ABSTRACT Active galactic nuclei (AGN) are powered by the accretion of discs of gas on to supermassive black holes (SMBHs). Stars and stellar remnants orbiting the SMBH in the nuclear star cluster (NSC) will interact with the AGN disc. Orbiters plunging through the disc experience a drag force and, through repeated passage, can have their orbits captured by the disc. A population of embedded objects in AGN discs may be a significant source of binary black hole mergers, supernovae, tidal disruption events, and embedded gamma-ray bursts. For two representative AGN disc models, we use geometric drag and Bondi–Hoyle–Littleton drag to determine the time to capture for stars and stellar remnants. We assume a range of initial inclination angles and semimajor axes for circular Keplerian prograde orbiters. Capture time strongly depends on the density and aspect ratio of the chosen disc model, the relative velocity of the stellar object with respect to the disc, and the AGN lifetime. We expect that for an AGN disc density $\rho \gtrsim 10^{-11}{\rm g\, cm^{-3}}$ and disc lifetime ≥1 Myr, there is a significant population of embedded stellar objects, which can fuel mergers detectable in gravitational waves with LIGO-Virgo and LISA.
Envisioning the next decade of Galactic Center science: a laboratory for the study of the physics and astrophysics of supermassive black holes
As the closest example of a galactic nucleus, the Galactic center (GC)
presents an exquisite laboratory for learning about supermassive black holes (SMBH) and their
environment. We describe several exciting new research directions that, over the next 10 years,
1
arXiv:1903.05293v1 [astro-ph.GA] 13 Mar 2019
hold the potential to answer some of the biggest scientific questions raised in recent decades: Is
General Relativity (GR) the correct description for supermassive black holes? What is the nature of
star formation in extreme environments? How do stars and compact objects dynamically interact
with the supermassive black hole? What physical processes drive gas accretion in low-luminosity
black holes? We describe how the high sensitivity, angular resolution, and astrometric precision
offered by the next generation of large ground-based telescopes with adaptive optics will help us
answer these questions. First, it will be possible to obtain precision measurements of stellar orbits in the Galaxy’s central potential, providing both tests of GR in the unexplored regime near
a SMBH and measurements of the extended dark matter distribution that is predicted to exist at
the GC. The orbits of these stars will also allow us to measure the spin of the SMBH. Second,
we will probe stellar populations at the GC to significantly lower masses than are possible today,
down to the brown more »
- Award ID(s):
- 1743747
- Publication Date:
- NSF-PAR ID:
- 10111872
- Journal Name:
- Astro2020: Decadal Survey on Astronomy and Astrophysics
- Volume:
- 51
- Issue:
- 3
- Sponsoring Org:
- National Science Foundation
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ABSTRACT Stars and stellar remnants orbiting a supermassive black hole (SMBH) can interact with an active galactic nucleus (AGN) disc. Over time, prograde orbiters (inclination i < 90°) decrease inclination, as well as semimajor axis (a) and eccentricity (e) until orbital alignment with the gas disc (‘disc capture’). Captured stellar-origin black holes (sBH) add to the embedded AGN population that drives sBH–sBH mergers detectable in gravitational waves using LIGO–Virgo–KAGRA or sBH–SMBH mergers detectable with Laser Interferometer Space Antenna. Captured stars can be tidally disrupted by sBH or the SMBH or rapidly grow into massive ‘immortal’ stars. Here, we investigate the behaviour of polar and retrograde orbiters (i ≥ 90°) interacting with the disc. We show that retrograde stars are captured faster than prograde stars, flip to prograde orientation (i < 90°) during capture, and decrease a dramatically towards the SMBH. For sBH, we find a critical angle iret ∼ 113°, below which retrograde sBH decay towards embedded prograde orbits (i → 0°), while for io > iret sBH decay towards embedded retrograde orbits (i → 180°). sBH near polar orbits (i ∼ 90°) and stars on nearly embedded retrograde orbits (i ∼ 180°) show the greatest decreases in a. Whethermore »
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Abstract We present the first Event Horizon Telescope (EHT) observations of Sagittarius A* (Sgr A*), the Galactic center source associated with a supermassive black hole. These observations were conducted in 2017 using a global interferometric array of eight telescopes operating at a wavelength of λ = 1.3 mm. The EHT data resolve a compact emission region with intrahour variability. A variety of imaging and modeling analyses all support an image that is dominated by a bright, thick ring with a diameter of 51.8 ± 2.3 μ as (68% credible interval). The ring has modest azimuthal brightness asymmetry and a comparatively dim interior. Using a large suite of numerical simulations, we demonstrate that the EHT images of Sgr A* are consistent with the expected appearance of a Kerr black hole with mass ∼4 × 10 6 M ⊙ , which is inferred to exist at this location based on previous infrared observations of individual stellar orbits, as well as maser proper-motion studies. Our model comparisons disfavor scenarios where the black hole is viewed at high inclination ( i > 50°), as well as nonspinning black holes and those with retrograde accretion disks. Our results provide direct evidence for the presence ofmore »
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