44 research outputs found
The influence of diffuse scattered light II. Observations of galaxy haloes and thick discs and hosts of BCGs
Studies of deep photometry of galaxies have presented discoveries of excess
light in surface-brightness and colour profiles at large radii in the form of
diffuse faint haloes and thick discs. In a majority of the cases, it has seemed
necessary to use exotic stellar populations or alternative physical solutions
to explain the excess. Few studies have carefully scrutinized the role of
scattered light in this context. I explore the influence of scattered light on
ground-based observations of haloes and thick discs around edge-on galaxies,
haloes around face-on disc galaxies, host galaxies around blue compact galaxies
(BCGs), and haloes around elliptical galaxies. Surface-brightness structures of
all considered types of galaxies are modelled and analysed to compare
scattered-light haloes and thick discs with measurements. I simulate the
influence of scattered light and accurate sky subtraction on simplified
S\'ersic-type and face-on disc galaxy models. All galaxy models are convolved
with both lower-limit and brighter point spread functions (PSFs); for a few
galaxies it was possible to use dedicated PSFs. The results show bright
scattered-light haloes and high amounts of red excess at large radii and faint
surface brightnesses for nearly all types of galaxies; exceptions are the
largest elliptical-type galaxies where the influence of scattered light is
smaller. Studies have underestimated the role of scattered light to explain
their surface-brightness profiles. My analysis shows surface-brightness
profiles that include scattered light that are very similar to and overlap
measurements at all radii. The derivation of physical properties of haloes,
thick discs, and BCG hosts from diffuse data is misleading since accurate and
radially extended PSFs are non-existent. Significantly improved analyses that
include new measurements of PSFs are required to study diffuse haloes further.Comment: 18 pages, 11 figures, and 15 pages + 11 figures in appendix, accepted
for publication in A&
The influence of diffuse scattered light I. The PSF and its role to observations of the edge-on galaxy NGC 5907
All telescopes and instruments are to some degree affected by scattered
light. It is possible to estimate the amount of such scattered light, and even
correct for it, with a radially extended point spread function (PSF). The outer
parts of the PSF have only rarely been determined, since they are faint and
therefore difficult to measure. A mostly complete overview of existing
properties and measurements of radially extended PSFs is presented, to both
show their similarities and to indicate how bright extended objects can be used
to measure the faintest regions. The importance of the far wings of the PSF and
their possible temporal variations are demonstrated in three edge-on galaxy
models. The same study is applied to the first edge-on galaxy where earlier
observations reveal a halo, NGC 5907. All PSFs were collected in two diagrams,
after they were offset or normalized, when that was possible.
Surface-brightness structures of edge-on galaxies were modelled and analysed to
study scattered-light haloes that result with an exponential disc. The models
were convolved with both a lower-limit PSF and a more average PSF. The PSF of
the observed data could be used in the case of NGC 5907. The comparison of the
PSFs demonstrates a lower-limit power-law decline at larger radii. The
analysis of the galaxy models shows that also the outer parts of the PSF are
important to correctly model and analyse observations and, in particular,
fainter regions. The reassessed analysis of the earlier measurements of NGC
5907 reveals an explanation for the faint halo in scattered light, within the
quoted level of accuracy.Comment: 17 pages, 9 figures, Astronomy & Astrophysics, in pres
Helical coronal ejections and their role in the solar cycle
The standard theory of the solar cycle in terms of an alpha-Omega dynamo
hinges on a proper understanding of the nonlinear alpha effect. Boundary
conditions play a surprisingly important role in determining the magnitude of
alpha. For closed boundaries, the total magnetic helicity is conserved, and
since the alpha effect produces magnetic helicity of one sign in the large
scale field, it must simultaneously produce magnetic helicity of the opposite
sign. It is this secondary magnetic helicity that suppresses the dynamo in a
potentially catastrophic fashion. Open boundaries allow magnetic helicity to be
