arXiv subjects

Jillian Bellovary

Publications and source records attributed to Jillian Bellovary.

At least 19 recordsLinked to original sources

arXiv2025

Handy Relation Between Binary Black Hole Merger Times and Host Galaxy Properties

Over the past 15 years, the evidence has clearly demonstrated that massive black hole (MBH) binary merger timescales depend strongly on the structural and kinematic properties of their host galaxy. Stellar density, gas content, shape and kinematics all play a role, combining in non-linear ways to effect the evolution of the binary. The binary properties themselves, such as eccentricity, mass ratio, and orbital plane, all matter as well. This makes it nontrivial to estimate accurate cosmological MBH binary merger rates, or to generate merger rate ranges that reflect the distribution of galaxy hosts and orbits. Using an extensive set of high-resolution direct N-body simulations in which the shape, structure, and kinematics of each galaxy host are directly informed by observations, we map out MBH binary merger timescales over a range of galaxy hosts and MBH binary orbits. This yields a convenient set of scaling relations to determine MBH binary merger timescales -- and the range of merger timescales -- as functions of basic observables. Such scaling relations can be readily employed as a subgrid model in cosmological or semi-analytic studies, for example, to model event rates for LISA or pulsar timing.

astro-ph.GA↗

arXiv2025

Intermediate Mass Ratio Inspirals in Milky Way Galaxies

A consequence of a non-zero occupation fraction of massive black holes (MBHs) in dwarf galaxies is that these MBHs can become residents of larger galaxy halos via hierarchical merging and tidal stripping. Depending on the parameters of their orbits and original hosts, some of these MBHs will merge with the central supermassive black hole in the larger galaxy. We examine four cosmological zoom-in simulations of Milky Way-like galaxies to study the demographics of the black hole mergers which originate from dwarf galaxies. Approximately half of these mergers have mass ratios less than 0.04, which we categorize as intermediate mass ratio inspirals, or IMRIs. Inspiral durations range from 0.5 - 8 Gyr, depending on the compactness of the dwarf galaxy. Approximately half of the inspirals may become more circular with time, while the eccentricity of the remainder does not evolve. Overall, IMRIs in Milky Way-like galaxies are a significant class of black hole merger that can be detected by LISA, and must be prioritized for waveform modeling.

astro-ph.GA↗

arXiv2025

Little Red Dots are Tidal Disruption Events in Runaway-Collapsing Clusters

I hypothesize a physical explanation for the "Little Red Dots" (LRDs) discovered by the James Webb Space Telescope (JWST). The first star formation in the universe occurs in dense clusters, some of which may undergo runaway collapse and form an intermediate mass black hole. This process would appear as a very dense stellar system, with recurring tidal disruption events (TDEs) as stellar material is accreted by the black hole. Such a system would be compact, UV-emitting, and exhibit broad H-alpha emission. If runaway collapse is the primary mechanism for forming massive black hole seeds, this process could be fairly common and explain the large volume densities of LRDs. In order to match the predicted number density of runaway collapse clusters, the tidal disruption rate must be on the order of 10^-4 per year. A top-heavy stellar initial mass function may be required to match observations without exceeding the predicted LambdaCDM mass function. The TDE LRD hypothesis can be verified with followup JWST observations looking for TDE-like variability.

astro-ph.GA↗

arXiv2024

An Enhanced Massive Black Hole Occupation Fraction Predicted in Cluster Dwarf Galaxies

The occupation fraction of massive black holes (MBHs) in dwarf galaxies offers interesting insights into initial black hole seeding mechanisms and their mass assembly history, though disentangling these two effects remains challenging. Using the {\sc Romulus} cosmological simulations we examine the impact of environment on the occupation fraction of MBHs in low mass galaxies. Unlike most modern cosmological simulations, {\sc Romulus} seeds MBHs based on local gas properties, selecting dense ($n>3$ cm$^{-3}$), pristine ($Z<3e-4Z_{\odot}$), and rapidly collapsing regions in the early Universe as sites to host MBHs without assuming anything about MBH occupation as a function of galaxy stellar mass, or halo mass, {\it a priori}. The simulations predict that dwarf galaxies with M$_{\star}<10^9$ M$_{\odot}$ in cluster environments are $\sim2$ times more likely to host a MBH compared to those in the field. The predicted occupation fractions are remarkably consistent with those of nuclear star clusters. Across cluster and field environments, dwarf galaxies with earlier formation times are more likely to host a MBH. While the MBH occupation function is similar between cluster and field environments at high redshift ($z>3$), a difference arises as late-forming dwarfs -- which do not exist in the cluster environment -- begin to dominate in the field and pull the MBH occupation fraction down for low mass galaxies. Additionally, prior to in-fall some cluster dwarfs are similar to progenitors of massive, isolated galaxies, indicating that they might have grown to higher masses had they not been impeded by the cluster environment. While the population of MBHs in dwarf galaxies is already widely understood to be important for understanding MBH formation, this work demonstrates that environmental dependence is important to consider as future observations search for low mass black holes in dwarf galaxies.

