Galaxy bulges and their black holes: a requirement for the quenching of star formation

One of the central features of the last 8 to 10 billion years of cosmic history has been the emergence of a well-populated red sequence of non-star-forming galaxies. A number of models of galaxy formation and evolution have been devised to attempt to explain this behavior. Most current models require feedback from supermassive black holes (AGN feedback) to quench star formation in galaxies in the centers of their dark matter halos (central galaxies). Such models make the strong prediction that all quenched central galaxies must have a large supermassive black hole (and, by association, a prominent bulge component). I show using data from the Sloan Digital Sky Survey that the observations are consistent with this prediction. Over 99.5% of red sequence galaxies with stellar masses in excess of 10^{10} M_{\sun} have a prominent bulge component (as defined by having a Sersic index n above 1.5). Those very rare red sequence central galaxies with little or no bulge (n<1.5) usually have detectable star formation or AGN activity; the fraction of truly quenched bulgeless central galaxies is <0.1% of the total red sequence population. I conclude that a bulge, and by implication a supermassive black hole, is an absolute requirement for full quenching of star formation in central galaxies. This is in agreement with the most basic prediction of the AGN feedback paradigm.Comment: 6 pages, 4 color figures (figure 1 is of slightly degraded quality). To appear in August 1 edition of the Astrophysical Journa

Omega from the skewness of the cosmic velocity divergence

We propose a method for measuring the cosmological density parameter $\Omega$ from the statistics of the divergence field, $\theta \equiv H^{-1} \div v$, the divergence of peculiar velocity, expressed in units of the Hubble constant, $H \equiv 100 h km/s/Mpc$. The velocity field is spatially smoothed over $\sim 10 h^{-1} Mpc$ to remove strongly nonlinear effects. Assuming weakly-nonlinear gravitational evolution from Gaussian initial fluctuations, and using second-order perturbative analysis, we show that \propto -\Omega^{-0.6} ^2. The constant of proportionality depends on the smoothing window. For a top-hat of radius R and volume-weighted smoothing, this constant is $26/7-\gamma$, where $\gamma=-d\log / d\log R$. If the power spectrum is a power law, $P(k)\propto k^n$, then $\gamma=3+n$. A Gaussian window yields similar results. The resulting method for measuring $\Omega$ is independent of any assumed biasing relation between galaxies and mass. The method has been successfully tested with numerical simulations. A preliminary application to real data, provided by the POTENT recovery procedure from observed velocities favors $\Omega \sim 1$. However, because of an uncertain sampling error, this result should be treated as an assessment of the feasibility of our method rather than a definitive measurement of $\Omega$.Comment: 16 pages + 2 figures, uuencoded postscript file, also available by anonymous ftp from ftp.cita.utoronto.ca in directory /cita/francis/div_skewness, CITA 94-1

Polarization dependence of emission spectra of multiexcitons in self-assembled quantum dots

We have investigated the polarization dependence of the emission spectra of p-shell multiexcitons of a quantum dot when the single particle level spacing is larger than the characteristic energy of the Coulomb interactions. We find that there are many degenerate multiexciton states. The emission intensities depend on the number of degenerate initial and final states of the optical transitions. However, unlike the transition energies, they are essentially independent of the strength of the Coulomb interactions. In the presence of electron-hole symmetry the independence is exact.Comment: 7 pages, 5 figures, published in Solid State Commu

