Galactic winds and circulation of the interstellar medium in dwarf galaxies

We study, through 2D hydrodynamical simulations, the feedback of a starburst on the ISM of typical gas-rich dwarf galaxies. The main goal is to address the circulation of the ISM and metals following the starburst. We assume a single-phase rotating ISM in equilibrium in the galactic potential generated by a stellar disc and a spherical dark halo. The starburst is assumed to occur in a small volume in the centre of the galaxy, and it generates a mechanical power of 3.8×10³⁹ or 3.8×10⁴⁰ erg s⁻¹ for 30 Myr. We find, in accordance with previous investigations, that the galactic wind is not very effective in removing the ISM. The metal-rich stellar ejecta, however, can be efficiently expelled from the galaxy and dispersed in the intergalactic medium.
Moreover, we find that the central region of the galaxy is always replenished with cold and dense gas a few 100 million years after the starburst, achieving the requisite for a new star formation event in ≈0.5–1 Gyr. The hydrodynamical evolution of galactic winds is thus consistent with the episodic star formation regime suggested by many chemical evolution studies.
We also discuss the X-ray emission of these galaxies and find that the observable (emission-averaged) abundance of the hot gas underestimates the real one if thermal conduction is effective. This could explain the very low hot-gas metallicities estimated in starburst galaxies.     

Evolution of a cloudy protogalaxy interacting with an early galactic wind and galaxy formation

We argue that a combined evidence from galactic and extragalactic studies suggests that a major star formation in giant galaxies is preceded by an evolutionary phase at which a strong galactic wind driven by the initial burst of star formation enriches the gaseous protogalaxy with metals and heats it up, so that the latter turns over from contraction to expansion. The result is the ejection of enriched material from the outer part of the protogalaxy into the intergalactic space, while the inner part, after a delay of about one to a few Gyr, finally contracts and cools down to form the galactic major stellar component (the hot model of galaxy formation). The paper presents a specific mechanism to produce a hot protogalaxy according to which an early galactic wind is imparting energy and momentum into a collapsing protogalaxy whose mass is contained mainly in clouds and only a small portion is in the intercloud gas that provides pressure confinement for the clouds. The model is then capable of accounting for the nearly equal mass and iron abundance in cluster giant galaxies and the intracluster gas provided the observationally plausible input parameters for giant galaxies and early galactic winds are adopted. It also predicts the formation of long-lived X-ray coronae with characteristics similar to those observed around giant ellipticals.The model specifies a characteristic length-scale that can be very naturally interpreted as a size for a stellar system to come; a very encouraging result is that it perfectly fits in with a typical size of giant ellipticals.     

Dwarf galaxies: Important clues to galaxy formation

The smallest dwarf galaxies are the most straight forward objects in which to study star formation processes on a galactic scale. They are typically single cell star forming entities, and as small potentials in orbit around a much larger one they are unlikely to accrete much (if any) extraneous matter during their lifetime (either intergalactic gas, or galaxies) because they will typically lose the competition with the much larger galaxy. We can utilise observations of stars of a range of ages to measure star formation and enrichment histories back to the earliest epochs. The most ancient objects we have ever observed in the Universe are stars found in and around our Galaxy. Their proximity allows us to extract from their properties detailed information about the time in the early Universe into which they were born. A currently fashionable conjecture is that the earliest star formation in the Universe occurred in the smallest dwarf galaxy sized objects. Here I will review some recent observational highlights in the study of dwarf galaxies in the Local Group and the implications for understanding galaxy formation and evolution. This revised version was published online in August 2006 with corrections to the Cover Date.     

An Analytic Model of Galactic Winds and Mass Outflows

Galactic winds and mass outflows are observed both in nearby starburst galaxies and in high-redshift star-forming galaxies. We develop a simple analytic model to understand the observed superwind phenomenon with a discussion of the model uncertainties. Our model is built upon the model of McKee & Ostriker for the interstellar medium. It allows one to predict how properties of a superwind, such as wind velocity and mass outflow rate, are related to properties of its starforming host galaxy, such as size, gas density and star formation rate. The model predicts a threshold of star formation rate density for the generation of observable galactic winds. Galaxies with more concentrated star formation activities produce superwinds with higher velocities. The predicted mass outflow rates are comparable to (or slightly larger than) the corresponding star formation rates. We apply our model to both local starburst galaxies and high-redshift Lyman break galaxies, and find its predictions to be in good agreement with current observations. Our model is simple and so can be easily incorporated into numerical simulations and semi-analytical models of galaxy formation.     