astro-ph.GA↗

arXiv2023

Astrophysics with the Laser Interferometer Space Antenna

The Laser Interferometer Space Antenna (LISA) will be a transformative experiment for gravitational wave astronomy, and, as such, it will offer unique opportunities to address many key astrophysical questions in a completely novel way. The synergy with ground-based and space-born instruments in the electromagnetic domain, by enabling multi-messenger observations, will add further to the discovery potential of LISA. The next decade is crucial to prepare the astrophysical community for LISA's first observations. This review outlines the extensive landscape of astrophysical theory, numerical simulations, and astronomical observations that are instrumental for modeling and interpreting the upcoming LISA datastream. To this aim, the current knowledge in three main source classes for LISA is reviewed; ultracompact stellar-mass binaries, massive black hole binaries, and extreme or intermediate mass ratio inspirals. The relevant astrophysical processes and the established modeling techniques are summarized. Likewise, open issues and gaps in our understanding of these sources are highlighted, along with an indication of how LISA could help making progress in the different areas. New research avenues that LISA itself, or its joint exploitation with upcoming studies in the electromagnetic domain, will enable, are also illustrated. Improvements in modeling and analysis approaches, such as the combination of numerical simulations and modern data science techniques, are discussed. This review is intended to be a starting point for using LISA as a new discovery tool for understanding our Universe.

gr-qc↗

arXiv2022

AGN quenching in simulated dwarf galaxies

We examine the quenching characteristics of $328$ isolated dwarf galaxies $\left(10^{8} < M_{\rm star}/M_\odot < 10^{10} \right)$ within the \Rom{} cosmological hydrodynamic simulation. Using mock observation methods, we identify isolated dwarf galaxies with quenched star formation and make direct comparisons to the quenched fraction in the NASA Sloan Atlas (NSA). Similar to other cosmological simulations, we find a population of quenched, isolated dwarf galaxies below $M_{\rm star} < 10^{9} M_\odot$ not detected within the NSA. We find that the presence of massive black holes (MBHs) in \Rom{} is largely responsible for the quenched, isolated dwarfs, while isolated dwarfs without an MBH are consistent with quiescent fractions observed in the field. Quenching occurs between $z=0.5-1$, during which the available supply of star-forming gas is heated or evacuated by MBH feedback. Mergers or interactions seem to play little to no role in triggering the MBH feedback. At low stellar masses, $M_{\rm star} \lesssim 10^{9.3} M_\odot$, quenching proceeds across several Gyr as the MBH slowly heats up gas in the central regions. At higher stellar masses, $M_{\rm star} \gtrsim 10^{9.3} M_\odot$, quenching occurs rapidly within $1$ Gyr, with the MBH evacuating gas from the central few kpc of the galaxy and driving it to the outskirts of the halo. Our results indicate the possibility of substantial star formation suppression via MBH feedback within dwarf galaxies in the field. On the other hand, the apparent over-quenching of dwarf galaxies due to MBH suggests higher resolution and/or better modeling is required for MBHs in dwarfs, and quenched fractions offer the opportunity to constrain current models.

