arXiv subjects

Freeke van de Voort

Publications and source records attributed to Freeke van de Voort.

67 records · Page 4Linked to original sources

arXiv2016

The Effect of Galactic Feedback on Gas Accretion and Wind Recycling

In the absence of galactic winds, the rate at which gas accretes onto galaxies is determined by the gravitational potential and by radiative cooling. However, outflows driven by supernovae and active galactic nuclei not only eject gas from galaxies, but also prevent gas from accreting in the first place. Furthermore, gas previously ejected from a galaxy can re-accrete onto (the same or a different) galaxy. Because this gas has a high metallicity, its cooling rate is relatively high, which will increase its chances to re-accrete. This complex interplay between gas inflows and outflows is discussed in this chapter. Wind recycling is found to be an important process that fuels galaxies at late times and the recycled gas has different properties than gas accreting for the first time. Quantitative conclusions, however, vary between studies, because the amount of wind recycling is dependent on the details of the feedback model. We discuss these differences, known caveats, and ways to make progress in understanding how galaxies are fed at low redshift.

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arXiv2016

The impact of stellar feedback on hot gas in galaxy haloes: the Sunyaev-Zel'dovich effect and soft X-ray emission

The thermal Sunyaev-Zel'dovich (SZ) effect and soft X-ray emission are routinely observed around massive galaxies and in galaxy groups and clusters. We study these observational diagnostics of galaxy haloes for a suite of cosmological `zoom-in' simulations from the `Feedback In Realistic Environments' project, which spans a large range in halo mass 10^10-10^13 Msun). We explore the effect of stellar feedback on the hot gas observables. The properties of our simulated groups, such as baryon fractions, SZ flux, and X-ray luminosities (L_X), are broadly consistent with existing observations, even though feedback from active galactic nuclei is not included. We make predictions for future observations of lower-mass objects for both SZ and diffuse X-ray measurements, finding that they are not just scaled-down versions of massive galaxies, but more strongly affected by galactic winds driven by star formation. Low-mass haloes (<~10^11 Msun) retain a low fraction of their baryons, which results in a strong suppression of the SZ signal. Our simulations therefore predict a scaling with halo mass that is steeper than self-similar for haloes less massive than 10^13 Msun. For halo masses <~10^12 Msun, L_X is time-variable and correlated primarily with the star formation rate (SFR). For these objects, the diffuse X-ray emission is powered mostly by galactic winds and the gas dominating the X-ray emission is flowing out with radial velocities close to the halo's circular velocity. For halo masses >~10^13 Msun, on the other hand, L_X is much less variable and not correlated with the SFR, because the emission originates from the quasi-hydrostatic, virialized halo gas.

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arXiv2016

Galaxy growth from redshift 5 to 0 at fixed comoving number density

Studying the average properties of galaxies at a fixed comoving number density over a wide redshift range has become a popular observational method, because it may trace the evolution of galaxies statistically. We test this method by comparing the evolution of galaxies at fixed number density and by following individual galaxies through cosmic time (z=0-5) in cosmological, hydrodynamical simulations from OWLS. Comparing progenitors, descendants, and galaxies selected at fixed number density at each redshift, we find differences of up to a factor of three for galaxy and interstellar medium (ISM) masses. The difference is somewhat larger for black hole masses. The scatter in ISM mass increases significantly towards low redshift with all selection techniques. We use the fixed number density technique to study the assembly of dark matter, gas, stars, and black holes and the evolution in accretion and star formation rates. We find three different regimes for massive galaxies, consistent with observations: at high redshift the gas accretion rate dominates, at intermediate redshifts the star formation rate is the highest, and at low redshift galaxies grow mostly through mergers. Quiescent galaxies have much lower ISM masses (by definition) and much higher black hole masses, but the stellar and halo masses are fairly similar. Without active galactic nucleus (AGN) feedback, massive galaxies are dominated by star formation down to z=0 and most of their stellar mass growth occurs in the centre. With AGN feedback, stellar mass is only added to the outskirts of galaxies by mergers and they grow inside-out.

