arXiv subjects

Claudio Dalla Vecchia

Publications and source records attributed to Claudio Dalla Vecchia.

102 records · Page 6Linked to original sources

arXiv2010

Metal-line emission from the warm-hot intergalactic medium: II. Ultraviolet

Approximately half the baryons in the local Universe are thought to reside in the warm-hot intergalactic medium (WHIM). Emission lines from metals in the UV band are excellent tracers of the cooler fraction of this gas. We present predictions for the surface brightness of a sample of UV lines that could potentially be observed by the next generation of UV telescopes at z<1. We use a subset of simulations from the OWLS project to create emission maps and to investigate the effect of varying the physical prescriptions for star formation, supernova and AGN feedback, chemodynamics and radiative cooling. Most models produce results in agreement within a factor of a few, indicating that the predictions are robust. Of the lines we consider, C III is the strongest line, but it typically traces gas colder than 10^5 K. The same is true for Si IV. The second strongest line, C IV, traces circum-galactic gas with T~10^5 K. O VI and Ne VIII probe the warmer (T~10^5.5 K and T~10^6 K, respectively) and more diffuse gas that may be a better tracer of the large scale structure. N V emission is intermediate between C IV and O VI. The intensity of all emission lines increases strongly with gas density and metallicity, and for the bright emission it is tightly correlated with the temperature for which the line emissivity is highest. In particular, the C III, C IV, Si IV and O VI emission that is sufficiently bright to be potentially detectable in the near future (>10^3 photon/s/cm^2/sr), comes from relatively dense (rho>10^2 rho_mean) and metal rich (Z>0.1 Z_sun) gas. As such, emission lines are highly biased tracers of the missing baryons and are not an optimal tool to close the baryon budget. However, they do provide a powerful means to detect the gas cooling onto or flowing out of galaxies and groups. (Abridged)

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arXiv2010

Metal-line emission from the warm-hot intergalactic medium: I. Soft X-rays

Emission lines from metals offer one of the most promising ways to detect the elusive warm-hot intergalactic medium (WHIM; 10^5 K 10^6 K). We find that the OVIII 18.97A is the strongest emission line, with a predicted maximum surface brightness of ~10^2 photon/s/cm^2/sr, but a number of other lines are only slightly weaker. All lines show a strong correlation between the intensity of the observed flux and the density and metallicity of the gas responsible for the emission. On the other hand, the potentially detectable emission consistently corresponds to the temperature at which the emissivity of the electronic transition peaks. The emission traces neither the baryonic nor the metal mass. In particular, the emission that is potentially detectable with proposed missions, traces overdense (rho>10^2rho_mean) and metal-rich (Z>0.1Z_sun) gas in and around galaxies and groups. While soft X-ray line emission is therefore not a promising route to close the baryon budget, it does offer the exciting possibility to image the gas accreting onto and flowing out of galaxies.

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arXiv2010

The case for AGN feedback in galaxy groups

[Abridged] The relatively recent insight that energy input from supermassive black holes (BHs) can have a substantial effect on the star formation rates (SFRs) of galaxies motivates us to examine its effects on the scale of galaxy groups. At present, groups contain most of the galaxies and a significant fraction of the overall baryon content of the universe. To explore the effects of BH feedback on groups, we analyse two high resolution cosmological hydro simulations from the OverWhelmingly Large Simulations project. While both include galactic winds driven by supernovae, only one includes feedback from BHs. We compare the properties of the simulated groups to a wide range of observational data, including hot gas radial profiles and gas mass fractions (fgas), luminosity-mass-temperature (L-M-T) scaling relations, K-band luminosity of the group and its central brightest galaxy (CBG), SFRs and ages of the CBG, and gas/stellar metallicities. Both runs yield entropy profiles similar to the data, while the run without AGN feedback yields highly peaked temperature profiles, in discord with the observations. Energy input from BHs significantly reduces fgas for groups with masses less than ~10^14 Msun, yielding fgas-T and L-T relations that are in agreement with the data. The run without AGN feedback suffers from the well known overcooling problem; the resulting K-band luminosities are much larger than observed. By contrast, the run that includes BH feedback yields K-band luminosities and CBG SFRs and ages in agreement with current estimates. Both runs yield very similar gas-phase metallicities that match X-ray data, but they predict very different stellar metallicities. Based on the above, galaxy groups provide a compelling case that BH feedback is a crucial ingredient in the formation of massive galaxies.

