44 research outputs found
Numerical modeling of the interstellar medium in galactic disks
We have been developing detailed hydrodynamic models of the global interstellar medium in the hope of understanding the mass and volume occupied by various phases, as well as their structure and kinematics. In our model, the gas is modeled by one fluid while representative Pop 1 stars are modeled by a second fluid. The two fluids are coupled in that the gas forms into stars at a rate given by a Schmidt law while stellar mass loss returns matter into the gas phase (on a time scale of 100 Myr). Also, the stars heat the gas through stellar winds and the gas cools through optically thin radiation. The time behavior of these two fluids is studied in two spatial dimensions with the Eulerian finite difference numerical hydrodynamic code Zen. The two spatial dimensions are along the plane of a disk (x, total length of 2 kpc) and perpendicular to the disk (z, total height of +/- 15 kpc) and a galactic gravitational field in the z direction, typical of that at the solar circle, is imposed upon the simulation; self-gravity and rotation are absent. For the boundary conditions, outflow is permitted at the top and bottom of the grid (z = +/- 15 kpc) while periodic boundary conditions are imposed upon left and right sides of the grid. As initial conditions, we assumed a gaseous distribution like that seen for the H1 by earlier researchers, although the results are insensitive to the initial conditions. We have run simulations in which the heating due to stars, parameterized as a stellar wind velocity, a, is varied from low (a = 150 km/s), to intermediate (a = 300 km/s), to high (a = 600 km/s). Since the intermediate case is roughly equivalent to the Galactic energy injection rate from supernovae, this summary will concentrate on results from this simulation
When Eyewitnesses Are Also Earwitnesses: Effects on Visual and Voice Identifications
In Experiment 1, subjects witnessed a mock crime either visually or both auditorily and visually. A visual lineup was conducted with either a guilty or an innocent suspect present. Identification accuracy of visual-only versus auditory-visual witnessed did not differ, although the diagnosticity ratio for the visual-only condition was more than twice as large. Thus, there was only limited support for auditory information interfering with encoding visual information. In Experiment 2, subjects witnessed a mock crime either auditorily or both auditorily and visually. A voice lineup was conducted with either a guilty or an innocent suspect present. Consistent with Yarmey’s (1986) prediction that visual information can interfere with encoding auditory information, guilty-suspect identification was significantly higher in the auditory-only condition
An H-alpha survey aiming at the detection of extraplanar diffuse ionized gas in halos of edge-on spiral galaxies. I. How common are gaseous halos among non-starburst galaxies?
In a series of two papers we present results of a new H-alpha imaging survey,
aiming at the detection of extraplanar diffuse ionized gas in halos of
late-type spiral galaxies. We have investigated a sample of 74 nearby edge-on
spirals, covering the northern and southern hemisphere. In 30 galaxies we
detected extraplanar diffuse emission at mean distances of |z|~1-2 kpc.
Individual filaments can be traced out to |z|<= 6 kpc in a few cases. We find a
good correlation between the FIR flux ratio (S_60/S_100) and the SFR per unit
area (L_FIR/D^2_25), based on the detections/non-detections. This is actually
valid for starburst, normal and for quiescent galaxies. A minimal SFR per unit
area for the lowest S_60/S_100 values, at which extended emission has been
detected, was derived, which amounts to (dE/dt)_A25^thres = (3.2+-0.5)x10E+40
erg/s/kpc^2. There are galaxies where extraplanar emission was detected at
smaller values of L_FIR/D^2_25, however, only in combination with a
significantly enhanced dust temperature. The results corroborate the general
view that the gaseous halos are a direct consequence of SF activity in the
underlying galactic disk.Comment: LaTeX, 12 pages, 7 figures, accepted for publication in A&
Kinematics of diffuse ionized gas in the disk halo interface of NGC 891 from Fabry-P\'erot observations
The properties of the gas in halos of galaxies constrain global models of the
interstellar medium. Kinematical information is of particular interest since it
is a clue to the origin of the gas. Here we report observations of the
kinematics of the thick layer of the diffuse ionized gas in NGC 891 in order to
determine the rotation curve of the halo gas. We have obtained a Fabry-P\'erot
data cube in Halpha to measure the kinematics of the halo gas with angular
resolution much higher than obtained from HI 21 cm observations. The data cube
was obtained with the TAURUS II spectrograph at the WHT on La Palma. The
velocity information of the diffuse ionized gas extracted from the data cube is
compared to model distributions to constrain the distribution of the gas and in
particular the halo rotation curve. The best fit model has a central
