The Flow of Gas Within the Bondi Radius of a Supermassive Black Hole:
The key to understanding the dynamics of ambient gas being gravitationally captured by a supermassive black hole lies in correctly modeling the behavior of the accreting gas once it falls within the gravitational influence of the black hole, the Bondi radius R_B = 2GM_BH/c_s², where M_BH is the mass of the black hole and c_s is the sound speed of the gas in the vicinity of R_B. In the absence of angular momentum, the black hole is predicted to gravitationally capture the ambient ISM surrounding it at a rate Ṁ_Bondi =4πλ (GM_BH)² ρ c_s^-3, where λ=0.25 for an adiabatic process and &rho is the density of the gas at R_B. For observed central &rho values of many nearby elliptical galaxies or Sgr A*, observed luminosities are orders of magnitude smaller than what would be predicted based on the Bondi accretion rate and assuming a standard 10% radiative efficiency. These observational results imply that the radiative efficiency of the infalling gas is exceedingly low and/or that much less material is accreted by the black hole than the Bondi formula implies, prompting the development of a host of theoretical efforts to explain the very low radiative efficiencies and/or accretion rates (such as ADAF, CDAF, and ADIOS models). Determining which (if any!) of these scenarios describes low-L_X black hole systems is of fundamental importance to our understanding of accretion physics and black hole demography. While ADAFs, CDAFs, and ADIOS models all predict that the compression of gas within the accretion flow within the Bondi radius leads to a T(R) ∝ R^-1 relation for the gas, the models predict significantly different radial density profiles. Thus, the density profile of hot gas flowing within R_B provides a crucial constraint on which accretion flow model is appropriate. The list of galaxies that are close enough and that harbor a SMBH massive enough for the Bondi radius of the black hole to be resolved with even Chandra is extremely short, and the best target is the S0 galaxy, the closest galaxy that harbors a >10⁹M_Sun black hole (with a Bondi radius of 5", or 235 pc). We were awarded a 1 million second X-ray Visionary Project (XVP) Chandra observation to derive the first temperature and density profile of hot gas inside the Bondi radius of a black hole. The projected temperature jumps significantly from ~0.3 keV beyond 5' to ~0.7 keV within ~4"-5", but then abruptly drops back to ~0.3 keV within ~3". This is contrary to the expectation that the temperature should rise toward the center for a radiatively inefficient accretion flow. A hotter thermal component of ~1 keV inside 3" (~150 pc) is revealed using a two-component thermal model, with the cooler ~0.3 keV thermal component dominating the spectra. We have argued that the softer emission comes from diffuse gas physically located within ~150 pc of the black hole. The density profile is broadly consistent with ρ(R) ∝ R^-1 within the Bondi radius for either the single temperature or the two-temperature model. The X-ray data along with physical reasoning argue against the absence of a black hole, supporting that we are witnessing the onset of the gravitational influence of the supermassive black hole.

Chandra+VLT image of NGC3115.

