We develop a general physical model for how galactic disks survive and/or are destroyed in mergers and interactions. Based on simple dynamical arguments, we show that gas primarily loses angular momentum to internal torques in a merger, induced by the gravity of the secondary. Gas within some characteristic radius, determined by the efficiency of this angular momentum loss (itself a function of the orbital parameters, mass ratio, and gas fraction of the merging galaxies), will quickly lose angular momentum to the stars sharing the perturbed host disk, fall to the center and be consumed in a starburst. We use a similar analysis to determine where violent relaxation of the pre-merger stellar disks is efficient on final coalescence. Our model describes both the dissipational and dissipationless components of the merger, and allows us to predict, for a given arbitrary encounter, the stellar and gas content of the material that will survive (without significant angular momentum loss or violent relaxation) to re-form a disk in the merger remnant, versus being dissipationlessly violently relaxed or dissipationally losing angular momentum and forming a compact central starburst. We test these predictions with a large library of hydrodynamic merger simulations, and show that they agree well (with small scatter) with the properties of simulated merger remnants as a function of merger mass ratio, orbital parameters, and gas distributions, in simulations which span a wide range of parameter space in these properties as well as prescriptions for gas physics, stellar and AGN feedback, halo and initial disk structural properties, redshift, and galaxy masses. We show that, in an immediate (short-term) sense, the amount of stellar or gaseous disk that survives or re-forms following a given interaction can be understood purely in terms of simple, well-understood gravitational physics, independent of the details of the ISM gas physics or stellar and AGN feedback. This allows us to demonstrate and quantify how these physics are in fact important, in an indirect sense, to enable disks to survive mergers, by lowering star formation efficiencies in low mass systems (allowing them to retain large gas fractions) and distributing the gas to large radii. The efficiency of disk destruction in mergers is a strong function of gas content -our model allows us to explicitly predict and demonstrate how, in sufficiently gas rich mergers (with quite general orbital parameters), even 1:1 mass-ratio mergers can yield disk-dominated remnants, and more realistic 1:3-1:4 mass-ratio major mergers can yield systems with < 20% of their mass in bulges. We discuss a number of implications of this modeling for the abundance and morphology of bulges as a function of mass and redshift, and provide simple prescriptions for the implementation of our results in analytic or semi-analytic models of galaxy formation.

We use a suite of semi-empirical models to predict the galaxy-galaxy merger rate and relative contributions to bulge growth as a function of mass (both halo and stellar), redshift, and mass ratio. The models use empirical constraints on the halo occupation distribution, evolved forward in time, to robustly identify where and when galaxy mergers occur. Together with the results of high-resolution merger simulations, this allows us to quantify the relative contributions of mergers with different properties (e.g. mass ratios, gas fractions, redshifts) to the bulge population. We compare with observational constraints, and find good agreement. We also provide useful fitting functions and make public a code a to reproduce the predicted merger rates and contributions to bulge mass growth. We identify several robust conclusions. (1) Major mergers dominate the formation and assembly of ∼ L * bulges and the total spheroid mass density, but minor mergers contribute a non-negligible ∼ 30%.(2) This is mass-dependent: bulge formation and assembly is dominated by more minor mergers in lower-mass systems. In higher-mass systems, most bulges originally form in major mergers near ∼ L * , but assemble in increasingly minor mergers. (3) The minor/major contribution is also morphology-dependent: higher B/T systems preferentially form in more major mergers, with B/T roughly tracing the mass ratio of the largest recent merger; lower B/T systems preferentially form in situ from minor minors. (4) Low-mass galaxies, being gas-rich, require more mergers to reach the same B/T as high-mass systems. Gas-richness dramatically suppresses the absolute efficiency of bulge formation, but does not strongly influence the relative contribution of major versus minor mergers. (5) Absolute merger rates at fixed mass ratio increase with galaxy mass. (6) Predicted merger rates agree well with those observed in pair and morphology-selected samples, but there is evidence that some morphology-selected samples include contamination from minor mergers. (7) Predicted rates also agree with the integrated growth in bulge mass density with cosmic time, but with factor ∼ 2 uncertainty in both -up to half the bulge mass density could come from non-merger processes. We systematically vary the model assumptions, totaling ∼ 10 3 model permutations, and quantify the resulting uncertainties. Our conclusions regarding the importance of different mergers for bulge formation are very robust to these changes. The absolute predicted merger rates are systematically uncertain at the factor ∼ 2 level; uncertainties grow at the lowest masses and high redshifts.

