ABSTRACT
We examine the rotation rates, sizes and star formation (SF) efficiencies of a representative population of simulated disc galaxies extracted from the Galaxies–Intergalactic Medium ...Interaction Calculation (GIMIC) suite of cosmological hydrodynamic simulations. These simulations include efficient, but energetically feasible supernova feedback, but have not been tuned in any way to produce ‘realistic’ disc galaxies. Yet, they generate a large number of discs, without requiring extremely high resolution. Over the wide galaxy stellar mass range, 9.0 ≲ log 10 M*(M⊙) < 10.5, the simulations reproduce the observed Tully–Fisher relation, the rotation curves of disc galaxies in bins of stellar mass, the mass–size relation of disc galaxies, the optical rotation to virial circular velocity ratio (‘Vopt/Vvir’) and the SF efficiencies of disc galaxies as inferred from stacked weak lensing and stacked satellite kinematics observations. They also reproduce the specific star formation rates of ∼L* galaxies but predict too low levels of SF for low‐mass galaxies, which is plausibly due to the finite resolution of the simulations. At higher stellar masses, log10M*(M⊙) > 10.6, the simulated galaxies are too concentrated and have too high SF efficiencies. We conjecture that this shortcoming reflects the neglect of feedback from accreting supermassive black holes in these simulations. We conclude that it is possible to generate a representative population of disc galaxies that reproduces many of the observed trends of local disc galaxies using standard numerical hydrodynamic techniques and a plausible implementation of the ‘subgrid’ astrophysical processes thought to be relevant to galaxy formation.
We use the Galaxies-Intergalactic Medium Interaction Calculation (gimic) cosmological hydrodynamic simulation at z = 0 to study the distribution and environmental dependence of neutral hydrogen (H i) ...gas in the outskirts of simulated galaxies. This gas can currently be probed directly in, for example, Lyα absorption via the observation of background quasars. Radio facilities, such as the Square Kilometre Array, will provide a complementary probe of the diffuse H i in emission and will constrain the physics underpinning the complex interplay between accretion and feedback mechanisms which affect the intergalactic medium. We extract a sample of 488 galaxies from a resimulation of the average cosmic density gimic region. We estimate the neutral hydrogen content of these galaxies and the surrounding intergalactic medium within which they reside. We investigate the average H i radial profiles by stacking the individual profiles according to both mass and environment. We find high H i column densities at large impact parameters in group environments and markedly lower H i densities for non-group galaxies. We suggest that these results likely arise from the combined effects of ram pressure stripping and tidal interactions present in group environments.
Abstract
This work presents a study of galactic outflows driven by stellar feedback. We extract main-sequence disc galaxies with stellar mass 109 ≤ M⋆/ M⊙ ≤ 5.7 × 1010 at redshift z = 0 from the ...highest resolution cosmological simulation of the Evolution and Assembly of GaLaxies and their Environments (EAGLE) set. Synthetic gas rotation velocity and velocity dispersion (σ) maps are created and compared to observations of disc galaxies obtained with the Sydney-AAO (Australian Astronomical Observatory) Multi-object Integral field spectrograph (SAMI), where σ-values greater than 150 km s−1 are most naturally explained by bipolar outflows powered by starburst activity. We find that the extension of the simulated edge-on (pixelated) velocity dispersion probability distribution depends on stellar mass and star formation rate surface density (ΣSFR), with low-M⋆/low-ΣSFR galaxies showing a narrow peak at low σ (∼30 km s−1) and more active, high-M⋆/high-ΣSFR galaxies reaching σ > 150 km s−1. Although supernova-driven galactic winds in the EAGLE simulations may not entrain enough gas with T <105 K compared to observed galaxies, we find that gas temperature is a good proxy for the presence of outflows. There is a direct correlation between the thermal state of the gas and its state of motion as described by the σ-distribution. The following equivalence relations hold in EAGLE: (i) low-σ peak ⇔ disc of the galaxy ⇔ gas with T <105 K; (ii)
high-σ tail ⇔ galactic winds ⇔ gas with T ≥105 K.
