An official website of the United States government
Abstract Quenching of star formation in the central galaxies of cosmological halos is thought to result from energy released as gas accretes onto a supermassive black hole. The same energy source also appears to lower the central density and raise the cooling time of baryonic atmospheres in massive halos, thereby limiting both star formation and black hole growth, by lifting the baryons in those halos to greater altitudes. One predicted signature of that feedback mechanism is a nearly linear relationship between the central black hole’s mass (MBH) and the original binding energy of the halo’s baryons. We present the increasingly strong observational evidence supporting a such a relationship, showing that it extends up to halos of massMhalo∼ 1014M⊙. We then compare current observational constraints on theMBH–Mhalorelation with numerical simulations, finding that black hole masses in IllustrisTNG appear to exceed those constraints atMhalo< 1013M⊙and that black hole masses in EAGLE fall short of observations atMhalo∼ 1014M⊙. A closer look at IllustrisTNG shows that quenching of star formation and suppression of black hole growth do indeed coincide with black hole energy input that lifts the halo’s baryons. However, IllustrisTNG does not reproduce the observedMBH–Mhalorelation because its black holes gain mass primarily through accretion that does not contribute to baryon lifting. We suggest adjustments to some of the parameters in the IllustrisTNG feedback algorithm that may allow the resulting black hole masses to reflect the inherent links between black hole growth, baryon lifting, and star formation among the massive galaxies in those simulations.
more »
« less
Young Star Clusters Dominate the Production of Detached Black Hole–Star Binaries
Abstract The recent discovery of two detached black hole–star (BH–star) binaries from Gaia’s third data release has sparked interest in understanding the formation mechanisms of these systems. We investigate the formation of these systems by dynamical processes in young star clusters (SCs) and via isolated binary (IB) evolution, using a combination of directN-body and population synthesis simulations. We find that dynamical formation in SCs is nearly 50 times more efficient per unit of star formation at producing BH–star binaries than IB evolution. We expand this analysis to the full Milky Way (MW) using a FIRE-2 hydrodynamical simulation of an MW-mass galaxy. Even assuming that only 10% of star formation goes into SCs, we find that approximately four out of every five BH–star systems are formed dynamically, and that the MW contains a total of ∼2 × 105BH–star systems. Many of these dynamically formed systems have longer orbital periods, greater eccentricities, and greater black hole masses than their isolated counterparts. For binaries older than 100 Myr, we show that any detectable system withe≳ 0.5 orMBH≳ 10M⊙canonlybe formed through dynamical processes. Our MW model predicts between 64 and 215 such detections from the complete DR4 Gaia catalog, with the majority of systems being dynamically formed in massive and metal-rich SCs. Finally, we compare our populations to the recently discovered Gaia BH1 and Gaia BH2, and conclude that the dynamical scenario is the most favorable formation pathway for both systems.
more »
« less
- Award ID(s):
- 2310362
- PAR ID:
- 10556483
- Publisher / Repository:
- IOP Publisher
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 965
- Issue:
- 1
- ISSN:
- 0004-637X
- Page Range / eLocation ID:
- 22
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract GW231123, the most massive binary black hole (BBH) merger detected by LIGO–Virgo–KAGRA, highlights the need to understand the origins of massive, high-spin stellar black holes (BHs). Dense star clusters provide natural environments for forming such systems, beyond the limits of standard massive star evolution to core collapse. While repeated BBH mergers can grow BHs through dynamical interactions (the so-called “hierarchical merger” channel), most star clusters with masses ≲106M⊙have escape speeds too low to retain higher-generation BHs, limiting growth into or beyond the mass gap. In contrast, BH–star collisions with subsequent accretion of the collision debris can grow and retain BHs irrespective of the cluster escape speed. UsingN-body (Cluster Monte Carlo) simulations, we study BH growth and spin evolution through this process, and we find that accretion can drive BH masses up to at least ∼200M⊙, with spins set by the details of the growth history. BHs up to about 150M⊙can reach dimensionless spinsχ ≳ 0.7 via single coherent episodes, while more massive BHs form through multiple stochastic accretion events and eventually spin down toχ ≲ 0.4. These BHs later form binaries through dynamical encounters, producing BBH mergers that contribute up to ∼10% of all detectable events, comparable to predictions for the hierarchical channel. However, the two pathways predict distinct signatures: hierarchical mergers yield more unequal mass ratios, whereas accretion-grown BHs preferentially form near-equal-mass binaries. The accretion-driven channel allows dense clusters with low escape speeds, such as globular clusters, to produce highly spinning BBHs with both components in or above the mass gap, providing a natural formation pathway to GW231123-like systems.more » « less