lost. Simulations are presented that allow an estimate of alpha in the presence
of open or closed boundaries, either with or without solar-like differential
rotation. In all cases the sign of the magnetic helicity agrees with that
observed at the solar surface (negative in the north, positive in the south),
where significant amounts of magnetic helicity can be ejected via coronal mass
ejections. It is shown that open boundaries tend to alleviate catastrophic
alpha quenching. The importance of looking at current helicity instead of
magnetic helicity is emphasized and the conceptual advantages are discussed.Comment: 8 pages, 7 figs, IAU Symp. 223, In: Multi-Wavelength Investigations
of Solar Activity. Eds: A.V. Stepanov, E.E. Benevolenskaya & A.G. Kosoviche
Hot bubbles of planetary nebulae with hydrogen-deficient winds I. Heat conduction in a chemically stratified plasma
Heat conduction has been found a plausible solution to explain discrepancies
between expected and measured temperatures in hot bubbles of planetary nebulae
(PNe). While the heat conduction process depends on the chemical composition,
to date it has been exclusively studied for pure hydrogen plasmas in PNe. A
smaller population of PNe show hydrogen-deficient and helium- and
carbon-enriched surfaces surrounded by bubbles of the same composition;
considerable differences are expected in physical properties of these objects
in comparison to the pure hydrogen case. The aim of this study is to explore
how a chemistry-dependent formulation of the heat conduction affects physical
properties and how it affects the X-ray emission from PN bubbles of
hydrogen-deficient stars. We extend the description of heat conduction in our
radiation hydrodynamics code to work with any chemical composition. We then
compare the bubble-formation process with a representative PN model using both
the new and the old descriptions. We also compare differences in the resulting
X-ray temperature and luminosity observables of the two descriptions. The
improved equations show that the heat conduction in our representative model of
a hydrogen-deficient PN is nearly as efficient with the chemistry-dependent
description; a lower value on the diffusion coefficient is compensated by a
slightly steeper temperature gradient. The bubble becomes somewhat hotter with
the improved equations, but differences are otherwise minute. The observable
properties of the bubble in terms of the X-ray temperature and luminosity are
seemingly unaffected.Comment: 11 pages, 11 figures, A&A in pres
Modeling the diffuse X-ray emission of Planetary Nebulae with different chemical composition
Based on time-dependent radiation-hydrodynamics simulations of the evolution
of Planetary Nebulae (PNe), we have carried out a systematic parameter study to
address the non-trivial question of how the diffuse X-ray emission of PNe with
closed central cavities is expected to depend on the evolutionary state of the
nebula, the mass of the central star, and the metallicity of stellar wind and
circumstellar matter. We have also investigated how the model predictions
depend on the treatment of thermal conduction at the interface between the
central `hot bubble' and the `cool' inner nebula, and compare the results with
recent X-ray observations. Our study includes models whose properties resemble
the extreme case of PNe with Wolf-Rayet type central stars. Indeed, such models
are found to produce the highest X-ray luminosities.Comment: 5 pages, 2 figures, to appear in proceedings of the IAU Symposium
283: "Planetary Nebulae: An Eye to the Future", Eds.: A. Manchado, L.
Stanghellini and D. Schoenberne
The central dynamics of M3, M13, and M92: Stringent limits on the masses of intermediate-mass black holes
We used the PMAS integral field spectrograph to obtain large sets of radial
velocities in the central regions of three northern Galactic globular clusters:
M3, M13, and M92. By applying the novel technique of crowded field 3D
spectroscopy, we measured radial velocities for about 80 stars within the
central ~ 10 arcsec of each cluster. These are by far the largest spectroscopic
datasets obtained in the innermost parts of these clusters up to now. To obtain
kinematical data across the whole extent of the clusters, we complement our
data with measurements available in the literature. We combine our velocity
measurements with surface brightness profiles to analyse the internal dynamics
of each cluster using spherical Jeans models, and investigate whether our data
provide evidence for an intermediate-mass black hole in any of the clusters.
