Peter Mitchell

The difference in shape between the observed galaxy stellar mass function and the predicted dark matter halo mass function is generally explained primarily by feedback processes. Feedback can shape the stellar-halo mass (SHM) relation by driving gas out of galaxies, by modulating the first-time infall of gas onto galaxies (i.e., preventative feedback), and by instigating fountain flows of recycled wind material. We present a novel method to disentangle these effects for hydrodynamical… Meer weergeven

The difference in shape between the observed galaxy stellar mass function and the predicted dark matter halo mass function is generally explained primarily by feedback processes. Feedback can shape the stellar-halo mass (SHM) relation by driving gas out of galaxies, by modulating the first-time infall of gas onto galaxies (i.e., preventative feedback), and by instigating fountain flows of recycled wind material. We present a novel method to disentangle these effects for hydrodynamical simulations of galaxy formation. We build a model of linear coupled differential equations that by construction reproduces the flows of gas onto and out of galaxies and haloes in the EAGLE cosmological simulation. By varying individual terms in this model, we isolate the relative effects of star formation, ejection via outflow, first-time inflow and wind recycling on the SHM relation. We find that for halo masses M_200 < 10^12 M⊙ the SHM relation is shaped primarily by a combination of ejection from galaxies and haloes, while for larger M200 preventative feedback is also important. The effects of recycling and the efficiency of star formation are small. We show that if, instead of M_200 , we use the cumulative mass of dark matter that fell in for the first time, the evolution of the SHM relation nearly vanishes. This suggests that the evolution is due to the definition of halo mass rather than to an evolving physical efficiency of galaxy formation. Finally, we demonstrate that the mass in the circum-galactic medium is much more sensitive to gas flows, especially recycling, than is the case for stars and the interstellar medium. Minder weergeven

With the Multi Unit Spectroscopic Explorer (MUSE), it is now possible to detect spatially extended Lyman α (Lyα) emission from individual faint (MUV ∼ -18) galaxies at redshifts, 3 < z < 6, tracing gas out to circumgalactic scales. To explore the implications of such observations, we present a cosmological radiation hydrodynamics simulation of a single galaxy, chosen to be typical of the Lyα-emitting galaxies detected by MUSE in deep fields. We use this simulation to study the origin and… Meer weergeven

With the Multi Unit Spectroscopic Explorer (MUSE), it is now possible to detect spatially extended Lyman α (Lyα) emission from individual faint (MUV ∼ -18) galaxies at redshifts, 3 < z < 6, tracing gas out to circumgalactic scales. To explore the implications of such observations, we present a cosmological radiation hydrodynamics simulation of a single galaxy, chosen to be typical of the Lyα-emitting galaxies detected by MUSE in deep fields. We use this simulation to study the origin and dynamics of the high-redshift circumgalactic medium (CGM). We find that the majority of the mass in the diffuse CGM is comprised of material infalling for the first time towards the halo centre, but with the inner CGM also containing a comparable amount of mass that has moved past first-pericentric passage, and is in the process of settling into a rotationally supported configuration. Making the connection to Lyα emission, we find that the observed extended surface brightness profile is due to a combination of three components: scattering of galactic Lyα emission in the CGM, in situ emission of CGM gas (mostly infalling), and Lyα emission from small satellite galaxies. The weight of these contributions vary with distance from the galaxy such that (1) scattering dominates the inner regions ( r<7kpc ), at surface brightness larger than a few 10-19 cgs, (2) all components contribute equally around r∼10kpc (or SB ∼ 10-19), and (3) the contribution of small satellite galaxies takes over at large distances (or SB ∼ 10-20). Minder weergeven

The role of galactic wind recycling represents one of the largest unknowns in galaxy evolution, as any contribution of recycling to galaxy growth is largely degenerate with the inflow rates of first-time infalling material, and the rates with which outflowing gas and metals are driven from galaxies. We present measurements of the efficiency of wind recycling from the EAGLE cosmological simulation project, leveraging the statistical power of large-volume simulations that reproduce a realistic… Meer weergeven

The role of galactic wind recycling represents one of the largest unknowns in galaxy evolution, as any contribution of recycling to galaxy growth is largely degenerate with the inflow rates of first-time infalling material, and the rates with which outflowing gas and metals are driven from galaxies. We present measurements of the efficiency of wind recycling from the EAGLE cosmological simulation project, leveraging the statistical power of large-volume simulations that reproduce a realistic galaxy population. We study wind recycling at the halo scale, i.e. gas that has been ejected beyond the halo virial radius, and at the galaxy scale, i.e. gas that has been ejected from the interstellar medium to at least ≈10 per cent of the virial radius. Galaxy-scale wind recycling is generally inefficient, with a characteristic return time-scale that is comparable to or longer than a Hubble time, and with an efficiency that clearly peaks at the characteristic halo mass of M200=1012M⊙ . Correspondingly, the majority of gas being accreted on to galaxies in EAGLE is infalling for the first time. Recycling is more efficient at the halo scale, with values that differ by orders of magnitude from those assumed by semi-analytical galaxy formation models. Differences in the efficiency of wind recycling with other hydrodynamical simulations are currently difficult to assess, but are likely smaller. We find that cumulative first-time gas accretion rates at the virial radius are reduced relative to the expectation from dark matter accretion for haloes with mass M200<1012M⊙ , indicating efficient preventative feedback on halo scales. Minder weergeven