astro-ph.GA↗

arXiv2022

A hidden population of massive black holes in simulated dwarf galaxies

We explore the characteristics of actively accreting MBHs within dwarf galaxies in the \textsc{Romulus25} cosmological hydrodynamic simulation. We examine the MBH occupation fraction, x-ray active fractions, and AGN scaling relations within dwarf galaxies of stellar mass $10^{8} < M_{\rm star} < 10^{10} M_\odot$ out to redshift $z=2$. In the local universe, the MBH occupation fraction is consistent with observed constraints, dropping below unity at $M_{\rm star} < 3\times10^{10} M_{\odot}$, $M_{\rm 200} < 3\times10^{11} M_\odot$. Local dwarf AGN in \textsc{Romulus25} follow observed scaling relations between AGN x-ray luminosity, stellar mass, and star formation rate, though they exhibit slightly higher active fractions and number densities than comparable x-ray observations. Since $z=2$, the MBH occupation fraction has decreased, the population of dwarf AGN has become overall less luminous, and as a result, the overall number density of dwarf AGN has diminished. We predict the existence of a large population of MBHs in the local universe with low x-ray luminosities and high contamination from x-ray binaries and the hot interstellar medium that are undetectable by current x-ray surveys. These hidden MBHs make up $76\%$ of all MBHs in local dwarf galaxies, and include many MBHs that are undermassive relative to their host galaxy's stellar mass. Their detection relies not only on greater instrument sensitivity but on better modeling of x-ray contaminants or multi-wavelength surveys. Our results indicate dwarf AGN were substantially more active in the past despite being low-luminosity today, and indicate future deep x-ray surveys may uncover many hidden MBHs in dwarf galaxies out to at least $z=2$.

astro-ph.GA↗

arXiv2021

Black Hole Growth and Feedback in Isolated Romulus25 Dwarf Galaxies

We investigate the effects of massive black hole growth on the structural evolution of dwarf galaxies within the Romulus25 cosmological hydrodynamical simulation. We study a sample of 228 central, isolated dwarf galaxies with stellar masses $M_{star} < 10^{10} M_\odot$ and a central BH. We find that the local $M_{BH} - M_{star}$ relation exhibits a high degree of scatter below $M_{star} < 10^{10} M_\odot$, which we use to classify BHs as overmassive or undermassive relative to their host $M_{star}$. Overmassive BHs grow through a mixture of BH mergers and relatively high average accretion rates, while undermassive BHs grow slowly through accretion. We find that isolated dwarf galaxies that host overmassive BHs also follow different evolutionary tracks relative to their undermassive BH counterparts, building up their stars and dark matter earlier and experiencing star formation suppression starting around $z=2$. By $z=0.05$, overmassive BH hosts above $M_{star} > 10^{9} M_\odot$ are more likely to exhibit lower central stellar mass density, lower HI gas content, and lower star formation rates than their undermassive BH counterparts. Our results suggest that overmassive BHs in isolated galaxies above $M_{star} > 10^{9} M_\odot$ are capable of driving feedback, in many cases suppressing and even quenching star formation by late times.

astro-ph.GA↗

arXiv2020

Getting Ready for LISA: The Data, Support and Preparation Needed to Maximize US Participation in Space-Based Gravitational Wave Science

The NASA LISA Study Team was tasked to study how NASA might support US scientists to participate and maximize the science return from the Laser Interferometer Space Antenna (LISA) mission. LISA is gravitational wave observatory led by ESA with NASA as a junior partner, and is scheduled to launch in 2034. Among our findings: LISA science productivity is greatly enhanced by a full-featured US science center and an open access data model. As other major missions have demonstrated, a science center acts as both a locus and an amplifier of research innovation, data analysis, user support, user training and user interaction. In its most basic function, a US Science Center could facilitate entry into LISA science by hosting a Data Processing Center and a portal for the US community to access LISA data products. However, an enhanced LISA Science Center could: support one of the parallel independent processing pipelines required for data product validation; stimulate the high level of research on data analysis that LISA demands; support users unfamiliar with a novel observatory; facilitate astrophysics and fundamental research; provide an interface into the subtleties of the instrument to validate extraordinary discoveries; train new users; and expand the research community through guest investigator, postdoc and student programs. Establishing a US LISA Science Center well before launch can have a beneficial impact on the participation of the broader astronomical community by providing training, hosting topical workshops, disseminating mock catalogs, software pipelines, and documentation. Past experience indicates that successful science centers are established several years before launch; this early adoption model may be especially relevant for a pioneering mission like LISA.

astro-ph.IM↗

arXiv2020

Orbital Migration of Interacting Stellar Mass Black Holes in Disks around Supermassive Black Holes II. Spins and Incoming Objects