attenuation tau_H-alpha=6, a dust scale length of 8.1 kpc, an ionized gas scale
length of 5.0 kpc. Above the plane the rotation curve lags with a vertical
gradient of -18.8 km/s/kpc. We find that the scale length of the H-alpha must
be between 2.5 and 6.5 kpc. Furthermore we find evidence that the rotation
curve above the plane rises less steeply than in the plane. This is all in
agreement with the velocities measured in the HI.Comment: A&A, in press. 13 pages, 19 figure
Dependence of radio halo properties on star formation activity and galaxy mass
We investigate the relation between the existence and size of radio halos,
which are believed to be created by star formation (SF) related energy input
into the interstellar medium, and other galaxy properties, most importantly
star formation activity and galaxy mass. Based on radio continuum and H-alpha
observations of a sample of seven late-type spiral galaxies we find a direct,
linear correlation of the radial extent of gaseous halos on the size of the
actively star-forming parts of the galaxy disks. Data of a larger sample of 22
galaxies indicate that the threshold energy input rate into the disk ISM per
unit surface area for the creation of a gaseous halo depends on the mass
surface density of the galaxy, in the sense that a higher threshold must be
surpassed for galaxies with a higher surface density. Because of the good
prediction of the existence of a radio halo from these two parameters, we
conclude that they are important, albeit not the only contributors. The
compactness of the SF-related energy input is also found to be a relevant
factor. Galaxies with relatively compact SF distributions are more likely to
have gaseous halos than others with more widespread SF activity. These results
quantify the so-called "break-out" condition for matter to escape from galaxy
disks, as used in all current models of the interstellar medium and first
defined by Norman and Ikeuchi (1989).Comment: accepted for publication in Astronomy & Astrophysic
HI in NGC 5433 and its Environment: High-Latitude Emission in a Small Galaxy Group
We present HI synthesis maps of the edge-on starburst NGC 5433 and its
environment, obtained with the VLA in its C and D configurations. The
observations and spectral model residuals of the main disc emission in NGC 5433
reveal 3 extraplanar features. We associate 2 of these features with coherent
extraplanar extensions across multiple spectral channels in our data, including
a complete loop in position-velocity space. Interpreting the latter as an
expanding shell we derive a corresponding input energy of 2 x 10^54 ergs,
comparable to that for the largest supershells found in the Galaxy and those in
other edge-on systems. NGC 5433 is in a richer environment than previously
thought. We confirm that KUG 1359+326 is a physical companion to NGC 5433 and
find two new faint companions, both with Minnesota Automated Plate Scanner
identifications, that we label SIS-1 and SIS-2. Including the more distant IC
4357, NGC 5433 is the dominant member of a group of at least 5 galaxies,
spanning over 750 kpc in a filamentary structure. A variety of evidence
suggests that interactions are occurring in this group. While a number of
underlying mechanisms are consistent with the morphology of the high-latitude
features in NGC 5433, we argue that environmental effects may play a role in
their generation.Comment: 18 pages including 10 figures. Accepted for publication in MNRAS. For
higher resolution Fig. 1, see http://www.astro.cornell.edu/~spekkens/papers/
v2: Proof-corrected cop
An H-alpha survey aiming at the detection of extraplanar diffuse ionized gas in halos of edge-on spiral galaxies II. The H-alpha survey atlas and catalog
In this second paper on the investigation of extraplanar diffuse ionized gas
in nearby edge-on spiral galaxies we present the actual results of the
individual galaxies of our H-alpha imaging survey. A grand total of 74 galaxies
have been studied, including the 9 galaxies of a recently studied sub-sample
(Rossa & Dettmar 2000). 40.5% of all studied galaxies reveal extraplanar
diffuse ionized gas, whereas in 59.5% of the survey galaxies no extraplanar
diffuse ionized gas could be detected. The average distances of this extended
emission above the galactic midplane range from 1-2 kpc, while individual
filaments in a few galaxies reach distances of up to |z| ~ 6 kpc. In several
cases a pervasive layer of ionized gas was detected, similar to the Reynolds
layer in our Milky Way, while other galaxies reveal only extended emission
locally. The morphology of the diffuse ionized gas is discussed for each galaxy
and is compared with observations of other important ISM constituents in the
context of the disk-halo connection, in those cases where published results
were available. Furthermore, we present the distribution of extraplanar dust in
these galaxies, based on an analysis of the unsharp-masked R-band images. The
results are compared with the distribution of the diffuse ionized gas.Comment: LaTeX, 21 pages, 7 figures, accepted for publication in A&A, figs.