Searching for Evidence for Intermediate-mass Black Holes (IMBHs) in Globular Clusters from the Tidal Disruption of Passing Stars in the Cluster:
The establishment of the existence of black holes with masses in the 100--10⁴ M_Sun range would represent a major advancement in our understanding of the formation of black holes and their relation to both star formation and galaxy formation. While the mechanism for how IMBHs could be formed in the field of a galaxy is well-known to be problematic, globular clusters provide a more natural environment for IMBHs to form, and indeed a vigorous debate has been raging in the literature over whether Galactic and M31 globular clusters exhibit the kinematic signs of the presence of an IMBH. If IMBHs exist within globular clusters, they would be expected to occasionally tidally disrupt a passing star. Following the short-lived Eddington flare phase expected from the accretion of shredded material onto the IMBH, a longer-lived phase characterized by diminished X-ray emission and optical line emission from the X-ray-illumination of the stellar debris that remains around the IMBH would be tell-tale signatures of a disruption event that occurred 10 to a few hundred years ago. So rather than searching for the short-lived (and therefore rare) Eddington phase, my strategy has been to target a large number of extragalactic globular clusters that harbor luminous X-ray sources, and search for optical emission lines from these clusters. One prime candidate found for a post-tidal disruption event is the X-ray source CXOJ033831.8-352604 in a globular cluster in the Fornax elliptical galaxy NGC1399. With Magellan spectra, we found strong [N II] and [O III] forbidden emission lines emanating from the cluster harboring this ultraluminous X-ray source (ULX). The width of the lines and line ratios coupled with the high X-ray luminosity rule out other more mundane explanations such as planetary nebula or supernova remnants as the source of the line emission, leaving tidal disruption as the only viable explanation. Surprisingly, both Hα and Hβ are missing from the spectrum, indicating that if this is X-ray-illuminated debris from from a tidal disruption event, the disrupted object was severely lacking in hydrogen. Our best-fit CLOUDY models predict the correct optical emission line ratios and X-ray luminosity in a situation where a 100-200 M_Sun black hole has disrupted a 0.65 M_Sun hydrogen-poor red clump star within the globular cluster (Clausen et al. 2012, MNRAS, 424, 1268). We have recently been awarded 10 orbit Hubble Space Telescope COS observation of this globular cluster to search for the model-predicted carbon, oxygen, and nitrogen ultraviolet lines, which will confirm or deny our tidal disruption model, and put good constraints on the mass of the black hole disruptor.

Chandra+HST/ACS image of the CXOJ033831.8-352604 in NGC1399 (bright point source in upper left).

Magellan/MaGe spectrum of the globular cluster harboring CXOJ033831.8-352604 showing strong, redshifted [O III] and [N II] emission lines (Irwin wt al. 2010, ApJ, 712, L1).

Identifying High Mass Concentrations on the Galaxy/Group Scale With Gravitational Lensing :
Strong gravitational lensing of background galaxies by foreground clusters/groups of galaxies is a powerful tool for determining the mass of the intervening structure. Until recently, the majority of strong lensing mass estimates have been for clusters with masses on the order of 10¹⁵ M_Sun. This is beginning to change as more data from the Sloan Digital Sky Survey (SDSS) emerges, and an increasing number of lensing events from groups of galaxies are being discovered. Lensing also provides an avenue for probing the high-mass end of the galaxy mass function. As the strength of lensing depends on the mass of the lensing system, lens searches should be particularly adept at finding lensing systems with high mass concentrations, such as fossil groups of galaxies, individual galaxies with very massive dark matter halos, and if we are lucky so-called "dark" clusters. One such lensing discovery by the SDSS is the Cheshire Cat gravitational lens. This unusual system is composed of arcs from four separate background galaxies lensed by two massive elliptical galaxies and a collection of ~50 much fainter galaxies (Δ m > 2 mag) at z=0.43. The massive elliptical "eyes" of the Cat differ in velocity by 1350 km s^-1, indicating that we might be seeing two fossil groups merging nearly along the line of sight. This was confirmed by our spectroscopic study of the ~50 galaxies surrounding the "eyes", for which we found a double-peaked velocity distribution, with each peak centered on each of the "eyes" The Chandra data for this system shows a very strong temperature peak above 5 keV in the core of the otherwise symmetric X-ray distribution, indicative of a shock, again consistent with a line of sight merger between two fossil groups. This appears to be the first example of merging fossil groups observed to date (a fossil group progenitor). We have begun a project to identify more examples of fossil group progenitors by using strong gravitational lenses as an earmark for finding the high mass concentrations indicative of both merging systems and fossil groups in general.

Gemini GMOS image of the Cheshire Cat gravitational lens (upper), and Hubble+Chandra contour image of the Cheshire Cat (lower).

Velocity distribution of galaxies showing concentrations around both "eyes" at z=0.428 and z=0.433 determined from GMOS spectroscopy (Irwin et al. 2015, ApJ, 806, 268).