The masses, rates, and spins of merging stellar-mass binary black holes (BBHs) detected by aLIGO and Virgo provide challenges to traditional BBH formation and merger scenarios. An active galactic nucleus (AGN) disk provides a promising additional merger channel, because of the powerful influence of the gas that drives orbital evolution, makes encounters dissipative, and leads to migration. Previous work showed that stellar mass black holes (sBHs) in an AGN disk migrate to regions of the disk, known as migration traps, where positive and negative gas torques cancel out, leading to frequent BBH formation. Here we build on that work by simulating the evolution of additional sBHs that enter the inner disk by either migration or inclination reduction. We also examine whether the BBHs formed in our models have retrograde or prograde orbits around their centers of mass with respect to the disk, determining the orientation, relative to the disk, of the spin of the merged BBHs. Orbiters entering the inner disk form BBHs with sBHs on resonant orbits near the migration trap. When these sBHs reach ~80 Msun, they form BBHs with sBHs in the migration trap, which over 10 Myr reach ~1000 Msun. We find 68% of the BBHs in our simulation orbit in the retrograde direction, which implies BBHs in our merger channel will have small dimensionless aligned spins, χ_eff. Overall, our models produce BBHs that resemble both the majority of BBH mergers detected thus far (0.66 to 120 Gpc^-3 yr^-1 ) and two recent unusual detections, GW190412 (~0.3 Gpc^-3 yr^-1 ) and GW190521 (~0.1 Gpc^-3 yr^-1 ).

astro-ph.HE↗

arXiv2020

The role of gravitational recoil in the assembly of massive black hole seeds

When two black holes merge, the asymmetric emission of gravitational waves provides an impulse to the merged system; this gravitational wave recoil velocity can be up to 4000 km s$^{-1}$, easily fast enough for the black hole to escape its host galaxy. We combine semi-analytic modeling with cosmological zoom-in simulations of a Milky Way-type galaxy to investigate the role of black hole spin and gravitational recoil in the epoch of massive black hole seeding. We sample four different spin distributions (random, aligned, anti-aligned, and zero spin), and compare the resulting merger rates, occupation fractions, and MBH-host relations with what is expected by excluding the effect of recoil. The inclusion of gravitational recoil and MBH spin in the assembly of MBH seeds can reduce the final $z=5$ MBH mass by up to an order of magnitude. The MBH occupation fraction, however, remains effectively unaltered due to episodes of black hole formation following a recoil event. While electromagnetic detections of these events are unlikely, LISA is ideally suited to detect gravitational wave signals from such events.

astro-ph.GA↗

arXiv2019

Building A Field: The Future of Astronomy with Gravitational Waves, A State of The Profession Consideration for Astro2020

Harnessing the sheer discovery potential of gravitational wave astronomy will require bold, deliberate, and sustained efforts to train and develop the requisite workforce. The next decade requires a strategic plan to build -- from the ground up -- a robust, open, and well-connected gravitational wave astronomy community with deep participation from traditional astronomers, physicists, data scientists, and instrumentalists. This basic infrastructure is sorely needed as an enabling foundation for research. We outline a set of recommendations for funding agencies, universities, and professional societies to help build a thriving, diverse, and inclusive new field.

astro-ph.IM↗

arXiv2019

Orbital Migration of Interacting Stellar Mass Black Holes in Disks around Supermassive Black Holes

The merger rate of stellar-mass black hole binaries (sBHBs) inferred by the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) suggests the need for an efficient source of sBHB formation. Active galactic nucleus (AGN) disks are a promising location for the formation of these sBHBs, as well as binaries of other compact objects, because of powerful torques exerted by the gas disk. These gas torques cause orbiting compact objects to migrate towards regions in the disk where inward and outward torques cancel, known as migration traps. We simulate the migration of stellar mass black holes in an example of a model AGN disk, using an augmented N-body code that includes analytic approximations to migration torques, stochastic gravitational forces exerted by turbulent density fluctuations in the disk, and inclination and eccentricity dampening produced by passages through the gas disk, in addition to the standard gravitational forces between objects. We find that sBHBs form rapidly in our model disk as stellar-mass black holes migrate towards the migration trap. These sBHBs are likely to subsequently merge on short time-scales. The process continues, leading to the build-up of a population of over-massive stellar-mass black holes. The formation of sBHBs in AGN disks could contribute significantly to the sBHB merger rate inferred by LIGO.

astro-ph.HE↗

arXiv2019

The Laser Interferometer Space Antenna: Unveiling the Millihertz Gravitational Wave Sky

The first terrestrial gravitational wave interferometers have dramatically underscored the scientific value of observing the Universe through an entirely different window, and of folding this new channel of information with traditional astronomical data for a multimessenger view. The Laser Interferometer Space Antenna (LISA) will broaden the reach of gravitational wave astronomy by conducting the first survey of the millihertz gravitational wave sky, detecting tens of thousands of individual astrophysical sources ranging from white-dwarf binaries in our own galaxy to mergers of massive black holes at redshifts extending beyond the epoch of reionization. These observations will inform - and transform - our understanding of the end state of stellar evolution, massive black hole birth, and the co-evolution of galaxies and black holes through cosmic time. LISA also has the potential to detect gravitational wave emission from elusive astrophysical sources such as intermediate-mass black holes as well as exotic cosmological sources such as inflationary fields and cosmic string cusps.

