arXiv subjects

Thorsten Naab

Publications and source records attributed to Thorsten Naab.

At least 217 records · Page 12Linked to original sources

arXiv2011

Discovery of an AGN-Driven Molecular Outflow in the Local Early-Type Galaxy NGC 1266

We report the discovery of a powerful molecular wind from the nucleus of the non-interacting nearby S0 field galaxy NGC 1266. The single-dish CO profile exhibits emission to +/- 400 km/s and requires a nested Gaussian fit to be properly described. Interferometric observations reveal a massive, centrally- concentrated molecular component with a mass of 1.1x10^9 Msuns and a molecular outflow with a molecular mass of 2.4x10^7 Msuns. The molecular gas close to the systemic velocity consists of a rotating, compact nucleus with a mass of about 4.1x108 Msuns within a radius of approximately 60 pc. This compact molecular nucleus has a surface density of \approx 2.7 \times 10^4 Msuns/pc^2, more than two orders of magnitude larger than that of giant molecular clouds in the disk of the Milky Way, and it appears to sit on the Kennicutt-Schmidt relation despite its extreme kinematics and energetic activity. We interpret this nucleus as a disk that confines the outflowing wind. A mass outflow rate of 13 Msuns/yr leads to a depletion timescale of about 85 Myr. The star formation in NGC 1266 is insufficient to drive the outflow, and thus it is likely driven by the active galactic nucleus (AGN). The concentration of the majority of the molecular gas in the central 100 pc requires an extraordinary loss of angular momentum, but no obvious companion or interacting galaxy is present to enable the transfer. NGC 1266 is the first known outflowing molecular system that does not show any evidence of a recent interaction.

astro-ph.CO↗

arXiv2011

Disruption of Star Clusters in the Interacting Antennae Galaxies

We reexamine the age distribution of star clusters in the Antennae in the context of N-body+hydrodynamical simulations of these interacting galaxies. All of the simulations that account for the observed morphology and other properties of the Antennae have star formation rates that vary relatively slowly with time, by factors of only 1.3 - 2.5 in the past 10^8 yr. In contrast, the observed age distribution of the clusters declines approximately as a power law, dN/dt \propto t^{gamma} with gamma = -1.0, for ages 10^6 yr \la t \la 10^9 yr. These two facts can only be reconciled if the clusters are disrupted progressively for at least 10^8 yr and possibly 10^9 yr. When we combine the simulated formation rates with a power-law model, f_surv \propto t^{delta}, for the fraction of clusters that survive to each age t, we match the observed age distribution with exponents in the range -0.9 \la delta \la -0.6 (with a slightly different delta for each simulation). The similarity between delta and gamma indicates that dN/dt is shaped mainly by the disruption of clusters rather than variations in their formation rate. Thus, the situation in the interacting Antennae resembles that in relatively quiescent galaxies such as the Milky Way and the Magellanic Clouds.

astro-ph.CO↗

arXiv2011

The effects of a hot gaseous halo in galaxy major mergers

Cosmological hydrodynamical simulations as well as observations indicate that spiral galaxies are comprised of five different components: dark matter halo, stellar disc, stellar bulge, gaseous disc and gaseous halo. While the first four components have been extensively considered in numerical simulations of binary galaxy mergers, the effect of a hot gaseous halo has usually been neglected even though it can contain up to 80% of the total gas within the galaxy virial radius. We present a series of hydrodynamic simulations of major mergers of disc galaxies, that for the first time include a diffuse, rotating, hot gaseous halo. Through cooling and accretion, the hot halo can dissipate and refuel the cold gas disc before and after a merger. This cold gas can subsequently form stars, thus impacting the morphology and kinematics of the remnant. Simulations of isolated systems with total mass M~10^12Msun show a nearly constant star formation rate of ~5Msun/yr if the hot gaseous halo is included, while the star formation rate declines exponentially if it is neglected. We conduct a detailed study of the star formation efficiency during mergers and find that the presence of a hot gaseous halo reduces the starburst efficiency (e=0.5) compared to simulations without a hot halo (e=0.68). Moreover we find cases where the stellar mass of the merger remnant is lower than the sum of the stellar mass of the two progenitor galaxies when evolved in isolation. This suggests a revision to semi-analytic galaxy formation models which assume that a merger always leads to enhanced star formation. We show that adding the hot gas component has a significant effect on the kinematics and internal structure of the merger remnants, like an increased abundance of fast rotators and an r^(1/4) surface brightness profile at small scales.

