The Nature of CO Emission From z~6 Quasars Narayanan, D., Li, Y., Cox, T. J., Hernquist, L., Hopkins, P., Chakrabarti, S., Davé, R., Di Matteo, T., Gao, L., Kulesa, C., Robertson, B., Walker, C. 2007, ApJS, 174, 13 (Theoretical) On the Evolutionary History of Stars and their Fossil Mass and Light Fardal, M. A., Katz, N., Weinberg, D. H., Davé, R. 2007, MNRAS, 379, 985. (Theoretical) Accretion, feedback and galaxy bimodality: a comparison of the GalICS semi-analytic model and cosmological SPH simulations Catteneo, A., Blaizot, J., Weinberg, D. H., Colombi, S., Davé, R., Devriendt, J., Guiderdoni, B., Katz, N., Keres, D. 2007, MNRAS, 377, 63. (Theoretical) Constraints on Physical Properties of z~6 Galaxies Using Cosmological Hydrodynamic Simulations Finlator, K., Davé, R. & Oppenheimer, B. D. 2007, MNRAS, 376, 1861. (Theoretical) The association between gas and galaxies II: The 2-point correlation function Wilman, R. J., Morris, S. L., Jannuzi, B. T., Davé, R., Shone, A. M. 2007, MNRAS, 375, 735. (Extragalactic) The Enrichment History of Baryons in the Universe Davé, R. & Oppenheimer, B. D. 2007, MNRAS, 374, 427. (Theoretical)
R. Dave, with student Ben Oppenheimer, have continues their investigations of the observational and physical impact of cosmic-scale galactic outflows. The primary focus has been on implementing several new physical processes such that our simulations are more robust down to the present epoch, z=0. These involved implementing an on-the-fly galaxy finder in order to more accurately calculate outflow parameters, and including a sophisticated chemical enrichment model that includes contributions from Type Ia supernovae and stellar mass loss. The resulting simulations are being used to investigate the nature of how gas flows in and out of galaxies, and to make predictions for upcoming observations with HST's Cosmic Origins Spectrograph. We have found some remarkable physical effects occur with the cycle of feedback in and out of galaxies, including a characteristic scale for outflows of around 100 physical kpc, and the introduction of a new phenomenon we dubbed halo fountains, in which for typical L* galaxies today gas is continually kicked into the halo and may be raining back down on the disk as high velocity clouds. These ideas open up a whole new range of testable predictions for galactic halo gas both in the Milky Way and external galaxies, and we are involved in a number of proposed studies to do so with HST. R. Dave, with student Kristian Finlator, continue to investigate implications for our new model of the galaxy mass-metallicity relation. In our model, the mass-metallicity relation is established as a consequence of the relationship between the amount of material ejected from the galaxy relative to the amount of stars formed. Our momentum-driven wind simulations that properly enrich the IGM naturally yield a correct mass-metallicity relation. This contrasts with the standard picture of the mass-metallicity relation in which it is governed by the competition between outflow velocity and the galaxy potential depth. We have been comparing our model predictions with more detailed observations from two groups, led by S. Ellison (Victoria) and L. Kewley (Hawaii), and continue to find that observed trends with respect to galaxy star formation rate and size are well-reproduced in our scenario, lending further support to our model. R. Dave, with student Kristian Finlator and F. Ozel (Physics) are developing a novel code for simulating radiative transfer and hydrodynamics in the early universe. The approach combines a long characteristics method to obtain accurate Eddington tensors with a short characteristics matrix solver with a fully implicit formulation to obtain accurate solutions for large timesteps. For expected applications, this code will scale well sublinearly with number of sources, while still maintaining full accuracy in a complex evolving 3-D matter distribution. These traits make it ideal for implementing into our hydrodynamic solver in order to do a full radiation hydro treatment of reionization, self-consistently including sources (galaxies) and sinks (the cosmological distribution of neutral gas), which will be the next phase of the project. This is Finlator's thesis project. R. Dave investigated what constraints may be placed on the stellar initial mass function from the evolution of the stellar mass-star formation rate relation for galaxies out to z~2. He finds that the observed evolution of this relation is in conflict with generic predictions of galaxy growth in hierarchical LCDM models. After considering numerous possible systematic effects, he determines that a plausible solution is that the IMF of typical star-forming galaxies evolves with redshift, such that it is more top-heavy at higher redshifts. A simple model of IMF evolution is constructed, and is shown to reconcile a number of other puzzling