astro-ph.IM↗

arXiv2019

The Social Sciences Interdisciplinarity for Astronomy and Astrophysics -- Lessons from the History of NASA and Related Fields

In this paper we showcase the importance of understanding and measuring interdisciplinarity and other -disciplinarity concepts for all scientists, the role social sciences have historically played in NASA research and missions, the sparsity of social science interdisciplinarity in space and planetary sciences, including astronomy and astrophysics, while there is an imperative necessity for it, and the example of interdisciplinarity between social sciences and astrobiology. Ultimately we give voice to the scientists across all fields with respect to their needs, aspirations and experiences in their interdisciplinary work with social sciences through an ad-hoc survey we conducted within the Astro2020 Decadal Survey scientific community.

astro-ph.IM↗

arXiv2019

The evolution of kicked stellar-mass black holes in star cluster environments II. Rotating star clusters

In this paper, we continue our study on the evolution of black holes (BHs) that receive velocity kicks at the origin of their host star cluster potential. We now focus on BHs in rotating clusters that receive a range of kick velocities in different directions with respect to the rotation axis. We perform N-body simulations to calculate the trajectories of the kicked BHs and develop an analytic framework to study their motion as a function of the host cluster and the kick itself. Our simulations indicate that for a BH that is kicked outside of the cluster's core, as its orbit decays in a rotating cluster the BH will quickly gain angular momentum as it interacts with stars with high rotational frequencies. Once the BH decays to the point where its orbital frequency equals that of local stars, its orbit will be circular and dynamical friction becomes ineffective since local stars will have low relative velocities. After circularization, the BH's orbit decays on a longer timescale than if the host cluster was not rotating. Hence BHs in rotating clusters will have longer orbital decay times. The timescale for orbit circularization depends strongly on the cluster's rotation rate and the initial kick velocity, with kicked BHs in slowly rotating clusters being able to decay into the core before circularization occurs. The implication of the circularization phase is that the probability of a BH undergoing a tidal capture event increases, possibly aiding in the formation of binaries and high-mass BHs.

astro-ph.GA↗

arXiv2019

Disentangling nature from nurture: tracing the origin of seed black holes

The origin and properties of black hole seeds that grow to produce the detected population of supermassive black holes are unconstrained at present. Despite the existence of several potentially feasible channels for the production of initial seeds in the high redshift universe, since even actively growing seeds are not directly observable at these epochs, discriminating between models remains challenging. Several new observables that encapsulate information about seeding have been proposed in recent years, and these offer exciting prospects for truly unraveling the nature of black hole seeds in the coming years. One of the key challenges for this task lies in the complexity of the problem, the required disentangling of the confounding effects of accretion physics and mergers, as mergers and accretion events over cosmic time stand to erase these initial conditions. Nevertheless, some unique signatures of seeding do survive and still exist in: local scaling relations between black holes and their galaxy hosts at low-masses; in high-redshift luminosity functions of accreting black holes; and in the total number and mass functions of gravitational wave coalescence events from merging binary black holes. One of the clearest discriminants for seed models are these high redshift gravitational wave detections of mergers from space detectable in the milliHertz range. These predicted event rates offer the most direct constraints on the properties of initial black hole seeds. Improving our theoretical understanding of black hole dynamics and accretion will also be pivotal in constraining seeding models in combination with the wide range of multi-messenger data.

astro-ph.HE↗

arXiv2019

Where are the Intermediate Mass Black Holes?

Observational evidence has been mounting for the existence of intermediate mass black holes (IMBHs, 10^2-10^5 Msun), but observing them at all, much less constraining their masses, is very challenging. In one theorized formation channel, IMBHs are the seeds for supermassive black holes in the early universe. As a result, IMBHs are predicted to exist in the local universe in dwarf galaxies, as well as wandering in more massive galaxy halos. However, these environments are not conducive to the accretion events or dynamical signatures that allow us to detect IMBHs. The Laser Interferometer Space Antenna (LISA) will demystify IMBHs by detecting the mergers of these objects out to extremely high redshifts, while measuring their masses with extremely high precision. These observations of merging IMBHs will allow us to constrain the formation mechanism and subsequent evolution of massive black holes, from the 'dark ages' to the present day, and reveal the role that IMBHs play in hierarchical galaxy evolution.

astro-ph.HE↗