arXiv subjects

David Garofalo

Publications and source records attributed to David Garofalo.

At least 37 records · Page 2Linked to original sources

arXiv2021

Powerful Jets from Radiatively Efficient Disks, a Decades-Old Unresolved Problem in High Energy Astrophysics

The discovery of 3C 273 in 1963, and the emergence of the Kerr solution shortly thereafter, precipitated the current era in astrophysics focused on using black holes to explain active galactic nuclei (AGN). But while partial success was achieved in separately explaining the bright nuclei of some AGN via thin disks, as well as powerful jets with thick disks, the combination of both powerful jets in an AGN with a bright nucleus, such as in 3C 273, remained elusive. Although numerical simulations have taken center stage in the last 25 years, they have struggled to produce the conditions that explain them. This is because radiatively efficient disks have proved a challenge to simulate. Radio quasars have thus been the least understood objects in high energy astrophysics. But recent simulations have begun to change this. We explore this milestone in light of scale-invariance and show that transitory jets, possibly related to the jets seen in these recent simulations, as some have proposed, cannot explain radio quasars. We then provide a road map for a resolution.

astro-ph.HE↗

arXiv2020

The emergence of the X-ray luminosity/cluster richness relation for radio galaxies

The idea that mergers are more likely in dense groups or clusters coupled with the assumption that such events lead to cold gas flows onto black holes, suggests a direct relationship between the radiative efficiency of an active galactic nucleus and environmental richness. Observations, however, increasingly challenge this and other basic expectations. Mounting evidence, for example, shows an inverse trend between near-Eddington accreting objects and environmental richness. Broken down by radio galaxy subgroup, recent work has explored connections between low excitation radio galaxies with Fanaroff-Riley II jet morphology (FRII LERGs) and other radio galaxies. We make contact with that work by adding a discussion of the recently discovered FR0 radio galaxies and show how to fit them in a picture in which FRII LERGs are not initial nor final phases in the lifetime of a radio galaxy, but de facto transition states. We describe how to understand the observed X-ray luminosity/cluster richness relation as a fundamental correlation on the nature of the jet-disk connection.

astro-ph.HE↗

arXiv2020

Evidence for radio loud to radio quiet evolution from red and blue quasars

Recent work on red and blue quasi-stellar objects (QSOs) has identified peculiar number distributions as a function of radio loudness that we explore and attempt to explain from the perspective of a picture in which a subset of the population of active galaxies evolves from the radio loud to the radio quiet state. Because the time evolution is slowed down by an order of magnitude or more for the radio quiet phase, the numbers of red and blue QSOs approach each other at the extreme end of the radio quiet range of radio loudness with larger numbers. The rapid time evolution of most radio loud phases, instead, makes the numbers similar but lower at the far radio loud end. At the midpoint of radio loudness, instead, the differences between red and blue QSOs experience their largest values which results from accretion rapidly spinning black holes down but subsequently spinning them up more slowly. Recovering these basic features of the observations is evidence for rapid evolution away from radio loudness and slow evolution in radio quiet states.

astro-ph.HE↗

arXiv2020

Spin of the M87 black hole

Spin measurement of the 6.5 billion solar mass black hole in M87 from the Event Horizon Telescope image is the latest in a series that span a wide range in values, but that tend to share the feature of co-rotation between the accretion flow and black hole. The spin paradigm for black holes predicts very high black hole spin which in that framework was produced in its last significant merger. High black hole spin appears to be ruled out in the gap paradigm, however, which predicts early formation with a mass already in excess of 4 billion solar masses. In this picture, the black hole experienced slow evolution as it departed from its original radio quasar phase and over billions of years not only failed to double its mass but also fell short of regaining its original high spin, such that it is now compatible with a co-rotating accreting black hole whose dimensionless spin fits best in the range 0.2 < a < 0.5.

astro-ph.HE↗

arXiv2020

Why radio quiet quasars are preferred over radio loud quasars regardless of environment and redshift

Evidence has accumulated suggesting the clustering of radio loud quasars is greater than for radio quiet quasars. We interpret these results in a context in which the fraction of radio loud quasar formation is less than or equal to that for radio quiet quasars for all environments and redshift. Because we assume that post-merger cold gas onto large black holes produces either a radio loud or a radio quiet quasar, we show that for largest black hole masses that live in largest dark matter halos, fRLQ approaches 0.5 from below but does not exceed it, such that in rich clusters the formation of a radio loud quasar tends to be equally likely to occur as a radio quiet quasar. In dark matter halos with smaller mass, by contrast, radio quiet quasars are more likely to form and the likelihood increases inversely with dark matter halo mass. As a result, averaging over a population of radio loud and radio quiet quasars will necessarily generate lower average black hole masses for the radio quiet subgroup. Hence, despite the fact that the formation of radio quiet quasars is preferred over radio loud quasars in any environment, at any mass scale, at any luminosity, or redshift, averaging over a range of radio loud quasars will give the appearance they are preferred in cluster environments over radio quiet quasars. We show how this also accounts for the order of magnitude difference in the total number of jetted active galaxies compared to non-jetted counterparts.

