The Basics

Education

  • PhD Astronomy, August 2006
    • University of Maryland, College Park
    • Advisor: Dr. M. Coleman Miller
    • Thesis: ‚ÄúGrowing Intermediate-Mass Black Holes with Gravitational Waves‚ÄĚ Download PDF
  • MS Astronomy, December 2002
    • University of Maryland, College Park
  • BA with Distinction Physics (Astrophysics concentration), May 1999
    • University of Pennsylvania

Appointments

  • Assistant Professor
    • University of Michigan
    • 2016‚Äďpresent
  • Assistant Research Scientist
    • University of Michigan
    • 2009‚Äď2015
  • Postdoctoral Fellow
    • University of Michigan
    • 2006‚Äď2009

Metrics

  • 157 publications
  • 40 first-author publications
  • 6297 citations total
  • 2116 first-author citations
  • h-index = 37
  • g-index = 80
  • Total funding = $1,424,053

Publications

Below is a list of my most recent publications automatically pulled from ADS, updated ~weekly (but may be out of date while I update to new ADS API), or on your command.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.

all publications

We present a stellar dynamical mass measurement of the supermassive black hole in the elliptical (E1) galaxy NGC 3258. Our findings are based on integral field unit spectroscopy from the Multi Unit Spectroscopic Explorer (MUSE) observations in narrow-field mode with adaptive optics and the MUSE wide-field mode, from which we extract kinematic information by fitting the Ca II and Mg $b$ triplets, respectively. Using axisymmetric, three-integral Schwarzschild orbit library models, we fit the observed line-of-sight velocity distributions to infer the supermassive black hole mass, the $H$-band mass-to-light ratio, the asymptotic circular velocity, and the dark matter halo scale radius of the galaxy. We report a black hole mass of $(2.2 pm 0.2)times10^9 rm M_{scriptscriptstyleodot}$ at an assumed distance of $31.9 rm Mpc$. This value is in close agreement with a previous measurement from Atacama Large Millimeter/submillimeter Array CO observations. The consistency between these two measurements provides strong support for both the gas dynamical and stellar dynamical methods.

The Australian, Chinese, European, Indian, and North American pulsar timing array (PTA) collaborations recently reported, at varying levels, evidence for the presence of a nanohertz gravitational-wave background (GWB). Given that each PTA made different choices in modeling their data, we perform a comparison of the GWB and individual pulsar noise parameters across the results reported from the PTAs that constitute the International Pulsar Timing Array (IPTA). We show that despite making different modeling choices, there is no significant difference in the GWB parameters that are measured by the different PTAs, agreeing within 1ŌÉ. The pulsar noise parameters are also consistent between different PTAs for the majority of the pulsars included in these analyses. We bridge the differences in modeling choices by adopting a standardized noise model for all pulsars and PTAs, finding that under this model there is a reduction in the tension in the pulsar noise parameters. As part of this reanalysis, we "extended" each PTA's data set by adding extra pulsars that were not timed by that PTA. Under these extensions, we find better constraints on the GWB amplitude and a higher signal-to-noise ratio for the Hellings‚ÄďDowns correlations. These extensions serve as a prelude to the benefits offered by a full combination of data across all pulsars in the IPTA, i.e., the IPTA's Data Release 3, which will involve not just adding in additional pulsars but also including data from all three PTAs where any given pulsar is timed by more than a single PTA.