-
Abstract The existence of compact stellar remnants in the mass range 2–5M⊙has long been debated. This so-called lower-mass gap (LMG) was initially suggested by the lack of low-mass X-ray binary observations with accretors about 2–5M⊙, but it has recently been called into question following newer observations, including an LMG candidate with a millisecond pulsar (MSP) companion in the dense globular cluster NGC 1851. Here, we model NGC 1851 with a grid of similar dense star clusters utilizing the state-of-the-art Monte CarloN-body code Cluster Monte Carlo, and we specifically study the formation of LMG black holes (BHs). We demonstrate that both massive star evolution and dynamical interactions can contribute to forming LMG BHs. In general, the collapse of massive remnants formed through mergers of neutron stars (NSs) or massive white dwarfs produces the largest number of LMG BHs among all formation channels. However, in more massive clusters, supernova core collapse can contribute comparable numbers. Our NGC 1851-like models can reproduce MSP—LMG BH binaries similar to the observed system. Additionally, the LMG BHs can also become components of dynamically assembled binaries, and some will be in merging BH–NS systems similar to the recently detected gravitational wave source GW230529. However, the corresponding merger rate is probably ≲1 Gpc−3yr−1.more » « less
-
Abstract Astrometry from the Gaia mission was recently used to discover the two nearest known stellar-mass black holes (BHs), Gaia BH1 and Gaia BH2. These objects are among the first stellar-mass BHs not discovered via X-rays or gravitational waves. Both systems contain ∼1M⊙stars in wide orbits (a≈ 1.4 au, 4.96 au) around ∼9M⊙BHs, with both stars (solar-type main sequence star, red giant) well within their Roche lobes in Gaia BH1 and BH2, respectively. However, the BHs are still expected to accrete stellar winds, leading to potentially detectable X-ray or radio emission. Here, we report observations of both systems with the Chandra X-ray Observatory, the Very Large Array (for Gaia BH1) and MeerKAT (for Gaia BH2). We did not detect either system, leading to X-ray upper limits ofLX< 9.4 × 1028andLX< 4.0 × 1029erg s−1and radio upper limits ofLr< 1.6 × 1025andLr< 1.0 × 1026erg s−1for Gaia BH1 and BH2, respectively. For Gaia BH2, the non-detection implies that the accretion rate near the horizon is much lower than the Bondi rate, consistent with recent models for hot accretion flows. We discuss implications of these non-detections for broader BH searches, concluding that it is unlikely that isolated BHs will be detected via interstellar medium accretion in the near future. We also calculate evolutionary models for the binaries’ future evolution using Modules for Experiments in Stellar Astrophysics, and find that Gaia BH1 will be visible as a symbiotic BH X-ray binary for 5–50 Myr. Since no symbiotic BH X-ray binaries are known, this implies either that fewer than ∼104Gaia BH1-like binaries exist in the Milky Way, or that they are common but have evaded detection.more » « less
-
Abstract Black hole–neutron star binaries are of interest in many ways: they are intrinsically transient, radiate gravitational waves detectable by LIGO, and may produceγ-ray bursts. Although it has long been assumed that their late-stage orbital evolution is driven entirely by gravitational wave emission, we show here that in certain circumstances, mass transfer from the neutron star onto the black hole can both alter the binary's orbital evolution and significantly reduce the neutron star's mass: when the fraction of its mass transferred per orbit is ≳10−2, the neutron star's mass diminishes by order unity, leading to mergers in which the neutron star mass is exceptionally small. The mass transfer creates a gas disk around the black holebeforemerger that can be comparable in mass to the debris remaining after merger, i.e., ~0.1M⊙. These processes are most important when the initial neutron star–black hole mass ratioqis in the range ≈0.2–0.8, the orbital semimajor axis is 40 ≲ a0/rg ≲ 300 (rg ≡ GMBH/c2), and the eccentricity is large ate0 ≳ 0.8. Systems of this sort may be generated through the dynamical evolution of a triple system, as well as by other means.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.3847/1538-4357/ad2f2c