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arXiv2011

Flow-Driven Cloud Formation and Fragmentation: Results From Eulerian and Lagrangian Simulations

The fragmentation of shocked flows in a thermally bistable medium provides a natural mechanism to form turbulent cold clouds as precursors to molecular clouds. Yet because of the large density and temperature differences and the range of dynamical scales involved, following this process with numerical simulations is challenging. We compare two-dimensional simulations of flow-driven cloud formation without self-gravity, using the Lagrangian Smoothed Particle Hydrodynamics (SPH) code VINE and the Eulerian grid code Proteus. Results are qualitatively similar for both methods, yet the variable spatial resolution of the SPH method leads to smaller fragments and thinner filaments, rendering the overall morphologies different. Thermal and hydro-dynamical instabilities lead to rapid cooling and fragmentation into cold clumps with temperatures below 300K. For clumps more massive than 1 Msun/pc, the clump mass function has an average slope of -0.8. The internal velocity dispersion of the clumps is nearly an order of magnitude smaller than their relative motion, rendering it subsonic with respect to the internal sound speed of the clumps, but supersonic as seen by an external observer. For the SPH simulations most of the cold gas resides at temperatures below 100K, while the grid-based models show an additional, substantial component between 100 and 300K. Independently of the numerical method our models confirm that converging flows of warm neutral gas fragment rapidly and form high-density, low-temperature clumps as possible seeds for star formation.

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arXiv2011

The ATLAS3D project - II. Morphologies, kinemetric features and alignment between photometric and kinematic axes of early-type galaxies

[Abridged] We use the ATLAS3D sample of 260 early-type galaxies (ETGs) to study the apparent kinematic misalignment angle, Psi, defined as the angle between the photometric and kinematic major axis. We find that 71% of nearby ETGs are strictly aligned systems (Psi > 5 deg), an additional 14% have 5 < Psi < 10 deg and 90% of galaxies have Psi < 15 deg. Taking into account measurement uncertainties, 90% of galaxies can be considered aligned to better than 5 deg, suggesting that only a small fraction of early-type galaxies (~10%) are not consistent with axisymmetry within the projected half-light radius. We identify morphological features such as bars and rings (30%), dust structures (16%), blue nuclear colours (6%) and evidence of interactions (8%). We use kinemetry to analyse the mean velocity maps and separate galaxies in two broad types of regular and non-regular rotators. We find 82% of regular rotators and 17% non-regular rotators, with 2 galaxies that we were not able to classify due to data quality. The non-regular rotators are typically found in dense regions and are massive. The majority of galaxies do not have any specific kinemetric features, but we highlight here the frequency of the kinematically distinct cores (7%) and the aligned double peaks in the velocity dispersion maps (4%). Most of the galaxies that are misaligned have complex kinematics and are non-regular rotators. While the trends are weak, there is a tendency that large values of Psi are found in galaxies at intermediate environmental densities and among the most massive galaxies in the sample. We suggest that the most common formation mechanism for ETGs preserves the axisymmetry of the disk progenitors. Less commonly, the formation process results in a triaxial galaxy with much lower net angular momentum.

astro-ph.CO↗

arXiv2011

The Atlas3D project -- IV: the molecular gas content of early-type galaxies

We have carried out a survey for 12CO J=1-0 and J=2-1 emission in the 260 early-type galaxies of the volume-limited Atlas3D sample, with the goal of connecting their star formation and assembly histories to their cold gas content. This is the largest volume-limited CO survey of its kind and is the first to include many Virgo Cluster members. Sample members are dynamically hot galaxies with a median stellar mass 3\times 10^{10} Msun; they are selected by morphology rather than colour, and the bulk of them lie on the red sequence. The overall CO detection rate is 56/259 = 0.22 \error 0.03, with no dependence on K luminosity and only a modest dependence on dynamical mass. There are a dozen CO detections among the Virgo Cluster members; statistical analysis of their H_2 mass distributions and their dynamical status within the cluster shows that the cluster's influence on their molecular masses is subtle at best, even though (unlike spirals) they seem to be virialized within the cluster. We suggest that the cluster members have retained their molecular gas through several Gyr residences in the cluster. There are also a few extremely CO-rich early-type galaxies with H_2 masses >= 10^9 Msun, and these are in low density environments. We do find a significant trend between molecular content and the stellar specific angular momentum. The galaxies of low angular momentum also have low CO detection rates, suggesting that their formation processes were more effective at destroying molecular gas or preventing its re-accretion. We speculate on the implications of these data for the formation of various sub-classes of early-type galaxies.

