arXiv subjects

Gerhard Hensler

Publications and source records attributed to Gerhard Hensler.

At least 37 records · Page 2Linked to original sources

arXiv2014

The Supernova-ISM/Star formation Interplay

Supernovae are the most energetic stellar events and influence the interstellar medium by their gasdynamics and energetics. By this, both also affect the star formation positively and negatively. In this paper, we review the complexity of investigations aiming at understanding the interchange between supernova explosions with the star-forming molecular clouds. Commencing from analytical studies the paper advances to numerical models of supernova feedback from superbubble scales to galaxy structure. We also discuss parametrizations of star-formation and supernova-energy transfer efficiencies. Since evolutionary models from the interstellar medium to galaxies are numerous and are applying multiple recipes of these parameters, only a representative selection of studies can be discussed here.

astro-ph.GA↗

arXiv2014

Comparative studies of Population Synthesis Models in the frame work of modified Strömgren filters

Evolutionary models form a vital part of stellar population research to understand their evolution, but despite their long history of development, they often misrepresent and misinterpret the properties of stellar population observed through broadband and spectroscopic measurements. With the growing numbers of these synthesis models, model comparison becomes an important analysis to choose a suitable model for upgrade. Along with the model comparison, we reinvestigate the technique of modified Strömgren photometry to measure reliable parameter-sensitive colours and estimate precise model ages and metallicities. The assessment of Rakos/Schulz models with GALEV and Worthey's Lick/IDS model find smaller colour variation: Δ(uz-vz) $\leq$ 0.056, Δ(bz-yz) $\leq$ -0.05 and Δ(vz-yz) $\leq$ 0.061. The study conveys a good agreement of GALEV models with the modified Strömgren colours but with poor UV model predictions with the observed globular cluster data, while the spectroscopic models perform badly due to the use of older isochrone and stellar spectral libraries with inaccurate/insufficient knowledge of various stellar phases and their treatment. Overall, the assessment finds modified Strömgren photometry well suited to study different types stellar populations by mitigating the effects of the age-metallicity degeneracy.

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arXiv2012

Collisionless Stellar Hydrodynamics as an Efficient Alternative to N-body Methods

For simulations that deal only with dark matter or stellar systems, the conventional N-body technique is fast, memory efficient, and relatively simple to implement. However when including the effects of gas physics, mesh codes are at a distinct disadvantage compared to SPH. Whilst implementing the N-body approach into SPH codes is fairly trivial, the particle-mesh technique used in mesh codes to couple collisionless stars and dark matter to the gas on the mesh, has a series of significant scientific and technical limitations. These include spurious entropy generation resulting from discreteness effects, poor load balancing and increased communication overhead which spoil the excellent scaling in massively parallel grid codes. We propose the use of the collisionless Boltzmann moment equations as a means to model collisionless material as a fluid on the mesh, implementing it into the massively parallel FLASH AMR code. This approach, which we term "collisionless stellar hydrodynamics" enables us to do away with the particle-mesh approach. Since the parallelisation scheme is identical to that used for the hydrodynamics, it preserves the excellent scaling of the FLASH code already demonstrated on peta-flop machines. We find the classic hydrodynamic equations and Boltzmann moment equations can be reconciled under specific conditions, allowing us to generate analytic solutions for collisionless systems using conventional test problems. We confirm the validity of our approach using a suite of demanding test problems, including the use of a modified Sod shock test. We conclude by demonstrating the ability of our code to model complex phenomena by simulating the evolution of a spiral galaxy whose properties agree with those predicted by swing amplification theory. (Abridged)

astro-ph.IM↗

arXiv2012

Do High-Velocity Clouds trace the Dark Matter subhalo population?

Within the cosmological concordance model, Cold Dark Matter (CDM) subhalos form the building blocks which merge hierarchically to more massive galaxies. Since intergalactic gas is accreted by massive galaxies, observable e.g. as high- velocity clouds (HVCs) around the Milky Way, with extremely low metallicities, these can be suggested to represent the baryonic content of primordial Dark Matter (DM) subhalos. Another possibility of their origin is that they stem from disrupted satellite galaxies, but in this case, these gas clouds move unaccompanied by a bound DM structure. Since HVCs are observed with long gas tails and with irregular substructures, numerical models are performed aiming at exploring their structure and compare them with observations. If HVCs are engulfed by DM subhalos, their gas must leave the DM gravitational potential and reflect this in their dynamics. On the other hand, the evolution and survival of pure gas models must be tested to distinguish between DM-dominated and DM-free clouds and to allow conclusions on their origin. The models demonstrate that purely baryonic HVCs with low masses are disrupted by ram-pressure stripping and Kelvin-Helmholtz instabilities, while more massive ones survive, losing their initially spherical shape and develop significant substructures including cometary elongations in the column density distribution ("head-tail structure"). On the contrary, HVCs with DM subhalos survive with more than 90% of their gas mass still bound and spherically shaped, approaching the Galactic disk like bullets. In addition, we find that velocity gradients along the cometary head-tail structures does not necessarily offer a possibility to distinguish between DM-dominated and purely gaseous HVCs. Comparison of models with observations let us conclude that HVCs are not embedded in a DM substructure and do not trace the cosmological subhalo population.

