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Register a new userObservational signatures of the black hole Mass Distribution in the Galactic Center
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Razieh Emami Meibody
Publication date
2019
fields
Physics
and research's language is
English
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We simulate the star cluster, made of stars in the main sequence and different black hole (BH) remnants, around SgrA* at the center of the Milky Way galaxy. Tracking stellar evolution, we find the BH remnant masses and construct the BH mass function. We sample 4 BH species and consider the impact of the mass-function in the dynamical evolution of system. Starting from an initial 6 dimensional family of parameters and using an MCMC approach, we find the best fits to various parameters of model by directly comparing the results of the simulations after $t = 10.5$ Gyrs with current observations of the stellar surface density, stellar mass profile and the mass of SgrA*. Using these parameters, we study the dynamical evolution of system in detail. We also explore the mass-growth of SgrA* due to tidally disrupted stars and swallowed BHs. We show that the consumed mass is dominated for the BH component with larger initial normalization as given by the BH mass-function. Assuming that about 10% of the tidally disrupted stars contribute in the growth of SgrA* mass, stars make up the second dominant effect in enhancing the mass of SgrA*. We consider the detectability of the GW signal from inspiralling stellar mass BHs around SgrA* with LISA. Computing the fraction of the lifetime of every BH species in the LISA band, with signal to noise ratio $gtrsim 8$, to their entire lifetime, and rescaling this number with the total number of BHs in the system, we find that the total expected rate of inspirals per Milky-Way sized galaxy per year is $10^{-5}$. Quite interestingly, the rate is dominated for the BH component with larger initial normalization as dictated by the BH mass-function. We interpret it as the second signature of the BH mass-function.
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A small cluster of massive stars residing in the Galactic center, collectively known as IRS13E, is of special interest due to its close proximity to Sgr A* and the possibility that an embedded intermediate-mass black hole (IMBH) binds its member stars. It has been suggested that colliding winds from two member stars, both classified as Wolf-Rayet type, are responsible for the observed X-ray, infrared and radio emission from IRS13E. We have conducted an in-depth study of the X-ray spatial, temporal and spectral properties of IRS13E, based on 5.6 Ms of ultra-deep Chandra observations obtained over 20 years. These X-ray observations show no significant evidence for source variability. We have also explored the kinematics of the cluster members, using Keck near-infrared imaging and spectroscopic data on a 14-yr baseline that considerably improve the accuracy of stars proper motions. The observations are interpreted using 3-dimensional hydrodynamical simulations of colliding winds tailored to match the physical conditions of IRS13E, leading us to conclude that the observed X-ray spectrum and morphology can be well explained by the colliding wind scenario, in the meantime offering no support for the presence of a putative IMBH. An IMBH more massive than a few $10^3{rm~M_odot}$ is also strongly disfavored by the stellar kinematics.
We investigate a mechanism for a super-massive black hole at the center of a galaxy to wander in the nucleus region. A situation is supposed in which the central black hole tends to move by the gravitational attractions from the nearby molecular clouds in a nuclear bulge but is braked via the dynamical frictions by the ambient stars there. We estimate the approximate kinetic energy of the black hole in an equilibrium between the energy gain rate through the gravitational attractions and the energy loss rate through the dynamical frictions, in a nuclear bulge composed of a nuclear stellar disk and a nuclear stellar cluster as observed from our Galaxy. The wandering distance of the black hole in the gravitational potential of the nuclear bulge is evaluated to get as large as several 10 pc, when the black hole mass is relatively small. The distance, however, shrinks as the black hole mass increases and the equilibrium solution between the energy gain and loss disappears when the black hole mass exceeds an upper limit. As a result, we can expect the following scenario for the evolution of the black hole mass: When the black hole mass is smaller than the upper limit, mass accretion of the interstellar matter in the circum-nuclear region, causing the AGN activities, makes the black hole mass larger. However, when the mass gets to the upper limit, the black hole loses the balancing force against the dynamical friction and starts spiraling downward to the gravity center. From simple parameter scaling, the upper mass limit of the black hole is found to be proportional to the bulge mass and this could explain the observed correlation of the black hole mass with the bulge mass.