astro-ph.CO↗

arXiv2011

The Atlas3D project - V. The CO Tully-Fisher relation of early-type galaxies

We demonstrate here using both single-dish and interferometric observations that CO molecules are an excellent kinematic tracer, even in high-mass galaxies, allowing us to investigate for the first time the CO Tully-Fisher relation of early-type galaxies. We compare the Tully-Fisher relations produced using both single-dish and interferometric data and various inclination estimation methods, and evaluate the use of the velocity profile shape as a criterion for selecting galaxies in which the molecular gas extends beyond the peak of the rotation curve. We show that the gradient and zero-point of the best-fit relations are robust, independent of the velocity measure and inclination used, and agree with those of relations derived using stellar kinematics. We also show that the early-type CO Tully-Fisher (CO-TF) relation is offset from the CO-TF of spirals by 0.98 \pm 0.22 magnitude at Ks-band, in line with other results. The intrinsic scatter of the relation is found to be \approx 0.4 magnitudes, similar to the level found in the spiral galaxy population. Next generation facilities such as the Large Millimeter Telescope (LMT) and the Atacama Large Millimeter/Sub-millimeter Array (ALMA) should allow this technique to be used in higher-redshift systems, providing a simple new tool to trace the mass-to-light ratio evolution of the most massive galaxies over cosmic time.

astro-ph.CO↗

arXiv2011

The ATLAS3D project - III. A census of the stellar angular momentum within the effective radius of early-type galaxies: unveiling the distribution of Fast and Slow Rotators

We provide a census of the apparent stellar angular momentum within 1 Re of a volume-limited sample of 260 early-type galaxies (ETGs) in the nearby Universe, using integral-field spectroscopy obtained in the course of the ATLAS3D project. We exploit the LambdaR parameter to characterise the existence of two families of ETGs: Slow Rotators which exhibit complex stellar velocity fields and often include stellar kinematically Distinct Cores (KDCs), and Fast Rotators which have regular velocity fields. Our complete sample of 260 ETGs leads to a new criterion to disentangle Fast and Slow Rotators which now includes a dependency on the apparent ellipticity (Epsilon). It separates the two classes significantly better than the previous prescription, and than a criterion based on V/Sigma: Slow Rotators and Fast Rotators have LambdaR lower and larger than kFSxSQRT(Epsilon), respectively, where kFS=0.31 for measurements made within 1 Re. We show that the vast majority of early-type galaxies are Fast Rotators: these have regular stellar rotation, with aligned photometric and kinematic axes (Paper II, Krajnovic et al. 2011}, include discs and often bars and represent 86% (224/260) of all early-type galaxies in the volume-limited ATLAS3D sample. Fast Rotators span the full range of apparent ellipticities from 0 to 0.85, and we suggest that they cover intrinsic ellipticities from about 0.35 to 0.85, the most flattened having morphologies consistent with spiral galaxies. Only a small fraction of ETGs are Slow Rotators representing 14% (36/260) of the ATLAS3D sample of ETGs. Of all Slow Rotators, 11% (4/36) exhibit two counter-rotating stellar disc-like components and are rather low mass objects (Mdyn<10^10.5 M_Sun). All other Slow Rotators (32/36) appear relatively round on the sky (Epsilon_e<0.4), tend to be massive (Mdyn>10^10.5 M_Sun), and often (17/32) exhibit KDCs.

astro-ph.CO↗

arXiv2010

The Two Phases of Galaxy Formation

Cosmological simulations of galaxy formation appear to show a two-phase character with a rapid early phase at z>2 during which in-situ stars are formed within the galaxy from infalling cold gas followed by an extended phase since z<3 during which ex-situ stars are primarily accreted. In the latter phase massive systems grow considerably in mass and radius by accretion of smaller satellite stellar systems formed at quite early times (z>3) outside of the virial radius of the forming central galaxy. These tentative conclusions are obtained from high resolution re-simulations of 39 individual galaxies in a full cosmological context with present-day virial halo masses ranging from 7e11 M_sun h^-1 < M_vir < 2.7e13 M_sun h^-1 and central galaxy masses between 4.5e10 M_sun h^-1 < M_* < 3.6e11 M_sun h^-1. The simulations include the effects of a uniform UV background, radiative cooling, star formation and energetic feedback from SNII. The importance of stellar accretion increases with galaxy mass and towards lower redshift. In our simulations lower mass galaxies ($M_* < 9e10 M_sun h^-1) accrete about 60 per cent of their present-day stellar mass. High mass galaxy ($M_* > 1.7e11 M_sun h^-1) assembly is dominated by accretion and merging with about 80 per cent of the stars added by the present-day. In general the simulated galaxies approximately double their mass since z=1. For massive systems this mass growth is not accompanied by significant star formation. The majority of the in-situ created stars is formed at z>2, primarily out of cold gas flows. We recover the observational result of archaeological downsizing, where the most massive galaxies harbor the oldest stars. We find that this is not in contradiction with hierarchical structure formation. Most stars in the massive galaxies are formed early on in smaller structures, the galaxies themselves are assembled late.

