“Fueling Nuclear Activity in Disk Galaxies: Starbursts and Monsters”Clayton H. Heller and Isaac ShlosmanUniversity of KentuckyThe Astrophysical Journal, 424: 84-1051994 March 20Journal Club Talk by Adam Mott2005 September 23Centers of Disk Galaxies●Some disk galaxies exhibit strong activity in their central regions, thought to be due to–accretion of material onto a central supermassive black hole (BH)●active galactic nuclei (AGNs)–formation of lots of massive stars in a short period of time●starburst galaxiesHow can material get to the center?●Both AGNs and starbursts require the processing of huge amounts of material in order to achieve the observed luminosities.–a few percent of the total mass of the galaxy–a significant fraction of the total ISM●How does the ISM get past the angular momentum barrier in order to fuel the central activity?–Shlosman, Begelman, & Frank (1990): global nonaxisymmetric instabilities–Simkin, Su, & Schwarz (1980): stellar bars channel gas toward centerGas Dynamics●Following Simkin, Su, & Schwarz (1980), much of the focus of past simulations was to model the flow of gas in a barred stellar potential, but...●“there are indications that the ga s plays an important if not dominant role in the dynamics of the central few hundred parcecs”–gas not only affected by gravity, but also by●stellar winds●supernovae ejectaLimitations of Past Simulations●Lack of resolution–weren't able to follow gas flows for more than a decade in radius●Two-dimensional●Most studies have ignored–self-gravity of the gas–imposition of an oval or spiral distortion to the gravitational potential–“back reaction” of the gasGoal of this Paper●To simulate the dynamics of a two component galactic disk (gas+stars) in a responsive halo, including the effects of–massive star formation–supernovae●Concentration on the inner disk, as close in as ~50 pc from the center.●“We aim at understanding the physical processes which lead to nuclear starbursts and to the rapid growth of the central BH.”Observational Evidence/Motivation●Although the observations suffer from low angular resolution, observations of CO, CN, & other molecules tell us that gas in the central kpc often forms “molecular bars” or nuclear rings.●These bars of molecular gas don't necessarily have to coexist with a stellar bar.●Central concentrations of molecular gas are associated with high star formation and black hole accretion.●Supports the idea of large-scale rearrangement of molecular gas.Numerical Method●Smooth Particle Hydrodynamics (SPH) is used to evolve the gas.–A finite set of “particles” are used to model the gas.–Not just treated as point particles; they are smoothed out according to a spherically symmetric function.●Gravity–Force acting on each mass is calculated by dividing all other masses into groupings of appropriate size.–“hierarchical TREE method”: N log N computing time●Multiple time step sizes (necessary to get the desired dynamic range in radius, density)Fig. 1: Testing the CodeColliding gas slabs(no gravity)Polytropic gas sphere in hydrostatic equilibriumInduced pulsation by expanding b) by 15%.Damping is due to “artificial viscosity.”Simulation correctly produced the expected pulsation frequency.Galaxy Simulations: Initial Conditions●Exponential disk (in both radius r and height z)–stars●scale lengths 2.85 kpc in r, 0.5 kpc in z●16,384 collisionless SPH particles–gas●scale length 0.25 kpc in z●8,192 collisional SPH particles●Spherical halo with 10,240 collisionless particles●Mass ratio (halo/disk) = 1●Relaxation to “virial equilibrium”Simple Model for Stellar Evolution●When the local density of unstable gas exceeds ~100 MSun/pc3, the SPH gas particle becomes an OB star.–Its stellar wind deposits a specified fraction (say 5%) of 3x1051 ergs to the surrounding ISM (manifest as an increase in local gas pressure) spread out evenly over its 106 yr main sequence lifetime.–Supernova! At the end of the MS lifetime, an equivalent amount of energy is released again, but this time over 104 yr. No remnant.●This “IMF” has only very massive stars & may overestimate energy deposition. Compensate by keeping efficiency low.Simple Model for Black Hole●Single SPH particle starts with “seed” mass of 5x107 MSun.–Accretes any gas particles within a radius of 20 pc.–Absorbed particle's momentum considered to be lost, and radiation from accretion process is not taken into account.Fig. 3 1% gas seed BH? Yes star formation? No1 time unit = 4.7x107 yrevolution of gasfinal BH mass = 6x108 solarFig. 4 10% gas seed BH? Yes star formation? No1 time unit = 4.7x107 yrevolution of gasfinal BH mass = 2.5x109 solarFig. 5 1% gas seed BH? Yes star formation? Yes1 time unit = 4.7x107 yrevolution of gasfinal BH mass = 6x108 solarFig. 6a 1% gas seed BH? Yes star formation? Yes1 time unit = 4.7x107 yrsame scenario as in Fig. 5highlights new stars onlyFig. 6b 10% gas seed BH? Yes star formation? Yes1 time unit = 4.7x107 yrhighlights new stars onlyFig. 71 time unit = 4.7x107 yr1% gas5% gasFig. 8 10% gas seed BH? Yes star formation? Yes1 time unit = 4.7x107 yrevolution of gasfinal BH mass = 2.5x109 solarFig. 9inflow rates at r = 1 kpcfraction of total gas within inner 1 kpc at the end of the simulationmedian BH accretion ratecircles: no star formationtriangles: star formationFig. 10 10% gas seed BH? No star formation? No1 time unit = 4.7x107 yrevolution of gasfinal central cloud mass = 4x109 solarFig. 11 10% gas seed BH? No star formation? Yes1 time unit = 4.7x107 yrevolution of gasOverall Trends●Models witho ut star formation–Whether or not you start with a seed BH, the inner 1 kpc becomes dominated by a few large clouds which eventually merge into a single massive object.–The accretion onto the central BH is sporadic in nature.–The capture and processing of clouds by the central BH results in remnant disks of radius 60-80 kpc (?).Trends, cont'd●Models with star formation–More mixing of gases within the stellar bar.–“Star formation which is concentrated at the apocenters of the gaseous circulation in the stellar bar and in the nuclear region.”–Very luminous (~1045-1046 erg/s) central starburst phase which lasts 107 yr–“The starburst phase coincides with both the gas becoming dynamically important and the catastrophic growth of the BH.”Thanks for