Below is a list of my most recent publications automatically pulled from ADS, updated ~weekly (but may be out of date while I update to new ADS API), or on your command. You may also want to check out my ADS listing, my arXiv listing, and my Orcid page.
In the published article, we presented inferences about the population of supermassive black hole (SMBH) binaries emitting at nanohertz gravitational-wave (GW) frequencies based on the 15 yr dataset from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). Here, we report two bugs in the astrophysical analysis software used for the published article. While parameter posteriors of our inference can be seen to have slight differences on close inspection, none of the conclusions from the original publication have changed, including the finding that the measured GW background is dominated by the most massive, high-mass-ratio SMBH binaries. Our corrected results show slightly less evidence for environmental interactions with SMBH binaries and indicate that the population of SMBH binaries contributing to the GW background occurs at slightly higher mass ratios and redshifts than indicated in the published article. Correcting these errors does not change the central conclusion that astrophysically motivated models of SMBH binary populations are able to reproduce both the amplitude and shape of the observed low-frequency GW spectrum. These errors did not affect any other analyses of the NANOGrav 15 yr dataset, including the evidence for a GW background presented in another of our published articles.
We present a stellar-dynamical mass measurement of the central black hole in the lenticular galaxy NGC 5102 (SA0<SUP>−</SUP>). Our analysis combines high-quality integral-field spectroscopy from the Very Large Telescope Multi Unit Spectroscopic Explorer with high-spatial- and high-spectral-resolution Hubble Space Telescope Imaging Spectrograph observations, using the Ca II triplet as a stellar kinematic tracer. We constrain the black hole mass with axisymmetric, three-integral Schwarzschild orbit-superposition models, incorporating surface-brightness measurements from Hubble Space Telescope F547M WFPC2 imaging. Assuming a distance of 3.66 Mpc, we find a black hole mass of (1.30−0.18+0.19)×106M⊙, which is within 1.7σ of a previous CO band-head-based Jeans anisotropic modeling result (M∙=(9.1−1.5+1.8)×105M⊙). Our measurement is also consistent with literature extrapolations of the M<SUB>∙</SUB>─σ<SUB>e</SUB> relation into the currently undersampled low-mass regime. The close agreement between these independent dynamical approaches provides external validation of the Jeans anisotropic modeling framework and supports the robustness of our Schwarzschild orbit-superposition result, bolstering confidence in future black hole mass measurements with this framework.
We quantify pulsar timing array (PTA) sensitivity to anisotropy in the gravitational wave background using the cross-correlation based Fisher information matrix in the pixel and spherical harmonic bases. We use a set of simulations to empirically determine scaling relations of a PTA's sensitivity to anisotropy with the number of pulsars $N_\mathrm{psr}$ in the array, the error $δt$ on the times of arrival, the frequency $f_\mathrm{GW}$ of the gravitational waves, and the angular scale $∆Ω$ of the anisotropy. The sensitivity scales approximately as $N_\mathrm{psr}^{0.8}$, $δt^{-0.08}$, and $∆Ω^{1.6}-∆Ω^{2.1}$ (depending on the ranges of $\ell$ and $m$ under consideration). In addition, we use realistic simulations to project the NANOGrav PTA sensitivity to a 30-year baseline and quantify the growth in sensitivity at several timeslices. Except at the lowest frequencies, we find negligible effect on sensitivity through increasing the observation duration only. Finally, we introduce a multi-resolution pixel basis motivated by the large dependence of the sensitivity on sky location, and demonstrate the operation of the basis through a set of injections and recoveries.
The observed gravitational-wave background (GWB) spectrum is higher in amplitude than model predictions by a factor of 2─3. Using a semi-analytic model, we evaluate the effect of a high-scatter supermassive black hole (SMBH) scaling relation (M<SUB>BH</SUB>─M<SUB>bulge</SUB>) on models of the nanohertz GWB. By implementing an intrinsic scatter of the M<SUB>BH</SUB>─M<SUB>bulge</SUB> relation, which is larger at higher redshift, but matches local observations, we find that the amplitude of GWB models increases to be consistent with the low-frequency end of the GWB spectrum. This amplitude increase is not uniform across frequencies, a strongly evolving scatter preferentially increases the number density of the most massive SMBHs which, in the GWB spectrum, minimizes the strength of the low-frequency turnover. Our models with positively evolving intrinsic scatter can reproduce the electromagnetically observed overmassive SMBHs at 4 < z < 6 without changing the M<SUB>BH</SUB>─M<SUB>bulge</SUB> normalization though we find that including moderate normalization evolution marginally improves fits to the GWB data. We conclude that the M<SUB>BH</SUB>─M<SUB>bulge</SUB> relation which best describes the available GWB and electromagnetic data sets has intrinsic scatter that evolves as ∊(z)=∊0+(0.56±0.4)log10(1+z) and normalization that evolves as α(z) = α<SUB>0</SUB>(1 + z)<SUP>0.84±0.35</SUP>. The results of this work imply that the M<SUB>BH</SUB>─M<SUB>bulge</SUB> relation we see today is not universal throughout cosmic time and that a diversity of seeding models and growth mechanisms may be at play in the early stages of SMBH─galaxy evolution.