astro-ph.HE↗

arXiv2019

A unified framework for X-shaped radio galaxies

We propose a radically different picture for X-shaped radio galaxies compared to existing models as transition objects between cold mode accreting low spinning retrograde black holes and low spinning prograde black holes. The model explains their smaller average black hole masses, their general aversion for cluster compared to isolated environments, the negligible difference in spectral index between primary and secondary jets despite a time difference in their formation, their absence among the most powerful radio quasars and radio galaxies, and their connection to the elusive FRI quasar class, among others. The key to their formation is cold gas accretion onto Schwarzschild black holes.

astro-ph.HE↗

arXiv2019

Merger signatures in low excitation radio galaxies

While no consensus governs our understanding of the origin of low redshift radio galaxies, the possibility that mergers may trigger accretion from hot cluster halo gas has spurred a recent search for such signatures. Evidence for mergers is at best tenuous, however, and even when found, generates more questions than answers. With scant evidence for minor mergers, some connection to major mergers is found in isolated environments but not where one would expect, i.e. in clusters. We provide an explanation for these recent results by Gordon et al (2019) on the relevance of major mergers in low excitation radio galaxies (LERGs) at low redshift. While LERGs are not the direct result of a merger, we describe how they form in clusters in only a few million years while that timescale is an order of magnitude longer in field environments. As a result of these different timescales, the average lifetime of a cluster LERG is estimated at an order of magnitude greater value than for field LERGs. Observing a LERG in the cluster environment, therefore, will tend to occur when greater time has passed since the major merger event that produced its high excitation radio galaxy ancestor, such that fewer signatures of that event remain visible. We provide simple estimates for the fraction of LERGs as a function of environment that are directly related to these timescales, obtaining a probability of about 7% that field LERGs will show merger signatures and 3% for clusters, showing that theory and observation match if major merger signatures remain visible for a few hundred million years.

astro-ph.HE↗

arXiv2019

The generalized hardness-intensity diagram for black hole and neutron star X-ray binaries

Over the past half century, X-ray and radio observations of accreting neutron stars and stellar mass black holes have yielded a rich observational picture with common features including state transitions and jet formation, but also sharp differences. While black hole X-ray binaries overwhelmingly suppress jets in so-called soft states, accreting neutron stars are less restrictive, with a soft state wind observed in some sources to co-exist with a jet. We propose an explanation for these differences that leads to a generalization of a foundational element, the hardness-intensity diagram of Fender et al (2004). The inverse relation between jets and winds fits into a picture that connects to prograde accretion while the possibility of counterrotation between accretion disk and compact object accounts for observed differences in accreting neutron stars. This picture comes with a surprising twist, which is that neutron stars embody the small-scale analog of FRII quasars, an idea that allows us to complete the scale invariant picture for the jet-disk connection.

astro-ph.HE↗

arXiv2019

The sub-Eddington boundary for the quasar mass-luminosity plane: A theoretical perspective

By exploring more than sixty thousand quasars from the Sloan Digital Sky Survey Data Release 5, Steinhardt & Elvis discovered a sub-Eddington boundary and a redshift-dependent drop-off at higher black hole mass, possible clues to the growth history of massive black holes. Our contribution to this special issue of Universe amounts to an application of a model for black hole accretion and jet formation to these observations. For illustrative purposes we include about 100,000 data points from the Sloan Digital Sky Survey Data Release 7 where the sub-Eddington boundary is also visible, and propose a theoretical picture that explains these features. By appealing to thin disk theory and both the lower accretion efficiency and the time evolution of jetted quasars compared to non-jetted quasars in our 'gap paradigm', we explain two features of the sub-Eddington boundary. First, we show that a drop-off on the quasar mass-luminosity plane for larger black hole mass occurs at all redshifts. But the fraction of jetted quasars is directly related to the merger function in this paradigm, which means the jetted quasar fraction drops with decrease in redshift, which allows us to explain a second feature of the sub-Eddington boundary, namely a redshift dependence of the slope of the quasar mass-luminosity boundary at high black hole mass stemming from a change in radiative efficiency with time. We are able to reproduce the mass dependence of, as well as the oscillating behavior in, the slope of the sub-Eddington boundary as a function of time. The basic physical idea involves retrograde accretion occurring only for a subset of the more massive black holes which implies that most spinning black holes in our model are prograde accretors. In short, this paper amounts to a qualitative overview of how a sub-Eddington boundary naturally emerges in the gap paradigm.