astro-ph.CO↗

arXiv2010

The Atlas3D project -- I. A volume-limited sample of 260 nearby early-type galaxies: science goals and selection criteria

The Atlas3D project is a multi-wavelength survey combined with a theoretical modeling effort. The observations span from the radio to the millimeter and optical, and provide multi-colour imaging, two-dimensional kinematics of the atomic (HI), molecular (CO) and ionized gas (Hbeta, [OIII] and [NI]), together with the kinematics and population of the stars (Hbeta, Fe5015 and Mgb), for a carefully selected, volume-limited (1.16*10^5 Mpc^3) sample of 260 early-type (elliptical E and lenticular S0) galaxies (ETGs). The models include semi-analytic, N-body binary mergers and cosmological simulations of galaxy formation. Here we present the science goals for the project and introduce the galaxy sample and the selection criteria. The sample consists of nearby (D<42 Mpc) morphologically-selected ETGs extracted from a parent sample of 871 galaxies (8% E, 22% S0 and 70% spirals) brighter than M_K<-21.5 mag (stellar mass M_Star>6*10^9 M_Sun). We analyze possible selection biases and we conclude that the parent sample is essentially complete and statistically representative of the nearby galaxy population. We present the size-luminosity relation for the spirals and ETGs and show that the ETGs in the Atlas3D sample define a tight red sequence in a colour-magnitude diagram, with few objects in the transition from the blue cloud. We describe the strategy of the SAURON integral-field observations and the extraction of the stellar kinematics with the pPXF method. We give an overview of the characteristics of the other main datasets already available for our sample and of the ongoing modelling projects.

astro-ph.CO↗

arXiv2010

Detailed Numerical Simulations on the Formation of Pillars around HII-regions

We study the structural evolution of turbulent molecular clouds under the influence of ionizing radiation emitted from a nearby massive star by performing a high resolution parameter study with the iVINE code. The temperature is taken to be 10K or 100K, the mean number density is either 100cm^3 or 300cm^3. Besides, the turbulence is varied between Mach 1.5 and Mach 12.5 and the main driving scale between 1pc and 8pc. We vary the ionizing flux by an order of magnitude. In our simulations the ionizing radiation enhances the initial turbulent density distribution and thus leads to the formation of pillar-like structures observed adjacent to HII regions in a natural way. Gravitational collapse occurs regularly at the tips of the structures. We find a clear correlation between the initial state of the turbulent cold cloud and the final morphology and physical properties of the structures formed. The most favorable regime for the formation of pillars is Mach 4-10. Structures and therefore stars only form if the initial density contrast between the high density unionized gas and the gas that is going to be ionized is lower than the temperature contrast between the hot and the cold gas. The density of the resulting pillars is determined by a pressure equilibrium between the hot and the cold gas. A thorough analysis of the simulations shows that the complex kinematical and geometrical structure of the formed elongated filaments reflects that of observed pillars to an impressive level of detail. In addition, we find that the observed line-of sight velocities allow for a distinct determination of different formation mechanisms. Comparing the current simulations to previous results and recent observations we conclude that e.g. the pillars of creation in M16 formed by the mechanism proposed here and not by the radiation driven implosion of pre-existing clumps.

