Christopher C. Hayward

WHAT DOES A SUBMILLIMETER GALAXY SELECTION ACTUALLY SELECT? THE DEPENDENCE OF SUBMILLIMETER FLUX DENSITY ON STAR FORMATION RATE AND DUST MASS

We perform 3-D dust radiative transfer (RT) calculations on hydrodynamic simulations of isolated and merging disk galaxies in order to quantitatively study the dependence of observed-frame submillimeter (submm) flux density on galaxy properties. We find that submm flux density and star formation rate (SFR) are related in dramatically different ways for quiescently star-forming galaxies and starbursts. Because the stars formed in the merger-induced starburst do not dominate the bolometric luminosity and the rapid drop in dust mass and more compact geometry cause a sharp increase in dust temperature during the burst, starbursts are very inefficient at boosting submm flux density (e.g., a & 16x boost in SFR yields a . 2x boost in submm flux density). Moreover, the ratio of submm flux density to SFR differs significantly between the two modes; thus one cannot assume that the galaxies with highest submm flux density are necessarily those with the highest bolometric luminosity or SFR. These results have important consequences for the bright submillimeter-selected galaxy (SMG) population. Among them are: 1. The SMG population is heterogeneous. In addition to merger-driven starbursts, there is a subpopulation of galaxy pairs, where two disks undergoing a major merger but not yet strongly interacting are blended into one submm source because of the large (& 15”, or � 130 kpc at z = 2) beam of single-dish submm telescopes. 2. SMGs must be very massive (M⋆ & 6×10 10 M⊙). 3. The infall phase makes the SMG duty cycle a factor of a few greater than what is expected for a merger-driven starburst. Finally, we provide fitting functions for SCUBA and AzTEC submm flux densities as a function of SFR and dust mass and bolometric luminosity and dust mass; these should be useful for calculating submm flux density in semi-analytic models and cosmological simulations when performing full RT is computationally not feasible. Subject headings: galaxies: high-redshift — galaxies: interactions — galaxies: starburst — infrared: galaxies — radiative transfer — submillimeter: galaxies

Star formation in galaxy mergers with realistic models of stellar feedback and the interstellar medium

We use simulations with realistic models for stellar feedback to study galaxy mergers. These high resolution (1 pc) simulations follow formation and destruction of individual GMCs and star clusters. The final starburst is dominated by in situ star formation, fueled by gas which flows inwards due to global torques. The resulting high gas density results in rapid star formation. The gas is self gravitating, and forms massive (~10^10 M_sun) GMCs and subsequent super-starclusters (masses up to 10^8 M_sun). However, in contrast to some recent simulations, the bulk of new stars which eventually form the central bulge are not born in superclusters which then sink to the center of the galaxy, because feedback efficiently disperses GMCs after they turn several percent of their mass into stars. Most of the mass that reaches the nucleus does so in the form of gas. The Kennicutt-Schmidt law emerges naturally as a consequence of feedback balancing gravitational collapse, independent of the small-scale star formation microphysics. The same mechanisms that drive this relation in isolated galaxies, in particular radiation pressure from IR photons, extend over seven decades in SFR to regulate star formation in the most extreme starbursts (densities >10^4 M_sun/pc^2). Feedback also drives super-winds with large mass loss rates; but a significant fraction of the wind material falls back onto the disks at later times, leading to higher post-starburst SFRs in the presence of stellar feedback. Strong AGN feedback is required to explain sharp cutoffs in star formation rate. We compare the predicted relic structure, mass profile, morphology, and efficiency of disk survival to simulations which do not explicitly resolve GMCs or feedback. Global galaxy properties are similar, but sub-galactic properties and star formation rates can differ significantly.

The formation of high‐redshift submillimetre galaxies

We describe a model for the formation of z ∼ 2 Submillimeter 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 observ ational constraints as physical input naturally yield objec ts 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 bir thcloud covering fraction. The most luminous (S850&15 mJy) sources are recovered by ∼10 13 M⊙ 1:1 major mergers with a birthcloud covering fraction close to unity, whereas more average SMGs (S850∼5‐7 mJy) may be formed in lower mass halos (∼5×10 12 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 hel p ascertain the robustness of these models.