Pulsar timing arrays (PTAs) use an array of millisecond pulsars to search for gravitational waves in the nanohertz regime in pulse time of arrival data. This paper presents rigorous tests of PTA methods, examining their consistency across the relevant parameter space. We discuss updates to the 15-year isotropic gravitational-wave background analyses and their corresponding code representations. Descriptions of the internal structure of the flagship algorithms Enterprise and PTMCMCSampler are given to facilitate understanding of the PTA likelihood structure, how models are built, and what methods are currently used in sampling the high-dimensional PTA parameter space. We introduce a novel version of the PTA likelihood that uses a two-step marginalization procedure that performs much faster in gravitational wave searches, reducing the required resources facilitating the computation of Bayes factors via thermodynamic integration and sampling a large number of realizations for computing Bayesian false-alarm probabilities. We perform stringent tests of consistency and correctness of the Bayesian and frequentist analysis methods. For the Bayesian analysis, we test prior recovery, simulation recovery, and Bayes factors. For the frequentist analysis, we test that the optimal statistic, when modified to account for a non-negligible gravitational-wave background, accurately recovers the amplitude of the background. We also summarize recent advances and tests performed on the optimal statistic in the literature from both gravitational wave background detection and parameter estimation perspectives. The tests presented here validate current analyses of PTA data.

We present the first results from the Revealing Low-Luminosity Active Galactic Nuclei (ReveaLLAGN) survey, a JWST survey of seven nearby LLAGNs. We focus on two observations with the Mid-Infrared Instrument (MIRI)'s Medium-Resolution Spectrometer of the nuclei of NGC 1052 and Sombrero (NGC 4594/M104). We also compare these data to public JWST data of higher-luminosity AGNs, NGC 7319 and NGC 7469. JWST clearly separates the AGN spectrum from the galaxy light even in Sombrero, the faintest target in our survey; the AGN components have very red spectra. We find that the emission-line widths in both NGC 1052 and Sombrero increase with increasing ionization potential, with FWHM > 1000 km s<SUP>‚ÄĎ1</SUP> for lines with ionization potential ‚Č≥ 50 eV. These lines are also significantly blueshifted in both LLAGNs. The high-ionization-potential lines in NGC 7319 show neither broad widths nor significant blueshifts. Many of the lower-ionization-potential emission lines in Sombrero show significant blue wings extending >1000 km s<SUP>‚ÄĎ1</SUP>. These features and the emission-line maps in both galaxies are consistent with outflows along the jet direction. Sombrero has the lowest-luminosity high-ionization-potential lines ([Ne V] and [O IV]) ever measured in the mid-infrared, but the relative strengths of these lines are consistent with higher-luminosity AGNs. On the other hand, the [Ne V] emission is much weaker relative to the [Ne III] and [Ne II] lines of higher-luminosity AGNs. These initial results show the great promise that JWST holds for identifying and studying the physical nature of LLAGNs.

The cosmic merger history of supermassive black hole binaries (SMBHBs) is expected to produce a low-frequency gravitational wave background (GWB). Here we investigate how signs of the discrete nature of this GWB can manifest in pulsar timing arrays through excursions from, and breaks in, the expected $f_{mathrm{GW}}^{-2/3}$ power-law of the GWB strain spectrum. To do this, we create a semi-analytic SMBHB population model, fit to NANOGrav's 15 yr GWB amplitude, and with 1,000 realizations we study the populations' characteristic strain and residual spectra. Comparing our models to the NANOGrav 15 yr spectrum, we find two interesting excursions from the power-law. The first, at $2 ; mathrm{nHz}$, is below our GWB realizations with $p$-value significance $p = 0.05$ to $0.06$ ($approx 1.8 sigma - 1.9 sigma$). The second, at $16 ; mathrm{nHz}$, is above our GWB realizations with $p = 0.04$ to $0.15$ ($approx 1.4 sigma - 2.1 sigma$). We explore the properties of a loud SMBHB which could cause such an excursion. Our simulations also show that the expected number of SMBHBs decreases by three orders of magnitude, from $sim 10^6$ to $sim 10^3$, between $2; mathrm{nHz}$ and $20 ; mathrm{nHz}$. This causes a break in the strain spectrum as the stochasticity of the background breaks down at $26^{+28}_{-19} ; mathrm{nHz}$, consistent with predictions pre-dating GWB measurements. The diminished GWB signal from SMBHBs at frequencies above the $26$~nHz break opens a window for PTAs to detect continuous GWs from individual SMBHBs or GWs from the early universe.

Grants & Awards

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.

Selected Recent Talks

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.