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Garching bei München, Germany

Cole D.R.,University of Central Lancashire | Debattista V.P.,University of Central Lancashire | Erwin P.,Max Planck Insitut fur Extraterrestrische Physik | Erwin P.,Universitats Sternwarte Munich | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2014

The role of gas in themass assembly at the nuclei of galaxies is still subject to some uncertainty. Stellar nuclear discs bridge the gap between the large-scale galaxy and the central massive objects that reside there. Using a high-resolution simulation of a galaxy forming out of gas cooling and settling into a disc, we study the formation and properties of nuclear discs. Gas, driven to the centre by a bar, settles into a rotating star-forming nuclear disc (ND). This ND is thinner, younger, kinematically cooler and more metal rich than the surrounding bar. The ND is elliptical and orthogonal to the bar. The complex kinematics in the region of the ND are a result of the superposition of older stars streaming along the bar and younger stars circulating within the ND. The signature of the ND is therefore subtle in the kinematics. Instead the ND stands out clearly in metallicity and age maps. We compare the model to the density and kinematics of real galaxies with NDs finding qualitative similarities. Our results suggest that gas dissipation is very important for forming nuclear structures.

Vollmer B.,11 Rue Of Luniversite | Davies R.I.,Max Planck Insitut fur Extraterrestrische Physik
Astronomy and Astrophysics | Year: 2013

Galactic gas-gas collisions involving a turbulent multiphase interstellar medium (ISM) share common ISM properties: dense extraplanar gas visible in CO, large linewidths (≳3;50 km s-1), strong mid-infrared H 2 line emission, low star formation activity, and strong radio continuum emission. Gas-gas collisions can occur in the form of ram pressure stripping caused by the rapid motion of a spiral galaxy within the intracluster medium, galaxy head-on collisions, compression of the intragroup gas and/or galaxy ISM by an intruder galaxy which flies through the galaxy group at a high velocity, or external gas accretion on an existing gas torus in a galactic center. We suggest that the common theme of all these gas-gas interactions is adiabatic compression of the ISM leading to an increase of the turbulent velocity dispersion of the gas. The turbulent gas clouds are then overpressured and star formation is quenched. Within this scenario we developed a model for turbulent clumpy gas disks where the energy to drive turbulence is supplied by external infall or the gain of potential energy by radial gas accretion within the disk. The cloud size is determined by the size of a continuous (C-type) shock propagating in dense molecular clouds with a low ionization fraction at a given velocity dispersion. We give expressions for the expected volume and area filling factors, mass, density, column density, and velocity dispersion of the clouds. The latter is based on scaling relations of intermittent turbulence whose open parameters are estimated for the circumnuclear disk in the Galactic center. The properties of the model gas clouds (~0.1 pc, ~100 M ⊙, Δv ≳ 6 km s-1) and the external mass accretion rate necessary for the quenching of the star formation rate due to adiabatic compression (Ṁ ~ 1-10 M⊙ yr-1) are consistent with those derived from high-resolution H2 2.12 μm line observations. Based on these findings, a scenario for the evolution of gas tori in galactic centers is proposed and the implications for star formation in the Galactic center are discussed. © ESO, 2013.

Bailey J.D.,Max Planck Insitut fur Extraterrestrische Physik | Landstreet J.D.,College Hill | Landstreet J.D.,University of Western Ontario
Astronomy and Astrophysics | Year: 2015

Context. In recent years, significant effort has been made to understand how the magnetic field strengths and atmospheric chemical abundances of Ap/Bp stars evolve during their main sequence lifetime by identifying a large number of Ap/Bp stars with accurately known ages. As a next step, these stars should be studied individually and in detail to offer further insight into the physics of how such main sequence stars evolve. Aims. We have obtained high resolution spectra using the ESPaDOnS spectropolarimeter and FEROS spectrograph of the chemically peculiar, magnetic Bp star HD 133652. Using these data, we present a simple magnetic field model and abundance determinations of He, O, Mg, Si, Ti, Cr, Fe, Pr, and Nd. Methods. Abundance analysis was performed using zeeman.f, a spectral synthesis program that includes the effects of magnetic fields on line formation. The magnetic field structure is approximated as a simple, co-linear multipole expansion that reproduces the observed variations of the line-of-sight magnetic field with phase. The abundance distribution of each element was modelled using a uniform abundance in each of the two magnetic hemispheres. Results. Using the new magnetic field measurements, we were able to refine the rotation period of HD 133652 to P = 2.30405 ± 0.00002 d. The abundance analysis reveals that the elements modelled (except He, O and Mg) are overabundant compared to the Sun; however most elements studied do not show substantial differences in the large-scale mean abundances between the two magnetic hemispheres. The individual line profiles are very complex and clearly indicate the presence of significant small-scale abundance variations on the stellar surface. Conclusions. These data are adequate to perform a useful investigation of the magnetic field structure and abundance distribution over the stellar surface. HD 133652 is now one of a growing list of hotter Bp stars of known age for which this type of analysis has been performed. © ESO 2015.

