Stellar Energy Feedback Research Articles

SPITZEROBSERVATIONS OF DUST EMISSION FROM H II REGIONS IN THE LARGE MAGELLANIC CLOUD

Massive stars can alter physical conditions and properties of their ambient interstellar dust grains via radiative heating and shocks. The HII regions in the Large Magellanic Cloud (LMC) offer ideal sites to study the stellar energy feedback effects on dust because stars can be resolved, and the galaxy's nearly face-on orientation allows us to unambiguously associate HII regions with their ionizing massive stars. The Spitzer Space Telescope survey of the LMC provides multi-wavelength (3.6 to 160 $\mu$m) photometric data of all HII regions. To investigate the evolution of dust properties around massive stars, we have analyzed spatially-resolved IR dust emission from two classical HII regions (N63 and N180) and two simple superbubbles (N70 and N144) in the LMC. We produce photometric spectral energy distributions (SEDs) of numerous small subregions for each region based on its stellar distributions and nebular morphologies. We use DustEM dust emission model fits to characterize the dust properties. Color-color diagrams and model fits are compared with the radiation field (estimated from photometric and spectroscopic surveys). Strong radial variations of SEDs can be seen throughout the regions, reflecting the available radiative heating. Emission from very small grains drastically increases at locations where the radiation field is the highest, while polycyclic aromatic hydrocarbons (PAHs) appear to be destroyed. PAH emission is the strongest in the presence of molecular clouds, provided that the radiation field is low.

Chemo-dynamical evolution of tidal dwarf galaxies. I. Method and IMF dependence

We present high-resolution simulations of tidal dwarf galaxies (TDG) to investigate their early chemo-dynamical evolution and test their survivability. In this work the simulation setup is introduced and the response of TDGs to self-consistent star formation (SF) and an external tidal field is examined. Throughout the simulation star cluster particles with variable masses down to $5\,M_{\odot}$ form, depending on the local gas reservoir. For low cluster masses $M_{\mathrm{cl}}$, the stellar initial mass function (IMF) is considered to be either filled or truncated at a maximal star mass $m_\mathrm{max}$ to represent the observed $m_{\mathrm{max}} - M_{\mathrm{cl}}$ relation (IGIMF theory). The evolution of TDGs with fully-populated and truncated IMFs are compared to study the impact of stellar energy feedback on their survivability. Both TDGs experience an initial starburst but after a dynamical time they evolve into dwarf galaxies with self-regulated and continuous SF. At this stage the truncated-IMF model contains about 6 times more stellar mass than the invariant IMF models, but the final bound gas mass is comparable in both models. In spite of their significantly different SF histories, both TDG models are not disrupted within the first 500 Myr. We conclude that TDGs can survive an early starburst, independent of the underlying IMF description, even though they do not harbor a stabilizing dark matter halo.

Formation and evolution of early-type galaxies – III. Dependence of the star formation history on the total mass and initial overdensity

We investigate the influence of the initial proto-galaxies over-densities and masses on their evolution, to understand whether the internal properties of the proto-galactic haloes are sufficient to account for the varied properties of the galactic populations. By means of fully hydrodynamical N-body simulations performed with the code EvoL we produce twelve self-similar models of early-type galaxies of different initial masses and over-densities, following their evolution from z \geq 20 down to z \leq 1. The simulations include radiative cooling, star formation, stellar energy feedback, a reionizing photoheating background, and chemical enrichment of the ISM. We find a strong correlation between the initial properties of the proto-haloes and their star formation histories. Massive (10^13M\odot) haloes experience a single, intense burst of star formation (with rates \geq 10^3M\odot/yr) at early epochs, consistently with observations, with a less pronounced dependence on the initial over-density; intermediate mass (10^11M\odot) haloes histories strongly depend on their initial over-density, whereas small (10^9M\odot) haloes always have fragmented histories, resulting in multiple stellar populations, due to the "galactic breathing" phenomenon. The galaxy models have morphological, structural and photometric properties comparable to real galaxies, often closely matching the observed data; even though some disagreement is still there, likely a consequence of some numerical choices. We conclude that internal properties are essentially sufficient to explain many of the observed features of early type galaxies, particularly the complicated and different star formation histories shown by haloes of very different mass. In this picture, nature seems to play the dominant role, whereas nurture has a secondary importance.