astro-ph.HE↗

arXiv2019

Resolving the radio loud/radio quiet dichotomy without thick disks

Observations of radio loud active galaxies in the XMM-Newton archive by Mehdipour & Costantini show a strong anti-correlation between the column density of the ionized wind and the radio loudness parameter, providing evidence that jets may thrive in thin disks. This is in contrast with decades of analytic and numerical work suggesting jet formation is contingent on the presence of an inner, geometrically thick disk structure, which serves to both collimate and accelerate the jet. Thick disks emerge in radiatively inefficient disks which are associated with sub-Eddington as well as super-Eddington accretion regimes yet we show that the inverse correlation between winds and jets survives where it should not, namely in a luminosity regime normally attributed to radio quiet active galaxies which are modeled with thin disks. This along with other lines of evidence argues against thick disks as the foundation behind the radio loud/radio quiet dichotomy, opening up the possibility that jetted versus non jetted black holes may be understood within the context of radiatively efficient thin disk accretion.

astro-ph.HE↗

arXiv2018

FR0 radio galaxies and their place in the radio morphology classification

So-called FR0 radio galaxies have recently emerged as a family of active galaxies with all the same properties as FRI radio galaxies except for their ratio of core to total emission which is about 30 times as high compared to FRI sources. We show how their properties fit within the gap paradigm as low prograde spinning black holes whose progenitors are powerful FRII quasars that transitioned rapidly from cold mode into advection dominated accretion in a few million years. The prediction is that if sufficient fuel exists, FR0 radio galaxies will evolve into full-fledged FRI radio galaxies and the observational dearth of FRI radio galaxies compared to FR0s at low redshift tells us about the supply of gas in the low redshift FR0s. Given the model prescription, this 5 to 1 FR0 to FRI ratio implies that at low redshift the FRII quasar class of active galaxies struggles to fuel its black hole beyond 1.3 times its original mass. In addition to this, we illustrate model prescriptions for the black hole mass, black hole spin, redshift and environment distribution for FR0 radio galaxies by fitting them within a paradigm that views them as a continuous class of active galaxies that are sandwiched between FRII quasars and FRI radio galaxies.

astro-ph.HE↗

arXiv2018

The distribution and lifetime of powerful radio galaxies as a function of environment and redshift

Correlations between jet power and active time for z < 0.1 high excitation and low excitation radio galaxies are explored as well as evidence in favor of a specific, non-random distribution for these objects including mid-infrared emitting radio galaxies as a function of environment and redshift. In addition, so-called weak line radio galaxies with FRII jet morphology have been identified as a class of active galaxies in the process of shutting down. This paper identifies common features between these seemingly disparate phenomena described above for the population of radio galaxies, and strings them together by way of a simple phenomenological framework that has shed light on the radio loud/radio quiet dichotomy, the jet-disk connection, and the distribution of all active galaxies as a function of redshift.

astro-ph.HE↗

arXiv2018

The redshift distribution of BL Lacs and FSRQs

Flat spectrum radio quasars (FSRQs) and BL Lacs are powerful jet producing active galactic nuclei associated with supermassive black holes accreting at high and low Eddington rates, respectively. Based on the Millennium Simulation, Gardner and Done (2014; 2018) have predicted their redshift distribution by appealing to ideas from the spin paradigm in a way that exposes a need for a deeper discussion on three interrelated issues: (1) an overprediction of BL Lacs compared to flat spectrum radio quasars; (2) a difference in FSRQ and BL Lac distributions; (3) a need for powerful but different jets at separated cosmic times. Beginning with Gardner and Done's determination of Fermi observable FSRQs based on the distribution of thermal accretion across cosmic time from the Millennium Simulation, we connect FSRQs to BL Lacs by way of the gap paradigm for black hole accretion and jet formation to address the above issues in a unified way. We identify a physical constraint in the paradigm for the numbers of BL Lacs that naturally leads to separate peaks in time for different albeit powerful jets. In addition, we both identify as puzzling and ascribe physical significance to a tail end in the BL Lac curve versus redshift that is unseen in the redshift distribution for FSRQs.