ON SIZES, KINEMATICS, M/L GRADIENTS, AND LIGHT PROFILES OF MASSIVE COMPACT GALAXIES AT z ∼ 2

We present a detailed analysis of the structure and resolved stellar populations of simulated merger remnants, and compare them to observations of compact quiescent galaxies at z � 2. We find that major merging is a viable mechanism to produce systems of � 10 11 Mand � 1 kpc size, provided the gas fraction at the time of final coalescence is high (� 40%), and provided that the progenitors are compact star-forming galaxies, as expected at high redshift. Their integrated spectral energy distributions and velocity dispersions are in good agreement with the observations, and their position in the (vmaj=�;�) diagram traces the upper envelope of the distribution of lower redshift early-type galaxies. The simulated merger remnants show time- and sightline-dependent M=L ratio gradients that result from a superposition of radially dependent stellar age, stellar metallicity, and extinction. The median ratio of effective radius in rest-frame V -band light to that in mass surface density is � 2 during the quiescent remnant phase. This is typically expressed by a negative color gradient (i.e., red core), which we expect to correlate with the integrated color of the system. Finally, the simulations differ from the observations in their surface brightness profile shape. The simulated remnants are typically best fit by high (n � 4) Sersic indices, whereas observed quiescent galaxies at z � 2 tend to be less cuspy (hni � 2:3). Limiting early star formation in the progenitors may be required to prevent the simulated merger remnants from having extended wings. Subject headings: galaxies: evolution, galaxies: formation - galaxies: structure - galaxies: stellar content

Submillimetre galaxies in a hierarchical universe: number counts, redshift distribution, and implications for the IMF

High-redshift submillimetre galaxies (SMGs) are some of the most rapidly star-forming galaxies in the Universe. Historically, galaxy formation models have had difficulty explaining the observed number counts of SMGs. We combine a semi-empirical model with 3-D hydrodynamical simulations and 3-D dust radiative transfer to predict the number counts of unlensed SMGs. Because the stellar mass functions, gas and dust masses, and sizes of our galaxies are constrained to match observations, we can isolate uncertainties related to the dynamical evolution of galaxy mergers and the dust radiative transfer. The number counts and redshift distributions predicted by our model agree well with observations. Isolated disc galaxies dominate the faint (S1.1 . 1 mJy, or S850 . 2 mJy) population. The brighter sources are a mix of merger-induced starbursts and galaxy-pair SMGs; the latter subpopulation accounts for � 30 50 per cent of all SMGs at all S1.1 & 0.5 mJy (S850 & 1 mJy). The mean redshifts are � 3.0 3.5, depending on the flux cut, and the brightest sources tend to b e at higher redshifts. Because the galaxy-pair SMGs will be resolved into multiple fainter sources by ALMA, the bright ALMA counts should be as much as 2 times less than those observed using single-dish telescopes. The agreement between our model, which uses a Kroupa IMF, and observations suggests that the IMF in high-redshifts starbursts need not be top-heavy; if the IMF were top-heavy, our model would over-predict the number counts. We conclude that the difficulty some models have reproducing the observed SMG counts is likely indicative of more general problems ‐ such as an under-prediction of the abundance of massive galaxies or a star formation rate‐stellar mass relation normalisation lower than t hat observed ‐ rather than a problem specific to the SMG population.

