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Supermassive Black Holes

Supermassive Black Holes

Supermassive black holes (SMBHs), with masses ranging from below 106 to above 1010 solar masses, are now known to be present in the centers of most and perhaps all large galaxies. The mass of the black hole (BH) is correlated with the stellar mass (MBH-Mbulge relation) and velocity dispersion (MBH-σ relation) of the bulge of the galaxy. These discoveries over the past 20 years have led to the popular idea that black holes and galaxies co-evolve and that feedback of active galactic nuclei (AGN) during the growth phases of the black hole strongly affects the gas content and star formation in the host galaxy (Ho 2004; Kormendy & Ho 2013). Understanding the formation and growth history of SMBHs, their influence on galaxy evolution, and the exotic phenomena of stellar dynamics and gas accretion in the SMBH environment has become a major theme in astronomy.

TMT’s capabilities for high angular resolution imaging and ultra-deep spectroscopy will provide unprecedented opportunities to advance numerous areas of SMBH science. These will include precision measurements of BH masses spanning a range of more than four orders of magnitude in mass, examining the relationships between SMBHs and their host galaxy environments, understanding the physical processes of the fueling and feedback of black holes and their redshift evolution, determining the early growth history of SMBHs through observations of high-redshift quasars, and carrying out fundamental tests of General Relativity through high-precision measurements of stellar orbits around the Galactic Center. In this section, we describe a few of the most exciting science cases for understanding the fundamental properties, accretion physics, and cosmological growth history of SMBHs.

The Galactic Center Black Hole
Simulated galactic center field

Simulated near-IR image (tint=20s) of the central 17''x17'' of the Galaxy, centered on the supermassive black hole, Sgr A* at the angular resolution of TMT. The image contains ~2105 stars down to K ~24 mag, including ~2500 known stars and a theorized population based on the observed GC radial profile and the K-band luminosity function of the Galactic bulge. The image includes photon, background, and read noise.

 

The Galactic Center's proximity makes it a unique laboratory for addressing issues in the fundamental physics of supermassive black holes (SMBHs) and their roles in galaxy formation and evolution. Current AO studies have transformed our understanding of the Galactic Center (GC). In the past decade, proper motion studies have determined the orbits of some individual stars moving within 0.04 pc of Sgr A*. These provide the strongest evidence for the existence of a central black hole in the Galactic Center and the best mass measurement in any galactic nuclei. However, the star density in this region is so high that source confusion has limited studies to the brightest stars, and introduces biases in astrometric and spectroscopic measurements. TMT’s gains in resolution and contrast will enable detection and mapping of the orbits of stars four magnitudes fainter. This will allow the detection of many stars closer to the black hole than we can observe (resolve) today. The measurement of the orbits of these short period stars will be essential for tests of General Relativity in this region.

Dynamical Detection and Demographics of SMBHs

The mass is the most fundamental yet hard-to-measure property of black holes. The MBH-Mbulge and MBH-σ relations we observe for galaxies at z ≈ 0 are reasonably tight for classical bulges and elliptical galaxies (intrinsic scatter 0.29 dex), but they are not for pseudo-bulges. For redshifts z>1 there are hints that both the MBH-Mbulge and MBH-σ relations evolve, in the sense that the black hole tends to be over-massive with respect to the galaxy, but these findings are uncertain and controversial, depending on highly indirect estimates of MBH and host galaxy parameters (e.g., Bennert et al. 2011; Schulze & Wisotzki 2014).

The most reliable method to measure the mass of a black hole is from the orbital dynamics of gas or stars in the region within which the black hole dominates the gravitational potential. This region is usually defined by the radius of the sphere of influence of the hole: rinfl = G MBH/σ2 = 13 pc (MBH/108 M¤)0.5. TMT’s IRIS instrument will provide the next great leap in observational capabilities for measurement of the kinematics of galaxy nuclei on sub-arcsecond scales. TMT’s AO capability will enable IRIS to achieve a spatial resolution close to the diffraction limit, 8 mas (λ/μm), which surpasses the spatial resolution of the HST by almost one order of magnitude. TMT is capable of spatially resolving the sphere of influence of a mass MBH at an angular distance up to DA = 335 Mpc (μm/λ) (MBH/108 M¤)0.5, which corresponds to a distance of 1 Mpc for MBH= 103 M¤ or 3 Mpc for MBH = 104 M¤. At a redshift of z = 0.1, this corresponds to MBH = 108 M¤, and at z = 0.4, MBH = 109 M¤. Black holes with MBH∼ 1010 M¤ can be detected at any redshift, provided that suitable dynamical tracers (stellar absorption lines or emission lines from ionized or molecular gas) are accessible in wavelength ranges that can be observed using IRIS.