astro-ph.SR↗

arXiv2010

The growth of dark matter halos: evidence for significant smooth accretion

We study the growth of dark matter halos in the concordance LCDM cosmology using several N-body simulations of large cosmological volumes. We build merger trees from the Millennium and Millennium-II simulations, covering a range 10^9-10^15 Msun in halo mass and 1-10^5 in merger mass ratio. Our algorithm takes special care of halo fragmentation and ensures that the mass contribution of each merger to halo growth is only counted once. This way the integrated merger rate converges and we can consistently determine the contribution of mergers of different mass ratios to halo growth. We find that all resolved mergers, up to mass ratios of 10^5, contribute only ~60% of the total halo mass growth, while major mergers are subdominant, e.g. mergers with mass ratios smaller than 3:1 (10:1) contribute only ~20% (~30%). This is verified with an analysis of two additional simulation boxes, where we follow all particles individually throughout cosmic time. Our results are also robust against using several halo definitions. Under the assumption that the power-law behaviour of the merger rate at large mass ratios can be extrapolated to arbitrarily large mass ratios, it is found that, independently of halo mass, ~40% of the mass in halos comes from genuinely smooth accretion of dark matter that was never bound in smaller halos. We discuss possible implications of our findings for galaxy formation. One implication, assuming as is standard that the pristine intergalactic medium is heated and photoionized by UV photons, is that all halos accrete >40% of their baryons in smooth "cold" T>~10^4K gas, rather than as warm, enriched or clumpy gas or as stars.

astro-ph.CO↗

arXiv2010

One moment in time - modeling star formation in the Antennae

We present a new high-resolution N-body/SPH simulation of an encounter of two gas-rich disk galaxies which closely matches the morphology and kinematics of the interacting Antennae galaxies (NGC 4038/39). The simulation includes radiative cooling, star formation and feedback from SNII. The large-scale morphology and kinematics are determined by the internal structure and the orbit of the progenitor disks. The properties of the central region, in particular the starburst in the overlap region, only match the observations for a very short time interval of ~20 Myr after the second encounter. This indicates that the Antennae galaxies are in a special phase only about 40 Myr after the second encounter and 50 Myr before their final collision. This is the only phase in the simulation when a gas-rich overlap region between the nuclei is forming accompanied by enhanced star formation. The star formation rate as well as the recent star formation history in the central region agree well with observational estimates. For the first time this new model explains the distributed extra-nuclear star formation in the Antennae galaxies as a consequence of the recent second encounter. The proposed model predicts that the Antennae are in a later merger stage than the Mice (NGC 4676) and would therefore lose their first place in the classical Toomre sequence.

astro-ph.CO↗

arXiv2009

Constraints on the relationship between stellar mass and halo mass at low and high redshift

We use a statistical approach to determine the relationship between the stellar masses of galaxies and the masses of the dark matter halos in which they reside. We obtain a parameterized stellar-to-halo mass (SHM) relation by populating halos and subhalos in an N-body simulation with galaxies and requiring that the observed stellar mass function be reproduced. We find good agreement with constraints from galaxy-galaxy lensing and predictions of semi-analytic models. Using this mapping, and the positions of the halos and subhalos obtained from the simulation, we find that our model predictions for the galaxy two-point correlation function (CF) as a function of stellar mass are in excellent agreement with the observed clustering properties in the SDSS at z=0. We show that the clustering data do not provide additional strong constraints on the SHM function and conclude that our model can therefore predict clustering as a function of stellar mass. We compute the conditional mass function, which yields the average number of galaxies with stellar masses in the range [m, m+dm] that reside in a halo of mass M. We study the redshift dependence of the SHM relation and show that, for low mass halos, the SHM ratio is lower at higher redshift. The derived SHM relation is used to predict the stellar mass dependent galaxy CF and bias at high redshift. Our model predicts that not only are massive galaxies more biased than low mass ones at all redshifts, but the bias increases more rapidly with increasing redshift for massive galaxies than for low mass ones. We present convenient fitting functions for the SHM relation as a function of redshift, the conditional mass function, and the bias as a function of stellar mass and redshift.

astro-ph.CO↗

arXiv2009

Can Gas prevent the Destruction of Thin Stellar Discs by Minor Mergers?

