T. Naab

High molecular gas fractions in normal massive star-forming galaxies in the young universe

Stars form from cold molecular interstellar gas. As this is relatively rare in the local Universe, galaxies like the Milky Way form only a few new stars per year. Typical massive galaxies in the distant Universe formed stars an order of magnitude more rapidly. Unless star formation was significantly more efficient, this difference suggests that young galaxies were much more molecular-gas rich. Molecular gas observations in the distant Universe have so far largely been restricted to very luminous, rare objects, including mergers and quasars, and accordingly we do not yet have a clear idea about the gas content of more normal (albeit massive) galaxies. Here we report the results of a survey of molecular gas in samples of typical massive-star-forming galaxies at mean redshifts <z> of about 1.2 and 2.3, when the Universe was respectively 40% and 24% of its current age. Our measurements reveal that distant star forming galaxies were indeed gas rich, and that the star formation efficiency is not strongly dependent on cosmic epoch. The average fraction of cold gas relative to total galaxy baryonic mass at z = 2.3 and z = 1.2 is respectively about 44% and 34%, three to ten times higher than in today’s massive spiral galaxies. The slow decrease between z ≈ 2 and z ≈ 1 probably requires a mechanism of semi-continuous replenishment of fresh gas to the young galaxies.

A study of the gas–star formation relation over cosmic time★

We use the first systematic data sets of CO molecular line emission in z∼ 1–3 normal star-forming galaxies (SFGs) for a comparison of the dependence of galaxy-averaged star formation rates on molecular gas masses at low and high redshifts, and in different galactic environments. Although the current high-z samples are still small and biased towards the luminous and massive tail of the actively star-forming ‘main-sequence’, a fairly clear picture is emerging. Independent of whether galaxy-integrated quantities or surface densities are considered, low- and high-z SFG populations appear to follow similar molecular gas–star formation relations with slopes 1.1 to 1.2, over three orders of magnitude in gas mass or surface density. The gas-depletion time-scale in these SFGs grows from 0.5 Gyr at z∼ 2 to 1.5 Gyr at z∼ 0. The average corresponds to a fairly low star formation efficiency of 2 per cent per dynamical time. Because star formation depletion times are significantly smaller than the Hubble time at all redshifts sampled, star formation rates and gas fractions are set by the balance between gas accretion from the halo and stellar feedback. In contrast, very luminous and ultraluminous, gas-rich major mergers at both low and high z produce on average four to 10 times more far-infrared luminosity per unit gas mass. We show that only some fraction of this difference can be explained by uncertainties in gas mass or luminosity estimators; much of it must be intrinsic. A possible explanation is a top-heavy stellar mass function in the merging systems but the most likely interpretation is that the star formation relation is driven by global dynamical effects. For a given mass, the more compact merger systems produce stars more rapidly because their gas clouds are more compressed with shorter dynamical times, so that they churn more quickly through the available gas reservoir than the typical normal disc galaxies. When the dependence on galactic dynamical time-scale is explicitly included, disc galaxies and mergers appear to follow similar gas-to-star formation relations. The mergers may be forming stars at slightly higher efficiencies than the discs.

Formation of Early-Type Galaxies from Cosmological Initial Conditions

We describe high-resolution smoothed particle hydrodynamics (SPH) simulations of three approximately M* field galaxies starting from ΛCDM initial conditions. The simulations are made intentionally simple, and include photoionization, cooling of the intergalactic medium, and star formation, but not feedback from AGNs or supernovae. All of the galaxies undergo an initial burst of star formation at z ≈ 5, accompanied by the formation of a bubble of heated gas. Two out of three galaxies show early-type properties at present, whereas only one of them experienced a major merger. Heating from shocks and PdV work dominates over cooling so that for most of the gas the temperature is an increasing function of time. By z ≈ 1 a significant fraction of the final stellar system is in place and the spectral energy distribution resembles those of observed massive red galaxies. The galaxies have grown from z = 1 → 0 on average by 25% in mass and in size by gas-poor (dry) stellar mergers. By the present day the simulated galaxies are old (≈10 Gyr), kinematically hot stellar systems surrounded by hot gaseous haloes. Stars dominate the mass of the galaxies up to ≈4 effective radii (≈10 kpc). Kinematic and most photometric properties are in good agreement with those of observed elliptical galaxies. The galaxy with a major merger develops a counter-rotating core. Our simulations show that realistic intermediate-mass giant elliptical galaxies with plausible formation histories can be formed from ΛCDM initial conditions even without requiring recent major mergers or feedback from supernovae or AGNs.