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arXiv2010

Impact of baryon physics on dark matter structures: a detailed simulation study of halo density profiles

The back-reaction of baryons on the dark matter halo density profile is of great interest, not least because it is an important systematic uncertainty when attempting to detect the dark matter. Here, we draw on a large suite of high resolution cosmological hydrodynamical simulations, to systematically investigate this process and its dependence on the baryonic physics associated with galaxy formation. The inclusion of baryons results in significantly more concentrated density profiles if radiative cooling is efficient and feedback is weak. The dark matter halo concentration can in that case increase by as much as 30 (10) per cent on galaxy (cluster) scales. The most significant effects occur in galaxies at high redshift, where there is a strong anti-correlation between the baryon fraction in the halo centre and the inner slope of both the total and the dark matter density profiles. If feedback is weak, isothermal inner profiles form, in agreement with observations of massive, early-type galaxies. However, we find that AGN feedback, or extremely efficient feedback from massive stars, is necessary to match observed stellar fractions in groups and clusters, as well as to keep the maximum circular velocity similar to the virial velocity as observed for disk galaxies. These strong feedback models reduce the baryon fraction in galaxies by a factor of 3 relative to the case with no feedback. The AGN is even capable of reducing the baryon fraction by a factor of 2 in the inner region of group and cluster haloes. This in turn results in inner density profiles which are typically shallower than isothermal and the halo concentrations tend to be lower than in the absence of baryons.

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arXiv2009

The physics driving the cosmic star formation history

We investigate the physics driving the cosmic star formation (SF) history using the more than fifty large, cosmological, hydrodynamical simulations that together comprise the OverWhelmingly Large Simulations (OWLS) project. We systematically vary the parameters of the model to determine which physical processes are dominant and which aspects of the model are robust. Generically, we find that SF is limited by the build-up of dark matter haloes at high redshift, reaches a broad maximum at intermediate redshift, then decreases as it is quenched by lower cooling rates in hotter and lower density gas, gas exhaustion, and self-regulated feedback from stars and black holes. The higher redshift SF is therefore mostly determined by the cosmological parameters and to a lesser extent by photo-heating from reionization. The location and height of the peak in the SF history, and the steepness of the decline towards the present, depend on the physics and implementation of stellar and black hole feedback. Mass loss from intermediate-mass stars and metal-line cooling both boost the SF rate at late times. Galaxies form stars in a self-regulated fashion at a rate controlled by the balance between, on the one hand, feedback from massive stars and black holes and, on the other hand, gas cooling and accretion. Paradoxically, the SF rate is highly insensitive to the assumed SF law. This can be understood in terms of self-regulation: if the SF efficiency is changed, then galaxies adjust their gas fractions so as to achieve the same rate of production of massive stars. Self-regulated feedback from accreting black holes is required to match the steep decline in the observed SF rate below redshift two, although more extreme feedback from SF, for example in the form of a top-heavy IMF at high gas pressures, can help.

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arXiv2009

The Origin of Extended Disk Galaxies at z=2

Galaxy formation models typically assume that the size and rotation speed of galaxy disks are largely dictated by the mass, concentration, and spin of their surrounding dark matter haloes. Equally important, however, are the fraction of baryons in the halo that collect into the central galaxy, as well as the net angular momentum that they are able to retain during its assembly process. We explore the latter using a set of four large cosmological N-body/gasdynamical simulations drawn from the OWLS (OverWhelmingly Large Simulations) project. These runs differ only in their implementation of feedback from supernovae. We find that, when expressed as fractions of their virial values, galaxy mass and net angular momentum are tightly correlated. Galaxy mass fractions, m_d=M_gal/M_vir, depend strongly on feedback, but only weakly on halo mass or spin over the halo mass range explored here (M_vir>1e11 h^{-1}M_sun). The angular momentum of a galaxy, j_d=J_gal/J_vir, correlates with m_d in a manner that is insensitive to feedback and that deviates strongly from the simple j_d = m_d assumption often adopted in semi-analytic models of galaxy formation. The m_d-j_d correlation implies that, in a given halo, galaxy disk size is maximal when the central galaxy makes up a substantial fraction (~20%-30%) of all baryons within the virial radius. At z=2, such systems may host gaseous disks with radial scale lengths as large as those reported for star-forming disks by the SINS survey, even in moderately massive haloes of average spin. Extended disks at z=2 may thus signal the presence of systems where galaxy formation has been particularly efficient, rather than the existence of haloes with unusually high spin parameter.