22-54 are only available in electronic form and figs. 2-11 + 17-20 are also
available at http://www.astro.rub.de/jrossa/ha-surve
Long-term X-ray variability of Swift J1644+57
We studied the X-ray timing and spectral variability of the X-ray source Sw
J1644+57, a candidate for a tidal disruption event. We have separated the
long-term trend (an initial decline followed by a plateau) from the short-term
dips in the Swift light-curve. Power spectra and Lomb-Scargle periodograms hint
at possible periodic modulation. By using structure function analysis, we have
shown that the dips were not random but occurred preferentially at time
intervals ~ [2.3, 4.5, 9] x 10^5 s and their higher-order multiples. After the
plateau epoch, dipping resumed at ~ [0.7, 1.4] x 10^6 s and their multiples. We
have also found that the X-ray spectrum became much softer during each of the
early dip, while the spectrum outside the dips became mildly harder in its
long-term evolution. We propose that the jet in the system undergoes precession
and nutation, which causes the collimated core of the jet briefly to go out of
our line of sight. The combined effects of precession and nutation provide a
natural explanation for the peculiar patterns of the dips. We interpret the
slow hardening of the baseline flux as a transition from an extended, optically
thin emission region to a compact, more opaque emission core at the base of the
jet.Comment: 16 pages, 12 figures. Accepted by MNRAS on 2012 Feb 11; minor
improvements in the introduction and discussion from the previous arXiv
versio
The Interstellar Environment of our Galaxy
We review the current knowledge and understanding of the interstellar medium
of our galaxy. We first present each of the three basic constituents - ordinary
matter, cosmic rays, and magnetic fields - of the interstellar medium, laying
emphasis on their physical and chemical properties inferred from a broad range
of observations. We then position the different interstellar constituents, both
with respect to each other and with respect to stars, within the general
galactic ecosystem.Comment: 39 pages, 12 figures (including 3 figures in 2 parts
A review of elliptical and disc galaxy structure, and modern scaling laws
A century ago, in 1911 and 1913, Plummer and then Reynolds introduced their
models to describe the radial distribution of stars in `nebulae'. This article
reviews the progress since then, providing both an historical perspective and a
contemporary review of the stellar structure of bulges, discs and elliptical
galaxies. The quantification of galaxy nuclei, such as central mass deficits
and excess nuclear light, plus the structure of dark matter halos and cD galaxy
envelopes, are discussed. Issues pertaining to spiral galaxies including dust,
bulge-to-disc ratios, bulgeless galaxies, bars and the identification of
pseudobulges are also reviewed. An array of modern scaling relations involving
sizes, luminosities, surface brightnesses and stellar concentrations are
presented, many of which are shown to be curved. These 'redshift zero'
relations not only quantify the behavior and nature of galaxies in the Universe
today, but are the modern benchmark for evolutionary studies of galaxies,
whether based on observations, N-body-simulations or semi-analytical modelling.
For example, it is shown that some of the recently discovered compact
elliptical galaxies at 1.5 < z < 2.5 may be the bulges of modern disc galaxies.Comment: Condensed version (due to Contract) of an invited review article to
appear in "Planets, Stars and Stellar
Systems"(www.springer.com/astronomy/book/978-90-481-8818-5). 500+ references
incl. many somewhat forgotten, pioneer papers. Original submission to
Springer: 07-June-201