SMBH masses can be probed using either stars or gas as dynamical tracers. The stellar-dynamical method is more general in that it can be applied to a much larger set of targets, but modeling the full distribution of stellar orbits in a galaxy is a formidable challenge. Complications include the presence of the dark matter halo, possible triaxial structure in the bulge, and stellar population gradients. These issues can be mitigated if the SMBH sphere of influence is very well resolved by the observations, and the exquisite angular resolution provided by TMT’s AO system will enable major improvements in the quality and accuracy of SMBH mass measurements even for nearby galaxies in which SMBHs have already been detected using currently available capabilities. Velocity dispersion around a 107 M¤ BH is ~67 km/s requiring a spectral resolution of R~9000 to properly characterize it. Measurement and modeling of the kinematics of a thin gas disk in circular rotation is far simpler, but only a small fraction of galaxies contain a circumnuclear gas disk in regular rotation that also has emission lines (from ionized or molecular gas) sufficiently bright for kinematic mapping. TMT and IRIS will have the capability to carry out both stellar-dynamical and gas-dynamical measurements of SMBH masses, and TMT’s enormous collecting area and exquisite angular resolution will be an extremely powerful combination.

Coevolution of Supermassive Black Holes and Galaxies

Measuring the host properties of AGNs is key to understanding the co-evolution of SMBHs and galaxies. However, it is also very challenging, particularly for AGNs at higher redshift. TMT's superb spatial resolution and sensitivity will bring us a major breakthrough in separating the host signatures from the bright unresolved nucleus. This can help us to better constrain the MBH-σ relation, particularly for AGNs at high redshift. Measuring the central velocity dispersion requires a strong spectral feature redshifted to an accessible wavelength. There are multiple absorption lines that can be used for this purpose. For example, the Ca II triplet at rest-wavelength of 0.85 μm, centering it in the JHK bands corresponds to redshifts of 0.4, 0.9, and 1.6, respectively. Given the expected spatial resolution of IRIS in each of these bands, the minimum resolved radius of influence of SMBH corresponds to 60, 100, and 150 pc at these redshifts. For comparison, note that the radii of influence for M87 (MBH ~ 3 x 109 M¤) and M31 (MBH ~ 7 x 107 M¤) are approximately 100 and 12 pc, respectively. Thus, it will be possible to probe the upper end of the SMBH mass function with TMT at these redshifts.

TMTwill also allow us to study in much more detail the mechanisms of feeding and feedback of AGNs. It will provide observations not only for nearby AGNs in unprecedented quality, but also to galaxies reaching redshifts out to z~2, where galaxies are most active, and AGN feedback is suspected of playing a major role in shaping galaxy evolution. In the mean time, TMT's high-spatial-resolution spectroscopy will reveal fainter AGNs at higher redshifts, allowing us to probe the evolution of AGN luminosity function over a far wider range of luminosities.

Binary and Merging SMBH in the nearby Universe
Stalling radius of merging black Holes

The stalling radius for merging black holes in the cores of nearby galaxies in the sample of Ravindranath et al. (2002). The stalling radii were computed using the measured black hole masses and the prescription given in Merrit (2006). TMT operating at the diffraction limit will be able to resolve binary black holes in a number of core ellipticals if black holes are stalled. TMT will not be able to probe stalling radii falling in the shaded region.

For the later stages of the merger, the finest spatial resolution is of paramount importance. In the hierarchical paradigm of galaxy formation and evolution, galaxy mergers bring their central massive black holes (BHs) together. Initially, the BHs are expected to have a separation of more than a kpc. The dynamical friction caused by stars then brings the two BHs closer on a time scale of 100 Myrs, making them a gravitationally bound binary system with a separation of about a parsec (Begelman et al. 1980).

