54 research outputs found
The large scale magnetic field structure of the spiral galaxy NGC 5775
In order to better determine the large-scale 3D-structure of magnetic fields
in spiral galaxies we present a Faraday rotation analysis of the edge-on spiral
galaxy NGC 5775. Deep radio-continuum observations in total power and linear
polarization were performed at 8.46 GHz with the VLA and the 100-m Effelsberg
telescope. They were analyzed together with archival 4.86 and 1.49 GHz
VLA-data. We thus can derive rotation measures from a comparison of three
frequencies and determine the intrinsic magnetic field structure. A very
extended halo is detected in NGC 5775, with magnetic field lines forming an
X-shaped structure. Close to the galactic disk the magnetic field is
plane-parallel. The scaleheights of the radio emission esimated for NGC 5775
are comaprable with other galaxies. The rotation measure distribution varies
smoothly on both sides along the major axis from positive to negative values.
From the derived distribution of rotation measures and the plane-parallel
intrinsic magnetic field orientation along the galactic midplane we conclude
that NGC 5775 has an 'even axisymmetric' large-scale magnetic field
configuration in the disk as generated by an \alpha \Omega -dynamo which is
accompanied by a quadrupolar poloidal field. The magnetic field lines of the
plane-parallel component are pointing 'outwards'. The observed X-shaped halo
magnetic field, however, cannot be explained by the action of the disk's
mean-field dynamo alone. It is probably due to the influence of the galactic
wind together with the dynamo action.Comment: 11 pages, 13 figures, A&A accepte
AGN feedback works both ways
Simulations of galaxy growth need to invoke strong negative feedback from
active galactic nuclei (AGN) to suppress the formation of stars and thus
prevent the over-production of very massive systems. While some observations
provide evidence for such negative feedback, other studies find either no
feedback, or even positive feedback, with increased star formation associated
with higher AGN luminosities. Here we report an analysis of several hundred AGN
and their host galaxies in the Chandra Deep Field South using X-ray and radio
data for sample selection. Combined with archival far infrared data as a
reliable tracer of star formation activity in the AGN host galaxies, we find
that AGN with pronounced radio jets exhibit a much higher star formation rate
than the purely X-ray selected ones, even at the same X-ray luminosities. This
difference implies that positive AGN feedback plays an important role, too, and
therefore has to be accounted for in all future simulation work. We interpret
this to indicate that the enhanced star formation rate of radio selected AGN
arises because of jet-induced star formation, as is hinted by the different jet
powers among our AGN samples, while the suppressed star formation rate of X-ray
selected AGN is caused by heating and photo-dissociation of molecular gas by
the hot AGN accretion disc.Comment: 21 pages, 9 figures, 3 tables, accepted for publication by Ap
The kinematics of the diffuse ionized gas in NGC 4666
The global properties of the interstellar medium with processes such as
infall and outflow of gas and a large scale circulation of matter and its
consequences for star formation and chemical enrichment are important for the
understanding of galaxy evolution. In this paper we studied the kinematics and
morphology of the diffuse ionized gas (DIG) in the disk and in the halo of the
star forming spiral galaxy NGC 4666 to derive information about its kinematical
properties. Especially, we searched for infalling and outflowing ionized gas.
We determined surface brightness, radial velocity, and velocity dispersion of
the warm ionized gas via high spectral resolution (R ~ 9000) Fabry-P\'erot
interferometry. This allows the determination of the global velocity field and
the detection of local deviations from this verlocity field. We calculated
models of the DIG distribution and its kinematics for comparison with the
measured data. In this way we determined fundamental parameters such as the
inclination and the scale height of NGC 4666, and established the need for an
additional gas component to fit our observed data. We found individual areas,
especially along the minor axis, with gas components reaching into the halo
which we interpret as an outflowing component of the diffuse ionized gas. As
the main result of our study, we were able to determine that the vertical
structure of the DIG distribution in NGC 4666 is best modeled with two
components of ionized gas, a thick and a thin disk with 0.8 kpc and 0.2 kpc
scale height, respectively. Therefore, the enhanced star formation in NGC 4666
drives an outflow and also maintains a thick ionized gas layer reminiscent of
the Reynold's layer in the Milky Way.Comment: 12 pages, 10 figures, 3 table
An AGN-starburst composite multi-messenger model of NGC 1068
Recent multi-wavelength observations indicate that some starburst galaxies
show a dominant nonthermal contribution from its central region. These active
galactic nuclei (AGN)-starburst composites are of special interest, as both
phenomena on their own are potential sources of the high-energetic cosmic rays
and their accompanied gamma-ray and neutrino emission. Here, we will focus on
NGC 1068, which is known since several years from its atypical radio-gamma-ray
correlation. Recently this source has also shown strong indications of high
energy neutrino emission. A first semi-analytical, two-component
multi-messenger model is presented that already gives some constraints on the
AGN-starburst composite characteristics of NGC 1068 and exposes the need to
include both starburst \emph{and} AGN corona to describe the multi-messenger
data
Sharpening up Galactic all-sky maps with complementary data - A machine learning approach
Galactic all-sky maps at very disparate frequencies, like in the radio and
-ray regime, show similar morphological structures. This mutual
information reflects the imprint of the various physical components of the
interstellar medium. We want to use multifrequency all-sky observations to test
resolution improvement and restoration of unobserved areas for maps in certain
frequency ranges. For this we aim to reconstruct or predict from sets of other
maps all-sky maps that, in their original form, lack a high resolution compared
to other available all-sky surveys or are incomplete in their spatial coverage.