The NANOGrav 15 yr (NG15) data set provides evidence for a gravitational-wave background (GWB) signal at nHz frequencies, which is expected to originate either from a cosmic population of inspiraling supermassive black hole binaries or new particle physics in the early Universe. A firm identification of the source of the NG15 signal requires an accurate reconstruction of its frequency spectrum. In this Letter, we provide such a spectral characterization of the NG15 signal based on a piecewise power-law (PPL) ansatz that strikes a balance between existing alternatives in the literature. Our PPL reconstruction is more flexible than the standard constant power-law model, which describes the GWB spectrum in terms of only two parameters: an amplitude A and a spectral index γ. Concurrently, it better approximates physically realistic GWB spectra—especially those of cosmological origin—than the free spectral model, since the latter allows for arbitrary variations in the GWB amplitude from one frequency bin to the next. Our PPL reconstruction of the NG15 signal relies on individual PPL models with a fixed number of internal nodes (i.e., constant power law, broken power law, doubly broken power law, etc.), which are ultimately combined in a Bayesian model average. The data products resulting from our analysis provide the basis for fast refits of spectral GWB models.
The accretion mechanism and SED of black holes at low luminosities are critical questions regarding BHs, but they are still poorly understood. We propose for Chandra data of 22 LLAGN, that combined with existing archival data on 9, will yield a complete data set of all 31 galaxies in the Gemini/NIFS AO LLP black-hole mass campaign. These data will enable massive science: 1. Create the best LLAGN SEDs by virtue of the amount and quality of the data available for a sample with a broad range of black hole masses and Eddington ratios. 2. Specifically study the L_X-L_NIR relation, which is critical for understanding LLAGN and for making the most use of upcoming JWST data. 3. Add value to the Gemini LLP black-hole mass campaign by measuring BH accretion when Gemini find BH mass upper limits.
We propose to carry out a large, uniform, survey for dual AGNs in distant galaxies using archival Chandra surveys. Currently, there is no systematic study of the evolution of dual AGN at high-z; and observational constraints on the dual AGN fraction in the nearby universe are higher than predicted from simulations, resulting in an inconsistent expected dual AGN rate at z>1. We will be able to accurately determine the dual AGN rate (<0.5%), as well as a measure the dual AGN fraction as a function of redshift (<2%). Our tight constraint on the dual AGN fraction as a function of z will allow us to statistically differentiate between the low- and high-end predictions for the fraction of dual AGN across cosmic time.
We propose to observe a sample of 50 nearby (z<0.037) AGN in order to measure the dual AGN fraction in the small-separation regime, where current angular resolution limits have prevented systematic analyses. We will analyze the sample with BAYMAX, a tool we’ve developed that uses a Bayesian framework to quantitatively evaluate whether a given source in a Chandra observation is a single or dual point source for separations <0.5”. We plan to combine this sample with archival observations of 36 AGN to constrain, for the first time, the nearby dual AGN fraction. The outcome of this study will be a measurement of the local dual AGN fraction, to within 2.8%, at smaller limiting separations (14-260 pc) than has ever been done before.
We propose to analyze a sample of 26 nearby (z<0.035) AGN in order to measure the dual AGN fraction in the small-separation regime, where current angular resolution limits have prevented systematic analyses. We will analyze the sample with BAYMAX, a tool we’ve developed that uses a Bayesian framework to quantitatively evaluate whether a given source in a Chandra observation is a single or dual point source for separations <0.5”. We plan to combine this sample with new observations of 55 AGN to constrain, for the first time, the nearby dual AGN fraction. The outcome of this study will be a measurement of the local dual AGN fraction, to within 2.8%, at smaller limiting separations (14-250 pc) than has ever been done before.
We propose a 100 ksec observation of the core of BCG 2261 to test for the presence of a recoiling SMBH. Binary SMBHs are thought to scour out cores in the host galaxy before coalescence of the black holes, which can lead to large recoils. Despite the importance of the connection between binary BHs, strong gravity, and galaxy evolution, it has never been conclusively observed. Without confirmation, we don’t know if binary SMBHs can create stellar cores achieve high recoil velocities. We can produce the first direct observational proof of a recoiling SMBH in BCG 2261, the strongest candidate to date to host a recoiling SMBH and an extreme stellar core. With a detection, we will finally have definitive observational evidence connecting core formation, gravitational waves, and binary BHs.