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arXiv2009

Chemical enrichment in cosmological, smoothed particle hydrodynamics simulations

(Abridged) We present an implementation of stellar evolution and chemical feedback for smoothed particle hydrodynamics (SPH) simulations. We consider the timed release of individual elements by both massive (Type II supernovae and stellar winds) and intermediate mass stars (Type Ia supernovae and asymptotic giant branch stars). We illustrate the results of our method using a suite of cosmological simulations that include new prescriptions for radiative cooling, star formation, and galactic winds. Radiative cooling is implemented element-by-element, in the presence of an ionizing radiation background, and we track all 11 elements that contribute significantly to the radiative cooling. We contrast two reasonable definitions of the metallicity of a resolution element and find that while they agree for high metallicities, there are large differences at low metallicities. We argue the discrepancy is indicative of the lack of metal mixing caused by the fact that metals are stuck to particles. We argue that since this is a (numerical) sampling problem, solving it using a poorly constrained physical process such as diffusion could have undesired consequences. We demonstrate that the two metallicity definitions result in redshift z = 0 stellar masses that can differ by up to a factor of two, because of the sensitivity of the cooling rates to the elemental abundances. We find that by z = 0 most of the metals are locked up in stars. The gaseous metals are distributed over a very wide range of gas densities and temperatures. The shock-heated warm-hot intergalactic medium has a relatively high metallicity of ~ 10^-1 Z_sun that evolves only weakly and is therefore an important reservoir of metals.

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arXiv2009

Galaxies-Intergalactic Medium Interaction Calculation --I. Galaxy formation as a function of large-scale environment

[Abridged] We present the first results of hydrodynamical simulations that follow the formation of galaxies to z=0 in spherical regions of radius ~20 Mpc/h drawn from the Millennium Simulation. The regions have overdensities that deviate by (-2, -1, 0, +1, +2)sigma from the cosmic mean, where sigma is the rms mass fluctuation on a scale of ~20Mpc/h at z=1.5. The simulations have mass resolution of up to 10^6 Msun/h, cover the entire range of large-scale environments and allow extrapolation of statistics to the entire 500 (Mpc/h)^3 Millennium volume. They include gas cooling, photoheating from an ionising background, SNe feedback and winds, but no AGN. We find that the specific SFR density at z <~ 10 varies systematically from region to region by up to an order of magnitude, but the global value, averaged over all volumes, reproduces observational data. Massive, compact galaxies, similar to those observed in the GOODS fields, form in the overdense regions as early as z=6, but do not appear in the underdense regions until z~3. These environmental variations are not caused by a dependence of the star formation properties on environment, but rather by a strong variation of the halo mass function from one environment to another, with more massive haloes forming preferentially in the denser regions. At all epochs, stars form most efficiently in haloes of circular velocity ~ 250 km/s. However, the star formation history exhibits a form of "downsizing" (even in the absence of AGN): the stars comprising massive galaxies at z=0 have mostly formed by z=1-2, whilst those comprising smaller galaxies typically form at later times. However, additional feedback is required to limit star formation in massive galaxies at late times.

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arXiv2009

Disk galaxies at z=2 in OWLS

We use the OWLS (OverWhelmingly Large Simulations) set of cosmological Nbody/gasdynamical simulations to study the properties of simulated galaxies at z=2. We focus on the effect of supernova feedback from evolving stars on the baryonic mass and angular momentum content of galaxies that assemble at the center of 1e11-3e12 h^{-1}M_\odot halos. Our main finding is that the mass and angular momentum of such galaxies are strongly coupled, in a way that is approximately independent of feedback: varying the feedback implementation leads, in a given halo, to large variations in galaxy mass but leaves the galaxy mass-angular momentum correlation largely unaltered. In particular, the ratio between the angular momentum of a galaxy and that of its surrounding halo (j_d=J_gal/J_vir) correlates closely with the galaxy mass (expressed in units of the virial mass of the halo; m_d=M_gal/M_vir). This correlation differs substantially from the m_d=j_d assumption commonly adopted in semianalytic models of galaxy formation. We use these results to infer the sizes of disk galaxies at z=2 expected in the LCDM scenario and to interpret recent observations of extended disks at z~2 by the SINS collaboration