The stellar metallicity distribution of disc galaxies and bulges in cosmological simulations

By means of high-resolution cosmological hydrodynamical simulations of Milky Way-like disc galaxies, we conduct an analysis of the associated stellar metallicity distribution functions (MDFs). After undertaking a kinematic decomposition of each simulation into spheroid and disc sub-components, we compare the predicted MDFs to those observed in the solar neighbourhood and the Galactic bulge. The effects of the star formation density threshold are visible in the star formation histories, which show a modulation in their behaviour driven by the threshold. The derived MDFs show median metallicities lower by 0.2-0.3 dex than the MDF observed locally in the disc and in the Galactic bulge. Possible reasons for this apparent discrepancy include the use of low stellar yields and/or centrally-concentrated star formation. The dispersions are larger than the one of the observed MDF; this could be due to simulated discs being kinematically hotter relative to the Milky Way. The fraction of low metallicity stars is largely overestimated, visible from the more negatively skewed MDF with respect to the observational sample. For our fiducial Milky Way analog, we study the metallicity distribution of the stars born "in situ" relative to those formed via accretion (from disrupted satellites), and demonstrate that this low-metallicity tail to the MDF is populated primarily by accreted stars. Enhanced supernova and stellar radiation energy feedback to the surrounding interstellar media of these pre-disrupted satellites is suggested as an important regulator of the MDF skewness.

RADIATION-DRIVEN FOUNTAIN AND ORIGIN OF TORUS AROUND ACTIVE GALACTIC NUCLEI

We propose a plausible mechanism to explain the formation of the so-called "obscuring tori" around active galactic nuclei (AGNs) based on three-dimensional hydrodynamic simulations including radiative feedback from the central source. The X-ray heating and radiation pressure on the gas are explicitly calculated using a ray-tracing method. This radiation feedback drives a "fountain", that is, a vertical circulation of gas in the central a few to tens parsecs. Interaction between the non-steady outflows and inflows causes the formation of a geometrically thick torus with internal turbulent motion. As a result, the AGN is obscured for a wide range of solid angles. In a quasi-steady state, the opening angles for the column density toward a black hole < 10^23 cm^-2 are approximately +-30 deg and +-50 deg for AGNs with 10% and 1% Eddington luminosity, respectively. Mass inflows through the torus coexist with the outflow and internal turbulent motion, and the average mass accretion rate to the central parsec region is 2x10^-4 ~ 10^-3, M_sun/yr this is about ten times smaller than accretion rate required to maintain the AGN luminosity. This implies that relatively luminous AGN activity is intrinsically intermittent or that there are other mechanisms, such as stellar energy feedback, that enhance the mass accretion to the center.

Radiation-driven Feedback to the ISM around AGNs

AbstractAlthough the “donut-like” obscuring molecular torus is often postulated to explain the type-1 and -2 dichotomy in AGNs, its physical origin is still unclear. We propose a plausible mechanism to explain the formation of the geometrically and optically thick torus, i.e. radiation-driven fountain. Using 3-D hydrodynamic simulations including radiative feedback from the central source, taking into account the X-ray heating and radiation pressure on the gas, we found that a vertical circulation of gas is generated in the central few to tens parsecs. Interaction between the non-steady outflows and inflows causes the formation of a geometrically thick torus with internal turbulent motion. As a result, the AGN is obscured for a wide range of solid angles. In a quasi-steady state, the opening angles for the column density toward a black hole &lt; 1023 cm−2 are approximately ± 30° and ± 50° for AGNs with 10% and 1% Eddington luminosity, respectively. Mass inflows through the torus coexist with the outflow and internal turbulent motion, although the average mass accretion rate to the central parsec region is about ten times smaller than the accretion rate required to maintain the assumed AGN luminosity. This implies that relatively luminous AGN activity is intrinsically intermittent or that there are other mechanisms, such as stellar energy feedback, that enhance the mass accretion to the center.