STATISTICAL PROPERTIES OF COLLISIONLESS EQUAL- AND UNEQUAL-MASS MERGER REMNANTS OF DISK GALAXIES

We perform a large parameter survey of collisionless N-body simulations of binary mergers of disk galaxies with mass ratios of 1?:?1, 2?:?1, 3?:?1, and 4?:?1, using the special-purpose hardware GRAPE. A set of 112 merger simulations is used to investigate the fundamental statistical properties of merger remnants as a function of the initial orientation and mass ratio of the progenitor disks. The photometric and kinematical properties of the simulated merger remnants are analyzed. The methods used to determine the characteristic properties are equivalent to the methods used for observations of giant elliptical galaxies. We take projection effects into account and analyze the remnant properties in a statistical way for comparison with observations. The basic properties of the remnants correlate with the mass ratio of the progenitor disks. We find that about 80% of the equal-mass merger simulations lead to slowly rotating merger remnants having * 0) isophotes. A distinct subclass of 4 out of 12 initial orientations leads to purely boxy remnants independent of orientation. The 1?:?1 mergers with other initial orientations show disky or boxy isophotes, depending on the viewing angle. Remnants with mass ratios of 3?:?1 and 4?:?1 have more homogeneous properties. They all rotate rapidly (maximum value of v/? = 1.2) and show a small amount of minor-axis rotation, consistent with models of isotropic or slightly anisotropic oblate rotators. If observed in projection, they would be interpreted as being supported by rotation. About 90% of the projected 3?:?1 and 4?:?1 remnants show disky isophotes. The 2?:?1 remnants show intermediate properties. Projection effects lead to a large spread in the data, in good agreement with observations. They do not change the fundamental kinematical differences between equal- and unequal-mass merger remnants. The correlation between isophotal twist and apparent ellipticity of every single merger remnant is in good agreement with observations. The amount of twisting strongly depends on the orientation of the remnant but is only weakly dependent on the mass ratio of the merger. The results of this study weaken the disk merger scenario as the possible formation mechanism of massive boxy giant ellipticals, as only equal-mass mergers with special initial orientations can produce purely boxy anisotropic merger remnants. Some orientations of 1?:?1 mergers can even lead to disky and anisotropic remnants that are either not observed or would be classified as S0 galaxies based on their morphology. In general, the properties of equal-mass (and 2?:?1) merger remnants are consistent with those of the observed population of giant ellipticals in the intermediate-mass regime between low-mass, fast-rotating, disky and bright, massive, boxy giant ellipticals. The 3?:?1 and 4?:?1 merger remnants, however, are in very good agreement with the class of low-luminosity, fast-rotating giant elliptical galaxies. Binary mergers of disk galaxies are therefore still very good candidates for being the main formation mechanism for intermediate- and low-mass giant ellipticals. The homogeneous class of massive boxy ellipticals most likely formed by a different process.

Dynamical properties of ultraluminous infrared galaxies. I. Mass ratio conditions for ULIRG activity in interacting pairs

We present first results from our Very Large Telescope large program to study the dynamical evolution of ultraluminous infrared galaxies (ULIRGs), which are the products of mergers of gas-rich galaxies. The full data set consists of high-resolution long-slit H- and K-band spectra of 38 ULIRGs and 12 QSOs (in the range 0.042 3 : 1 typically do not force enough gas into the center to generate ULIRG luminosities.