Gutierrez L.,National Autonomous University of Mexico | Gutierrez L.,Institute of Astrophysics of Canarias | Gutierrez L.,University of La Laguna | Erwin P.,Max Planck Insitut fur Extraterrestrische Physik | And 7 more authors.
Astronomical Journal | Year: 2011

We present azimuthally averaged radial profiles of R-band surface brightness for a complete sample of 47 early-type, unbarred galaxies, as a complement to our previous study of early-type barred galaxies. Following very careful sky subtraction, the profiles can typically be determined down to brightness levels well below 27 mag arcsec-2 and in the best cases below 28 mag arcsec-2. We classified the profiles according to the scheme used previously for the barred sample: TypeI profiles are single unbroken exponential radial declines in brightness; TypeII profiles (" truncations") have an inner shallow slope (usually exponential) which changes at a well-defined break radius to a steeper exponential; and TypeIII profiles ("antitruncations") have an inner exponential that is steeper, giving way to a shallower outer (usually exponential) decline. By combining these profiles with previous studies, we can make the first clear statements about the trends of outer-disk-profile types along the Hubble sequence (including both barred and unbarred galaxies), and their global frequencies. We find that TypeI profiles are most frequent in early-type disks, decreasing from one-third of all S0-Sa disks to barely 10% of the latest-type spirals. Conversely, TypeII profiles (truncations) increase in frequency with Hubble type, from only 25% of S0 galaxies to 80% of Sd-Sm spirals. Overall, the fractions of TypeI, II, and III profiles for all disk galaxies (Hubble types S0-Sm) are 21%, 50%, and 38%, respectively; this includes galaxies (8% of the total) with composite TypeII+III profiles (counted twice). Finally, we note the presence of bars in 10 galaxies previously classified (optically) as "unbarred." This suggests that 20% of optically unbarred galaxies are actually barred; the bars in such cases can be weak, obscured by dust, or so large as to be mistaken for the main disk of the galaxy. © 2011. The American Astronomical Society. All rights reserved.

Erwin P.,Max Planck Insitut fur Extraterrestrische Physik | Erwin P.,Universitats Sternwarte Munich | Debattista V.P.,University of Central Lancashire
Monthly Notices of the Royal Astronomical Society | Year: 2013

We show that direct detection and measurement of the vertically thickened parts of bars (so-called 'boxy' or 'peanut-shaped' bulges) are possible not only for edge-on galaxies but also for galaxies with moderate inclinations (i< 70°), and that examples are relatively common in the nearby Universe. The analysis of a sample of 78 nearby, moderately inclined (i ≲ 65°) early-type (S0-Sb) barred galaxies shows that the isophotal signature of the box/peanut can usually be detected for inclinations as low as i ∼ 40° - and in exceptional cases down to i ∼ 30°. In agreement with the predictions from N-body simulations, the signature is most easily detectable when the bar's position angle is within ∼50° of the galaxy major axis; in particular, galaxies where the bar lies very close to the minor axis do not show the signature clearly or at all. For galaxies with i= 40°-65° and relative angles <45°, we find evidence for the signature ≈2/3 of the time; the true frequency of box/peanut structures in bars may be higher. Comparison with N-body models also allows us to link observed photometric morphology with 3D physical structures, and thus estimate the relative sizes of box/peanut structures and bars. For our local sample, we find that box/peanut structures range in radial size (measured along the bar major axis) from 0.4 to 3.8 kpc (mean 1.5 ± 0.9 kpc) and span 0.26-0.58 of the bar length (mean of 0.38 ± 0.08). This is a clear observational confirmation that when bars thicken, it is not the entire bar which does so, but only the inner part. This technique can also be used to identify galaxies with bars which have not vertically thickened. We suggest that NGC 3049 and IC 676 may be particularly good examples, and that the fraction of S0-Sb bars which lack box/peanut structures is at least ∼13 per cent. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

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