The chemical composition of matter in the early universe

The results of the computations of the chemical evolution for a galaxy cluster are presented. The matter exchange between galaxies and intergalactic medium is taken into account. Two dependences of star formation rate on time are considered: (i) monotonously decreasing dependence characteristic of elliptical galaxies, (ii) dependence having two peaks associated with creation of spiral galaxy subsystems, with suppression of star formation at the period between maxima. It is assumed that galactic ejection is due to explosions of II-type supernova with massesm5M , and that the accretion on to a galaxy depends but weakly on the time. By comparing the obtained results with total combination of available observations, it is established that the rate of gaseous exchange between a galaxy and intergalactic medium should be rather large: 0.03M _gal Gyr^–1. Besides, the activity of each type of galaxy leads to an approximately equal enrichment of intergalactic gas by new elements synthesized in the stars. The existence of a large accretion on to the Galaxy leads to the decrease of primordial deuterium abundance by a factor of no more than 2 during the galaxy evolution time. It enables us to assume that the standard Big Bang model with baryon density parameter _b 0.1 may be considered as true.     

Cosmic ray propagation and the star formation history of NGC 1961

We present new radio continuum data at four frequencies for the supermassive, peculiar galaxy NGC 1961. These observations allow us to separate the thermal and non-thermal radio emission and to determine the non-thermal spectral index distribution. This spectral index distribution in the galactic disc is unusual: at the maxima of the radio emission the synchrotron spectrum is very steep, indicating aged cosmic ray electrons. Away from the maxima the spectrum is much flatter. The steep spectrum of the synchrotron emission at the maxima indicates that a strong decline of the star formation rate has taken place at these sites. The extended radio emission is a sign of recent cosmic ray acceleration, probably by recent star formation. We suggest that a violent event in the past, most likely a merger or a collision with an intergalactic gas cloud, has caused the various unusual features of the galaxy.     

Warm and Hot Gas and the Evolution of Dwarf Galaxies

This paper discusses results of multi-wavelength studies of low-mass galaxies with outflows. As a case study the extreme low-metallicity dwarf galaxy I Zw 18 is presented. The results from HST imaging, long-slit echelle spectroscopy and X-ray imaging are discussed in the context of chemical enrichment of dwarf galaxies. If outflows work primarily as a temporary repository of metals and not as a loss of metals into the intergalactic medium, we show that galactic winds can explain the relatively low N/O scatter seen in nearby low-metallicity dwarf galaxies as well as the apparent increase of N/O scatter observed in damped Ly_α systems with metallicities near that of I Zw 18and below. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cluster magnetic fields from galactic outflows

We performed cosmological, magnetohydrodynamical simulations to follow the evolution of magnetic fields in galaxy clusters, exploring the possibility that the origin of the magnetic seed fields is galactic outflows during the starburst phase of galactic evolution. To do this, we coupled a semi-analytical model for magnetized galactic winds as suggested by Bertone, Vogt & Enßlin to our cosmological simulation. We find that the strength and structure of magnetic fields observed in galaxy clusters are well reproduced for a wide range of model parameters for the magnetized, galactic winds and do only weakly depend on the exact magnetic structure within the assumed galactic outflows. Although the evolution of a primordial magnetic seed field shows no significant differences to that of galaxy cluster fields from previous studies, we find that the magnetic field pollution in the diffuse medium within filaments is below the level predicted by scenarios with pure primordial magnetic seed field. We therefore conclude that magnetized galactic outflows and their subsequent evolution within the intracluster medium can fully account for the observed magnetic fields in galaxy clusters. Our findings also suggest that measuring cosmological magnetic fields in low-density environments such as filaments is much more useful than observing cluster magnetic fields to infer their possible origin.     

Feedback and metal enrichment in cosmological SPH simulations – II. A multiphase model with supernova energy feedback