We present measurements of galactic outflow rates from the EAGLE suite of cosmological simulations. We find that gas is removed from the interstellar medium (ISM) of central galaxies with a dimensionless mass loading factor that scales approximately with circular velocity as V_c^−3/2 in the low-mass regime where stellar feedback dominates. Feedback from active galactic nuclei causes an upturn in the mass loading for halo masses >1012M⊙ . We find that more gas outflows through the halo… Meer weergeven

We present measurements of galactic outflow rates from the EAGLE suite of cosmological simulations. We find that gas is removed from the interstellar medium (ISM) of central galaxies with a dimensionless mass loading factor that scales approximately with circular velocity as V_c^−3/2 in the low-mass regime where stellar feedback dominates. Feedback from active galactic nuclei causes an upturn in the mass loading for halo masses >1012M⊙ . We find that more gas outflows through the halo virial radius than is removed from the ISM of galaxies, particularly at low redshifts, implying substantial mass loading within the circum-galactic medium. Outflow velocities span a wide range at a given halo mass/redshift, and on average increase positively with redshift and halo mass up to M_200 ∼ 10^12 M⊙ . Outflows exhibit a bimodal flow pattern on circum-galactic scales, aligned with the galactic minor axis. We present a number of like-for-like comparisons to outflow rates from other recent cosmological hydrodynamical simulations, and show that comparing the propagation of galactic winds as a function of radius reveals substantial discrepancies between different models. Relative to some other simulations, EAGLE favours a scenario for stellar feedback where agreement with the galaxy stellar mass function is achieved by removing smaller amounts of gas from the ISM, but with galactic winds that then propagate and entrain ambient gas out to larger radii. Minder weergeven

We analyse the properties of circum-galactic gas around simulated galaxies in the redshift range z ≥ 3, utilising a new sample of cosmological zoom simulations. These simulations are intended to be representative of the observed samples of Lyman α (Ly α) emitters recently obtained with the multi unit spectroscopic explorer (MUSE) instrument (halo masses ∼10^10 - 10^11 M⊙). We show that supernova feedback has a significant impact on both the inflowing and outflowing circum-galactic medium (CGM)… Meer weergeven

We analyse the properties of circum-galactic gas around simulated galaxies in the redshift range z ≥ 3, utilising a new sample of cosmological zoom simulations. These simulations are intended to be representative of the observed samples of Lyman α (Ly α) emitters recently obtained with the multi unit spectroscopic explorer (MUSE) instrument (halo masses ∼10^10 - 10^11 M⊙). We show that supernova feedback has a significant impact on both the inflowing and outflowing circum-galactic medium (CGM) by driving outflows, reducing diffuse inflow rates, and by increasing the neutral fraction of inflowing gas. By temporally stacking simulation outputs, we find that significant net mass exchange occurs between inflowing and outflowing phases: none of the phases are mass-conserving. In particular, we find that the mass in neutral outflowing hydrogen declines exponentially with radius as gas flows outwards from the halo centre. This is likely caused by a combination of both fountain-like cycling processes and gradual photoionization/collisional ionization of outflowing gas. Our simulations do not predict the presence of fast-moving neutral outflows in the CGM. Neutral outflows instead move with modest radial velocities (∼50 km s^-1), and the majority of the kinetic energy is associated with tangential rather than radial motion. Minder weergeven

It is now possible for hydrodynamical simulations to reproduce a representative galaxy population. Accordingly, it is timely to assess critically some of the assumptions of traditional semi-analytic galaxy formation models. We use the Eagle simulations to assess assumptions built into the Galform semi-analytic model, focussing on those relating to baryon cycling, angular momentum and feedback. We show that the assumption in Galform that newly formed stars have the same specific angular momentum… Meer weergeven

It is now possible for hydrodynamical simulations to reproduce a representative galaxy population. Accordingly, it is timely to assess critically some of the assumptions of traditional semi-analytic galaxy formation models. We use the Eagle simulations to assess assumptions built into the Galform semi-analytic model, focussing on those relating to baryon cycling, angular momentum and feedback. We show that the assumption in Galform that newly formed stars have the same specific angular momentum as the total disc leads to a significant overestimate of the total stellar specific angular momentum of disc galaxies. In Eagle, stars form preferentially out of low specific angular momentum gas in the interstellar medium (ISM) due to the assumed gas density threshold for stars to form, leading to more realistic galaxy sizes. We find that stellar mass assembly is similar between Galform and Eagle but that the evolution of gas properties is different, with various indications that the rate of baryon cycling in Eagle is slower than is assumed in Galform. Finally, by matching individual galaxies between Eagle and Galform, we find that an artificial dependence of AGN feedback and gas infall rates on halo mass doubling events in Galform drives most of the scatter in stellar mass between individual objects. Put together our results suggest that the Galform semi-analytic model can be significantly improved in light of recent advances. Minder weergeven

