arXiv:0711.3478 [ps, pdf, other] Title: Resolving Gas Dynamics in the Circumnuclear Region of a Disk Galaxy in a Cosmological Simulation Authors: Robyn Levine, Nickolay Y. Gnedin, Andrew J.S. Hamilton, Andrey V. Kravtsov Comments: 16 pages (includes appendix), submitted to ApJ. Figures here are at low resolution; for higher resolution version, download this http URL Subjects: Astrophysics (astro-ph) Using a hydrodynamic adaptive mesh refinement code, we simulate the growth and evolution of a galaxy, which could potentially host a supermassive black hole, within a cosmological volume. Reaching a dynamical range in excess of 10 million, the simulation follows the evolution of the gas structure from super-galactic scales all the way down to the outer edge of the accretion disk. Here, we focus on global instabilities in the self-gravitating, cold, turbulence-supported, molecular gas disk at the center of the model galaxy, which provide a natural mechanism for angular momentum transport down to sub-pc scales. The gas density profile follows a power-law scaling as r^-8/3, consistent with an analytic description of turbulence in a quasi-stationary circumnuclear disk. We analyze the properties of the disk which contribute to the instabilities, and investigate the significance of instability for the galaxy's evolution and the growth of a supermassive black hole at the center. [8] arXiv:0711.3792 [ps, pdf, other] Title: Interaction-induced star formation in a complete sample of 10^5 nearby star-forming galaxies Authors: Cheng Li, Guinevere Kauffmann, Timothy Heckman, Y. P. Jing, Simon D. M. White Comments: 13 pages, 12 figures, submitted for publication in Monthly Notices Subjects: Astrophysics (astro-ph) We investigate the clustering properties of a complete sample of 10^5 star-forming galaxies drawn from the SDSS DR4. On scales less than 100 kpc, the amplitude of the correlation function exhibits a strong dependence on the specific star formation rate of the galaxy. We interpret this as the signature of enhanced star formation induced by tidal interactions. We then explore how the average star formation rate in a galaxy is enhanced as the projected separation r_p between the galaxy and its companions decreases. We find that the enhancement depends strongly on r_p, but very weakly on the relative luminosity of the companions. The enhancement is also stronger in low mass galaxies than in high mass galaxies. In order to explore whether a tidal interaction is not only sufficient, but also necessary to trigger enhanced star formation in a galaxy, we compute background subtracted neighbour counts for the galaxies in our sample. The average number of close neighbours around galaxies with low to average values of SFR/M* is close to zero. At the highest specific star formation rates, however, more than 40% of the galaxies in our sample have a companion within a projected radius of 100 kpc. Visual inspection of the highest SFR/M* galaxies without companions reveals that more than 50% of these are clear interacting or merging systems. We conclude that tidal interactions are the dominant trigger of enhanced star formation in the most strongly star-forming systems. Finally, we find clear evidence that tidal interactions not only lead to enhanced star formation in galaxies, but also cause structural changes such as an increase in concentration. [5] arXiv:0711.4077 [ps, pdf, other] Title: The dark matter halo of NGC 1399 - CDM or MOND? Authors: T. Richtler (1), Y. Schuberth (1 and 2), M. Hilker (3), B. Dirsch (1), L. Bassino (4), A. J. Romanowsky (1 and 5) ((1) Universidad de Concepción, (2) Universität Bonn, (3) ESO, Garching, (4) Universidad Nacional de La Plata, (5) UCO/Lick Observatory) Comments: 4 pages, 2 figures, accepted for publication as a Letter in A&A Subjects: Astrophysics (astro-ph) Central galaxies in galaxy clusters may be key discriminants in the competition between the cold dark matter (CDM) paradigm and modified Newtonian dynamics (MOND). We investigate the dark halo of NGC 1399, the central galaxy of the Fornax cluster, out to a galactocentric distance of 80 kpc. The data base consists of 656 radial velocities of globular clusters obtained with MXU/VLT and GMOS/Gemini, which is the largest sample so far for any galaxy. We performed a Jeans analysis for a non-rotating isotropic model. An NFW halo with the parameters r_s = 50 kpc and rho_s = 0.0065 M_sun/pc^3 provides a good description of our data, fitting well to the X-ray mass. More massive halos are also permitted that agree with the mass of the Fornax cluster as derived from galaxy velocities. We compare this halo with the expected MOND models under isotropy and find that additional dark matter on the order of the stellar mass is needed to get agreement. A fully radial infinite globular cluster system would be needed to change this conclusion. Regarding CDM, we cannot draw firm conclusions. To really constrain a cluster wide halo, more data covering a larger radius are necessary. The MOND result appears as a small-scale variant of the finding that MOND in galaxy clusters still needs dark matter. arXiv:0711.4099 [ps, pdf, other] Title: Contribution to Unresolved Infrared Fluctuations from Dwarf Galaxies at Redshifts of 2-3 Authors: Ranga-Ram Chary, Asantha Cooray, Ian Sullivan Comments: 12 pages, 3 figures, submitted to ApJ Letters Subjects: Astrophysics (astro-ph) In order to understand the origin of clustered anisotropies detected in Spitzer IRAC images, we stack the Spitzer IRAC/Great Observatories Origins Deep Survey (GOODS) images at pixel locations corresponding to faint, z_{AB}~27 mag, optical sources with no obvious IR counterparts. We obtain a stacked median flux of 130+/-5 nJy at 3.6 microns. We also use the wealth of multi-wavelength data in GOODS to measure the stacked spectrum of these sources from the ultraviolet to near-infrared bands. The median flux spectrum is consistent with a L<0.03 L_{*,UV} galaxy population at z~2.5. They produce a 3.6 