The star formation main sequence and stellar mass assembly of galaxies in the Illustris simulation

Understanding the physical processes that drive star formation is a key challenge for galaxy formation models. In this paper, we study the tight correlation between the star formation rate (SFR) and stellar mass of galaxies at a given redshift, how halo growth influences star formation, and star formation histories of individual galaxies. We study these topics using Illustris, a state-of-the-art cosmological hydrodynamical simulation of galaxy formation. Illustris reproduces the observed relation (the star formation main sequence, SFMS) between SFR and stellar mass at redshifts z = 0 and 4, but at intermediate redshifts of z ≃ 1–2, the simulated SFMS has a significantly lower normalization than reported by observations. The scatter in the relation is consistent with the observed scatter. However, the fraction of outliers above the SFR–stellar mass relation in Illustris is less than that observed. Galaxies with halo masses of ∼10^12 M_⊙ dominate the SFR density of the Universe, in agreement with the results of abundance matching. Furthermore, more-massive galaxies tend to form the bulk of their stars at high redshift, which indicates that ‘downsizing’ occurs in Illustris. We also studied the star formation histories of individual galaxies, including the use of a principal component analysis decomposition. We find that for fixed stellar mass, galaxies that form earlier have more-massive black holes at z = 0, indicating that star formation and black hole growth are tightly linked processes in Illustris. While many of the properties of normal star-forming galaxies are well reproduced in the Illustris simulation, forming a realistic population of starbursts will likely require higher resolution and probably a more sophisticated treatment of star formation and feedback from stars and black holes.

A deep ALMA image of the Hubble Ultra Deep Field

We present the results of the first, deep Atacama Large Millimeter Array (ALMA) imaging covering the full ≃4.5 arcmin2 of the Hubble Ultra Deep Field (HUDF) imaged with Wide Field Camera 3/IR on HST. Using a 45-pointing mosaic, we have obtained a homogeneous 1.3-mm image reaching σ1.3 ≃ 35 μJy, at a resolution of ≃0.7 arcsec. From an initial list of ≃50 > 3.5σ peaks, a rigorous analysis confirms 16 sources with S1.3 > 120 μJy. All of these have secure galaxy counterparts with robust redshifts (〈z〉 = 2.15). Due to the unparalleled supporting data, the physical properties of the ALMA sources are well constrained, including their stellar masses (M*) and UV+FIR star formation rates (SFR). Our results show that stellar mass is the best predictor of SFR in the high-redshift Universe; indeed at z ≥ 2 our ALMA sample contains seven of the nine galaxies in the HUDF with M* ≥ 2 × 1010 M⊙, and we detect only one galaxy at z > 3.5, reflecting the rapid drop-off of high-mass galaxies with increasing redshift. The detections, coupled with stacking, allow us to probe the redshift/mass distribution of the 1.3-mm background down to S1.3 ≃ 10 μJy. We find strong evidence for a steep star-forming ‘main sequence’ at z ≃ 2, with SFR ∝M* and a mean specific SFR ≃ 2.2 Gyr−1. Moreover, we find that ≃85 per cent of total star formation at z ≃ 2 is enshrouded in dust, with ≃65 per cent of all star formation at this epoch occurring in high-mass galaxies (M* > 2 × 1010 M⊙), for which the average obscured:unobscured SF ratio is ≃200. Finally, we revisit the cosmic evolution of SFR density; we find this peaks at z ≃ 2.5, and that the star-forming Universe transits from primarily unobscured to primarily obscured at z ≃ 4.

How to distinguish starbursts and quiescently star-forming galaxies: the ‘bimodal’ submillimetre galaxy population as a case study

