Ram pressure stripping of spiral galaxies in clusters

We use three-dimensional SPH/ N -body simulations to study ram pressure stripping of gas from spiral galaxies orbiting in clusters. We find that the analytic expectation of Gunn & Gott, relating the gravitational restoring force provided by the disc to the ram pressure force, provides a good approximation to the radius at which gas will be stripped from a galaxy. However, at small radii it is also important to consider the potential provided by the bulge component. A spiral galaxy passing through the core of a rich cluster, such as Coma, will have its gaseous disc truncated to ∼4 kpc, thus losing ∼80 per cent of its diffuse gas mass. The time-scale for this to occur is a fraction of a crossing time ∼10⁷ yr. Galaxies orbiting within poorer clusters, or inclined to the direction of motion through the intracluster medium, will lose significantly less gas. We conclude that ram pressure alone is insufficient to account for the rapid and widespread truncation of star formation observed in cluster galaxies, or the morphological transformation of Sabs to S0s that is necessary to explain the Butcher–Oemler effect.     

Ram pressure stripping the hot gaseous haloes of galaxies in groups and clusters

We use a large suite of carefully controlled full hydrodynamic simulations to study the ram pressure stripping of the hot gaseous haloes of galaxies as they fall into massive groups and clusters. The sensitivity of the results to the orbit, total galaxy mass, and galaxy structural properties is explored. For typical structural and orbital parameters, we find that ∼30 per cent of the initial hot galactic halo gas can remain in place after 10 Gyr. We propose a physically simple analytic model that describes the stripping seen in the simulations remarkably well. The model is analogous to the original formulation of Gunn & Gott, except that it is appropriate for the case of a spherical (hot) gas distribution (as opposed to a face-on cold disc) and takes into account that stripping is not instantaneous but occurs on a characteristic time-scale. The model reproduces the results of the simulations to within ≈10 per cent at almost all times for all the orbits, mass ratios, and galaxy structural properties we have explored. The one exception involves unlikely systems where the orbit of the galaxy is highly non-radial and its mass exceeds about 10 per cent of the group or cluster into which it is falling (in which case the model underpredicts the stripping following pericentric passage). The proposed model has several interesting applications, including modelling the ram pressure stripping of both observed and cosmologically simulated galaxies and as a way to improve present semi-analytic models of galaxy formation. One immediate consequence is that the colours and morphologies of satellite galaxies in groups and clusters will differ significantly from those predicted with the standard assumption of complete stripping of the hot coronae.     

Ram-pressure stripping of disc galaxies orbiting in clusters – II. Galactic wakes

We present 3D hydrodynamical simulations of ram-pressure stripping of a disc galaxy orbiting in a galaxy cluster. In this paper, we focus on the properties of the galaxies' tails of stripped gas. The galactic wakes show a flaring width, where the flaring angle depends on the gas disc's cross-section with respect to the galaxy's direction of motion. The velocity in the wakes shows a significant turbulent component of a few     . The stripped gas is deposited in the cluster rather locally, i.e. within     from where it was stripped. We demonstrate that the most important quantity governing the tail density, length and gas mass distribution along the orbit is the galaxy's mass-loss per orbital length. This in turn depends on the ram pressure as well as the galaxy's orbital velocity.
For a sensitivity limit of     in projected gas density, we find typical tail lengths of     . Such long tails are seen even at large distances (0.5 to     ) from the cluster centre. At this sensitivity limit, the tails show little flaring, but a width similar to the gas disc's size.
Morphologically, we find good agreement with the H  i tails observed in the Virgo cluster by Chung et al. 2007 . However, the observed tails show a much smaller velocity width than predicted from the simulation. The few known X-ray and Hα tails are generally much narrower and much straighter than the tails in our simulations. Thus, additional physics like a viscous intracluster medium (ICM), the influence of cooling and tidal effects may be needed to explain the details of the observations.
We discuss the hydrodynamical drag as a heat source for the ICM but conclude that it is not likely to play an important role, especially not in stopping cooling flows.     

Origin of disc lopsidedness in the Eridanus group of galaxies

The H  i surface density maps for a sample of 18 galaxies in the Eridanus group are Fourier analysed. This analysis gives the radial variation of the lopsidedness in the H  i spatial distribution. The lopsidedness is quantified by the Fourier amplitude A ₁ of the m = 1 component normalized to the average value. It is also shown that in the radial region where the stellar disc and H  i overlap, their A ₁ coefficients are comparable. All the galaxies studied show significant lopsidedness in H  i . The mean value of A ₁ in the inner regions of the galaxies (1.5–2.5 scalelengths) is ≥ 0.2. This value of A ₁ is twice the average value seen in the field galaxies. Also, the lopsidedness is found to be smaller for late-type galaxies; this is opposite to the trend seen in the field galaxies. These two results indicate a different physical origin for disc lopsidedness in galaxies in a group environment compared to the field galaxies. Further, a large fraction (∼30 per cent) shows a higher degree of lopsidedness ( A ₁≥ 0.3). It is also seen that the disc lopsidedness increases with the radius as demonstrated in earlier studies, but over a radial range that is two times larger than done in the previous studies. The average lopsidedness of the halo potential is estimated to be ∼10 per cent, assuming that the lopsidedness in H  i disc is due to its response to the halo asymmetry.     