Observations of the galaxy cluster MACS J2135-0102 (z cl =0.325) were made with the LABOCA 870µm bolometer camera 29 on the APEX telescope on 2009 May 8 for a total of 3.2hours (1200 seconds on-source) in excellent conditions (PWV=0.35-0.40mm). We used a 6 arcminute spiral pattern scan, centered at α:21:35:12.706 δ:-01:01:43.27 (J2000). For flux calibration, Mars and Uranus were both observed immediately prior to the science observations. The data were reduced using the minicrush reduction package, which includes temperature drift correction, flat-fielding, opacity correction, bad bolometer masking and de-spiking 29 . The final map appears flat, and has an r.m.s. of 3.5mJy/beam. Including systematic effects, we estimate calibration and fitting uncertainties as ~4% and 6% respectively. Visual inspection of the image shows a bright 30σ source centered at α:21:35:11.6 δ:-01:02:52.0 with an 870µm flux of 106.0±3.5mJy. Given the uncertainties in the calibration we adopt the flux of the SMG as S 870 =106.5±7.0mJy, and derive S 870 =24.2±7.0mJy for the counter-image.We followed up SMMJ2135-0102 with the Submillimetre APEX Bolometer CAmera (SABOCA, Siringo et al., in prep.) on the APEX telescope on UT 2009 September 20 and 21. SABOCA is a 37 superconducting Transition Edge Sensing (TES) bolometer array with hexagonal layout and twobeam separation on sky Its filter transmission curve is optimised to cover the 350µm window, and has a central wavelength of 352µm (852 GHz) for flat spectrum sources. SABOCA operates at a temperature of 300mK and is installed in the Cassegrain cabin of APEX. We observed SMMJ2135-0102 in a 20"x20" raster of spirals with 35 seconds duration in order to obtain a fully sampled map.

We have used the Submillimeter Array to image a flux limited sample of seven submillimeter galaxies, selected by the AzTEC camera on the JCMT at 1.1 mm, in the COSMOS field at 890 µm with ∼ 2 ′′ resolution. All of the sources-two radio-bright and five radio-dim-are detected as single point-sources at high significance (> 6σ), with positions accurate to ∼ 0.2 ′′ that enable counterpart identification at other wavelengths observed with similarly high angular resolution. All seven have IRAC counterparts, but only two have secure counterparts in deep HST/ACS imaging. As compared to the two radio-bright sources in the sample, and those in previous studies, the five radio-dim sources in the sample (1) have systematically higher submillimeter-to-radio flux ratios, (2) have lower IRAC 3.6-8.0 µm fluxes, and (3) are not detected at 24µm. These properties, combined with size constraints at 890 µm (θ ∼ < 1.2 ′′), suggest that the radio-dim submillimeter galaxies represent a population of very dusty starbursts, with physical scales similar to local ultraluminous infrared galaxies, and an average redshift higher than radio-bright sources.

We describe a model for the formation of z∼ 2 submillimetre galaxies (SMGs) which simultaneously accounts for both average and bright SMGs while providing a reasonable match to their mean observed spectral energy distributions (SEDs). By coupling hydrodynamic simulations of galaxy mergers with the high‐resolution 3D polychromatic radiative transfer code sunrise, we find that a mass sequence of merger models which use observational constraints as physical input naturally yield objects which exhibit black hole, bulge and H2 gas masses similar to those observed in SMGs. The dominant drivers behind the 850 μm flux are the masses of the merging galaxies and the stellar birth cloud covering fraction. The most luminous (S850≳ 15 mJy) sources are recovered by ∼1013 M⊙ 1:1 major mergers with a birth cloud covering fraction close to unity, whereas more average SMGs (S850∼ 5–7 mJy) may be formed in lower mass haloes (∼5 × 1012 M⊙). These models demonstrate the need for high spatial resolution hydrodynamic and radiative transfer simulations in matching both the most luminous sources as well as the full SEDs of SMGs. While these models suggest a natural formation mechanism for SMGs, they do not attempt to match cosmological statistics of galaxy populations; future efforts along this line will help ascertain the robustness of these models.