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arXiv2016

Cold-Mode Accretion: Driving the Fundamental Mass-Metallicity Relation at z~2

We investigate the star formation rate (SFR) dependence on the stellar mass and gas-phase metallicity relation at z=2 with MOSFIRE/Keck as part of the ZFIRE survey. We have identified 117 galaxies (1.98 < z < 2.56), with $8.9\leq$log(M/M$_{\odot}$)$\leq11.0$, for which we can measure gas-phase metallicities. For the first time, we show discernible difference between the mass-metallicity relation, using individual galaxies, when deviding the sample by low ($<10$~M$_{\odot}$yr$^{-1}$) and high ($>10$~M$_{\odot}$yr$^{-1}$) SFRs. At fixed mass, low star-forming galaxies tend to have higher metallicity than high star-forming galaxies. Using a few basic assumptions, we further show that the gas masses and metallicities required to produce the fundamental mass--metallicity relation, and its intrinsic scatter, are consistent with cold-mode accretion predictions obtained from the OWLS hydrodynamical simulations. Our results from both simulations and observations are suggestive that cold-mode accretion is responsible for the fundamental mass-metallicity relation at $z=2$ and demonstrates the direct relationship between cosmological accretion and the fundamental properties of galaxies.

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arXiv2015

The creation and persistence of a misaligned gas disc in a simulated early-type galaxy

Massive early-type galaxies commonly have gas discs which are kinematically misaligned with the stellar component. These discs feel a torque from the stars and the angular momentum vectors are expected to align quickly. We present results on the evolution of a misaligned gas disc in a cosmological simulation of a massive early-type galaxy from the Feedback In Realistic Environments project. This galaxy experiences a merger which, together with a strong galactic wind, removes most of the original gas disc. The galaxy subsequently reforms a gas disc through accretion of cold gas, but it is initially 120 degrees misaligned with the stellar rotation axis. This misalignment persists for about 2 Gyr before the gas-star misalignment angle drops below 20 degrees. The time it takes for the gaseous and stellar components to align is much longer than previously thought, because the gas disc is accreting a significant amount of mass for about 1.5 Gyr after the merger, during which the angular momentum change induced by accreted gas dominates over that induced by stellar torques. Once the gas accretion rate has decreased sufficiently, the gas disc decouples from the surrounding halo gas and realigns with the stellar component in about 6 dynamical times. During the late evolution of the misaligned gas disc, the centre aligns faster than the outskirts, resulting in a warped disc. We discuss the observational consequences of the long survival of our misaligned gas disc and how our results can be used to calibrate merger rate estimates from observed gas misalignments.

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arXiv2014

Characterizing simulated galaxy stellar mass histories

Cosmological galaxy formation simulations can now produce rich and diverse ensembles of galaxy histories. These simulated galaxy histories, taken all together, provide an answer to the question "how do galaxies form?" for the models used to construct them. We characterize such galaxy history ensembles both to understand their properties and to identify points of comparison for histories within a given galaxy formation model or between different galaxy formation models and simulations. We focus primarily on stellar mass histories of galaxies with the same final stellar mass, for six final stellar mass values and for three different simulated galaxy formation models (a semi-analytic model built upon the dark matter Millennium simulation and two models from the hydrodynamical OverWhelmingly Large Simulations project). Using principal component analysis (PCA) to classify scatter around the average stellar mass history, we find that one fluctuation dominates for all sets of histories we consider, although its shape and contribution can vary between samples. We correlate the PCA characterization with several $z=0$ galaxy properties (to connect with survey observables) and also compare it to some other galaxy history properties. We then explore separating galaxy stellar mass histories into classes, using the largest PCA contribution, k-means clustering, and simple Gaussian mixture models. For three component models, these different methods often gave similar results. These history classification methods provide a succinct and often quick way to characterize changes in the full ensemble of histories of a simulated population as physical assumptions are varied, to compare histories of different simulated populations to each other, and to assess the relation of simulated histories to fixed time observations.