astro-ph.GA↗

arXiv2012

Self-gravitating equilibrium models of dwarf galaxies and the minimum mass for star formation

We construct a series of model galaxies in rotational equilibrium consisting of gas, stars, and a fixed dark matter (DM) halo and study how these equilibrium systems depend on the mass and form of the DM halo, gas temperature, non-thermal and rotation support against gravity, and also on the redshift of galaxy formation. For every model galaxy we find the minimum gas mass M_g^min required to achieve a state in which star formation (SF) is allowed according to contemporary SF criteria. The obtained M_g^min--M_DM relations are compared against the baryon-to-DM mass relation M_b--M_DM inferred from the \LambdaCDM theory and WMAP4 data. Our aim is to construct realistic initial models of dwarf galaxies (DGs), which take into account the gas self-gravity and can be used as a basis to study the dynamical and chemical evolution of DGs. Rotating equilibria are found by solving numerically the steady-state momentum equation for the gas component in the combined gravitational potential of gas, stars, and DM halo using a forward substitution procedure. We find that for a given M_DM the value of M_g^min depends crucially on the gas temperature T_g, gas spin parameter α, degree of non-thermal support σ_eff, and somewhat on the redshift for galaxy formation z_gf. Depending on the actual values of T_g, α, σ_eff, and z_gf, model galaxies may have M_g^min that are either greater or smaller than M_b. Galaxies with M_DM \ga 10^9 M_sun are usually characterized by M_g^min \la M_b, implying that SF in such objects is a natural outcome as the required gas mass is consistent with what is available according to the \LambdaCDM theory. On the other hand, models with M_DM \la 10^9 M_sun are often characterized by M_g^min >> M_b, implying that they need much more gas than available to achieve a state in which SF is allowed. Abridged.

astro-ph.CO↗

arXiv2012

Morphological Mutations of Dwarf Galaxies

Dwarf galaxies (DGs) are extremely challenging objects in extragalactic astrophysics. They are expected to originate as the first units in Cold Dark-Matter cosmology. They are the galaxy type most sensitive to environmental influences and their division into multiple types with various properties have invoked the picture of their variant morphological transformations. Detailed observations reveal characteristics which allow to deduce the evolutionary paths and to witness how the environment has affected the evolution. Here we review peculiarities of general morphological DG types and refer to processes which can deplete gas-rich irregular DGs leading to dwarf ellipticals, while gas replenishment implies an evolutionary cycling. Finally, as the less understood DG types the Milky Way satellite dwarf spheroidal galaxies are discussed in the context of transformation.

astro-ph.CO↗

arXiv2012

Ages and Metallicities of Cluster Galaxies in A779 using Modified Strömgren Photometry

In the quest for the formation and evolution of galaxy clusters, Rakos and co-workers introduced a spectrophotometric method using the modified Strömgren photometry. But with the considerable debate toward the project's abilities, we re-introduce the system after a thorough testing of repeatability of colors and reproducibility of the ages and metallicities for six common galaxies in the three A779 data sets. A fair agreement has been found between the modified Strömgren and Strömgren filter systems to produce similar colors (with the precision of 0.09 mag in (uz-vz), 0.02 mag in (bz-yz), and 0.03 mag in (vz-vz)), ages and metallicities (with the uncertainty of 0.36 Gyr and 0.04 dex from the PCA and 0.44 Gyr and 0.2 dex using the GALEV models). We infer that the technique is able to relieve the age-metallicity degeneracy by separating the age effects from the metallicity effects, but still unable to completely break. We further extend this paper to re-study the evolution of galaxies in the low mass, dynamically poor A779 cluster by correlating the luminosity (mass), density, radial distance with the estimated age, metallicity, and the star formation history. Our results distinctly show the bimodality of the young, low-mass, metal-poor population with the mean age of 6.7 Gyr (\pm 0.5 Gyr) and the old, high-mass, metal-rich galaxies with the mean age of 9 Gyr (\pm 0.5 Gyr). The method also observes the color evolution of the blue cluster galaxies to red, and the downsizing phenomenon. Our analysis shows that the modified Strömgren photometry is very well suited for studying low- and intermediate-z clusters, as it is capable of observing deeper with better spatial resolution at spectroscopic redshift limits, and the narrowband filters estimate the age and metallicity with lesser uncertainties compared to other methods that study stellar population scenarios.