We study the effect of dissipational gas physics on the vertical heating and thickening of disc galaxies during minor mergers. We produce a suite of minor merger simulations for Milky Way-like galaxies. This suite consists of collisionless simulations as well as hydrodynamical runs including a gaseous component in the galactic disc. We find that in dissipationless simulations minor mergers cause the scale height of the disc to increase by up to a factor of ~2. When the presence of gas in the disc is taken into account this thickening is reduced by 25% (50%) for an initial disc gas fraction of 20% (40%), leading to a final scale height z0 between 0.6 and 0.7 kpc, regardless of the initial scale height. We argue that the presence of gas reduces disc heating via two mechanisms: absorption of kinetic impact energy by the gas and/or formation of a new thin stellar disc that can cause heated stars to recontract towards the disc plane. We show that in our simulations most of the gas is consumed during the merger and thus the regrowth of a new thin disc has a negligible impact on the z0 of the post merger galaxy. Final disc scale heights found in our simulations are in good agreement with studies of the vertical structure of spiral galaxies where the majority of the systems are found to have scale heights of 0.4 kpc < z0 < 0.8 kpc. We also found no tension between recent measurements of the scale height of the Milky Way thin disc and results coming from our hydrodynamical simulations. We conclude that the existence of a thin disc in the Milky Way and in external galaxies is not in obvious conflict with the predictions of the CDM model.

astro-ph.CO↗

arXiv2009

The evolution of Black Hole scaling relations in galaxy mergers

We study the evolution of black holes (BHs) on the M_BH-sigma and M_BH-M_bulge planes as a function of time in disk galaxies undergoing mergers. We begin the simulations with the progenitor black hole masses being initially below (Delta log M_BH=-2), on (Delta log M_BH=0) and above (Delta log M_BH=0.5) the observed local relations. The final relations are rapidly established after the final coalescense of the galaxies and their BHs. Progenitors with low initial gas fractions (f_gas=0.2) starting below the relations evolve onto the relations (Delta log M_BH=-0.18), progenitors on the relations stay there (Delta log M_BH=0) and finally progenitors above the relations evolve towards the relations, but still remaining above them (Delta log M_BH=0.35). Mergers in which the progenitors have high initial gas fractions (f_gas=0.8) evolve above the relations in all cases (Delta log M_BH=0.5). We find that the initial gas fraction is the prime source of scatter in the observed relations, dominating over the scatter arising from the evolutionary stage of the merger remnants. The fact that BHs starting above the relations do not evolve onto the relations, indicates that our simulations rule out the scenario in which overmassive BHs evolve onto the relations through gas-rich mergers. By implication our simulations thus disfavor the picture in which supermassive BHs develop significantly before their parent bulges.

astro-ph.CO↗

arXiv2009

Fully compressive tides in galaxy mergers

The disruptive effect of galactic tides is a textbook example of gravitational dynamics. However, depending on the shape of the potential, tides can also become fully compressive. When that is the case, they might trigger or strengthen the formation of galactic substructures (star clusters, tidal dwarf galaxies), instead of destroying them. We perform N-body simulations of interacting galaxies to quantify this effect. We demonstrate that tidal compression occurs repeatedly during a galaxy merger, independently of the specific choice of parameterization. With a model tailored to the Antennae galaxies, we show that the distribution of compressive tides matches the locations and timescales of observed substructures. After extending our study to a broad range of parameters, we conclude that neither the importance of the compressive tides (~15% of the stellar mass) nor their duration (~ 10 Myr) are strongly affected by changes in the progenitors' configurations and orbits. Moreover, we show that individual clumps of matter can enter compressive regions several times in the course of a simulation. We speculate that this may spawn multiple star formation episodes in some star clusters, through e.g., enhanced gas retention.

astro-ph.CO↗

arXiv2009

The Effects of the Ionizing Radiation Background on Galaxy Evolution

We find that the amount and nature of the assumed ionizing background can strongly affect galaxy formation and evolution. Galaxy evolution simulations typically incorporate an ultraviolet background which falls off rapidly above z=3; e.g., that of Haardt & Madau (1996). However, this decline may be too steep to fit the WMAP constraints on electron scattering optical depth or observations of intermediate redshift (z ~ 2-4) Ly-alpha forest transmission. As an alternative, we present simulations of the cosmological formation of individual galaxies with UV backgrounds that decline more slowly at high redshift: both a simple intensity rescaling and the background recently derived by Faucher-Giguere (2009), which softens the spectrum at higher redshifts. We also test an approximation of the X-ray background with a similar z-dependence. We find for the test galaxies that an increase in either the intensity or hardness of ionizing radiation generically pushes star formation towards lower redshifts: although overall star formation in the simulation boxes is reduced by 10-25%, the galaxies show a factor of ~2 increase in the fraction of stars within a 30 kpc radius that are formed after z=1. Other effects include late gas inflows enhanced up to 30 times, stellar half-mass radii decreased by up to 30%, central velocity dispersions increased up to 40%, and a strong reduction in substructure. The magnitude of the effects depends on the environmental/accretion properties of the particular galaxy.

astro-ph.GA↗