We have developed a new scheme to treat a multiphase interstellar medium in smoothed particle hydrodynamics simulations of galaxy formation. This scheme can represent a co-spatial mixture of cold and hot ISM components, and is formulated without scale-dependent parameters. It is thus particularly suited to studies of cosmological structure formation where galaxies with a wide range of masses form simultaneously. We also present new algorithms for energy and heavy element injection by supernovae, and show that together these schemes can reproduce several important observed effects in galaxy evolution. Both in collapsing systems and in quiescent galaxies our codes can reproduce the Kennicutt relation between the surface densities of gas and of star formation. Strongly metal-enhanced winds are generated in both cases with ratios of mass-loss to star formation which are similar to those observed. This leads to a self-regulated cycle for star formation activity. The overall impact of feedback depends on galaxy mass. Star formation is suppressed at most by a factor of a few in massive galaxies, but in low-mass systems the effects can be much larger, giving star formation an episodic, bursty character. The larger the energy fraction assumed available in feedback, the more massive the outflows and the lower the final stellar masses. Winds from forming discs are collimated perpendicular to the disc plane, reach velocities up to  ∼1000 km s⁻¹  , and efficiently transport metals out of the galaxies. The asymptotically unbound baryon fraction drops from >95 per cent to ∼30 per cent from the least to the most massive of our idealized galaxies, but the fraction of all metals ejected with this component exceeds 60 per cent regardless of mass. Such winds could plausibly enrich the intergalactic medium to observed levels.     

Temperature and abundance profiles of hot gas in galaxy groups – II. Implications for feedback and ICM enrichment

We investigate the history of galactic feedback and chemical enrichment within a sample of 15 X-ray bright groups of galaxies, on the basis of the inferred Fe and Si distributions in the hot gas and the associated metal masses produced by core-collapse and Type Ia supernovae (SNe). Most of these cool-core groups show a central Fe and Si excess, which can be explained by prolonged enrichment by SN Ia and stellar winds in the central early-type galaxy alone, but with tentative evidence for additional processes contributing to core enrichment in hotter groups. Inferred metal mass-to-light ratios inside r ₅₀₀ show a positive correlation with total group mass but are generally significantly lower than in clusters, due to a combination of lower global intracluster medium (ICM) abundances and gas-to-light ratios in groups. This metal deficiency is present for products from both SN Ia and SN II, and suggests that metals were either synthesized, released from galaxies or retained within the ICM less efficiently in lower mass systems. We explore possible causes, including variations in galaxy formation and metal release efficiency, cooling out of metals, and gas and metal loss via active galactic nuclei (AGN) – or starburst-driven galactic winds from groups or their precursor filaments. Loss of enriched material from filaments coupled with post-collapse AGN feedback emerges as viable explanations, but we also find evidence for metals to have been released less efficiently from galaxies in cooler groups and for the ICM in these to appear chemically less evolved, possibly reflecting more extended star formation histories in less massive systems. Some implications for the hierarchical growth of clusters from groups are briefly discussed.     

Formation and evolution of elliptical galaxies and QSO activity

We present the results of a numerical code that combines multi-zone chemical evolution with 1D hydrodynamics to follow in detail the evolution and radial behaviour of gas and stars during the formation of elliptical galaxies. We use the model to explore the links between the evolution and formation of elliptical galaxies and QSO activity. The knowledge of the radial gas flows in the galaxy allows us to trace metallicity gradients, and, in particular, the formation of a high-metallicity core in ellipticals. The high-metallicity core is formed soon enough to explain the metal abundances inferred in high-redshift quasars. The star formation rate and the subsequent feedback regulate the episodes of wind, outflow and cooling flow, thus affecting the recycling of the gas and the chemical enrichment of the intergalactic medium. The evolution of the galaxy shows several stages, some of which are characterized by a complex flow pattern, with inflow in some regions and outflow in other regions. All models, however, exhibit during their late evolution a galactic wind at the outer boundary and, during their early evolution, an inflow towards the galactic nucleus. The characteristics of the inner inflow could explain the bolometric luminosity of a quasar lodged at the galactic centre as well as the evolution of the optical luminosity of quasars.     

Sunyaev–Zel'dovich distortion from early galactic winds

We consider the distortion in the cosmic microwave background (CMB) resulting from galactic winds at high redshift. Winds outflowing from galaxies have been hypothesized to be possible sources of metals in the intergalactic medium, which is known to have been enriched to 10^−2.5 Z_⊙ at z ∼3. We model these winds as functions of mass of the parent galaxy and redshift, assuming that they activate at a common initial redshift, z _in, and calculate the mean y -distortion and the angular power spectrum of the distortion in the CMB. We find that the thermal Sunyaev–Zel'dovich (SZ) effect resulting from the winds is consistent with previous estimates. The distortion arising from the kinetic SZ (kSZ) effect is, however, found to be more important than the thermal SZ (tSZ) effect. We find that the distortion resulting from galactic winds is an important contribution to the power spectrum of distortion at very small angular scales ( l ∼10⁴). We also find that the power spectrum resulting from clustering dominates the Poisson power spectrum for l ≤(4–5)×10⁵. We show explicitly how the combined power spectrum from wind dominates over that of clusters at 217 GHz, relevant for PLANCK . We also show how these constraints change when the efficiency of the winds is varied.     