The growth of the angular momentum L of protogalaxies induced by tidal torques is reconsidered. We adopt White's formalism and study the evolution of L in Lagrangian coordinates; the motion of the ...fluid elements is described by the Zel'dovich approximation. We obtain a general expression for the ensemble expectation value of the square of L in terms of the first and second invariant of the inertia tensor of the Lagrangian volume Γ enclosing the collapsing mass of the proto-object. We then specialize the formalism to the particular case in which Γ is centred on a peak of the smoothed Gaussian density field and approximated by an isodensity ellipsoid. The result is the appropriate analytical estimate for the rms angular momentum of peaks to be compared against simulations that make use of the Hoffman-Ribak algorithm to set up a constrained density field that contains a peak with given shape. Extending the work of Heavens & Peacock, we calculate the joint probability distribution function for several spin parameters and peak mass M using the distribution of peak shapes, for different initial power spectra. The probability distribution for the rms final angular momentum 〈Lf2〉1/2 on the scales corresponding to common bright galaxies, M≈ l011 M⊙, is centred on a value of ≈ 1067 kg m2 s−1, for any cosmologically relevant power spectrum, in line with previous theoretical and observational estimates for Lf. Other astrophysical consequences are discussed. In particular, we find that typical values 〈 λ2 〉1/2 ≈ 0.1 of the dimensionless spin parameter for peaks smoothed on galactic scales and of height v ∼ 1, usually associated with late-type galaxies, may be recovered in the framework of the Gaussian peak formalism. This partially relaxes the importance attributed to dissipative processes in generating such high values of centrifugal support for spiral galaxies. In addition, the values of the specific angular momentum versus mass — as deduced from observations of rotational velocities and photometric radii of spiral galaxies — are well fitted by our theoretical isoprobability contours. In contrast, the observed lower values for the specific angular momentum for ellipticals of the same mass cannot be accounted for within our linear-regime investigation, highlighting the importance of strongly non-linear phenomena to explain the spin of such objects.
We have measured redshifts for 243 z≈ 3 quasars in nine Very Large Telescope (VLT) Visible Imaging and Multi-Object Spectrograph (VIMOS) Lyman-break galaxy (LBG) redshift survey areas, each of which ...is centred on a known bright quasar. Using the spectra of these quasars, we measure the cross-correlation between neutral hydrogen gas causing the Lyα forest and 1020 LBGs at z≈ 3. We find an increase in neutral hydrogen absorption within ≈5 h
−1 Mpc of a galaxy in agreement with the results of Adelberger et al. The Lyα-LBG cross-correlation can be described by a power law on scales larger than 3 h
−1 Mpc. When galaxy velocity dispersions are taken into account, our results at smaller scales (<2 h
−1 Mpc) are also in good agreement with the results of Adelberger et al. There is little immediate indication of a region with a transmission spike above the mean intergalactic medium value which might indicate the presence of star formation feedback. To measure the galaxy velocity dispersions, which include both intrinsic LBG velocity dispersion and redshift errors, we have used the LBG-LBG redshift-space distortion measurements of Bielby et al. We find that the redshift-space transmission spike implied in the results of Adelberger et al. is too narrow to be physical in the presence of the likely LBG velocity dispersion and is likely to be a statistical fluke. Nevertheless, neither our nor previous data can rule out the presence of a narrow, real-space transmission spike, given the evidence of the increased Lyα absorption surrounding LBGs which can mask the spike's presence when convolved with a realistic LBG velocity dispersion. Finally, we identify 176 C iv systems in the quasar spectra and find an LBG-C iv correlation strength on scales of 10 h
−1 Mpc consistent with the relation measured at ≈Mpc scales.