Do dwarf galaxies form in tidal tails

The formation of tidal dwarf galaxies (TDGs) inside tidal arms of interacting disc galaxies has been studied with N-body and N-body/Smoothed Particle Hydrodynamics (SPH) simulations at different resolutions. In pure N-body simulations, no bound objects are formed at high resolution. At low resolution, bound objects can form in tidal tails in agreement with previous work. We conclude that TDGs are not likely to form by pure collisionless collapse in tidal tails. However, the presence of a sufficiently massive and extended gas component in the progenitor disc supports the formation of bound stellar objects in the tidal arms. Our results clearly favour a dissipation supported scenario in which the formation of TDGs is induced by the local collapse of gas which then triggers the collapse of the stellar component.

iVINE – Ionization in the parallel tree/sph code VINE: first results on the observed age‐spread around O‐stars

We present a three-dimensional, fully parallelized, efficient implementation of ionizing ultraviolet (UV) radiation for smoothed particle hydrodynamics (SPH) including self-gravity. Our method is based on the SPH/TREE code VINE. We therefore call it iVINE (for Ionization + VINE). This approach allows detailed high-resolution studies of the effects of ionizing radiation from, for example, young massive stars on their turbulent parental molecular clouds. In this paper, we describe the concept and the numerical implementation of the radiative transfer for a plane-parallel geometry and we discuss several test cases demonstrating the efficiency and accuracy of the new method. As a first application, we study the radiatively driven implosion of marginally stable molecular clouds at various distances of a strong UV source and show that they are driven into gravitational collapse. The resulting cores are very compact and dense exactly as it is observed in clustered environments. Our simulations indicate that the time of triggered collapse depends on the distance of the core from the UV source. Clouds closer to the source collapse several 10 5 yr earlier than more distant clouds. This effect can explain the observed age spread in OB associations where stars closer to the source are found to be younger. We discuss possible uncertainties in the observational derivation of shock front velocities due to early stripping of protostellar envelopes by ionizing radiation.

Orbital structure of collisionless merger remnants: on the origin of photometric and kinematic properties of elliptical and S0 galaxies

We present a detailed investigation of the relation between the orbital content of merger remnants and observable properties of elliptical and SO galaxies. Our analysis is based on the statistical sample of collisionless mergers of disc galaxies with different mass ratios and orbital parameters, published by Naab & Burkert. We use the spectral method by Carpintero & Aguilar to determine the orbital content of every remnant and correlate it with its intrinsic shape, and its projected kinematic and photometric properties. We discuss the influence of the bulge component and varying pericentre distances. The classified orbit families are box orbits, minor-axis tubes, inner and outer major-axis tubes and boxlets. In general, box orbits dominate the inner parts of the remnant. Major and minor-axis tubes become dominant at intermediate radii and boxlets at large radii. The two most abundant orbit classes are the minor-axis tubes and the box orbits. Their ratio seems to determine the basic properties of a remnant. On average, the fraction of minor-axis tubes increases by a factor of 2 from a merger mass ratio of 1:1 to 4:1, whereas the fraction of box orbits decreases by 10 per cent. At a given mass the central velocity dispersion of a remnant scales with the ratio of minor-axis tubes to box orbits. Interestingly, the division line between rotational supported systems and pressure supported objects, (υ maj /σ 0 )* = 0.7, turns out to coincide with a box to minor-axis tube ratio of unity. The observed h 3 -υ/σ anticorrelation for ellipticals cannot be reproduced by collisionless merger remnants. We propose that this can only be reconciled by an additional physical process that significantly reduces the box orbit content. Remnants that are dominated by minor-axis tube orbits have predominantly discy projections. Boxy remnants have always a box to minor-axis tube ratio larger than one. This study will enable us to identify observed ellipticals that could have formed, in the collisionless limit, by gas-poor disc mergers. In addition, it demonstrates how observable properties of spheroidal stellar systems are connected with their intrinsic orbital structure.

