Spectroscopic Signatures of the Tidal Disruption of Stars by Massive Black Holes

During the tidal disruption of a star by a massive black hole (BH) of mass MBH <~ 10^7 Msun, stellar debris falls back to the BH at a rate well above the Eddington rate. A fraction of this gas is subsequently blown away from the BH, producing an optically bright flare of radiation. We predict the spectra and spectral evolution of tidal disruption events, focusing on the photoionized gas outside this outflow's photosphere. The spectrum will show absorption lines that are strongly blueshifted relative to the host galaxy, very broad (0.01-0.1c), and strongest at UV wavelengths (e.g., C IV, Ly alpha, O VI), lasting ~ 1 month for a 10^6 Msun BH. Meanwhile, supernovae in galactic nuclei are a significant source of confusion in optical surveys for tidal disruption events: we estimate that nuclear Type Ia supernovae are two orders of magnitude more common than tidal disruption events at z ~ 0.1 for ground-based surveys. Nuclear Type II supernovae occur at a comparable rate but can be excluded by pre-selecting red galaxies. Supernova contamination can be reduced to a manageable level using high-resolution follow-up imaging with adaptive optics or the Hubble Space Telescope. Our predictions should help optical transient surveys capitalize on their potential for discovering tidal disruption events.Comment: 14 pages, 5 figures; accepted for publication in MNRAS; some added discussion in Section

Paired Teaching for Faculty Professional Development in Teaching

Paired (or co-) teaching is an arrangement in which two faculty are collaboratively responsible for all aspects of teaching a course. By pairing an instructor experienced in research-based instructional strategies (RBIS) with an instructor with little or no experience in RBIS, paired teaching can be used to promote the adoption of RBIS. Using data from post-course interviews with the novice instructors of four such arrangements, we seek to describe factors that make for effective professional development in teaching via paired teaching. We suggest that the novice instructor’s approach to the paired teaching and their previous teaching experience are two aspects which mediate their learning about teaching. Additionally, the structure of the pair-taught course and the sequence of teaching assignments for the novice instructor both likely play roles in facilitating the adoption of RBIS by novice instructors. We discuss these results within the framework of cognitive apprenticeship

Fossil Gas and the Electromagnetic Precursor of Supermassive Binary Black Hole Mergers

Using a one-dimensional height integrated model, we calculate the evolution of an unequal mass binary black hole with a coplanar gas disk that contains a gap due to the presence of the secondary black hole. Viscous evolution of the outer circumbinary disk initially hardens the binary, while the inner disk drains onto the primary (central) black hole. As long as the inner disk remains cool and thin at low $\dot{M}_{\rm ext}$ (rather than becoming hot and geometrically thick), the mass of the inner disk reaches an asymptotic mass typically \sim 10^{-3}-10^{-4}\Msun. Once the semimajor axis shrinks below a critical value, angular momentum losses from gravitational waves dominate over viscous transport in hardening the binary. The inner disk then no longer responds viscously to the inspiraling black holes. Instead, tidal interactions with the secondary rapidly drive the inner disk into the primary. Tidal and viscous dissipation in the inner disk lead to a late time brightening in luminosity $L\propto t_{\rm minus}^{5/4}$, where $t_{\rm minus}$ is the time prior to the final merger. This late time brightening peaks $\sim 1$ day prior to the final merger at $\sim 0.1 L_{\rm Edd}$. This behavior is relatively robust because of self regulation in the coupled viscous-gravitational evolution of such binary systems. It constitutes a unique electromagnetic signature of a binary supermassive black hole merger and may allow the host galaxy to be identified if used in conjunction with the Laser Interferometric Space Antenna (LISA) localization.Comment: 11 pages, 6 figures, submitted to MNRA

Evolution of the bar fraction in COSMOS: quantifying the assembly of the Hubble sequence

We have analyzed the redshift-dependent fraction of galactic bars over 0.2 < z < 0.84 in 2157 luminous face-on spiral galaxies from the COSMOS 2 deg^2 field. Our sample is an order of magnitude larger than that used in any previous investigation, and is based on substantially deeper imaging data than that available from earlier wide-area studies of high-redshift galaxy morphology. We find that the fraction of barred spirals declines rapidly with redshift. Whereas in the local universe about 65% of luminous spiral galaxies contain bars (SB+SAB), at z ~ 0.84 this fraction drops to about 20%. Over this redshift range the fraction of strong bars (SBs) drops from about 30% to under 10%. It is clear that when the universe was half its present age, the census of galaxies on the Hubble sequence was fundamentally different from that of the present day. A major clue to understanding this phenomenon has also emerged from our analysis, which shows that the bar fraction in spiral galaxies is a strong function of stellar mass, integrated color and bulge prominence. The bar fraction in very massive, luminous spirals is about constant out to z ~ 0.84, whereas for the low-mass, blue spirals it declines significantly with redshift beyond z = 0.3. There is also a slight preference for bars in bulge-dominated systems at high redshifts that may be an important clue toward the coevolution of bars, bulges, and black holes. Our results thus have important ramifications for the processes responsible for galactic downsizing, suggesting that massive galaxies matured early in a dynamical sense, and not just as a result of the regulation of their star formation rate