We present the initial imaging and spectroscopic data acquired as part of the Very Large Telescope (VLT) VIMOS Lyman-break galaxy Survey. UBR (or UBVI) imaging covers five ≈36 × 36 arcmin2 fields ...centred on bright z > 3 quasi-stellar objects (QSOs), allowing ≈21 000 2 < z < 3.5 galaxy candidates to be selected using the Lyman-break technique. We performed spectroscopic follow-up using VLT VIMOS, measuring redshifts for 1020 z > 2 Lyman-break galaxies and 10 z > 2 QSOs from a total of 19 VIMOS pointings. From the galaxy spectra, we observe a 625 ± 510 km s−1 velocity offset between the interstellar absorption and Lyman α emission-line redshifts, consistent with previous results. Using the photometric and spectroscopic catalogues, we have analysed the galaxy clustering at z≈ 3. The angular correlation function, w(θ), is well fitted by a double power law with clustering scalelength, r
0= 3.19+0.32
−0.54
h
−1 Mpc and slope γ= 2.45 for r < 1 h
−1 Mpc and r
0= 4.37+0.43
−0.55
h
−1 Mpc with γ= 1.61 ± 0.15 at larger scales. Using the redshift sample we estimate the semiprojected correlation function, w
p(σ), and, for a γ= 1.8 power law, find r
0= 3.67+0.23
−0.24
h
−1 Mpc for the VLT sample and r
0= 3.98+0.14
−0.15
h
−1 Mpc for a combined VLT+Keck sample. From ξ(s) and ξ(σ, π), and assuming the above ξ(r) models, we find that the combined VLT and Keck surveys require a galaxy pairwise velocity dispersion of ≈700 km s−1, higher than ≈400 km s−1 assumed by previous authors. We also measure a value for the gravitational growth rate parameter of β(z= 3) = 0.48 ± 0.17, again higher than that previously found and implying a low value for the bias of b= 2.06+1.1
−0.5. This value is consistent with the galaxy clustering amplitude which gives b= 2.22 ± 0.16, assuming the standard cosmology, implying that the evolution of the gravitational growth rate is also consistent with Einstein gravity. Finally, we have compared our Lyman-break galaxy clustering amplitudes with lower redshift measurements and find that the clustering strength is not inconsistent with that of low-redshift L* spirals for simple 'long-lived' galaxy models.
The first generation of star-forming haloes Reed, Darren S.; Bower, Richard; Frenk, Carlos S. ...
Monthly notices of the Royal Astronomical Society,
10/2005, Letnik:
363, Številka:
2
Journal Article
Recenzirano
Odprti dostop
We model gas cooling in high-resolution N-body simulations in order to investigate the formation of the first generation of stars. We follow a region of a Lambda cold dark matter (ΛCDM) universe ...especially selected to contain a rich cluster by the present day. The properties of the dark haloes that form in these subsolar mass-resolution simulations are presented in a companion paper by Gao et al. The first gas clouds able to cool by molecular hydrogen (H2)-line emission collapse at extremely high redshift, z ≈ 47, when the mass of the dark halo is 2.4 × 105 h−1 M⊙. By z ≈ 30, a substantial population of haloes are capable of undergoing molecular hydrogen cooling although their ability to form stars is dependent on the efficiency of feedback processes such as dissociating Lyman—Werner radiation. The mass of the main halo grows extremely rapidly and, by z ≈ 36, its virial temperature has reached 104 K, at which point gas cooling becomes dominated by more effective atomic processes. By z ≈ 30, a small ‘group’ of such potential galaxies will have formed unless prevented from doing so by feedback processes. By this redshift, massive (≳100 M⊙) Population III stars are able to ionize gas well beyond their own host halo and neighbouring H ii regions can percolate to form an ionized superbubble. Such patches would be too widely separated to contribute significantly to reionization at this time. The large number density of early cooling haloes in the pre-reionized universe raises the exciting prospect that this ultra-early generation of stars may be observable as gamma-ray bursts or supernovae.
The thermal history of the intracluster medium (ICM) is complex. Heat input from cluster mergers, from active galactic nuclei (AGN) and from winds in galaxies offsets and may even prevent the cooling ...of the ICM. Consequently, the processes that set the temperature and density structure of the ICM play a key role in determining how galaxies form. In this paper, we focus on the heating of the ICM during cluster mergers, with the eventual aim of incorporating this mechanism into semi-analytic models for galaxy formation.