micron absolute background intensity between 0.1 and 0.35 nW m^{-2} sr^{-1} and the clustered IR light could account for ~30-50% of fluctuation power in the IR background at 4 arcminute angular scales. Although the redshift distribution of these sources is unknown, they appear to contain between 5-20% of the co-moving stellar mass density at z~2.5. arXiv:0704.0646 (cross-list from gr-qc) [ps, pdf, other] Title: The Mathematical Universe Authors: Max Tegmark (MIT) Comments: Replaced to match accepted Found. Phys. version, 31 pages, 5 figs; more details at this http URL Subjects: General Relativity and Quantum Cosmology (gr-qc); Astrophysics (astro-ph); High Energy Physics - Theory (hep-th) I explore physics implications of the External Reality Hypothesis (ERH) that there exists an external physical reality completely independent of us humans. I argue that with a sufficiently broad definition of mathematics, it implies the Mathematical Universe Hypothesis (MUH) that our physical world is an abstract mathematical structure. I discuss various implications of the ERH and MUH, ranging from standard physics topics like symmetries, irreducible representations, units, free parameters, randomness and initial conditions to broader issues like consciousness, parallel universes and Godel incompleteness. I hypothesize that only computable and decidable (in Godel's sense) structures exist, which alleviates the cosmological measure problem and help explain why our physical laws appear so simple. I also comment on the intimate relation between mathematical structures, computations, simulations and physical systems. arXiv:0711.4116 [ps, pdf, other] Title: Star Formation at Zero and Very Low Metallicities Authors: Jonathan C. Tan (1), Christopher F. McKee (2) ((1) Dept. of Astronomy, University of Florida; (2) Depts. of Physics and Astronomy, UC Berkeley) Comments: 16 pages, including 11 figures, Review paper to appear in "First Stars III", eds. B. O'Shea, A. Heger and T. Abel Subjects: Astrophysics (astro-ph) We describe how star formation is expected to proceed in the early metal-free Universe, focusing on the very first generations of stars. We then discuss how the star formation process may change as the effects of metallicity, external radiative feedback, and magnetic and turbulent support of the gas become more important. The very first stars (Pop III.1) have relatively simple initial conditions set by cosmology and the cooling properties of primordial gas. We describe the evolution of these stars as they grow in mass by accretion from their surrounding gas cores and how the accretion process is affected and eventually terminated by radiative feedback processes, especially HII region expansion and disk photoevaporation. The ability of the protostar and its disk to generate dynamically important magnetic fields is reviewed and their effects discussed. Pop III.1 star formation is likely to produce massive (~100-200Msun) stars that then influence their surroundings via ionization, stellar winds, and supernovae. These processes heat, ionize and metal-enrich the gas, thus altering the initial conditions for the next generation of star formation. Stars formed from gas that has been altered significantly by radiative and/or mechanical feedback, but not by metal enrichment (Pop III.2) are expected to have significantly smaller masses than Pop III.1 stars because of more efficient cooling from enhanced HD production. Stars formed from gas that is metal-enriched to levels that affect the dynamics of the collapse (the first Pop II stars) are also expected to have relatively low masses. We briefly compare the above star formation scenarios to what is known about present-day star formation. arXiv:0711.4123 [ps, pdf, other] Title: The Orbital Distribution of Satellite Galaxies Authors: Stéphane Herbert-Fort, Dennis Zaritsky, Yeun Jin Kim, Jeremy Bailin, James E. Taylor Comments: 14 pages, 13 figures; Accepted for publication in MNRAS Subjects: Astrophysics (astro-ph) We measure the distribution of velocities for prograde and retrograde satellite galaxies using a combination of published data and new observations for 78 satellites of 63 extremely isolated disc galaxies (169 satellites total). We find that the velocity distribution is non-Gaussian (>99.9% confidence), but that it can be described as the sum of two Gaussians, one of which is broad (sigma = 176 \pm 15 km/s), has a mean prograde velocity of 86 \pm 30 km/s, and contains ~55% of the satellites, while the other is slightly retrograde with a mean velocity of -21 \pm 22 km/s and sigma = 74 \pm 18 km/s and contains ~45% of the satellites. Both of these components are present over all projected radii and found in the sample regardless of cuts on primary inclination or satellite disc angle. The double-Gaussian shape, however, becomes more pronounced among satellites of more luminous primaries. We remove the potential dependence of satellite velocity on primary luminosity using the Tully-Fisher relation and still find the velocity distribution to be asymmetric and even more significantly non-Gaussian. The asymmetric velocity distribution demonstrates a connection between the inner, visible disc galaxy and the kinematics of the outer, dark halo. The reach of this connection, extending even beyond the virial radii, suggests that it is imprinted by the satellite infall pattern and large-scale effects, rather than by higher-level dynamical processes in the formation of the central galaxy or late-term evolution of the satellites. arXiv:0711.4204 [ps, pdf, other] Title: Clustering properties of a type-selected volume-limited sample of galaxies in the CFHTLS Authors: H. J. McCracken, O. Ilbert, Y. Mellier, E. Bertin, L. Guzzo, S. Arnouts, O. Le Fèvre, G. Zamorani Comments: 16 pages, 18 figures, accepted for publication in Astronomy and Astrophysics Subjects: Astrophysics (astro-ph) (abridged) We present an investigation of the clustering of i'AB<24.5 galaxies in the redshift interval 0.2