The frequency of dual AGNs at low galaxy/black hole mass is poorly constrained. Thus we lack a full physical understanding of the connection between galaxy mergers and AGN activity and therefore merger-driven feedback. In particular, it is unknown whether or not LLAGN can be triggered by mergers instead of only by stochastic processes. We will address this with a 50 ksec observation to test for a dual AGN in SDSS J0914+0853, a low-mass (MBH 10^6.3), dual LLAGN candidate based on serendipitous, shallow Chandra imaging. The 15-ksec data showed two X-ray sources, but the nature of the secondary source is ambiguous because of 10% pile-up and potential PSF artifacts. With deeper, short-frame-rate Chandra observations at a new roll angle, we can unambiguously determine if the secondary is real.
Coming Soon
Supermassive black holes, once thought to be theoretical novelties, are now considered to play a major role in many astrophysical phenomena including galaxy evolution. Now that we live in the era of gravitational wave observations, it is interesting to look forward to a time when we can detect gravitational waves from supermassive black hole coalescence. A major question remains: Do supermassive black holes merge? I will review the case for supermassive black holes as active players in the universe, focusing on the black hole outflows. Then I will concentrate on my group’s recent work searching for dual and binary AGNs along with recent developments: (1) closer inspection of time-domain-identified binary candidates; (2) a Bayesian framework for determining duality in a Chandra observation; and (3) spectroscopic and time-domain identification of low-mass-ratio binary AGN.
Supermassive black holes, once thought to be theoretical novelties, are now considered to play a major role in many astrophysical phenomena including galaxy evolution. Now that we live in the era of gravitational wave observations, it is interesting to look forward to a time when we can detect gravitational waves from supermassive black hole coalescence. A major question remains: Do supermassive black holes merge? I will review the case for supermassive black holes as active players in the universe, focusing on black hole outflows. Then I will concentrate on my group’s recent work searching for dual and binary supermassive black holes along with recent developments: (1) closer inspection of time-domain-identified binary candidates; (2) a Bayesian framework for determining duality in a Chandra observation; and (3) spectroscopic and time-domain identification of low-mass-ratio binaries.
Supermassive black holes, once thought to be theoretical novelties, are now considered to play a major role in many astrophysical phenomena including galaxy evolution. Now that we live in the era of gravitational wave observations, it is interesting to look forward to a time when we can detect gravitational waves from supermassive black hole coalescence. A major question remains: Do supermassive black holes merge? I will review the case for supermassive black holes as active players in the universe, focusing on the black hole outflows. Then I will concentrate on my group’s recent work searching for dual and binary supermassive black holes along with recent developments: (1) closer inspection of time-domain-identified binary candidates; (2) a Bayesian framework for determining duality in a Chandra observation; and (3) spectroscopic and time-domain identification of low-mass-ratio binaries.
Supermassive black holes, once thought to be theoretical novelties, are now considered to play a major role in many astrophysical phenomena including galaxy evolution. Now that we live in the era of gravitational wave observations, it is interesting to look forward to a time when we can detect gravitational waves from supermassive black hole coalescence. A major question remains: Do supermassive black holes merge? I will review the case for supermassive black holes as active players in the universe, focusing on the black hole outflows. Then I will focus on my recent work searching for dual and binary AGNs along with recent developments: (1) closer inspection of time-domain-identified binary candidates; (2) a Bayesian framework for determining duality in a Chandra observation; and (3) spectroscopic and time-domain identification of low-mass-ratio binary AGN.
Supermassive black holes, once thought to be theoretical novelties, are now considered to play a major role in many astrophysical phenomena including galaxy evolution. Now that we live in the era of gravitational wave observations, it is interesting to look forward to a time when we can detect gravitational waves from supermassive black hole coalescence. A major question remains: Do supermassive black holes merge? I will review the case for supermassive black holes role in the universe, focusing on the black hole mass scaling relations. Then I will focus on my recent work searching for dual and binary AGNs along with recent developments: (1) closer inspection of time-domain-identified binary candidates; (2) a Bayesian framework for determining duality in a Chandra observation; and (3) spectroscopic and time-domain identification of low-mass-ratio binary AGN.
Supermassive black holes, once thought to be theoretical novelties, are now considered to play a major role in many astrophysical phenomena including galaxy evolution. Now that we live in the era of gravitational wave observations, it is interesting to look forward to a time when we can detect gravitational waves from supermassive black hole coalescence. A major question remains: Do supermassive black holes merge? I will review the case for supermassive black holes role in the universe, focusing on the black hole mass scaling relations. Then I will introduce a new, empirical scaling relation that can be used for black hole mass estimation. Finally I will discuss the prospects and pitfalls of searching for dual and binary AGNs along with recent devlopments. These include (1) closer inspection of time-domain-identified binary candidates; (2) a Bayesian framework for determining duality in a Chandra observation; and (3) spectroscopic and time-domain identification of low-mass-ratio binary AGN.