We generate and examine a suite of non-radiative hydrodynamic simulations of mergers in which the initial temperature and density structure of the systems are set using realistic scaling laws. Our collisions cover a range of mass ratios and impact parameters, and consider both systems composed entirely of gas (these reduce the physical processes involved), and systems comprising a realistic mixture of gas and dark matter. We find that the heating of the ICM can be understood relatively simply by considering evolution of the gas entropy during the mergers. The increase in this quantity in our simulations closely corresponds to that predicted from scaling relations based on the increase in cluster mass.
We examine the physical processes that succeed in generating the entropy in order to understand why previous analytical approaches failed. We find the following. (i) The energy that is thermalized during the collision greatly exceeds the kinetic energy available when the systems first touch. The smaller system penetrates deep into the gravitational potential before it is disrupted. (ii) For systems with a large mass ratio, most of the energy is thermalized in the massive component. The heating of the smaller system is minor and its gas sinks to the centre of the final system. This contrasts with spherically symmetric analytical models in which accreted material is simply added to the outer radius of the system. (iii) The bulk of the entropy generation occurs in two distinct episodes. The first episode occurs following the collision of the cores, when a large shock wave is generated that propagates outwards from the centre. This causes the combined system to expand rapidly and overshoot hydrostatic equilibrium. The second entropy generation episode occurs as this material is shock heated as it recollapses. Both heating processes play an important role, contributing approximately equally to the final entropy. This revised model for entropy generation improves our physical understanding of cosmological gas simulations.
Numerical simulations indicate that the smooth, photoionized intergalactic medium (IGM) responsible for the low column density Lyα forest follows a well-defined temperature-density relation, which is ...well described by a power law . We demonstrate that such an equation of state results in a power-law cut-off in the distribution of linewidths (b-parameters) as a function of column density (N) for the low column density (N≲1014.5 cm−2) absorption lines. This explains the existence of the lower envelope that is clearly seen in scatter plots of the b(N) distribution in observed QSO spectra. Even a strict power-law equation of state will not result in an absolute cut-off because of line blending and contamination by unidentified metal lines. We develop an algorithm to determine the cut-off, which is insensitive to these narrow lines. We show that the parameters of the cut-off in the b(N) distribution are strongly correlated with the parameters of the underlying equation of state. We use simulations to determine these relations, which can then be applied to the observed cut-off in the b(N) distribution to measure the equation of state of the IGM. We show that systematics that change the b(N) distribution, such as cosmology (for a fixed equation of state), peculiar velocities, the intensity of the ionizing background radiation and variations in the signal-to-noise ratio, do not affect the measured cut-off. We argue that physical processes that have not been incorporated in the simulations, e.g. feedback from star formation, are unlikely to affect the results. Using Monte Carlo simulations of Keck spectra at z = 3, we show that determining the slope of the equation of state will be difficult, but that the amplitude can be determined to within 10 per cent, even from a single QSO spectrum. Measuring the evolution of the equation of state with redshift will allow us to put tight constraints on the reionization history of the Universe.
Is criminal disenfranchisement compatible with a democratic political order? This article considers this question in light of a recently developed view that criminal disenfranchisement is justified ...because it expresses our commitment to democratic values. We call this view expressive disenfranchisement and refer to the general conception in which it is grounded as democratic expressivism. Contra supporters of expressive disenfranchisement, we argue that democratic expressivism does not offer a sound justification of criminal disenfranchisement. Additionally, we argue that, insofar as one really cares about answering serious criminal wrongs via an expression of democratic values, criminal disenfranchisement should be abandoned and replaced with a policy that temporarily obliges the relevant criminals to vote. Democratic expressivists should, in other words, move from supporting the disenfranchisement of serious offenders to endorsing a policy of compulsory criminal voting for a finite period of time.
PDF