astro-ph.GA↗

arXiv2011

The collisions of high-velocity clouds with the galactic halo

Spiral galaxies are surrounded by a widely distributed hot coronal gas and seem to be fed by infalling clouds of neutral hydrogen gas with low metallicity and high velocities. We numerically study plasma waves produced by the collisions of these high-velocity clouds (HVCs) with the hot halo gas and with the gaseous disk. In particular, we tackle two problems numerically: 1) collisions of HVCs with the galactic halo gas and 2) the dispersion relations to obtain the phase and group velocities of plasma waves from the equations of plasma motion as well as further important physical characteristics such as magnetic tension force, gas pressure, etc. The obtained results allow us to understand the nature of MHD waves produced during the collisions in galactic media and lead to the suggestion that these waves can heat the ambient halo gas. These calculations are aiming at leading to a better understanding of dynamics and interaction of HVCs with the galactic halo and of the importance of MHD waves as a heating process of the halo gas.

astro-ph.GA↗

arXiv2011

The Morphological Origin of Dwarf Galaxies

Dwarf galaxies (DGs) serve as extremely challenging objects in extragalactic astrophysics. Their origin is expected to be set as the first units in CDM cosmology. Nevertheless they are the galaxy type most sensitive to environmental in uences and their division into multiple types with various properties have invoked the picture of their variant morphological transformations. Detailed observations reveal characteristics which allow to deduce the evolutionary paths and to witness how the environment has affected the evolution. Here we refer to general morphological DG types and review some general processes, most of which deplete gas-rich irregular DGs. Moreover, the variety of pecularities is brie y refered, but cannot be comprehensively analyzed because of limited paper space.

astro-ph.CO↗

arXiv2011

Evolution of the Milky Way halo by accretion of dwarf satellite galaxies

Within the Cold Dark Matter scenario the hierarchical merging paradigm is the natural result to form massive galactic halos by the minor mergers of sub-halos and, by this, inherently their stellar halo. Although this must be also invoked for the Milky Way, the context of chemical and kinematic coherence of halo stars and dwarf spheroidal galaxies is yet unsolved a focus of present-day research. To examine this issue we model the chemo-dynamical evolution of the system of satellites selected from the cosmological Via Lactea II simulations to be similar for the Milky Way environment but at an early epoch.

astro-ph.CO↗

arXiv2010

Supernova Feedback on the Interstellar Medium and Star Formation

Supernovae are the most energetic stellar events and influence the interstellar medium by their gasdynamics and energetics. By this, both also affect the star formation positively and negatively. In this paper, we review the development of the complexity of investigations aiming at understanding the interchange between supernovae and their released hot gas with the star-forming molecular clouds. Commencing from analytical studies the paper advances to numerical models of supernova feedback from superbubble scales to galaxy structure. We also discuss parametrizations of star-formation and supernova-energy transfer efficiencies. Since evolutionary models from the interstellar medium to galaxies are numerous and apply multiple recipes of these parameters, only a representative selection of studies can be discussed here.

astro-ph.GA↗

arXiv2010

Accretion-regulated star formation in late-type galaxies

We develop a four-phase galaxy evolution model in order to study the effect of accretion of extra-galactic gas on the star formation rate (SFR) of a galaxy. Pure self-regulated star formation of isolated galaxies is replaced by an accretion-regulated star formation mode. The SFR settles into an equlibrium determined entirely by the gas accretion rate on a Gyr time scale.

astro-ph.CO↗

arXiv2010

Cosmological radiative transfer for the line-of-sight proximity effect

We study the proximity effect in the Ly-a forest around high redshift quasars as a function of redshift and environment employing a set of 3D radiative transfer simulations. The analysis is based on dark matter only simulations at redshifts 3, 4, and 4.9 and, adopting an effective equation of state for the baryonic matter, we infer the HI densities and temperatures in the cosmological box. The UV background (UVB) and additional QSO radiation with Lyman limit flux of L_{νLL} = 1e31 and 1e32 erg / Hz s are implemented with a radiative transfer code until an equilibrium configuration is reached. We analyse mock spectra originating at the QSO in the most massive halo, in a random filament and in a void. The proximity effect is studied using flux transmission statistics, in particular with the normalised optical depth. Beyond a radius of r > 1 Mpc / h from the quasar, we measure a transmission profile consistent with geometric dilution of the QSO ionising radiation. A departure from geometric dilution is only seen, when strong absorbers intervene the line-of-sight. The cosmic density distribution around the QSO causes a large scatter in the normalised optical depth. The scatter decreases with increasing redshift and increasing QSO luminosity. The mean proximity effect provides an average signal that is biased by random large scale density enhancements on scales up to r \approx 15 Mpc / h. The distribution of the proximity effect strength provides a measure of the proximity effect along individual lines of sight. It shows a clear maximum almost without an environmental bias. Therefore it can be used for an unbiased estimate of the UVB. Differing spectral energy distributions between the QSO and the UVB modify the profile which can be reasonably well corrected analytically. A few Lyman limit systems have been identified that prevent the detection of the proximity effect due to shadowing.