We present an analysis of the predictions made by the GALFORM semi-analytic galaxy formation model for the evolution of the relationship between stellar mass and halo mass. We show that for the standard implementations of supernova feedback and gas reincorporation used in semi-analytic models, this relationship is predicted to evolve weakly over the redshift range 0 < z < 4. Modest evolution in the median stellar mass versus halo mass (SHM) relationship implicitly requires that, at fixed… Meer weergeven

We present an analysis of the predictions made by the GALFORM semi-analytic galaxy formation model for the evolution of the relationship between stellar mass and halo mass. We show that for the standard implementations of supernova feedback and gas reincorporation used in semi-analytic models, this relationship is predicted to evolve weakly over the redshift range 0 < z < 4. Modest evolution in the median stellar mass versus halo mass (SHM) relationship implicitly requires that, at fixed halo mass, the efficiency of stellar mass assembly must be almost constant with cosmic time. We show that in our model, this behaviour can be understood in simple terms as a result of a constant efficiency of gas reincorporation, and an efficiency of SNe feedback that is, on average, constant at fixed halo mass. We present a simple explanation of how feedback from active galactic nuclei (AGN) acts in our model to introduce a break in the SHM relation whose location is predicted to evolve only modestly. Finally, we show that if modifications are introduced into the model such that, for example, the gas reincorporation efficiency is no longer constant, the median SHM relation is predicted to evolve significantly over 0 < z < 4. Specifically, we consider modifications that allow the model to better reproduce either the evolution of the stellar mass function or the evolution of average star formation rates inferred from observations. Minder weergeven

It has been argued that the specific star formation rates of star-forming galaxies inferred from observational data decline more rapidly below z = 2 than is predicted by hierarchical galaxy formation models. We present a detailed analysis of this problem by comparing predictions from the GALFORM semi-analytic model with an extensive compilation of data on the average star formation rates of star-forming galaxies. We also use this data to infer the form of the stellar mass assembly histories of… Meer weergeven

It has been argued that the specific star formation rates of star-forming galaxies inferred from observational data decline more rapidly below z = 2 than is predicted by hierarchical galaxy formation models. We present a detailed analysis of this problem by comparing predictions from the GALFORM semi-analytic model with an extensive compilation of data on the average star formation rates of star-forming galaxies. We also use this data to infer the form of the stellar mass assembly histories of star-forming galaxies. Our analysis reveals that the currently available data favour a scenario where the stellar mass assembly histories of star-forming galaxies rise at early times and then fall towards the present day. In contrast, our model predicts stellar mass assembly histories that are almost flat below z = 2 for star-forming galaxies, such that the predicted star formation rates can be offset with respect to the observational data by factors of up to 2-3. This disagreement can be explained by the level of coevolution between stellar and halo mass assembly that exists in contemporary galaxy formation models. In turn, this arises because the standard implementations of star formation and supernova feedback used in the models result in the efficiencies of these process remaining approximately constant over the lifetime of a given star-forming galaxy. We demonstrate how a modification to the time-scale for gas ejected by feedback to be reincorporated into galaxy haloes can help to reconcile the model predictions with the data. Minder weergeven

The estimated stellar masses of galaxies are widely used to characterise how the galaxy population evolves over cosmic time. If stellar masses can be estimated in a robust manner, free from any bias, global diagnostics such as the stellar mass function can be used to constrain the physics of galaxy formation. We explore how galaxy stellar masses, estimated by fitting broad-band spectral energy distributions (SEDs) with stellar population models, can be biased as a result of commonly adopted… Meer weergeven

The estimated stellar masses of galaxies are widely used to characterise how the galaxy population evolves over cosmic time. If stellar masses can be estimated in a robust manner, free from any bias, global diagnostics such as the stellar mass function can be used to constrain the physics of galaxy formation. We explore how galaxy stellar masses, estimated by fitting broad-band spectral energy distributions (SEDs) with stellar population models, can be biased as a result of commonly adopted assumptions for the star formation and chemical enrichment histories, recycled fractions and dust attenuation curves of galaxies. We apply the observational technique of broad-band SED fitting to model galaxy SEDs calculated by the theoretical galaxy formation model GALFORM, isolating the effect of each of these assumptions. We find that, averaged over the entire galaxy population, the common assumption of exponentially declining star formation histories does not, by itself, adversely affect stellar mass estimation. However, we also show that this result does not hold when considering galaxies that have undergone a recent burst of star formation. We show that fixing the metallicity in SED fitting or using sparsely sampled metallicity grids can introduce mass-dependent systematics into stellar mass estimates. We find that the common assumption of a star-dust geometry corresponding to a uniform foreground dust screen can cause the stellar masses of dusty model galaxies to be significantly underestimated. Finally, we show that stellar mass functions recovered by applying SED fitting to model galaxies at high redshift can differ significantly in both shape and normalisation from the intrinsic mass functions predicted by a given model. Given these differences, our methodology of using stellar masses estimated from model galaxy SEDs offers a new, self-consistent way to compare model predictions with observations. Minder weergeven