The massive binary BHs will eventually spiral in and merge to form a single central SMBH of the new galaxy (Milosavljevic & Merritt 2003, Di Matteo et al. 2005). However, binary coalescence via emission of gravitational waves requires that binary must first shrink to much smaller radii (~0.001-0.01 pc) from the parsec scale separation where standard dynamical friction stops acting. Possible solutions to the so called "final parsec problem" include the interaction of the binary with the surrounding stars and gas e.g., slingshot ejection of approaching stars can shrink the binary orbit further. The resulting gravitational binding energy is released to the surrounding stars. N-body simulations show that this process can convert a steep power-law cusp into a shallow power-law cusp within the radius of gravitational influence of the BHs (Milosavljevic & Merritt 2001). Successive mergers will further decrease the density in the central regions and thus forming cores and resulting in mass deficit in bright elliptical galaxies (Ravindranath et al. 2002, Milosavljevic et al. 2002). The density in the central regions can become very low, thus stalling the binary orbit for a few Gyrs. Thus, a large fraction of core elliptical galaxies are likely to have stalled binary BHs. At present, only the total black hole mass has been measured for these ellipticals. The current generation telescopes do not have the requisite spatial resolution to determine if the cores of these ellipticals have stalled binary BHs or a single merged BH. TMT will be able to probe down to the expected stalling radii of binary BHs and reveal dual nuclei within the cores if the nuclei are active. Also stellar orbits in the innermost regions around binary BHs are expected to be different than the orbits around single BHs. Thus, study of stellar motions in the nearest core ellipticals should also provide evidence for binary BHs even in the case of non-active nuclei.

Since the presence of an active galactic nucleus is unambiguous evidence for a central massive BH, observations of double active nuclei will provide the direct evidence for the presence of binary BHs. Superior angular resolution available with Chandra and Hubble Space Telescope (HST) have enabled direct observation of a pair of active nuclei separated at ~1.4 kpc in the ultra-luminous infrared galaxy NGC 6240 (Komossa et al. 2003, Max et al. 2007). TMT with improved spatial resolution and sensitivity is expected to detect pairs of active nuclei in many active galaxies.

In dual AGN systems both central engines may be buried inside the obscuring dusty structure and/or host galaxy. In such cases, NIR (1-2.5 μm) polarimetry with the TMT offers a powerful way to identify obscured dual AGN systems through the conical centrosymmetric polarization pattern centered on each AGN. For example, the polarization pattern detected in the 10” (600 pc) central regions of NGC 1068 (Cappetti et al. 1995, Simpson et al. 2002) is the signature of a central point source whose radiation is polarized by dust and/or electron scattering within the ionization cones, which if rotated by 90 degrees point directly to the illuminating source. In the case where the ionization cones can be resolved, the polarization pattern will indicate two slightly separated centers that can be identified as dual nucleus AGN.

Time Variability, Probing the Structure and Processes in the Central Engine

TMT will see first light in an exciting era for time domain astronomy. Particularly exciting for the study of supermassive black holes is the identification and study of Tidal Disruption Events (TDEs), which occur when a star on an orbit around SMBH makes a close approach and is ripped apart by tidal forces. Once the stellar debris rains down on the black hole, soft X-ray and UV radiation characterized by a 105 K blackbody emerges from the inner accretion region. Reprocessing of this radiation in the debris results in a fraction of the emission emerging in the optical. Large numbers of TDEs can be detected by the current big transient surveys like the Palomar Transit Factory (PTF, Arcavi et al., 2014), and many more will be found in the next generation of large-area surveys such as with the LSST. Observations of TDEs are important because they in principle provide a means of measuring the masses and spins of black holes in optically ‘normal’ galaxies.

The TMT will allow the detection and study of TDEs up to much higher redshifts than previously possible. Because of the “negative” K-correction (TDEs emitting primarily in the restframe UV with a characteristic 105 K black body) TDEs will be visible by TMT to redshifts of 6 or larger, enabling constraints on SMBH properties and evolution over a vast range of cosmic time.

The study of a large number of TDEs will help to anchor the observational signatures to precisely-measured black hole masses. While analytic models and numerical simulations exist, there are still large uncertainties in the conversion from a measured light curve to a black hole mass estimate – the ultimate goal of TDE studies. Finding TDEs in a nearby galaxy with an independent measurement of the black hole mass, either from the M-σ relation or better yet maser disk kinematics will provide a better calibration of the models. With current observational capabilities the probability of finding such nearby events is low, and measuring their lightcurves is challenging. But, with LSST and the greater follow-up sensitivity and angular resolution of the TMT, it should be possible to build up a sample of “low-z” TDEs.

Contact
The science case for each science theme is developed and implemented by a dedicated international science development team (ISDT). Scientists from around the world interested to join a particular ISTD should contact the respective convener. 

Conveners
Matt Malkan (UCLA)
Yue Shen (University of Illinois, Urbana-Champagne)

 

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