high-precision N-body simulations taking into account deviations from pure Newtonian gravitational interaction via a post-Newtonian development up to 2.5 order, which is the one accounting for energy released by gravitational wave emission. The violent dynamics of the system leads to various successive merger events to grow a single object containing ∼25 per cent of the total cluster mass before partial dispersal of the cluster, and to generate, in different bursts, a significant quantity of gravitational wave emission. If generalized, the present results suggest a mechanism of mass growth up to the scale of a supermassive black hole.

The Far Ultraviolet M-dwarf Evolution Survey. I. The Rotational Evolution of High-energy Emissions*

Abstract M-dwarf stars are prime targets for exoplanet searches because of their close proximity and favorable properties for both planet detection and characterization. However, the potential habitability and atmospheric characterization of these exoplanetary systems depends critically on the history of high-energy stellar radiation from X-rays to NUV, which drive atmospheric mass loss and photochemistry in the planetary atmospheres. With the Far Ultraviolet M-dwarf Evolution Survey, we have assessed the evolution of the FUV radiation, specifically eight prominent emission lines, including Lyα, of M-dwarf stars with stellar rotation period and age. We demonstrate tight power-law correlations between the spectroscopic FUV features, and measure the intrinsic scatter of the quiescent FUV emissions. The luminosity evolution with rotation of these spectroscopic features is well-described by a broken power law, saturated for fast rotators, and decays with increasing Rossby number, with a typical power-law slope of −2, although likely shallower for Lyα. Our regression fits enable FUV emission-line luminosity estimates relative to bolometric from known rotation periods to within ∼0.3 dex, across eight distinct UV emission lines, with possible trends in the fit parameters as a function of source layer in the stellar atmosphere. Our detailed analysis of the UV luminosity evolution with age further shows that habitable-zone planets orbiting lower-mass stars experience much greater high-energy radiative exposure relative the same planets orbiting more massive hosts. Around early- to mid-M dwarfs, these exoplanets, at field ages, accumulate up to 10–20× more EUV energy, relative to modern Earth. Moreover, the bulk of this UV exposure likely takes place within the first Gyr of the stellar lifetime.