observations, from the evolution of colors in cluster galaxies to the abundance of carbon-enhanced metal-poor stars in the Milky Way halo. R. Dave investigated the impact of galactic outflows on the enrichment and thermodynamic state of intragroup gas, focusing on observable X-ray properties. He found that outflows make a small but noticeable difference on enrichment and entropy, and both bring the simulations into better agreement with observations than in the no-outflow case. Specifically, outflows are able to effectively enrich the intragroup medium and increase the core entropy to observed levels, while reducing the amount of stars formed to match data. While the differences are subtle, forthcoming increased samples of X-ray groups will be able to more precisely test this scenario. The implication would be that no exotic process is needed to enrich the ICM, and no AGN feedback is needed to increase its entropy; both things can be achieved using the same outflows required to enrich the IGM. Assistance was provided by student S. Sivanandam, who developed a code to calculate X-ray luminosities from simulation particles. R. Dave, with Desika Narayanan (CfA) and a group at Harvard led by T. J. Cox and L. Hernquist, have compared models of z~6 massive galaxies hosting quasars with observations of such systems in CO line emission, using a CO line radiative transfer code developed by Desika applied to galaxy mergers simulations from Yuexing Li and the Harvard group. The predictions from this were in consistent with various observations of CO line profiles, although the line widths predicted in the models were significantly larger than seen. We considered various selection effects that could favor seeing low-linewidth systems, and concluded that while the single such detected system was not likely, it was not incredibly unlikely either. As statistics of such high-z millimeter observations increase, this model can be more stringently tested. R. Dave, with student Desika Narayanan, Bok Fellow Yancey Shirley, and the Harvard group led by T. J. Cox, are investigating the fundamental driver of the relationship between molecular line emission (CO and HCN) and star formation rate in galaxies. Using simulations and Desika's non-LTE line transfer code, we found that the assumption of a Kennicutt Law in the neutral gas, combined with non-LTE radiative transfer effects in the molecular emission, produces a slope in the SFR-CO/HCN relation that becomes shallower with increasing critical density for a particular transition. Hence CO 1-0 follows the Kennicutt slope of 1.5, while higher CO transitions or HCN 1-0 that trace dense gas follow a slope closer to linear. These trends are consistent with observations. The latter linear trend has been used to argue that the Kennicutt relation is not fundamental, and instead the star formation rate simply follows the molecular gas density linearly. In this model, even denser transitions such as HCN 3-2 would continue to show a linear slope with SFR. However, in Desika's model, the linear slope is purely a coincidence, and HCN 3-2 should actually show a sublinear slope. Initial observations by Yancey and collaborators seem to confirm Desika's prediction. R. Dave, with undergraduate student David Hernandez, investigated the Lyman alpha absorber-galaxy cross-correlation function, to study its evolution and compare with observations. They find a strong finger of God effect as observed, and show that the evolution of the correlation function evolves very slowly with redshift. R. Dave, with B. Barton (Irvine), J.-D. Smith (Arizona), C. Papovich (Arizona), and J. Jensen (Hawaii) continued their survey for z=8.2 Lyman alpha emitters using a custom-built narrow band filter for NIRI on Gemini. Due to weather, not enough data was taken for a meaningful search, but the survey continues with the hope of identifying the first confirmed z>7 object. A byproduct of the survey has been the identification of 6 OII emitters, which is actually 4x the number expected in that field. We are attempting to assess the significance of that result through a careful cosmic variance analysis. R. Dave, with G. Williger, L. Haberzettl, and J. Lauroesch (Louisville) and the LQG team, continued an observing project on the Clowes-Campusano Large Quasar Group at z~1.2. We have obtained optical and NIR imaging, optical and UV spectroscopy of ~20 quasars within the 2 square degree field, and multiobject spectroscopy of galaxies in the fields. The goal is to study the relationship between star formation, mass, environment, absorption line systems, and AGN within an overdense region during the critical epoch in the universe where the color-overdensity relationship first appears. Data collection is ongoing, including using the Bok with 90prime. We had a 7-night run using several Windhorst medium-band filters to improve photometric redshift estimates for the galaxy survey.