Recent gamma-ray and cosmic-ray observations have put strong constraints on the amount of primordial black holes (PBHs) in our universe. In this article, we use the archival radio data of the inner Galactic Centre to constrain the PBH to dark matter ratio for three different PBH mass distributions including monochromatic, log-normal and power-law. We show that the amount of PBHs only constitutes a very minor component of dark matter at the Galactic Centre for a large parameter space.
Recent gravitational wave (GW) observations by LIGO/Virgo show evidence for hierarchical mergers, where the merging BHs are the remnants of previous BH merger events. These events may carry important clues about the astrophysical host environments of the GW sources. In this paper, we present the distributions of the effective spin parameter ($chi_mathrm{eff}$), the precession spin parameter ($chi_mathrm{p}$), and the chirp mass ($m_mathrm{chirp}$) expected in hierarchical mergers. Under a wide range of assumptions, hierarchical mergers produce (i) a monotonic increase of the average of the typical total spin for merging binaries, which we characterize with ${bar chi}_mathrm{typ}equiv overline{(chi_mathrm{eff}^2+chi_mathrm{p}^2)^{1/2}}$, up to roughly the maximum $m_mathrm{chirp}$ among first-generation (1g) BHs, and (ii) a plateau at ${bar chi}_mathrm{typ}sim 0.6$ at higher $m_mathrm{chirp}$. We suggest that the maximum mass and typical spin magnitudes for 1g BHs can be estimated from ${bar chi}_mathrm{typ}$ as a function of $m_mathrm{chirp}$. The GW data observed in LIGO/Virgo O1--O3a prefers an increase in ${bar chi}_mathrm{typ}$ at low $m_mathrm{chirp}$, which is consistent with the growth of the BH spin magnitude by hierarchical mergers, at $sim 2 sigma$ confidence. A Bayesian analysis suggests that 1g BHs have the maximum mass of $sim 15$--$30,M_odot$ if the majority of mergers are of high-generation BHs (not among 1g-1g BHs), which is consistent with mergers in active galactic nucleus disks and/or nuclear star clusters, while if mergers mainly originate from globular clusters, 1g BHs are favored to have non-zero spin magnitudes of $sim 0.3$. We also forecast that signatures for hierarchical mergers in the ${bar chi}_mathrm{typ}$ distribution can be confidently recovered once the number of GW events increases to $gtrsim O(100)$.
The discovery of gravitational wave radiation from merging black holes (BHs) also uncovered BHs with masses in the range of ~20-90 Msun, which upon their merger became even more massive ones. In contrast, the most massive Galactic stellar-mass BH currently known has a mass ~21 Msun. While low-mass X-ray binaries (LMXBs) will never independently evolve into a binary BH system, and binary evolution effects can play an important role explaining the different BH masses found through studies of LMXBs, high-mass X-ray binaries, and gravitational wave events, (electromagnetic) selection effects may also play a role in this discrepancy. Assuming BH LMXBs originate in the Galactic Plane where massive stars are formed, we show that both the spatial distribution of the current sample of 20 Galactic LMXBs with dynamically confirmed BH masses, and that of candidate BH LMXBs, are both strongly biased to sources that lie at a large distance from the Galactic Plane. Specifically, most of the confirmed and candidate BH LMXBs are found at a Galactic height larger than 3 times the scale height for massive star formation. In addition, the confirmed BHs in LMXBs are found at larger distances to the Galactic Center than the candidate BH LMXBs. Interstellar absorption makes candidate BH X-ray binaries in the Plane and those in the Bulge close to the Galactic Center too faint for a dynamical mass measurement using current instrumentation. Given the observed and theoretical evidence for BH natal and/or Blaauw kicks, their relation with BH mass and binary orbital period, and the relation between outburst recurrence time and BH mass, the observational selection effects imply that the current sample of confirmed BH LMXBs is biased against the most massive BHs.
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