The violent interstellar medium of the Magellanic Cloud System

AbstractThe interstellar gas of the Magellanic System is subject to the harassment of tidal interactions on galaxy-wide scales and stellar energy feedback on sub-galactic scales. Hisurveys of the Magellanic System have produced spectacular images of the tidally displaced interstellar gas in the Magellanic Bridge and Streams. Multi-wavelength observations of the interstellar gas in the Magellanic Clouds have revealed gas components in physical conditions ranging from cold molecular cloud to hot ionized coronal gas. While stellar energy feedback is responsible for heating and dispersing interstellar gas, it can also compress ambient cloud to form stars. I will useChandra,XMM-Newton,FUSE,HST,Spitzer,ATCA, and other ground-based observations to illustrate the interplay among massive stars, interstellar medium, and star formation.

SAURON’s Challenge for the Major Merger Scenario of Elliptical Galaxy Formation

The intrinsic anisotropy δ and flattening of simulated merger remnants is compared with elliptical galaxies that have been observed by the SAURON collaboration, and that were analyzed using axisymmetric Schwarzschild models. Collisionless binary mergers of stellar disks and disk mergers with an additional isothermal gas component, neglecting star formation, cannot reproduce the observed trend δ = 0.55 (SAURON relationship). An excellent fit of the SAURON relationship for flattened ellipticals with ⩾ 0.25 is, however, found for merger simulations of disks with gas fractions ≥20%, including star formation and stellar energy feedback. Massive black hole feedback does not strongly affect this result. Subsequent dry merging of these merger remnants does not generate the slowly rotating SAURON ellipticals, which are characterized by low ellipticities < 0.25 and low anisotropies. These objects therefore might not have been shaped by a final major merger of either early-type galaxies or disks. We show, however, that stellar spheroids resulting from multiple, hierarchical mergers of star-bursting subunits in a cosmological context are in excellent agreement with the low ellipticities and anisotropies of the slowly rotating SAURON ellipticals and their observed trend of δ with . The numerical simulations indicate that the SAURON relation might be a result of strong violent relaxation and phase mixing of multiple, kinematically cold stellar subunits, with the angular momentum of the system determining its location on the relation.

Molecular cloud turbulence and the star formation efficiency: enlarging the scope

AbstractWe summarize recent numerical results on the control of the star formation efficiency (SFE), addressing the effects of turbulence and the magnetic field strength. In closed-box numerical simulations, the effect of the turbulent Mach number ${\cal M}_{\rm s}$ depends on whether the turbulence is driven or decaying: In driven regimes, increasing ${\cal M}_{\rm s}$ with all other parameters fixed decreases the SFE, while in decaying regimes the converse is true. The efficiencies in non-magnetic cases for realistic Mach numbers ${\cal M}_{\rm s}$ 10 are somewhat too high compared to observed values. Including the magnetic field can bring the SFE down to levels consistent with observations, but the intensity of the magnetic field necessary to accomplish this depends again on whether the turbulence is driven or decaying. In this kind of simulations, a lifetime of the molecular cloud (MC) needs to be assumed, being typically a few free-fall times. Further progress requires determining the true nature of the turbulence driving and the lifetimes of the clouds. Simulations of MC formation by large-scale compressions in the warm neutral medium (WNM) show that the generation of the clouds' initial turbulence is built into the accumulation process that forms them, and that the turbulence is driven for as long as accumulation process lasts, producing realistic velocity dispersions and also thermal pressures in excess of the mean WNM value. In simulations including self-gravity, but neglecting the magnetic field and stellar energy feedback, the clouds never reach an equilibrium state, but rather evolve secularly, increasing their mass and gravitational energy until they engage in generalized gravitational collapse. However, local collapse events begin midways through this process, and produce enough stellar objects to disperse the cloud or at least halt its collapse before the latter is completed. Simulations of this kind including the missing physical ingredients should contribute to a final resolution of the MC lifetime and the origin of the low SFE problems.

Energy Crisis in the Superbubble DEM L192 (N51D)

Superbubbles surrounding OB associations provide ideal laboratories in which to study the stellar energy feedback problem, because the stellar energy input can be estimated from the observed stellar content of the OB associations, and the interstellar thermal and kinetic energies of superbubbles are well defined and easy to observe. We have used DEM L192, also known as N51D, to carry out a detailed case study of the energy budget in a superbubble, and we find that the expected amount of stellar mechanical energy injected into the interstellar medium, (18 ± 5) × 1051 ergs, exceeds the amount of thermal and kinetic energies stored in the superbubble, (6 ± 2) × 1051 ergs. Clearly, a significant fraction of the stellar mechanical energy must have been converted into other forms of energy. The X-ray spectrum of the diffuse emission from DEM L192 requires a power-law component to explain the featureless emission at 1.0-3.0 keV. The origin of this power-law component is unclear, but it may be responsible for the discrepancy between the stellar energy input and the observed interstellar energy in DEM L192.