The star formation rate in disc galaxies: thresholds and dependence on gas amount

We reassess the applicability of the Toomre criterion in galactic discs and we study the local star formation law in 16 disc galaxies for which abundance gradients are published. The data we use consist of stellar light profiles, atomic and molecular gas (deduced from CO with a metallicity-dependent conversion factor), star formation rates (from Hα emissivities), metallicities, dispersion velocities and rotation curves. We show that the Toomre criterion applies successfully to the case of the Milky Way disc, but it has limited success with the data of our sample; depending on whether or not the stellar component is included in the stability analysis, we find average values for the threshold ratio of the gas surface density to the critical surface density in the range 0.5–0.7. We also test various star formation laws proposed in the literature, i.e. either the simple Schmidt law or modifications of it, that take into account dynamical factors. We find only small differences among them as far as the overall fit to our data is concerned; in particular, we find that all three star formation laws (with parameters derived from the fits to our data) match observations in the Milky Way disc particularly well. In all cases we find that the exponent n of our best-fitting star formation rate has slightly higher values than in other recent works and we suggest several reasons that may cause that discrepancy.     

Chemo-spectrophotometric evolution of spiral galaxies – II. Main properties of present-day disc galaxies

We study the chemical and spectrophotometric evolution of galactic discs with detailed models calibrated on the Milky Way and using simple scaling relations, based on currently popular semi-analytic models of galaxy formation. We compare our results with a large body of observational data on present-day galactic discs, including disc sizes and central surface brightness, Tully–Fisher relations in various wavelength bands, colour–colour and colour–magnitude relations, gas fractions versus magnitudes and colours and abundances versus local and integrated properties, as well as spectra for different galactic rotational velocities. Despite the extremely simple nature of our models, we find satisfactory agreement with all those observables, provided that the time-scale for star formation in low-mass discs is longer than for more massive ones. This assumption is apparently in contradiction with the standard picture of hierarchical cosmology. We find, however, that it is extremely successful in reproducing major features of present-day discs, like the change in the slope of the Tully–Fisher relation with wavelength, the fact that more massive galaxies are on average 'redder' than low-mass ones (a generic problem of standard hierarchical models) and the metallicity–luminosity relation for spirals. It is concluded that, on a purely empirical basis, this new picture is at least as successful as the standard one. Observations at high redshifts could help to distinguish between the two possibilities.     

Wakes of ram-pressure-stripped disc galaxies

We present a sample of 30 wide-angle tailed radio galaxies (WATs) that we use to constrain the jet speeds in these sources. We measure the distribution of the jet-sidedness ratios for the sample, and assuming that the jets are beamed, the jet speeds in the range (0.3–0.7) c are obtained. Whilst the core prominence of the sample, which ought to be a reliable indicator of beaming, shows little correlation with the jet sidedness, we argue that due to the peculiar nature of the WATs, core prominence is unlikely to be a good indicator of beaming in these sources. We further show that if the jets are fast and light, then the galaxy speeds required to bend the jets into C-shapes such as those seen in 0647+693 are reasonable for a galaxy in a merging or recently merged cluster.     

Galaxy evolution in Hickson compact groups: the role of ram-pressure stripping and strangulation

Galaxies in compact groups tend to be deficient in neutral hydrogen compared to isolated galaxies of similar optical properties. In order to investigate the role played by a hot intragroup medium (IGM) for the removal and destruction of H  i in these systems, we have performed a Chandra and XMM–Newton study of eight of the most H  i deficient Hickson compact groups. Diffuse X-ray emission associated with an IGM is detected in four of the groups, suggesting that galaxy–IGM interactions are not the dominant mechanism driving cold gas out of the group members. No clear evidence is seen for any of the members being currently stripped of any hot gas, nor for galaxies to show enhanced nuclear X-ray activity in the X-ray bright or most H  i deficient groups. Combining the inferred IGM distributions with analytical models of representative disc galaxies orbiting within each group, we estimate the H  i mass-loss due to ram-pressure and viscous stripping. While these processes are generally insufficient to explain observed H  i deficiencies, they could still be important for H  i removal in the X-ray bright groups, potentially removing more than half of the interstellar medium in the X-ray bright HCG 97. Ram pressure may also have facilitated strangulation through the removal of galactic coronal gas. In X-ray undetected groups, tidal interactions could be playing a prominent role, but it remains an open question whether they can fully account for the observed H  i deficiencies.     