Additionally, we want to investigate the commonalities and differences that the
ISM components exhibit over the electromagnetic spectrum. We build an
-dimensional representation of the joint pixel-brightness distribution of
maps using a Gaussian mixture model and see how predictive it is: How well
can one map be reproduced based on subsets of other maps? Tests with mock data
show that reconstructing the map of a certain frequency from other frequency
regimes works astonishingly well, predicting reliably small-scale details well
below the spatial resolution of the initially learned map. Applied to the
observed multifrequency data sets of the Milky Way this technique is able to
improve the resolution of, e.g., the low-resolution Fermi LAT maps as well as
to recover the sky from artifact-contaminated data like the ROSAT 0.855 keV
map. The predicted maps generally show less imaging artifacts compared to the
original ones. A comparison of predicted and original maps highlights
surprising structures, imaging artifacts (fortunately not reproduced in the
prediction), and features genuine to the respective frequency range that are
not present at other frequency bands. We discuss limitations of this machine
learning approach and ideas how to overcome them
Evidence for a large off-centered galactic outflow and its connection to the extraplanar diffuse ionized gas in IC 1553
Aims. We analyze a MUSE optical integral field spectrum of the star-forming
edge-on galaxy IC 1553 in order to study its extraplanar diffuse ionized gas
(eDIG) and the processes shaping its disk-halo interface. Methods. We extracted
the optical emission line properties from the integral field spectrum and
generated the commonly used emission line diagnostic diagrams in order to
analyze the ionization conditions and the distribution of the eDIG.
Furthermore, we performed gravitational potential fitting to investigate the
kinematics of a suspected galactic outflow. Results. We find that the eDIG
scale height has a maximum value of approximately 1.0 kpc and decreases roughly
linearly with the radial distance from the galactic center in projection. The
ionization state of the eDIG is not consistent with a pure photoionization
scenario and instead requires a significant contribution from shock ionization.
This, in addition to the gas kinematics, strongly suggests the presence of a
galactic scale outflow, the origin of which lies at least 1.4 kpc away from the
galactic center. The inferred shock velocity in the eDIG of approximately 225
km s-1 is comparable to the escape velocity estimated from our potential
modelling. The asymmetric distribution of currently star-forming clusters
produces a range of different ionization conditions in the eDIG. As a result,
the vertical emission line profiles vary quantitatively and qualitatively along
the major axis of the galaxy. This analysis illustrates that it is crucial in
studies of the eDIG to use observations that take the spatial and kinematical
distributions into account, such as those done with integral field units, to
form an accurate picture of the relevant physical properties.Comment: Accepted for publication by Astronomy & Astrophysics on August 1,
202
Detecting Quasars in Large-Scale Astronomical Surveys
We present a classification-based approach to identify quasi-stellar radio
sources (quasars) in the Sloan Digital Sky Survey and evaluate its performance
on a manually labeled training set. While reasonable results can already be
obtained via approaches working only on photometric data, our experiments
indicate that simple but problem-specific features extracted from spectroscopic
data can significantly improve the classification performance. Since our
approach works orthogonal to existing classification schemes used for building
the spectroscopic catalogs, our classification results are well suited for a
mutual assessment of the approaches' accuracies.Comment: 6 pages, 8 figures, published in proceedings of 2010 Ninth
International Conference on Machine Learning and Applications (ICMLA) of the
IEE
The Effect of Light Scattering by Dust in Galactic Halos on Emission Line Ratios
We present results from Monte Carlo simulations describing the radiation
transfer of line emission, produced both by HII regions in the disk
and in the diffuse ionized gas (DIG), through the dust layer of the galaxy
NGC891. This allows us to calculate the amount of light originating in the HII
regions of the disk and scattered by dust at high , and compare it with the
emission produced by recombinations in the DIG. The cuts of photometric and
polarimetric maps along the -axis show that scattered light from HII regions
is still 10\% of that of the DIG at ~pc, whereas the the degree of
linear polarization is small (\%). The importance of these results for the
determination of intrinsic emission line ratios is emphasized, and the
significance and possible implications of dust at high are discussed.Comment: 12 pages, LaTeX (aasms4.sty), 2 figures; ApJ Letters, accepted, June
5t
A Hubble Space Telescope WFPC2 Investigation of the Disk-Halo Interface in NGC891
We present deep narrowband observations with high spatial resolution of
extraplanar diffuse ionized gas in the halo of NGC891, obtained with the WFPC2
on-board the HST. Our H-alpha observations, centered on the northern part of
NGC891, reveal an extended gaseous halo, which fills almost the entire field of
view of our WFPC2 observation. The majority of the H-alpha emission is diffuse.
Several discrete features (e.g., filaments) are visible as well. Some of the
filaments reach distances of up to 2.2 kpc above the galactic plane, and are
extremely collimated, even at high galactic latitudes. We compare the
morphology of these filaments with theoretical models, which describe possible
transport mechanisms in a general way. We also investigate extraplanar dust
features, which are best visible in unsharp-masked images of our broadband
F675W image, and we compare them to the spatial distribution of DIG filaments.
Individual dust features, however, are not spatially correlated with diffuse
ionized gas counterparts, such as individual filaments. Quite interestingly,
the orientation of the dust features changes from being mostly aligned
perpendicular to the disk at low galactic latitudes, to a parallel alignment at
high |z|. We compare the diffuse ionized gas distribution to the hot ionized
medium, traced by X-ray observations performed by Chandra. There exists a good
correlation of the presence of the warm and hot ionized gas, in particular, an
X-ray bright region at |z| ~ 1-1.5 kpc fills the entire northern halo region,
whereas the intensity in the midplane is considerably depressed. We also
compare the sizes of individual H-alpha emission line features in the midplane
of NGC891 with similar structures that are known in our Milky Way and in the
LMC.Comment: LaTeX, 15 pages, 11 figures, accepted for publication in The
Astronomical Journal, paper with high resolution figures can be accessed at
http://www.astro.rub.de/jrossa/ngc891
- …