HIGH-REDSHIFT STAR-FORMING GALAXIES: ANGULAR MOMENTUM AND BARYON FRACTION, TURBULENT PRESSURE EFFECTS, AND THE ORIGIN OF TURBULENCE

The structure of a sample of high-redshift (z ~ 2), rotating galaxies with high star formation rates and turbulent gas velocities of σ ≈ 40-80 km s -1 is investigated. Fitting the observed disk rotational velocities and radii with a Mo et al. (MMW) model requires unusually large disk spin parameters λ d > 0.1 and disk-to-dark halo mass fractions of m d ≈ 0.2, close to the cosmic baryon fraction. The galaxies segregate into dispersion-dominated systems with 1 ≤ υ max /σ ≤ 3, maximum rotational velocities υ max ≤ 200 km s -1 , and disk half-light radii r 1/2 ≈ 1-3 kpc, and rotation-dominated systems with υ max > 200 km s -1 , υ max /σ > 3, and r 1/2 ≈ 4-8 kpc. For the dispersion-dominated sample, radial pressure gradients partly compensate the gravitational force, reducing the rotational velocities. Including this pressure effect in the MMW model, dispersion-dominated galaxies can be fitted well with spin parameters of λ d = 0.03-0.05 for high disk mass fractions of m d ≈ 0.2 and with λ d = 0.01-0.03 for m d = 0.05. These values are in good agreement with cosmological expectations. For the rotation-dominated sample, however, pressure effects are small and better agreement with theoretically expected disk spin parameters can only be achieved if the dark halo mass contribution in the visible disk regime (2-3 x r 1/2 ) is smaller than predicted by the MMW model. We argue that these galaxies can still be embedded in standard cold dark matter halos if the halos do not contract adiabatically in response to disk formation. In this case, the data favor models with small disk mass fractions of m d = 0.05 and disk spin parameters of λ d ≈ 0.035. It is shown that the observed high turbulent gas motions of the galaxies are consistent with a Toomre instability parameter Q = 1 which is equal to the critical value, expected for gravitational disk instability to be the major driver of turbulence. The dominant energy source of turbulence is then the potential energy of the gas in the disk.

VINE—A NUMERICAL CODE FOR SIMULATING ASTROPHYSICAL SYSTEMS USING PARTICLES. I. DESCRIPTION OF THE PHYSICS AND THE NUMERICAL METHODS

We present a numerical code for simulating the evolution of astrophysical systems using particles to represent the underlying fluid flow. The code is written in Fortran 95 and is designed to be versatile, flexible and extensible, with modular options that can be selected either at the time the code is compiled or at run time through a text input file. We include a number of general purpose modules describing a variety of physical processes commonly required in the astrophysical community and we expect that the effort required to integrate additional or alternate modules into the code will small. In its simplest form the code can evolve the dynamical trajectories of a set of particles in two or three dimensions using a module which implements either a Leapfrog or Runge-Kutta-Fehlberg integrator, selected by the user at compile time. The user may choose to allow the integrator to evolve the system using individual timesteps for each particle or with a single, global time step for all. Particles may interact gravitationally as N-body particles, and all or any subset may also interact hydrodynamically, using the Smoothed Particle Hydrodynamic (SPH) method by selecting the SPH module. A third particle species can be included with a module to model massive point particles which may accrete nearby SPH or N-body particles. Such particles may be used to model, e.g., stars in a molecular cloud. Free boundary conditions are implemented by default, and a module may be selected to include periodic boundary conditions. We use a binary ‘Press’ tree to organize particles for rapid access in gravity and SPH calculations. Modules implementing an interface with special purpose ‘GRAPE’ hardware may also be selected to accelerate the gravity calculations. If available, forces obtained from the GRAPE coprocessors may be transparently substituted for those obtained from the tree, or both tree and GRAPE may be used as a combination GRAPE/tree code. The code may be run without modification on single processors or in parallel using OpenMP compiler directives on large scale, shared memory parallel machines. We present simulations of several test problems, including a merger simulation of two elliptical galaxies with 800000 particles. In comparison to the Gadget-2 code of Springel (2005), the gravitational force calculation, which is the most costly part of any simulation including self-gravity, is ∼ 4.6 − 4.9 times faster with VINE when tested on different snapshots of the elliptical galaxy merger simulation when run on an Itanium 2 processor in an SGI Altix. A full simulation of the same setup with 8 processors is a factor of 2.91 faster with VINE. The code is available to the public under the terms of the Gnu General Public License. Subject headings: methods: numerical — methods: N-body simulations — galaxies: interactions