In recent work, we have suggested that the high-redshift (z∼ 2–4) bright submillimetre galaxy (SMG) population is heterogeneous, with major mergers contributing both at early stages, where quiescently star-forming discs are blended into one submm source (‘galaxy-pair SMGs’), and at late stages, where mutual tidal torques drive gas inflows and cause strong starbursts. Here we combine hydrodynamic simulations of major mergers with 3D dust radiative transfer calculations to determine observational diagnostics that can distinguish between quiescently star-forming SMGs and starburst SMGs via integrated data alone. We fit the far-infrared (FIR) spectral energy distributions of the simulated galaxies with the optically thin single-temperature modified blackbody, the full form of the single-temperature modified blackbody and a power-law temperature distribution model. The effective dust temperature, Td, and power-law index of the dust emissivity in the FIR, β, derived can significantly depend on the fitting form used, and the intrinsic β of the dust is not recovered. However, for all forms used here, there is Td above which almost all simulated galaxies are starbursts, so a Td cut is very effective at selecting starbursts. Simulated merger-induced starbursts also have higher LIR/Mgas and LIR/LFUV than quiescently star-forming galaxies and lie above the star formation rate–stellar mass relation. These diagnostics can be used to test our claim that the SMG population is heterogeneous and to observationally determine what star formation mode dominates a given galaxy population. We comment on applicability of these diagnostics to ultraluminous IR galaxies (ULIRGs) that would not be selected as SMGs. These ‘hot-dust ULIRGs’ are typically starburst galaxies lower in mass than SMGs, but they can also simply be SMGs observed from a different viewing angle.

The star-forming molecular gas in high-redshift Submillimetre Galaxies

We present a model for the CO molecular line emission from high redshift Submillimeter Galaxies (SMGs). By combining hydrodynamic simulations of gas rich galaxy mergers with the polychromatic radiative transfer code, SUNRISE, and the 3D non-LTE molecular line radiative transfer code, TURTLEBEACH, we show that if SMGs are typically a transient phase of major mergers, their observed compact CO spatial extents, broad line widths, and high excitation conditions (CO SED) are naturally explained. In this sense, SMGs can be understood as scaled-up analogs to local ULIRGs. We utilize these models to investigate the usage of CO as an indicator of physical conditions. We find that care must be taken when applying standard techniques. The usage of CO line widths as a dynamical mass estimator from SMGs can possibly overestimate the true enclosed mass by a factor �1.5-2. At the same time, assumptions of line ratios of unity from CO J=3-2 (and higher lying lines) to CO (J=1-0) will oftentimes lead to underestimates of the inferred gas mass. We provide tests for these models by outlining predictions for experiments which are imminently feasible with the current generation of bolometer arrays and radio-wave spectrometers.

K+A GALAXIES AS THE AFTERMATH OF GAS-RICH MERGERS: SIMULATING THE EVOLUTION OF GALAXIES AS SEEN BY SPECTROSCOPIC SURVEYS

Models of poststarburst (or “K+A”) galaxies are constructed by combining fully three-dimensional hydrodynamic simulations of galaxy mergers with radiative transfer calculations of dust attenuation. Spectral line catalogs are generated automatically from moderate-resolution optical spectra calculated as a function of merger progress in each of a large suite of simulations. The mass, gas fraction, orbital parameters, and mass ratio of the merging galaxies are varied systematically, showing that the lifetime and properties of the K+A phase are strong functions of merger scenario. K+A durations are generally . 0.1-0.3 Gyr, significantly shorter than the commonly assumed 1 Gyr, which is obtained only in rare cases, owing to a wide variation in star formation histories resulting from different orbital and progenitor configurations. Combined with empirical merger rates, the model lifetimes predict rapidlyrising K+A fractions as a function of redshift that are consistent with results of large spectroscopic surveys, resolving tension between the observed K+A abundance and that predicted when one assumes the K+A duration is the lifetime of A stars (� 1 Gyr). The simulated spectra are spatially resolved on scales of about 1 kpc, and indicate that a centrally-concentrated starburst causes the Balmer absorption strengths to increase towards the central few kiloparsecs of the remnant. The effects of dust attenuation, viewing angle, and aperture bias on our models are analyzed. In some cases, the K+A features are longer-lived and more pronounced when AGN feedback removes dust from the center, uncovering the young stars formed during the burst. In this picture, the K+A phase begins during or shortly after the bright starburst/AGN phase in violent mergers, and thus offers a unique opportunity to study the effects of quasar and star formation feedback on the gas reservoir and evolution of the remnant. Analytic fitting formulae are provided for the estimates of K+A incidence as a function of merger scenario. Subject headings: galaxies:evolution, galaxies:interactions, galaxies:starburst, methods:numerical