Enhanced Momentum Feedback from Clustered Supernovae

Young stars typically form in star clusters, so the supernovae (SNe) they produce are clustered in space and time. This clustering of SNe may alter the momentum per SN deposited in the interstellar medium (ISM) by affecting the local ISM density, which in turn affects the cooling rate. We study the effect of multiple SNe using idealized 1D hydrodynamic simulations which explore a large parameter space of the number of SNe, and the background gas density and metallicity. The results are provided as a table and an analytic fitting formula. We find that for clusters with up to ~100 SNe the asymptotic momentum scales super-linearly with the number of SNe, resulting in a momentum per SN that can be an order of magnitude larger than for a single SN, with a maximum efficiency for clusters with 10-100 SNe. We argue that additional physical processes not included in our simulations -- self-gravity, breakout from a galactic disk, and galactic shear -- can slightly reduce the momentum enhancement from clustering, but the average momentum per SN still remains a factor of 4 larger than the isolated SN value when averaged over a realistic cluster mass function for a star-forming galaxy. We conclude with a discussion of the possible role of mixing between hot and cold gas, induced by multi-dimensional instabilities or preexisting density variations, as a limiting factor in the buildup of momentum by clustered SNe, and suggest future numerical experiments to explore these effects.Comment: 19 pages, 26 figures, revised to reflect accepted version. Discussion regarding resolution effects has changed; additional analysis into galactic and gravitational effects has been adde

Instability of Supersonic Cold Streams Feeding Galaxies II. Nonlinear Evolution of Surface and Body Modes of Kelvin-Helmholtz Instability

As part of our long-term campaign to understand how cold streams feed massive galaxies at high redshift, we study the Kelvin-Helmholtz instability (KHI) of a supersonic, cold, dense gas stream as it penetrates through a hot, dilute circumgalactic medium (CGM). A linear analysis (Paper I) showed that, for realistic conditions, KHI may produce nonlinear perturbations to the stream during infall. Therefore, we proceed here to study the nonlinear stage of KHI, still limited to a two-dimensional slab with no radiative cooling or gravity. Using analytic models and numerical simulations, we examine stream breakup, deceleration and heating via surface modes and body modes. The relevant parameters are the density contrast between stream and CGM ($\delta$), the Mach number of the stream velocity with respect to the CGM ($M_{\rm b}$) and the stream radius relative to the halo virial radius ($R_{\rm s}/R_{\rm v}$). We find that sufficiently thin streams disintegrate prior to reaching the central galaxy. The condition for breakup ranges from $R_{\rm s} < 0.03 R_{\rm v}$ for $(M_{\rm b} \sim 0.75, \delta \sim 10)$ to $R_{\rm s} < 0.003 R_{\rm v}$ for $(M_{\rm b} \sim 2.25, \delta \sim 100)$. However, due to the large stream inertia, KHI has only a small effect on the stream inflow rate and a small contribution to heating and subsequent Lyman-$\alpha$ cooling emission.Comment: The main astrophysical results are Figure 22 and Figure 23. Final 7 pages are appendices. Accepted to MNRA

Toy Models for Galaxy Formation versus Simulations

We describe simple useful toy models for key processes of galaxy formation in its most active phase, at z > 1, and test the approximate expressions against the typical behaviour in a suite of high-resolution hydro-cosmological simulations of massive galaxies at z = 4-1. We address in particular the evolution of (a) the total mass inflow rate from the cosmic web into galactic haloes based on the EPS approximation, (b) the penetration of baryonic streams into the inner galaxy, (c) the disc size, (d) the implied steady-state gas content and star-formation rate (SFR) in the galaxy subject to mass conservation and a universal star-formation law, (e) the inflow rate within the disc to a central bulge and black hole as derived using energy conservation and self-regulated Q ~ 1 violent disc instability (VDI), and (f) the implied steady state in the disc and bulge. The toy models provide useful approximations for the behaviour of the simulated galaxies. We find that (a) the inflow rate is proportional to mass and to (1+z)^5/2, (b) the penetration to the inner halo is ~50% at z = 4-2, (c) the disc radius is ~5% of the virial radius, (d) the galaxies reach a steady state with the SFR following the accretion rate into the galaxy, (e) there is an intense gas inflow through the disc, comparable to the SFR, following the predictions of VDI, and (f) the galaxies approach a steady state with the bulge mass comparable to the disc mass, where the draining of gas by SFR, outflows and disc inflows is replenished by fresh accretion. Given the agreement with simulations, these toy models are useful for understanding the complex phenomena in simple terms and for back-of-the-envelope predictions.Comment: Resubmitted to MNRAS after responding to referee's comments; Revised figure