Supermassive black holes, star formation and downsizing of elliptical galaxies

The overabundance of Mg relative to Fe, observed in the nuclei of bright ellipticals, and its increase with galactic mass, poses a serious problem for all current models of galaxy formation. Here, we improve on the one-zone chemical evolution models for elliptical galaxies by taking into account positive feedback produced in the early stages of supermassive central black hole growth. We can account for both the observed correlation and the scatter if the observed anti-hierarchical behaviour of the AGN population couples to galaxy assembly and results in an enhancement of the star formation efficiency which is proportional to galactic mass. At low and intermediate galactic masses, however, a slower mode for star formation suffices to account for the observational properties.     

Star formation and chemical evolution in smoothed particle hydrodynamics simulations: a statistical approach

In smoothed particle hydrodynamics (SPH) codes with a large number of particles, star formation as well as gas and metal restitution from dying stars can be treated statistically. This approach allows one to include detailed chemical evolution and gas re-ejection with minor computational effort. Here we report on a new statistical algorithm for star formation and chemical evolution, especially conceived for SPH simulations with large numbers of particles, and for parallel SPH codes.
For the sake of illustration, we also present two astrophysical simulations obtained with this algorithm, implemented into the Tree-SPH code by Lia & Carraro .
In the first simulation, we follow the formation of an individual disc-like galaxy, predict the final structure and metallicity evolution, and test resolution effects. In the second simulation we simulate the formation and evolution of a cluster of galaxies, to demonstrate the capabilities of the algorithm in investigating the chemo-dynamical evolution of galaxies and of the intergalactic medium in a cosmological context.     

Star Formation in the Outskirts of Disk Galaxies

The far outer regions of galactic disks allow an important probe of both star formation and galaxy formation. I discuss how observations of HII regions in these low gas density, low metallicity environments can shed light on the physical processes which drive galactic star formation. The history of past star formation at large radii, as traced by observations of old and intermediate-age stars, constrains the epoch at which the highest angular momentum regions of disks were in place; first results for the M31 disk suggest this occured a significant (≳ 8 Gyr) time ago. This revised version was published online in September 2006 with corrections to the Cover Date.     

The Role of Magnetic Fields in Spiral Galaxies

Interstellar magnetic fields are strong: up to 25μG in spiralarms and 40μG in nuclear regions.In the spiral galaxy NGC 6946 the average magnetic energy densityexceeds that of the thermal gas. Magnetic fields control the evolution of denseclouds and possibly the global star formation efficiency in galaxies.Gas flows and shocks in spiral arms and bars are modified by magneticfields. Magnetic forces instar-forming circumnuclear regions are able to drive mass inflow towardsthe active nucleus. Magnetic fields are essential for the propagationof cosmic rays and the formation of galactic winds and halos.     

Chemical evolution of the intracluster gas

Recent X-ray observations have shown that intracluster medium has non-primordial composition. Iron lines have been detected. We present preliminary results on modelling of the chemical evolution of the intracluster medium in galaxy clusters. We consider in detail the galactic winds driven by supernovae, taking into account the binding energy of the galactic gas. We try to explain the metallicity gradient observed in the Perseus cluster from morphological segregation of galaxies in the inner part of the cluster.     

The dark matter problem in disc galaxies

In the generic CDM cosmogony, dark-matter haloes emerge too lumpy and centrally concentrated to host observed galactic discs. Moreover, discs are predicted to be smaller than those observed. We argue that the resolution of these problems may lie with a combination of the effects of protogalactic discs, which would have had a mass comparable to that of the inner dark halo and be plausibly non-axisymmetric, and of massive galactic winds, which at early times may have carried off as many baryons as a galaxy now contains. A host of observational phenomena, from quasar absorption lines and intracluster gas through the G-dwarf problem, point to the existence of such winds. Dynamical interactions will homogenize and smooth the inner halo, and the observed disc will be the relic of a massive outflow. The inner halo expanded after absorbing energy and angular momentum from the ejected material. Observed discs formed at the very end of the galaxy formation process, after the halo had been reduced to a minor contributor to the central mass budget and strong radial streaming of the gas had died down.     

Galactic winds: a short review

Galactic winds are important in recycling energy and metals in galaxies. They are also suspected to be the primary mechanism in the enrichment of the intergalactic medium. New observations are revealing the ubiquity of galactic winds, particularly at high redshift. We review the observational evidence for them in nearby star-forming and active galaxies and in the high-redshift universe.