astro-ph.CO↗

arXiv2010

Modelling the chemical evolution

Advanced observational facilities allow to trace back the chemical evolution of the Universe, on the one hand, from local objects of different ages and, secondly, by direct observations of redshifted objects. The chemical enrichment serves as one of the cornerstones of cosmological evolution. In order to understand this chemical evolution in morphologically different astrophysical objects models are constructed based on analytical descriptions or numerical methods. For the comparison of their chemical issues, as there are element abundances, gradients, and ratios, with observations not only the present-day values are used but also their temporal evolution from the first era of metal enrichment. Here we will provide some insight into basics of chemical evolution models, highlight advancements, and discuss a few applications.

astro-ph.CO↗

arXiv2009

Did the Milky Way dwarf satellites enter the halo as a group?

The dwarf satellite galaxies in the Local Group are generally considered to be hosted in dark matter subhalos that survived the disruptive processes during infall onto their host halos. It has recently been argued that if the majority of satellites entered the Milky Way halo in a group rather than individually, this could explain the spatial and dynamical peculiarities of its satellite distribution. Such groups were identified as dwarf galaxy associations that are found in the nearby Universe. In this paper we address the question whether galaxies in such associations can be the progenitors of the Milky Way satellite galaxies. We find that the dwarf associations are much more extended than would be required to explain the disk-like distribution of the Milky Way and Andromeda satellite galaxies. We further identify a possible minor filamentary structure, perpendicular to the supergalactic plane, in which the dwarf associations are located, that might be related to the direction of infall of a progenitor galaxy of the Milky Way satellites, if they are of tidal origin.

astro-ph.CO↗

arXiv2009

Galactic outflows and the chemical evolution of dwarf galaxies

Galactic winds in dwarf galaxies are driven by the energy released by supernova explosions and stellar winds following an intense episode of star formation, which create an over-pressured cavity of hot gas. Although the luminosity of the star formation episode and the mass of the galaxy play a key role in determining the occurrence of the galactic winds and the fate of the freshly produced metals, other parameters play an equally important role. In this contribution we address the following questions (i) What is the late evolution of superbubbles and what is the final fate of the superbubble cavities? (ii) How does the multi-phase nature of the ISM, in particular the coexistence of hot gas with embedded clouds, affect the development of galactic winds? (iii) What is the relation between the flattening of a galaxy and the development of bipolar galactic winds?

astro-ph.GA↗

arXiv2008

Interaction of massive stars with their surroundings

Due to their short lifetimes but their enormous energy release in all stages of their lives massive stars are the major engines for the comic matter circuit. They affect not only their close environment but are also responsible to drive mass flows on galactic scales. Recent 2D models of radiation-driven and wind-blown HII regions are summarized which explore the impact of massive stars to the interstellar medium but find surprisingly small energy transfer efficiencies while an observable Carbon self-enrichment in the Wolf-Rayet phase is detected in the warm ionized gas. Finally, the focus is set on state-of-the-art modelling of HII regions and its present weaknesses with respect to uncertainties and simplifications but on a perspective of the requested art of their modelling in the 21st century.

astro-ph↗

arXiv2008

Modeling the Disk (three-phase) Interstellar Medium

The evolution of galactic disks from their early stages is dominated by gasdynamical effects such as gas infall, galactic fountains, and galactic outflows, and further more. The influence of these processes is only understandable in the framework of diverse gas phases differing in their thermal energies, dynamics, and element abundances. To trace the temporal and chemical evolution of galactic disks, it is therefore essential to model the interstellar gasdynamics combined with stellar dynamics, the interactions between gas phases, and star-gas mass and energy exchanges as detailed as possible. This article reviews the potential of state-of-the-art numerical schemes like Smooth-Particle and grid-based hydrodynamics as well as the inherent processes as of star-formation criteria and feedback, energy deposit and metal enrichment by stars and on the influence of gas-phase interactions on the galactic gas dynamics and chemistry.

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