A study of the kinematics of unusually H I‐rich galaxies

We examine the H I kinematics of the “Bluedisk” ensemble of 48 galaxies selected from the Sloan Digital Sky Survey and observed in H I with the Westerbork Synthesis Radio Telescope. The sample consists of 25 galaxies with a high H I mass fraction and a comparatively large control sample comprising 23 galaxies of comparable stellar mass, stellar mass surface density, redshift, and inclination. By studying the H I velocity fields of these galaxies, we investigate whether there are signatures of ongoing gas accretion: i.e. global asymmetries and indications for warping and kinematical lopsidedness. We find no enhanced kinematical asymmetries between the H I‐rich sample and the control sample galaxies, indicating no significant difference in kinematical signatures such as warping and lopsidedness. Furthermore, we find no difference in position angle and systemic velocity offset with respect to the optical between both sub‐samples. We therefore do not find compelling evidence for enhanced global asymmetry of the H I‐excess galaxies ensemble properties in comparison to the control sample galaxies. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Richness dependence of the recent evolution of clusters of galaxies

We revisit the issue of the recent dynamical evolution of clusters of galaxies using a sample of Abell, Corwin & Olowin (ACO) clusters with   z < 0.14  , which has been selected such that it does not contain clusters with multiple velocity components nor strongly merging or interacting clusters, as revealed in X-rays. We use as proxies of the cluster dynamical state the projected cluster ellipticity, velocity dispersion and X-ray luminosity. We find indications for a recent dynamical evolution of this cluster population, which however strongly depends on the cluster richness. Poor clusters appear to be undergoing their primary phase of virialization, with their ellipticity increasing with redshift with a rate  dε/d z ≃ 2.5 ± 0.4  , while the richest clusters show an ellipticity evolution in the opposite direction (with  dε/d z ≃−1.2 ± 0.1  ), which could be due to secondary infall. When taking into account sampling effects due to the magnitude-limited nature of the ACO cluster catalogue we find no significant evolution of the cluster X-ray luminosity, while the velocity dispersion increases with decreasing redshift, independent of the cluster richness, at a rate  dσ _v /d z ≃−1700 ± 400 km s⁻¹  .     

Stochasticity in N-body simulations of disc galaxies

We demonstrate that the chaotic nature of N -body systems can lead to macroscopic variations in the evolution of collisionless simulations containing rotationally supported discs. The unavoidable stochasticity that afflicts all simulations generally causes mild differences between the evolution of similar models but, in order to illustrate that this is not always true, we present a case that shows an extreme bimodal divergence. The divergent behaviour occurs in two different types of code, and is independent of all numerical parameters. We identify and give explicit illustrations of several sources of stochasticity, and also show that macroscopic variations in the evolution can originate from differences at the round-off error level. We obtain somewhat more consistent results from simulations in which the halo is set-up with great care compared with those started from more approximate equilibria, but we have been unable to eliminate diverging behaviour entirely because the main sources of stochasticity are intrinsic to the disc. We show that the divergence is only temporary and that halo friction is merely delayed, for a substantial time in some cases. We argue that the delays are unlikely to arise in real galaxies, and that our results do not affect dynamical friction constraints on halo density. Stochastic variations in the evolution are inevitable in all simulations of disc–halo systems, irrespective of how they were created, although their effect is generally far less extreme than we find here. The possibility of divergent behaviour complicates comparison of results from different workers.     

Ram pressure stripping of disc galaxies orbiting in clusters – I. Mass and radius of the remaining gas disc

We present the first 3D hydrodynamical simulations of ram pressure stripping of a disc galaxy orbiting in a galaxy cluster. Along the orbit, the ram pressure that this galaxy experiences varies with time. In this paper, we focus on the evolution of the radius and mass of the remaining gas disc, and compare it with the classical analytical estimate proposed by Gunn & Gott. We find that this simple estimate works well in predicting the evolution of the radius of the remaining gas disc. Only if the ram pressure increases faster than the stripping time-scale, the disc radius remains larger than predicted. However, orbits with such short ram pressure peaks are unlikely to occur in other than compact clusters. Unlike the radius evolution, the mass-loss history for the galaxy is not accurately described by the analytical estimate. Generally, in the simulations the galaxy loses its gas more slowly than predicted.     

Exploring the evolution of spiral galaxies

We have constructed a family of simple models for spiral galaxy evolution to allow us to investigate observational trends in star formation history with galaxy parameters. The models are used to generate broad-band colours from which ages and metallicities are derived in the same way as the data. We generate a grid of model galaxies and select only those that lie in regions of parameter space covered by the sample. The data are consistent with the proposition that the star formation history of a region within a galaxy depends primarily on the local surface density of the gas but that one or two additional ingredients are required to explain the observational data fully. The observed age gradients appear steeper than those produced by the density dependent star formation law, indicating that the star formation law or infall history must vary with galactocentric radius. Furthermore, the metallicity–magnitude and age–magnitude correlations are not reproduced by a local density dependence alone. These correlations require one or both of the following: (i) a combination of mass dependent infall and metal enriched outflow, or (ii) a mass dependent galaxy formation epoch. Distinguishing these possibilities on the basis of current data is extremely difficult.