On the Formation of Giant Holes in the Interstellar Medium by Disk Instabilities

Recently, it has been suggested, based on two-dimensional simulations, that giant holes in the intercloud medium may be a consequence of the nonlinear evolution of combined gravitational and thermal instabilities alone, without any stellar energy feedback. Here we investigate the vertical structure of the gas in the disk. It is found that in the strict two-dimensional mapping, the H I disk in starless models would be extremely thin, a few parsecs, because of the excessive radiative cooling. Thus, additional physical processes should be invoked in order to have both significant density substructure, produced by large-scale instabilities, and admissible scale heights. Different possibilities are explored, but none are found satisfactory, indicating that models based mainly on gravitational plus thermal instabilities present severe difficulties in explaining the observed giant holes in shell-filled galaxies.

Numerical Models of the Multiphase Interstellar Matter with Stellar Energy Feedback on a Galactic Scale

High-resolution two-dimensional hydrodynamic simulations for the interstellar matter (ISM) in a galactic disk are enhanced to include explicitly star formation and the feedback effects from supernovae and stellar winds. A globally stable multiphase ISM is formed, in which filamentary and clumpy structure is a characteristic feature. We find a new component of 106-108 K gas that is a direct consequence of the energy input from the feedback. The total supernovae rate in the system varies by an order of magnitude over a timescale of 106 yr. The evolution of the supernovae rate exhibits chaotic behavior because the star formation is triggered by supernovae explosions in the inhomogeneous interstellar medium. We also find that, in spite of its very complicated spatial structure, the multiphase ISM exhibits a one-point probability density function (pdf) that is a perfect lognormal distribution over four decades in density, 102-106 M☉ pc-2. The lognormal pdf is very robust even in regions with frequent bursts of supernovae. Low-density regions or cavities (<10 M☉ pc-2), on the other hand, exhibit the normal Gaussian distribution. These characteristic pdf's are achieved over a local dynamical scale. The energy spectra are E(k) ∝ k-3 without feedback and E(k) ∝ k-2 including stellar energy feedback.

Formation of Cavities, Filaments, and Clumps by the Nonlinear Development of Thermal and Gravitational Instabilities in the Interstellar Medium under Stellar Feedback

Based on our high-resolution two-dimensional hydrodynamical simulations, we propose that large cavities may be formed by the nonlinear development of the combined thermal and gravitational instabilities, without need for stellar energy injection in a galaxy modeling the Large Magellanic Cloud (LMC). Our numerical model of star formation allows us to follow the evolution of the blast waves due to supernovae in the inhomogenous, multiphase, and turbulent-like media self-consistently. Formation of kiloparsec-scale inhomogeneity, such as cavities as seen in the observed H I map of the LMC, is suppressed by frequent supernovae (the average supernova rate for the whole disk is ~0.001 yr-1). However, the supernova explosions are necessary for the hot component (Tg > 106-107 K). Position-velocity maps show that kiloparsec-scale shells/arcs formed through nonlinear evolution in a model without stellar energy feedback have kinematics similar to explosive phenomena, such as supernovae. We also find that dense clumps and filamentary structure are formed as a natural consequence of the nonlinear evolution of the multiphase interstellar medium (ISM). Although the ISM on a small scale looks turbulent-like and transient, the global structure of the ISM is quasi-stable. In the quasi-stable phase, the volume filling factor of the hot, warm, and cold components are ~0.2, ~0.6, and ~0.2, respectively. We compare observations of H I and molecular gas of the LMC with the numerically obtained H I and CO brightness temperature distributions. The morphology and statistical properties of the numerical H I and CO maps are discussed. We find that the cloud mass spectra of our models represent a power-law shape, but their slopes change between models with and without the stellar energy injection. We also find that the slope depends on the threshold brightness temperature of CO.