Key questions about Galactic Center dynamics

I discuss four key questions about Galactic Center dynamics, their implications for understanding both the environment of the Galactic MBH and galactic nuclei in general, and the progress made in addressing them. The questions are (1) Is the stellar system around the MBH relaxed? (2) Is there a "dark cusp" around the MBH? (3) What is the origin of the stellar disk(s)?, and (4) What is the origin of the S-stars?Comment: Invited overview lecture in "The Galactic Center, a window to the nuclear environment of disk galaxies" (Shanghai 19-23/10/2009). To appear in ASP Conf. Proc. Ser. "Galactic center workshop 2009" ed. Mark Morris (12 pp 5 fig

The Signature of Microlensing in QSO Variability-Redshift Correlations

A recently discovered inverse correlation between QSO redshift and long-term continuum variability timescales was suggested to be the signature of microlensing on cosmological scales (Hawkins 1993). A general theoretical method for calculating such correlations is presented and applied to various lensing scenarios in the framework of $\Lambda = 0$ Friedmann cosmologies. It is shown that the observed timescales can be strongly influenced by the observational limitations: the finite duration of the monitoring campaign and the finite photometric sensitivity. In most scenarios the timescales increase with source redshift, $z_s$, although slower than the $1+z_s$ time dilation expected of intrinsic variability. A decrease can be obtained for an extended source observed with moderate sensitivity. In this case, only lenses no further away than several hundreds Mpc participate in the lensing. The resulting optical depth is too small to explain the common long-term QSO variability unless an extremely high local lens density is assumed. These results do not support the idea that the reported inverse correlation can be attributed to microlensing of a uniform QSO sample by a uniform distribution of lenses. The possibility of using observations at various wavelengths and QSO samples at various positions to identify microlensing in QSO variability is also discussed.Comment: Self-unpacking, uuencoded postscript file, 10 pages with 7 figures included. Accepted for publication by the MNRAS

Stellar dynamics and tidal disruption events in galactic nuclei

The disruption of a star by the tidal field of a massive black hole is the final outcome of a chain of complex dynamical processes in the host galaxy. I introduce the "loss cone problem", and describe the many theoretical and numerical challenges on the path of solving it. I review various dynamical channels by which stars can be supplied to a massive black hole, and the relevant dynamical relaxation / randomization mechanisms. I briefly mention some "exotic" tidal disruption scenarios, and conclude by discussing some new dynamical results that are changing our understanding of dynamics near a massive black hole, and may well be relevant for tidal disruption dynamics.Comment: Invited review talk presented in the "Tidal Disruption events and AGN outbursts" workshop, 25-27 June 2012, ESAC, Madrid, Spain. To appear in the EPJ web of conferences, Editor: R. Saxton. 7 p

Bloated Stars as AGN Broad Line Clouds: The Emission Line Profiles

The Bloated Stars Scenario proposes that AGN broad line emission originates in the winds or envelopes of bloated stars (BS). Alexander and Netzer (1994) established that ~ 5e4 BSs with dense, decelerating winds can reproduce the observed emission line spectrum and avoid rapid collisional destruction. Here, we use the observed properties of AGN line profiles to further constrain the model parameters. In the BS model, the origin of the broad profiles is the stellar velocity field in the vicinity of the central black hole. We use a detailed photoionization code and a model of the stellar distribution function to calculate the BS emission line profiles and compare them to a large sample of AGN CIV, CIII] and MgII profiles. We find that the BSs can reproduce the general shape and width of typical AGN profiles as well as the line ratios if (i) The ionizing luminosity to black hole mass ratio is low enough. (ii) The broad line region size is limited by some cutoff mechanism. (iii) The fraction of the BSs in the stellar population falls off roughly as r^-2. (iv) The wind density and/or velocity are correlated with the black hole mass and ionizing luminosity. Under these conditions the strong tidal forces near the black hole play an important role in determining the line emission properties of the BSs. Some discrepancies remain: the calculated BS profiles tend to have weaker wings than the observed ones, and the differences between the profiles of different lines are somewhat smaller than those observed.Comment: 13 pages with 10 figures, LaTeX using mn.sty and epsf.sty, to appear in MNRA