The surface brightness profiles reveal that all three clusters are consistent
with a core profile, although shallow cusps cannot be excluded. We find that
spherical Jeans models with a constant mass-to-light ratio provide a good
overall representation of the kinematical data. A massive black hole is
required in none of the three clusters to explain the observed kinematics. Our
1sigma (3sigma) upper limits are 5300 M_sun (12000 M_sun) for M3, 8600 M_sun
(13000 M_sun) for M13, and 980 M_sun (2700 M_sun) for M92. A puzzling
circumstance is the existence of several potential high velocity stars in M3
and M13, as their presence can account for the majority of the discrepancies
that we find in our mass limits compared to M92.Comment: accepted for publication in A&A, 20 pages, 15 figures, tables D1 to
D6 only available at CD
Spectral characterisation of inertial particle clustering in turbulence
Clustering of inertial particles is important for many types of astrophysical and geophysical turbulence, but it has been studied predominately for incompressible flows. Here, we study compressible flows and compare clustering in both compressively (irrotationally) and vortically (solenoidally) forced turbulence. Vortically and compressively forced flows are driven stochastically either by solenoidal waves or by circular expansion waves, respectively. For compressively forced flows, the power spectrum of the density of inertial particles is a useful tool for displaying particle clustering relative to the fluid density enhancement. Power spectra are shown to be particularly sensitive for studying large-scale particle clustering, while conventional tools such as radial distribution functions are more suitable for studying small-scale clustering. Our primary finding is that particle clustering through shock interaction is particularly prominent in turbulence driven by spherical expansion waves. It manifests itself through a double-peaked distribution of spectral power as a function of Stokes number. The two peaks are associated with two distinct clustering mechanisms; shock interaction for smaller Stokes numbers and the centrifugal sling effect for larger values. The clustering of inertial particles is associated with the formation of caustics. Such caustics can only be captured in the Lagrangian description, which allows us to assess the relative importance of caustics in vortically and compressively forced turbulence. We show that the statistical noise resulting from the limited number of particles in the Lagrangian description can be removed from the particle power spectra, allowing us a more detailed comparison of the residual spectra. We focus on the Epstein drag law relevant for rarefied gases, but show that our findings apply also to the usual Stokes drag.Spectral characterisation of inertial particle clustering in turbulencepublishedVersio
Towards DIB mapping in galaxies beyond 100 Mpc. A radial profile of the 5780.5 diffuse interstellar band in AM 1353-272 B
Diffuse Interstellar Bands (DIBs) are non-stellar weak absorption features of
unknown origin found in the spectra of stars viewed through one or several
clouds of Interstellar Medium (ISM). Research of DIBs outside the Milky Way is
currently very limited. Specifically spatially resolved investigations of DIBs
outside of the Local Group is, to our knowledge, inexistent. Here, we explore
the capability of the high sensitivity Integral Field Spectrograph, MUSE, as a
tool to map diffuse interstellar bands at distances larger than 100 Mpc. We use
MUSE commissioning data for AM 1353-272 B, the member with highest extinction
of the "The Dentist's Chair", an interacting system of two spiral galaxies.
High signal-to-noise spectra were created by co-adding the signal of many
spatial elements distributed in a geometry of concentric elliptical half-rings.
We derived decreasing radial profiles for the equivalent width of the
5780.5 DIB both in the receding and approaching side of the companion
galaxy up to distances of 4.6 kpc from the center of the galaxy.
Likewise, interstellar extinction, as derived from the Halpha/Hbeta line ratio
displays a similar trend, with decreasing values towards the external parts.
This translates into an intrinsic correlation between the strength of the DIB
and the extinction within AM 1353-272 B consistent with the current existing
global trend between these quantities when using measurements for both Galactic
and extragalactic sight lines. Mapping of DIB strength in the Local Universe as
up to now only done for the Milky Way seems feasible. This offers a new
approach to study the relationship between DIBs and other characteristics and
species of the ISM in different conditions as those found in our Galaxy to the
use of galaxies in the Local Group and/or single sightlines towards supernovae,
quasars and galaxies outside the Local Group.Comment: 4 pages, 4 figures, accepted for publication as a Letter in Astronomy
and Astrophysics; Received 10 February 2015 / Accepted 20 February 2015 ;
English corrections include