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arXiv2008

Simulating galactic outflows with kinetic supernova feedback

Feedback from star formation is thought to play a key role in the formation and evolution of galaxies, but its implementation in cosmological simulations is currently hampered by a lack of numerical resolution. We present and test a sub-grid recipe to model feedback from massive stars in cosmological smoothed particle hydrodynamics simulations. The energy is distributed in kinetic form among the gas particles surrounding recently formed stars. The impact of the feedback is studied using a suite of high-resolution simulations of isolated disc galaxies embedded in dark halos with total mass 10^{10} and 10^{12} Msol/h. We focus in particular on the effect of pressure forces on wind particles within the disc, which we turn off temporarily in some of our runs to mimic a recipe that has been widely used in the literature. We find that this popular recipe gives dramatically different results because (ram) pressure forces on expanding superbubbles determine both the structure of the disc and the development of large-scale outflows. Pressure forces exerted by expanding superbubbles puff up the disc, giving the dwarf galaxy an irregular morphology and creating a galactic fountain in the massive galaxy. Hydrodynamic drag within the disc results in a strong increase of the effective mass loading of the wind for the dwarf galaxy, but quenches much of the outflow in the case of the high-mass galaxy.

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arXiv2007

On the relation between the Schmidt and Kennicutt-Schmidt star formation laws and its implications for numerical simulations

When averaged over large scales, star formation in galaxies is observed to follow the empirical Kennicutt-Schmidt (KS) law for surface densities above a constant threshold. While the observed law involves surface densities, theoretical models and simulations generally work with volume density laws (i.e. Schmidt laws). We derive analytic relations between star formation laws expressed in terms of surface densities, volume densities, and pressures and we show how these relations depend on parameters such as the effective equation of state of the multiphase interstellar medium. Our analytic relations enable us to implement observed surface density laws into simulations. Because the parameters of our prescription for star formation are observables, we are not free to tune them to match the observations. We test our theoretical framework using high-resolution simulations of isolated disc galaxies that assume an effective equation of state for the multiphase interstellar medium. We are able to reproduce the star formation threshold and both the slope and the normalisation of arbitrary input KS laws without tuning any parameters and with very little scatter, even for unstable galaxies and even if we use poor numerical resolution. Moreover, we can do so for arbitrary effective equations of state. Our prescription therefore enables simulations of galaxies to bypass our current inability to simulate the formation of stars. On the other hand, the fact that we can reproduce arbitrary input thresholds and KS laws, rather than just the particular ones picked out by nature, indicates that simulations that lack the physics and/or resolution to simulate the multiphase interstellar medium can only provide limited insight into the origin of the observed star formation laws.

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arXiv2004

Quenching Cluster Cooling Flows with Recurrent Hot Plasma Bubbles

The observed cooling rate of hot gas in clusters is much lower than that inferred from the gas density profiles. This suggests that the gas is being heated by some source. We use an adaptive-mesh refinement code (FLASH) to simulate the effect of multiple, randomly positioned, injections of thermal energy within 50 kpc of the centre of an initially isothermal cluster with mass M_200=3x10^(14) Msol and kT=3.1 keV. We have performed eight simulations with spherical bubbles of energy generated every 10^8 years, over a total of 1.5 Gyr. Each bubble is created by injecting thermal energy steadily for 10^7 years; the total energy of each bubble ranges from 0.1--3x10^(60) erg, depending on the simulation. We find that 2x10^(60) erg per bubble (corresponding to a average power of 6.3x10^(44) erg/s) effectively balances energy loss in the cluster and prevents the accumulation of gas below kT=1 keV from exceeding the observational limits of 30 Msol/yr. This injection rate is comparable to the radiated luminosity of the cluster, and the required energy and periodic timescale of events are consistent with observations of bubbles produced by central AGN in clusters. The effectiveness of this process depends primarily on the total amount of injected energy and the initial location of the bubbles, but is relatively insensitive to the exact duty cycle of events.

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