Different theoretical methodologies lead to order-of-magnitude variations in predicted galaxy-galaxy merger rates. We examine how this arises, and quantify the dominant uncertainties. Modeling of dark matter and galaxy inspiral/merger times contribute factor ∼ 2 uncertainties. Different estimates of the halo-halo merger rate, the subhalo "destruction" rate, and the halo merger rate with some dynamical friction time delay for galaxy-galaxy mergers, agree to within this factor ∼ 2, provided proper care is taken to define mergers consistently. There are some caveats: if halo/subhalo masses are not appropriately defined the major merger rate can be dramatically suppressed, and in models with "orphan" galaxies and under-resolved subhalos the merger timescale can be severely overestimated. The dominant differences in galaxy-galaxy merger rates between models owe to the treatment of the baryonic physics. Cosmological hydrodynamic simulations without strong feedback and some older semi-analytic models, with known discrepancies in mass functions, can be biased by large factors (∼ 5) in predicted merger rates. However, provided that models yield a reasonable match to the total galaxy mass function, the differences in properties of central galaxies are sufficiently small to alone contribute small (factor ∼ 1.5) additional systematics to merger rate predictions. But variations in the baryonic physics of satellite galaxies in models can also have a dramatic effect on merger rates. The well-known problem of satellite "over-quenching" in most current semi-analytic models (SAMs)-whereby SAM satellite populations are too efficiently stripped of their gas-could lead to order of magnitude underestimates of merger rates for lowmass, gas-rich galaxies. Models in which the masses of satellites are fixed by observations (or SAMs adjusted to resolve this "over-quenching") tend to predict higher merger rates, but with factor ∼ 2 uncertainties stemming from the uncertainty in those observations. The choice of mass used to defined "major" and "minor" mergers also matters: stellar-stellar major mergers can be more or less abundant than halo-halo major mergers by an order of magnitude. At low masses, most true major mergers (mass ratio defined in terms of their baryonic or dynamical mass) will appear to be minor mergers in their stellar mass ratio-observations and models using just stellar criteria could underestimate major merger rates by factors ∼ 3 − 5. We discuss the uncertainties in relating any merger rate to spheroid formation (in observations or theory): in order to achieve better than factor ∼ 3 accuracy, it is necessary to account for the distribution of merger orbital parameters, gas fractions, and the full efficiency of merger-induced effects as a function of mass ratio.

We present results from a continuing interferometric survey of high-redshift submillimeter galaxies with the Submillimeter Array, including high-resolution (beam size ∼ 2 arcsec) imaging of eight additional AzTEC 1.1mm selected sources in the COSMOS Field, for which we obtain six reliable (peak S/N > 5 or peak S/N > 4 with multiwavelength counterparts within the beam) and two moderate significance (peak S/N > 4) detections. When combined with previous detections, this yields an unbiased sample of millimeter-selected SMGs with complete interferometric followup. With this sample in hand, we (1) empirically confirm the radio-submillimeter association, (2) examine the submillimeter morphology -including the nature of submillimeter galaxies with multiple radio counterparts and constraints on the physical scale of the far infrared -of the sample, and (3) find additional evidence for a population of extremely luminous, radio-dim submillimeter galaxies that peaks at higher redshift than previous, radio-selected samples. In particular, the presence of such a population of high-redshift sources has important consequences for models of galaxy formationwhich struggle to account for such objects even under liberal assumptions -and dust production models given the limited time since the Big Bang.

We present a physical model for the origin of z ∼ 2 dust-obscured galaxies (DOGs), a class of high-redshift ultraluminous infrared galaxies (ULIRGs) selected at 24 μm which are particularly optically faint (F 24 μm /F R > 1000). By combining N-body/smoothed particle hydrodynamic simulations of high-redshift galaxy evolution with 3D polychromatic dust radiative transfer models, we find that luminous DOGs (with F 24 0.3 mJy at z ∼ 2) are well modelled as extreme gas-rich mergers in massive (∼5 × 10 12 -10 13 M ) haloes, with elevated star formation rates (SFR; ∼500-1000 M yr −1 ) and/or significant active galactic nuclei (AGN) growth (Ṁ BH 0.5 M yr −1 ), whereas less luminous DOGs are more diverse in nature. At final coalescence, merger-driven DOGs transition from being starburst dominated to AGN dominated, evolving from a 'bump' to a power-law (PL) shaped mid-IR (Infrared Array Camera, IRAC) spectral energy distribution (SED). After the DOG phase, the galaxy settles back to exhibiting a 'bump' SED with bluer colours and lower SFRs. While canonically PL galaxies are associated with being AGN dominated, we find that the PL mid-IR SED can owe both to direct AGN contribution and to a heavily dust obscured stellar bump at times that the galaxy is starburst dominated. Thus, PL galaxies can be either starburst or AGN dominated. Less luminous DOGs can be well-represented either by mergers or by massive (M baryon ≈ 5 × 10 11 M ) secularly evolving gas-rich disc galaxies (with SFR 50 M yr −1 ). By utilizing similar models as those employed in the submillimetre galaxy (SMG) formation study of Narayanan et al., we investigate the connection between DOGs and SMGs. We find that the most heavily star-forming merger-driven DOGs can be selected as submillimetre galaxies, while both merger-driven and secularly evolving DOGs typically satisfy the BzK selection criteria. The model SEDs from the simulated galaxies match observed data reasonably well, though Mrk 231 and Arp 220 templates provide worse matches. Our models provide testable predictions of the physical masses, dust temperatures, CO linewidths and location on the M BH -M bulge relation of DOGs. Finally, we provide public SED templates derived from these simulations.