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arXiv2014

Galactic r-process enrichment by neutron star mergers in cosmological simulations of a Milky Way-mass galaxy

We quantify the stellar abundances of neutron-rich r-process nuclei in cosmological zoom-in simulations of a Milky Way-mass galaxy from the Feedback In Realistic Environments project. The galaxy is enriched with r-process elements by binary neutron star (NS) mergers and with iron and other metals by supernovae. These calculations include key hydrodynamic mixing processes not present in standard semi-analytic chemical evolution models, such as galactic winds and hydrodynamic flows associated with structure formation. We explore a range of models for the rate and delay time of NS mergers, intended to roughly bracket the wide range of models consistent with current observational constraints. We show that NS mergers can produce [r-process/Fe] abundance ratios and scatter that appear reasonably consistent with observational constraints. At low metallicity, [Fe/H]<-2, we predict there is a wide range of stellar r-process abundance ratios, with both supersolar and subsolar abundances. Low-metallicity stars or stars that are outliers in their r-process abundance ratios are, on average, formed at high redshift and located at large galactocentric radius. Because NS mergers are rare, our results are not fully converged with respect to resolution, particularly at low metallicity. However, the uncertain rate and delay time distribution of NS mergers introduces an uncertainty in the r-process abundances comparable to that due to finite numerical resolution. Overall, our results are consistent with NS mergers being the source of most of the r-process nuclei in the Universe.

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arXiv2013

Soft X-ray and ultra-violet metal-line emission from the gas around galaxies

(Abridged) A large fraction of the gas in galactic haloes has temperatures between 10^4.5 and 10^7 K. At these temperatures, cooling is dominated by metal-line emission if the metallicity Z>~0.1 Zsun. We explore the detectability of several lines using large cosmological, hydrodynamical simulations. We stack surface brightness maps centred on galaxies to calculate the expected mean surface brightness profiles for different halo masses. Assuming a detection limit of 10^-1 photon s^-1 cm^-2 sr^-1, proposed X-ray telescopes can detect O VIII emission from z=0.125 out to 80% of the virial radius (Rvir) of groups and clusters and out to 0.4Rvir for haloes with masses Mhalo=10^12-13 Msun. Emission lines from C VI, N VII, O VII, and Ne X can be detected out to smaller radii, 0.1-0.5Rvir. With a detection limit of 10^-20 erg s^-1 cm^-2 arcsec^-2, future UV telescopes can detect C III emission out to 0.2-0.6Rvir at z=0.25. C IV, O VI, Si III, and Si IV can be seen out to 0.1-0.2Rvir for Mhalo>10^12 Msun. Optical HI H-alpha emission is comparable in strength to C III emission. At z=3 it may be possible to observe C III out to 0.2-0.3Rvir and other rest-frame UV lines out to ~0.1Rvir for Mhalo>10^11 Msun with upcoming optical instruments. Metal-line emission is typically biased towards high density and metallicity and towards the temperature at which the emissivity curve of the corresponding metal line peaks. The bias is similar for the different soft X-ray lines considered, whereas it varies strongly between different UV lines. Active galactic nucleus (AGN) feedback can change the inner surface brightness profiles significantly, but it generally does not change the radius out to which the emission can be observed. Metal-line emission is a promising probe of the warm and hot, enriched gas around galaxies and provides a unique window into the interactions between galaxies and their gaseous haloes.

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arXiv2012

Properties of gas in and around galaxy haloes

We study the properties of gas inside and around galaxy haloes as a function of radius and halo mass, focussing mostly on z=2, but also showing some results for z=0. For this purpose, we use a suite of large cosmological, hydrodynamical simulations from the OverWhelmingly Large Simulations project. The properties of cold- and hot-mode gas, which we separate depending on whether the temperature has been higher than 10^5.5 K while it was extragalactic, are clearly distinguishable in the outer parts of massive haloes (virial temperatures >> 10^5 K. The differences between cold- and hot-mode gas resemble those between inflowing and outflowing gas. The cold-mode gas is mostly confined to clumpy filaments that are approximately in pressure equilibrium with the diffuse, hot-mode gas. Besides being colder and denser, cold-mode gas typically has a much lower metallicity and is much more likely to be infalling. However, the spread in the properties of the gas is large, even for a given mode and a fixed radius and halo mass, which makes it impossible to make strong statements about individual gas clouds. Metal-line cooling causes a strong cooling flow near the central galaxy, which makes it hard to distinguish gas accreted through the cold and hot modes in the inner halo. Stronger feedback results in larger outflow velocities and pushes hot-mode gas to larger radii. The gas properties evolve as expected from virial arguments, which can also account for the dependence of many gas properties on halo mass. We argue that cold streams penetrating hot haloes are observable as high-column density HI Lyman-alpha absorption systems in sightlines near massive foreground galaxies.

astro-ph.CO↗

arXiv2012

Cold accretion flows and the nature of high column density HI absorption at redshift 3

