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Register a new userSwift coalescence of supermassive black holes in cosmological mergers of massive galaxies
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Fazeel M. Khan
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Supermassive black holes (SMBHs) are ubiquitous in galaxies with a sizable mass. It is expected that a pair of SMBHs originally in the nuclei of two merging galaxies would form a binary and eventually coalesce via a burst of gravitational waves. So far theoretical models and simulations have been unable to predict directly the SMBH merger timescale from ab-initio galaxy formation theory, focusing only on limited phases of the orbital decay of SMBHs under idealized conditions of the galaxy hosts. The predicted SMBH merger timescales are long, of order Gyrs, which could be problematic for future gravitational wave searches. Here we present the first multi-scale $Lambda$CDM cosmological simulation that follows the orbital decay of a pair of SMBHs in a merger of two typical massive galaxies at $zsim3$, all the way to the final coalescence driven by gravitational wave emission. The two SMBHs, with masses $sim10^{8}$ M$_{odot}$, settle quickly in the nucleus of the merger remnant. The remnant is triaxial and extremely dense due to the dissipative nature of the merger and the intrinsic compactness of galaxies at high redshift. Such properties naturally allow a very efficient hardening of the SMBH binary. The SMBH merger occurs in only $sim10$ Myr after the galactic cores have merged, which is two orders of magnitude smaller than the Hubble time.
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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 $0%$ to $6%$, their overall active fraction, ${cal A}$, 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 $5%$ to $22%$, and increasing with the stellar mass of the host as ${cal A} sim(log_{10}M_{star})^{4.5}$. If dwarfs are characterized by low-metallicity environments, the active fraction may reach $sim 30%$ for the most massive hosts. For galaxies with stellar mass in the range $10^7<M_{star} [M_{odot}]<10^{10}$, 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 $sim 3$ times larger than current X-ray surveys.
The coalescence of massive black hole binaries (BHBs) in galactic mergers is the primary source of gravitational waves (GWs) at low frequencies. Current estimates of GW detection rates for the Laser Interferometer Space Antenna and the Pulsar Timing Array vary by three orders of magnitude. To understand this variation, we simulate the merger of equal-mass, eccentric, galaxy pairs with central massive black holes and shallow inner density cusps. We model the formation and hardening of a central BHB using the Fast Multiple Method as a force solver, which features a $O(N)$ scaling with the number $N$ of particles and obtains results equivalent to direct-summation simulations. At $N sim 5times 10^5$, typical for contemporary studies, the eccentricity of the BHBs can vary significantly for different random realisations of the same initial condition, resulting in a substantial variation of the merger timescale. This scatter owes to the stochasticity of stellar encounters with the BHB and decreases with increasing $N$. We estimate that $N sim 10^7$ within the stellar half-light radius suffices to reduce the scatter in the merger timescale to $sim 10$%. Our results suggest that at least some of the uncertainty in low-frequency GW rates owes to insufficient numerical resolution.
We exploit the recent, wide samples of far-infrared (FIR) selected galaxies followed-up in X rays and of X-ray/optically selected active galactic nuclei (AGNs) followed-up in the FIR band, along with the classic data on AGN and stellar luminosity functions at high redshift z>1.5, to probe different stages in the coevolution of supermassive black holes (BHs) and host galaxies. The results of our analysis indicate the following scenario: (i) the star formation in the host galaxy proceeds within a heavily dust-enshrouded medium at an almost constant rate over a timescale ~0.5-1 Gyr, and then abruptly declines due to quasar feedback; over the same timescale, (ii) part of the interstellar medium loses angular momentum, reaches the circum-nuclear regions at a rate proportional to the star formation and is temporarily stored into a massive reservoir/proto-torus wherefrom it can be promptly accreted; (iii) the BH grows by accretion in a self-regulated regime with radiative power that can slightly exceed the Eddington limit L/L_Edd< 4, particularly at the highest redshifts; (iv) for massive BHs the ensuing energy feedback at its maximum exceeds the stellar one and removes the interstellar gas, thus stopping the star formation and the fueling of the reservoir; (v) afterwards, if the latter has retained enough gas, a phase of supply-limited accretion follows exponentially declining with a timescale of about 2 e-folding times. We show that the ratio of the FIR luminosity of the host galaxy to the bolometric luminosity of the AGN maps the various stages of the above sequence. Finally, we discuss how the detailed properties and the specific evolution of the reservoir can be investigated via coordinated, high-resolution observations of starforming, strongly-lensed galaxies in the (sub-)mm band with ALMA and in the X-ray band with Chandra and the next generation X-ray instruments.
We summarize what large surveys of the contemporary universe have taught us about the physics and phenomenology of the processes that link the formation and evolution of galaxies and their central supermassive black holes. We present a picture in which the population of AGN can be divided into two distinct populations. The Radiative-Mode AGN are associated with black holes that produce radiant energy powered by accretion at rates in excess of ~1% of the Eddington Limit. They are primarily associated with less massive black holes growing in high-density pseudo-bulges at a rate sufficient to produce the total mass budget in these black holes in ~10 Gyr. The circum-nuclear environment contains high density cold gas and associated star-formation. Major mergers are not the primary mechanism for transporting this gas inward; secular processes appear dominant. Stellar feedback will be generic in these objects and strong AGN feedback is seen only in the most powerful AGN. In Jet-Mode AGN the bulk of energetic output takes the form of collimated outflows (jets). These AGN are associated with the more massive black holes in more massive (classical) bulges and elliptical galaxies. Neither the accretion onto these black holes nor star-formation in their host bulge is significant today. These AGN are probably fueled by the accretion of slowly cooling hot gas that is limited by the feedback/heating provided by AGN radio sources. Surveys of the high-redshift universe are painting a similar picture. (Abridged).
We present a study of relations between the masses of the central supermassive black holes (SMBHs) and the atmospheric gas temperatures and luminosities measured within a range of radii between $R_{rm e}$ and 5$R_{rm e}$, for a sample of 47 early-type galaxies observed by the {it Chandra X-ray Observatory}. We report the discovery of a tight correlation between the atmospheric temperatures of the brightest cluster/group galaxies (BCGs) and their central SMBH masses. Furthermore, our hydrostatic analysis reveals an approximately linear correlation between the total masses of BCGs ($M_{rm tot}$) and their central SMBH masses ($M_{rm BH}$). State-of-the-art cosmological simulations show that the SMBH mass could be determined by the binding energy of the halo through radiative feedback during the rapid black hole growth by accretion, while for the most massive galaxies mergers are the chief channel of growth. In the scenario of a simultaneous growth of central SMBHs and their host galaxies through mergers, the observed linear correlation could be a natural consequence of the central limit theorem.
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