astro-ph.HE↗

arXiv2018

Magnetic reconnection and Blandford-Znajek process around rotating black holes

We provide a semi-analytic comparison between the Blandford-Znajek (BZ) and the magnetic reconnection power for accreting black holes in the curved spacetime of a rotating black hole. Our main result is that for a realistic range of astrophysical parameters, the reconnection power may compete with the BZ power. The field lines anchored close to or on the black hole usually evolve to open field lines in general relativistic magnetohydrodynamic (GRMHD) simulations. The BZ power is dependent on the black hole spin while magnetic reconnection power is independent of it for the force-free magnetic configuration with open field lines adopted in our theoretical study. This has obvious consequences for the time evolution of such systems particularly in the context of black hole X-ray binary state transitions. Our results provide analytical justification of the results obtained in GRMHD simulations.

astro-ph.HE↗

arXiv2017

Magnetic Fields Threading Black Holes: restrictions from general relativity and implications for astrophysical black holes

The idea that black hole spin is instrumental in the generation of powerful jets in active galactic nuclei and X-ray binaries is arguably the most contentious claim in black hole astrophysics. Because jets are thought to originate in the context of electromagnetism, and the modeling of Maxwell fields in curved spacetime around black holes is challenging, various approximations are made in numerical simulations that fall under the guise of 'ideal magnetohydrodynamics'. But the simplifications of this framework may struggle to capture relevant details of real astrophysical environments near black holes. In this work, we highlight tension between analytic and numerical results, specifically between the analytically derived conserved Noether currents for rotating black hole spacetimes and the results of general relativistic numerical simulations (GRMHD). While we cannot definitively attribute the issue to any specific approximation used in the numerical schemes, there seem to be natural candidates, which we explore. GRMHD notwithstanding, if electromagnetic fields around rotating black holes are brought to the hole by accretion, we show from first principles that prograde accreting disks likely experience weaker large-scale black hole-threading fields, implying weaker jets than in retrograde configurations.

physics.gen-ph↗

arXiv2016

Scale-invariant jet suppression across the black hole mass scale

We provide a schematic framework for understanding observations of jet suppression in soft state black hole X-ray binaries based on the Blandford-Payne process and the net magnetic flux threading the black hole. Due to the geometrical thinness of soft state disks, mass-loading of field lines is ineffective compared to both geometrically thick disks as well as thin disks with greater black hole threading flux, a simple physical picture that allows us to understand the weakness of jets in radiatively efficient thin disks accreting in the prograde direction around high-spinning black holes. Despite a simplicity that forbids insights into the complexity of turbulent-driven evolution or the physics of the observed short-term time variability, we show how the breadth of this framework is such that it can serve as a coarse-grained foundation for understanding black hole accretion and jet formation across the mass scale.

astro-ph.HE↗

arXiv2016

Possible Evolution of Supermassive Black Holes from FRI quasars

We explore the question of the rapid buildup of black hole mass in the early universe employing a growing black hole mass-based determination of both jet and disk powers predicted in recent theoretical work on black hole accretion and jet formation. Despite simplified, even artificial assumptions about accretion and mergers, we identify an interesting low probability channel for the growth of one billion solar mass black holes within hundreds of millions of years of the Big Bang without appealing to super Eddington accretion. This result is made more compelling by the recognition of a connection between this channel and an end product involving active galaxies with FRI radio morphology but weaker jet powers in mildly sub-Eddington accretion regimes. While FRI quasars have already been shown to occupy a small region of the available parameter space for black hole feedback in the paradigm, we further suggest that the observational dearth of FRI quasars is also related to their connection to the most massive black hole growth due to both these FRIs high redshifts and relative weakness. Our results also allow us to construct the AGN luminosity function at high redshift, that agree with recent studies. In short, we produce a connection between the unexplained paucity of a given family of active galactic nuclei and the rapid growth of supermassive black holes, two heretofore seemingly unrelated aspects of the physics of active galactic nuclei.

astro-ph.HE↗

arXiv2015

Reconciling AGN-star formation, the Soltan argument, and Meier's paradox

We provide a theoretical context for understanding the recent work of Kalfountzou et al (2014) showing that star formation is enhanced at lower optical luminosity in radio loud quasars. Our proposal for coupling the assumption of collimated FRII quasar jet-induced star formation with lower accretion optical luminosity, also explains the observed jet power peak in active galaxies at higher redshift compared to the peak in accretion power, doing so in a way that predicts the existence of a family of radio quiet AGN associated with rapidly spinning supermassive black holes at low redshift, as mounting observations suggest. The relevance of this work lies in its promise to explain the observed cosmological evolution of accretion power, jet power, and star formation, in a way that is both compatible with the Soltan argument and resolves the so-called `Meier Paradox'.

astro-ph.GA↗