Simulations predict that galaxies grow primarily through the accretion of gas that has not gone through an accretion shock near the virial radius and that this cold gas flows towards the central galaxy along dense filaments and streams. There is, however, little observational evidence for the existence of these cold flows. We use a large, cosmological, hydrodynamical simulation that has been post-processed with radiative transfer to study the contribution of cold flows to the observed z=3 column density distribution of neutral hydrogen, which our simulation reproduces. We find that nearly all of the HI absorption arises in gas that has remained colder than 10^5.5 K, at least while it was extragalactic. In addition, the majority of the HI is rapidly falling towards a nearby galaxy, with non-negligible contributions from outflowing and static gas. Above a column density of N_HI = 10^17 cm^-2, most of the absorbers reside inside haloes, but the interstellar medium only dominates for N_HI > 10^21 cm^-2. Haloes with total mass below 10^10 Msun dominate the absorption for 10^17 10^17 cm^-2 are closely related to star formation: most of their HI either will become part of the interstellar medium before z=2 or has been ejected from a galaxy at z>3. Cold accretion flows are critical for the success of our simulation in reproducing the observed rate of incidence of damped Lyman-alpha and particularly that of Lyman limit systems. We therefore conclude that cold accretion flows exist and have already been detected in the form of high column density HI absorbers.

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arXiv2011

The drop in the cosmic star formation rate below redshift 2 is caused by a change in the mode of gas accretion and by AGN feedback

The cosmic star formation rate is observed to drop sharply after redshift z=2. We use a large, cosmological, smoothed particle hydrodynamics simulation to investigate how this decline is related to the evolution of gas accretion and to outflows driven by active galactic nuclei (AGN). We find that the drop in the star formation rate follows a corresponding decline in the global cold-mode accretion rate density onto haloes, but with a delay of order the gas consumption time scale in the interstellar medium. Here we define cold-mode (hot-mode) accretion as gas that is accreted and whose temperature has never exceeded (did exceed) 10^5.5 K. In contrast to cold-mode accretion, which peaks at z~3, the hot mode continues to increase to z~1 and remains roughly constant thereafter. By the present time, the hot mode strongly dominates the global accretion rate onto haloes. Star formation does not track hot-mode halo accretion because most of the hot halo gas never accretes onto galaxies. AGN feedback plays a crucial role by preferentially preventing gas that entered haloes in the hot mode from accreting onto their central galaxies. Consequently, in the absence of AGN feedback, gas accreted in the hot mode would become the dominant source of fuel for star formation and the drop off in the cosmic star formation rate would be much less steep.

astro-ph.CO↗

arXiv2011

The rates and modes of gas accretion on to galaxies and their gaseous haloes

(Abridged) We study the rate at which gas accretes onto galaxies and haloes and investigate whether the accreted gas was shocked to high temperatures before reaching a galaxy. For this purpose we use a suite of large cosmological, hydrodynamical simulations from the OWLS project. We improve on previous work by considering a wider range of halo masses and redshifts, by distinguishing accretion onto haloes and galaxies, by including important feedback processes, and by comparing simulations with different physics. The specific rate of gas accretion onto haloes is, like that for dark matter, only weakly dependent on halo mass. For halo masses Mhalo>>10^11 Msun it is relatively insensitive to feedback processes. In contrast, accretion rates onto galaxies are determined by radiative cooling and by outflows driven by supernovae and active galactic nuclei. Galactic winds increase the halo mass at which the central galaxies grow the fastest by about two orders of magnitude to Mhalo~10^12 Msun. Gas accretion is bimodal, with maximum past temperatures either of order the virial temperature or <~10^5 K. The fraction of gas accreted on to haloes in the hot mode is insensitive to feedback and metal-line cooling. It increases with decreasing redshift, but is mostly determined by halo mass, increasing gradually from less than 10% for ~10^11 Msun to greater than 90% at 10^13 Msun. In contrast, for accretion onto galaxies the cold mode is always significant and the relative contributions of the two accretion modes are more sensitive to feedback and metal-line cooling. The majority of stars present in any mass halo at any redshift were formed from gas accreted in the cold mode, although the hot mode contributes typically over 10% for Mhalo>~10^11 Msun. Galaxies, but not necessarily their gaseous haloes, are predominantly fed by gas that did not experience an accretion shock when it entered the host halo.

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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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